catalysis and automotive pollution control, proceedings of the first international symposium (capoc...

Studies in Surface Science and CatalysisAdvisory Editors: B. Delman and J.T. Yates

Vol. 30

CATALYSIS ANDAUTOMOTIVEPOLLUTION CONTROLProceedings of the First International Symposium (CAPOC I),Brussels, September 8-11, 1986

Ed itors

A. Crucq and A. FrennetUnite de Recherche sur la Catalyse, Universite libre de Bruxelles, Brussels, Belgium

ELSEVIER Amsterdam - Oxford - New York - Tokyo 1987

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Printed in The Netherlands

IX

STUDIES IN SURFACE SCIENCE AND CATALYSISAdvisory Editors: B. Delmon, Unlversite Catholique de Louvain, Louvain-Ia-Neuve, Belgium

J.T. Yates, University of Pittsburgh, Pittsburgh, PA, U.S.A.

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Volume 14

Preparation of Catalysts I. Scientific Bases for the Preparation of HeterogeneousCatalysts. Proceedings of the First International Symposium held at the SolvayResearch Centre, Brussels, October 14-17, 1975edited by B. Delmon, P.A. Jacobs and G. PonceletThe Control of the Reactivity of Solids. A Critical Survey of the Factors that In-fluence the Reactivity of Solids, with Special Emphasis on the Control of the Chem-ical Processes in Relation to Practical Applicationsby V.V. Boldyrev, M. Bulens and B. DelmonPreparation of Catalysts II. Scientific Bases for the Preparation of HeterogeneousCatalysts. Proceedings of the Second International Symposium, Louvain-Ia-Neuve,September 4-7, 1978edited by B. Delmon, P. Grange, P. Jacobs and G. PonceletGrowth and Properties of Metal Clusters. Applications to Catalysis and the Photo-graph ic Process. Proceedings of the 32nd International Meeting of the Societe deChimie Physique, Villeurbanne, September 24-28, 1979edited by J. BourdonCatalysis by Zeolites. Proceedings of an International Symposium organized by theInstitut de Recherches sur la Catalyse - CNRS - Villeurbanne and sponsored by theCentre National de la Recherche Scientifique, Ecully (Lyon), September 9-11, 1980edited by B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier andH. PraliaudCatalyst Deactivation. Proceedings of the International Symposium, Antwerp,October 13-15, 1980edited by B. Delmon and G.F. FromentNew Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis,Tokyo, June 30-July 4,1980. Parts A and Bedited by T. Seiyama and K. TanabeCatalysis by Supported Complexesby Yu.1. Yermakov, B.N. Kuznetsov and V.A. ZakharovPhysics of Solid Surfaces. Proceedings of the Symposium held in Bechyne, September29-0ctober 3, 1980edited by M. LaznickaAdsorption at the Gas-5olid and Liquid-5olid Interface. Proceedings of an Inter-national Symposium held in Alx-en-Provence, September 21-23, 1981edited by J. Rouquerol and K.S.W. SingMetal-Support and Metai·Additive Effects in Catalysis. Proceedings of an Interna-tional Symposium organized by the Institut de Recherches sur la Catalyse - CNRS -Villeurbanne and sponsored by the Centre National de la Recherche Scientifique,Ecully (Lvonl.Beprember 14-16, 1982edited by B. Imelik, C. Naccache, G. Couduriar, H. Praliaud, P. Meriaudeau,P. Gallezot, G.A. Martin and J.C. VedrineMetal Microstructures in Zeolites. Preparation - Properties - Applications.Proceedings of a Workshop, Bremen, September 22-24,1982edited by P.A. Jacobs, N.!. Jaeger, P. Jir(l and G. Schulz·EkloffAdsorption on Metal Surfaces. An Integrated Approachedited by J. BenardVibrations at Surfaces. Proceedings of the Third International Conference, Asilomar,CA, September 1-4, 1982edited by C.R. Brundle and H. Morawitz

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Heterogeneous Catalytic Reactions Involving Molecular Oxygenby G.I. GolodetsPreparation of Catalysts III. Scientific Bases for the Preparation of HeterogeneousCatalysts. Proceedings of the Third International Symposium, Louvain-Ia-Neuve,September 6-9, 1982edited by G. Poncelet, P. Grange and P.A. JacobsSpillover of Adsorbed Species. Proceedings of the International Symposium,Lyon-Villeurbanne, September 12-16,1983edited by G.M. Pajonk, S.J. Teichner and J.E. GermainStructure and Reactivity of Modified Zeolites. Proceedings of an InternationalConference, Prague, July 9-13, 1984edited by P.A. Jacobs, N.I. Jaeger, P. Jiru, V.B. Kazansky and G. Schulz-EkloffCatalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium onCatalysis, Quebec, P.Q., September 30-0ctober 3, 1984edited by S. Kaliaguine and A. MahayCatalysis by Acids and Bases. Proceedings of an International Symposium organizedby the Institut de Recherches sur la Catalyse-CNRS-Villeurbanne and sponsoredby the Centre National de la Recherche Scientifique, Villeurbanne (Lyon),September 25-27, 1984edited by B. Imelik, C. Naccache, G. Coudurier, V. Ben Taarit and J.C. VedrineAdsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, BruneiUniversity, Uxbridge, June 28-29, 1984edited by M. Che and G.C. BondUnsteady Processes in Catalytic Reactorsby Vu.Sh. MatrosPhysics of Solid Surfaces 1984edited by J. KoukalZeolites: Synthesis, Structure, Technology and Application. Proceedings of theInternational Symposium, Portoroz-Pcrtorose, September 3-8, 1984edited by B. Drzaj, S. HoCevar and S. PejovnikCatalytic Polymerization of OIefins. Proceedings of the International Symposiumon Future Aspects of Olefin Polymerization, Tokyo, July 4-6,1985edited by T. Keii and K. SogaVibrations at Surfaces 1985. Proceedings of the Fourth International Conference,Bowness-on-Windermere, September 15-19, 1985edited by D.A. King, N.V. Richardson and S. HollowayCatalvtic Hvdrogenationedited by L. CervenyNew Developments in Zeolite Science and Technology. Proceedings of the 7thInternational Zeolite Conference, Tokyo, August 17-22, 1986edited by V. Murakami, A. lijima and J.W. WardMetal Clusters in Catalysis.edited by B.C. Gates, L. Guczi and H. KnozingerCatalysis and Automotive Pollution Control. Proceedings of the First InternationalSymposium (CAPaC I), Brussels, September 8-11, 1986edited by A. Crucq and A. Frennet

FOREWORD

In June 1984 the EEC Commission proposed new standards ofpermissible exhaust gas from motor vehicles to be introduced in Europe; thesestandards were approved by the Ministers of the Environment one year later. As thecontrol of automotive pollution is at present mainly a catalytic problem, we thoughtthis was a good opportunity to organize an International Symposium on the subjectand an organizing committee composed of people engaged in catalytic research in thedifferent Belgian Universities was constituted.

As the symposium was the first one to be organized at international levelin this otherwise very restricted scientific field, this decision may have initiallyappeared somewhat risky, but was justified by the success of the four-daysymposium, with 177 people attending. Most participants came from the EEecountries, with large delegations from Belgium (33), France (32), West Germany(26), the United Kingdom (16) and the Netherlands (10) but we must note the size ofthe U.S. (20) and Swedish (10) delegations and the interest shown by people comingfrom Australia, China, Finland, Hungary, Japan, Switzerland and Venezuela. About60% of the participants came from industry, mainly from the car and oil industriesand catalyst manufacturers.

The number of abstracts submitted was not very large (38) but as notedby the Paper Selection Committee and as the reader of the Proceedings will be able tojudge for himself, the quality and the scientific interest of the papers presented areexceptional, and this was also true of the discussions following the presentation; un-fortunately these discussions are not published.

The introduction of the new EEC standards raised some controversy inthe industries concerned as well as in public opinion. That is why the organizerschose to devote the first day of the conference to a general introduction to theproblem of pollution by exhaust gas. Seven invited lectures were presented and arepublished in these Proceedings, dealing with the effects of exhaust gas on humanhealth and the environment, with the economical and legislative problems associatedwith the new EEC standards, and with the points of view of the oil and motorindustries. The first day ended with a round table, with the participation of W.D.J.Evans, C. Gerryn, W. Groenendaal, H.Henssler, K. Taylor and M. Walsh; theensuing general discussion, which is unfortunately not published, was verystimulating.

The topics to be dealt with during the catalytic sessions included not onlythe catalytic converters, but also such problems as specific pollution control of dieselengines, synthesis of adequate fuels, and additives adapted to catalytic converters.Surprisingly, very few papers (3) were submitted and presented on these subjects,whereas 24 papers were devoted to fundamental and applied studies on catalyticconverters, support preparation and base metal catalysts.

Finally the organizers have been strongly encouraged by many parti-cipants to hold a follow-up symposium in a not-too-short delay of 2 to 3 years. Wehope the CAPOC II Conference will generate the same interest as CAPOC I, theProceedings of which are contained in this volume.

XI

XII

ACKNOWLEDGEMENTS

The Organizing Committee is greatly indebted to Mr Ducarme, "Ministrede l'Environnement de l'Executif Regional Wallen", for his support and interest tothis symposium and who accepted to give the opening address.

The organizers also greatly appreciated the cooperation of the members ofthe organizing committee. In this respect, we are particularly grateful to W. Hecq, E.Cadron, M. Campinne and E. Derouane for the active part they have taken in theorganization. The suggestions and advices of A. Derouane, G. Froment, A. Germain,G. Poncelet were very helpful.

Special thanks are due to the members of the paper selection committee fortheir important contribution in selecting the proposed papers with conscientiousness(W.DJ. Evans, G. Leclercq, G. Maire, A. Pentenero, V. Ponec, M. Prigent).

The Organizing Committee is indebted to all the authors of the lecturesdelivered during the introductory session who analyzed various points of view relatedto the general problem of pollution by motor vehicles exhaust gases : health,environment, economics.

It is a pleasure to acknowledge the stimulating action of C. Gerryn as well inthe organization of the symposium as in the introductory session.

We also are grateful to K. Taylor for her outstanding general introductorylecture on the problem of exhaust catalysts. Special thanks to W.DJ. Evans for hisactive part in the paper selection committee and the scientific advisory board and whogave a remarkable general lecture on the exhaust catalyst.

The Organizing Committee acknowledges the authors who presentedpapers, the Chairmen and all the participants who made the symposium fruitful.

The Organizing Committee wants to associate with these acknowledgementsthe members of the "Unite de Recherche sur la Catalyse" of the "Universite Libre deBruxelles" who contributed in various degrees to the success of this symposium:J.-M.Bastin, M.Cogniaux, L.Degols, J.-P.Demiddeleer, P.Moisin, B.Parmentier,G. Thiry, M.-N. Zauwen.

We are indebted to the authorities of the "Universite Libre de Bruxelles''who agreed that this meeting could be held in the facilities of the "Institut deSociologie".

The organizers,

AFRENNETChairman of the

Organizing Committee

ACRUCQSecretary of the

Organizing Committee

THE ORGANIZING COMMITTEE ACKNOWLEDGES THEFINANCIAL SUPPORT OF :

Minlstere de I'Environnement de l'Executif Regional Wallon

Federation BeIge des Industries de l'AutomobiIe et du Cycle(FEBlAC)

Solvay & Cie S.A.

Societe Chimique de Belgique

Banque Bruxelles Lambert

XIII

XIV

A. FULL CONGRESS

Andersson, Lennart

Andersson, Soren

Ashworth, Richard

Baker, RG.

Baresel, D.

Bauwens, Jean

Bennett, C.O.

Berndt, Malte

Blanchard, G.

Block, Jochen

Bordes, Elisabeth

Boulhol, Olivier

Boulinguiez (Mrs)

Bradt, Willy

Brandt, Gerhard

LIST OF PARTICIPANTS

Univ. Chalmers GoteborgSweden

EKANobelABSweden

T.!. Cheswick SilencersUnited Kingdom

Univ. FlindersAustralia

Rob. BoschWest Germany

Cockerill Materials Ind.Belgium

Univ. ConnecticutU.S.A.

Doduco K.G.West Germany

Rhone- PoulencFrance

Fritz Haber Inst.West Germany

Univ. CompiegneFrance

Ag. Qual. AirFrance

ElfFrance

ClaytonBelgium

Ethyl Mineral AdditivesWest Germany

xv

Cairns, J. UKAEA HarwellUnited Kingdom

Campinne, M. Ecole Royale Militaire, BrusselsBelgium

Chapelet Letourneux, Gilbert ElfSolaizeFrance

Cheng San Univ. CompiegneFrance

Chiron, Mireille INRETSFrance

Colbourne, D. ShellWest Germany

Collette, Herve FNDP, NamurBelgium

Cooper, Barry 1. Johoson MattheyUSA

Courtine, Pierre Univ. CompiegneFrance

Crucq, Andre ULB, BrusselsBelgium

Darville FNDP, NamurBelgium

Davies, MJ. UKAEA HarwellUnited Kingdom

Deakin, Alan FordUnited Kingdom

Degols,Luc ULB, BrusselsBelgium

Delmon, Bernard UCL, Louvain La NeuveBelgium

Dettling,1.e. EngelhardUSA

XVI

Donnelly, Richard G. W.R. Grace & CoUSA

Douglas. J.M.K. Johnson MattheyUnited Kingdom

Doziere, Richard IFPFrance

Druart, Guy Soc. Bel. Gaz PetroleBelgium

Dubas, Henri Ciba-GeigySwitzerland

Duprez,D. Univ. PoitiersFrance

Durand. Daniel IFPFrance

Engler DegussaWest Germany

Evans, W.DJ. Johnson MattheyUnited Kingdom

Finck, Francois Univ. L. Pasteur, StrasbourgFrance

Fisher Galen B. General MotorsUSA

Fitch, Frank Laporte InorganicsUnited Kingdom

Fitoussi Rhone PoulencFrance

Foster, Al BPUnited Kingdom

Fougere UTACFrance

Frennet, Alfred ULB, BrusselsBelgium

XVII

Frestad, Arne EKANobelABSweden

Froment, G. Univ. GentBelgium

GandhiH.S. FordUSA

Garin, F. Univ. L. Pasteur, StrasbourgFrance

Garreau Rhone-PoulencFrance

GermainA. Univ. LiegeBelgium

Gerryn, Claude FordBelgium

Girard, Philippe ElfSolaiseFrance

Gonzalez-Velasco, Juan R. Univ. Pais Vasco BilbaoSpain

Gottberg, Ingemar VolvoSweden

Gould David, G. FordUnited Kingdom

Groenendaal, Willem Strategic Analysis EuropeThe Netherlands

Grootjans, J. LabofinaBelgium

Haas, Jurgen DornierWest Germany

Hammer, Hans BrennstoffchemieWest Germany

Harrison, Brian Johnson MattheyUnited Kingdom

XVIII

Havil Univ. Paris 6France

Hawker, P.N. Johnson MattheyUnited Kingdom

Hecker, William C. Univ. Brigham Young, ProvoUSA

Hecq, Walter ULB, BrusselsBelgium

Hegedus, L. Louis W.R. Grace & CoUSA

Held, Wolfgang VolkswagenWest Germany

Henssler, H. EEC

Herz, Richard Univ. California San DiegoUSA

Hickey, C. (Mrs) Esso PetroleumUnited Kingdom

Howitt, John S. Coming Glass WorksUSA

Imai, Tamotsu SignalUSA

Impens,R. Fac. Agronomique, GemblouxBelgium

Ing,Hok UTACFrance

Jacobs, Peter KUL,LeuvenBelgium

Jagel, Kenneth I. EngelhardUSA

Johansen, Keld TopseeDenmark

XIX

Jourde, Jean-Pierre RenaultFrance

Joustra, A.H. ShellThe Netherlands

Kaczmarec Rhone PoulencFrance

Kapsteyn, F. Univ. Amsterdam,The Netherlands

Kilpin, Michael FordUnited Kingdom

Koberstein, E. DegussaWest Germany

Kruger HoechstWest Germany

Kruse, Norbert Fritz Haber InstituteWest Germany

Kuijpers, E.G.M. VEGThe Netherlands

Laine. J. Inst. Ven. Invest. CientificasVenezuela

Le Normand, F. Univ. L. Pasteur, StrasbourgFrance

Leclercq, Ginette Univ. LilleFrance

Leclercq, Lucien Univ. LilleFrance

Lehmann, Ulrich Condea ChemieWest Germany

Lester, George R. SignalUSA

Li Wan (Mrs) Univ. BeijingChina

Lienard, Georges ULB, BrusselsBelgium

xx

Lin Peyian (Mrs) Univ. HefeiChina

Lowendahl, L. Univ. Chalmers GoteborgSweden

Mabilon IFPFrance

Maire, G. Univ. L. Pasteur, StrasbourgFrance

Maret, Dominique PeugeotFrance

Marinangeli, Richard E. SignalUSA

MarseII, Lars Saab-Scania ABSweden

Mathieu, Veronique FNDP,NamurBelgium

Maxant, Genevieve (Mrs) Comptoir Lyon Alemand LouyotFrance

Merian, Ernest JournalistChemosphere/IAEACISAGUFSwitzerland

Mesters.C, ShellThe Netherlands

Meunier, Guillaume Univ. L. Pasteur, StrasbourgFrance

Moles,P.J. Magnesium ElektronUnited Kingdom

Mottier, Michel Henri ConsultantSwitzerland

Murphy, Michael General MotorsEur. Techn. CenterG.D. Luxembourg

Naudin, Thierry PeugeotFrance

XXI

Niemantsverdriet, J.W. Fritz Haber InstituteWest Germany

Nieuwenhuys, B.E. Univ, LeidenThe Netherlands

Nortier, P. Rhone-PoulencFrance

Odenbrand, I. Univ. LundSweden

Otterstedt, I.A. Univ. Chalmers, GoteborgSweden

Oudet, Francois Univ, CompiegneFrance

Pentenero, Andre Dniy. NancyFrance

Pernicone, Nicolas Institute G. DoneganiItaly

Poncelet, G. DCL, Louvain La NeuveBelgium

Ponec, V. Dniy. LeidenThe Netherlands

Praliaud, Helene (Mrs) IRC, VilleurbanneFrance

Prigent, Michel IFPFrance

Questiaux, Daniel LabofinaBelgium

Rinckel, Francis PeugeotFrance

Roche.Rene PSA-ERFrance

Salanne, Simo KemiraOyFinland

Schay, Zoltan Inst. Isotopes, BudapestHungary

XXII

Schwaller Univ. L. Pasteur, StrasbourgFrance

Seip, Ulrike (Mrs) MANWest Germany

Senamaud, Jean Michel RenaultFrance

Shelef, Mordecai FordUSA

Shinjoh, H. ToyotaJapan

Singoredjo, L. Univ. Amsterdam,The Netherlands

Skoldheden, Per VolvoSweden

Slater, Hawes AC Spark PlugUSA

Smailes, R. UKAEA HarwellUnited Kingdom

Soustelle, M. Ecole des Mines, St EtienneFrance

Sposini, Mario EcofuelItaly

Stohr,H. Grace GmbHWest Germany

Tauzin PSA-ERFrance

Taylor, x.c General MotorsUSA

Tsuchitani, Kazuo Shokubai KagakuJapan

Tuenter,G. Neth. Energy Res. Found.The Netherlands

Umehara,K. NGKEuropeWest Germany

XXIII

Vaccari, Angelo Univ. BolognaItaly

Van Delft, F.C.MJ.M. Univ. LeidenThe Netherlands

Vandervoort, Philippe Toyota Motor Corp.Belgium

Virta Pirrko (Mrs) KemiraOyFinland

Walsh Michael P. ConsultantUSA

Wan, C.Z. EngelhardUSA

Weber, Kurt H. VolvoSweden

Wolf, Eduardo Univ. Notre DameUSA

Wolsing, Wilhelm Engelhard Kali Chemie Autocat.West Germany

Yamazaki Takayuki Nissan Motor Co LtdBelgium

Zhao, Jiusheng Univ. TianjinChina

Zink, Uwe Coming KeramikWest Germany

B. 1ST DAY INTRODUCTORY SESSION ONLY

Crate

De Nil, A.

Jensen, Bent

Luck, Lucien

Volvo Car CorporationBelgium

AnalisBelgium

CEFICBelgium

General Motors ContinentalBelgium

XXIV

Machej UCL, Louvain-La-NeuveBelgium

MacKinley EEC

Norcross, Geoffrey Intern. Prof. Assoc. Envir. AffairsBelgium

Rasson, Andre Austin Rover DistributionBelgium

WiIlems,H. Johnson MattheyBelgium

Evans,P.W. Molycorp SARLFrance

Yonehara Kiyoshi Nippon Shokubai Kagaku Co.Japan

Searles R.A Johnson Matthey Chemicals,Div. AutocatalystsUnited Kingdom

Maegerlein Degussa AG Dpt AC/GKAWest Germany

Brunoli, Joseph A Signal Automotive Products -Norplex EuropaWest Germany

Hulsmann Ford Werke AG.West Germany

Maegerlein Degussa AG Dpt AC/GKAWest Germany

Ogata,Hideo Mitsubishi Motor Corp.West Germany

Schneider, Dietrich Ford Werke AG.West Germany

von Salmuth, H.D. Ford Werke A.G.West Germany

A. Crucq and A. Frennet (Editors), Catalysis and Automotive Pollution Control1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

EFFECTS OF MOTOR VEHICLE POLLUTANTS ON HEALTH

M.CHIRONINRETS,I09 Av.Salvadn~ Allende BP 75,69b72 BRON Cedex France

INTRODUCTION

The characteristic feature of pollution due to road traffic is its widesp:eading such that the whole population is affected, including children,

invalids, old people and pregnant women.On the other hand,the durations of exposure may vary

within wide li~its. Thus the traffic can be continuous in

some areas and very intermittent in others while the displacement of people canvary to a great extent. The pollutants can also be prevented from dispersingbecause of local configurations or unfavourable weather conditions. Further-more it should be noted how certain pollutants can accumulate in the body inthe absence of the long periods free from exposure that are required for themto be eliminated and how it is impossible to protect people suffering from someparticular sensitivity or illness from the effects of pollution.

All this must be borne in mind when considering the effects of motorvehicle pollutants on health.

There is also the obvious difference between the evaluation of the effectof a pollutant dispersed in the environment as a whole and one that is dis-persed in an industrial area where both the level of pollution and the durationof exposure are known, where the total duration of exposure cannot in any caseexceed 45 years and where an individual can be withdrawn from the risk at any

time.For pollutants in the form of a gas the dispersion is very rapid for the

usual weather conditions and the exposure decreases with distance from thevehicle exhaust systems. Thus the people exposed to the greatest levels ofpollution are first of all the drivers of the motor vehicles, then thosemaking use of two-wheeled vehicles and finally the pedestrians.

Pollutants in the form of particles on the other hand settle very quicklyand the level of atmospheric pollution falls very rapidly on moving away fromthe vehicles. However the particles land on the ground and in water and canaccordingly find their way into food, this giving rise to pollution at adistance which can even affect people living in country areas.

CARBON MONOXIDEThis is the pollutant for which the effects on the human organism

are the most well understood.

1

2

The carbon monoxide in the atmosphere originates to a large extent frommotor vehicles and is almost completely due to them in the vicinity of streets.

In some very polluted and poorly ventilated areas carbon monoxideconcentrations of 50 to 100 ppm can persist for several hours and theindividuals that are obliged to remain in such areas because of their work areexposed to high levels of pollution solely because of motor vehicle traffic.

It can be assumed that daily averages of 30 ppm apply for an individualtravelling by car in town and exceptionally of 80 ppm for someone standingat a heavily polluted point (not taking _into account the inside of a tunnel).

The action in the human organism is well understood: the carbon monoxidereplaces the oxygen on attaching itself to the normal haemoglobin. Thus itinhibits the normal respiratory function of the haemoglobin which is totransport the oxygen contained in the air to the body tissues.

The affinity of carbon monoxide for haemoglobin is 250 times greater thanthat of oxygen. A permanent balance is established between the carbon monoxidein the atmosphere and that in the blood; there is no accumulation in theorganism and the carbon monoxide is completely rejected on expiring air whenthe atmospheric concentration is zero. The speed of attachment or rejectionof the carbon monoxide depends in particular on the level of pulmonaryventilation. Curves have been produced showing how the concentration of carbonmonoxide in the blood (in terms of the proportion of carboxyhaemoglobin) varieswith that in the atmosphere, the duration of eA~osure and the pulmonaryventilation (curves produced on referring to Coburn and Forster's equation).See Ref.l and figures 1 & 2

3

The consequences of hypoxia (reduction in the transport of oxygen to thetissues) can be classified into three different categories:

a) For fairly high concentrations of carbon monoxide (greater than 50 ppm)persisting for several hours, functional but unspecific disorders can beobserved, mainly headaches, asthenia, giddiness and nausea.

b) For lower concentrations, of the order of those normally experienced bytown dwellers, the hypoxia can be sufficient to give rise to an hypoxia attackin the case of subjects already suffering from ischaemic arteriopathy. Thesesubjects cannot compensate for the reduction in the carriage of oxygen by anincreased flow of air. Such attacks can occur in the coronary, cerebral ordistal region. A critical level of 2.5 per cent of carboxyhaemoglobin has beenestablished by the W.H.O. for this type of attack, corresponding to a longduration carbon monoxide concentration of about 13 ppm.

c) The third effect, again in the case of low carbon monoxide concentrations,is to accelerate the formation of atheroma plaques corresponding to a prematureageing of the arteries. It has not been possible to define a limiting concen-tration for this effect since the accumulation of cholesterol in the arteriesfalls when the supply of oxygen is greater than normal. Thus any increase inthe supply of oxygen is beneficial.

NITROGEN OXIDES , OZONE AND OXIDIZING PHOTOCHEMICAL DERIVATIVESThe nitrogen oxides concentrations in towns can amount to about 1 ppm

during peak traffic hours. Under the action of solar radiation the N02dissociates into NO and atomic oxygen which gives rise to the formation ofozone 03' The organic molecules react with the ozone to form freeradicals which in turn act as a catalyst for the oxidation of the NO and thehydrocarbons. Thus the irradiated exhaust gases are "biologically more active",that is to say the total oxidising power is increased as well as the concen-tration of irritant aldehydes.

The nitrogen oxides together with the photo-oxidising fog, the actionof which is similar to that of the ozone ,act as irritating agents so faras the pulmonary aveola are concerned. The active surface agent is oxidisedand there is an inflammatory reaction.

A certain adaptation of the organism has been observed in the case of shortduration exposures.

The oxidising agents favour the onset of pulmonary infections and theinduction of respiratory allergies.

For people in good health, the results of epidemiological studies haveindicated that the average concentration of N02 over a 24 hour period shouldnot exceed 0.05 ppm.

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It should be noted however that in the case of more sensitive individuals,particularly those suffering of asUuna,this value is to high but there is alack of data foY' the establishment of a more suitable value .

The peak concentrations, given the results of studies for this type ofpollution, should amount to 0.25 ppm of ':02 two to three times a week for aperiod of one hour.

HYIJROCARBONSA large number of hydrocarbon compounds are emitted by the vehicles either

as a result of a simple evaporation before combustion or of an incompletecombustion

Some studies have been concerned with particular elements or a group ofcompounds and others with the petrol vapour as a whole.

In all cases the studies have revealed evidence of mutagenic or carcin-ogenic action, eii::her on bacteria,on cell cultures or on living animals

The responsible products are mainly benzene and its homologues and thearomatic polycyclic hydrocarbons.

For the amounts encountered in the environment it is impossible to quanti-fy the effects of the different carcinogenic agents that are present.

The limiting exposure is often expressed in the f'o rm of a maximum amountthat may be inhaled during a lifetime, as in the case of radiation. Thisamount is then converted to a maximum acceptable concentration.

For example, the maximum amount of a-B.P. (a-Benzo Pyrene) that may beinhaled is 12 to 16 rug corresponding to a maximum acceptable concentration ofO.1 5/, g/ m3.

Of the different aromatic hydrocarbons a-B.P. has been the subject ofmost studies but is not the most carcinogenic.

It should be noted however that the subject of chemical carcinogenesis isstill not well understood and there are multiple interactions between thedifferent pollutants whether they are of alimentary, domestic or environ-mental nature.

Just as the combined effects of alcohol and tobacco are much greater thanthe sum of their individual effects, it is likely that there are a number ofinteractions between carcinogenic chemicals.

Thus it does not make much sense to establish limiting values for eachchemical given the fact that they have a combined effect.

It should also be pointed out here that significant inhalations ofhydrocarbons are possible in the vicinity of petrol filling stations.

DIESEL EXHAUST PARTICLES

These particles when viewed under an electron microscope are in the

form of clusters of smaller round sub-particles formed during combustion

that subsequently have sticked together. The average diameter of the

particles lies between 0.2 to 0.3 microns. They each have a nucleus of

pratically pure carbon surrounded by adsorbed hydrocarbons.

The particles, due to their small diameter, penetrate deep into the

lungs as far as the alveoli. Some 80 per cent of the inhaled particles

are retained in the lungs for long, almost indefinite, periods of time.

Thus the lungs fill up with "dust".

The diesel exhaust particles, as well as the hydrocarbons that are

extracted from them, have a mutagenic effect in the laboratory but it

has not been possible to quantify this effect as a result of epidemiolo-

gical studies.

HEAVY METALS (excluding lead)

Motor vehicles emit a number of metals: chromium, manganese, barium,

vanadium, iron, aluminium, cadmium, nickel,aso.

However it is difficult to determine the contribution of the motor

vehicles to this type of pollution.

Many of these metals are toxic as it has been recognised in indus-

trial medicine. In particular cadmium, nickel and chromium are carcinogenic

while manganese is toxic so far as the nervous system is concerned.

However it is unlikely that any of these elements have any detec-

table effect when considered separately.

LEAD

Lead pollution so far as man is concerned is of purely artificial

origin.

Lead additives pollute the atmosphere, the ground, water, vegetatim and finally

animals and msn. In the vicinity of roads the pollutim, extends for sane hundred of

meters. Beymd that distance, the levels are 10 to 30 times less than the levels in urban

areas but are nevertheless still mainly due to the transfer over short or long distances

of pollutants due to the motor vehicles.

The fact that additives are responsible for most of the lead cmtent in the air, in

dust and even in most of our food has allowed to estimate, as a result of a study of the

intake by the mrren organism, that at least fJJ per cent of the lead in the body comes fran

lead alkyls. Other food or food related sources (timed foods, capsules, filters, water

pipes) playa much less important role than is generally believed. In areas where the

traffic is important the contribution of the motor vehicle can account for

7

8

80 per cent of the lead in the human body.Lead, at the observed levels of exposure is acting on the proto-

porphyrin of the red corpuscles, whose increase in number is an indication ofa restriction on the synthesis of haemoglobin. Such an increase can be detect-ed for lead concentrations in the blood as low as about 15~g/dl, a frequently

I

observed value (a concentration of 35rg/dl or less is considered as normal)However this effect, although detectable, cannot be regarded as a

pathological one in the absence of any anemia.The most important effect, so far as public health is concerned, is the

insidious one on the development of childrens' brains, with particular con-sequences for their intelligence (in terms of I~'s) and behaviour.

It is common for children to ingest lead in a particular way - on raisingdirty hands and objects to their mouths likely to be contaminated with highlead content dust in areas where the traffic is important.

100

90

80

~ 70

~i 60

'"~~

50

a 40...'"......;l:

30!c...~

~ 20o

10

50 60 70 60 90 _ 00 = = _VERBAl LO.

fig.1.Cumulative frequency distributions of verbal10 scores in high and low lead subjects(ref.3)

9

AIJ)EHYIJESThese irritate the upper respiratory tracts and eyes. The aldehyde

content in the exhaust of petrol engined vehicles give rise to concentrationsin the atmosphere that are already at the limit established for irritanteffects (0.1 ppm).

Formaldehyde is classed as a mutagenic substance. The limiting concen-tration must accordingly be set very low and this is the emission which is ofmost concern to the public health specialists when considering the use ofalcoholic fuels.

ALCOHOLS: ETHANOL AND METHANOL

Ethanol, when inhaled in the small concentrations in the atmosphere thatcould arise in the case of the use of partially alcoholised fuels, does notappear to constitute a public health risk.

Nethanol on the other hand is very toxic as was recognised quite recentlyin connection with the adulteration of wines (the ingestion of only a fewmillilitres can be fatal). Nethanol can penetrate into the organism via thelung~ or skin. It accumulates in the body and the maximum acceptable con-centrations in the absence of periods of non-exposure for the eliminationof the poison, is very low (3ppm).

The methanol is oxidised within the organism into formaldehyde and then intoformic acid and these substances are the real poisons. Ethanol is destroyed bythe same enzymes thai: a t t.ack the methanol.Thus the presence of ethanol caninhibit the formation of formaldehyde and formic acid and can therefore beregarded as an antidote.

Nethanol (and its metabolic waste products) for low rates of exposure cancause irritation and damage to the eyes (optic nerve) while chronic exposurecan lead to a permanent decrease in visual acuity.

CONCLUSIONS

On considering the possibility of decreasing the emission of pollutants asa result of catalytic action we can class the substances emitted by motorvehicles into three categories:

a) The concentrations of carbon monoxide, nitrogen monoxide and oxidizingderivatives are, under normal conditions, at the limit of any detectable effectson health. An appreciable reduction in the emission of these substances wouldresult in negligible concentrations for the general public (not countingprofessional exposures).

b) Lead is not eliminated from the enviTonment nor fTom the humanorganism and its insidious action on the development of childrens' brains calls

10

for a cautious approach.remain in people's bloodbeing already present in

Even if lead additives are eliminated, lead willfor a long time, to a large extent as a result of itthe environment and in living beings as a result of

previous motor vehicle emissions.c) In the case of mutagenic or carcinogenic pollutants it is impossible to

establish a safe level of concentration" as we know almost nothing about

their combined action. Some 80 per cent of cancers have been attributed topollution in general. There is probably some cell repairing activity for verylow concentrations but we have no precise knowledge of this. The best that wecan do in these circumstances is to ensure that the total amount of carcin-ogenic pollutants in the environment, i.e. of benzene, aromatic polycyclichydrocarbons, diesel exhaust particles and formaldehyde is kept as low aspossible.

Coburn R.F. ,Forster R.E.,Kane P.B. ,Considerations of the physiologicalvariables that determine the blood carboxyhemoglobin concentration inman ,J. of clinical invest igat ions: vol 44,11, p , 1899-191 ('-; 1965

2Joumard R.,Chiron M.,Vidon R.,La fixation du monoxyde de carbone surl'hemoglobine et ses effets sur l'homme,Institut de Recherche desTransports,Bron.France.Oct 1983

3 Needleman B.L.,Leviton L.A.,Bellinger D.,Lead associated intellectualdeficit.,New England J.Med.,306:367 ,1982

A. Crucq and A. Frennet (Editors), Catalysis and Automotive Pollution Control1987 Elsevier Science Puhl-shers B.V., Amsterdam - Printed in The Netherlands

AUTOMOTIVE TRAFFICRisks for the Environment

by R. IMPENS

Departernent de Biologie vegetale,Faculte des Sciences Agronomiques de l'Etat, Gemboux (Belgique)

ABSTRACT

Automotive traffic generates a lot of air pollutants, some metalliccontaminants and causes troubles, not only for the roadside environment but also forthe terrestrial and aquatic ecosystems.

The exhaust gases of vehicle's engines contain mainly carbon monoxide anddioxide, nitrogen oxides, a few sulfur dioxide, a great number of hydrocarbons, ororganic carbon derivates, and some heavy metals particulates.

Some of these compounds are directly toxic for living organisms, when theyoccur in a closed environment such as inside the car, tunnels, subterranean car parks,or rooms; but they are harmless when emitted in open space, when natural diffusionconditions are sufficient to prevent high concentrations in the air.

Other emitted gases will interact with oxidants (e.g. 03) to form new labilecompounds, which have a high phytotoxic activity at low concentrations (p.A.N.,andphotochemical smogs).

These oxidants, obtained by photochemical reactions in the atmosphere,may be involved in the widespread dieback and decline of forests in both Europe andNorth America. The 03 and photooxidants theory, and its influence on aciddeposition, will be shortly presented and discussed.

Heavy metals contamination of soil, water and plant materials, nearhighways is well known, and there's a trend to accelerate the reduction of leadaddition in the fuels.

The vicinity of heavy traffic roads, is a source for important troubles toterrestrial and aquatic ecosystems. Some examples of these will be discussed for theirdirect or indirect effects on animal, microbiological or plant lifes.

The regular use of deicing salts, essentially sodium and calcium chlorides,in winter period, affects the resistance to drought stress of trees and crops, andincreases the sensitivity of plants to parasitic diseases.

The compaction of soils near the road is involved in anaerobic conditionsnear the roots of trees, which will be followed by an important dieback.

The risks for environment alterations could be prevented and reduced byclean motors, with a drastic reduction of gaseous pollutants. The lead problem will beprogressively resolved by the new European standards of lead addition to fuels; butthe lead already present in soils will remain a threat for some sensitive crops andforages.

A passive protection of roadside contamination could be obtained by green

11

12

screens, containing resistant and rustic shrubs and trees, which will filter the air andact as efficient sinks for dust and heavy metals particles.

Due to aerial long distance transport and photochemical reactions,prevention of damages to forests request more attention. The solution is reducedemissions ofthe precursors oflethal compounds: clean motors are wanted...

Other risks for the roadside environment (chlorides, asphyxic conditions,etc.) are not directly involved with air pollutants emissions: disastrous landscapemodifications by speedways construction are more fundamental.

1. INTRODUCTION

Automotive traffic generates a lot of gaseous air pollutants, some metalliccontaminants, asbestos, and causes troubles not only to the roadside environment butalso for the terrestrial and aquatic ecosystems.

Three major pollutions emanate from the highway: smog, noise and dust.Effects of noise have ominous portent for the enjoyment of life by the

human race, and are already affecting our health.The exhaust gases of vehicle's engines contain mainly carbon monoxide

(CO) and dioxide (COz), nitrogen oxides (N0x)' a great number of hydrocarbons(HC)n or organic carbonaceous derivates, a few sulphur dioxide, particles and soot(Table 1).

Table 1Average exhaust gas composition of an Otto test engine

Compound

COzHZO°zNOx

% by Volume

12.810.5

1.00.5

Compound

CONZHzHydrocarbons

% by Volume

2.376.0

0.40.1

(in V.D.I. Richtlinic 2282)

The emitted quantities are correlated to the traffic density. Estimations aremade with different criteria: the total amount of emitted pollutants (Table 2) or therelative importance of traffic pollution in the global pollution pattern (Table 3).

---------------------------

Table 2Estimation of the emissions due to automotive traffic

in Belgium (year 1977)

13

Type of Number offuel vehicles

co Pb++ Hr- CI-

Results given in 103 T. (from Hecq and Sempoux 1980)

Gasoline 3.0 x 106Diesel 0.5 x 106

140043

109 9011 39

3.813.0

1.8 0.9 0.7

Table 3Estimation of the emissions of S02 and NOx

in France (year 1982)

Pollutant Industry Transport Power Domesticplants use

S02 1157 KT 57.5 KT 933.3 KT 230.1 KT(48.7%) (2.4%) (39.2%) (9.7%)

NOx 254 KT 648.0 KT 240.0 KT 140.0 KT(19.0%) (52.0%) (18.0%) (11.0%)

Results given in 1()3 T. (or %) - (from C1TEPA 1983)

The conditions of these emissions are well known, an important literature isdevoted to correlate the pollutions with the type of engine, type of fuel, the speed ofthe car, the driving cycle, etc. (Sibenaler1972).

Other parameters of the pollutions are :

- the type of traffic, and the emissions level of each vehicle- the traffic capacity- the wind velocity- the wind direction- the atmospheric stability- the type of site- the distance from the source (Joumard and Vidon 1970).

2. DESCRIPTION OF THE EMITTED POLLUTANTS

2.1. Carbon oxides (COx)Carbon monoxide is one of the three most common products of fuel

combustion, carbon dioxide and water vapor are the other two. Most of the CO in theatmosphere results from incomplete combustion ofcarbonaceous materials.

14

Carbon monoxide is quite stable in the atomosphere and is probablyconverted to C02, but the rate of this conversion (not known exactly) is low. Its apoisonous inhalent and no other toxic gaseous air pollutant is found at such relativelyhigh concentrations in the urban atmosphere.

Carbon monoxide is dangerous because it has a strong affinity forhemoglobin. The major risks for human or animal health are when CO is emitted inconfined or enclosed spaces (inside the car, in tunnels or subterranean car-parks, etc.)where it will accumulate and reach the toxic levels.

There are few data on eventual risks for plants. Fluckiger (1979) reportsan increase of peroxydase activity and of ethylen synthesis by birches (Betula pendula)growing near highways. An early abscission ofleaves is observed too.

Carbon dioxide is a normal component of air, it is an important material forplant life - emitted by all living organisms during the respiration and fixed inphotosynthesis by green plants. Normal concentrations in the air are ranging from300 to 380 ppm. Concentrations, which could be toxic are rarely observed (a volcanicemission, occurred recently in Cameroun, contradicts this optimistic opinion).

2.2. Nitrogen oxides (NOx)Oxides of nitrogen are an important group of air contaminants, produced

during the high temperature combustion of gasoline in the engine.The combustion fixes atmospheric nitrogen to produce first nitrogen

monoxide (NO), which will be converted in nitrogen dioxide (N02)' This oxidation israther rapid at high concentration, the rate is much slower at low concentrations. Insunlight, especially in presence of organic material (hydrocarbons), this conversion isgreatly accelerated.

By gasoline powered engines, NOx emissions increase with average speed(Pearce, 1986 -Joumard, 1986).

The hazards associated with nitrogen oxides are:

- a direct noxious effect on the health and well being of people;- a direct phytotoxic effect on plant communities. The measure NOx concentrations in

the air, are generally always low, and don't cause plant damages, except when theyare associated with other gaseous air pollutants as sulfur dioxide or ozone;

- an indirect effect : due to photochemical oxidation of organic material, with anabundant production of toxic compounds.

2.3. HydrocarbonsAn analysis of hydrocarbons and other organic compounds emitted in

exhaust gas of a four cylinder otto engine is listed in Table 4 (Becker KH. et al,1985).

The composition of car exhaust and of the organic fraction, is "in the road"condition quite variable and strongly dependant on the mode of driving.

Among the substances responsible for photochemical air pollution are

insaturated hydrocarbons (faster reactors), saturated hydrocarbons (slower reactors),aromatics and aldehydes. Automobile exhaust is the major source; howeverhydrocarbons and other organic gases are also expelled during the production,refining and handling of gasoline.

2.4. OxidantsThe general terms "oxidants" and "photochemical air pollutants" include a

large number of trace compounds, results of reactions between primary pollutants(NO, N02 and hydrocarbons) under the action of sunlight.

Important reaction products (or secondary pollutants) are ozone (03),peroxyacetyl nitrate (p.A.N.), higher oxides of nitrogen, aldehydes and ketones, aswell as several gaseous and/or particle-bound inorganic and organic acids.

The effects of photochemical pollutants are mainly:

- Plant damage: with a definite economic significance, because the damages to cropsand forests. Some cultivated species are very susceptible to ozone and P.A.N (ex.tobacco and grape).There is considerable evidence that chronic exposure of a variety of plants toconcentrations below these that cause irreversible damage, adversely affects plantgrowth, and decreases the resistance of plants to climatic stresses and parasiticdiseases, and finally induces a progressive dieback.

- deterioration of materials: ex. fast cracking of stretched rubber products.- eye irritation and health hazards.- decrease in visibility.

These oxidants could be involved in the forest dieback; this theory will belater discussed.

2.5. ParticlesA large number of extremely fine particles are emitted from automobile

exhaust systems, with approximately 70 percent in the size range of 0,02 to 0,06micron. These particles consist of the both inorganic and organic compounds of highmolecular weight. The quantity of solid and droplet material produced in the exhaustamounts to a few milligrams per gram of gasoline burned (Rose 1962).

15

16

Table 4Volatile organic emissions of an Otto engine (Dulson 1981)

Compound % by mass of total Compound % by mass of totalorganic emissions organic emissions

Methane 7.0 2-Methylpentane 1.1Ethine 10.9 3-Methylpentane 0.8Ethene 15.7 n-Hexane 1.0Ethane 1.6 Benzene 12.7Propene 0.2 2-Methylhexane 0.7Propane 1.1 3-Ethylpentane 0.6Acetaldehyde 0.7 n-Heptane 0.4n-Butane 1.8 Toluene 18.9Butenes 0.7 1,I-Dimethylhexane 0.3Acetonitrilite 1.3 Ethylbenzene 2.1Acetone 0.9 m-, p-Xylene 6.7

I Isopentane 5.2 o-Xylene 1.8t~entane 1.4 Trimethylbenzenes 4.0

--_..._._~---~-_. ._-----_.

Most gasoline contain lead additives, which provide the antiknockcharacteristics that are required by present-day high compression engines. The mostcommon additives contain tetra-ethyl lead or tetra-methyl lead together with organicchlorides and bromides.

Lead as a pollutant in the air, on plants and in soils has elicited increasingattention during the last twenty years. The dispersion of this heavy metal in theterrestrial and aquatic ecosystems is well known, and the hazards, associated toincreasing concentrations oflead in water, crops, forages and soils are well known.

Legislative measures (quality standards of fuels) and regulations willprogressively prohibit the use of alkyl-lead additions in fuels, and reduce the risks oflead contamination of the food-chain, but there will stilI remain an important problemof soil, sediments and water contamination by lead.

Other heavy metals: Fe, Cu, Cd, Zn and Cr, are emitted by automotivetraffic, due to panelbody alterations, tyres, brakes systems etc. Asbestos dusts could bereleased by brake-linings or clutch facings.

3. EXAMPLES OF POLLUTIONS DUE TOAUTOMOTIVE TRAFFIC

3.1. Gaseous air compounds acting as primary pollutants.In 1974, a National Commission for Environment near Highways was

created under leading of Dr E. MANNAERT.The first objectives were to measure air pollution, dust deposition and lead

contamination, due to automotive traffic near motorways.The research was performed by our colleagues of the BECEWA (Rijks

University Gent) in association with our laboratory (Gembloux).Six different sampling sites were choiced along the heavy loaded "Ostend-

Brussels-Liege" highway. The sites differ by the traffic density and the road profile,all of them were in rural areas.

Four gaseous air pollutants were measured at increasing distances from themotorway: CO, NOx, light and heavy hydrocarbons. Additional but sporadicmeasurements of 3-4 benzopyrene were made in only one sampling site (10 Km WofBrussels).

Deposited dusts, and soots were collected too.The results of these researches were published in a confidential report (1.

Vandenbossche et al, 1976). As an example, we compare NOx distribution in the air,in flat country - near Gent with an average traffic density of ± 10 000 cars and± 3 000 lorries during a 7h period (Fig. 1) and near Liege (traffic density ± 3 000cars, ± 1 100 lorries during the same period) (Fig.2).

The major influences on air pollutants dispersion are traffic capacity, winddirection, type of site and the distance from the source.

3.2. Lead contamination.A research collaboration between the "Green project" and the Plant Biology

Department of Gembloux Faculty started in 1972.The aims of this research were to collect informations about lead emission

by exhaust gases of cars, and to survey the fallout of lead particles near highways andprevent any contamination of the food chain.

A survey of lead deposition on vegetation gives a lot of information on thelevel of contamination and on the various factors affecting the dust depositionpatterns.

3.2.1. TechniquesMore than 20 sites were located near Belgian highways, in rural areas, some

other sites were chosen in Brussels (parks and avenues).During five years, every month (every fortnight during the summer

period), samples of soil, grass, tree leaves and vegetables were collected. Ten yearsago, we started a programme of sampling (soil and grasses) to survey the efficiency ofa windbreak. Vaselinated plates were placed: before, in- and behind windbreaks tofollow the deposition of lead particles and dust.

After being dried and extracted with a 1/1 HCI03 - HN03 solution, thesamples are analysed for their heavy metals content.

In all samples. Pb, Zn and Cd are determined by pulse polarography(Delcarte et aI1973) or by flame spectrometric atomic absorption. All the results, inthe following tables and figures, are given in p.p.m. (mg/kg dry weight). Oursampling sites are located in a map (see Fig.3). A rural site, chosen far away from anyroad, serves as a control area, where samples are collected to measure the backgroundlevels of the studied heavy metals.

17

18

160

140

120

100

80

60

40

20

ppb NOx ....- -./' "1\

/ \/ '\I "",,_ ....... ~_-~\

I '" ''',/ ,/ ,

I / 1\ \,/I

/ \/ /.-'-'~.~. .......... \1 <,

Ii ~ \il \.I' '.j ! -,

;1 -,"

jf "'~'

IIJ''l'i

";~I.)

4 2 1 3DISTANCE FROM THE HIGHWAY: (!)&@ :16.5 M ; @&@ 33 M .

Caption Date Meteo Traffic density from10 am to 17 omCars Lorries

06.11.74 N.E - 5.5.0 9.686 x 2.959 x(1.6m/s)

----- 13.11.74 5.5.0 (6.6m/s) 9.652 3.180------ 17.04.75 5.5.0(3.1 rn/s) 9.586 i 2.959 x

FIG. 1 DISPERSION OF NOx - MOTORWAY OSTEND BRUSSELS LIEGE

SITE NEAR GENT •

100

80

60

40

20

o

ppb NOx

,"" ",,I \

i ,,I \

I \ / ".I \ \I 1\,/ \!

I r; \.I V\ \

I / " \I,

I /' »- \ \I r-, " \.5/ .I ~ ~ '.

/ \/ V // <, .~

.// ' ..>: .' '<..........~./ .......... r-.4 2 1 3

19

DISTANCE FROM THE HIGHWAY ~ & ~ : 18 M ;~ : 41 M ;~: 37 M .

Traffic density fromCaption Date MIHeo 10 am to 17 pm

Cars Lorries

9.09.74 E.S.E-SW(2.8m! ) 2994 x 1136 x

-'-'- 11. 09.74 E. (2.7mJs) 2994 x 1136 x------- 18.12.74 W.SW (7.0m/s) 2984 1036

FIG.2 DISPERSION OF NOx - MOTORWAY OSTEND BRUSSELS LIEGE

SITE NEAR LIEGE

20

BELGIQUE

FIG.3 SAMPLING SITES .

3.2.2. Importance of the lead contaminationThe first aims of the study were to evaluate the levels of lead contamination

in the roadside environment. To secure a good survey, we sample ubiquitous plantspecies e.g. : Tussilago farfara L., Plantago major L., Plantago lanceolata L.,Ligustrum sp. and Trifolium repens L.

Lead is present in a relative narrow zone along either side of the road. Wecompare lead presence in superficial soil, in plantain tissues (after or without washingof the samples in a 10% vol. RCI solution) (see Fig. 4) (Impens et af 1972)

400

360

320

zeo

240

200

160

'20

eo

40

Pb. concentration.ppm/DW.

" Total leG in SOl 1o Total lead in plantain

o IIashee! out lead (plantain

15 10 5 Road -0 15 20 25 30 35 IJ:)

Distance from the road Right( in meters)

21

FIG.4 LEAD CONCENTRATION NEAR THE BRUXELLES-NAMUR HIGHWAY .

22

The distribution of lead is influenced by:

- the distance to the road: on varying distances from highway, the lead content of soiland herbage can rise to 10 to 20 times the normal content and remains noticeable ata distance of at least 120 m from the highway. In centerstrips, concentrations oflead are very high, this is caused by washing away of lead containing dust to thecenter and the side-strips by rain.

- the duration of exposition: there is a progressive increase of lead accumulation onleaves, from early spring time to the autumn days. In some needles of Pinus nigraL., we found about 1 500 to 2 300 ppm of lead.

- traffic density: there is a clear correlation between the density of traffic and theamount oflead deposited.

- climatic conditions: rain or snowfalls cause an important washout of the depositeddust on leaves, and a progressive contamination of soil, rivers and ponds.

- profile of the road: highways are built in flat country, or as sunk roads or enbankedroads. One of our difficulties was to find, on the same highway, similar conditionsof traffic density associated with different road profiles. The deposition of lead isinfluenced by the prevailing winds in the three different profiles. Lead gradients inthe road transect are more perturbed in sunk roads (see Fig.S).

- plant species: the anatomy of leaves (presence of cuticle, hairs, etc.), the habit ofthe plant, the pattern of growth and the ability for root absorption of heavy metalshave to be considered.

- the state of growth of the vegetation is very important. There is an increase in leadin the above ground portion of plants, when active growth shows a minimum.

3.2.3. Lead deposition in townsStreets, avenues and parks are heavily contaminated by the exhaust gases of

cars. Grasses in lawns contain more than 150 ppm of lead. Shrubs and ornamentaltrees are important sinks for the aerial lead and may accumulate other heavy metals ontheir leaves (Impens et aI1972).

In busy avenues, the street dust has a lead content ranging from 500 ppm to2 500 ppm and plani screens are effective to reduce the lead burden in gardens. Somesimilar results were obtained in British cities (Davies 1978).

3.2.3. Lead deposition and food chain contaminationLead is transferred to the soil, plant or animal via sedimentation, impaction,

precipitation or inhalation. The roadside environment receives metal particles of allsizes classes, the larger ones by sedimentation, and the smaller ones by the latterprocesses.

The natural lead content of the soil always causes a certain concentration oflead in plants; but the absorption of this metal by roots being not very important, mostof the contamination of leaves, stems, and fruits is due to impaction and precipitationand not to absorption and translocation from roots to epigeal organs (Ter Haar G.,

Pb. conc. Flat.mg/kg (DW)

Excavated.360 ................

340 ----- Enbanked.320

300

260

260

240

220

200

leo160

140

120

100

60

60

40

20 .............

100m SCm 25m 5m" 5m 25m SCm 100m

LEFT CENTER RIGHT

FIG.5 LIEGE-BRUXELLES HIGHWAY.LEAD CONTAMINATION OF GRASSES.

23

24

1970; 1mpens et al1976; Davies, 1978).Commercial crops consumed by man (e.g. letttuce, spinach, cabbage, beans,

peas and other vegetables) show a highly significant increase in surface coated lead,when growing near highways and therefore, are no more suitable for consumption,not even if 50 per cent of the deposited dust can be removed by an acid washing (seeTable 5).I--.-.-- ..--.~-_·_·_-··_·- - ..-_...- ...-. ----.- ---

I[ Table 5. CONTAMINATION OF VEGETABLES NEAR BELGIAN

HIGHWAYS

On the other hand, protected edible parts of plants like seeds, tubers(potatoes), bulbs (onions), roots (radish and carrots) show hardly any increase in lead.It's an evidence that lead salts are immobile in the soil and largely unavailable toplant's roots.

The washout of lead, in artificial conditions, with solutions of Hel(10% vol) shows that some vegetables have an important retention ability for leadparticles.

The concentration of lead in grass is of special interest, because animals mayeat it, and high concentrations of lead are found in grass, hay or silage from pasturesnear highways.

Absorption by the alimentary tract of surface coated lead on forage seemsnot to be considerable, most of the lead being eliminated in the faeces (Delcarte et al,1974).

The Committee "Legislation of Foodstuff' of the E.E.C. has proposed aguideline value of 10 ppm lead in the dry matter of foods. This value seems difficult toberespected in some large areas.

3.3. A complex pollution: acid rainForest decline is now widely considered to be one of the most important

environmental problems in the northern hemisphere. This forest decline isunprecedented in severity and geographical extent, and at the present time, it cannotbe attributed to a known cause.

Until the beginning of the 1980's, acidic precipitation was blamed as themajor cause. The term "acid rain" has become so popular in recent years that itincludes all types of pollution - induced injury occurring in ecosystems.

In Belgium, the first damages were observed and recognized in our easternforests of Hertogenwald and Eifel, near the German border. These forests containmainly Norways spruce (Picea abies (L.) Karst) and some Silver fir (Abies alba,Mille) and Common beech (Fagus silvestris L.).

These forests are of great economic value, the mean altitude of these regionsis relatively high (from 450 to 700 m). The climatic conditions are severe,characterized by cold winters, snowing conditions hard frost, and long foggy periods.The soils are poor with acidic reaction.

Damages to Norway spruces appear preferably on dominant trees.Symptoms are:

- severe yellowing, especially of exposed needles occurs usually beginning with theoldest;

- needle loss is observed, usually starting with oldest needles;- secondary shoots in upper crown are dropped by the tree;- adventitious buds develop in increased manner;- growth is inhibited, forming shorter needles;- fine roots are dying and the mycorrhiza regeneration capacity is reduced.

However, the symptoms vary slightly from region to region (Krause, 1983,Laitat and Impens, 1985).

There is still a great deal of argument about the main causes of this dieback,the main hypotheses are briefly discussed.

a. Direct effects of sulphuric and nitric acid are not generally considered to beprimarily responsible for the damage.

b. The acid rain hypothesis was postulated by Ulrich in 1979 (Ulrich et al., 1979) andstates that wet and dry disposition of acids leads to chemical reactions in the soil,destroying the buffering system and eventually leading to mobilization of toxicions: aluminium and manganese.This process is enhanced by severe leaching of magnesium- and calcium ions due toincreased H+ input and corresponding leaching processes.

c. The stress hypothesis, formulated by Schutt in 1983 (Schutt et al., 1983) postulatesthat the total impact of air pollutants in the past decades and their combination ofeffects lead to a severe loss in vitality, and increases predisposition to climaticstress

25

26

of (frost, heat, water deficiency etc.) and plant pathogens.d. Ozone hypothesis, ozone and other photooxidants may be involved in the decline

(Arndt et al, 1982). The following mode of action was proposed (Krause et al,1983). Exposure to elevated ozone levels damages all membranes, resulting in anincreased leaching of ions by acid fog and acid rain. Ozone also causes damage tochlorophyll, and thus reduction of the photosynthesis. Root growth is inhibited,reducing the ability to compensate for leached nutrients by increased uptake.

Ozone levels in damaged forests in the FRG are often remarkably high of 80-110 ug rrr-' (Black Forest) and 60-90 ug m-3 (Bavarian Forest).

In Vielsalm (Belgian Ardennes) we have recorded during last July dailyconcentrations of Oj ranging from 40 to 80 ~g m-3.

Blank (1985) reports that there is only one data set which gives a clear ideaof the trend in natural background ozone concentrations in Europe, over the past 20years. On the Isle of Rugen (in the Baltic, off the German Democratic Republic) anarea not affected by any local pollution, ozone levels increased by 60% between 1956and 1977. A similar trend has been reported for two other stations in forest areas ofEast Germany.

This coincides with the steady increase of NOx and hydrocarbon emissionsfrom motor vehicles, which react with sunlight to produce ozone during transportover long distances from urban areas.

The ozone and photochemical compounds then affect remote areas,particularly at higher altitudes.

However, there are some gaps in the photochemical oxidant hypothesis.Probably, the decline is caused by ozone acting in combination of other stress as acidmist, drought, frost or pathogens.

e. Climate and pathogens hypothesis. Climatic factors may be involved in the forestdieback, in association with chronic exposure to air pollutants. The series of dryand hot years experienced during the last decade has certainly affected tree vitalityand reduced the resistance to pollutants and pathogens effects.

Hot summers have often been followed by harsh winters, with early or latedeep frost periods. These drastic and sudden changes in temperature and wateravailability may open the way for secondary cause of decline: fungal infections orsome other noy yet identified diseases.

4. OTHER RISKS FOR THE ROADSIDE ENVIRONMENT

Roadside environment is exposed not only to exhaust gases and metallicparticles, but also to other stresses caused by the automotive traffic. Two of theseenvironmental troubles will bebriefly discussed:

10 chloride contamination of soil, stormwater runoff, and plants;20 anaerobic conditions in the soil due to compaction and vibration.

In winter when it is freezing large quantities of de-icing salts are spread onthe roads in order to make the traffic easier - NaCI and CaCl2 are principallyconcerned -. De-icing salts could be phytotoxic.

We have made observations - in natural conditions near motorways, andperformed experiences in a tree nursery, to compare the sensitivity of some treespecies. In our trials, rates of salt application were comparable with those used forroad de-icing in Belgium (30 or 60 g/m2 for a dose).

There is an important increase of chloride concentrations in the upper soil(till 60 em depth), water collected near treated motorways is enriched in chlorideions.

CaCl2 induces phytotoxic effects, commonly expressed as foliar necroses,which appear at the leaf tips and margins (Paul et at. 1984). Sensitivity of trees tofungal diseases is enhanced.

Both direct and indirect effects of de-icing salts must be considered whendeciding on tree plantings along the margins of motorways,

Poor aeration conditions in the soil may result from impeded exchangebetween the soil-gas-phase and the atmosphere (e.g. due to a sealed soil surface) soilcompaction, and natural gas pipes leakages or from exceptionally high biologicalactivity in the soil (Impens.Delcarte, 1979).

Anaerobic soil conditions, whatever their causes may be, are especiallydisastrous for trees and shrubs. Root growth is inhibited while root respiration anduptake of water and nutrients are reduced.

The oxidation-reduction status of the soil is closely related to oxygenpresence. Under anaerobic conditions, nitrates, iron, manganese and sulfates can bereduced. Presence of sulfides, reduced manganese and nitrites is nearly always relatedwith declining or dead trees.

Another secondary effect of compacted soils is a severe waterstressresponsible of an important dieback. This waterstress may be increased by chloridesaccumulation in the soil.

5. CONCLUSION

We should have the right to demand clean air conditions near motorwaysand a drastic reduction of all emissions of noxious exhaust gases and particles.

The risks for environment alterations could be prevented and reduced byclean motors. Awaiting for this future clean engine, there is a request for new airquality standards, and the extension of legal control of vehicle emissions. The EEChas been the prime mover in the determination of standards - Member States havebeen required to frame their national laws within the EEC framework - Europe and

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28

North America are both moving towards tighter emission control standards andunleaded gasoline.

Some other lands, as Switzerland, where forest decline is important, areplanning new legal dispositions with a drastic reduction of speed on highways with thehope of limited emissions of gaseous pollutants.

If lead pollution will be progressively reduced by the nextcoming Europeanstandards of lead addition to fuels, the lead already present in soils will remain a threatfor some sensitive crops and forages.

A passive protection of roadside contamination could be obtained by greenscreens, implanted along the motorways. But some engineers have an irrational fearof trees, demanding they be kept far from the roadway for driver safety.

These green screens must be designed before the building of the road, theymust be planned with resistant and rustic shrubs and trees, which will filter the air andact as efficient sinks for dust and heavy metals particles. Some of these species mayserve as bioindicators of air or soil pollution.

Due to aerial long distance transport and photochemical reactions,prevention of damages to forests request more attention. The solution is reducedemissions of the precursors of toxic compounds: clean motors are wanted.

It is a hard job, but it has to be done.

6. BIBLIOGRAPHY

BECKER, K.H., W.FRICKE, J. LOBEL and U. SCHURATH. 1985: Formation,transport and control of photochemical oxidants in air pollution byphotochemical oxidants, Ed. R. GUDERIAN, Ecological studies vol. 52.Springer Verlag, Heidelberg 1985.

BLANK, L.W., 1985: A new type of forest decline in Germany. Nature, vol. 314n? 6009: pp. 311-314.

ClTEPA. 1983: in Anonyme: Agence pour la qualite de l'Air France. 1983. 32pp.DAVIES, B.E., 1978: Plant available lead and other metals in British garden soils.

Science of total Environment, 9: 243-262.FLUCKIGER, A., 1979: Premature senescence in plants along a motorway. Env.

Pollution 20 (3): 171.HECQ, W. et L. SEMPOUX. 1980: Aspects techniques et economiques de la lute

contre la pollution atmospherique dans le secteur du transport routier.Poll. Atm.,juil.-sept. 1980: 299-312.

IMPENS, R., Z. M'VUNZU and P. NANGNNT. 1972: Determination du plomb surla vegetation Ie long des autoroutes. Analytical letters 6 (3): 253-264.

IMPENS, R. and E. DELCARTE. 1979: Survey of urban trees in Brussels, Belgium.J. Arboriculture 5 (8): 169-176.

JOUMARD, R. et VIDON. 1979: Niveaux de pollution en bordure des autoroutes etvoies rapides urbaines. Poll. atm., avril-juin 1979: 149-152.

JOUMARD, R 1986: Influence of speed limits on road and motorways onpollutants emissions. in Highway pollution. Second Intern.Symp. 7-11 July 1986, London 58-67.

KRAUSE, G.H.M., 1983: Forests effects in West Germany. Symp. Airpollution and the Productivity of the Forest. Washington D.C.Oct. 4-5, 1983,32 pp.

KRAUSE, G.H.M., B. PRINZ and K.D. JUNG. 1983: NeuereUntersuchungen zur Aufklarung immissionsbedingterWaldschaden. VDI-Bericht W 500, 257-266, VDI-VerlagGmbH, Dusseldorf.

LAITAT, E. et R. IMPENS. 1985: Surveillance du deperissement des foretsen Belgique. in Poll. atmospherique n" 105: 16-23.

PAUL, R, M. ROCHER and R IMPENS; 1984: Influence des epandages deCaCIZ sur le sorbier, l'erable, le tilleul et le platane. Bull. Soc.Roy. Bot. Belg. 117: 277-284.

PEARCE, T.C., 1986: Vehicle emissions at high speed. in Highway pollution,Second Intern. Symp. 7-11 July 1986. London: 48-57.

PRINZ, B., G.H.M. KRAUSE and H. STRATMANN. 1982: VorlaufigerBericht der Landesanstalt fur Immissionsschutz tiberUntersuchungen zur Aufklarung der Waldschaden in derBundesrepublik Deutschland. LIS-Bericht Nr. 28: 154 p.Landesanstalt fur Immissionsschutz des Landes NW, WallneyerStr. 6, 4300 Essen 1.

ROSE, A.H., 1962: Automotive exhaust emissions. in Air Pollution A.C.Stem Ed. Vol. 2 pp. 40-80. Academic Press NY 1962.

SCHUTT, 0., W. KOCH, H. BLASCHKE, KJ. LANG, H.J. SCHUCK and S.SIMMERER. 1983: So stirbt der Wald - Schadbilder undKrankenheitsverlauf. BLV-Verlagsgesellschaft Miinchen-Wien-Zurich.

SIBENALER, E., 1972: La pollution par les emissions des moteurs acombustion interne et allumage par etincelle. Symp. Intern"problemes sanitaires poses par le Pb dans l'environnement", 2-6 oct. 1972. Amsterdam. 808-72F: 159 pp + annexes.

ULRICH, B., R. MAYER and P.K. KHANNA. 1979: Deposition vonLuftverunreinigungen und ihre Auswirkungen inWaldokosysternen im Solling. Schriften a.d. Forstl.Versuchsanstalt, Band 58. J.D. Saverlander's Verlag,Frankfurt/M.

VANDENBOSSCHE, 1. and R IMPENS. 1976: Verslage over leefmilieulangs autowegen 1974-1976. Ministere des Travaux Publics,Bruxelles, 342 pp.

29

A. Crucq and .\. Frennet (Editors), Catalysis and Automotive Pollution Control1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Ne therlands

CATALYSIS IN MODERN PETROLEUM REFINING

by J. GROOTJANS

LABOFINA SA, Feluy, Belgium.

ABSTRACT

The progressive lead phase out in motorspirits, deeper conversion schemeson crudes of poorer quality and more stringent regulations on polluant emissionsdetermine to a large extent the ongoing research in catalysis related to petroleumrefining.

From the inspection of the gasoline pool of a conversion type refinery, it isclear that major contributions with respect to octane optimization, may be expectedfrom the fluid catalytic cracker and the downstream upgrading of its products. Thedevelopment of zeolites contributes very substantially to these goals, both by theirintroduction into FCC catalysts and their use in the upgrading of some of the sidestreams.

The latter is illustrated by a new process developed at Labofina : low valueC3-C4 olefinic streams are converted on a zeolitic catalyst to a light olefinic gasoline,particularly suited to be etherified with methanol. This combination process offersmany advantages over the present commercial processes.

INTRODUCTION

The oil refining industry is obviously not the trendsetter with respect toregulations on polluant emissions. These regulations, whether they are related toautomotive or heating applications are translated into some of the finished productspecifications. Other specifications are dictated by the end use and the marketplace.

The task of the oil refining industry is to use in the most profitable way theavailable resources (crude oil and refining processes) in order to satisfy the demandsof finished specification products. During the last 15 years, the oil industry has beenfacing many challenging problems that are well known by the public. Many refineriesextended and adapted their processing units in order to produce from less expensiveheavy crude oils, more valuable and cleaner white products. However, the whiteproducts obtained by conversion processes generally require further upgrading inorder to meet the final specifications.

The progressive lead phase out in motorspirits makes it far morecomplicated. The refiner has to produce more octane barrils from components whichare much poorer and more difficult to upgrade. In this paper we aim to illustrate how

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32

catalysis, and zeolitic catalysis in particular, allows for some major breakthroughs.

FLUID CATALYTIC CRACKING

In a conversion type refining, the fluid catalytic cracker (FCC) producesdirectly and through downstream upgrading some 30 to 50% of the gasoline poolcomponents. The normal feedstock of the FCC is straight run vacuum gasoil. Deeperconversion routes, metal passivators and the development of more performinghydrotreating catalysts, allow for the production of additional feedstocks:

- atmospheric residues,- visbroken and coker vacuum gasoils- solvent deasphalted oils- hydrometallized residuesFCC catalyst manufacturers are facing the challenge of developing

materials that convert these more refractory feedstocks, while yielding crackedgasoline of improved octane numbers. Two concepts are being used in the design of anoctane analyst:

- Shape selective zeolite:A shape selective zeolite cracks the low octane paraffinic components out of thegasoline boiling range, and therefore enhances the octane numbers at the expense ofdecreased gasoline yield. The LPG olefinic fragments can however be converted topremium gasoline components in downstream upgrading units.

- Zeolites with reduced hydrogen transfer activity:These zeolites of the faujasite type have as well good hydrogenation asdehydrogenation activity. Slowing down the hydrogen transfer versus the crackingactivity is established via controlling the Si/Al ratio during their synthesis and thenatural dealuminating process which takes place during the hydrothermalequilibration.

The equilibrated unit cell size is well related to the Si/AI ratio, and is aconvenient tool in selecting and controlling these octane catalysts.

Slowing down the hydrogen transfer favors the production of olefins.Heavy naphtenic compounds are converted into aromatic gas oil components underfast hydrogen transfer, but into aromatic gasoline components when the hydrogentransfer is slowed down relative to the rate of cracking. These catalysts thereforecontribute in two ways to increased gasoline octane.

On the commercial scale we see effectively a gain of a few RON points whenusing these catalysts. The gain on the motor octane MaN is in general much lesspronounced.

DOWNSTREAM UPGRADING

More octane enhancement can be achieved through further processing ofsome of the FCC products. The traditional ones are :- iso-butane/butenes alkylation (HF and H2S04 catalyzed)- catalytic oligomerization of propylene and butenes (H3P04 on Kieselghur)- dimerization of propylene and butenes (lFP's DIMERSOL)

Alkylation is a mature process, and we see little incentive for new catalyticsystems, unless they would allow to carry out the reaction without isobutane excess.The olefins condensation processes produce blending stocks of good RON's butsomewhat low MaN's:

33

r---------·---·--·------------ ---

, RON MON (RON + MONhf-- -.--.-------.-.---- ---- ....-!

---II,

C4 alkylateOligomerisate (C4)Dimerisate cq)

93-9596-97

97

92-9481-82

79-82

93.589

89

More recently, new options have been made available:-MTBE:

The FCC produces about 1.5 wt% on feed of isobutylene. Isobutylene is easilyetherified with methanol into MTBE. MTBE has excellent octane numbers (RON ==117, MaN = 101), but obviously, even if all i-C4 could be recovered from the FCC,the total MTBE product would be less than 2.5% of the gasoline blend.

-TAME:The FCC produces roughly 2.5 wt% on feed of tertiary amylenes. They are alsoreadily etherified on cationic resins into TAME. The octane numbers for TAME aresomewhat lower than for MTBE : RON = 112, MaN =99. In blends one finds thatpart of the MTBE may be substituted by TAME without penalty on the blend octanenumbers. Again, full recovery and etherification of the tertiary amylenes wouldyield a total TAME product representing less than 3.5% of the gasoline pool.

- Heavy ethers:Processes are being proposed that aim to etherify the total light catalytic gasoline.The tertiary olefins become however rapidly more difficult to convert withincreasing carbon number.

:34

Still other techniques are gaining interest in optimizing the octane barrilfrom FCC derived products such as :

- butene-1 to butene-2 isomerization for better alkylate- reforming of the low octane heart cut of the cat cracked gasoline.

If the European gasoline demand for high MaN remains steady, esthers willincreasingly contribute when lead anti-knocks progressively disappear.

A NEW COMBINAnON PROCESS

Refineries that have access to isobutylene streams from steam cracking mayface the problem that the existing alkylation and possibly catalytic condensation unitscannot take the normal butenes which are contained in the pyrolysis stream.

Skeletal isomerization of normal butenes is an active research domain, buthas not yet found an industrial realization.

Also in a conversion type refinery, there are several streams containingsubstantial amounts of olefins which are not upgraded: for instance, the propylenesplitter bottom.

Labofina developed a combination process that very effectively contributesto the octane barril :

In the first step of the process, propylene and/or n-butenes are converted tospecies boiling in the gasoline range. The catalyst is a special shape selective zeolite,operating conditions are mild and the space velocity is exceptionally high.

On a propylene feedstock, very substantial amounts of isobutylene arefound in the reactor effluent. On a n-butene feedstock, attractive yields ofpolypropylene are obtained as well as iso-butylene.

Material balances and product distributions are presented in Table 1. Forcomparison, the same analyses are given for the oligomerization on phosphoric acid.

At a conversion of 90% on n-butenes, the differences in selectivity betweenboth systems are striking. The ranges reflect the influences of the operatingconditions. It is stressed that the shape selective zeolite is very slowly deactivated bycoke lay-down. Cycle times of several months are readily obtained, regeneration iscarried out by simple coke burning. On an octane basis, it is clear that the gasolineobtained on phosphoric acid is superior. The zeolite however produces essentiallygasoline species boiling in the C4-C7 range, a substantial part being tertiary olefins.The phosphoric acid produces dominantly dimers and trimers.

Feed Type:

Catalyst:

Table 1 : Step 1Material balances at 90% conversion

C4 Effluent from a MTBE processn-C4content: 50 wt%

Shape SelectiveZeolite

Phosphoric Acidon Kieselguhr

35

Yield on feed (wt%)

PropyleneIso-butyleneGasoline

Gasoline analysis(Vol % distillation)

36-98°C98-150°C150 - 195°C

,> 195°C!

3.7 - 15.33.0 - 6.3

38.3 - 23.4

Typical range

oo

45.0

36

In a second processing step, the depropanized effluent of the zcoliticconversion is etherified with methanol on a cationic resin.

Table 2 summarizes the global material balances, and again gives thecomparison with the phosphoric acid process.

-----._----_.-r---- .---._--I

Table 2:Overall material balances after etherification

Labofina cornbina- -r Phosphoric acidtion process on Kieselguhr

wt% vol% wt% vol%_.---------- ~.~_. --------_...'" .._,.--,---

50.0 50.0 50.0 50.050.0 50.0 50.0 50.0

4.3 3.2

104.3 103.2 100.0 100.0

IN:

Total

I +N-ButaneN-ButenesMethanol

f-----.---------------+-----

OUT:

Light endsPropyleneI +N-ButaneN-ButenesMTBETAMEHeavy ethersOlefinic gasoline

Total

0.98.3 9.6

50.0 50.05.0 5.05.5 4.42.0 1.62.3 1.9

30.3 24.4

104.3 96.9

50.05.0

45.0

100.0

50.05.0

36.1

91.1

Table 3 gives the analysis of the final etherified gasoline. Emphasis is given on theblend octane numbers since these reflect how this component will perform in thegasoline pool. For a complex refinery with alkylation and a phosphoric acidoligomerization process, the linear programming simulation selects the combinationprocess while shutting down the phosphoric acid oligomerization unit. The alkylationremains at full capacity.

Table 3:Analysis of the gasoline obtained by the Labofina combination process

Specific Gravity d1J 0.7515

WT%of ETHERS:MTBE 17.6TAME 6.3HEAVY ETHERS 7.3

Total 31.2

Reid Vapor Pressure, KPA (PSI) 35.9 (5.2)

Vol % distilled at 100°C 49.0

RON 98 100MON 83 85

Blend RON- Base at RON =94.8 unleaded 96 100- Base at RON =90.9 (0.15g/l TEL) 96 100

BlendMON- Base at MON =83.5 unleaded 85 90- Base at MON =84.0 (0.15 gil TEL) 86 90

CONCLUSION

The fluid catalytic cracker plays a key role in the increased octane demandresulting from the progressive lead phase out.

Zeolites contribute substantially, both in the main cracking process and thedownstream upgrading of cracked products.

As an example, a new combination process has been discussed that comparesvery favorably to the traditional condensation processes.

;17

.\. Cruce and A. Frennet (Editors), Catalysis and Automotive Pollution Control1987 Elsevier Science Publishers B.Y., Amsterdam - Printed in The Netherlands

THE POINT OF VIEW OF THE AUTOMOBILE INDUSTRYPrevention is better than cure

by Claude GERRYN

FORD of Europe Inc,2 Boulevard de la Woluwe, 1150 Brussels, Belgium,

ABSTRACT

Emphasizing on the fact that prevention is better than cure, it is shown thatthe development of engines such as the lean bum engine, that produce only low levelsof polluting gases and thus requires only simple oxidation catalytic converters or evenno catalytic converter at all, appears much more promising, from an economicalviewpoint - lower buying price, cheaper maintenance, lower fuel consumption-, thanthe complex technology of the 3-way catalytic converters.

The new EEC standards are criticized because their introduction in a tooshort delay gives at best a half hearted support to- and at worse results in a slowingdown of- the development, still under way, of the lean bum technology.

Finally attention is also drawn on the fact that the use of 3-way catalyticconverters may result in substituting some forms of pollution by others notnecessarily less harmful: examples of such substitution are given.

THANK YOU and good afternoon Mr President, Ladies and Gentlemen!Let me first tell you how honoured and pleased I am to be with you today:- honoured to have the privilege of addressing such a select group of experts

and decision-makers- pleased to be able to present our views on a topic of such pressing

importance to us all.After reading the impressive list of outstanding papers and knowing most

authors present, prepared to share their expertise with us, I am sure a betterunderstanding of the differences and disagreements on this -international debate willevolve at the end of this symposium: and I wish, therefore, to thank and congratulatethe Universite Libre de Bruxelles for taking this initiative.

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Let me quote our Vice-Chairman Walter Hayes in his key-note speechduring the FIA Round Table in Bournemouth in May this year: and I quote

"It would seem to me that anybody who does not want a strong economy, a safe andclean environment and the most efficient possible use of national and internationalresources is a very strange person indeed. Anybody who does not want sweeterengines, better roads, safer cars, more responsible drivers and happy owners is notreally operating on all his mental cylinders."

Unquote

However, there is no doubt that a large part of the increasing pressure foraction to protect the environment has come about as a result of increasing publicawareness of the issues, but unfortunately this is not always based on real facts or it

clear understanding of the problem.Public opinion is being more effectively marshalled by pressure groups and

this trend is not lost on the politicians. This results all too often in action whoseprimary function appears to be to satisfy the political need, to be seen to be "doingsomething". Realistic evaluation and assessment of the potential effectiveness of the"something" and of its benefit is rarely feasible before implementation, and resultingbenefits are difficult to identify.

It is accepted that governments have an obligation to serve broad nationaland international needs on complex environmental matters and act as clearing housesfor ideas and programmes. Business, for its part, commands managerial andorganisational abilities and can mobilise the scientific and technological resourcesrequired to solve these problems.

It is imperative that these two great segments of society should rest on afirm foundation of knowledge and understanding, especially in the field of "publicproblem solving". Realism, without any sign of false sentimentality, should be thebase for action and it is not so much what can be done which should determine theroute to follow but what needs to be done, with the reasons why and when.

We do recognise the international dimension of many environmentalproblems but these .have to be tackled by coordinated action - bilateral as well asmultilateral - between industry, governments and their respective internationalrepresentative bodies, organisations and the public concerned.

We are indeed committed to conducting our operations in anenvironmentally sound manner and to reducing - as far as technically feasible - anyundesirable effect of our products on the environment, but at the same time we have tofulfill the imperative of economic growth and have to produce commercially viablemotor vehicles. We too are breathing in this world, want to optimise the use of scarceresources, want environmentally favourable energy options.

Our professional burden does not immunise us from undesirable effectsbut... it does tend to sharpen our perceptions and make us more acutely aware if

compared with our other social partners of the balance of advantages/disadvantagesthat must be weighed in order to be clear on the value to society of any given controlmeasure. This concern is unfortunately often misinterpreted as obstructiveness.

Allow me to remind you briefly of the six commanding ground rules whichseem necessary to me to realise within our society sensibly-balanced environmentalprogress:* First, governments, in a national or international context, should carefully assess,

before developing and implementing environmental policies, the need for suchpolicies, the potential methods of achievement, and their impact on industry.

* Second, due to the wide range and complexity of problems raised byenvironmental protection measures, the closest possible contact and consultationbetween industry and government should be sought.

* Third, any environmental protection measures envisaged must be technicallysound and economically acceptable, reviewed in a framework of global approachand where at least safety and energy-use are topics to be included in the globalappreciation.

* Fourth, care must also be taken to avoid substituting one form of pollution foranother.

* Fifth, the costs of control requirements, with the resulting benefit to theenvironment, must be part of the decision-making process. These costs, whetherabsorbed in the first place by the state or by industry itself, must ultimately beborne by the taxpayer or the consumer, i.e. the general public.

* Sixth, especially in the case of motor vehicles, to avoid distortion of trade and to

enable cost-effective solutions to be found, exhaust emission legislation in Europeshould be, if not totally harmonised, at least accepted by all West Europeangovernments, including those who are not EC members.

However, this cannot mean a global-worldwide-conformity ofenvironmental pollution limits, since each case has to be judged on its own merits, inits own geographical context, within its road infrastructure, existing town-planningand layout, attitude of the population and their pattern of behaviour.

Having said this, and to enable us to examine one of the key questions to themotor industry, namely the political dimension and commercial consideration givento the environmental pollution issue in Europe, we should review step by step in howfar the basic ground rules have been respected:

In view of the combined "political/emotional" dimension in this instance, itis superfluous to dwell on the assessment of need or the government/industryconsultation aspects (the two first commanding ground rules).

Whereas the EEC Commission for example can be praised for its effort inseeking through its ERGA Committee a common Europe-wide compromise which istechnically and economically feasible, individual national governments have,unfortunately, for political considerations or other reasons - such as prospects foradditional employment - diluted the effort available for the exploration of those needsand the means to meet them. The ongoing dialogue between industry and government

41

42

is thereby diminished.It was only on June 27/28, 1985 - after about two years of uncoordinated tug-

of-war - that an agreement could be formulated by the EC Council of EnvironmentalMinisters - with the exception of Denmark - on the basis of an EC Commissionproposal specifying the exhaust emission levels and the introduction dates.

Though tough, we were lad that a compromise appeared to have beenreached, avoiding a division of the common automobile market. However, thecontinuing reservation more than one year later, of its position by Denmark and theapparent unwillingness so far of Sweden, Switzerland and Austria to recognise the ECproposal - be it only as an alternative to their national legislation - is a cause forconcern.

Now what compromise is in our view technically sound and economicallyacceptable? The third commanding rule:

In general, requirements must be so framed that they do not preventinnovation; they must not present unrealistic or arbitrary standards. It must be bornein mind that however necessary the control of motor vehicle emissions and noise maybe - and no one would deny the necessity - there are other considerations, such assafety, cost, reliability, which also have to be balanced.

There are many major legislative requirements affecting the vehicle, andnone of these can be handled in total isolation - all have some interaction on the others,there is a "knock-on" effect, so that a "solution" in one area raises a new task inanother.

Furthermore, there is a balance to be struck between what can be done andhow soon it can be done. The greater the pressure on the technical resources, thehigher the cost of meeting the requirements. Inevitably, if too much of amanufacturer's engineering capacity is applied to one objective, other perhaps equallydesirable objectives may have to be abandoned, or at least postponed.

The constraints defining vehicle design are three-fold:- legislative constraints, or what we must do- market demands, what we would like to do- resource constraints, what we are able to do.Regulation on health/safety, construction and use, trade and economic

policy, taxation preferences, all come under the general heading of governmentpolicy, part of the legislative constraints.

Economic factors, styling preferences, pricing, running and maintenancecosts, performance, comfort, reliability, durability, quality at large, are key tocustomer satisfaction, the market demand.

Enginnering and manufacturing resources, research and productdevelopment, technological development, skilled manpower, production cost andprofitability, are some of the indispensable logistics to be looked at in the context of

resource constraints.All in all, a tremendous job before a final decision can be reached on any

new vehicle design.Due to the many different disciplines involved: design, testing, tooling,

finance, manufacturing, homologation and marketing, the gestation period for amajor new design - engine or vehicle - is some sixty months.

Please allow me for a few minutes to focus on engineering or, to be moreprecise, on the present technological developments available for immediateapplication:

If we analyse the means to control exhaust emissions, there are two apparentbasic ways in which pollution caused by motor vehicles can be further reduced:* by not generating emissions, developing more efficient cleaner-burning engines

which control their emission levels inside the combustion chamber (prevention)or

* by not letting them escape, uncleansed, into the environment, using "hang on"equipment to treat the exhaust gases after they leave the engine, and which I wouldcall (cure).

Prevention being better than cure, how far can one expect to go with thepreventive measure and how good is today's cure?

In the first case, the aim is to construct the engine and its combustionchamber, and control the combustion process, in such a way that the creation ofundesirable components in the exhaust gases - CO, HC and NOx - is minimised, whilstat the same time its economy is improved and adequate power still developed. One ofthe techniques is known as "HCLB" - high compression lean bum. This developmentof engines much leaner than stoichiometric is already finding its way into production.Examples are Ford's 1100 Fiesta, 1.6 Emax Sierra, Volkswagen's new Golf, Jaguar'srevised V12 and, of course, the recently introduced 1.8 I Sierra, and the 1.41 and1.6 I Escort and Orion.

Under the maxim "consume less, fume less" offering petrol economyimprovements of up to 20% over current - 1983/1984 - engines, this route has thecapability in the future of reducing NOx by 60% from the uncontrolled situation, andapproximately 90% for CO. Thus, the "best of both worlds" appears possible. The"prove out" process has begun. It is to be hoped that the legislators will give us enoughtime to further work on it and enough confidence to further invest in it.

In the second case, the "curing" or "clean up" approach, various techniques have beenevolved to reduce the emission: closed crankcase ventilation systems, exhaust after-burning, catalytic converters, etc, whereby the control systems, and the technology,become more complex.

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There is no doubt that a three-way catalyst is extremely effective inreducing emissions: NOx emissions by up to 80% from the uncontrolled levels, andHC and CO by over 95%. However, such techniques inevitably affect the operation ofthe vehicle, its performance, its fuel consumption, its driveability. They may alsodemand specific engine operating conditions in order to be effective, such as :

- rich mixtures to use after-burning techniques- stoichiometric air/fuel mixture ratios to ensure the functioning of a

three-way catalyst.Catalyst systems are complex in themselves, requiring complex control

technology and skilled use of expensive equipment to ensure their continuedeffectiveness, care in fuel and vehicle use, and regular skilled maintenance, whichmay be beyond the capability of the third and fourth owners.

In addition, some further investigation on potential side-effects during itsparticular use seems needed :

concerning potential odour problems caused by sulphurtrioxyde, or the allegedcarcinogenic effect, perhaps falsely or unjustly attributed to the active platinumleaving some types of catalysts, fire hazards as recently reported in the Germanpress where in Cologne a fire, caused by a vehicle equipped with a catalyticconverter (Audi 100), caused damage to the extent of 70.000 ECUs or3.150.000 BFr.

Anyhow, how should we now - more specifically - judge the compromisereached at EEC level from a technical and economic viewpoint?

The proposed standards have to be regarded as very tough indeed. This isparticularly so for large cars. On the basis of vehicle tests carried out by the Germangovernment/UBA/, 10% of cars presently available in the US would fail to complywith the new standards.

By effectively requiring three-way catalysts, a duplication of effort andexpense is imposed, especially on the manufacturers who are furthest advanced onlean-bum. This duplication of investment in product development and manufacturingwill add to the industry's costs, at a time when it is least able to afford it. To have oneso-called "green vehicle" in each of the car model ranges is costing Ford for even thislimited programme some 250 million ECUs or dollars, which is equivalent toBFr 11 billion 250 million, and is involving up to 10% of European research anddevelopment manpower.

For the "large" cars, over 2.0 litres, the fitting of three-way catalysts will bevirtually necessary since this is the only certain way of ensuring compliance with theemission levels required. This means that all these cars will require considerablechanges to the floor pan and exhaust system to accomodate the catalyst, plus additionalheat protection - both for the occupants, the interior trim and for grass, which maycatch fire as mentioned earlier, due to the heat given off by the catalyst - which worksin the range of 350 to 800 degrees Celsius. Also required will be either multi-point or

central, single-point fuel-injection or an electronically controlled carburettor, plus amicroprocessor, and oxygen-lambda-sensor in the exhaust pipe, probably secondaryair pumps and other equipment. Catalyst systems will add approximately 850ECUs/dollars or BFr 40.000 to the pre-tax showroom "customer" price of these over2.0 litre cars. Indications are that fuel consumption of the car will be up to 5-10%worse, and, although servicing intervals will remain unchanged, they will be verymuch more expensive.

Looking at real world conditions, in Germany a Scorpio 2 I injection in itscatalyst version costs 1.340 ECUs or BFr 60.500 more than the one without a catalyticconverter. An Orion/Escort 1.080 ECUs or about BFr 50.000 more, and a Sierra 21only 755 ECUs or BFr 34.000 more.

If comparing a 1.6 injection catalyst engine of 66 kW power and with amanual five-speed gearbox with a 1.6 carburettor non-catalyst engine of the same66 kW power and the same manual five-speed gearbox, the fuel economy for the non-catalyst versus the catalyst engine is just over 10%. It should, however, be mentionedthat the catalyst engine runs on 91 RON fuel where the non-catalyst version is tunedto 96.5 RON fuel. The price difference between normal and super grade fuel shouldbe brought into the final cost equation here to the advantage of the vehicle equippedwith a catalytic converter.

For "medium" cars of 1.4 to 2.0 litre capacity, the standards defined willmost probably require the most complex and costly of the possible lean-burnsolutions, greatly reducing the incentive to develop this new European technology.The lean-bum technology offers the prospect of a lasting substantially improvedenvironmental impact with improved fuel economy and cost-of-ownership.

Ford expects that an oxidation catalyst will be required, with lean-burnengines, either an "open-loop" - without microprocessor control - or "closed-loop" -with microprocessor control. Similar floor pan and exhaust system changes as for

three-way catalysts will be required, but the engine equipment will vary greatly.Some "medium" cars, which are of relatively light weight compared to the enginecubic capacity, may use a normal carburettor coupled with a simple computerisedignition system, plus "open-loop" oxidation catalyst. The showroom - or customer -price effect of this lay-out could be about 350 ECUs or dollars, or BFr 16.000,

before taxes. Other "medium" cars, where the power to weight ratio is lessfavourable, or where other constraints exist, may need similar fuel injection orelectronically controlled carburettors and microprocessors to the three-way catalyst.Here the objective would be to maintain the engine in the leanbum air to fuel range ofover 18:1, whereas the three-way system maintains it close to 14.7:1. The showroomprice effect of such systems will be slightly lower than the three-way systems notedearlier for "large" cars.

But for both systems, however, "open-loop" or "closed loop" with lean-bum engines, Ford would expect to obtain an improvement in fuel consumption of

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between 12 and 20%. Service intervals may either remain as now or become even lessfrequent and, for the "open-loop" system, it will remain relatively simple andrelatively cheap. "Closed-loop" systems or those involving injection equipment maybecome more expensive.

"Small" cars, of under 1.4 litres capacity, will generally be able to use lean-bum technology alone (although for some markets outside the EC, a catalyst solutionmay be necessary). Showroom price effects will be minimal, possibly in the order of150 ECUs/dollars/(BFr 8.000) or less, depending on the type of electronic ignitionequipment fitted. Servicing is unlikely to be different to today's cars, and fuelconsumption may improve by 12% or so. This possible improvement is especiallymarked in comparison with the fuel consumption penalty associated with three-waycatalysts.

It is hoped that the second phase for small cars with introduction dates1992/1993 and for which the emission levels still have to be decided will enable us tocontinue with the lean-bum technology.

The manufacturing investment for one new lean-bum engine at Dagenham,approaching 250 million ECUs or dollars, or BFr 11 billion 250 million, plus afurther 60 million ECUs or dollars or BFr 2 billion 700 million for design anddevelopment gives an indication of just how great the cost to Ford will be during aperiod when profitability, to put it mildly, is less than satisfactory.

The complexity of the matter is even more clearly demonstrated if one takesinto account the need to avoid substituting one form of pollution for another. It givesfurther weight to the argument why industry cannot simply agree to any "quick shot"solutions: there are simply no easy answers to complicated questions.

For example. it is recognised that an increasing concentration of C02 in theatmosphere causes a warming trend, leading to climatic changes in the next century,of sufficient magnitude to produce major physical, economic and social dislocationson a world-wide scale.

Absorbing heat radiation from the earth's surface, trapping it, andpreventing it from dissipating into space, plays a critical role in maintaining theearth's heat balance.

Since it looks as if the global atmospheric C02 concentrations could doublebefore the middle of next century, an average annual increase in global surfacetemperatures of about 2-3 degrees Celsius and possibly as much as 7-10 degreesCelsius could occur in the North Polar region during the winter. Changes in rainfallpatterns, desertification, higher sea levels, and so forth ...could be expected. OnJune 23, 1986, the Energy, Research and Technology Committees of the EuropeanParliament adopted Mr Fitzsimon's report on the measures to be taken against

increasing C02 concentration in the atmosphere to avoid - as was said - an ecologicalcatastrophe. A catalytic converter oxydising the harmful CO and HC to the so-calledharmless C02 is in view of this problem not a benefactor but a producer of a potentialhazard to world environment.

Taking another example of a possible shift from one form of pollution toanother is the major increase in the sales of diesel-powered passenger cars.

Due to the uncertainty on the final national legislation and on sufficientavailability Europe-wide of unleaded fuel, a vast number of our customers, in anunderstandable move to be independent of political bargaining, are giving preferenceto diesel engines. In this case, smoke and particulates which are characteristics of thediesel engine will increasingly need to be controlled in order to avoid another seriesof concerns. These concerns are recognised however and development of particulateand gaseous emission standards for diesel engined vehicles is already well advanced atthe EC Commission, since for both a final proposal for a Council directive wassubmitted to the Council in the month of June.

For the gaseous emissions for heavy commercial diesel vehicles over 3.5 tgross vehicle mass, the proposal is equivalent to the United Nations regulation R 49but with the levels reduced by 20% for CO and NOx and 30% for HC, and this startingfor new engine homologation on April 1, 1988 and for new registrations onOctober 1, 1990.

Concerning the particulates emissions for diesel passenger cars, theproposal is based on the US measurement method, transposed into the European testprocedure and intended to be introduced in two stages, first large cars: October 1,1988 for new models and October 1, 1989 for new registrations, followed by mediumand small cars: from October 1991 for new models and 1993 for new registrations.

Legislating satisfactory and consistent diesel fuel quality will also enhancethe environmental impact of diesel vehicles. Unlike vehicle legislation which wouldaffect only new vehicle designs, attention to fuel quality would also benefit theenvironmental performance of the existing diesel vehicle park.

Please allow me to interject here that changes in diesel or gasoline fuelquality in the recent past, pressures on refiners resulting in more secondaryprocessing, reduction of lead in leaded gasoline, the introduction of unleaded gasolineand the introduction of three-way catalyst systems - all mean that a pan-Europeanspecification for both fuels is both timely and appropriate.

Methanol. ethanol and other organic or oxygenate compounds added togasoline to make up for a decrease in lead and thus to improve its anti-knock qualitycould also create health and environment problems.

Aldehydes, polycyclic aromatic compounds, benzene, ethene, organicacids: some of them are known as potential respiratory, eye and skin irritants, otherscould mutate cells or cause cancer under specific conditions, dependent onconcentration of dose... etc. - and, though there is considerable uncertainty as to the

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magnitude of the health risks, one should take them into account in any final appraisal.Replacement of asbestos by another product able to respond to equal heat

and friction characteristics may well result in a similar atmospheric trace pollutant.It is essential that, before any alternative solution is enforced, one should -

as far as technically possible - be convinced about its advantages and healthcharacteristics, in other words, its benefits to the environment as well as to itsperformance, durability, and resistance to deterioration which has to be equal or atleast similar to the one being replaced, and - it has to be economically acceptable.

All the costs for making this wide variety of vehicles and fortesting/approving them are to be paid by "the consumer". His ability to pay forcomplex technology, including periodic replacement or maintenance, has to be takeninto account. It is all part of the balance that needs to be kept between the desirable andthe inappropriate, the necessary and the ideal.

The estimates prepared by the EC Commission that the annual cost to theCommunity for the emission issue alone could exceed ten billion ECDs is believed tobe accurate. Ford would support the view that all of the implications of the use ofthreeway catalysts - and to some extent oxidation catalysts - have NOT been fullyexplored.

The concentration of the major, non-communist, supply sources for the rawmaterials in one country - South Africa - is also a cause for some concern, as are therecent reports of a fourfold increase in the price of rhodium, an essential constituentof catalysts. From less than $ 300 an ounce in 1984 to over $ 1.150 an ounce at thebeginning of 1985, falling below $ 800 an ounce mid 1985, to climb again to $ 1.100end 1985. This shows the instability and uncertainty which exist with the much-neededraw material for the present generation catalyst: the noble metal. The capability ofmeeting the potential automotive demand for rhodium from known reserves is also indoubt.

Also platinum - about two grarnmes goes into each catalyst - stood in Augustat the highest levels since 1980: prices have more than doubled from a low of $ 237an ounce last year to $ 545 an ounce, after reaching $ 560. New surges to $ 600-700an ounce are not implausible.

If the entire European auto market were to use catalysts, estimates of anadditional annual demand of 500.000 ounces of platinum, 150.000 ounces ofpalladium and 30.000 ounces of rhodium have been put forward, depending on thenumber of automobiles sold in the EEC in a given year.

The increases in total metal requirements from 1985 levels are expectedthen to be about 19% for platinum and 17% forrhodium.

Not being an economist, I would nevertheless tend to predict in thosecircumstances some rather drastic price increases on the bullion market.

And, remember, first cost is never the whole story: costs have a nasty habitof coming back and biting the purchaser again and again, by way of increasedmaintenance, replacement and general operating costs.

Finally, allow me to underline the need to avoid any sort of unique "pioneerrole" or "cavalier seul" behaviour of an individual government.

European harmonisation of the environmental exhaust emission legislationis a basic demand from industry - to improve the general quality standard, throughtess complexity and thus fewer line disturbances, and to avoid distortion ofcompetition and trade between the European countries and to create - throughexcellence and economy of scale - the best possible product for the best possible price.

It appears, however, that we are confronted with at least four differentemission standards across Europe - 15.04, EC fifth amendment, Swedish A 10,US 1983/1987 - and this is, to say the least, to be deplored. Even those countries whohave collectively agreed to go the 83 US route have different dates/pro-cedures/conformity requirements, all of which increase the manufacturer's burdenwith no environmental benefit.

We have had to undertake the radical re-engineering of no fewer than thirty-seven current and future engine applications. The cost of this enterprise is certainlynot less than 200 million ECUs or dollars.

No manufacturer, or administration for that matter, has a bottomless pot ofgold, and the consumer, equally, has to operate within financial constraints.

The cleanest, quietest, road vehicle in the world is of no use if few peoplecan afford to buy it, run it, use it.

In the end, this all adds up to compromise:The EC compromise might be assessed as giving at best half-hearted support

for the new European technology of lean-burn. It is something - and in view of thecircumstances - possibly the only political option left.

We have an expression in Belgium: "Qui aime bien, chatie bien'V''Eengoede vader spaart de roede niet"I"You always hit the child you love the most!" Thismay after all be the rationale behind it. Should this give us hope for the future?

Thank you, Mr President.

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CONTROL OF DIESEL PARTICULATE EMISSIONS IN EUROPE

by Michael P. Walsh

Formerly Director of the U.S, Environmental Agency's Office of Mobile Source AirPollution Control, currently an environmental consultant in the motor vehiclepollution field.Address : 2800 N Dinwiddie Street, Arlington, Virginia 22207, U.S.A.

ABSTRACT

The greater use of diesel equipped vehicles for private cars and all categories ofcommercial vehicles is the major trend observed worldwide over the last decade in themotor vehicle field. While the energy advantages of the diesel are unquestioned,concerns began to grow during the 1970's over the environmental consequences ofincreased dieselization. Although inherently cleaner than gasoline engines from thestandpoint of carbon monoxide (CO) and evaporative hydrocarbons (HC), dieselsproduce more aldehydes, sulfur oxides (because of the higher sulfur content in dieselfuel than in gasoline) and nitrogen oxides. Offensive smoke and odor emissions arealso a problem. Most importantly, however, uncontrolled diesels emit significantamounts of particulate. These particles are a direct health concern as well as a serioussource of overall environmental degradation. The purpose of this presentation is toreview the information regarding adverse health and environmental consequencesassociated with diesel particulate. In addition, possible control strategies will besummarized.

I. Background

The greater use of diesel equipped vehicles for private cars and allcategories of commercial vehicles is the major trend observed worldwide over the lastdecade in the motor vehicle field. While, for the first three quarters of this century,the gasoline fueled internal combustion engine (ICE) powered the automobileindustry to ever greater peaks of prosperity, the dramatic increase in fuel pricesspurred by the OPEC oil embargo and reinforced by the later Iranian crisis, sent theworld's automotive engineers searching for a more fuel efficient alternative. For thefirst time a potential market opportunity for an alternative engine was created - onewhich promised significantly better fuel efficiency than the conventional gasolinefueled, otto cycle powerplant.

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Not surprisingly, almost all eyes focused on the diesel, a powerplant whichhad been in common usage on trucks for almost as long as the otto cycle was used incars. It was familiar, reliable, had the nucleus of a fuel distribution system in place,and most importantly had demonstrated advantages in fuel efficiency. Across the fullrange of driving conditions, diesel fuel economy was at least 25 percent better thangasoline cars of the same weight and size. In stop and go urban traffic, the efficiencyadvantage rose to 35 or 40 percent.

Worldwide diesel car production increased from about 1.3% of the totalpassenger car market in 1976 to 5.4% in 1983. With commercial vehicles, worldwidepenetration in both vehicle categories is continuing to grow.

While the energy advantages of the diesel are unquestioned, concerns beganto grow during the 1970's over the environmental consequences of increaseddieselization. Although inherently cleaner than gasoline engines from the standpointof carbon monoxide (CO) and evaporative hydrocarbons (He), diesels produce morealdehydes, sulfur oxides (because of the higher sulfur content in diesel fuel than ingasoline) and nitrogen oxides. Offensive smoke and odor emissions are also aproblem. Most importantly, however, diesels emit substantial amounts of fineparticulate.

Because of this, during its 1985 deliberations regarding motor vehiclepollution issues, the European Community Environmental Ministers asked theCommission to develop a proposal to control diesel particulate emissions. Thoughoriginally intended by the end of 1985, it was not possible for the Commission to meetthis deadline. However, during June, the Commission approved proposals for twonew directives aimed at reducing air pollution caused by diesel powered vehicles.Unfortunately, only one, dealing with passenger cars, addressed particulate emissionsand even this did little more than maintain the status quo.

The purpose of this paper is to review the reasons why control of dieselparticulate emissions is urgently needed, especially in Europe, to show that thetechnology is available to reduce these emissions and to illustrate the potential impactof introducing this technology in Europe.

II. Health and Environmental Concernswith Diesel Particulate

Uncontrolled diesels emit approximately 30 to 70 times more particulatethan gasoline-fueled engines equipped with catalytic converters and burning unleadedfuel. These particles are a concern from several standpoints:

1. Many areas already experience unhealthy air quality levels for total suspendedparticulate (TSP) matter. Most TSP comes from stationary sources but diesels

contribute. These particles in urban air are of concern because a strong correlationbetween suspended particulate and variations in infant mortality and total mortalityrates has been established. Further, clear evidence emerges from the body ofepidemiological literature that implicates particles in aggravating disease amongbronchitics, asthmatics, cardiovascular patients and people with influenza. Anysignificant increase in diesel particulate emissions would add to the difficulty ofsolving this problem.

2. Beyond the overall impact on TSP, diesel particles raise a special health concernbecause they are very small (averaging about 0.2 microns in size). SmalI particles,which are much more likely to be deposited in the deepest recesses of the lung(alveolar region) and which require much longer periods of time to be clearedfrom the respiratory tract, have a greater potential to adversely affect humanhealth than larger particles. In addition, when emitted, they remain suspended inthe air near the breathing zones of people for long periods of time. For thesereasons, the Harvard University Health Effects Project recently concluded that"particulate pollution should be a public health concern because, even at currentambient concentrations, it may be contributing to excess mortality and morbidity.Furthermore, our recent analyses .... indicate that fine particles (FP) and sulfates(S04=) are among the most harmful particles to public health."

3. In addition, diesel particulate has also been singled out as especially hazardous andtoxic because of its composition. The U.S. EPA has noted that up to 10,000chemicals may be adsorbed on the surface of diesel particles and drawn deep intothe lung with them. Many of these chemical compounds are known to be mutagenicin short term bioassays, and to be capable of causing cancer in laboratory animals.Based on an exhaustive multiyear program of in vitro and in vivo studies by EPAand others focusing on the comparative potency of diesel particulate with otherknown human carcinogens, EPA estimated the risk to range from 0.26 x 10-6 to1.4 x 10-6 lung cancers per person per year due to a constant lifetime exposure toone microgram per cubic meter of diesel particulate. Since total national urbanexposure to diesel particulate in the United States was estimated to range from 3 to5 micrograms per cubic meter by 1995, it is easy to see why this has been a cause ofgreat concern. Two new animal studies, one sponsored by General Motors andanother underway at Lovelace Inhalation Toxicology Research Institute labora-tories appear to add further evidence of the cancer risk.

A recent study conducted by rno found similar problems in Europe. "As atentative order of magnitude estimate for the mutagenicity of European exhaust,the following emission factors may be assumed:

~I - g~~~~ne, no catalyst---3~~ ~~~. ~~~i~ --.. ----(\~~~ ~~~~~~~:~~~~IGasoline, with catalyst 10 000 rev/km (100 000 rev/liter) "I-------_. ~ .._-_ .. _.._------. -_.._ .. _ ... _-------_._~

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Because as noted in the Harvard project "most of the toxic trace metals, organics,or acidic materials emitted from automobiles or fossil fuel combustion arc highlyconcentrated in the fine particle fraction" and since diesel engine penetration inEurope is much greater than in the United States, the potential cancer risk is alsosubstantially greater.

Other epidemiologic studies have tended to reinforce these concerns. For example,a 1983 study of heavy construction workers found positive trends in lung cancer bylength of union membership and a higher than expected rate among retirees.Further, a pilot study of U.S. railroad workers, conducted by researchers atHarvard, indicated that the risk ratio for respiratory cancer in diesel exposedsubjects relative to unexposed subjects could be as great as 1.42, i.e., the possibilityof developing cancer may be 42 percent greater in individuals exposed to dieselsthan in individuals which are not exposed. The follow up study which has now beencompleted appears to be equally alarming - "Using multiple logistic regression toadjust for smoking and asbestos exposure, workers age 64 or less at the time ofdeath with lung cancer had increased relative odds (1.2 - 1.4, P less than 0.05) ofhaving worked in diesel exhaust exposed jobs."

Clearly, it is prudent to conclude that greatly increased numbers of diesels withoutsubstantial particulate controls could result in a significant increase in cancer risksin Europe as well as elsewhere. Further, since the diesel car population in theCommunity is projected to grow from today's 5.8 million to about 15 million by1995, without substantial controls the risk will increase tremendously.

4. While health issues have been the cause of most concern, diesel and other particlescan also become a nuisance, degrade aesthetics and material usage through soilingand may contribute directly, or in conjunction with other polluants, to structuraldamage by means of corrosion or erosion.

5. Impairment of visibility has been widely noted as an adverse effect of increasedparticulates. Diesel particles because of their composition (primarily carbonbased) and size (in the size range of 0.2 microns) are very high light absorbers andscatterers and therefore have the potential to be especially harmful to visibility.

During late 1985, the results of several new studies were presented whichincreased concerns regarding adverse health effects from diesel particulate emissions.In particular;

1. Stoeber (Fraunhofer Institute) reported on carcinogenicity in rodents after longterm high dose diesel inhalation. On both mice and rats, malignant tumorsincreased with exposure to diesel exhaust. With the mice, however, gaseous phase

emissions seemed most important whereas with the rats the particles seemed to bethe main cause.

2. Brightwell (Batelle-Geneva) reported that unfiltered diesel exhaust produced anincrease in lung tumor incidence from 19% to 40%; gasoline emissions reportedlyshowed no effect.

3. In a summary presentation, McClellan (Lovelace) described the issue as no longerwhether diesel exhaust is carcinogenic but rather under what conditions and howmuch.

III. Diesel Smoke and Particulate Control Outside Europe

Because of these various problems associated with diesel smoke andparticulate, control programs have been underway for many years. This next sectionwill review the history of these programs to date. In general, one can note that theinitial focus was on smoke control because it was clearly visible and a nuisance. As theevidence has grown in recent years regarding the serious health and environmentalproblems, more attention has focused on control of the particles themselves.

Smoke is composed primarily of unburned carbon particles from the fueland usually results when there is an excess amount of fuel available for combustion.This condition is most likely to occur under high engine load conditions such asacceleration and engine lugging when the engine needs additional fuel for power.Further, a common maintenance error, failure to clean or replace a dirty air cleaner,may produce high smoke emissions because it can choke off available air to the engineresulting in a lower than optimum air-fuel mixture. Vehicle operation can also beimportant since smoke emissions from diesel engines are minimized by selection ofthe proper transmission gear to keep the engine operating at the most efficient speeds.Moderate accelerations and lower highway cruising speed changes as well as reducedspeed for hill climbing also minimize smoke emissions.

United StatesU.S. emission control requirements for smoke from engines used in heavy

duty trucks and buses were first implemented for the 1970 model year. These opacitystandards were specified in terms of percent of light allowed to be blocked by thesmoke in the diesel exhaust (as determined by a light extinction meter). Heavy dutydiesel engines produced during model years 1970 through 1973 were allowed a lightextinction of 40 percent during the acceleration phase of the certification test and20 percent during the lugging portion; 1974 and later model years are subject tosmoke opacity standards of 20 percent during acceleration, 15 percent duringlugging, and 50 percent at maximum power.

It appeared to the EPA during the early 1970's that before very significant

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pollution controls on trucks and buses could actually be brought about a new testprocedure encompassing truly representative modes of usage in urban areas wasneeded. A multiyear effort to develop such a test was therefore initiated. While thiswork was underway, EPA became alarmed by the sudden growth in diesel cars whichstarted during the late 1970's. Even though trucks and buses were clearly moreimportant sources of particulate than cars and light trucks at that time, EPA concludedthat the latter vehicles were very significant and that it was possible to initiate controlson these vehicles more quickly than on trucks and buses.

Accordingly, the first diesel exhaust particulate standards in the world wereestablished for cars and light trucks in an EPA rulemaking published on March 5,1980. Standards of 0.6 grams per mile (0.37 g/km) were set for all cars and lighttrucks starting with the 1982 model year dropping to 0.2 grams per mile (0.12 g/km)and 0.26 (0.16) for 1985 model year cars and light trucks, respectively. In early1984, EPA delayed the second phase of the standards from 1985 to 1987 model year.Almost simultaneously, California decided to adopt its own diesel particulatestandards - 0.4 grams per mile (0.25 g/km) in 1985,0.2 (0.12) in 1986 and 1987, and0.08 (0.05) in 1989.

Less than one year later, in January 1981, EPA formally proposed similarparticulate standards for trucks and buses. A comprehensive urban truck and bus testprocedure had been developed by that time and analysis clearly showed that smokecontrols were inadequate to bring about truly significant particulate reductions.

A four year delay ensued before final action was taken by EPA. During thistime, a new Administration at EPA reevaluated the need for diesel particulate controlas well as the newly developed truck test procedure. These reevaluations reached thesame fundamental conclusions as the earlier work - truck and bus controls isextremely important because the pollutants involved endanger the public health andenvironment and trucks are a major contributor to those pollutants; as a result, thefirst particulate standards for heavy duty diesel engines were promulgated by the U.S.EPA earlier this year. Standards of 0.60 grams per Brake - Horsepower - Hour(g/bhph) (0.80 grams per kilowatt-hour) were adopted for 1988 through 1990 modelyears, 0.25 (0.34) for 1991 through 1993 model years and 0.10 (0.13) for 1994 andlater model years. Because of the special need for bus control in urban areas, the 0.10(0.13) standard for these vehicles will go into effect in 1991, three years earlier thanfor heavy duty trucks.

These standards are required to be met over the full life of the vehicle orengine, rather than over half the life as is the case with cars. Also, EPA based thestandard on the new "transient" test referenced above rather thanon the old "steady-state" test because the transient test is much more representative of the manner inwhich trucks are driven in cities.

CanaanIn March of 1985, in parallel with a significant tightening of gaseous

emissions standards, Canada adopted the V.S. particulate standards for cars and lighttrucks (0.2 and 0.26 grams per mile, respectively) to go into effect in the 1988 ModelYear. Since then, Canada has initiated a review of truck controls and is consideringadoption of V.S. standards for these vehicles as well.

JapanJapan does not currently regulate exhaust particulate emissions from diesel

engines. However, smoke standards have applied to both new and in-use vehicles since1972 and 1975, respectively. The maximum permissible limits for both are 50 percentopacity; however, the new vehicle standard is the more stringent because smoke ismeasured at full load, while in-use vehicles are required to meet standards under theless severe no-load acceleration test.

Smoke standards versus particular standardsWhile smoke standards provide a limited degree of emission control, by not

focusing on particulate levels over an average driving cycle and because they arefairly lenient, their effect actually reducing particulate emitted is somewhat limited. Itis safe to say that particulate emissions throughout the world outside the U.S. remainvirtually uncontrolled at the present time.

IV. The European Response To Date

A. Common Market

Smoke limits similar to those described above in the United States and Japanhave been in effect in Europe for many years. However, recognizing that theserequirements are not adequate, during its 1985 deliberations regarding motor vehiclepollution issues, the European Community Environmental Ministers asked theCommission to develop a proposal to control diesel particulate emissions.

Specifically, the proposed standards are:

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Type approval1.3 g/test

Conformity of production1.7 g/test

These standards, which many European produced vehicles are alreadyachieving, are intended to be introduced on the following timetable:

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Vehicle type

over 2 000 cm32 000 cm3 or lessD.I.

·······_-_·_--1Introduction date

New models New cars I

October 1988 October 1989 IOctober 1991 october. 1993 [October 1994 October 1996

---------

Studies conducted by Germany have indicated that theapproximate conversion rate between the US and ECE tests is about 3, i.e., 0.2 gramsper mile on the US test is roughly equivalent to about 0.6 grams per test on the ECEtest.

Approximate conversions are summarized below:

Test procedure USGrams/mile

ECEGrams/test Grams/kilometer

0.60.2

0.08

B. Non Common Market European Countries

1.80.6

0.24

0.450.150.06 .~

In stark contrast to the Common Market, several other European countrieshave been cooperating in moving toward more significant diesel particulaterequirements. Sweden has already adopted the US passenger car standard to go intoeffect in 1989 and Switzerland and Austria are likely to do so in the near future. Thesecountries are also looking hard at more stringent requirements for trucks and buses.

V. Impact of the Commission Proposal On Common MarketEmissions

Overall diesel car sales increased by 21.3% across Europe from 1984 to1985. As illustrated below, the increase was even greater in some countries.

Country 1985 % Market 1984 % Change1984-1985

Belgium-Luxembourg 95000 26.4 96900 -1.9Denmark 10400 6.6 10300 0.2France 264800 15.0 240400 10.1Ireland 8400 14.2 6000 41.7Italy 438600 25.1 425300 3.1Netherlands 71300 14.4 61000 17.0Spain 124900 22.6 126700 -1.4United Kingdom 66200 3.6 45100 46.8 I

West Germany 530800 22.3 321800 64.9 J

It is especially ironic that West Germany has encouraged the growth ofhigh particulate emitting diesel cars by allowing them to qualify for "low pollution"tax incentives without any requirement that they meet the same particulate levels asGennan models exported to the US. In part as a result of these tax incentives, dieselcar sales in Germany have accelerated in the last year, as is illustrated in Figure 1.Fortunately, in adopting its low pollution tax policies earlier this year, theNetherlands did not provide similar tax reductions for uncontrolled diesels.

Because of the high growth rate for diesel vehicles in Europe and themodest particulate reductions proposed by the Commission, it appears that overallparticulate emissions will grow tremendously throughout the next twenty five years.This is illustrated in Figure 2 which plots motor vehicle particulate during this period.This figure shows that emissions from all categories of vehicles will continue to growunder the Commission proposal.

Even these projections may be understating the potential problem, however,as concerns have been growing that diesel fuel quality may deteriorate significantly inthe future in Europe. Should this happen, particulate emissions will likely rise evenfurther.

VI. What Is Possible

Light duty vehiclesFortunately, it is possible to do something about these problems. Two major

approaches exist for meeting stringent diesel particulate standards: enginemodifications to lower engine out emission levels, and trap-oxidizers and theirassociated regeneration systems. Engine modifications include changes in combustionchamber design, fuel injection timing and spray pattern, turbocharging, and the use ofexhaust gas recirculation. Further particulate controls appear possible throughgreater use of electronically controlled fuel injection which is currently under rapiddevelopment. Using such a system, signals proportional to fuel rate and pistonadvance position are measured by sensors and are electronically processed by theelectronic control system to determine the optimum fuel rate and timing.

Exhaust aftertreatment generally consists of a filter or trap to capture theparticulate and a regeneration system to convert it to less harmful materials; Trapoxidizer prototype systems have shown themselves capable of 70 to 90 percentreductions from engine out particulate emissions rates and with proper regenerationthe ability to achieve these rates for high mileage. Systems have now started to beintroduced commercially.

Figure 3 shows the distribution of emission results for 1986 model cars inthe United States. Compared to an average emission rate of 0.6 grams per mile in1980, it can be seen that current emissions now average about 0.2 grams per mile. (It

59

60

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32 .....------------------------,

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CALENDAR YEAR

FIG.1 DIESEL SALES IN GERMANY.

-·c~

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CALENDAR YEAR

FIG.2 ESTIMATION OF PARTICULATE EMISSIONS FROM 1985 TO 2010 IN EUROPEFOLLOWING ECE COMMISSION PROPOSAL.

is important to note that this reduction occurred at that same time that NOx emissionswere literally cut in half, from 2.0 grams per mile to 1.0 grams per mile.) The clean-est vehicles in this figure are equipped with the first generation trap oxidizer systems,which are designed to capture and burn the particles. Daimler Benz introduced twomodels equipped with these systems in California and neighboring Western states for1985 Model Year. To date, over 20,000 vehicles equipped with these systems haveentered commercial service without significant problems. As recently noted byDaimler Benz in full page advertisements in the Washington Post and the New YorkTimes, "an ingenious trap oxidizer ... virtually eliminates visible diesel exhaustemissions." Further, field tests by Volkswagen of a trap system and an on board fueladditive regeneration system on vehicles operated up to 25,000 miles has provenpotential to achieve a 0.08 gram per mile level. Some 1986 Mercedes models werecertified in California below 0.08 (while simultaneously achieving less than 1.0 gramper mile NOx)'

Truck and bus control technologiesWhile not yet as far advanced, control technologies for trucks and buses are

similar to those for light duty vehicles: engine modifications to lower engine outemission levels, and trap-oxidizers and their associated regeneration systems.

As noted earlier, there was a four year delay between EPA's initial proposalfor heavy duty truck particulate control and final EPA action. During this period,most manufacturers reduced their particulate control development work. However,two manufacturers which did continue their efforts, Daimler-Benz and Volvo White,made significant progress, with Daimler Benz predicting trap availability in 1990 andVolvo White in 1991. Daimler-Benz's position was based upon what appears to be themost advanced development and test program of any heavy duty manufacturer.Current applications of its traps on urban buses have already demonstrated a servicelife of 100,000 miles.

When adopting standards in March of 1985, EPA emphasized its optimismthat the particulate standards would be achieveable in spite of the limited work done todate by heavy truck manufacturers. First, the Agency noted that "review of newinformation submitted on the subject of trap oxidizer feasibility indicates for lightduty diesels, continued progress has been made in solving the various technicaldifficulties associated with traps. Daimler-Benz has already introduced traps on lightduty vehicles in California, and Volkswagen and other manufacturers will do so in the1986 Model Year." Secondly, regarding heavy duty vehicles, EPA noted that "whatlittle work has been done also indicates progress. Traps are not fully developed today,but they were not expected to be. The important issue is whether they can reasonablybe expected to be available for future standards, and on this issue, EPA's position isunchanged. In fact, the new data which were included in manufacturers' commentswere extremely promising, and EPA is confident in its projections of successfulapplication of traps to heavy duty engines." Extensive cost-benefit analyses carried

61

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o NO. + rllTICILl1E

0.9

0.8

- 0.7•-<,w• 0.6·~

0.5

VIZ 0.40VI 0.3VI

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04250 3625 3500 3000 2625

TEST WEIGHT

FIG. 3 DISTRIBUTION OF EMISSIONS FOR 1986 MODEL CARS IN U.S.

E22J nIL l CAR I ~ lllCE CUI ~ COMMEAClll

240-· 220..200·:: 180..· 160·..· 140·..w 120-··.. 100-VI 80w

l-e:( 60....~u 40~II' 20e:(Q. 0

1990 1995 2000 2005 2010

CALENDAR YEAR

FIG.4 ESTIMATION OF PARTICULATE EMISSIONS FROM 1985 TO 2010 IN EUROPEFOLLOWING U.S. STANDARDS.

out on heavy duty vehicles has shown that truck and bus controls are even more cost-beneficial than passenger car standards.

One additional point is worth noting. For diesel buses especially, because oftheir long lifetimes and their intensive usage in densely populated urban areas, retrofitis a very attractive strategy and is being pursued by many areas around the world.

VII. Tighter Standards Could Help Europe

If the European Community were to require the introduction of standardssimilar to those being introduced in the United States, it could continue to enjoy thebenefits of the diesel engine while reducing the environmental consequences. Forexample, Figure 4 shows what could happen if US type standards were introduced inEurope. In making these projections, the US standards for commercial vehicles wereassumed to be introduced in 1995, the large car standards in 1989 and the small carstandards in 1993.

The contrast between the Commission proposal and US type standards isfurther illustrated in Figures 5, 6, 7 and 8. The figures show that not only can overallemissions be reduced, even with fairly large vehicle population growth, the pollutionfrom individual categories can also be substantially lowered.

VIII. Conclusions

Based on the information summarized above, it seems clear that the adversehealth and environmental consequences of diesel particulate emissions are sufficientlyserous to justify control to the limits of technological feasibility. In addition,technology has been sufficiently developed for cars and light trucks to achieve the1987 U.S. standards (0.2 grams per mile and 0.26 grams per mile, respectively) andlooks extremely promising for the 1989 California standard of 0.08 grams per mile.Finally, substantial progress has also occurred for truck and bus controls, both newand used.

In view of this, and in view of the even greater need for particulate controlin Europe than in the US because of the much greater proportion of the fleet poweredby diesel engines, a much stronger proposal is warranted than that recently profferedby the Commission. It should include the following key elements:

1. The mandatory particulate standard should be 0.6 grams per test.2. New truck and bus controls, similar to those recently adopted by the US EPA,

should be adopted.3. Because of the high public exposure to emissions from urban buses as well as the

long lifetime of the existing fleet, the Community should do all possible toencourage the retrofit of urban buses with advanced particulate controls.

64

o CnlllllU + US STAIDARD

2010200519951990

400 ~-------------------------:JlJ

380

360

340

320

300

280

260

240

220

200

180

160

140

120 +-----,------r-----,--------,----------11985

"·o~o·c···"--

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CALENDAR YEAR

FIG.S ESTIMATION OF PARTICULATE EMISSIONS IN EUROPE FROM 1985 TO 2010.COMPARISON BETWEEN ECE COMMISSION PROPOSAL AND U.S. STANDARDS.

( ALL DIESEL VEHICLES )

E22l US STAlDIIG ~ C.IIISII ••

·c·L-··~··c-···~.........VIwl-e(.....:JU

I-et:e(Q.

130-,-------------------------,120

110

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90

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70

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50

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0...J..£."""-t..:......>..L-...J.L....Lj~:u..----.Lr-L+_"_.:u..---'-L...L.~~-1.L...L+_=>....::..L-..lL.L+~

1985 1990 1995 2000 2005 2010

CALENDAR YEAR

FIG.6 ESTIMATION OF PARTICULATE EMISSIONS IN EUROPE FROM 1985 TO 2010.COMPARISON BETWEEN ECE COMMISSION PROPOSAL AND U.S. STANDARDS.

(SMALL CARS )

~ US STUOIiOS

201020052000199519901985

5

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30

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a 140-cM 120···~ 100........Vlw 80l-e:(

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CALENDAR YEAR


(COMMERCIAL VEHICLES )

66

In addition, member countries which have adopted tax incentives toencourage consumers to purchase low pollution vehicles should either exclude dieselsfrom any tax credits or require them at a minimum to achieve 0.6 grams per test(BCE test) or 0.2 grams per mile (US test) particulate to qualify for low pollution taxcredits.

IX. References

1. "The Benefits and Costs of Light Duty Diesel Particulate Controls," Michael P.Walsh, SAE #830179

2. "The Benefits and Costs of Light Duty Diesel Particulate Controls II," Michael P.Walsh, SAE, February 1984

3. "The Benefits and Costs of Light Duty Diesel Particulate Controls III - The UrbanBus," Michael P. Walsh, SAE, February 1985

4. "The Benefits and Costs of Light Duty Diesel Particulate Controls IV - The In-Use Urban Bus," Michael P. Walsh, SAE, February 1986

5. "Cancer Incidence Among Members of A Heavy Construction EquipmentOperators Union With Potential Exposure To Diesel Exhaust Emissions,"Submitted to Coordinating Research Council by Environmental Health Associates,18 April, 1983

6. Mortality Among Members of A Heavy Construction Equipment OperatorsUnion With Potential Exposure To Diesel Exhaust Emissions," Submitted toCoordinating Research Council by Environmental Health Associates, 18 April,1983

7. "Relation of Air Pollution To Mortality: an Exploration Using Daily Data for 14London Winters, 1958-1972", Mazumdar, Schimmel, Higgins, Electric PowerResearch Institute, Palo Alto, 1980

8. "Diesel Cars, Benefits, Risks and Public Policy," National Academy of Sciences,December 1981

9. "Review of Recent Information Regarding Carcinogenicity of Diesel EngineEmissions", Pepelko to Gray, U.S. EPA, June 14, 1985

10. U.S. Environmental Protection Agency, Heavy Duty Diesel ParticulateRegulations, Draft Regulatory Analysis, Approved by Michael P. Walsh,December 23, 1980

11. "Impact of Light Duty Diesels On Visibility in California," Trijonis, March 198212. "Diesel Exhaust Odor and Irritants: A Review," Nicholas P. Cernansky, Journal

of the Air Pollution Control Association, February 198313. U.S. Environmental Protection Agency, Control of Air Pollution From New

Motor Vehicles and New Motor Vehicle Engines; Gaseous Emission andParticulate Emission Regulations, Federal Register, March 15, 1985

14. "Trap-Oxidizer Technology For Light-Duty Diesel Vehicles: Feasibility, Costsand Present Status," Weaver and Miller, Report to EPA by Energy and ResourceConsultants, March 1983

15. "Diesel Technology", National Research Council, Report of the TechnologyPanel of the Diesel Impacts Study Committee, National Academy of Sciences,1982

16. "Draft Environmental Guidelines On The Diesel Vehicle," Clavel and Walsh,United Nations Environment Program, March 1983

t7. "Benefits of Reducing Odors From Diesel Vehicles: Results Of A ContingentValuation Survey," Prepared for Environmental Protection Agency by CharlesRiver Associates, March 1983

18. "Diesel Exhaust and Air Pollution," TNO, Netherlands Organization ForApplied Scientific Research, January 1986

19. "Begrenzung der Partikelemission von Dieselfahrzeugen im Rahmen dereuropaischen Abgasvorschriften", Umweltbundesarnt, October 1985

20. CCMC Manufacturers' Measurements of Particulates On Diesel EnginedPassenger Cars, November 1985

21. "Health Effects of Airborne Particles," Ozkaynak and Spengler, Health EffectsProject Staff, Harvard University, February 1986

22. OECD, "Road Research Programme, Impact of Heavy Freight Vehicles, FinalReport," September 28, 1982

23. Shenker, Smith, Munoz, Woski, Speizer, "Lung Cancer Among Diesel ExposedRailroad Workers, Results of a Pilot Study," Harvard School of Public Health,1982

24. Schenker, Oral Statement, American Lung Association Convention, May 198425. U.S. Environmental Protection Agency, Federal Register, March 5, 1980.

Standard for Emission of Particulate, Regulation for Diesel Fueled Light DutyVehicles and Light Duty Trucks

26. U.S. Environmental Protection Agency, Regulatory Analysis of the Light DutyDiesel Vehicles, U.S. Environmental Protection Agency, February 20,1980

67


THE PROBLEMS INVOLVED IN PREPARING AND UPHOLDINGUNIFORM EXHAUST-GAS STANDARDS WITHIN THE COMMONMARKET

by H. Henssler

Commission of the European Communities,Directorate Internal Market and Industrial Affairs200 rue de la Loi, B-1049 Brussels, Belgium.

ABSTRACT

Uniform exhaust emission standards for passenger cars and light dutyvehicles exist since 1970 in the Common Market. In June 1985 the Council ofMinisters for the Environment agreed by majority a further decisive reduction ofthese standards. The future European emission standards are based on the principle ofequivalence of their environmental effect with that of the current US standards, takingaccount of the European conditions in particular with regard to the composition of thecar fleet and its operating characteristics. Like the preceding steps of the EEC exhaustemission regulations, the new standards refer to the present European test procedurewhich however at a later stage should be completed by a test cycle representing extra-urban driving conditions.

The new European emission standards will apply to passenger cars with amaximum mass up to 2,500 kg having not more than 6 seats. The limit values and theeffective dates are differentiated according to 3 categories of engine capacity.

The presentation describes the aims, the development and the rationale ofthe EEC exhaust emission regulations and also gives a summary of their legal bases.

THE EEC'S LEGAL BASES AND METHODS OF PROCEDURE

1. In March 1970 the Council of Ministers of the European Communities adopted, asthe second separate directive of the EEC type-approval procedure, directive70/220/EEC "on the approximation of the laws of the Member States relating tomeasures to be taken against air pollution by gases from positive-ignition enginesof motor vehicles". Since this date, uniform exhaust emission standards existthroughout the whole Community for the concerned category of vehicles.

2. The present contribution is intended to describe the aims, development andsignificance of the European Communities' exhaust-gas standards. It appearssuitable, by way of an introduction, first of all to describe the essential aspects ofthe EEC's legal bases and methods of procedure.

3. A fundamental aim of the 1957 Treaty of Rome establishing the EuropeanEconomic Community is the creation of a common internal market among the

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Member States! within which cinzens, goods and services may cross borderswithout let or hindrance. Naturally, if goods are to move freely, first of allcustoms barriers and then the so-called non-tariff barriers or technical barriers totrade must be removed.

The major non-tariff barrier to the cross-frontier trade in motor vehicles isconstituted by the Member States' type-approval procedures, together with theirvarious technical requirements and administrative practices. It has for goodreasons been impossible simply to do away with each Member State's proceduresand to replace them with an EEC type approval procedure which, however, wasneeded if barriers to trade of this type were to be avoided.

4. Use was therefore made of "optional harmonization" whereby the EEC type-approval is established in parallel to the Member State procedures, but does notreplace them. From this results the possibility of a choice at two levels:

- Member States may retain, alongside the EEC provisions which they are boundto introduce, divergent national provisions, and- Manufacturers may, where Member States decide to retain national provisions,choose whether they wish to manufacture their products in accordance with EECor Member State provisions.

The relevant national authorities must approve vehicle types, permit vehicles to besold on their territory and to enter service if they comply with EEC provisions.However, they may deliver such approval on the basis of other criteria, such as anynational technical requirements already existing.

5. The European Communities issue their technical and administrative standards inthe motor vehicle field in the form of "directives". A directive is one of the legalinstruments made available to the executive of the Communities under the Treatyof Rome in order that it may perform its function. It is addressed to Member Statesand for them its aim is binding, while the choice of means of implementation is leftopen to them.

Under normal circumstances four European institutions work together inpreparing a directive: the Commission, the European Parliament, the Economicand Social Committee and the Council of Ministers. Their respective functions arelaid down in Article 100 of the Treaty of Rome: the Commission has the right ofinitiative i.e. it proposes directives, while the Parliament and Economic and SocialCommittee deliver opinions on it and the Council adopts and issues the directive.

IThe founder members of the EEC: Belgium, Federal Republic of Germany, France, Italy, Luxembourg and theNetherlands. Subsequent accession of: Denmark, Greece, Ireland, Portugal, Spain and the United Kingdom.

LINKS BETWEEN EEC AND ECE STANDARDS

6. Before we go into individual detail on the development of the Community exhaust-gas standards, it would seem appropriate to make one or two basic remarksconcerning the relationship between the Commission of the EuropeanCommunities and the United Nations' Economic Commission for Europe (ECE),Geneva as regards vehicle regulations.

Under the 1958 Geneva Convention the ECE has adopted a number of suchregulations, which, however, are not embedded in a complete type-approvalprocedure. As signatories to this Convention the majority of the EEC MemberStates base their national type-approval systems more or less on these regulations.The Commission has therefore been forced to take them into account. Theadvantages of technically equivalent standards within the Community and in themuch wider area covered by the Geneva requirements are obvious. The EEC hasthus decided to transpose the technical requirements of the ECE regulations intothe corresponding separate directives where these are relevant to its type-approvalprocedure.

DEVELOPMENTS SO FAR AS REGARDS THE EEC'S EXHAUST GASSTANDARDS

7. The need for effective exhaust gas regulations arose on both sides of the Atlantic inthe late 60's. This caused the European Community to adopt its Directive70/220/EEC in 1970. In content, it is equivalent to ECE Regulation 15. Like theAmerican approach the European provisions limited emissions only of carbonmonoxide (CO) and unburnt hydrocarbons (HC) for health-policy reasons, ormore precisely because of their effects on the quality of the air ingested in largeconurbations. Dynamic methods of measurements which were representative ofthe type of operation which vehicles were subjected to in inner city areas weredevised to back-up the relevant approval tests.

8. It is nowadays difficult to quantify the improvement of vehicle emission behaviourintended by this first generation of exhaust gas standards since there has been nostatistically reliable study of the preceding period i.e, there has been no basis forreliable comparison. However, it can be taken that these standards correspond tothe state of the art at the time and thus on average made a certain improvement tothe relevant emissions by the overall vehicle fleet.

9. Later on these standards were tightened up several times in both the United Statesand Europe. The fact that in the US also the driving cycle and in Europe thesampling and analysis methods were altered makes it difficult to carry out a directevaluation of the reduction in vehicle emissions thus achieved. It can be said overall

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that nowadays, following the three reduction stages set out in the EEC Directives inline with amendments 01, 03 and 04 to ECE Regulation 15, the European approvaltests require emissions that are 50% lower than the original limit values for COand He. The corresponding reductions in the United States are about 90%.

10.Initially nitrogen oxide (NO) emissions by vehicles were not covered by Europeanor American legislation. However, they were soon identified in the United States ascontributors to the well-known Los Angeles "smog" and were limited from 1973onwards. Europe reacted somewhat hesitantly, as public health hazards due toclimatic conditions of this type were still largely unfamiliar. Admittedly here too,a considerable increase in NOx emissions was recorded, not least as a result of themotor industry's efforts to meet the legal requirements concerning CO and HCemissions, but also to reduce fuel consumption - since the first energy crisis wasnow in full swing. The relevant state of the art was therefore enshrined in the limitvalues set out in Directive 77/102/EEC (ECE Regulation 15.02). After two furtherreductions the European standards lay roughly 30% below the original limitvalues. In this instance the United States have achieved a cut of 65%.

l1.Basically the European approach can be described as progress through continuousadaptation of its legislation to the state of the art achieved by the European motorindustry. Conversely American Legislation had from the outset been shaped bypolitical objectives such as Senator Musky's 1970 demand that vehicle emissions bereduced by 90% within ten years.

EEC REACTION TO THE GERMAN STEPS TOWARDS THEINTRODUCTION OF "LOW POLLUTION" VEHICLES - ORIGINS OFTHE "LUXEMBOURG COMPROMISE" OF 27 JUNE 1985

12.The steps in Germany towards the introduction of low-pollution vehicles madeexhaust gas regulations a political issue in Europe as well. Owing to the legalsituation at the start in the EEC this initiative had of necessity to result in adiscussion of an appropriate amendment to the exhaust gas directive.

The Commission was at pains to place the proposals required of it on an objectivebasis. It convened the Working Party known as ERGA (Evolution of Regulations,Global Approach) which, as part of its global approach, was first of all maderesponsible for identifying the relationships between vehicle emissions and airquality, for describing possible technical ways of reducing those emissions and forexamining their economic impact. Under a second mandate the Working Partythen mapped out strategies for the EEC-wide introduction of unleaded petrol. Theknow-how which the, in this form certainly unique, Working Party acquired wasused by the Commission to form the basis for its June 1984 proposals on the

introduction of unleaded gasoline and on the reduction of the permissible exhaustgas emissions from motor vehicles. The latter, moreover, was based on theprinciple of equivalence with the environmental effect of the current US exhaustgas standards while taking account of European conditions with regard to motorvehicle fleet and its operating characteristics.

These proposals received the approval of the majority of the Member States at themeetings of their environment ministers of 20 March and 27 June 1985. However,since Denmark and Greece entered fundamental reservations regarding the future"European exhaust emission standards" agreed by the majority, it has not beenpossible even now to adopt this directive formally.

UNDERLYING ASPECTS OF THE COMMUNITY'S FUTUREEXHAUST GAS STANDARDS

13.We will now go on to the underlying aspects of the exhaust gas standards agreed bythe majority. These will apply to passenger cars having a maximum permissiblemass of up to 2500 kg, while the limit values and dates of entry into force aredivided up into three engine capacity classes.

The pollutants covered are carbon monoxide (CO), unburnt hydrocarbons (HC)and nitrogen oxide (N0x)'

14.1t is assumed that vehicles with an engine capacity of more than 2 litresalready exist in a US version, or else that the appropriate catalytic convertertechnology can be applied to them without raising technical and economicproblems. The new European standards for this class are based on measurementstaken from vehicles for which a US exhaust gas certificate has been issued and aresuch that basically vehicles of this type can also be issued with EEC type approval.

For the approval of new vehicle types the following limit values have beenestablished: CO:25g!test, for the combined emissions of HC and NOx: 6.5g1test, forNOx: 3.5g/test.

For the control of production conformity a certain tolerance to these values isgranted which results in the following limit values: for CO: 30g/test, for thecombined HC and NOx emissions: 8.1g!test and for NOx: 4.4g!test. These limitvalues will apply to new vehicle types from October 1988 on and to all vehiclesregistered for the first time from October 1989 on.

15.Essentially the same technology can apply to vehicles having an enginecapacity between 1.4 and 2 litres, but it should be possible to provide cheaper

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alternatives and in particular lean-bum engines having a centralized fuel supply,plus oxidation type catalytic convertors or equivalent. Therefore, and becausestatistically speaking mid-range vehicles cover a shorter annual distance than top-of-the-range vehicles, slightly higher limit values apply to CO and the combinedemissions of HC and NOx' while additional flexibility is gained by dispensing witha separate NOx limit value.

For the approval of new vehicle types the following limit values apply: for CO:30g/test, for the combined emissions of HC and NOx' 8g/test. The correspondinglimit values for the control of production conformity are: for CO: 36g/test, for thecombined emissions ofHC and NOx' 109/test.

These limit values will be implemented as from October 1991 for new vehicletypes and as from October 1993 for all vehicles registered for the first time.

16.0wing to a lack of experience by the European motor industry in meeting the USstandards where vehicles have an engine capacity of less than 1.4 I, it is notconsidered possible to lay down equivalent European standards for the moment.For the time being limit values corresponding to the current state of the art will beapplied to vehicles in this category.

For the approval of new vehicle types these values are:For CO: 45g/test, for the combined emissions of HC and NOx: 15g/test and forNOx: 6g/test.For the control of production conformity the following limit values apply:For CO: 54g/test, for the combined emissions of HC and NOx: 199/test and forNOx: 7.5g/test.

These limit values will be implemented as from October 1990 for new vehicletypes and as from October 1991 for all vehicle registered for the first time. Beforethe end of 1987 a fmal European standard for this vehicle class will be laid downwhich then will apply, from October 1992, to new vehicle types and from October1993, to all vehicles placed in service for the first time.

17.The limit values referred to are based on the current European urban drivingcycle, which is considered by the majority of the Member States basically still to berepresentative of the traffic conditions in European conurbations. The expansionof the European test procedure by adding driving conditions representative of caroperation outside built up areas -considered by most of those involved to bedesirably in the longer term - is to be decided upon by the end of 1987.

I8.For private cars with an engine capacity of 1.4 litres and more the directiveprovides, as an alternative to the European test procedure limited in time, fortransposition of the relevant parts of the American certification process - basicallythe FfP-75 driving cycle. The underlying notion here is that manufacturers withvehicle types meeting the US specifications will first of all have to adapt these toEuropean test conditions before they may, on this basis, apply for type approval,while on the other hand it is desirable for environmental protection purposes tooffer such vehicles on the European market in the near future. The limit values arethose of the 1983 federal "'49 states") standards, i.e.: for CO: 2.llg/km, for HC:0.25g/km and for NOx: 0.62g/km. These limit values will apply both to type-approval and to control of production conformity according to the Americansampling procedure.

19.As it is aware of the specific problems affecting diesel engines and moreparticularly the meeting of stringent NOx limit values, the majority of the MemberStates agreed to the Commission's proposal to apply the limit values for vehicles ofbetween 1.4 and 2 litres engine capacity to all diesel cars larger than l.4litres.This is also intended to secure the necessary flexibility in the subsequentestablishment of limit values for particulate emissions from diesel engines forwhich in the meantime the Commission has presented a proposal. For vehiclesequipped with a direct injection diesel-engine of a capacity between 1.4 and 2litresthe new European emission standards will only apply from October 1993 (newtypes) and October 1996 (first registrations) respectively. Thereby the MemberStates intend to grant an additional lead-time for those manufacturers who aredeveloping such engine concepts in view of a further improvement of fueleconomy as well as exhaust emissions of future diesel car generations.

20.Equally, for private cars equipped with automatic transmissions exceptionalprovisions have been agreed, provided that such cars are derived from models withmanual transmissions for which an EEC approval has already been granted. In thiscase, the automatic transmission version will be approved against limit valueswhich result from the multiplication of the above-mentioned limit valuesmultiplied with a factor of 1.2 for the combined HC and NOx emissions and I.3 forthe NOx emissions.

21.With a view to the rapid introduction of unleaded petrol it has also been agreed thatall new car types subject to approval which have engines larger than 2 litres mustbe designed for the exclusive use of such fuels from 1 October 1988 and those withengines of less than 2 litres from 1 October 1989. Moreover, from 1 October1990, Member States of the Community may in general terms prohibit theapproval of new vehicles which are unable to run on unleaded fuel. Where amanufacturer can demonstrate considerable technical difficulty in converting

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vehicles from leaded to unleaded furl those vehicles will be exempted and the datesdecided for the implementation of the new emission standards will apply.

22.As mentioned before, the new European emission standards apply to private carsof not more than 6 seats and a total mass of not more than 2,500kg. It is thiscategory of vehicles - except those cars having an engine capacity of less than 1.4litres - with which European manufacturers have been able to gain experience ofthe technology required to meet the US standards.

For all other vehicles covered by the scope of the present directive, notably thoseof category N1 (Tight duty trucks") it has been agreed to follow an approachanalogous to that chosen for the private cars below 1.4 litres i.e. to introduceinterim emission standards. These consist of the limit values of Directive83/351/EEC, the application of which to the concerned vehicles will imply areduction of their HC and NOx emissions in the order of 25% compared to thepresent situation. These interim standards will apply from 1 October 1989 to newvehicle types and from 1 October 1990 to all new vehicles put into service.

In 1987 the Council will decide, on proposal of the Commission, the definitiveEuropean standards which should apply to these vehicles in 1993/1994.

ASSESSMENT OF THE "LUXEMBOURG AGREEMENT"

23.The fact that the majority decisions of March and June 1985 aim at an intrinsicEuropean solution to the environment problems caused by road traffic must bewelcomed without reservation in Brussels.

24.For several reasons the - apparently so obvious - direct transposition of the USexhaust gas regulation would quite definitely not have been a feasible solution forthe Community as a whole.

One thing we can mention here is the completely different legal frameworks of theAmerican and European regulatory systems. For example, options, such as thatopen to the US approval authority, of suspending the application of stringentexhaust gas standards either in general terms of in respect of individualmanufacturers, or, as in the case of the commercial-vehicle exhaust gas standardsnow proposed of permitting manufacturers to "offset" these is not provided foreither in the EEC's type approval procedure or in those of the individual MemberStates.

Secondly, the passenger car market in the USA cannot be compared with that in theCommunity. For example, vehicles with engines smaller than l.4litres capture aninsignificant part of the US market, whereas in Europe their market share is

somewhat over 50%. Similarly, diesel cars have virtually disappeared from the USmarket but currently account for 14% of new registrations in the Community, andthe figure is rising.

25.The compromise effected via the "Luxembourg Agreement" takes overall accountof European conditions. On one hand it enables motor manufacturers within theCommunity to back up existing know-how in Europe with proven US technology.On the other hand, it leaves them the option where the greatest turnover is, i.e. inmid-range and small cars, of developing cheaper alternative solutions which offera considerable reduction in fuel consumption alongside lower emission values,thereby making them particularly attractive to a wide range of customers. Themarketing of low-pollution vehicles on a voluntary basis on the German marketcan be taken a') confirmation that the industry has grasped this opportunity.

26.As a precautionary measure the "Luxembourg Agreement" seems to guaranteeachievement of the environmental-protection target. The European CO and HCemission standards for top-of-the-range vehicles are 87% lower, and in the case ofNOx emissions 70% lower than the original 1970 limit values. These values areroughly 80%n3% below the original values in the case of mid-range vehicles.

In order of magnitude these figures are fully in line with the reduction aimed at bythe standards laid down in the United States. In addition, calculations based on theavailable documents on the vehicle fleet, the mode of operation of the individualvehicle classes and the specific emissions from these show that the overall NOxemissions from the EEC fleet will reach the American level of roughly 1.5 milliontonnes per year, when predominantly made up of vehicles meeting the new EECstandards i.e. roughly around the year 2000.

EXTENSION OF THE EUROPEAN EMISSIONS REGULATIONS ONOTHER POLLUTANTS AND EACH VEHICLE CATEGORY

27.Pursuant its undertakings at the Environment Councils of 27 June an28 November 1985 relating to the gaseous emissions of passenger cars and lightcommercial vehicles covered by Directive 70/220/EEC, the Commission presentedon 23 June 1986 two further proposals on motor vehicle emissions to the Council.

28.The first proposal concerns the particulate emissions of diesel enginesequipping the motor vehicles covered by the above-mentioned Directive.Particulate emissions are besides smoke - which, since 1972 has been controlled bya Community Directive - a specific problem of diesel engines. The limitation ofthese emissions appears today the more necessary as the diesel car pare isincreasing rapidly and is forecasted to reach about 15 million units in the mid-90'swhich shall becompared with the present 6 million units.

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The proposal is based on the presently available though limited knowledge of theperformances of the best available diesel technology in the European motorindustry as a whole. Furthermore, it takes into account the lack of accuracy andreproductibility which result from the simple transposal, into the European testprocedure, of the sampling and analysis method of the current US legislation whichis at present the only codified method available.

The limit value proposed for the type-approval of new car models, independent ofthe weight and engine size, is 1.3g per European test as defined in Directive70/220jEEC. The limit value proposed for conformity of production, orotherwise, that which every new car must meet on its first registration, is 1.7g/test.These limit values are proposed to be implemented at the dates agreed at theLuxembourg Council for the implementation of the new European standards forgaseous emissions. By that means the Commission intends to assure that the motorindustry can concentrate its resources on adapting its production to the newCommunity requrements as a whole and that the administrative procedures relatedto the type-approval of modified car models will be limited to what is strictlynecessary.

29.The second proposal concerns the emissions of gaseous pollutants (CO, HC,NOx) from the diesel engines of commercial vehicles and buses of allweight classes. The contribution of the emission of these vehicles - which untilnow in Europe are not controlled except for smoke - to the general air pollutionbecomes more important since the continuing regulatory efforts of the Communityto reduce the emissions of light motor vehicles take effect. Here again, the absenceof any control of the concerned emissions of heavy vehicles in the Member Statesresults in very little data about the emission performance of big diesel engines andthe possibilities of their improvement in this respect.

The present proposal is based on a regulation ("R49") of the EconomicCommission for Europe of the United Nations, from which it takes over the testprocedure. The limit values of this regulation, however, no longer correspond tothe state of the art in diesel technology and an across the board reduction for allthree pollutants concerned appeared possible. Hence, the proposal contains limitvalues which for CO and NOx are 20% below those of R49 and for HC 30% belowthe R49 limit. In absolute figures, the limit values proposed are

11.2 g/kWh for CO2.4 g/kWh for HC and

14.4 g/kWh for NOx

It is proposed that these limit values will have to be complied with both by newvehicle types, at the latest on 1.4.1988, and by all new vehicles from 1.10.1990onwards.

30.FURTHER DEVELOPMENT

This presentation aimed at the explanation of the past developments and the presentsituation of the Communities' exhaust emission regulations. It needs to beunderstood that these regulations are in a permanent process ofevolution.

On the political level, initiatives are being taken to allow the "Luxembourgagreements" to become operational, to adopt the proposed particulate standardsand the standards for gaseous emissions of commercial vehicles.

On the technical level expert talks are going on about the future test cyclerepresenting extra-urban driving conditions, about the definitive emissionstandards for passenger cars below 1.4 litres engine capacity and for light dutyvehicles as well as about possible European requirements relating to the durabilityof anti-pollution devices and evaporation losses. These latter discussions shouldallow the Commission to present, by the end of 1987, appropriate regulatoryproposals for the concerned matters to the Member States.

79

"\. Crucq and A. Frennet (Editors), Catalysis and A utomot ioe Pollution Control1987 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

THE MARKET FOR CAR EXHAUST CATALYSTS IN WESTERNEUROPEA Review of Trends and Developments

by Willem GROENENDAAL

STRATEGIC ANALYSIS-EUROPE, Brussels, Belgium

ABSTRACT

The application of catalysts to control the emission of harmful componentsin the exhaust gases of cars equipped with gasoline engines is an establishedtechnology in North America and Japan since the early seventies. In the last yearalone, some 14 million catalyst equipped cars were sold.

In Western Europe the development started much later and last year majordecisions were taken which will determine the shape of the future European market.

The paper will discuss the options in pollution abatement technologyavailable under the EC regulations and the major role catalysts are expected to play.Estimates will be presented on the future catalyst demand both in the EC and otherWest European countries. Cost breakdowns for threeway catalyst manufacture will beincluded.

Apart from catalysts manufacturers with an established position in NorthAmerica a number of new suppliers may well emerge in Western Europe.

Announced catalyst and carrier manufacturing capacities will be comparedwith future market requirements.

The demand for lead free gasoline in Western Europe is determined by thesize of the catalysts equipped car population and government measures to stimulate theuse. The expected demand developments will be discussed and the technology toproduce lead free gasoline will be highlighted.

INTRODUCTION

Car exhaust catalyst systems have been installed in the majority of the newcars in the USA since model year 1975 and on all new gasoline fueld cars since 1981.The use of catalysts in the exhaust system of passenger cars and light duty commercialvehicles is considered the most practical way to comply with emission controlstandards in the USA and Japan. The technology is considered mature andproven (Ref. 1).

In the last year alone some 14 million new catalyst equipped cars were soldand in total some 130 million catalyst cars are now in use in the world.

Until 1985 catalyst equipped cars manufactured in Western Europe wereexported to the USA and Japan. In 1985 legislation came in force in West Germanyand Switzerland which encouraged the sales of cars with low levels of emissions of

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pollutants. Also in that year lead free gasoline started to become available in mostWestern European countries. In 1985 fifty thousand new catalyst cars were sold inWestern Europe.

This paper will discuss the options in pollution abatement technologyavailable to meet the EC regulations agreed upon in 1985 and the major role catalystsare expected to play. Estimates will be presented on the future catalyst demandboth in the EC and other Western European countries. Cost breakdowns forcatalysts will be included, which will show the major impact of the preciousmetal price and the metal content of the catalyst.

Apart from catalyst manufacturers with an established position in NorthAmerica new suppliers may well emerge in Western Europe. Announced catalyst andcarrier manufacturing capacities will be compared with future market re-quirements.

The demand for lead free gasoline in Western Europe is determined bygovernment measures to stimulate its use and the size of the catalyst equipped carpopulation. The expected development of demand will be discussed and thetechnology to produce lead free gasoline will be highlighted.

POLLUTANT EMISSION REGULATIONS AND LEGISLATION

The reduction of pollutants in automobile exhaust and in industrial wasteand stack gases is of major concern in Western Europe. This was fueled in the lastyears by the extensive forest die-back caused by acid rain. Government policiesare to achieve substantial reductions in the emissions of hydrocarbons (HC's), CO,S02, NOx and particulate matter. Road traffic is one of the major contributors to theman made emissions of HC's, CO and NOx' International and national regulations,legislation and incentives coming into force will substantially reduce these emissions.

Since 1970substantial reductions in emissions of pollutants from passengercars already were achieved in the countries of the European Community (EC): HCemissions were reduced by 60%, CO by 70% and NO by 30% (2). These reductionswere obtained by engine modifications.

In the USA and Japan the reductions in emissions have been much higher.This was caused by the stringent legislation which made the use of exhaust catalystsunavoidable.

In June 1985 EC regulations were announced which set a timetable for adrastic reduction of pollutants from automobile exhausts in the next ten years. Thesenew standards differ from the US an Japanese standards.

In the EC the passenger car fleet has been divided into three enginecylinder volume classes: below 1.4 liter (1), between 1.4 and 21 and above 21.The year in which the new emissions standard has to be met and the level of theemission differ for each class.

The so-called Stockholm Group are the other countries in Western Europe,

that is the Nordic countries, Austria and Switzerland. Within the next years thesecountries will adopt the US '83 emission standards. However, the timetables aredifferent. Denmark is a member of both groups of countries.

The testing procedures used for measuring pollutant levels and the methodschecking whether emission standards are achieved differ in the USA, EC and Japan.The EC test, R 15n5, lasts 13 minutes from cold start and the maximum speed is50 kilometers (km) per hour. The US test, FrP 75, lasts 41 minutes and themaximum speed is 90 km per hour. In the US regulations pollutant levels should bebelow the legal limit during a period of 50,000 miles with the same catalyst installed.

The forthcoming EC directive is summarised in Table 1.

Table 1EC Car Exhaust Gas Emission Directive

Decided on June 28, 1985Passenger cars for less than 6 persons and a weight below 2,500 kg

8:J

Engine size

> 2 liter

(except Diesel)

1.4 - 21iter

< 1.4 liter

> 2 liter

Proposed datefor implementation

1.10.88 new models1.10.89 all new cars

1.10.91 new models1.10.93 all new cars

Stage 1:1.10.90 new models1.10.91 all new cars

Stage 2 :1.10.92 new models1.10.93 all new cars

Proposedemission limitsgr/test maximum*

CO :25.0NOx 3.5HC+NOx : 6.5

CO :30.0NOxHC +NOx : 8.0

CO :45.0NOx 6.0HC +NOx : 15

Decision in 1987on limits

30.04.48.1

36.0

10.0

54.07.519

* EC test R ISn3 for respectively new modelapprovaland productionchecks.

Each member state is free to decide whether and when it will adoptthese EC regulations and delays in implementation could occur in some countries.

Standards for maximum levels of particulate emissions are being developedand will be decided upon in 1987. Also an EC test cycle with a high speed section isunder development.

84

Tax incentives have been created in West Germany and the Netherlandsto stimulate the early introduction of low emission cars before the EC regulationsbecome effective.

The announced legislation for the Stockholm Group of countries issummarized in Table 2.

Table 2STOCKHOLM GROUP EMISSION LEGISLATION

Country Date of implementation

Austria 25.05.86 voluntary + new diesel01.01.87 all new cars> 1.5101.01.88 all new cars < 1.5 I

Switzerland 01.10.86 all new vehiclesbelow 2500 kg

01.10.87 all new vehiclesbelow 3500 kg

Sweden 01.10.86 voluntary01.10.88 all new cars

Finland, Norway similar to Sweden

Standard

USA'83USA'83USA'83

USA'77

USA'83USA'83USA'83

USA '83 regulations, valid in 49 States and from 1.09.87 in Canada, are asfollows: Emission limits light duty vehicles; HC: 0.41, CO: 3.4, NOx: 1.0 andparticulate 0.20 (in gr/mile).

Comparisons between the US limits and the existing and forth-coming EC limits have been made (Ref. 2). The relation between the two isstrongly influenced by the type of exhaust clean up system used. In Table 3 thecomparison between the US and EC limits has been expressed according to the EC testmethod, which results in a wide spread of the US emission limits.

Engine size

Table 3Comparison between US and EC Standards

US limits are expressed in EC terms as gr/testEC

Pollutant US '83 Current Forthcoming

85

> 2 liter

104 - 2liter

< 104 liter

* Stage 1

CO 15 - 30 80 - 100 30HC+NOx 4- 9 26 - 29 8

CO 15 - 30 70 - 80 36HC+NOx 4- 9 24 - 26 10

CO 15 - 30 70 54 *HC+NOx 4- 9 24 19 * I

II

-_._-_._-,._---------~------

For the engine class above 21iter the forthcoming EC limits are close to theUS standards. For the smaller two classes the EC standards are less stringent.

EMISSION REDUCTION CONCEPTS

The engine exhaust gas contains the products of the incomplete combustionof LPG, gasoline or diesel fuel. The typical composition of the exhaust gas of agasoline engine is given in Figure 1.

Figure 1Typical exhaust gas composition from a gasoline engine

NO. HC THREEWAY LEAN BURNCATALYSIS

I'l'l4000 2DOO

3000 t5CO

ZOOO 1000

1I I I

U 1•• 7 IS

1,2III

I,.I20

I22

86

The current US emission limits for light duty vehicles are achieved forgasoline fueled cars by engines equipped with a controlled threeway catalystsystem including an oxygen sensor and fuel injection. For the older enginetypes dual bed reduction/oxidation catalyst systems are often applied (Ref. 3).

Cars with diesel engines meet the standards without exhaust catalysts.Except in California diesel cars meet the particulte emission standards. Soot traps orfilters are generally installed in new diesel cars sold in California.

In general the less stringent EC emission limits offer the possibility to applya wider range of emission reduction techniques (Ref. 4).

Threeway catalysis controlled and uncontrolledDual bed reduction/oxidation catalysisLean burn engine with oxidation catalysisExhaust gas recycle and oxidation catalysis or thermal oxidation

The effectiveness and the costs of the various methods vary greatly.Apart from the reduction in emissions, another advantage of the new lean

burn engine technology is a reduction in fuel consumption. However only a smallnumber of car manufacturers have opted for this route.

For the three EC engine classes, the following techniques are consideredlikely to meet the forthcoming emission limits.

Cars with above 2 liter gasoline enginesThe emission limits by model year 1989/1990 are close to the US standards

and therefore the same systems can be applied. Controlled threeway monolith typecatalysts, predominantly single bed, are the preferred choice (Ref. 5, 6, 7). Thecatalyst formulations are modified to cope with European driving conditions(Ref.8,9).

Cars in the 1.4 - 2 liter engine classFor this class of cars the forthcoming emission limits must be met by model

year 1992/1994. This class can be divided in two groups:

- Heavy and high performance vehicles. Single bed controlled threewaymonolity catalysts with fuel injection or a high performance carburator are themost likely choice.

- Light vehicles.Uncontrolled single bed threeway monolith catalysts, internal orexternal exhaust gas recycle followed by an oxidation catalyst or even thermaloxidation, or a lean burn engine followed by an oxidation catalyst are beingconsidered and/or applied.

Cars with engines below 1.4 literA large number of car types already meet the Stage 1 requirements of the

forthcoming EC standards for model year 1990/1991. The current types which do notmeet Stage 1 standards are being adapted. Catalysts are expected to play aninsignificant role.

Stage 2 standards for model year 1993/1994, to be decided in 1987, areanticipated to be comparable or less stringent than those for the 1.4 - 2 liter engineclass. Catalysts are expected to play a significant role. However, in view ofthe increasing costs of Platinum (pt) and particularly Rhodium (Rh), emissionreduction systems are likely to be chosen which minimize the use of these preciousmetals.

There are many car manufacturers in Europe and also manyimporters and each produces a number of engine families. In model year 1984,some 373 different engine families were sold in West Germany (Ref. 10),thus there is a a very wide variety of conditions with which to cope. The solutionsselected will be engine specific and chosen from the range of options discussed above.

WEST EUROPEAN CAR MARKETThe total number of passenger cars manufactured in Western Europe

in 1985 was 11.5 million, an increase of 4% compared with 1984; about 0.6 millionwere exported to the USA. Total worldwide production was 32.7 million cars.

Total sales of new cars in the EC in 1985 was 9.5 million, an increase of3% over the previous year, of which 0.9 million cars were imported from Japan.West Europe in total recorded new car sales of 10.5 million in 1985 of which 10.7%were Japanese imports. Diesel cars represented 17% of sales in the EC and 15.9% ofthe total cars sales in Western Europe.

The distribution of the 1985 and 1990 car sales over the emission classesin the EC is estimated (forecasted) as follows:

I=n'ty~ Engine size, liter % of Sales I1985 1990

Gasoline below 1.4 50% 48% I1.4 - 2 29 28

IlDi,,,,l

above 2 4 4

17 20 jThe distribution varies from country to country with a very high

percentage of small cars in Italy and France and relatively high diesel car populationsin West Germany and Italy.

Total car sales by 1990 in the EC are expected to exceed 10 million units.

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CATALYST TECHNOLOGY AND COSTS

Catalyst technologySince the introduction of the exhaust emission controls in the US in the mid-

seventies, catalyst technology has developed steadily and the European practicereflects the state of the art. The composition of a typical European catalyst, theoperating conditions and performance are given in the following Table 4.

---_._. _....._-------_._------

Table 4Typical European Threeway Catalyst

Composition

Carrier

Washcoat

Metals

Bulk density

OPERATING CONDITIONS

Temperature

Space velocity

Catalyst to enginecylinder volume ratio:

PERFORMANCE

Cordierite monolith with 400 passagesper square inch and a waIl thicknessof 0.15 mm.

20% wt. pseudo-boehmite promoted witha.o. lanthanides, to improve the hightemperature stability and the adhesionto the carrier.

Pt + Rh: 35-40 gr/cu ft (1.24-1.41 gr/l),Pt/Rh wt. ration: 5.

0.45 gr/l,

300-900C

100,000 - 200,000 Ill. h

0.8 - 1.5

Controlled within; A= 0.99 +/-0.06Conversion in %:Fresh; HC: above 80%, CO and NOx: above 70%

Uncontrolled within: A= 1.05 +/-0.2Conversion in %:Fresh; HC: min. 50%, avo 70%; CO min. 20%, avo 55%; NOx min.lO%, avo 50%

The precious metal content of a typical threeway catalyst in the USAcurrently is 20gr/cu ft (Ref. 6). To maintain a catalyst life similar to the US standardof a minimum 50,000 miles at the anticipated much higher remaining lead content(max. 13 mg/l) of European lead free gasolines, the precious metal content willhave to be higher.

Instead of ceramic monoliths, high temperature Ni metal-basedhoneycomb carriers are available (Ref. 11, 12). In view of their overall weightand size advantages they are used as pre-catalysts and as retrofits. However on a priceper volume basis, including canning, ceramic material currently is 2-3 times lessexpensive.

The continuing price increases (see below) of Pt and particularly Rh, bothindispensable components in a threeway catalyst, stimulated renewed R&D efforts tosubstitute at least in part, both products by less costly metals. To date these effortshave not resulted in a catalyst with equal peformance. Catalyst life in particular wasimpaired.

Catalyst costsThe sales price of threeway catalysts is for a major part determined by the

precious metal costs, while for the less expensive oxidation catalyst, the impact is farless pronounced. This is illustrated in the following Figures 2 and 3.

Figure 2Cost build-up for a Typical European Threeway Catalyst

1.3 liter ceramic carrier with 1.24 grlI Pt + Rh (ratio 5)Price: $ 47 per unit (large quantities)

89

OTHER 12,5 %

STOCKS+LOSSES .,1 %

~"""S ~3, & \

90

Figure 3Cost build-up for a Typical European Oxidation Catalyst

1 liter ceramic carrier with 1.24 gr/l Pt + Pd (ratio 8)Price: $ 30 per unit (large quantities)

STDCKS+LDSSES 3,9%

PLATINUM 49,5%

PALLADIUM 0,7%

18, 2 %

Because of the smaller size and the much lower price of Pd anoxidation catalyst is about 64% of the cost of a threeway catalyst.

The prices for the relevant platinum group metals during the last threeyears are given in Figure 4,

Figure 4Monthly Average Prices of Platinum, Palladium

and Rhodium 1985-1986.(source : Metals Bulletin)

10 128

oz

2 41985

$ per1200....--------------:;-------,11001000

900 m800 !~700 ~600 1II500 II

400 I

300200100

o

• p l at inum • palladium

The increased prices of Pt and Rh increased the cost for a typical Europeanthreeway catalyst during the last half year by about 40% in US dollars.

Worldwide consumption of precious metals in car exhaust catalyst ((Ref.13) in 1985 is given in Table 6.

Table 6Precious Metals Consumption in Auto Catalysts· 1985

Thousand Troy Ounces

Region Pt Pd Rh

North America 650 190 96Japan 175 100 30Rest of Western World (incl.Europe) 50 9

Total 875 290 135

Percentage of total demand 31% 11% 54%

In 1985 close to 85% of the Pt supply of the Western World originated fromSouth Africa.

Based on the catalyst requirements for export models and the expectedpenetration of catalyst cars in Western Europe the precious metal requirementsfor auto catalysts is forecast to be 375,000 oz in Europe by 1994 (Ref. 13).

CATALYST AND CARRIER MANUFACTURING CAPACITY ANDDEMAND

The US car industry mainly applies autocatalysts on ceramic monoliths;only OM uses pellets for 30% of its production. The two main manufacturers ofceramic monoliths are Coming, USA and NOK, Japan. The technology was developedby Coming in the early seventies.

Four companies manufacture and supply monolith-based catalysts in theUSA: Degussa, Engelhard, Johnson Matthey and Allied-Signal (UOP). In Japan andEurope the car industry applies only monolithic type catalysts.

The total number of local catalysts manufacturers in Japan is seven; threeare subsidiaries of car companies.

The total local manufacturing capacity appears more than adequate tosupply both existing and future requirements.

In Western Europe the industry has been tooling up to meet the risingdemand. Total planned catalyst manufacturing capacity is ten million unitsof which about seven million either exists or is under construction. Degussa and JMC

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92

have existing facilities, Engelhard Kali-Chemie has a plant under construction andAllied-Signal has announced the construction of a plant in France. In West GermanyHeraeus and Doduco supply catalysts for retrofitting existing cars. Three other firmshave indicated their intention to build manufacturing facilities.

Both Corning and NGK have cordierite monolith manufacturing facilitiesunder construction in Europe with an initial total capacity of 6.5 million units. Thetotal investment costs for these two plants are DM 140 million

The current manufacturing capacity in Western Europe for metalmonoliths is around 0.5 million.

Future requirements for car exhaust catalysts in Western Europe for thenext 5 to 10 years are extremely difficult to forecast as:

- Emission limits for cars below 1.4 I stage 2 have not been decided- EC regulations are mandatory and the degree of compliance is not

known.

Therefore only ranges can be given for the expected future requirements.In Table 7 our forecasts are presented until 1994, the year the EC regulations areexpected to become fully effective.

Table 7Demand Forecast for Car Exhaust Catalysts

Western Europe: 1986-1994

Year

198619901994

Minion units

1.5 - 22.5 - 35.0 - 9

On the basis of this forecast the announced manufacturing capacity inWestern Europe is adequate for the anticipated demand, including exports. With anaverage size of 1.3 liter and a precious metal load of 1.3g/l per unit, 380,000 oz of Pt,Rh and Pd are required for 7 million catalyst units.

MOTOR GASOLINE QUALITY REQUIREMENTS

The quality of the transport fuels, motor gasoline and diesel fuel in the next10-15 years will be strongly influenced, apart from market demands, by:

- Environmental legislation, directly and indirectly- Whitening of the barrel

Critical aspects of gasoline quality for the performance of the engine are

octane and volatility. High octane is required for high compression engines withhigh performance and low fuel consumption.

The current premium grade in Western Europe has an average ResearchOctane Number (RON) of about 97.5, regular is above 91 RON. From the 100 milliontonnes of gasoline consumed annually, 25% is regular, but there are large differencesin consumption patterns, ranging from 50% regular in West Germany to 95%premium in Italy.

The estimated average Western European pool RON without lead additive isabout 92.5. The average lead based octane increase is 3 to 3.5.

The mounting concern in the last years on the negative health effects of leadin the environment has resulted in the reduction of the gasoline lead contentand the promotion of the use of lead free gasoline.

The lead content of motor gasolines by January 1, 1987, will be reduced tomax. 0.15gr/1 in most European countries, the remaining countries are expected tofollow in the early nineties.

The EC has agreed on a timetable for the introduction of new cars fueledwith lead free gasoline only:

- by 1.10.89 all new models- by 1.10.91 all new cars

Lead free gasoline has been available on a limited basis in all countries since1985.

The government policies in Western Europe are far from uniform: insome countries tax incentives have resulted in the replacement of regular by lead freeregular; other countries promote by tax incentives the use of lead free super and stillothers only make lead free super and regular available. In the latter case marketforces result in virtually no sales of lead free gasoline (the price is higher) which intum reduces availability.

There are doubts in the minds of consumers whether it is wise to buy acatalyst equipped car or even to use lead free gasoline. Given time a more uniformsupply pattern will emerge.

Two grades of lead free gasoline have been introduced; Euro Super withRON 95 and Regular with RON 90-91. Specifications for both are given below:

93

Property

Research Octane Number (RON)Motor Octane Number (MON)Density at 15C, g/mlSulfur content, % wtBenzene content, % volLead content, gil

Regular

min.900r91min. 80 or 82.50.70- 0.79Below 0.1Below 5.0Below 0.013

Super

min. 95min. 850.70 - 0.79Below 0.1Below 5.0Below 0.013

94

Within the next 10 to 15 years lead compounds will disappear as a gasolineoctane improver and some 250 million octane tons per year (6 million octane barrelsper day) are required to fill this gap. The current estimated average gasolinecomposition in Western Europe (Ref. 14) is illustrated in Figure 5 below. The twomain blending components are reformate and cat cracked gasoline.

By reforming paraffins and naphtenes present in naphtha into isoparaffinsand aromatics the octane number is greatly improved. The main product of theconversion of heavy crude oil fractions in a catalytic cracker is gasoline.

The light naphtha part in the average gasoline includes raffinate fromaromatics extraction. Under the heading high octane light components: alkylate,polygasoline, isomerisate and oxygenates (alcohols and ethers) have been takencombined.

Figure 5Estimated Average Gasoline Composition

Western Europe 1986

BUTANE 3.0 %

CAT.CR.GASOLINE 33.0 %

HIGH OCTANELIGHT COMP.

UGHT NAPHTA 8.0 %

REFORMATE 49.0 %

The whitening of the barrel, we anticipate, will continue and will result in alower percentage of straight run naphtha (derived) components and a higher per-centage of cat cracker and hydrocracker derived components in the gasoline (Ref.15).

The increase in octane number required to replace lead can be achieved in anumber of ways (Ref. 16), all of which increase the manufacturing costs; somerequire substantial additional investment. At the current state of the art and prices theaverage cost of an octane-ton is about $ 1.8. The most attractive but limited octaneenhancement is obtained by the use of octane boosting catalysts in the cat cracker.

The technology to meet the octane level for total lead free gasoline is available andthe time schedule leaves sufficient room for refiners to make the required adaptationsand investments.

REFERENCES

1. M.P. Walsh, Experience in the United States with Automobile Emission Control,Platinum Metals Review, July 1986, vol. 30, n03, 106-115.

2. K.H. Neumann, Emission reduction by modern engine design, VDI-Bericht 531,Duesseldorf 1984. (In German).

3. P. Oeser and W. Brandstetter, Fundamentals of Catalyst Systems for S.l.Engines, MTZ Motortechnische Zeitschrift, vol 45, 5/84, 1-6 (In German).

4. Catalysts - Meeting new challenges in a $ 2.5 billion global business, ChemicalWeek, vol. 138, n026, June 251986,20-71.

5. H.D. Schuster, J. Abthoff and C. Noller, Concepts of Catalyst Exhaust EmissionControl for Europe, SAE paper 852095.

6. W.B. Williamson et al, Durability of Automotive Catalysts for EuropeanApplications, SAE paper 852097.

7. W.DJ. Evans and AJJ. Wilkins, Single Bed, Three Way Catalysts in theEuropean Environment, SAE paper 852096.

8. E. Koberstein, B.H. Engler and H. Voelker, Catalytic Automotive ExhaustPurification - The European Situation 1985, SAE paper 852094.

9. BJ. Cooper and TJ. Truex, Operational criteria affecting the design ofthermally stable single bed catalysts, SAE paper 850128.

10. Eurosystem Vehicle Registration Report (Germany). November 29,1984.11. P. Oeser et al, Catalytic control of exhaust emissions by metal supported precious

metal catalysts.12. M. Nonnemann,Metal Supports for Exhaust Gas Catalysts, SAE paper 850131.13. G.G. Robsom, Platinum 1986, Johnson Matthey pIc.14. W. Groenendaal, What is new for Fluid Cat Cracking - Outlook in Western

Europe - Katalistiks 7th annual symposium, 1986.15. S. Bernstein, European Automotive Fuels for the 80's and 90's, SAE paper

845047.16. J.A. Weiszmann et al, Pick your option for higher octane, Hydrocarbon

Processing, June 1986,41-45.

95

,\, Crucq and A, Frennet (Editors), Catalysis and Automotive Pollution Control1987 Elsevier Science Publishers B,V" Amsterdam -- Printed in The Netherlands

AUTOMOBILE CATALYTIC CONVERTERS

K. C. TAYLORPhysical Chemistry Dept., General Motors Research Labs, Warrer" MI (USA)

INTRODUCTIONThe automobile is identified as one source of emissions of hydrocarbons,

carbon monoxide, and nitrogen oxides to the atmosphere. Stationary sourcessuch as conventional power plants are another. Concern for the danger of

these substances to public hea lth in urban areas has led to the development of

motor vehicle emission regulations by industrialized nations, in general.

Current and proposed regulations have been designed to improve air quality by

reducing the impact of automobile exhaust on smog formation and on carbon

monoxide levels, and more recently to reduce acid deposition.The relationship between automobile exhaust emission levels and stationary

pollutant sources and air quality is not a direct one. Complex mathematicalmodels have been developed for predicting trends in air quality. These modelsinclude as input information on vehicle populations, atmospheric chemistry,

meteorological variables, and other variables which can impact on the air

quality of an urban area. Predicting the level of control needed to meet airquality goals is complicated by the multiple inputs to the atmosphere in urban

areas.

EMISSION CONTROL REQUIREMENTS

The emission control needs of countries differ and different emissionlimits for passenger cars and for trucks have been established throughout theworld. Exhaust emission standards for vehicles for countries where regula-

tions have been set cannot be compared directly because the tests on which the

emissions are measured differ; however, the range of control for passengercars regulated can be viewed by expressing the limits as the intended percent

reduction from the uncontrolled level as shown in Table 1. For example, the

U.S. Federal standards represent a reduction form uncontrolled levels of 96%

for hydrocarbons and carbon monoxide and 76% for nitrogen oxides. Standardshave also been established for gasoline fueled commercial vehicles and for

diesel fueled vehicles.

97

98

TABLEExhaust emission standardsPassenger cars [1 J

--~~"---"-------

Country

CO

Percent Reduction 1

HC NO x

U.S. 2 96 96 76Canada 2,3 70 80 24Australia2 82 86 24Japan (10-mode and 11-mode) 95 93 92Europe (ECE R15.04)4 70 50

c;Sweden, Switzerland~ 67 72 24SWitzerland5,6 87 88 51

1Percent reductions for U.S., Canada, Australia, Sweden and Switzerland

based on 1960 U.S. uncontrolled models.2Measured on 1975 U.S. FTP.3Enforced in 1986.

4Standards vary with vehicle weight.

5Measured on 1972 U.S. FTP.6Standards effective October 1, 1986.

The exhaust emission standards (limits) and the emission test proceduresfor passenger cars are listed in Tables 2 and 3, respectively. The current

Federal U.S. standards for passenger cars are 0.41 g/mile hydrocarbons, 3.4

g/mile carbon monoxide, and 1.0 g/mile nitrogen oxides. Different exhaustemission control standards have been set for the State of California. The

standards for nitrogen oxides for California are stricter than the Federalstandards, optional 0.7 g/mile and a primary standard of 0.4 g/mile. Start-

ing in 1989 passenger cars must be certified at the primary standard in

California. The current Federal U.S. HC/CO/NOx requirements for light dutytrucks are 0.80/10/2.3 g/mile, respectively. The light duty truck dieseleXhaust particulate standard is 0.6 g/mile, dropping to 0.26 g/mile effec-

tive in 1987. Additional requirements apply to heavy duty vehicles.

',ABLE 2

Exhaust emissio~ sta~dards

99

Passe~ger cars

COU', try

[ 1 ]

coStandard (g/km)

HC NOx

4.4 g/test 7.0 g/testHC+NOx: 19-28 g/test

U.S. 1

Canada 1,2

Australia 1

Japan (10-mode) (imported)

Japan (10-mode) (domestic)

Japa~ (ll-mode) (imported)Japan (11-mode) (domestic)Europe (ECE R15.04)3

. 4Sweden, SWitzerlandSwitzerland 1 ,5

canada 7, SWitzerland8,

Sweden 9

Saudia Arabia, Israel,Singapore (ECE R15.03)3

Korea 6

2.11

15.5

9.32.72.1

85.0 g/test

60.0 g/test58-110 g/test

24.2

9.3

2.1

65-143 g/test

2.11

0.62

1. 930.93

0.390.259.50 g/test

2.1

0.9

0.62

6-9.6 g/test

0.25

0.251.2

1.90.48

0.256.00 g/test

1.91.2

0.25

8.5-13.6 g/test

0.62

lMeasured on 1975 U.S. FIP.2Enforced in 1986.3Standards vary with vehicle weight.

4Measured on 1972 U.S. FIP.

5Standards effective October 1, 1986.6Light duty vehicles, effective July, 1987.7Standards effective September, 1987.

8Standards effective October, 19879Standards effective 1989 MY.

100

TABLE 3Emissio~ test proceduresPassenger cars [1J

Exhaust Emiss l or.s Evap. Emi ss ions Test 1Cou'1try Dri v ir.g Cycle Samplir;g Methods Mettl0ds Fuels

U.S. 1975 U.S. FTP CVS, FlO3 SHED~ UL,D

Ca'1ada 19'75 U.S. FTP CVS, FID3 Trap UL,D

Australia 2 1975 U.S. FTP CVS, FID3 SHED~ UL,D

Japan 10-Mode CVS, FID3 Trap 5 UL,D11 -t~ode CVS, FID UL

6-Mode6 CVS, HFID DEurope ECE (R15. 04) CVS, FID L,D(International) ECE (R15.03) Big Bag, NDIR L,D

Sweden, Switzerland 1972 U.S. FTP CVS, FlO L,DSwitzer land 9 1975 U.S. FTP CVS, FID ULSaud i Arab ia ECE (R15. 03) Big Bag, NDIR SHED 4,7 LIsrae 1, Singapore ECE (R15.03) Big Bag, NDIR L

8 1975 U.S. FID3 SHED~Korea FTP CVS, UL

1UL = unleaded gasoline; L leaded gasoline; D diesel.2Enforced in 1986.

3Heated FID used for diesel vehicles.

4SHED = Sealed Housing for Evaporative Determination.5Gasoline vehicles only.6 b .May e chassis or engine dy'1amometer procedure.7Tr ap measurements accepted.

8Light duty vehicles, effective July, 1987.9Effective October, 1986.

101

Because exhaust emissions vary as a function of such factors as the driving

mode and the ambient conditions, the exhaust emissions of a vehicle are com-pared with the standards using an established driving cycle and sampling

method. Like the emission limits these emission test procedures are a key

part of the emission regulations. While test methods and instrumentation for

sampling the exhaust have been virtually standardized, significant differencesexist in the driving cycle schedules required in Europe, Japan, and the United

States. These variations greatly dilute the complex emission control develop-

ment process and require costly triplication of compliance verification.

Today's arguments for cycle differences are weak; good progress has been madeduring the last twenty years in the development of efficient road systems and

improvement of traffic flow, both urban and rural. The growing division with-

in Europe on this sUbject suggests that a fresh look at a harmonized drivingcycle is entirely appropriate. Vehicle speed vs time traces for the drivingcycles for the U.S., Japan, and Europe are shown in Figure 1. Compared with

the European cycle, the U.S. cycle goes to higher top speeds, has a cold start

and hot start, and is a longer test.The third key part of emission regulations is the protocol for evaluating

vehicle compliance with the emission standards. Common to emission regula-tions world wide is the requirement to give evidence of compliance with emis-

sion regUlations and obtain government approval prior to vehicle production.

This requirement generally takes the form of sUbmitting descriptive documentsand test data on a prototype vehicle which indicates that the vehicle meetsthe emission regulations. In the U.S., for example, prototype passenger cars

accumulate mileage using a standardized driving schedule in order to predict

the emissions performance of vehicles during the 50,000 mile compliance

period. This procedure is called the AMA durability schedule. A vehiclerepresentative of each engine family must be tested. The number of different

engine families sold in the U.S. in 1984 was 192; an equivalent categorization

showed 119 in Japan, 78 in Australia, and 373 in West Germany [1J. In theU.S., Sweden, and Switzerland the governments may run their own verification

tests on vehicles submitted for certification. In Japan, the government wit-nesses the testing. In saudi Arabia and Canada the manufacturer's test re-sults are not required as part of the documentation.

All emission regulations specify that the production vehicle must be bUiltto match the preprOduction prototype vehicle for which emission performance

was extensively tested and shown to comply with the regulations. Furthermore,

the emissions of the production vehicles must comply with the regulations. Todemonstrate compliance of production vehicles, some regulations require

102

UNITED STATES 1972 FTP, 1975 FTP

I ~~ u S 1572 FTP only-------

"'0 f------,.,.-----------------'---

.

.J)lUS"';~POOI) \

""'~~~:HO<) .•• i \2S~~...iL..L.C.--~..LJLJ. Jl! fli!! 1,\ AL:-:~ o 2~O 300 "'~O ~~O 6~O 7~O ll~O 9~O 10'00 ti oo 1200 TJOO~~~o· I I I , I I

TIM~ ISECONDS • --- .....<,. ,. '!>Q ...<,,~ .... --- 101.1 ,~" ,,," d .. ".,pl'''\1 """ • \372 UCO"<l'---

Tt,.".. ".,. d·. I , •• 12.,., 10 m'''~' ..

JAPAN " MODE100

EUROPE EcE R1504. EEC 78/665

100 200 300 400 500 600 700

tE l TIMEfSECONOSdmup lot 25 second$ Ij- ~I on. C'yc'- .. 120 second,

Tal.'I.1I .nd----1umpling tilne .. 505 uconds

75

50

VEHICLE SPEEDkm/hr 25

- A V, \\\/'

- , '''''':1nAAA ~An &!A ~I I ,do 2~O )~O 460 s60 660 760 ebo

11=_..m", ",.~:::::';~:~:,o" ~1--- ....,P''''9 1''''' •• 780UCO''d.

101,1 .. """,•• 82D 'HO"'"

JAPAN 10 MODE100

75

soVEHICLE SPEED

kmlhr

w.rm up 'Of15 minule", one eyeN! '" 135 seconds

1-------------_1- Total samp1in9 lime .. 1005 seccods

- Total leSl lim •• 1140 s.conds

Fig. 1. Driving cycles for various emission regulatious.(Reproduced with permission from reference [1J.)

103

testing of selected production vehicles just following production or testingof showroom vehicles or testing of vehicles at low mileage.

Only the U.S. has a formal program for evaluating the emissions of vehiclesin-use after extended mileage accumulation by vehicle owners. The U.S.

Environmental Protection Agency or its contractors do emissions testing of

privately owned vehicles. The manufacturer, however, maintains responsibilityfor the emissions performance of the vehicle for 50,000 miles or 5 years.

Manufacturers may be forced to recall vehicle families with emissions signifi-

cantly exceeding the standards.Numerous other details of world wide emission regulations and procedures

will not be reviewed here. An argument has been made for simplifying andharmonizing the testing and compliance procedures which are used to demon-

strate compliance with the emission standards in different countries [lJ.

Harmonization could allow manufacturers which market vehicles worldwide to

expend resources optimizing emissions control performance, rather than induplication of testing and development activities.

CATALYTIC CONVERTERS

It is the responsibility of vehicle manufacturers to develop technologies

for' meeting emission regulations. The automobile catalytic converter is the

only technology available for meeting the most stringent standards. This

technology has been extensively reviewed in the technical literature [2J andonly a brief review of the historical developments will be given here. Cur-

rently catalytic converters are needed to meet emission standards for passen-

ger cars sold in the U.S., Canada, Australia, and Japan. Also emission limitsfor most light duty commercial vehicles sold in the U.S., Japan, and Canadahave resulted in catalysts being used.

Oxidation Catalysts

Automobile catalytic converters have been used in the U.S. since 1974 (1975model year vehicles) in order to meet regulated emission standards. In the

early 1970's the use of catalysts for automobile exhaust emission control

represented totally new catalyst technology. The development of this technol-

ogy is an environmental success story. Environmental and political issuescame together to force technology development. Use of the catalytic converterfor controlling emissions freed up constraints on engine parameter settings.

Earlier less stringent emission standards had been met without the use of

catalytic converters by changing engine calibrations including leaning theair/fuel ratio and retarding the spark. General Motors felt that any further

compromise in engine calibration would lead to unacceptable fuel consumption

1M

ana driveability. By electing to use catalysts on 1975 model year vehicles,

driveability was improved and fuel efficiency that had been lost during the

years 1968-1974 was recovered.Initially catalytic converters were used to control just carbon monoxide

and hydrocarbons. The nitrogen oxide standard of 3.1 g/mile and later 2

g/mile could be met by using exhaust gas recirculation which leads to the

formation of less nitrogen oxide in the engine. The catalysts used through

1980 were oxidation catalysts containing the noble metals platinum and palla-

dium. A typical catalyst used by General Motors contained 0.05 oz t noble

metal per converter, with a 5/2 ratio of platinum to palladium. New platinum

mines were opened in South Africa to supply noble metals for these catalysts.

A development which paved the way for the catalytic converter was the re-

moval of lead from gasoline. General Motors had successfully argued for theavailability of unleaded gasoline which was necessary to prevent contamination

of the catalyst. Actually removing the lead from gasoline had other benefits

as well. First, lead emissions were eliminated. Lead salts are a major

source of gasoline fueled automobile particulate emissions. The presence oflead salts in the enviromnent have posed a potential health concern. Second,

emissions of unburned hydrocarbons were reduced because of additional oxida-

tion which occurs in the exhaust system and lower production of unburnedhydrocarbons in the combustion chamber. Third, lead salt deposits which have

the potential to plug or alter the calibration of EGR systems were eliminated.Fourth, vehicle maintenance requirements for engine oil, spark plugs, and

eXhaust systems were reduced through the elimination of both lead deposits and

of the acids produced.

Two basic catalyst structures were used, distinguished by the configurationof the catalyst support. The two support types are alumina pellets and alumina

coated ceramic monoliths (Figure 2). The pellets are approximately 1/8th inch

in diameter and are composed of thermally stable transitional alumina. Themonoliths are made of a ceramic material such as cordierite(2Mg,2AI 203,5Si02)·

A cross-sectional view of the pellet-type catalytic converter designed byAC Spark Plug Division of General Motors is shown in Figure 3. The catalytic

converter consists of an inlet plenum, a narrow louvered catalyst bed, and an

exhaust plenum. Exhaust gases flow in at the top of the converter through adecreasing inlet plenum, pass through the catalyst bed, and exit through an

increasing outlet plenum on the bottom. This design ensures flow uniformity,low restriction, and minimized catalyst movement.

105

Fig. 2. Catalyst supports.

Fig. 3. Cross-sectional view of pellet-type catalytic converterdeveloped by AC Spark Plug Division of General Motors.

The two sizes of pelleted catalytic converters us~d by General Motors in

1975 were 160 cu in (2.6 L) and 260 cu in (4.3 L). These converters are shownin Figure 4. The converter shell and internal parts were a stainless steel

which provided needed high temperature durability and corrosion resistance atlow cost.

The wide use of catalytic converters in 1975 was met with some concern and

criticism. Initial concerns were focused on the potential high exhaust systemsurface temperatures to cause fires. Extensive testing conducted by vehiclemanufacturers and by others indicated that exhaust system temperatures were no

106

Fig. 4. Pellet type catalytic converters (A) 160 cu in, (B) 260 cu in.

more of a fire hazard than the temperatures produced in the exhaust systems of

earlier automobiles without catalytic converters. In fact, in 1977 the U.S.National Highway Transportatior. Administration announced that catalytic con-

verters did not present an unreasonable risk.

A second concern was that under some conditions sulfur dioxide in exhaustcould be emitted as sulfuric acid as a result of catalytic oxidation over the

noble metal catalyst. To answer this concern General Motors conducted a 350-car test designed to simulate sulfate emissions on a busy expressway. The

U.S. Environmental Protection Agency, other vehicle manufacturers, and severalindependent environmental monitoring organizations participated in the experi-

ment. This experiment showed conclusively that ambient levels of sulfuric

acid under this worse-case simulated exposure situation were far below thresh-old levels known to produce adverse health effects.

Three-way Catalysts

Since 1981, more complex emission control systems have been used in the

U.S. in order to satisfy the stricter 1 g/mile emission requirement for nitro-

gen oXides. Exhaust gas recirculation alone was no longer sufficient to con-trol nitrogen oxides. Meeting this new nitrogen oxide emission standard to-

gether with the hydrocarbon and carbon monoxide standard required a new cata-lyst and a totally new approach to emission control.

Now hydrocarbons, carbon monoxide, and nitrogen oXides are removed simul-

taneously over the same catalyst.

107

sc + 02 --- CO 2 + H20+ 02 CO 2+ CO & H2 ----- N2 + CO 2 & H20

The hydrocarbo~s and carbon mor,oxide are oxidized to CO 2 and H20 while nitric

oxide is reduced to nitroge~. Simulta~eous co~version of all three pollutantsover a single catalyst has led to the ~ame three-way catalyst.

The noble metal rhodium combined with pLat inurn has the property to do both

sets of reactions if the catalytic converter is operated at an air/fuel ratio

close to the stoichiometrically balanced composition of 14.6. A schematicdiagram illustrating the principle of operation of a rhodium containing three-

way catalyst is shown in Figure 5. Under co~ditions more reduci~g than the

14.6 air-fuel ratio, the conversion efficiency of the catalyst for reducingnitrogen oxides is high and the conversion efficiency of the catalyst foroxidizing hydrocarbons and carbon monoxide declines. Under conditions more

oxidizing than 14.6, the efficiency of the catalyst for oxidizing carbon mon-

oxide and hydrocarbons is high and conversion of ~itrogen oxides declines.

CO~HC

15

--dual3-woy

NOx

ZO -

~ 50sVi

?; 1,0coC>'-'

100

11,Air I fuel rolio

Fig. 5. Efficiency scan for a dual-bed catalyst and a three-way catalyst.

Rhodium is an essential ingredient in this catalyst and is found in allcurrent exhaust cataysts which convert nitrogen oxides. Many different cata-lyst compositions are used as three-way catalysts and the noble metal content

per converter varies widely. Noble metal usage in current catalysts is in therange 0.03-0.1 02 t/converter platinum, 0.005-0.017 02 t rhodium, and 0-0.1 02

t/converter palladium. The rhodium to platinum ratio in all three-way cata-lysts exceeds the mine ratio of these metals. Three-way catalysts used in the

U.S. contain platinum and rhodium at Pt/Rh = 10/1 if not higher rhodium. The

108

mir.e ratio for these metals is approximately Pt/Rh ~ 16."/1. Three-way cata-

lysts targeted for Europe co~tai~ approximately Pt/Rh ~ 5/1 a~d high ~oble

metal per converter (e.g., 0."1 oz t per converter). High r.ob Io metal loadir:gsare co~sidered "ecessary because of the high lead levels expected to be

present in u~leaded gasoline in Europe. The current high cost of the ~oble

metals and the demand that expandi~g world wide adoption of automobile exhaustcatalysts places on their availability requires that noble metal exhaust

catalysts be prepared and used most effectively.

Plati~um is an effective oxidation catalyst for carbon monoxide and thecomplete oxidation of hydrocarbons. Palladium also promotes the oXidation of

carbon monoxide and hydrocarbons but is more sensitive to poiso~ing thanplati~um in the exhaust e~vironment. Both platinum and palladium promote the

reduction of nitric oxide but are less effective than rhodium. In addition tothe noble metals, three-way catalysts contain the base metal cerium andpossibly other additives such as lanthanum, nickel or iron. These base metaladditives are believed to improve catalyst performance by extending co~version

during the rapid air-fuel ratio perturbations and help to stabilize thealumina support against thermal degradation.

In order to provide the proper stoichiometrically balanced exhaust gas

composition reqUired for use of the three-way catalyst, an air/fuel ratio

control system had to be developed for the vehicle. Closed-loop electronicair-fuel ratio control required the installation of an exhaust oxygen sensor

and an on-board microprocessor to provide the necessary control capability.

The continuous air-fuel ratio adjustments result in small 0.5-4 hertzperturbations of the exhaust composition with an amplitude of approximately+0.5 air-fuel ratio.

A diagram of the control system components is shown in Figure 6. The

exhaust oxygen sensor is placed ahead of the catalyst. The on-board

microprocessor receives signals from the oxygen sensor and a number of other

sensors and generates output signals which are used to control engine air-fuel, spark timing, transmission converter clutch, and a variety of otherengine and drivetrain functions. This system was first used primarily with

carburetors which over time are being replaced by fuel injection control.The three-way catalytic converter has to respond to a wide range of exhaustconditions because exhaust emissions vary as a function of the driving mode.

Typical engine out exhaust emissions for a passenger car are in the range

0.04-0.4 vol% hydrocarbons, 0.03-2.5 vol% carbon monoxide, and 0.0-0.2 vol%nitrogen oxides. Exhaust gas temperatures at the inlet to the catalytic

converter are typically 350-500 C for a warmed up catalytic converter. At

ELECTRONIC CONTROLMOOULE

THROTfLE BODYINJECTOH SYSTEM ~

\\.-11

DISTRIBUTOR __(,1JAIR CLEANER -- '==_

~~~

MASS AIR'LOW SENSOR

\

EXHAUSTOXYGEN SENSOR -

COOLANT SENSOR

MANIFOLD A8SOl U T[PRESSURE SENSOH

~---VAPOR CANISTER',----------",

TORQUE CONVERTERCLUTCH CONTRClL

Fig. 6. Closed-loop emission control system on a three-waycatalyst equipped vehicle.

start-up, however, the catalyst is cool and no reactions occur until the

catalyst is heated to operating temperature by the hot exhaust gases.

General Motors first marketed three-way catalytic converter systems inCalifornia during the 1978 model year and expanded their use in California

during the 1979 and 1980 model years. The California program allowed a"phasing-in" of this new technology prior to introduction to the full U.S.market in 1981. A simi lar "phase-in" opportun ity is proposed for Germany and

Austria by designating the strictest emission standards for only the largestpassenger cars.

Two types of catalytic converters are currently being used for meeting the

passenger car emission standards in the U.S.: three-way converters and dual-bed converters. Both converters contain three-way catalysts, but with the

dual-bed converter the three-way catalyst is followed by an air injection/oxidation catalyst system. As for the earlier oxidation catalysts two formsof catalyst support are used: pellets (thermally stable transitional alumina)

and monoliths (cordierite honeycombs coated with a thin alumina washcoat).Figure 7 shows four catalytic converters currently being used by GeneralMotors.

CATALYST DURABILITY

In the U.S. exhaust catalysts must have the durability to maintain high

activity for 50,000 miles or 5 years. The U.S. Federal regulations require

110

a b

C

Fig. 7. AC Spark Plug(A) 170 cu in(B) 170 cu in(C) 160 cu in( D) 260 cu in

d

three-way catalytic converters.dual bed monolith (three-way + oxidizing).three-way monolith.three-way pellet.three-way pellet (trucks).

that the exhaust emissions of passenger cars not exceed the standards within

this compliance period, and the automobile manufacturers maintain responsi-bility for meeting the emission standards. Because catalysts do deactivatewith use, the ability to withstand mild deactivation is built into the design

of the catalyst as well as the entire emission control system on a vehicle.

This is done by setting up vehicles to operate well below the standards at lowmileage, to select materials which are durable in the exhaust environment, and

to prevent accessibility to vehicle adjustments which could alter emissions.All catalysts are not expected to experience the same deactivation in use

because of the wide range of veh ic Ie operat i ng cond i tions. Vehic Ie manufac-turers have developed engine-dynamometer tests which are used for screeningcatalysts submitted from catalyst suppliers. On these tests the catalysts are

exposed to a range of operating conditions and temperatures in order to assess

activity and durability during a simulated aging schedule. Catalysts are se-lected for further testing on vehicles based on their performance on these

initial durability tests. Overall catalyst selection is based on performance

criteria. Vehicle manufacturers set noble metal loadings and the support

type, but the exact catalyst formulation including base metal additives isdesigned by the catalyst suppliers and this information is generally proprie-tary.

The major mechanisms of deterioration of automobile exhaust catalysts are

thermal damage due to exposure of the catalysts to very high temperatures,

poisoning by contaminants in the exhaust, and mechanical damage of the cata-lyst support. Research aimed at identifying and understanding the nature of

the deterioration and the impact on performance has included post-mortem

111

studies of used catalysts [3J a.o simulated aging studies ie; which catalystperformance is examined following exposure of the catalyst to high temperatureand/or catalyst poisons [~-12J. In general, accelerated aging studies haverevealed that exposure of catalysts to high temperature oxidizing conditionsdamage CO conversion whereas catalyst poisoning damages hydrocarbon oxidation

[12J. Examination of used catalysts generallY reveals a number of changes andexcept for severely damaged catalysts no single factor correlates clearly with

per formance.

Exposure to high temperatures can damage catalysts by sintering the noblemetal particles, resulting in a decrease in the fraction of the noble metal

available for catalytic reactions. Low temperature activity of the catalystis most impaired by noble metal sintering. High temperatures can also promote

damaging interactions between the noble metals (alloy formation) and interac-

t ior.s between base metal (includ mg the catalyst support) and noble metalcomponents of the catalyst [1~,15J. Vehicle conditions which can produce high

catalyst temperatures are, for example, repeated misfire resulting in theoxidation of large amounts of unburned fuel over the catalyst. High catalyst

temperatures are of concern for European catalyst applications since top driv-ing speeds permitted in Germany are higher than in the U.S.

Oxidizing conditions have been observed to damage three-way catalysts at

lower temperatures than reducing conditions. A platinum-rhodium three-waycatalyst (base metal additives present but not identified) aged on an engine

dynamometer was deactivated more readily (at lower temperature) during a brief

exposure to lean air-fuel ratios than to rich air-fuel ratios [13J. Activity

loss as measured at 600 F at stoichiometry was appreciable fOllowing only 20minutes exposure to lean exhaust at 1600 F [13J.

Excessively high temperatures can damage the catalyst support. The ceramic

monolith may melt forming channels for the exhaust to pass through the conver-

ter without contacting the catalyst. High temperatures can potentially damagethe alumina support by promoting transition to alpha-alumina and loss of sur-

face area. Mechanical loss of catalyst support material can result from den-

sification and cracking of the monolith wash coat leading to poor adhesion of

the catalyst layer to the ceramic monolith. Other mechanisms of loss areabrasion and breaking of catalyst pellets.

Typical catalyst poisons are lead and phosphorus. Lead is present at very

low levels in unleaded gasoline. Typical lead levels are 0.003 glgal although

0.05 glgal is the maximum allowed lead level in unleaded fuel. Lead is notbelieved to be a major catalyst poison at the 0.003 glgal level. On the otherhand, use of leaded fuel will poison three-way catalysts, and catalyst activ-

ity is not fully recovered upon changing back to unleaded fuel. Figure 8

112

>-ozwou..u..w

oI

100~------

80

60

40

20

oo

100

J5

1-10

- -0 ----

>-ozwou::u..w

oo

80

60

40

20

oo

100

----,5

---{j- -- '~--{J--:--._--- {j----

----T----

10

__ ---{} -'_0' _ ~ )

J15

>-ozwou::I.L..W

xo2:

80

60

40

20

---- -1')

Legendo UN LEAD FUEL

• LEAD FUEL

-, OBS. DATA

oo

-- - J ------,-------

5 10ODOMETER-MILES-WOO

T15

Fig 8. Converter efficiency during intermittent lead use.(Reproduced with permission from reference [4J.)

113

snows how the activity of a typical three-way catalyst is impaired during and

following intermittent operation with leaded fuel (1.2 g/gal) during 15,000miles of vehicle operation [4J. The converter efficiency of the control vehi-

cle was virtually unchanged at 94% for hydrocarbons, 95% for carbon monoxide,

and 66% for nitrogen oxides [4J. Following the misfueling shown here thecarbon monoxide emission level recovered to an acceptable level. The hydro-

carbon and nitrogen oxide emissions did not recover to passing values [4J.Fuel switching can be a reason why some used vehicles fail to meet emission

standards. lei a 1984 survey conducted by the U.S. Environmental Protection

Agency 14% of the vehicles tested were found to have been misfueled by usingleaded gasoline in catalyst-equipped vehicles. The survey also showed that

fuel swi tch Ing was higher in areas wi th no inspect ion-rna intenance (I/M) pro-

gram (19% fuel switching) compared with areas with liM programs (10% fuel

switching) [16J. These findings argue in favor of inspection programs which

check for proper maintenance of vehicle emission control systems and that all

components are present. This same survey showed that at least one emission

control component had been tampered with on 21% of the vehicles examined [16J.Tampering involved the catalytic converter itself, the EGR valve, altered

!'iller neck inlets, disabled air pumps and evaporative systems, and tamperingwith PVC's [1 6J. Phase down of the amount of lead allowed in leaded gasoline

which began in July, 1985 (from 1.10 glgal to 0.50 glgal and further to 0.10glgal in January, 1986) will reduce lead emissions to the environment andfue I-sw itch ing.

Phosphorus is recognized as a potential poison of automobile exhaust cata-

lysts. Phosphorus levels in gasoline are very low (0.2 mg/l), and fuel-de-

rived phosphorus at these levels does not damage three-way catalysts. Phos-phorus is present in high concentrations in engine oils (1.2 gil) and is the

source of phosphorus contamination of catalysts [9,10J. Phosphorus derived

from engine oil reacts strongly with the alumina support and tends to accumu-

late at the outer edge of the catalyst pellet in the same location as thenoble metals (Figure 9). Phosphorus can deposit on catalysts in more than one

chemical form and poisoning is not reversed by thermal treatments [6J. Phos-

phorus poisoning of catalysts has been studied extensively in simulated poi-soning studies (e.g., 6, 9-11).

Fuel-derived sulfur does not interfere with the performance of noble metal

exhaust catalysts as strongly as it does with base-metal catalysts. Compati-bility with SUlfur dioxide was one of the reasons for selecting noble metal

catalysts. Fuel contaminants such as organo-silicon compounds have been found

114

Io

I50

I100

~m

Fig. 9. Scanning electron micrograph trace of phosphorus and aluminumprofiles for a used (37 000 miles) three-way catalyst pellet.The vertical scale is concentration in arbitrary units.

to degrade both catalysts and oxygen sensors [7J. Manganese fuel additives

have been shown to impair three-way catayst activity [17J.

FUTURE ISSUES

The regulatory agenda in future years which could result in new regula-tions is likely to be driven by specific issues as in the past. We might

imagine that relationships between health and air quality would be high on

this list. Attention to specific air toxics such as benzene has been ofconcern recently to the California Air Resources Board. Attainment of the air

quality standard for ozone has been difficult in many areas of the country andwill likely continue to be so for several years. Inspection and maintenance

programs are intended to assist ozone attainment. The contribution of automo-

bile eXhaust emissions to acid deposition has been cited as a reason for pro-pOSing more stringent emission controls for nitrogen oxides, in spite of the

very small contribution of nitrogen oxide from passenger cars to the acidity(4.7% of the total acidity in the Eastern U.S.).

In the U.S. regulatory emphasis at the present time is on in-use perfor-mance. The pre-production accelerated durability tests cannot fully duplicate

the same distribution of performance as in-use vehicles. Large numbers ofthree-way catalysts introduced in 1981 and following years are now reaching

50.000 miles so that field performance can be evaluated.

The supply of noble metals for three-way catalysts and particularly the

rhodium supply is of concern to manufacturers. The rhodium use ie, platinum-

rhodium three-way catalysts exceeds the naturally occurring ratio of these

metals. Automobile catalytic converters are a large user of noble metals and

this imbalance in the use of platinum and rhodium can influence the price and

availability of rhodium. Noble metal recovery from spent automob ile exhaust

catalysts is currently a source of platinum and palladium and can be expected

to be a source of rhodium after 1990.

ACKNOWLDEGMENTS

The author wishes to thank David R. Monroe, Se H. Oh, and Michael J.

D'Aniello, Jr. (General Motors Research Laboratories), Gerald J. Barnes and

Mike C. Myal (General Motors Environmental Activities Staff), and Michael P.

Murphry (General Motors Luxembourg Operations S.A.) for their assistance with

the preparation of this manuscript.

REFERENCES

G.J. Barnes and R.J. Donohue, A Manufacturers's View of World EmissionsRegulations and the Need for Harmonization of Procedures, Society ofAutomotive Engineers Paper No. 850391 (February, 1985).

2 K.C. Taylor, Automobile catalytic Converters. Springer-Verlag, Berlin,1984.

3 R.K. Herz, E.J. Shinouskis, A. Datye and J. Schwank, Ind. Eng. Chem.Prod. Res. Dev , , 24, (1985) 6.

4 B.R. McIntyre and L.J. Faix, Lead Detection in Catalytic EmissionSystems and Effects on Emissions," Society of Automotive Engineers PaperNo. 860488 (February, 1986).

5 G. Kim, M.V. Ernest and S.R. Montgomery, Ind. Eng. Chern. Prod. Res.Dev , , 24, 525 (1984).

6 G.C. Joy, F.S. Molinaro and E.H. Homeler, "Influence of Phosphorus onThree-Component Control Catalysts: Catalyst Durability andCharacterization Studies," Society of Automotive Engineers Paper No.852099 (October, 1985).

7 H.S. Gandhi, W.B. Williamson, R.L. Coss, L.A. Marcotty and D. Lewis,"Silicon Contamination of Automotive Catalysts," Society of AutomotiveEngineers Paper No. 860565 (February, 1986).

8 W.B. Williamson, H.S. Gandhi, M.E. Szpilka and A. Deakin, "Durability ofAutomoti ve catalysts for European Applications," Soc iety of Automoti veEngineers Paper No. 852097 (October, 1985).

9 F. Car-ace io 10 and J.A. Spearot, "Eng ine Oi 1 Phosphorus Effects ofCatalytic Converter Performance in Federal Durability and High-SpeedVehicle Tests," Society of Automotive Engineers Paper No. 770637 (June,1977) •

10 F. Caracciolo and J.A. Spearot, "Engine Oil Additive Effects on theDeterioration of a Stoichiometric Emissions Control (C-4) System,"Society of Automotive Engineers Paper No. 790941 (OCtober, 1979).

115

116

11 D.R. Monroe, "Phosphorus and Lead Poisioning of Pelle ted Three-WayCatalysts," Society of Automotive Engineers Paper No. 800859 (,Jur,e,1980).

12 B.J. Cooper and T.J. Truex, "Operational Criteria Affecting the Designof Thermally Stable Single-Bed Three-Way Catalysts," Society ofAutomotive Engineers Paper No. 850128.

13 R.H. Hammerle and C.H. Wu, "Effect of High Temperatures on Three-WayAutomotive Catalysts," Society of Automotive Engineers Paper No. 840549(February, 1984).

14 K. Otto, W.B. Williamson and H S. Gandhi, Ceramic Eng. and Sci. Proc.,2, (1981) 352.

15 B.J. Cooper, Platinum Metals Rev., 27 (1983) 146.16 Helen Kahn, Automotive News, p. 50, November 4, 1985.17 J. Duncan and J. N. Braddock, "Combustor Study of the Deacti vat ion of a

Three-Way Catalyst by Lead and Manganese," Society of AutomotiveEng ineers Paper No. 841408 (oc tober, 1984).

A. Crucq and A. Frennet (Editors), Catalysis and Automotive Pollution Control© 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

ASPECTS OF AUTOMOTIVE CATALYST PREPARATION, PERFORMANCE AND DURABILITY

117

2 3B. J. COOPER, W. D. J. EVANS and B. HARRISON

lJohnson Matthey PIc, Catalytic Systems Division, 456 Devon Park Drive, Wayne,PA 19087 (United States of America)

2Johnson Matthey PIc, Catalytic Systems Division, Orchard Road, Royston,Hertfordshire SG8 5HE (United Kingdom)

3Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading, RG49NH(United Kingdom)

ABSTRACTThe development of legislative controls on petrol engined passenger cars in

the USA and Western Europe is reviewed. The application of catalytic controlstrategies to these requirements is discussed.

The principle components of modern exhaust emission control catalysts areidentified. They comprise (a) a ceramic substrate, (b) a high surface area washcoat, (c) base metal promoters and/or stabilisers and (d) platinum group metalseither singly or in combination.

The functional role of these components is discussed and their interactionreviewed from the materials technology standpoint. Aspects of catalystperformance and durability influenced by preparation factors are discussed withparticular reference to factors (b), (c) and (d).

LEGISLATIVE REVIEWThe increasingly urban nature of industrialised society has resulted in

deterioration of air quality and generated political pressure for control of

atmospheric pollution. Many states have introduced measures to reduce emissionsincluding latterly those from vehicle sources.

During the early 1940's significant environmental problems were occurring

with increasing frequencies in the Los Angeles basin. In the early 1950's the smog

problem was related (ref. 1) to photochemical interaction of nitrogen oxides (NOx),hydrocarbons (HC) and oxygen. Surveys established that a high proportion of man

made emissions in that locality were derived from the motor vehicle.These conclusions, supported by numerous studies, provoked intensive research

into methods of emission control. Notable contributors were Eugene Houdry who, in

1949, invented a form of the ceramic monolith now in almost universal use and theInter Industries Emission Control Programme led by Ford and Mobil which, during the

1960' s, defined the emissions control system which would be required to meet severe

regulations.Political pressures derived from an increasingly powerful and vocal

environmental lobby culminated in 1970 in the US Clean Air Act (ref. 2), which

118

included progressively more stringent regulations covering inter alia, emissionsof CO, HC and NOx. This targetted a reduction of approximately 90% in emissionsrelative to an uncontrolled average late 1960 model year vehicle.

Features of the legislation were introduction of lead free gasoline in 1974, arequirement for emissions control systems to be effective for at least 50,000 miles,and the definition of a test cycle and procedure to measure emissions.

Intervention of international fuel crises during the 1970's caused some easing ofthe timetable and emissions limits, the historical development being summarised in

Table 1.

TABLE 1

Development of U.S. Federal Emissions Regulations

Model Year CO HC NOx (g/mile)

1970 34.0 4.1 4.01975 15.0 1.5 3.11980 7.0 0.41 2.11981 7.0 0.41 1.01983 3.4 0.41 1.0

The increasing stringency of the limits required progressive introduction of

catalytic control strategies beginning in 1975.

Subsequent to introduction of this legislation, standards of similar severity(involving a different test procedure) were introduced rapidly in Japan. More

recently Australia (from January 1986) has adopted the US 1975 Federal limits.Universally, the solution to emissions control from motor vehicles for the US

market has included a platinum group metal catalyst. This has created, over a 12

year period, the largest single application for catalysts and certainly the largest

application of platinum group metals (Fig. 1) (ref. 3).

The complex political development of Europe relative to the US and Japan has

resulted in a different and more fragmented approach to the problem of control ofemissions from motor vehicles (refs. 4,5). European nations under the auspices of

the United Nations Economic Commission for Europe (ECE) have developed a unique test

cycle (ECE R15-04), sampling and measurement protocol. Although the sampling andmeasuring protocols are now similar to the US Federal Test Procedure (FTP-75) the

driving cycle is radically different. Thus, for the ECE-15 test, maximum andaverage speeds are 50 and 18.7 km/hr respectively with approximately 31% at idle.This simulates city driving in congested conditions. In contrast, the FTP-75

simulates urban driving, typical of that in the Los Angeles basin. Maximum andaverage speeds are 91 and 34 km/hr respectively, with 18.4% at idle.

119

RHODIUM DEMAND IN THEWESTERN WORLD 1985

PLATINUM DEMAND IN THEWESTERN WORLD 1985

Petroleum1% Glass Electrical

5% 7%

Total Demand =2,810,000 oz

Chemical18%Glass

6% Electrical9%

Total Demand = 250,000 oz

Fig. 1 Rhodium and Platinum useage by major application.

As in the USA, limits were progressively lowered and refinements made to the

test procedure (ref. 6). However, in the USA a single standard applies to all

passenger vehicles whereas in Europe standards have traditionally been related tovehicle inertia weight.

Currently regulation ECE R15-04 is in force (ref. 7) and has been adopted by theEuropean Economic Community (EEC) and by most other European States. The

requirements of this regulation are lax relative to contemporary US and Japanese

limits.By 1984, after several years of gradual reductions in emission levels, the

political climate, notably in West Germany, favoured a much more rapid change. The

West German proposals required introduction of three way catalysts and necessitateduse of unleaded fuel. After a lengthy period of debate, a compromise solution was

developed by the 'EEC' which substantially diluted the original proposals. The

'final proposals' (ref. 8) were published in June 1985 and entail progressive

introduction of standards (Table 2.)As a separate issue it had already been agreed that unleaded fuel should be made

available throughout the community from 1989. This date may be anticipated and the

fuel specification will be 95 RON, O.013g/litre lead (max.)The directive resulting from these proposals will be based upon the concept of

optional harmonisation. It will permit, but not require, Member States to adopt

national legislation in line with it.There remains considerable controversy surrounding the 'final proposals'.

There is strong polarisation with respect to identification of Phase 2 standards forsmall vehicles targetted for January 1st 1987.

120

TABLE 2

Final Proposals for European Common Market Automobile Emission Control Standards

New Models All New Cars

Over 2 li tres Oct. 1988 Oct. 19891.4 - 2 litres Oct. 1991 Oct. 1993Less than 1. 4L

Stage 1 Oct. 1990 Oct. 1991Stage 2 Oct. 1992 Oct. 1993

Engine Capacity Date of Introduction Emissions, g/test

CO (HC+NOx) NOx

25 6.5 3.530 8

45 15 6To be decided by 1987

Over one year after publication of the 'final proposals' there remains no

immediate likelihood of ratification. Nevertheless, West Germany has taken the

lead in promoting National Standards supported, during a voluntary introductory

period, by significant fiscal measures. In contrast, UK, France and Italy are not

expected to adopt or make the proposals mandatory for some time.

The schism within the EEC is mirrored by further divisions reflecting the wide

range of national interests of non-member states. Thus, Sweden has announced a

proposal to adopt US 1983 standards from 1989.

The dis pari t y between emission test procedures, allowable tail pipe emissions

and local market conditions conspire to prevent a universal solution to world wide

certification of any given vehicle. Consequently, even though a basic vehicle may

be utilised in several markets, there are generally significant differences in

subsystems to cope with, e.g. different emissions constraints. In consequence,

vehicles of European manufacture may be produced in several specifications. Thus,

models may be produced to Japanese specification involving an oxidation catalyst,

to US specification involving TWC and to a range of European specification involving

no catalyst at all. This substantially magnifies the capital and human resources

required to maintain a broad market presence.

CONTROL STRATEGIES

The emissions from conventional spark ignition engines are strongly dependent

on air:fuel (A/F) ratio. No single operating regime exists within which levels of

emissions of all pollutants is low.

In practical terms this has constrained the development of only three basic

control strategies (refs 9,10) in the context of stringent legislation. These are

all based upon application of supported platinum group metal catalysts. Thestrategies are:

(1) Removal of HC and CO by use of an oxidation catalyst (COC) generally containing

121

Pd or Pt/Pd with other means of reducing NOx emissions, e.g. exhaust gasrecirculation. This strategy normally entails a slightly lean tune andsecondary air injection. The extent of NOx reduction is determined bydriveability considerations, limiting applicability to less demandingrequirements.

(2) A combination of sequential reduction of NOx, over what is essentially a threeway catalyst (TWC), followed by oxidation of residual CO and HC over a COC after

injection of secondary air. This procedure requires a rich tune to providethe necessary net reducing atmosphere in the first catalyst, has an adverseimpact on fuel economy and is not likely to be favoured in the European Context.

(3) Removal of pollutants by use of a TWC. This can be achieved using a Pt/Rhformulation but only if the engine management system controls the fuellingclosely at the stoichiometric point (A/F : 14.7: 1; A: 1). Current European

practise for US models is unique in utilising only the single bed TWC and

electronic multipoint fuel injection, under oxygen sensor control, forimplementation of this strategy.These strategies as applied in the USA market, which can be implemented by a

variety of routes, were recently reviewed by Duleep (ref. 10). A strong trend

towards the single bed TWC operating under closed loop control of electronic fuelinjection was noted.

Strategies for the emerging European market have been reviewed recently by

Evans et al (ref. 11).A significant benefit of a lean fuelling strategy is improved fuel economy.

This has motivated intensive research into lean burn technology involving reliable

operation at high air:fuel ratios typically in excess of 20:1 (refs 12,13). Acorollary of such operation would be substantially reduced NOx emissions, (ref. 14)

albeit at higher NOx levels than a comparable vehicle fitted with a TWC, but at the

expense of an increase in HC. Operation of conventional engines at high air/fuel

ratios is limited by onset of pre-ignition, rapid torque fluctuations, fast

deterioration of the engine and poor driveability.Thus far it has not been demonstrated that acceptable driveability can be

achieved for a vehicle operating at 20-22:1 A/F other than by a very high level ofequipment, i.e. total electronic closed loop management with multipoint fuel

injection. However, even at that level, it is not possible to achieve severelegislation limits without provision of a COC to remove hydrocarbon species (ref.

13). Nevertheless, it is evident that substantial progress has been achieved andthat in the European context a fourth control strategy is potentially available for

mid-range vehicles.

122

CATALYSTS FOR AUTOMOTIVE APPLICATION

Catalyst technology was developed in the mid period of this century for

chemical process operations. In such applications the catalyst is generally sitedin a fixed bed reactor and after commissioning operates in a more or less steadystate mode for a long period of time. Furthermore, space considerations are

normally a minor factor in the design of the catalyst and reactor; space velocities

are generally quite low with large catalyst volumes being employed. Economic

considerations associated with selectivi ty and yield generally dictate tight

control of space velocity, temperatures and protection of the bed from poisons.Addi tionally complex reac tors, often wi th recycle or interbed cooling, are

practical solutions to maintaining the required yields.The situation in a motor vehicle could not be more different. The duty of the

automoti ve catalyst comprises a series of 'commissionings' followed by opera tion ina highly perturbed fashion. In the USA, the mandatory cold start and 50,000 mile

durability requirement demands operation of the catalyst at low temperatures.During actual operation the catalyst would be subjected to extremes of gas flow andtemperature with large variations in concentration of pollutants over the load-

speed envelope of the vehicle.In the emerging European market the situation is even more complex. Thus,

vehicles are generally much smaller but average and maximum speeds are higher.However, the lower speed test cycle and consequent cooler exhaust gas temperature

requires high catalyst activity at low temperatures. Consequently the operatingtemperature requirement is even broader than that for the US market (ref. 11).

In addition to the highly non steady state operation, uncontrolled poisoningis a major threat to the catalyst. The principal poisons are lead, sulphur,

phosphorus and zinc (refs. 15-18). The latter two species are generally derivedfrom lubricating oil, principally from the anti-scuff agent ZDDP. Very few

examples of significant catalyst deterioration in service have been reported due toZn/P poisoning (ref. 19).

Lead and sulphur are derived from the fuel and there is a complex equilibrium

dependent upon temperatures and gas composition controlling theabsorption/desorption of these poisons. In the case of lead, extended trials havedemonstrated the feasi bili ty (ref. 20) of successful operation of oxidation

catalysts on leaded fuel. However, it has been noted that in the decade since

introduction of lead-free fuel in the USA, residual lead levels have fallendramatically. In that market, where leaded and unleaded fuels are both available,

incidents of poisoning reflect contamination of distribution equipment or

deliberate misfuelling (refs. 21,22). Sulphur may also be derived from lube oil

but its impact in the sense of poisoning is low on PGM catalysts. Interaction withcatalyst components can, however, influence secondary/unregulated emissions of

sulphur bearing species such as sulphate (refs. 23-26).A further major difference with respect to chemical process reactors is the

critical need to achieve low pressure drop to minimise power losses. Thisrequirement conflicts to a large extent with those for high activity, Le. good heat

and mass transfer.In the early phases of the emerging market, the dominant technology for

achieving the total requirement derived from conventional fixed bed pelletedcatalyst technology, albeit with special high aspect ratio beds to minimise power

losses. However, widespread use was made of an alternative technology based on amulticellular ceramic substrate or monolith (ref. 27). Due to persistentdurability problems with pellet bed reactors the monolithic support catalyst has

become the dominant technology accounting for perhaps 95% of all new vehiclesystems.

The monolith has strong, porous, thin walls supporting an array of parallel

channels presenting a high geometric surface area. The high open area andstructure promote laminar flow, limiting pressure drop at high flow rate. Use of alow expansion body based upon Cordierite provides a high degree of thermal shock and

strength while offering a high maximum operating temperature.

Major advances in ceramic extrusion technology and processing have enabled

substantial advances in product quality. In consequence a wide range of shapes,sizes and cell dimensions are available (ref. 27).

Although ceramic monolith based catalysts dominate the global market, there

has been significant interest in Europe latterly in metallic monoliths (refs. 28-

30). The reduced wall thickness offers specific advantages in conversion in

applications where space is at a premium or ceramic based solutions are notpossible. Several major applications now exist (ref. 31) but presently cost

factors remain a major determinant in favour of ceramics.

However, it is not possible to achieve the combination of strength and thermalshock resistance required for a ceramic monolith together with the high specific

surface area required for catalysis. This surface area is applied to the monolith,generally in the form of an aqueous suspension of a highly porous material - the wash

coat. Its characteristics, along with those of the underlying support, have a keyrole in determining the activity and durability of the catalyst system.

Accordingly the key first stage of manufacturing a monolithic type catalyst is

formulation of the wash coat and uniform application over the internal surface ofthe monolith. Although commercial processes are proprietary with little detail

available, the coating is generally fixed, by calcination, at elevated temperature.

The second key activity is application of precious metals and promoters, foreconomic reasons generally from solution or dispersion. After drying, reductionor calcination processes are used to fix the precious metal. In principle. the

124

precious metal may be included with the wash coat.

The catalytic species of current automoti ve catalysts are balanced mixtures of

precious metals and promoters selected, as discussed previously, on the basis of

application. Precious metals are favoured due to high catalytic activity and

selectivity, particularly at low temperatures (as experienced with cold start

tests). Additionally their supported dispersions are relatively stable at high

temperatures and exhibit good resistance to poisoning.

The idealised requirements of the three chemical constituents of the catalyst

must be met in a manner which allows economic manufacture by routes compatible with

mass production. Subsequent sections are concerned with each of the three key

components (wash coat, base metal promoters and precious metals) and examine the

influence of preparation on performance.

WASH COAT

An autocatalyst wash coat must provide a high, stable surface area upon which

crystallites of precious metals and promoters can be dispersed. The overall

stability of the catalyst is to a large extent dependent upon that of the wash coat in

terms of surface area and adhesion.

Washcoats generally comprise mixtures of stabilisers, promoters and alumina.

Alumina forms the bulk of the wash coat, frequently in excess of 90%, and accordingly

its stability is crucial. Preparation of wash coats is proprietary but generally

involves formation of a high solids dispersion of activated alumina. Such

dispersions are generally produced by milling or use of high shear mixers.

Addi tions of dispersing agents, e t c , , are necessary to provide the surface tension

and flow properties required to allow penetration of a 400 cpsi monolith and achieve

uniform coating of cell walls.

Choice of alumina precursor has a significant impact on stability of surface

area (ref. 32). This is illustrated in Fig. 2 for activated aluminas derived from

Boehmite and Gibbsite, the two major industrial raw materials commonly available.

It is readily apparent that activated aluminas derived from boehmite are the

most thermally stable in the principle temperature ranges of interest.

Additionally•. transitional aluminas derived from gibbsite undergo major reordering

of the lattice at lower temperatures than ¥' alumina with significant implications

for shrinkage as well as the surface area changes noted above.

The inherent stability of aluminas can be further improved by addition of other

oxides (ref. 32). Base metals can act as promoters and in an ideal si tua tion would

fulfill a dual role. Fig. 3 shows the change in surface areas for boehmite derived

activated aluminas as a function of temperature. It may be seen that addition of

barium retards phase transformation and consequent loss of surface area to well

above 10000C.

125

GibbsiteX: X~

Boe'hmitea

200Surface

Aream'g

100

o400 600 800 1000 1200

Temperature c

Fig. 2. Surface area thermal stability and phase transformations for transitionalaluminas derived from Gibbsite and Boehmite.

100

40

20

o '--.,jL---,__,.--_r--r-.,.-'Fresh 750 1000 1200

Temperature C

Fig. 3. Thermal stability of surface area of ~ alumina - metal oxide mixtures.

The benefits of such improved stability in terms of catalyst performance is

illustrated in Fig. 4 for unstabilised and barium stabilised palladium and rhodium

catalysts after ageing under the specified conditions. In each case significantly

improved performance is achieved.

126

NOx(B)20

Pretreatment950 C/ 1% 0,/10% H,O' lHour

Pert 1.00 «\) 1.00 HzJCO

___-----,I--}~- HCt: 80o'iiiQ; 60>c8 40if!

Test Hrs 300

20'

100,-----'--"'-'-....:...:'-'--;;::-::-'-"1

(A)

c 80'o'iiiQ; 60>s:8 40if!

0.96 0.98 1.00 1.02 1.04Equivalence Ratio (r.)

__ (1) Pd/AI ,0 3- (2) Pd/Ba/AI,03

0.96 0.98 1.00 1.02 1.04Equivalence Ratio (Al

__ (1) Rh/AI,03- (2) Rh/Ba/AI,03

Fig. 4. Static engine based selectivity test showing the. influence of bariumstabiliser on catalyst performance (A) for palladium based catalysts after 300 hrsengine ageing (800 0C max, ) and (B) for Rh based catalysts after hydrothermal ageingfor 1 hr at 9500C.

We must now consider a complex series of trade-offs that are involved in the

application of the wash coat to the substrate. In simple terms the considerationsare as follows. The wash coat provides the means for a highly dispersed catalytic

material to maintain a high surface area. Therefore, for a given loading of

catalytic material, a higher quantity of wash coat will result in a more stable

dispersion. This is because, over the higher total surface area present, there

will be fewer next neighbour interactions between the precious metal components.Therefore, coalescence sintering will be reduced. In addition to this effect, the

100 -- 100

80

//~~~80c: c:

0 0'iii 60 'iii 60i i> >c: 40 c: 400 0U

.- / ICO --I o<P 20 >..- NOx~ <P 20 ..:.--

(Al HC ......0 0

0.96 0.98 1.00 1.02 0.96

Lambda Value

'"~O - -

NOx~(6) HC .....'

0.98 1.00 1.02

Lambda Value

Fig. 5. Static engine based selectivity test showing the influence of wash coatloading on the performance of a 5:1 Pt/Rh TWC after ageing for 200 hrs on an 8 modecycle (peak temperature 850°C, 3mgL-llead) Catalyst A contains 68 percent by weightof the wash coat deposited on Catalyst B.

127

washcoat acts as a poison sink and the higher the surface area of wash coat present,

the better the catalyst will resist the effects of poisons. The effects of wash

coat loading on catalyst activity are illustrated in figure 5.

Clearly the activity of the catalyst with a high wash coat loading and

therefore higher surface area is better.

Fig. 6 shows the activity pattern for a series of catalysts, differing solely

in wash coat loading, after thermal pretreatment in a wet oxidising gas and

subsequent 150 hours engine ageing in a perturbed ageing cycle. CO and NOx

conversion shows a significant dependence upon wash coat loading in this test.

100 I~NOX I3 co !mHC I90co

"iii 80

~o 70()

'" 60

50 1111x 1.20X 1.42X 1.51X

Relative Washcoat loading

Fig. 6. Static engine test data showing the effect of wash coat loading onconversion at ~ = 0.995 in a selectivity test after extended ageing (150 hrs).

In addition to surface area stability, the wash coat must maintain good

adhesion to the monolith at high loadings over the operating envelope. In

principle, this can be achieved by increasing the solids content of the dispersion

or repeated coatings. However, close process control must be exerted over the

application process which otherwise becomes a source of adhesion problems. Thus,

packing of solid particles during removal of occluded water by drying may provoke

shrinkage cracks. Thermal cycling during processing may provoke delamination and

loss of wash coat. Prevention of premature failure due to these mechanisms

requires tight control over all aspects of wash coat preparation and application.

Retention of high activity during service is critically dependent upon

maintaining integrity of the wash coat/monolith bond. However, even initially

well bonded coatings can be susceptible to deterioration due to frequent, rapid,

high temperature cycling. The influence of thermal ageing at 13500C

on an

initially highly adherent coating is shown in Fig. 7. Severe shrinkage has

occurred due to major changes in surface area and the alumina phase. This problem

may be overcome by inclusion of phase stabilisers (Fig. 8) whi~h defer and reduce

such changes.

128

Fig. 7. Optical micrograph ofwash coat after sintering at hightemperature showing severeshrinkage.

Fig. 8. Optical micrograph ofstable wash coat after hightemperature exposure showing freedomfrom shrinkage cracking.

Benefits derived from these improvements may be seen from comparison of the

hydrocarbon breakthrough for two otherwise identical catalyst systems after 350

hours operation at temperatures up to 8000C

(for 80% of the time) when exposed to-1

fuel containing 3mgL lead (Table 3).

TABLE 3

Effect of Washcoat Type on the Durability of Pt/Pd Catalysts for HydrocarbonOxidation

% Unconverted Hydrocarbonat 25 Hrs at 355 Hrs

Coating A (Figure 7)

Coating B (Figure 8)

13

11

18

14

Coating A (Fig. 7) shows approximately twice the rate of deterioration of thatfor B (Figure 8).

In addition to the specific features relating to activity and catalyst

durability, it is critical that the wash coat does not adversely impact upon the

129

overall performance of the monolith.

During normal service the monolith support is subjected to frequent thermal

cycling. Typically, exhaust gas temperature can reach several hundreds of degrees

celsius in less than a minute from a cold start. In most converter designs the flow

distribution is non uniform with flow concentrated over the central region. This,

coupled with highly exothermic reactions, results in development of strong axial

and radial thermal gradients. Radial gradients due to the relatively cool outer

skin are accentuated in the increasingly favoured non-cylindrical type converter.

These rapidly fluctuating temperature gradients may induce a catastrophic failure

of the ceramic as a result of thermal shock. Low expansion bodies have demonstrated

ability to resist thermal shock during service life in the USA but such problems

would be expected to be more severe in Europe due to different, more severe, driving

patterns and a growing tendency to move the catalyst nearer to the exhaust manifold.

Such problems can, however, be overcome by careful design of catalyst, converter and

exhaust train (ref. 33).

Fatigue type studies of thermally induced failures of ceramic monoliths have

been the subject of intensive investigation (refs. 34,35). However, the

statistical nature of brittle fracture and the difficult nature of the property

measurements has provoked development of a number of empirical tests. The most

useful of these is the burner type test in which the unit is heated rapidly from room

temperature to a predetermined high temperature and subsequently rapidly cooled by

shutting off the fuel. After a fixed number of cycles the unit is removed and

examined visually and accoustically for fracture. If unbroken, it is retested at a

higher temperature until failure is experienced. This temperature is

characteristic of the thermal shock resistance. As with all strength tests of

brittle materials, it is essential that a statistically significant sample is taken

as a measure of the mean property and dispersion.

The thermal shock characteristics as determined by a burner type test for raw

monolith and various types of coated catalyst are shown in Fig. 9. It may be seen

that a coating of washcoat to early formulations resulted in a marked degradation in

failure temperatures to a barely acceptable level. This is attributed to the large-6 0

differential in coefficient of thermal expansion of cordierite (10 x 10 / C) and-6 0

alumina (60 x 10 / C) resulting in thermal stresses at the monolith/wash coat

interface.

One method of preventing such interaction is precoating the monolith (ref. 36)

with an organic material which is subsequently removed during calcination (to fix

the wash coat). The effectiveness of such processes, which have been widely

practised for several years, is shown in Fig. 9 where the differential is reduced to

30/40oC.

800

130

oenCIJeOlCIJ 1000oQ.

ECIJI-CIJ...:;,

C\JLL.c:C\JCIJ:E

~ Min. Spec. Value

IIllI Pre-treat Cat.

4 x 6 inch

D Substrate II '74 CatalystmNew Catalyst

4.66 x 6 inch

SUBSTRATE ICATALYST SIZE

Fig. 9. Mean thermal shock failure temperature (burner test, minimum 15 units) for400 cell ceramic monoliths and catalysts of various types.

However, there are inherent disadvantages due to additional raw materials and

extra process costs. Furthermore process control is more difficult and the total

wash coat deposit feasible on a unit basis is much reduced. In consequence thisprovides an artificial and undesirable limitation on activity, durability and

poison resistance. In response to these limitations, a new process has been

developed which minimises surface interactions without resort to precoats. The

data shown in Fig. 9 indicate that this technology enables the benefits ofstabilised high wash coat levels to be achieved without adverse impact on thermalshock characteristics.

BASE METAL PROMOTERS/STABILISERSThe critical role of Rh in the performance of single-bed three-way catalysts

and its extreme sensitivity to deactivation by exposure to high temperature lean

operation, dictates that any new catalyst development must address the issues of Rhperformance and stability. Rh deactivation in three-way catalysts, after exposure

to high temperature lean ageing has been attributed to a strong Rh/AlZ03

interaction

(Ref. 38). Additional work (Refs. 39,40) has shown this interaction can beeliminated, with substantial improvements in thermal stability, by supporting the

Rh on zirconia. Unfortunately, the incorporation of Rh/ZrOZ

into three-waycatalysts requires complex manufacturing methods which are not suitable for high

speed production. An alternative approach is suggested by work that indicatesRh/Al

Z03interaction may occur preferentially at the grain boundaries of the

support (ref. 41). We have thus chosen to incorporate a stabilizer into the alumina

support system designed to preferentially block this interaction.

Although these results showed that substantial stabilization can be achieved

they also demonstrated the major problem of utilization of single-bed three-way

catalysts - CO and NOx performance around the stoichiometric air/fuel ratio.

Extensive testing of base metal stabilisers failed to secure the desired

improvement in performance. However, incorporation of base metal promoters in

three-way catalysts has been shown to improve CO and NOx performance in the region of

the stoichiometric air/fuel ratio. The two mos t widely used and studied promoters

are nickel and cerium (refs. 42-46). Their influence at equivalent total promoter

loading is shown in Fig. 10.

Conversion 0.02 wt% RhEtf.ciency

(%)

100

80

60

40

20

-_.... Ce Promoted-- Unpromoted- - - NilCe Promoted

co

...~ NOx

.96 .98 1.00 1.02Equivalence Ratio

1.04

Fig. 10. Performance of unpromoted, Ni/Ce and Ce only promoted 0.02 wt. % Rhcatalysts after ageing at 9800C in 1% 02' 10% 02 atmosphere for 1 hr.

A substantial increase in performance, particularly in the stoichiometric

region, is noted for both promoted systems. In that respect the ceria only system

shows superior stability. This, at least in part, can be attributed to the reaction

of nickel and alumina to form nickel aluminate (ref. 42) at elevated temperatures.

That effect, and increasing concern wi th regard to environmental impact of nickel,

has resulted in a trend away from use of that element.

The activity/stability relationships of such catalysts has been further

explored by synthetic gas studies using a reactor system cy~ling between rich and

lean conditions as shown in Table 4.

Under lean condi tions the promoter type and loading has very li ttle impact on

performance or thermal stability. Under rich conditions the promoter type and

loading affects both fresh performance and thermal stability. Substitution of

cerium-only for nickel/cerium results in a dramatic improvement in fresh CO

performance wi th a further more modest improvement seen from an increase in cerium

132

loading. After thermal ageing the "arne performance trend" are obs er ved . However.

only the high cerium ca t aLy s t doe" not show a large drop in performance in comparison

to the fre"h "tate.

TABLE 4

Tr ans I en t performance of f r esh and aged ca t a l ys r s (0.24% Pt/0.05% Rh) under lean andrich conditions. Temperature 400oC. GHSV 100,000 hr- l• gas compo s Lt Lons - base mixof 1200 ppm HC (C 3H6). 500 ppm NO, 14.0% COZ' 0.17% HZ and 10% HZO pI us either rich"pike 2.0% CO, 0.5% O2 for 4 s ec . or lean "pike 0.5% CO, 2.0% O2 for 10 sec. BalanceN2·

Lean Spike Rich Spike(% conver"ion) (% conver"ion)

HC CO NOx HC CO NOx

Ce/Ni Fresh 98 89 34 88 51 50Promoter Aged'" 96 86 30 72 24 46

Ce Promoter Fresh 100 89 39 86 71 54Aged'" 95 87 32 84 46 48

ZX Ce Promoter Fresh 99 89 37 86 76 54Aged'" 96 87 34 81 74 50

"'750oC I 10% H20 I Air I 5 hrs.

The origin of this large effect on CO performance has been explored bymeasuring the rich spike CO performance wi th and without H20 present. COconversions under rich condi tions, after hydrothermal ageing at 9000C in 1% oxygenfor four hours are shown in Table 5.

TABLE 5

Performance of hydrothermally aged 0.16 wt% Pt/0.03 wt% Rh catalysts containingceria promoter in the presence and absence of water vapour. (Conditions otherwiseas shown in Table 4).

CO COConversion (%) with H2O Conversion (%) without H2O

IX Ce Promot ar 54 49

2X Ce Promoter 64 49

6X Ce Promoter 70 49

133

Variation in cerium promoter level has no effect on CO performance when H °isZ

absent from the feedgas stream. With HZO present in the feedgas CO performance is

higher and increases with increased cerium loading. This is consistent with an

enhancement of the water-gas shift reaction upon addi tion of. cerium to Pt/Rh three-

way catalysts. This enhanced performance is at least partially transient in nature

with CO conversions dropping below 50% under steady state conditions.

These results show that a Pt/Rh catalyst system, based upon a stabilized

alumina wash coat designed to minimize the adverse effects of strong Rh/AlZ03

interactions and a high cerium promoter level for enhanced CO performance and

stability, should result in significantly improved three-way catalyst performance

and durability.

This conclusion was confirmed by separate static engine ageing of replicate

catalyst units under stoichiometric, lean and high temperature lean conditions.

Data for the first two conditions (entailing a maximum temperature of 7600C

for 17%

of the cycle) are similar; that for lean ageing is shown in Fig. ll(A). The ceria-

only catalyst shows enhanced stability in the stoichiometric region. Data for the

much more severe high temperature lean cycle is shown in Fig. ll(B).

O ...........,....-.--..,.....--r"-~...,....-,...._..,....._.,.----'

ConversionEfficiency (%)

0.16 wt% PtlO.03 wt% Rh

.98 1.00 1.02Equivalence Ratio

.96o

(B)20

60

40

80

100

ConversionEfficiencr-y:....;.(%....;.)---------------,

0.16 wt% PtlO.03 wt% RhCOHC

.98 1.00 1.02 1.04Equivalence Ratio

.96

60

40

20(A)

80

100

Fig. 11. Performance of high Ce promoted (solid lines) and mixed Ni/Ce promoted(broken lines) Pt/Rh Catalysts after lean ageing at (A) 7600C and (B) 10500C peaktemperatures.

This cycle, which involved lean excursions (0.3% excess oxygen), provokes much

greater deterioration of the catalyst. However, the high ceria system shows

superior stability relative to the mixed promoter system.

PRECIOUS METAL COMPONENT

In the design of an automotive exhaust catalyst the method of precious metal

incorporation plays an important role in the activity, selectivity, durability and

cost effectiveness of the system. In addition, the support material, together with

appropriate stabilisers and promoters, can playa significant role in determining

134

the precious metal location, dispersion and activity. The contribution of some of

these has been mentioned above. This section examines the deposition of precious

metals with particular reference to those presently most commonly found in

automotive catalysts namely platinum and rhodium.

There are a number of possible methods of deposition of the metals onto support

materials; these include impregnation, absorption or ion exchange, co-

precipitation with the support and vapour deposition. Vapour deposition is not

practical on economic grounds and co-precipitation, often used for the preparation

of base metal catalysts, cannot be used because of the problems of recycling and

recovery. Thus precious metal catalysts are usually prepared by the impregnation

or ion exchange of metal salts onto the support materials. A schematic

representation of the ion exchange process is shown below.

Ion Exchange of Metal Salt onto Support

Cationic exchange

Anionic exchange

IS-OH+ + C+

IS-OC+ + H+I

I _S-A + (OH)

C+

S

2+ 2+ 2+Pt(NH3)4 ' Pd(NH 3)4 ' [Rh(NH 3)SClj2- 2- 2-PtCl6 ' PdCl 4 ,RhCl 6

support surface

High pH promotes cation exchange, low pH promotes anion exchange. As the pH is

lowered in a cation exchange regime, interaction between precious metal and the

support decreases until the process can be considered a simple impregnation. The

same process occurs as the pH is raised under anion exchange condi t Lons .

Impregnation is considered a pore wetting process only, the salt being deposited on

the support as the solvent is removed by drying. This has the advantage that the

salt solution is not selectively depleted in precious metal during a continuous

process. If there are ion exchange processes, depletion does occur and the

solution requires frequent monitoring and metal replenishment. Ion exchange does,

however, have the advantage of the potential for selective metal placement whilst

impregnation generally gives a uniform dispersion.

The firing stage, following ion exchange or impregnation of the precious

metal, is an important one in the catalyst preparation. Depending upon temperature

and atmosphere the precursor salt decomposes to ei ther the metal or an oxide. The

effects that can be achieved are illustrated in figures 12(A) and (B) where

decomposition products, particle size and the light-off temperature (for carbon

monoxide) are plotted against firing temperature for salts of platinum and rhodium.

135

The results shown in Figure 12(A) are for platinum deposited on alumina via the

precursor platinum (II) tetrammine chloride. Apparently some CO oxidation occurs

even on the undecomposed precursor, although this may be due to CO enhanced

reduction of the salt. As the firing temperature is increased the precursor goes

through several stages of decomposition, during which the CO oxidation light-off

temperature also increases. The most noticeable effect, however, is the sharp

increase in particle size and light-off temperature when the precursor is fully

decomposed to the metal. Hence, platinum, which does not have an oxide phase stableo

above 400 C, sinters rapidly as the metal and the oxidation kinetics (which are

negative order for CO over platinum) come into play.

TGAResult !-:L:+~~;--------i

300

(Al 200

200 400 600 800 1000

Firing Temperature (OC)

TGA r--t--?f--.:r"'-------,Result I----,~+",.".,.....,~-:;....;:~---{ 330

300~o

tlIl-'0250Ls:

Ol:.J

L..-_,-----,..----._.....,._.....,._-:-/:200200 400 600 300 1000 1200

Firing Temperature (OC)

Fig. 12. Curves showing correlation between metal crystallite size, light offtemperature for CO oxidation and calcination temperature and composition foralumina supported catalysts prepared from (a) platinum tetrammine (chloride) and(b) Claus' salt (1%Rh/A1203)

In contrast, when Claus' salt ( [Rh(NH3\CljC12

) is used as the precursor for

rhodium, the initial decomposition product upon calcination is rhodium metal which

retains a relatively low particle size (Fig. 12(B)). As the temperature is

increased rhodium is converted to rhodium (III) oxide and particle growth increases

markedly. Thus, rhodium sinters as the oxide and a parallel, although not entirely

coincident, increase occurs in CO oxidation light-off temperature.

Thus far, only one precursor of each of the precious metals has been discussed

in the context of the calcination process. In practice, a number of precursors are

available and these can play a major role in determining metal location and

dispersion (ref. 47). The effect of precursor on rhodium dispersion on alumina is

shown in Table 6 where the absorption of NO is used as a measure, of dispersion.

The multiple absorption of NO on rhodium is characteristic of the highly

136

dispersed metal (refs. 37,48) and has also been observed for CO, 0z and HZ (refs.49,50). The ratio of NO to Rh would not normally be expected to be greater than Z.

TABLE 6

The effect of precursor on Rhodium Dispersion (1% Rh on alumina)

Precursor NO/Rh

[Rh(NH3)5CljClz 0.81

[Rh(Cl)6](NH4)3 0.96

Rhodium nitrate 1. 54

Rhodium sulphate 1.78

The dispersion of a precious metal on a support material is also strongly

dependent on the metal loading and the atmosphere in which the catalyst is fired.

These effects are illustrated in figure 13 where NO uptake is plotted against

rhodium loading on alumina for catalysts prepared from Claus' salt and rhodiumchloride. For each precursor, three firing atmospheres, i.e. nitrogen,

hydrogen/nitrogen and air, were investigated. A major difference between the two

precursors is immediately apparent. The catalyst prepared from Claus' salt does

not show a progressive increase in NO uptake above a critical rhodium loading. This

can be related to the relatively low solubility of Claus' salt compared to rhodiumchloride. At higher concentrations, the former crystallises, or sinters as the

salt, during the drying process prior to firing.

,..:w~';" 1.0Eco(;E.. 0.1~

.2~:;..E 0.01

"s:ooz

--- N2.... Air

100.001 L- --~----.J

0.01 0.1 1.0Rh loading...u mole m-2 (B.E.T.)

Fig. 13. Effect of concentration on rhodium dispersion using (A) [Rh(NH3)5CljCIZand (B) Rh C13 as precursors.

137

A second difference between the two is the behaviour when the catalysts arefired in air. Claus' salt initially decomposes to rhodium metal but in the presence

of air is converted to the oxide which sinters rapidly. Thus a worse dispersion of

rhodium is observed when Claus' salt is fired in air than when it is fired in nitrogen

or hydrogen/nitrogen. In the case of rhodium chloride a superior overall rhodium

dispersion is achieved and air firing is not so detrimental to dispersion as it is

for the ammine complex. These observations can again be explained in terms of the

decomposition chemistry of the precursor. Newkirk and McKee (ref. 51) have studied

the decomposition of rhodium chloride, both unsupported and supported on alumina,

in a hydrogen atmosphere. The salt is reduced to the metal at temperatures belowo

200 C and, in the case of the supported material, the hydrogen chloride evolved is

strongly adsorbed by alumina and is not released until temperatures in excess of

600 0C. The decomposition of rhodium chloride in air is slow and produces lower

chlorides or oxychlorides which retard the sintering process. Nitrogen firing is

also likely to produce a lower chloride content.

The role of the support material in determining the activity and selectivity of

precious metal catalysts is critical and there is now a significant literature on

metal support interactions. The effect may be illustrated for rhodium by

considering alumina and ceria as support phases. In the case of alumina the metal

support interaction was investigated by firing 1%Rh/AIZ03

samples in air over a

range of temperatures (table 7).

TABLE 7

The effect of alumina phase and ageing (8 hr s in air at the specified temperature) onrhodium dispersion (1%Rh/A1Z03 ex [Rh(NH3)5CljC1Z)

ALUMINA PHASE

Gamma

Delta

Theta

AGEING TEMP. °c NO/Rh

450 0.86

650 0.42

850 0.00

450 0.74

650 0.40

850 0.00

450 0.33

650 0.Z8

850 0.00

138

The rhodium dispersion becomes progressively worse on the higher temperatureand, therefore, lower surface area alumina phases. NO uptake also falls sharply asthe ageing temperature of each Rh/ Al

Z03phase is increased. The lower NO uptake can

be explained partially by rhodium sintering (as the oxide) and also by a metal

support interaction (Ref. 36). The interaction is less for the high temperature,

less reactive alumina phases but even here NO absorption is not measurable afterageing at 850

0C.The rhodium/alumina interaction is also observed when

temperature programmed reduction (TPR) is performed (Fig. l4(A) and (B).

B4.3

r---.,..---------, 18.

.4 L-_,....-_,....-_,....-_,....-----'

~ 2.8

'"::l;:-2.32~:e-:Cl.64

".:<..~1.36e

"'"e .88't>,..:r

200 400 600 800Temperature Deg. Celsius

200 400 600 800 1000Temperature Deg. Celsius

Fig. 14. Temperature programmed reduction traces for (A) 1% Rh/AIZ03 and (B) 1%Rh/CeOZ catalysts.

Rhodium begins to reduce at relatively low temperatures but the reduction peako

shows a very long tail and reduction is not complete until 800 C. In contrast, when

rhodium is supported on ceria the metal support interaction is weaker and reductionis complete by 2500C, the other peak in this system being assigned to the partial

reduction of ceria itself (Fig. 14(B)). Thus, in preparing precious metalcatalysts, careful attention must be paid to the choice of the support material

since this strongly influences activity, selectivity and durability.In addi tion to individual precious metal/ support interactions, those between

metals themselves must also be considered. Thus, it has been established that Ptand Rh can form alloys, surface enrichment of which, with oxidised Rh species, is

adverse to high activity (ref. 52). Thus, preparative methods must target

carefully the juxtaposition of all key components for optimum performance anddurabili t y ,

CONCLUDING REMARKSHigh performance automotive emission control catalysts are a combination of

the compromises required by the sometimes opposing requirements of their highly

139

dynamic operating environment. In consequence there is no universal solution to

emission control. Choice of support, chemical componen t s and careful control overinteractions is crucial to activity and durability.

Current generation systems achieve high activity and stability by combination

of stabilisers/promoters, controlled dispersion and targetting of precious metal

components to optimise metal support interactions. Over the 12 years of vehicle

application thus far accumulated, substantial improvements have been achieved in

performance, reflecting extensive investment in Research and Development. Over

that relatively short period this has established automotive applications as the

largest single application of heterogeneous catalysts and the principal consumer of

platinum group metals.

During that interval, the scientific basis of heterogeneous catalysis has

advanced substantially. New and improved techniques, e s g , temperature programmed

methods such as TPR and TPO, EXAFS, etc. have become more readily available and have

been/are being applied more widely, together with establi~hed tools to examine

metal-supported interactions. Such techniques have proved of immense value in a

sector previously dominated by empirical techniques which nevertheless remain of

great importance. Although much has been achieved there remain major challenges

from established markets (USA, Japan), large emerging markets (Europe, Australia,

'Korea) and potential markets in developing countries such as Brazil. Notable among

them are the economic and strategic requirements to reduce the absolute and relative

proportions of precious metals without compromising performance. Although

significant progress has been achieved, it is evident that such increasingly

demanding requirements can be met only as a result of improved scientific

understanding of these complex interactions.

ACKNOWLEDGEMENT

The data reviewed in this paper is a selection from that of many workers in the

Research and Development Laboratories of Johnson Matthey world wide. The

particular contribution of Drs. T. Truex and P. N. Hawker in preparation of this

review is gratefully acknowledged.Figures 4, 7, 9, 10 and 11 and Tables 4 and 5 are published by kind permission of

SAE from paper SAE 850128 (ref. 46).

Figure 13 and Tables 6 and 7 are reproduced by kind permission of Kodansha Lt d , ,

Tokyo, from Proceedings of 7th Int. Congo Cat. 1980 (ref. 47).

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Office, 1977, Serial No. 95-11.3. G. G. Robson, Platinum 1986, Johnson Matthey PIc., May 1986, pp 26, 42 and 44.

140

4. W. Berg, Evolution of Motor Vehicle Emission Control Legislation Leading tothe Catalyst Car?, SAE 850384.

5. M. P. Walsh, Global Trends in Motor Vehicle Air Pollution Control, SAE 850383.6. C. de Boer and J. A. Jeyes, The Interaction of Fuel Economy and Emission Control

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10. K. G. Duleep, Future Automotive Emission Control and Strategy, SAE 841244.11. W. D. J. Evans and A. J. J. Wilkins, Catalytic Emission Control Strategies for

Europe, Sci. Total Environ., In Press.12. S. Matsushita, T. Inoue, K. Wakanishi, N. Kato and N. Kobayashi, Development of

the Toyota Lean Combustion System, SAE 850044.13. L. C. van Beckhoven, R. C. Rijkboer and P. van Slaten, Air Pollution by Road

Traffic - Problems and Solutions in the European Context, SAE 850387.14. Y. Kimbara, K. Shinoda, H. Koide and N. Kobayashi, NOx Reduction is Compatible

with Fuel Economy Through Toyota's Lean Combustion System, SAE 851210.15. W. B. Williamson, H. S. Gandhi, M. E. Heyde and G. A. Zawaki, Deactivation of

Three Way Catalysts by Fuel Contaminants - Lead, Phosphorous and Sulphur, SAE79094.

16. R. H. Hammerle and Y. B. Graves, Lead Accumulation on Automotive, SAE 830270.17. B. Harrison, J. R. Taylor, A. F. Diwell and A. Salathiel, Lead Species in

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18. B. J. Cooper, B. Harrison, E. Shutt and 1. Lichtenstein, The Role of Rhodium inPlatinum/Rhodium Catalysts for Carbon Monoxide/Hydrocarbon/Nitrogen Oxides(NOx) and Sulphate Emission Control - The Influence of Oxygen on CatalystPerformance, SAE 770367.

19. W. B. Williamson, J. Perry, R. L. Gross, H. S. Gandhi and R. E. Beason. CatalystDeactivation due to Glaze Formation from Oil Derived Phosphorous and Zinc, SAE841406.

20. A. F. Diwell and B. Harrison, Car Exhaust Catalyst for Europe, Platinum MetalsReview 25(4) (1981) pp 142-151.

21. B. D. McNutt, D. Elliot and R. Dalla, Patterns of Vehicle Misfuelling in 1981and 1982, SAE 841345.

22. R. B. Michael, Misfuelling Emissions of Three Way Catalyst Vehicles, SAE841354.

23. W. R. Pierson, R. H. Hammerle and J. T. Kummer, Sulfuric Acid Aerosol Emissionsfrom Catalyst Equipped Cars, SAE 740287.

24. B. J. Cooper, E. Shutt and P. Oeser, Sulphate Emissions from AutomobileExhaust, Platinum Metals Review, 20 (2)(1976) 20.

25. C. M. Urban and R. J. Garbe, Exhaust Emissions from Malfunctioning Three WayCatalyst Equipped Automobiles, SAE 800511.

26. L. R. Smith and F. M. Black, Characterisation of Exhaust Emissions fromPassenger Cars Equipped with Three Way Catalyst Systems, SAE 800822.

27. J. S. Howitt, Thin Wall Ceramics as Monolithic Catalyst Supports, SAE 800082.28. C. A. Dulieu, W. D. J. Evans, R. J. Larbey, A. M. Verrall, A. J. J. Wilkins and J.

H. Pavey, Metal Supported Catalysts for Automotive Applications, SAE 770299.29. A. S. Pratt and J. A. Cairns, Noble Metal Catalysts on Metallic Substrates,

Platinum Metals Review 21(3) (1977) pp 2-11.30. M. Nonnenmann, Metal Supports for Exhaust Gas Catalysts, SAE 850131.31. H. Schuster, J. Abthoff and C. Noller, Concept of Catalytic Control for Europe,

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R. Gauguin, M. Graulier and D. Pap pee , Thermally Stable Carriers, Catalystsfor Control of Automotive Pollutants, Ed. J. E. McEvoy, ACS Series 143,American Chemical Society, Washington D.C., 1975, pp 147-160.W. D. J. Evans and A. J. J. Wilkins, Single Bed, Three Way Catalysts in, theEuropean Environment, SAE 852096.S. T. Gulati, Effects of Cell Geometry on Thermal Shock Resistance of CatalyticMonoliths, SAE 75071.J. D. Helfinstine and S. T. Gulati, High Temperature Fatigue in CeramicHoneycomb Supports, SAE 852100.Corning Glass Works, U.S. Patent 4,532,228.H. C. Yao, S. .Japa r and M. Sheleef, Surface Interations in the System Rh/ A1203,J. Cat. 50 (1977) 407.H. C. Yao, H. K. Stepren and H. S. Gandhi, Metal Support Interactions inAutomotive Exhaust Catalysts: Rh-Wash Coat Interaction, J. Cat. 61(1980)547.H. K. Stepren, W. B. Williamson and H. S. Gandhi, Development of ThermalResistant Rhodium Catalysts, SAE 800843.J. V. Minkiewiez, B. J. Cooper and M. R. Baxter, Zirconia Supported Pt/Rh ThreeWay Catalysts for High Temperature Operation, AIChE Summer National Meeting,Detroit, Mich. 1981.T. Wangand L. D. Schmidt, Intraparticle Redispersion of Rh and Ptr-Rh Particleson Si02 and Al 203 by Oxidation Reduction Cycling, J. Cat. 70(1981)187.B. J. Cooper and L. Keck, NiO Incorporation in Three Way Catalyst Systems, SAE800461.G. Kim, Ceria Promoted Three Way Catalysts for Auto Exhaust Emission Control,Ing.Eng.Chem.Prod.Res.Dev. 21(1982)267-274.E. C. Su, C. N. Montreuil and W. G. Rothschild, Oxygen Storage Capacity ofMonolithic Three Way Catalysts, Applied Catalysis 17(1985)75.C. Z. Wan and J. C. Dettling, Effective Rhodium Utilisation in AutomotiveExhaust Catalysts, SAE 860566.B. J. Cooper and T. J. Truex, Operational Criteria Affecting the Design ofThermally Stable Single Bed Three Way Catalysts, SAE 850128.B. Harrison, J. P. Heffer and F. King, Rhodium Containing Automobile ExhaustCatalysts, Proceedings of 7th Int.Cong.Cat.Tokyo 1980, pp 768-779.E. A. Hyde, R. Rudham and C. H. Rochester, .JvChem s Soc , , Faraday Trans.1,80(1984)531.S. E. Wanke and N. A. Dougharty, J.Cat., 24(1872)367.E. Kibuchi, K. Ito, T. Ino and Y. Morita, J.Cat., 46(1977)382.A. E. Newkirk and D. W. McKee, J.Cat., 11(1968)370.G. J. K. Acres, The Characterisation of Catalysts. Platinum Metals Review, 24(1)( 1980) pp 14-25.

A. Crucq and A. Frennet (Editors), Catalysis and Automotive Pollution Control!D 1987 Elsevier Science Publishers B.V .. Amsterdam -- Printed in The Netherlands

TITRATIONS OF CARBON MONOXIDE AND OXYGEN ON A PLATINUM ON SILICA CATALYST

143

C. O. BENNETT, L. M. LAPORTA, and M. B. CUTLIPDepartment of Chemical Engineering, University of Connecticut, Storrs,Connecticut, 06268, USA.

ABSTRACTIn the present work we study the reactions of CO with preadsorbed oxygen and

a1so the subsequent react i on of oxygen with preadsorbed CO. The cata1yst is0.12% Pt/Si02 (Cabosil) which has a fraction exposed of 0.47. As thetemperature is changed from 250C to 1920C the surface CO/Pts ratio decreasesfrom 0.85 to 0.68. However, at the same time the ratio of 0 atoms to Ptsurface atoms changes from about 0.53 to 1.62. Thus at 1920C to ratio of 0 tototal Pt atoms is 0.76.

The increase of O/Pt s with increasing temperature is explained by theformation of subsurface PtOx' The oxide formation rate is structure sensitive;it forms at appreciable rates only for highly dispersed Pt such as that used inthis study.

INTRODUCTIONThi s work has been undertaken as part of a program to add to our knowledge

of the oxidation of CO over Pt via models based on elementary steps(refs.l,2,3,4). Here we measure the reaction of CO(g) with adsorbed oxygen,and 02(g) with adsorbed CO. These processes, although not elementary steps,are simpler to analyze than the full reaction. In the present study we areconcerned only with the amounts of adsorbed CO and oxygen. As such, themeasurements are equivalent to the titration of one adsorbed species by theother gaseous species. We are particlarly interested in the effect oftemperature. The fraction exposed (FE) of the Pt particles supported on Si0

2(Cabosil) may have an effect on the titrations also.

At about 250C we have found that both titration reactions proceed slowly, sothat it is convenient to work at higher temperatures. It is known that oxygenreacts re l uctantly with a Pt surface covered by CO (refs .5,6,2). In addition,when CO reacts wi th an oxygen-covered surface, onl y part of the oxygen may beremoved at ambient temperature (refs.7 ,8,9). Our titrations are performed at600<T<192 0C, a range in which each constituent appears to be completely removedby the other.

Titrations have long been used for the determination of the FE of supportedmetals, usually involving the couple H

2-0 2(refs.l0,11,12). These methods are

144

based on an assumed stoichiometry of one adsorbed oxygen atom per exposed Pt

atom (O/P\ = 1.0). It is inconvenient to measure the H20

produced (some may

be adsorbed on the support), and the 02 disappearance has not usually been

measured. Thus the amount of H2

consumed is the basis of the calculation ofthe fraction exposed, along with the assumed stoichiometry. The observed

departures from O/Pts

1.0 for Pt/ 5i02

systems has been extensively

investigated (refs.I3,I4).

For s i l i ca-supported metal s , for which CO2

is not adsorbed on the support,

it is attractive to use the couple CO-02•

Then one can measure for the

oxygen-covered surface the CO disappearance and the CO2

production, and for the

CO-covered surfaces, the ° disappearance and the CO production. Now there is2 2no need to assume a stoi chi ometry; the amounts of adsorbed CO and 02 are

measured directly, and the carbon and oxygen balances can be verified. It is

st i l l necessary to assume non-dissociative adsorption of CO (true for most

metal s except iron and other typical methanation catalysts) and dissociative

adsorption of ° (true in general at T>250C).

Of course any chemisorption2 -method requi res the use of a ratio O/Pt , H/Pt , or CO/Pt , supposedly obtained

s s sfrom experiments on polycrystalline foils of pure metal. As the particle size

decreases, the di stribution of exposed crystal faces may change, and other

geometic and electronic effects may come into play. In other words, the ratios

(adsorbed gas/surface metal atoms) may be structure sensitive. Thus a

calibration of chemisorption methods against physical methods (EXAFS. electron

microscopy) is always desirable (ref.I5).

Titrations by CO of oxygen-covered Pt powder, Pt/C, and Pt/Al 0 have been0 2 3

performed at 25 C by Wentrcek et a l , (refs.I6,I7). However, they used the

stoichiometry usual at that time, i ,e , O/Pt =CO/Pt = 1.0, which lead Flynns s

and Wanke (ref.I8) to criticize their methods. This assumed stoichiometry is

not consistent with other results for polycrystalline Pt obtained by the same

group (ref.I9,20). In these articles it is proposed that CO/Pt = 0.75 andO/Pts = 0.5, at 250C. s

It is well established that subsurface or even bulk platinum oxides can form

at temperatures as low as 25 0C (refs.I3,I4,21,22) for highly dispersed

supported Pt. For bulk Pt, oxides also form, although higher temperatures are

required for detectable rates. The thermodynamics of the formation and

dissociation of s-PtO on a Pt wire have been studied by Berry (ref .23), whoo 2 -1 0 -1 -1

finds ilH =-42 kcal mol and lIS = 49 cal mol K , from which Table 1 can

be estab1 i shed.

Berry (ref.23) found that on a plot of rate (positive for oxidation) versus

temperature, an isobar starts at a negligible rate at low temperature, rises as

the kinetics become more favorable as the temperature increases, pa~ses througha maximum and then falls to zero at the temperature given in Table 1 for the

145

particular isobar. Further increase in temperature produces negative rates,i.e., PtOz dissociation. This is all completely normal. Thus in ourexperiments (PO = 30 mbar) we can expect to form Pt 0z at temperatures below474°C, if the kfnetics are favorable.

Other oxides may form, for example Pt304 (ref.Z1) or Pt ° (ref.Z4).

TABLE 1Equi] i br i um for Pt + Or Pt 0z

Oxygen partialPressure, mbar

Z16100301010-110-310- 5

Temperature,°c530507474446349Z74Z15

For bulk platinum there is some possibility that siliconor other impuritiesmay segregate to the surface and serve as a getter for oxygen, as discussed inrecent articles (refs.Z5,Z6,Z7), but this process occurs at temperatures above5000C, far above the range used in our work. In addition, for ourhighly-dispersed Pt, segregation would not appreciably change the surfacecomposition.

EXPERIMENTAL METHODSThe O.lZ wt % Pt/Cabosil M-5 catalyst was made by standard incipient wetness

techniques, using H/tC1 6 HZO as the precursor. After drying the powder waspressed into disks about O.lmm thick, as for infrared samples. These were thenbroken up and sieved so that the typical particles have a largest dimension ofabout 0.8mm. Thus a bed of such particles causes a low pressure drop, andintraparticle concentration and temperature gradients are also very low. ThePt loading is so low that X-ray diffraction did not lead to any usefulinformation about the particle size of the Pt crystallites.

Ultra-high purity HZ' He, 0z' and CO were used to make the necessarymixtures: 1 mol % CO, balance He, and 3 mol %0z' balance He. Ar was sometimesused as a tracer. The inert gases were passed through suitable traps to reduceimpurities especially HZO and 0z' to a minimum. However, it was found thatpassage of even the highly purified inert gases over a disk in the i r cell(typically 10mg) which had been saturated with CO at Z50C led to a continuousdecrease in the ir band for adsorbed CO. Thus it was preferable to do thetitration experiments in a large reactor with recycle (gradientless

146

conditions), so that the amount of impurities which might react with the

catalyst during purges was small compared to the amount of adsorbed CO. The

titration experiments were done with 4.0g of catalyst at a flow rate of 34

mL/min to the reactor; recirculation was obtained by a metal bellows pump. A

switch from Ar to He and back indicated a well mixed reactor of 32 mL volume.The fl ow system was arranged as al ready described (ref .2) so that a step

function in the composition of the feed to the reactor could be produced with a

rise time of about 2s. The composition of the effluent from the reactor was

measured by a mass spectrometer with a continuous inlet system. The infrared

system has also been described (ref.2).

Before starting any CO/02

cycles, the catalyst was reduced in flowing H2

at285

0Cfor 15h, and then cooled in helium to the desired temperature.

RESULTS

Some experiments were performed by direct switches between 1% CO and 3% 0 ,2

without any intervening inert-gas purge. After a few cycles, a reproducible

result illustrated by Fig. 1 is obtained at 600C. At zero time, at the left

edge of Fig. 1, the surface is saturated with oxygen at 600C.

At this time the

feed of 1% CO reaches the reactor, but the 02(g) concentration falls slowly

because of the residence time in the reactor (t = 32mL/34mL min -1 = 1 min).r

Thus gas phase O2

and CO react during the initial period, and the COconcentration rise is delayed. As the gaseous oxygen is depleted, the CO then

consumes the surface oxygen and then occupies the Pt surface up to its

saturation coverage at 600C.

1288 10

2d-JU N

~ 05 0" ~0 0

U 1~0

1.0 ......--~------fM--:--~-----,3

147

The right side of Fig. 1 shows the next switch, as 0 (g) replaces CO(g) over2

the CO-covered surface. As al ready observed in other studies (refs .5,6,2)

oxygen finds very few adjacent sites for dissociative adsorption until thegaseous CO concentration falls to a critical level, so that a small decrease inCO coverage can occur. At this point there is a sudden production of CO

2and

consumption of oxygen.The results of Fig. 1 permit the calculation, through material balances, of

the amounts of CO and ° adsorbed based on the various curves of Fig. l.However, the calculations involve the subtraction from each other of several

integrated quantities. The adsorbed quantities calculated in alternative wayswere not in satisfactory agreement because of their being based on relatively

small differences of various pairs of measured quantities. Rather than pursuethe description of this process here, we turn to a different set of

experiments, which have given satisfactory results.

The interpretation of the data becomes much more staightforward when an

inert gas purge is used between the two gas mixtures; the results at 600C

areshown in Fig. 2. Although no oxygen desorbs into an inert gas from anoxygen-covered surface, such may not be the case with CO. We defer a detailed

discussion of this matter until after an analysis of the results of the type

shown in Fig. 2•.1.0r--~=-""'------lf---:::;1111'--------r3

ONU~ 0.5..oo~o

2

16 0MIN

Fig. 2. Response curves with inert gas purge. T = 600C.

In Fig. 2, the CO2

peak on the left corresponds to the amount of O(ads) onthe surface after exposure to 02 in the previous cycle corresponding to the

right side of Fig. 2. The mols of ° ads, NO* ' is given, for small

concentrations of the active gases by

148

( 1)

-1 -1where q is the flow rate, mL min and [C02J is mol mL During the sameperiod, the amount of CO which reacts plus that which adsorbs is given by

NCO = q J'([Ref] - [COJ) dt = B (2)o

Here the concentration [Ref], shown only in the left side of Fig. 2, is theresponse the CO would have if it did not react or adsorb. This curve can becalculated from the res i dence time obtained from inert gas swiches as alreadymentioned. In all these calculations the flow rate and concentrations aremeasured at the same reference (ambient) temperature.

From the right side of Fig. 2 we can calculate

and

NO = q f([RefJ - [02J) dt = 0o

(3)

(4 )

where NO is the sum of the oxygen reacted with the preadsorbed CO* and theoxygen adsorbed. Thus there are two ways to calculate NCO*:

i) NCO* C, CO2 production (5 )

ii) NCO* = B - A, CO disappearance

Similarly, there are two ways to calculate NO*:

(6 )

i ) NO* A, CO2 production (7)

ii) NO* = 20 - C, 0 disappearance (8)

The titration experiments were done at 600 C (Fig. 2), 1000C, 140°C and 1920C

(Fi g. 3).

149

1.0.,.....-----"""":::::::=--..."...---:::::-------~

'"ou~ 0 5

~ .oo~o

8 12 16

01\1~o

Fig. 3. Response curves with inert gas purge. T = 1920C.

We next consider some evidence as to whether any CO* desorbed during theinert gas purge at the various temperatures. These results indicate that thereis no measurable desorption.

1. After saturation of the surface by CO(g), the feed is switched to inertgas, and [COJ is measured as a function of time (not shown in Figs. 2. and 3.).Then the amount of CO desorbed can be calculated by Eq. (2); a negative amountwould indicate desorption. In all cases the quantity is zero to withinexperimental error.

2. By using Eqs. (1-6), the quantities of CO* have been calculated by thetwo methods, Eqs. (5) and (6), and the results are shown in Fig. 4. Since Eq ,(5) gives the amount of CO~ after the inert gas purge, and Eq. (6) gives theamount of CO adsorbed with CO in the gas phase, the good agreement of the twomethods for CO* shown in Fig. 4 indicates that negligible CO desorbs during thepurge.

3. The same switches of feed concentration have been used in theflow-through infrared cell, for the same 4 temperatures. After the switch to

-1CO, the absorbance of the single CO band observed at about 2070 cm has beenmeasured, and corrected for emi ss i on. In Fig. 6 thi s infrared measurement ofthe quantity of CO* has been compared with that obtained from Fig. 4, and it isevident that the agreement is good.

150

l-

'"tf)

~S ~

~-t( 2 8

0-~

0 x

U-l0L~ II

0 40 120 2mDEGREES C

Fig. 4. Quantity of CO adsorbed obtained by oxygen titration after exposure atCO, both at T,oC. x , calculated from CO disappearance; 0, calculated from CO 2production.

~o 120DEGREES C

200

Fig. 5. Quantity of 0 adsorbed obtained by CO titration after exposure to 02'both at TOC. x , calculated from 02 disappearance; 0, calculated from CO2production.

By using Eqs , (1-4) and Eqs. (7-8), the quantities of 0* shown in Fig. 5

have been found. Again the quantities of 0* calculated by Eqs. (7) and (8) are

in reasonable agreement, and we know that oxygen should not desorb.

151

f---

~ 2/r--J<:f---<{o~018() ...Jo~:J..,

028

woz-r

O.aJ~oIf)en-r

o 50 100 150DEGREES C

200

Fig. 6. Comparison of mass spectrometer(MS) data and infrared (IR) data for COadsorption. The absorbance scale is made so that the two curves give the sameordinate at 600 C.

Having given these arguments for the validity of the results of Figs. 4 and5, we present in Table 2 the amounts adsorbed at all the temperatures.Included in Table 2 are the adsorbed quantities estimated at 25°C byextrapolation of the curves of Figs. 4 and 5. Satisfactory measurements werenot possible at 250C because the reaction rates were too low.

TABLE 2Adsorption of O

2and CO

Temperature, Oxygen°c umol/g cat. O/Pt

Carbon Monox i deumol/g cat. CO/Pt

6010014019225*

1.792.574.034.681.56

0.290.420.660.760.25

2.402.282.161.952.47

0.390.370.350.320.40

* Extrapolated values.Oxygen partial pressure, 30 mbarCO part i a1 pressure, 10 mbar

It is of interest to find also the values of CO/Pt and O/Pt and theirs srefers to a surface (exposed) atom of

chosen a reference value of CO/Pt sThis choice is based on the work of

variation with temperature. Pts

platinum. To compute these values we have0.85 at 250C, as in a previous study (2).

152

Freel (ref.28) and of Nishiyama and Wise (ref.20). It is then straightforward

to calculate the values in Table 3.

TAI3LE 3Surface concentrations

Temgerature, O/Pt s CO/PtsC

60 0.62 0.83100 0.89 0.79140 1.40 0.74192* 1.62 0.68

25 0.53 0.85

* Extrapolated values

3 it has been assumed that at 25°C the measured

0.5O/Pt sin

COl Pt =

so that the fraction exposed, FE = Pt IPt,sbe 0.53. This value agreesat 25°C O/Pt

sthe literature,

In preparing Table

0.40 corresponds to CO/Pt = 0.85,sis 0.47. Thi s then requi res thatwith that generally proposed

(refs.l0,12,16,18,19,20).

DISCUSS ION

As the temperature of O2

adsorption is raised, the CO2

peak produced startsto show a shoulder, and in Fig. 3 a second peak is visible. As suggested byHerz and Shinouskis (ref.9), it is reasonable to assign the more reactive peak

to surface 0*, and the later peak to oxygen from subsurface oxide. Since the

formation of bulk oxide is thermodynamically favored over the temperature rangeof the present work, the increasing oxygen isotherm of Fig. 5 is explained bythe kinetics of this activated process.

o 0The reference values (25 C) of CO/Pt = 0.85 and O/Pt = 0.5 at 25 Care

s sbased on studies for polycrystalline Pt. For Pt (110) a ratio COlO of unityhas been found (refs.5,29). The influence of other faces in polycrystalline

samples apparently changes this ratio to about 1.7.

The results of Table 3, in which O/Pt goes up to 1.62 at 1920C,

alsossupport the idea of the formation of a subsurface oxide. Several infraredstudies also support this interpretation. When supported Pt is oxidized at

oabout 250 C and then cool ed to room temperature, CO can be adsorbed on thesurface without oxidizing all the Pt, as already mentioned. This type ofexperiment shows a band at about 2120 em-I, interpreted as arising from CO

adsorbed on oxidized platinum (ref.7,8,9).

Salmeron et al , (ref .30) have exposed Pt single crystal surfaces to 02 at

70Uoc and above. They propose that oxygen di sso1ves in pl at i num; .t nen when the

153

crystal is cooled to lower temperature, diffusion of the oxygen toward the

surface is slow but its virtual pressure is high, so that subsurface oxides are

formed. Yeates et al , (ref.31) have proposed that oscillations in the CO/02

reaction on Pt single crystals at about 300°C are driven by coupling between

the surface reaction and the oxidation and reduction of the subsurface layers

of Pt. However at 3000C

the oxidation and reduction seem too slow for bulk Pt

for thi s explanat ion to be val id (refs .30,32). On the other hand, for highly

dispersed, supported Pt , for which oxidation and reduction are rapid, this

model might be appropriate. As pointed out in the review by Razon and Schmitz

(ref.Z7), it is also difficult to evaluate the effects of traces of impurities

1 ike Si , In some cases rapid surface reconstruction may be the driving force

for oscillations (ref.32).

ACKNOWLEDGEMENTS

We are grateful to the National Science Foundation for support under grant

No. CPE 8210100-01.

We also thank Dr. J. S. Chung for assistance in the experiments.

REFERENCES

1 C.O.Bennett, Catal. Rev.-Sci. Eng., 13 (1976) 121.2 S.M. Dwyer and C.O. Bennett, J. Catal. 75, (1982) 275.3 M.G. Goodman, M.B. Cutlip, C.N. Kenney, W. Morton and D. Mukesh, Surface

ser.. 120 (1982) L543.4 D. Mukesh, M.B. Cutlip, M. Goodman, C.N. Kenney and W. Morton, Chem. Eng.

Sci. 37 (1982) 1807.5 H.P. Bonzel and R. Ku , Surface Sci. 33 (1982) 91.6 H.P. Bonzel and J.J. Burton, Surface Sci. 52, (1925) 223.7 H. Heyne and F.C. Tompkins, Trans. Faraday Soc. 63, (1967) 1274.8 E. Kikuchi, P.C. Flynn and S.E. Wanke, J. Catal. 34, (1974) 131.9 R.K. Herz and E.J. Shinouskis, Appl. Surface Sci., 19 (1984) 373.

10 J.E. Benson and M. Boudart, J. Catal. 4, (1965) 704.11 D.E. Mears and R.C. Hansford, J. Cat a l , 9, (1967) 125.12 J .R. Wil son and W.K. Hall, J. Catal , 17, (1970) 190.13 T. Uchijima, J.M. Herrmann, Y. Inoue, R.L. Burwell, Jr .; J.B. Butt and J.B.

Cohen, J. Cat al , 50 (1977) 478.14 M. Kobayashi, Y. Inoue, N. Takahashi, R.L. Burwell, Jr., J.B. Butt and J.B.

Cohen, J. Cat a l , 64, (1980) 74.15 B.J. Kip, F.B.M. Duivenvoorden, D.C. Konigsbergh and R. Prins, In

preparation.16 P. Wentrcek, K. Kimoto and H. Wise, J. Catal. 33 (1973) 279.17 P Wentrcek and H. Wise, J. Catal. 36, (1975) 247.18 P.C. Flynn and S.E. Wanke, J. Cat a l , 36, (1975) 244.19 B.J. Wood, N. Endow and H. Wise, J. Cat a l , 18, (1970) 70.20 Y. Nishiyama and H. Wise, J. Catal. 32 (1974) 50.21 R.K. Nandi, F. Molinaro, C. Tang, J.B. Cohen, J.B. Butt and R.L. Burwell,

Jr., J. Cat a l , 78, (1982) 289.

154

22 T. Fukishima, J.R. Katzer, D.E. Sayers and J. Cook, 7th Inter. Conqr , onCatalysis, Tokyo, 1980, paper Al.

23 R.J. Berry, Surface Sci. 76, (1978) 415.24 C.G. Vayenos and J.N. Michaels, Surface Sci. 120 (1982) L405.25 H.P. Bonzel, A.M. Franken and G. Pireng, Surface Sci. 104, (1981) 625.26 H. Niehus and G. Comsa , Surface Sci. 102 (1981) Ll4.27 L.F. Razon, R.A. Schmitz, Cat a l , Rev.-Sci. Eng. 28(1), (1986) 89.28 J. Freel, J. Cat al , 25, (1972) 149.29 R. Dueres and R.P. Merrill, Surface Sci 55, (1976) 227.30 M. Salmeron, L. Brewer and G.A. Somorjai, Surface Sci. 112, (1981) 207.31 R.C. Yeates, J.E. Turner, A.J. Gellman and G.A. Somorjai, Surface Sci. 149,

(1985) 175.32 M.P. Cox, G. Ertl, R. Imbihl and J. R"tistig, Surface Sci. 134, (1983) L517.

A. Crucq and A. Frennet (Editors), Catalysis and Automotive Pollution Control(i:~' 1987 Elsevier Science Publishers B.V. Amsterdam - Printed in The Netherlands

THE A/F WINDOW WITH THREE-WAY CATALYSTS. KINETIC AND SURFACE

INVESTIGA TlONS.

E. KOBERSTEIN and G. WANNEMACHER

Physico-Chemical Research Dept., Degussa AG, ZN Wolfgang, P.O.Box 1345,

6450 Hanau 1 (Federal Republic of Germany)

ABSTRACT

155

Kinetic measurements, infrared surface investigations under running reaction condi-tions and model calculations for the system CO, NO and 02 on precious metal catalystsare reported. Sorption behavior of these compounds and moss transfer influencesdetermine the width of A/F windows on the leon side. Larger increases of the leon A/Fwindow width can only be attained in a compromise by reducing absolute reaction rates.

INTRODUCTION

Satisfactory conversion of all three pollutants (HC, CO, NO) by means of three-way

catalysts is only possible if the oxygen partial pressure in exhaust gas - with three-way

concepts determined by the A/F ratio - is kept within certain limits: the so-called A/F

window (see Fig. 1).

['/,1 Conversion In on Integrol reoctor

II

I I100 ---.-.W-~ o co

0-0-0-00...-""'1 o HC

80 0/ ·t • NOx

0--"-- /~ i !\60

/ 'I40 I Io I I

/' II20 o I I '\I I

II ~I I

14.1 14.4 14.7 15.0 15.3 AIF

Fig. 1. A/F window with three-way catalysts.

1fi6

Modern engine concepts (fuel cut, lean operation, etc.) require catalysts operating

with leaner mixtures, i.e., with regard to NOx conversions an extension into the lean

range is desirable.

Experimentally, catalysts have been found which show considerable differences re-

garding this property (see Fig. 2). A more or less pronounced conversion maximum for

NOx as function of temperature is found with all three-way catalysts.

1'1,) NO- Conversion inon Integrol reactor

100

80

60

40

20

GHSV ~ 10000 h-'T ~ 3300 [

o-L--.----,---r----.----,---,---,-_14.7 15.3 15.9 AF

Fig. 2. Catalysts with different A/F window width in the lean range.

To find the reasons for this behavior and to get hints on the principal parameters

limiting A/F window enlargements, some basic investigations were initiated, part of

which are described in this paper.

As a first approach, the study was confined to the system: CO, 0Z' NO and HZ and

precious metal model catalysts (Pt, Rh on ')'-alumina support) as well as technical

automotive exhaust catalysts.

The specific influence of non-precious metal oxide additives which definitely influ-

ence A/F windows will be reported in another study.

157

EXPERIMENT AL

c::atalyst samples

TABLE 1

cordierite monolith 400 cells/inch 2

100 gil -y-alumina + NPM-oxide additives1.3 gil PtIRh = 5: 1

Cat. No.1:

Cat. No.2:

Cat. No.3:

somples were cut out of 0 commercially used three-waytype OM 2966support:washcoat:precious metal:

-y-alumina pellets (spheres; d 3 rnrn) 0.3 % Pt

-y-alumina pellets (spheres; d 3 mm) 0.3 % Rh

catalyst

Cat. No.4: model catalyst: Rh on non-porous a-A1 20 3 spheres (d 3 mm)

Cat. No.5 and 6: samples for IR-measurementself-supporting -y-alumina cylindrical pellets(d =13 rnrn, height approx. 0.1 mm)precious metal: 5 % Pt (Cat. No.5)

5 % Rh (Cat. No.6)

Kinetic measurements

Integral reactar measurements. Far integral reactor measurements either engine test

units or laboratory equipment with model gas mixtures were used. Both are described in

(ref. 1).

Differential reactor measurements. The apparatus used is indicated in Fig. 3. Defined

gas mixtures from a dosing device could be switched either to a differential reactor

with outer loop (reactor A) or to a reactor with internal loop (Berty reactor/reactor B)

(ref. 2). Both reactors could be operated with high gas circulation (> 15: 1) guaranteeing

differential reactor conditions. Reactor A consisted of a quartz tube (18 mm diameter)

which could be heated up to 1100 °e by a Pt tube furnace. A correspondingly cut piece

of monolith catalyst (length 5 mm) - with about 25 open channels - or a corresponding

pellet layer were fixed within the quartz tube. A disadvantage of this device are re-

maining temperature gradients which can be avoided with the Berty reactor. On the

other hand, reactor B could only be heated to a maximum temperature of 550°C. For

monolithic catalysts a piece of 48 mm diameter and 5 mm length was fitted into the

Berty reactar tube•.Pellets were pierced and stringed on a stainless steel wire.

Both reactors could be switched alternatively to the analytical systems indicated for

CO, CO 2, °2, NO analysis. The curves plotted in Fig. 5 - 12 represent reaction rates

(moles converted divided by residence time and geometric surface area of catalyst).

Infrared spectroscopy. The system for measuring surface infrared spectra (see Fig. 4)

consists of a section for dosing model gases (in this case: CO, °2, NO), two IR

measuring cells, the vacuum pumps, a quadrupol mass spectrometer for gas analysis and

an infrared spectrophotometer (here: IR Perkin Elmer 325). The measurement cells can

158

(AU.

CO I Co, - IR-Analywr01 - 4nalySfr

110 - o.m;hJllil1P.Ql'lP4nolyst'

A100UIl

B4l)(fh

~YSI J I ~r~'" I

~0

I~.:T:'-~ , CATAlYSI

i660l/h r.- ~

'f ~ ~ lURliINE

C")

fl) ~'~~ "G; ,~I)

Fig. 3. Differential reactor experimental set-up.

Fig. 4. Infrared equipment: 1: pressure gauge; 2: pressure gauge (high vacuum);3: mass spectrometer; 4: oil trap; 5: adsorption trap; 6: pressure converter;7: high vacuum valves; 8: gate valves; 9: saturator; 10: capillary tubes;Pl, P2: rotary pump; P3: turbo pump.

159

be brought into the IR spectrometer olternatively. Cell A is constructed according to

(ref. 3) and allows measurements during running reactions on disk-shaped catalyst

sampels up to a temperature of 530°C. With cell B according to (ref. 4) samples can

be treated up to 1200 °C aut of the IR beam and then brought back into the

spectrometer without interrupting the' vacuum or gas atmosphere.

KINETIC INVESTIGATIONS

While the A/F window on the "rich" side is limited by stoichiometry (oxygen con-

centrations approaching zero) and means to shift this side of the A/F window to

"richer" conditions are well-known (e.g., surface oxygen storage; enhancement of the

water-gas shift reaction, e tc.), the "leon" side limit is obviously defined by a compe-

tition of reaction rates for oxidizing carbon monoxide, hydrocarbons and hydrogen by

nitrogen oxides or oxygen.

Due to their importance in actual exhaust gas, reactions a) and b) and their combi-

nation were selected for this special study.

a) CO + 1/2 02

b) CO + NO

(1)

(2)

Nearly identical kinetic results are obtained with pellet or monolith samples, Le.;

there is practically no difference in this regard between the outer layer of a pellet and

the coating of a monolith, if ')'-alumina and activation are the some.

Reaction a): CO + 1/2 02 - CO2The results of kinetic measurements in the differential reactor described above are

shown in Fig. 5 and 6. Above temperatures of approx. 400°C on apparent first-order

kinetic for CO and for 02 under the reaction conditions indicated is found until

complete consumption of one reaction partner near the surface enforces zero-order

kinetic. This is due to rate limitation by boundary layer diffusion. At comparatively low

temperatures « approx. 250°C) self-poisoning occurs for CO, while on apparent first-

order kinetic is measured for °2,

The corresponding Arrhenius plots (see Fig, 7) in the temperature range between

100°C and 800 °C - usual operating temperatures of automotive exhaust catalysts -

show four clearly separated sections which are interpreted as follows:

1) Ea - 100 kJ/mol: chemical reaction controlled

2) Ea 25 kJ/mol: alumina (wcshcoor ) pore diffusion controlled

3) Ea 6 kJ/mol: boundary loyer diffusion controlled

4) Homogenous gas reaction

160

r.[mol CDlm1s1

0.15

1 ~ fOOoC" PCG ~ GO)baro POI ~ D.015bor

0.1

0.05

I

0.01I

0.02Iom , .

0.04POI[bar]P" [bar 1

Fig. 5. Kinetics: reaction 0) (CO + 1/2 02

r,.lO':mdCDIm's]

-. CO2) T > 400°C. Cat. 1.

4.5

3

1.5

0.01 0.02 0.03 0.04Po,[bil'!Pco Ib..-J

Fig. 6. Kinetics: reaction c) (CO + 1/2 02 -. CO 2) T < 250°C. Cat. 1.

161

P(0: G. G' barPO z: G.C:bor

JOGO':,

1.'89000 [

-10

-8

Fig. 7. Kinetics: Arrhenius plot for reoction a). Cat. 1.

Reaction b): CO + NO - 1/2 N2 + CO 2Fig. 8 to 10 present the data measured with reaction b). At high temperatures very

similar curves are found compared with reaction a), which is due to the controlling

mass transfer influence (see Fig. 8). There is no difference whether the reaction rate is

measured as function of carbon monoxide concentrotion or NO concentration. At low

temperotures different kinetics result depending on whether NO or CO is varied, while

the other component is kept constant (see Fig. 9).

r,(mol rO/m1sJ

o,ms 1,500 0 Co P.o' 0,002 bar

" Pco' 0.002 bar

0,1

0.005

0,001 0,002 0,003i

0,004 Pco [bar]P 10 Ibor)

Fig. 8. Kinetics: reaction b) (CO + NO - l/? N2 + CO 2) T > 400°C. Cat. 1.

162

r, -10'[mol(Olm's]

o3

2T ,24GO(

o p '0' 0,002 bar., p [0 ' 0,G02 bar

0,001 0,002 0,003 0,004 p [0 lbcrlP,o[bor]

Fig. 9. Kinetics: reaction b) (CO + NO ---+ 1/2 N2 + CO 2) T < 250°C. Cat. 1.

lnr,Prc' 0,005 bar

P MO' G.005 tnr

-8

-10

_12 9000(

0,8 I,D6000C 5000 ( :OOO(

1,2 1,4 1,6

Fig. 10. Kinetics: Arrhenius plat for reaction b). Cat. 1.

The Arrhenius plot (see Fig. 10) is olso cornporoble and interpreted as for reaction

a), with the exception that no homogenous gas phase reaction (step 4) could be

detected.

Combined reactions a) and b)

Fig. 11 to 12 show "Arrhenius diagrams" where reaction rates dnCO/dt resp. dnNO/dt

under the reaction conditions indicated are plotted against the reciprocal temperature.

Parameters are: fresh and aged technical catalyst 1 (Pt /Rh); high-surface (porous) and

low-surface (non-porous) catalyst; single precious metals Pt and Rh. In all cases a

similar pattern is obtained: When CO conversion becames boundary layer diffusion

controlled, the reaction rate for NO x canversion begins to drop. The difference between

the abso)ute reaction rates for reactian a) and b) is considerably larger for pure

platinum cam pared with pure rhodium or Pt/Rh combinations. As could be expected, the

curves for the aged catalyst are shifted to higher temperatures.

The pattern described above is most clearly shown with high-surface (parous)

catalysts, while low-surface (non-porous) catalysts give nearly identical reaction rates

on Rh over a large temperature range, resulting in relatively higher NO x conversions.

The latter catalyst also gives higher NOx conversions in the lean range, increasing the

A/F window width (see Fig. 2). It must be pointed out that the absolute reaction rates

per geometric catalyst surface are of course much greater with the high-surface

catalyst.

Inri/co10(101,,"0)

-3

-4

-5

-6

1.0 1.2 1.4 1.6

CoU fresh a CO - NOaged 0 CO -NO

Fig. 11. Kinetics: Arrhenius plot for combined reactions a) and b);PC O = 0.01 bar; Po 0.0065 bar; P NO = 0.001 bar;

2Cat. 1: fresh and aged.

164

In rs,coIn(10r~oi

[01.2Cot4

o co • NOo CC .. NO

1,61,41.21,0

-]

-4

-5

-6-7

-8L---r-----"--.--~-,___---"--.._--'--,_---

Fig. 12. Kinetics: Arrhenius plot for combined reactions a) and b);P CO= 0.01 bar; Po = 0.0065 bar; P NO = 0.001 bar;Cat. 2 and 4. 2

INFRARED SURFACE SPECTROSCOPY

With the equipment described in chapter "infrared spectroscopy" the absorbance of

the Pt-CO resp. Rh-CO bands on catalyst 5 re sp, 6 were measured as function of

temperature and oxygen partial pressure under running reaction conditions. The OfF

value (ratio: oxidant/fuel) was changed either by oxygen or nitrogen oxide variation.

The results are shown in Fig. 13 to 17.

For reaction a) similar patterns are obtained for Me-CO absorbance as functian of

oxygen partial pressure and temperature with metallic (reduced) catalysts 5 and 6 (Pt

resp. Rh). With Pt at low temperatures, CO coverage also in the lean range is found,

while at higher temperatures and increasing oxygen partial pressures a step function

indicating a sudden CO depletion close to stoichiometry was detected. In the case of

rhodium the only difference are comparable CO coverages at lower temperatures and a

higher density of the step function with regard to oxygen partial pressure. In case of

reaction a) the CO absorbance, Le., the CO coverage, is completely reversible.

Reaction b) shows a different behavior. While on catalyst 5 (Pt) CO coverage shews

a similar pattern as with reaction a), it is not further reversible with increasing tem-

peratures. Measurements at lower temperatures after high temperature exposure

indicate only small absorbances. Obviously, a large part of the surface is now blocked

16.5

by some reaction intermediate which still has to be characterized. On reduced rhodium

(catalyst 3) rapid CO depletion is found at lower temperatures and at very low NO

partial pressures, indicating a displacement of CO by NO or by an intermediate

product. With increasing temperatures the step function mentioned above is formed

again. After heating of catalyst 6 for 4 hours at 800°C in air ("oxidized Rh"), hardly

any Me-CO absorbance could be measured. This confirms the reversible poisoning effect

of Rh by oxygen measured in integral reactors.

If 'half of the carbon monoxide is replaced by hydrogen in case of reaction a), a

considerable shift of the Pt-CO absorbance "step" into the lean range is found (see

Fig. 17).

Cot.5(PI}

ZOOO[I

~\( :-.0,011 0,012 0,013

pOllbal0,01

ca- l/Z OZ- CO 2Pco ~ 0,02 bor

0,0090,008

26'lJO( ~\JOOO[

0,8 +--..l-'+~ ~J4O"( \0,6 -&=-=--"T-oA....L..o:::::::::--....ll:I'[~cc

0,4 ---'4"ii?C-:)\'--'"0,2 46lJO~

0,1 \

1,0

1.2

AbsorboncePI- [·0

v~Z100[m"

Fig. 13. Infrared: Me-CO absorbance under running reaction conditions for Cat. 5 (Pt);reaction a).

166

AbsorbarcePI-e-ov,2100cm- 1

3

CO· NO --C02+ 1/ 2N2P[0 ' 0,02 bar

150"C/

o - 300°C2

<1 - 250°C

0-----__-

~ 300°C (afte.-400°Cl~--------.~_ "-350°C" /400"C

0.01 0.02 0,03 0,04 P Holbar]

Fig. 14. Infrared: Me-CO absorbance under running reaction conditions for Cot. 5 (Pt);reaction b).

Fig. 15. Infrared: Me-CO absorbance under running reaction conditions for Cat. 6 (Rh);reaction a).

AOscrOonce ~h I ;: ~ 'xv,ZO?Ocm'

0.6 ISGOC2S00C

co +NO-COz+'/zN)p :~' --'

167

0,1.

0,2

0.01 0.02 0.03 0.04 P.o 'bnrl

Fig. 16. Infrared: Me-CO absorbance under running reaction conditions for Cat. 6 (Rh),reaction b).

0,2

0,005 0.01 0,015 0,02 P02[bad

Fig. 17. Infrared: Me-CO absorbance under running reaction conditions for Cat.5 (Pt ),reaction a) with 50 % CO replaced by H2.

168

MODEL CALCULAnONS

To illustrate tendencies, not to calculate absolute data, some simplified model

calculations of reaction a) were made starting with a Langmuir-Hinshelwood kinetic (an

Eley-Rideal kinetic would not change the basic conclusion regarding the width of A/F

windows):

r' (3)

The following data were determined from our kinetic measurements « 250 °C) by

non-linear regression:

13 3k 1 : 2.7 x 10m Imol . s

K1 0.36 m 3/mol

K 2 0

Ea 1 : 90000 J/molEa Z : 16000 J/mol

For calculating the concentration profile as function of channel length and radius the

model of the isothermic tube reactor with catalytically coated wall and laminar flow

(ref. 5) was used with the fallowing simplifications:

1. No entrance disturbance.

Z. Only radial, no axial diffusion.

3. No flow changes by mol changes.

4. Mass transfer only by gas pore diffusion within the catalytic layer.

5. Identical diffusion coefficients for CO and 0Z"

6. Deff - 1II 0 D (as determined on ),-AI Z0 3 pellets).

Given these conditions the following equations were used:

1. Free gas volume:

(4)

w(r) w V!7rR Zc

169

2. Catalytic layer:

~ 2) ( ~d c. 1 dc.

De ff (d) + r cd-- r'

3. Boundary conditions:

( dC') (dC. )dr

l

~ T Cat.De ff D

~ dCi) _ ( dc. )--- 0 and __1_ 0dr r:O dr R +d'

c

(5)

(6)

(7)

4. Langmuir-Hinshelwood kinetic (see equation 1)

The equation systems were solved numerically by a computer programm. Fig. 18

shows one example .f,or mass transfer determined conditions.

ReM""]

Fig. 18. Model calculation: CO concentration as function of monolith channel length andradius for reaction a);

CCO: 0.4 mol/m 3; Co 0.25 mol/m 3.

2

170

To characterize carbon monoxide and oxygen coverages during the stationary

reaction on the precious metal surface as well, reaction rate r' and the coverages for

carbon monoxide 8 C O and oxygen 80 were calculated using literature data (ref. 7) for

elementary reactions of the Langmuir Hinshelwood kinetic. The reaction rates obtained

are comparable with reaction rates derived from kinetic measurements justifying the

literature data used.

CO

°2CO ads + ° -ads

COa ds2 0ads

CO 2

(8)

(9)

(10)

The following equations are applied for the single reactions:

8 . Cov 2

CO adsorption: rCO,ads

02 adsorption: r O,ads

CO desorption: r CO,des

= f27rR~CO . S' CJ CO ' 8 v' CCO

= 2 {;~TMo . S . (1°22

kCO,des . exp [-Eo CO,de/R T] 8 C O

(11)

(12)

(13)

LH reaction: (14)

(15)

Fig. 19 shows an example for a certain reaction condition. (T=250oC)

0.6

0.4

eo

It-----f0.2

XI 20 J() 40 50it l~m I

0.40.2 0.3r lmml

0.1

0.1

So~-(on[l.'fllfOIJOr>

[maflmJJ0.3-r--__

O)r---_~~

Fig. 19. Model calculation: CO and 02 concentration gradients as function of channelradius (different scale for wcsficoct and gas volume) and CO resp. ° coverages.

171

DISCUSSION

Kinetic measurements, infrared investigations and the model calculations give a

consistent result, which allows one to understand the factors determining the width of

A/F windows on the lean side. These factors are the sorption behavior of carbon

monoxide, oxygen and nitrogen oxide as function of temperature and partial pressures

and mass transfer influences controlled by the porous structures of the washcoat resp.

the boundary layer gas diffusion.

Looking upon the situation from the point of view of a precious metal cristallite

down in the porous )'-alumina structure - or a differential catalyst element - at low

temperatures its surface is blocked by CO on Pt and NO or a reaction intermediate on

Rh. This explains the kinetics shown in Fig. 6 and 9 (c.q., self-poisoning by CO). With

increasing temperature, reaction begins and quickly accelerates until mass transfer

phenomena are rate-limiting. This leads to considerable differences between the local

concentrations just above the precious metal surface and the concentration in the outer

gas volume. This phenomenon causes a shift of the NO x conversion curve in the direc-

tion of stoichiometry - i.e., a reduction of A/F window width in the lean range - with

integral reactors. As long as the local CO concentration is high enough - which is al-

ways the case under rich conditions - CO is adsorbed and reactions a) and b) proceed. A

small local surplus of oxygen leads to a rapid depletion of CO (step function) which

immediately stops the NO x conversion.

The concentration gradients of the reducing agents caused by mass transfer can be

flattened by adding a reducing gas with high diffusion coefficient such as hydrogen

(Fig. 17).

In a monolith or a pellet layer this consideration for a differential catalyst element

has to be extended over the whole reactor, where temperatures and concentrations are

changing considerably. Thus the influence of hydrogen is hardly to be detected with

integral reactors, probably due to the fact that the very high reaction rate leads to a

rapid hydrogen consumption at the entrance, leaving no more hydrogen in the following

sections. Starting with a rich mixture in the system CO, NO, O2 finally leaves a CO

residue inside the catalyst, enabling a high CO coverage and thus also an NO

conversion. In the case of a lean starting mixture a surplus of oxygen remains, leading

to an abrupt decrease in coverage around the stoichiometric point which stops NO

conversion. This means that only a part of the catalyst is available for NOx conversion

when starting with a lean mixture.

By lowering the absolute reaction rate (e.q., low temperature) or by reducing the

diffusion resistance (non-porous catalyst), the negative influence of mass transfer on

the A/F window width can be counterbalanced. For the system studied here it has to be

concluded that only a compromise between A/F window width in the lean range and

absolute reaction rote can be attained.

172

LEGEND

B : surface coverage

Bv fraction of the vacant sites

a : sticking probability

r'

rS

C

Rcw

w

V

D

De ffd'

R

T

M

S

c s

: rate of reaction inside the catalyst

: rate of reaction referred to the geometric surfaceof the catalyst

: rate of surface react~on (Langmuir-Hinshelwood

: radial coordinate in the tube reactor

: axial coordinate in the tube reactor

: concentration

: radius of the open channel

: gas velocity

: average gas velocity

volume flow

diffusion coefficient

: effective diffusion coefficient in the catalyst

: thickness of the washcoat

: gas constant: temperature

: molecular weight

: area of 1 mol surface metal atoms

: surface metal atoms concentration

[ mol/m\

[ mol/m 2s

[ s- 1 ]

[m ][ m ]

[ mol/m 3

[m ][ m/s ]

[ m/s ]

[ m3/s ]

[ m 2/s ]

[ m2/s ][ m ]

[ J/mol . k ][K ][ kg/mol

[ assumed value:

4 . 104 m2/mol

[ mol/m 3 ]

ACI<NOWLEDGMENT

We acknowledge the assistance of Mr. G. Kunz and Mr. G. Stein.

REFERENCES

1 E. Koberstein, Chemie in unserer Zeit, 18 (1984) 37-45.2 J.M. Berty, Chem. Eng. Progr., 70 (1974) 78.3 E. Gallei, E. Schadow, Rev. Sci. Instr., 45 (1974) 1504.4 H. Knozinqer et aI., Chem. Ing. Techn., 42 (1970) 548.5 D. Boeker, E. Wicke, Ber. Bunsenges. Phys. Chem., 89 (1985) 629.6 G. Brauer, F. Fetting, Chern, Ing. Techn., 36 (1964) 921.7 CT. Campbell, S.K. Shi, J.M. White, Appl. Surf. Sci., 2 (1979) 382;

CT. Campbell, J.M. White, J. CataI., 54 (1978) 289;P.A. Thiel, E.D. Williams, J.T. Jates, W.H. Weinberg, Surf. Sci., 84 (1979) 54;W.L. Winterbottem, Surf. Sci., 37 (1973) 195;W. Adlhoch, Dissertation, Kinetik der Reaktion zwischen CO und NO anpolykristallinem Platin bei niedrigen DrUcken, TH Karlsruhe, 1978;S.H. Oh, G.B. Fisher, J.E. Carpenter, D.W. Goodman, to be published inJ. Catal.

.\. Crucq and A. Frcnnet (Editors), Catalysis and A utomotiue Pollution Control1987 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

ELEMENTAL STEPS DURING THE CATALYTIC DECOMPOSITION OF NO OVER STEPPED SINGLECRYSTAL SURFACES OF PLATINUM AND RUTHENIUM

N. KRUSE and J.H. BLOCKFritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-60-1000 Berlin 33

ABSTRACT

173

The interaction of NO with stepped single crystal surfaces of Pt and Rufield emitters has been studied. Pulsed Field Desorption Mass Spectrometry(PFDMS) is employed to perform time resolved measurements. It is found that the

-5 -4NO adsorption (p = 1.3xl0 to 6.7xl0 Pa, T = 523 to 605 K) occurs molecularlyon the high index planes of Pt with (111) orientation of the terraces, whereasmolecular as well as dissociative adsorption is found to occur on the high indexplanes with (001) orientation of the terraces. By varying the repetition rate ofthe field pulses from 1 Hz up to 1 kHz (corresponding to a field free reactiontime between 1 sand 1 ms) kinetic data of the adsorption, thermal desorptionand decomposition of NO are obtained. The rate parameters for the first order

-14thermal desorption on stepped Pt(lll) are: Ed =139kJ/mol, ~ = 3xlO s. The ini-tial stages of NO dissociation on stepped Pt(OOl) follow a complex kineticmechanism which can be understood on the basis of structural autocatalysis. Onstepped Ru(OOI) the dissociation proceeds according to first order kinetics. Inaddition, the oxygen deposition and accumulation leads to strong oxidation ofRu. This is evidenced by desorption of RUO~+ ions (x up to 3). A field induceddecomposition of NO on Pt is observed and leads to high ionic rates of N20+ andN;. No marked face specifity has been found so far for this reaction.

INTRODUCTION

Adsorpti'on, thermal desorption and decomposition are elemental steps occur-ing during the catalytic reduction of NO over noble metals. Current interest inbasic research work concerns the elucidation of the kinetics of these steps andtheir dependence on the surface structure of the catalyst.

The platinum metal is one of the components in three-way automotive cata-lysts. It is mainly involved in oxidation reactions but may contribute to NO re-moval as well. Ruthenium has gathered interest as a candidate for NO decomposi-tion. This metal is not used so far in practical applications since it is un-

174

stable and forms volatile oxides, Ru03 and Ru04·Various surface sensitive techniques have been employed under ultrahigh va-

cuum conditions to study the interaction of NO with Pt and Ru single crystalsurfaces (for a review see ref.1). Recently, the Pt(410) plane has been found todissociate NO (ref.2), whereas the flat (Ill) plane is inactive which is in ac-cord with theoretical considerations (ref.3). The plane to plane variations inthe NO interaction with Pt have prompted us to use field emitter tips with theirwell defined crystallography as model catalysts and to perform face specificmeasurements in a time resolved manner by employing pulsed field desorption massspectrometry (PFDMS). This method has been shown in a number of previous papers(see e.g. ref.4-7) to provide valuable kinetic data on surface processes.

EXPERIMENTAL

A detailed description of the experimental set-up has been given by Blockand Czanderna (ref.7). The basic principles are as follows. The catalyst is pre-pared in the form of a field emitter tip by electrochemically etching a thinwire (0 ~ 0.1 mm). At its apex this field emitter closely resembles a hemisphereand its surface can be imaged in real space with atomic lateral resolution byfield ion microscopy (FIM).

In PFDMS the emitter is located at a distance of ~ 0.1 mm in front of anelectrode. High voltage pulses produce field pulses with heights up to 50Vjnm athalf widths of ~ 100 ns and variable repetition rates ~ 100 kHz. The strong elec-trical field of the pulses causes molecules which are adsorbed at the emittersurface to desorb in the form of ions. Even metal atoms may be ruptured fromtheir lattice site positions in this manner. The chemical identification of thedesorbed ions is accomplished by time-of-flight mass spectrometry. A probe holetechnique selects a small area of the emitter surface, containing a minimum of afew atomic sites up to several hundred of them. By tilting the emitter differentcrystallographic planes can be sampled.

Between the field pulses, during the reaction time, tR,(see fig.1) no fieldis applied for measurements of kinetic parameters. An arbitrary steady electri-cal field can be applied in order to study the field dependence of a given che-mical reaction.

The surface temperatures are measured by a thermocouple spotwelded to theemitter base. The Pt field emitter is obtained by electrochemically etching awire (99.99 % purity) of 0.1 mm 0 in dilute solutions of KCN at 2-4 V ac. Ru wascut by spark erosion from a boule and etched in dilute HC1. Cleaning of the sur-faces was performed in situ by cycles of heat treatment and field evaporation.

Kinetic data of surface processes can be obtained as sketched schematicallyin fig. 1. While the emitter surface is continuously dosed by nitric oxide at a

175

i _-.-,~

time

llJlJ.J ~ lshort ::leci1Jm leng ~;<

ea

!JLl!\l!/17~

1/c:~c::e

t;l!'.e

Fig. 1. Time scheme of the field pulses leading to desorption at a field strengthFD which is the sum of FR (steady field, also called "reaction" field) and Fp(pulsed field). Below: Schematic diagram illustrating the development of the sur-face coverage with field free reaction time; a steady surface coverage may resultafter a time T.

steady gas pressure, adsorption takes place only in the field free reaction in-terval, t R, between the pulses. The next field pulse desorbs the adsorbed layerand analyses its composition. If desorption is complete, the measured ion inten-sity, i.e. the number of ions detected with each pulse (ions per pulse), direct-ly represents the surface concentration of the species within the monitored areabefore the pulse. The new reaction period starts with zero coverage, and thelonger t R is, the more the adsorption process proceeds. It may happen that ther-mal desorption occurs between the pulses at long t R and sufficiently high tempe-rature. On the other hand the probability for chemical reactions increases withrising adsorbate concentrations. The kinetics of these processes can be studiedby systematically varying t R. This is normally done in the range tR=100~s ... ls.

Surface diffusion can disturb the measurements and mask true reaction kine-tics. While the adsorbate at the apex of the emitter is desorbed by the pulses,the adsorbate at the shank is not. This may lead to influx of mobile species in-

176

to the monitored area during t R. Such an influence has been observed in parti-cular cases (ref.6,S), and prevented by high enough field pulse amplitudes andrepetition rates.

RESULTS

For all of our studies the emitter surfaces are continously dosed by NO at-5 -4a steady gas pressure between 1.3xlO Pa and 6.7xlO Pa. In any case, under mere

pulsed field conditions, the desorption of the adsorbed layer leads to the de-tection of high amounts of NO+. The substrate material is concomitantly fieldevaporated at lower rates in form of Ptn+ and Ru n+ (n=1,2). Since NO may chemi-cally dissociate on surfaces with a certain geometrical arrangement of the atoms(details see below), various metal oxide species, MeOx' as well as mere 0 atomscan be desorbed as ions. The NO dissociation can be promoted by steady electricalfields giving high amounts of N20+ and N~.

We first present results of probe hole measurements on the stepped surfacein the vicinity of the (001) pole of a Ru emitter tip. Next we compare these re-sults with those obtained for the stepped region close to the (001) pole of a Ptemitter. Results of the field induced decomposition of NO over stepped Pt(lll)will be reported.

It has been pointed out in the experimental section of the paper that kine-tic data of surface processes can only be obtained if all adsorbed molecules arecompletely desorbed by each field pulse. This requirement can be checked fromfield strength variation measurements. Fig. 2 displays the results of such an ex-periment performed with a Ru emitter. The measurements are performed at a pres-sure p = 6.7xlO- 4pa NO and a temperature T = 552 K. A repetition rate of thepulses f = 4 Hz, i.e. t R = 0.25 s, is applied. No steady electrical field ispresent during t R. Up to a field strength FO ~27 V/nm the different ionic spe-cies display an uniform trend of increasing intensities. The onset values forfield desorption are different. At low field strengths the NO+ intensities domi-nate (for FO< 19 V/nm NO+ is the only species in the mass spectrum). VariousRu-oxides, RUO;+ (x=1 ... 3), appear subsequently. RUO~+ is seen first, RUO~+ andRu02+ come up later. The occurrence of these species proves dissociation of NOadto take place with subsequent build-up of an oxide layer.

At low field strength the adsorbed layer remains nearly untouched by thefield pulses. Only a small portion is desorbed but immediately refilled by con-tinuous impingement of NO molecules. Thus, the adsorbed layer has a steady con-centration. The oxidation state of the surface (and sub-surface region) is highand one of the characteristics under these conditions is the relatively highamount of RUO~+ ions. Nevertheless, there are still enough sites available formolecular adsorption of NO as evidenced by the occurrence of ~igh NO+ desorptionrates.

177

20 22 24 26 28desorption field s~rengr;, [V/nm)

Fig. 2. Dependence of ion intensities on desorption field strength during reac-tion of NO with stepped Ru(OOl). NO pressure: 6.7xlO- 4pa; Ru tip temperature:552 K; reaction time: 0.25 s, i.e. pulse repetition rate: 4 Hz.

For rising field strength values the desorption probability of the adsorbedlayer and, consequently, the ionic rates of the various species increase. How-ever, for a field strength FD > 27 V/nm this trend only holds for the Ru02+ ionwhile the ionic rates of all other species decrease. The observed behaviour pro-vides evidence for a strong structural change of the surface layer. Desorptionat high field strengths lowers the surface oxide level considerably, and thehigh oxidation state cannot be restored during the reaction time, t R=0.25 s, atT = 552 K. Hence, the ionic rates of the high index RUO~+ species decrease. Thisdecrease at high FD is more pronounced for RUO~+ than for RUO~+

The destruction of the oxide layer is associated with the creation of oxygenvacancy sites. NO~d may dissociate at these sites as evidenced by increasing in-tensities of Ru02 and decreasing intensities of NO+.

At the temperature T = 552 K thermal desorption of NOad also occurs duringt R=O.25 K. This process competes with the dissociation reaction, thus only thoseNO molecules which are neither thermally desorbed nor yet decomposed can be de-tected.

178

Interestingly, we find no mass spectrometric evidence for Nad emerging fromthe decomposition of NO ad. This suggests either fast recombination with subse-quent thermal desorption as N2 or diffusion out of the monitored area into theneighbouring surface regions.

Field strength variation measurements have also been performed for NO on Pt.The detailed results will be published elsewhere. It is found that the chemicaldissociation of NOad is strongly face dependent.While the stepped surface regionwith (001) orientation of the terraces (furtheron denoted as stepped (001)) isactive in N-O bond breaking, the stepped (111) surface is not. 0ad formation onstepped Pt(OOl) leads to the detection of 0+ and PtO n+ (n=1,2) ions. PtOx spe-cies from high index planes are not present in the mass spectra.

In order to obtain kinetic data of the NO decomposition reaction over steppedRu(OOl) and stepped Pt(OOl) we performed reaction time variation measurements.The results of this comparative study are displayed in fig. 3. In the Pt case weset T = 543 K. At this temperature both the decomposition as well as the thermaldesorption are competitive within the measurable time scale,and a kinetic analy-sis of these processes can be performed. The field strength amounts to FO=30V/nmfor measurements on Ru and to FO = 28 V/nm for those on Pt. These high fieldstrength values have been chosen in order to ensure high desorption probabili-ties of the adsorbed species. In fact, it has been found that the ion intensi-ies of Ru02+ and Pt02+ do not increase any further above these field strengthvalues. Oesorption of NOad from Pt leads to saturated NO+ intensities for FO> 20V/nm (ref.g). Thus, under these conditions, the measured intensities reflect thesurface concentrations of 0ad and NOad within the monitored area, respectively.Since the high index RUO~+ species are only slightly above the detection limitat FO = 30 V/nm and T = 503 K, they are not plotted in fig. 3.

The measured Ru02+ intensities and, consequently, the Dad concentrationsfollow a straight-line tR-dependence in fig. 3. Thus,the NOad decomposition withsubsequent oxygen deposition occurs at a constant rate. The time proportional in-crease of the Ru02+ intensity proves the dissociation to proceed independent onthe concentration of 0ad'

A completely different behaviour is found for pta+ (and a+ which is notplotted here). This species appears with a delay time of ~ a.1 s and we concludethat the Na decomposition is inhibited at shorter reaction times. The slope of thepta+ intensities is steep at t R ~ a.1 s but reduces with increasing t R values,This behaviour provides evidence that the dissociation in its initial stages fol-lows a complicated kinetic mechanism.

In fig, 4 we present the time dependence of both Na+ as well as pta+ inten-sities. Within the measured time scale the Na+ ions are always more abundant thanthe pta+ (and a+) ions. Thus, molecular adsorption prevails over dissociative ad-sorption. Moreover, for short t R, only molecular adsorption is observed. A suf-

179

,,/

r,impinqernan t rate ..//

rw/',,,

,

-310

110 -

) Ions/ 10aOpulses10<

Fig. 3. Dependence of Ru02+ and PtO+ ion intensities on reaction time, t R·NO pressure: 1.3xlO-5pa; impingement rate:0.19 molecules/s into the monitoredarea; tip temperatures: 503 K for Ru, 543 K for Pt; pulsed field strength: 30V/nm for Ru, 28 V/nm for Pt. The ion intensities refer to the same size of themonitored area.

ficient accumulation of NOad is necessary for the dissociation to occur.Closer inspection of the molecular adsorption reveals some interesting ki-

netic details. The NOad concentration increases linearly with rising t R valuesbut levels off later on. This behaviour is characteristic for thermal desorptionof NOad during t R, If we assume adsorption to be associated with a constantsticking probability and thermal desorption to obey first order kinetics, dc/dt=-CiT, and, consequently, c = ~ (l_e-t / T), the mean lifetime T before thermaldesorption occurs, is given by the time t R where the NOad concentration reachesthe (I-lie) level of its equilibrium value, e, at long times. At T = 543 K wefind T = 0.2 s, i,e. k = 5 s-l for the first order rate constant.

The NO+ intensity increase, i.e. the slope dc/dt, at short times defines

180

i

-110

I s I

-310

o10 -

Ions2 /1000pulses

10

110 -

/

//

/

~e- ••-impingement rate,"~. •'.~IO/e"?

'e'... ~/

,".''e','.

,'. NO·/e'':/";//e

';/fe10-1

Fig. 4. Dependence of NO+ and PtO+ intensities on reaction time, tR.NO pressure:1.3x10-5pa; T = 543 K, FD = 28 V/nm; T = mean lifetime of molecular adsorbed NO;impingement rate: 0.19 molecules/s into the monitored area.

adsorption rate. Comparison with the impingement rate from the gas phase (checkedby NO+ dc field ionization) yields sticking probabilities, s, between 0.6 and0.85 which are in reasonable agreement with the values s~ 0.6 found by Bonzel etal. (ref.10) on stepped Pt[4(100)x(111)J at the low coverage limit.

Similarly, the dissociation probabilities,w, can be calculated by comparingthe PtO+ intensities with the impingement rate. Within the measured time range,w is small. At a time t R =1 s, w is of the order of a few percent only. The dis-sociation probability of NO on stepped Ru(OOl) is higher and amounts to a con-stant value of more than 5 %(see Fig. 3). Note that the surface oxide coverageon both substrates, Ru as well as Pt, is far below a monolayer. Thus, the wvalues refer to the low coverage limit.

As already mentioned,NOad dissociation does not occur on the stepped Pt(lll)surface. Only molecular adsorption of NO is observed. A reaction time variation

-Jrrr

5.0

4.0

30

2.0

600

~\

K

181

us,

.~.I.,~

Fig. 5. Temperature dependences of the mean lifetime,T, of NO on pt; the valuesrefer to measurements on a stepped surface region with (111) orientation of theterraces.

measurement has been performed at various temperatures between 523 Kand 602 Kin order to probe the kinetics of adsorption and thermal desorption. The detail-ed results are published elsewhere (ref.9). By evaluating the temperature de-pendence of ' the mean lifetimes, T , according to Frenkel's equation,T=TO exp(Ed/kT), the activation energy,Ed, for thermal desorption and the preex-ponential term, TO' have been determined (see fig.5). We find Ed = 139 kJ/mol

-14and TO= 3x10 s.These rate parameters are in good agreement with those obtained by other

authors using different experimental techniques (ref.11,12). In PFDMS always themost stable adsorption state (with the longest lifetime) is sampled provideddiffusion and conversion into this state are fast processes.We conclude that our

182

o 2 4 6 8 10 12steady field strenqrh [V/nm j

Fig. 6. Field induced decomposition of NO on Pt, measured on a stepped surfacewith (Ill) orientation of the terraces;the steady field strength,FR, is variedand the pulsed field is adjusted so that the desorption field is constant at

-5FD = 24 V/nm; NO pressure: 6.7xIO Pa, T = 296K.

rate data are largely determined by steps since these sites exert the strongestbonding to adspecies. The same conclusion has been drawn by other authors fromtheir results (ref.II-I3). In fact, adsorption on perfectly flat Pt(III) is as-sociated with a binding energy of 105 kJ/mol, the most probable value obtainedby Serri et al. (ref.I4) from their theoretical model.

The results presented so far have been obtained under mere pulsed fieldconditions where the various processes and their kinetics are not influenced bythe probing pulses. However, steady electrical fields are frequently observed tochange the chemical reactivity of the adsorbed species. This influence may becomeapparent in form of fragmentation, association, charge transfer processes, etc.

We studied the field dependence of the NO adsorption by probing the steppedPt(III) surface region where, by mere pulsed fields, only NO+ desorption occurred(besides some field evaporation of the substrate material). The results are dis-played in fig. 6. The measurements are performed by increasing the steady field,FR, and adjusting the pulsed field, Fp' such that the total desorption field is

183

kept constant and amounts to FD = 24 V/nm. The reaction time is t R = 10 ms andthe surface temperature is set equal to T = 296 K.

In the lower steady field range only NO+ ions are detected. At moderatefields, however, various other species, N20+,N; and O+,appear additionally. Theintensities of these species increase steeply with rising FR values. The onsetsfor N20+ and 0+ desorption are coincident at FR ~4V/nm, the N; ions come upsomewhat later. The intensities of N20+ are observed to reach about the samelevel as those of NO+ for small FR' Further augmentation of the field strength isassociated with decreasing NO+ rates. Both N; and 0+ intensities increase con-tinuously within the measured range of FR values, however, the 0+ ions are muchless abundant.

The general result of fig. 6 is the field induced decomposition of NO. Thechemical nature of the products and their high ionic intensities prove this pro-cess to occur in the adsorbed layer rather than in the gas phase near the sur-face where the field strengths are high too. We note that the ionic traces re-main essentially the same for increasing surface temperatures as long as thereis no loss of NOad due to thermal desorption.

The surface oxide level in the monitored area is low as evidenced by thesmall 0+ ionic rates. This is unexpected in view of the stoichiometry of thefield induced decomposition. Further investigations are currently performed inorder to clarify this point.

DISCUSSION

We studied the elemental steps of the interaction and reaction of NO over Ptand Ru.

Results are presented for stepped surfaces with terrace orientations (100)and (111) on Pt and (001) on Ru. In fact, these planes, together with variousothers, form the surface of a field emitter tip which can be regarded as a cata-lyst particle with a diameter of about 20 to 200 nm. Although our probe hole mea-surements sample only a few atomic sites (up to about 200) the detailed crystal-lography of the probed area, i.e. terrace widths and step site symmetries, is notknown because the concomitant removal of substrate atoms by field evaporation(from kink site positions) during the measurements causes continuous alterationsof the morphology.

The observed structure sensitivity of platinum in the decomposition reactionof NO is in accord with the known catalytic behaviour of the metal. While thestepped Pt(lll) surface adsorbs NO only in its molecular form, the stepped Pt(lOO)surface also decomposes NO. Although step sites are generally regarded as activein N-O bond breaking we must conclude that this does not apply always. Banholzeret al.(ref.3) developed a model in order to explain the plane to plane variations

184

of the catalytic decomposition activity. This model is based on symmetry conser-vation rules for chemical reactions.

The flat Pt(IOO) clean surface is fairly unique in that it may exist in se-veral phases (ref.15-17). The (lxl) phase (bulk lattice structure) is metastableand observed to transform above 400 K into a quasi hexagonal structure (5x20 or"hex"). This reconstruction is removed by adsorption of NO (ref.IO) or othergases (ref.15,18). In fact, the reversible phase transition may cause severalinteresting phenomena during heterogeneous catalysis. Ertl et al. (ref.19) haveshown that oscillating rates in the CO 2 production during CO oxidation aredriven by the CO-induced phase transition.

In the present study on the stepped Pt(lOO) clean surface the ideal "hex"phase with its large unit cell has not been observed so far by field ion micros-copy. LEED studies of the Pt[4(lOO)x(lll)]and Pt[9(lOO)x(lll)] (ref.lO) sur-faces, however, give evidence for a reconstruction modulated by the steps.

Our results on stepped Pt(lOO) show molecular adsorption and decompositionof NO to be sequential steps. The decomposition does not begin before the NOadconcentration has reached a certain level. In fig. 4 this level corresponds tocaverages less than 1 %of a monolayer. The time dependence of the PtO+ inten-sities shows that not only the surface oxygen concentration, but also its pro-duction rate increases with time.

The results on stepped Pt(lOO) can be understood on the basis of structuralautocatalysis. We suggest that the reconstruction of the surface is removed bysufficiently high NOad concentrations. This leads the decomposition to startimmediately and to accelerate in an autocatalytic manner. Behm et al. (ref.20),in their detailed study of the CO induced hex~ (lxl) conversion of flat Pt(lOO),find COad concentrations of 5 %of a monolayer sufficient for this transition tooccur in a patch-like manner. In our studies the steps play an important roleas intermediate trapping sites for molecular adsorbed NO. Thus, the concentrationof NOad at step sites is much higher than at terrace sites. For an estimatedstep density of 10 % in the monitored area we expect, at t R = 0.1 s, several %of the step sites to be covered by NOad. The role of steps in the reconstructionof the Pt(lOO) surface is not clear so far. Bonze1 et a1. (ref.lO) report on astabilization effect of the (lxl) phase by steps. This finding calls for a ri-gorous analysis employing field ion microscopy as well as probe hole PFDMS withsmaller selected areas than used so far in our work.

Since the NOad decomposition occurs at steps the continuous oxygen deposi-tion at these sites prevents molecular adsorption. Under reaction conditions thesurface oxide level is always small, and the molecular adsorption (and thermaldesorption) is the prevailing process. The initial stages of oxide build-up haveobviously not been studied so far by other authors. Auger (AES1, XPS or tempera-ture programmed desorption (TPD) are not sensitive enough to sample this pro-

185

cess. Surface nitrogen has not been found in the present measurements. This isexplained by rapid recombination and immediate thermal desorption of molecularnitrogen.

The surface oxide level on stepped Pt(100) is always lower than on steppedRu(OOl). When the pulsed field strength is lowered, the adsorbed layer is notcompletely removed any longer.This leads to Dad accumulation at the surface,butonly on the Ru sample is the oxidation so strong that high index RUO~+ (x upto 3) can be desorbed. In a recent study of Ru oxidation by gaseous 0z (ref.Zl)we also found these ions. In addition, the corresponding neutral molecules arefound to be mobile species on top of an oxide layer. It is likely that these mo-lecules act as intermediates in the reaction towards the volatile Ru04 molecule.

The intense oxidation of Ru is in accordance with the known behaviour ofthe metal under the "real" conditions of heterogeneous catalysis.It is remarkablethat the NO decomposition reaction under our experimental conditions,i.e. at lowpressures, also leads to a high oxidation state of the surface (and sub-surface)region.

The field induced decomposition of NO over Pt is an interesting reactionphenomenon and presents another example for drastically changing chemical reac-tivities in high external electrostatic fields. It should be noted that we findno marked face specificity of the metal in this reaction. The presented data re-fer to the stepped Pt(lll) surface which shows no catalytic activity in N-O bondbreaking under zero field conditions. Recently, Kiskinova et al. (ref.ZZ) re-ported the potassium promoted NO decomposition over macroscopic Pt(lll) singlecrystal surfaces to produce nitrogen and oxygen as well as NZO and NO Z due tosecondary reactions. The potassium, adsorbed as K+, is considered as an elec-tronic promoter which transfers electrons into the antibonding Zn molecular or-bitals of the adsorbed NO molecule.This leads to weakening of the N-O bond and,finally, to its scisson. The field assisted NO decomposition cannot be ex-plained by electronic effects only. The adsorption of NO is associated with anincrease of the work function, thus there is a net electron transfer to the ad-sorbate (ref.Z3). The electrostatic interaction of the resulting dipole with the(positive) external field may be associated with a bending of the molecule andits ultimate dissociation. Adjacent NOad species may associate during this pro-cess so that the detection of high amounts of NZO+ and N~ becomes conceivable.

ACKNOWLEDGEMENT

This work was partially supported by the Sonderforschungsbereich (Sfb 6) atthe Freie Universitat Berlin.

186

REFERENCES1 W. Egelhoff, Jr., in D.A. King and D.P. Woodruff (Eds.), The Chemical

Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. IV, Elsevier,New York, 1982, p. 397.

2 Y.O. Park, W.F. Banholzer and R.J. Masel, Appl. Surf. Sci., 19 (1984) 145.3 W.F. Banholzer, Y.O. Park, K.M. Mak and R.J. Masel, Surf. Sci., 28 (1983)

176.4 D.B. Liang, G. Abend, J.H. Block and N. Kruse, Surf. Sci., 126 (1983) 392.5 N. Kruse, G. Abend and J.H. Block, Z. Phys. Chem. N.F., 144 (1985) 1.6 N. Kruse, Surf. Sci., in press.7 J.H. Block and A.W. Czanderna, in A.W. Czanderna (Ed.), Methods and Pheno-

mena, Vol. I, Elsevier Scientific Publ. Comp., 1975, p. 379.8 D.L. Cocke, G. Abend and J.H. Block, Int. J. Mass Spectrom. Ion Phys., 24

(1977) 271.9 N. Kruse, G. Abend and J.H. Block, Surf. Sci., submitted.

10 H.P. Bonzel, G. Broden and G. Pirug, J. Catal., 53 (1978) 96.11 T.H. Lin and G.A. Somorjai, Surf. Sci., 102 (1981) 573.12 C.T. Campbell, G. Ertl and J. Segner, Surf. Sci., 115 (1983) 309.13 J.A. Serri, M.J. Cardillo and G.E. Becker, J. Chem. Phys., 77 (1982) 2175.14 J.A. Serri, J.C. Tully and M.J. Cardillo, J. Chem. Phys., 79 (1983) 1530.15 A.E. Morgan and G.A. Somorjai, J. Chem. Phys., 51 (1969) 3309.16 P.R. Norton, J.A. Davies, O.K. Creber, C.W. Sitter and T.E. Jackman, Surf.

Sci., 108 (1981) 205.17 K. Heinz, E. Lang, K. Strauss and K. MUller, Surf. Sci., 120 (1982) L401.18 M.A. Barteau, E.I. Ko and R.J. Madix, Surf. Sci., 102 (1981) 99.19 G. Ertl, P.R. Norton and J. RUstig, Phys. Rev. Lett., 49 (1982) 177.20 R.J. Behrn, P.A. Thiel, P.R. Norton and G. Ertl, J. Chern. Phys. 78 (1983)

7437.21 G.K. Chuah, D.L. Cocke, N. Kruse, G. Abend, T. Kessler and J.H. Block, J.

de Physique, C2,47 (1986) 359.22 M. Kiskinova, G. Pirug and H.P. Bonzel, Surf. Sci., 140 (1984) 1.23 M. Kiskinova, G. Pirug and H.P. Bonzel, Surf. Sci., 136 (1984) 285.

A. Crucq and A. Frennct (Editors), Catalysis and Automotive Pollution Control1987 Elsevier Science Publishers B.V .. Amsterdam - Printed in The Netherlands

PERIODIC OPERATION EFFECTS ON AUTOMOTIVE NOBLE METAL CATALYSTS--- REACTION ANALYSIS OF BINARY GAS SYSTEMS ---

187

H. Shinjoh, H. Muraki and Y. FujitaniToyota Central Research and Development Laboratories, Inc., Nagakute-cho, Aichi-gun, Aichi-ken, 480-11, Japan.

ABSTRACT

Catalytic activities and periodic operation effects in various binary gassystems (CO-0 2 , NO-CO, NO-H 2 , C3H6-0 2 , and C3Ha-02) over Pt, Pd, and Rh/ct-A1 20 3were compared. In all reaction systems, periodic operation effects were found tosome extent. That is, the conversion improved in the cycling feed compared tothe static one. The periodic operation effects occurred most noticeably forcatalysts having lower catalytic activity as a result of the difference ofadsorption capability between the two reactants.

INTRODUCTIONAutomotive three-way catalysts which simultaneously control NOx, CO, and HC

emissions are designed to operate in the stoichiometric conditions of automotiveexhaust gas. However, it is well-known that the stoichiometry of exhaust gaschanges continuously between oxidizing and reducing atmospheres because of thestep-like response characteristics of oxygen sensor equipment. Recently, thisdynamic behaviour has been analyzed. Taylor et al (Ref. 1) reported the effectsof both symmetric and asymmetric air/fuel ratio cycles during conversion bythree-way catalysts. Schlatter (Ref. 2) and Herz et al (Ref. 3-6) studied therole of Ce02 added to three-way noble metal catalyst under dynamic conditions.The authors (Ref. 7) have investigated the behaviour of automotive noble metalcatalysts in the cycled model feedstreams which simulated engine exhaust gas.We observed that the reaction using noble metal catalysts, particularly Pt andPd, under cycling conditions was superior to that under static conditions, andthat catalyst perfomance depends on the cycling period and feedstream com-position. These phenomena may be used to improve the activity of three-way cata-lysts by selection of suitable periodic conditions. Further, it is veryimportant to clarify the mechanism of the periodic operation effects overvarious noble metal catalysts. Our object was to investigate the performance ofconversions over noble metal catalysts under periodic conditions.

Accordingly, we have systematically investigated the various binary gas reac-tion systems, such as CO-0 2 (Ref. 8), CO-NO (Ref. 9), HC-0 2 (Ref. 10), over

188

noble metal catalysts. For example, the periodic operation effects were par-ticularly observed in CO oxidation: the conversion depended on cycling period,and the existence of maximum conversion at a particular cycling period was con-firmed.

Following the investigation of binary gas systems under periodic conditionsas mentioned above, we examined NO reduction with Hz over noble metal catalysts.While NO-Hz reaction may be considered as a main process of NO reduction in thereducing atmosphere of automotive exhaust gas, little is known so far concerningsupported noble metal catalysts. Koblinski et al (Ref. 11) studied NO reductionwith H2 and CO over noble metal catalysts. They demonstrated that H2 is a moreeffective reducing agent than CO. Yao et al (Ref. 12-13) showed the relationshipbetween reactivities of catalysts in NO reduction with H2, and noble metal con-tent of catalyst used. But, the dynamic behaviour of noble metal catalysts inNO-Hz systems has not studied at all.

This paper introduces: (1) the detailed behaviour of the NO-Hz reaction underboth static and dynamic conditions over three kinds of noble metal catalysts(Pt, Pd, and Rh/a-Al Z03), and (2) the relationships between catalytic activityand periodic operation effects obtained from the various binary gas systems overthe same catalysts. Finally, in order to explore the role of periodic operation,the concept of self-poisoning of reactants contained in the binary gas systemsis postulated, allowing reasonable interpretation of our results obtained underthe cycling conditions.

EXPERIMENTALCatalyst

All catalysts were prepared by a conventional impregnation method usinga-Al 203 (pellet size: 2-3 mm¢, BET surface area: 10 mZ/g, bulk density: 0.79g/cm3). Details of the catalyst preparation was reported elsewhere (Ref. 7).Loading amount of Pt, Pd, and Rh was 0.05 g/l (0.006 wt%) as noble metal.

Experimental proceduresThe laboratory reactor system was a conventional flow system with a tubular

fixed-bed reactor as shown in Figure 1. A characteristic feature of this reactorsystem was its ability to change the feedstreams to the catalyst bed quickly sothat the feedstreams can be rapidly cycled between two different gas composi-tions. The cycling period was varied between 0 and 2.0 seconds.

In this work, NO and Hz were fed periodically to the main stream of Nz(carrier gas). The time-average concentrations of NO and Hz were changed from0.1 to 0.6 vol% and from 0.1 to 3.0 vol%, respectively. The ranges in these con-centrations correspond to those in automotive exhaust gas. Chemically pure Hz(99.999%) from Nippon Sanso Co. and NO (99%) from Takachiho Chemical Co. were

189

used without further purification.

MASS FLOWCONTROLLER

NO

SiCCATALYST

ANALYZERS

Figure 1. Schematic diagram of system for simulating catalyst to oscillatingfeedstream compositions.

The relative activities of catalysts used were expressed in terms of percen-tage NO and H2 conversion as a function of catalyst bed temperature (100 -600°C) at a constant flow rate (SV: 30,000/h) for both static and cyclingexperiments.

RESULTS AND DISCUSSIONNO-H 2 reaction under static conditions

The reduction of NO with H2 was examined under static conditions (cyclingperiod: 0 second). The feedstream composition of premixed gas was fixed at thestoichiometric 'ratio for converting NO to N2 • The concentrations of NO and H2

were 0.3 vol%.Figure 2 shows activity data obtained for reduction of NO with H2 over the

three supported noble metal catalysts. By arbitrarily taking the temperature at50% NO conversion as a measure of catalytic activity, the activity sequence wasPt > Pd > Rh.

In contrast, Koblinski et al (Ref. 11) showed, in the same reaction system,that the activity sequence was Pd > Pt > Rh > Ru. The discrepancy between theseresults may be due to the different catalyst supports: the present study used

190

inactive a-Al Z03 while Koblinski et al used active Al Z03.

100

~ NO-H2° static.c0(/\ 50L.Q)

>c0u0z

00 400

Temperature (OC )

Figure 2. NO conversion data of noble metal catalysts in NO-Hz reaction understatic conditions.

Otto et al (Ref. 13) studied the NO-Hz reaction over Pt and Rh catalysts andfound that, at a given temperature, Pt exceeds Rh in the turnover frequency bytwo orders of magnitude. They considered this resulted from the differentgeometrical surface structure of Rh and Pt catalysts. Rh remains oxidized to alarge degree under the conditions of these rate measurements and thus displaysfewer active reaction sites. The higher affinity of Rh for oxygen has recentlybeen shown (Ref. 12-15). Consistent with this concept is the fact that theamount of NO chemisorbed on an oxidized surface is smaller than that on areduced one. This explains why Rh is less active than Pt in the NO-Hz reaction.It is also probable that Pd remains more oxidized than Pt, but to a lesserextent than Rh under present experimental conditions.

The reaction products detected in the NO-Hz system, Nz, NzO, NH 3' and HzO,were similar to those in previous studies (Ref. 11). The reaction path could beestimated from the amount of consumed reactants ~C(NO) and ~C(Hz), or theirratio R, where R=[~C(Hz)/~C(NO)J. That is, the following set of reactions mayoccur simultaneously:

191

NO + 0.5 H2 -+ 0.5 N20 + 0.5 H2O (A)NO + H2 -+ 0.5 N2 + H2O ( B)NO + 2.5 H2 -+ NH 3 + H2O (C)

Thus for NO reduction processes, when reaction (A) occurs alone, the value ofR is 0.5. Similarly, for reactions (B) and (C), the values are 1.0 and 2.5respectively.

Figure 3 shows the ratio of consumed reactants, R, as a function of reactiontemperature. The behaviour of the three catalysts differed from one another. Inthe case of Pt and Pd catalysts, the value depended on catalyst bed temperature.However, the opposite was found in the case of Rh. Over Pt and Pd catalystsbelow 200°C, the value of R was smaller than 1.0. Therefore the main products ofNO reduction with H2 might be N20 and N2• On the other hand, over 500°C, themain products might be NH 3 and N2• With increasing temperature, the main reac-tion paths may change gradually from (A) to (8) and further to (C) in presentexperimental conditions.

2

NO-H20z ......... Ptu ..........-."<;J -~'------ Pd<, ~~9 Rh....... /

/N /

I II

UI

"<;J

00 200 400 600

Temperature (OC )

Figure 3. The relationship between R[~C(H2)/~C(NO)J and reaction temperatureover noble metal catalysts in the NO-H 2 reaction.

NO reduction with H2 under dynamic conditionsA similar activity examination of NO reduction with H2 was conducted under

dynamic conditions. The feedstreams of NO and H2 to the catalyst bed were sym-metrically cycled to give periods from 0.2 to 2.0 second.

192

Figure 4 illustrates the effect of cycling period on NO conversion at severaltemperatures over a Rh catalyst. At lower temperatures (below 300°C), theperiodic operation effect was most noticeable.

RhNO-Hz

,...0

0-

C0

Ul 50...<lJ>C0U

0Z

0 1.0 2.0

Per iad (s e c.)

Figure 4. The effect of cycling period on NO conversion at several temperaturesover a Rh catalyst.

The maximum conversions are clearly observed for somewhat lower temperaturesand the optimum period for the maximum conversion decreases with increasing tem-perature. Similar phenomena were observed in the systems of CO-O z (Ref. 8),CO-NO (Ref. 9), and HC-O z (HC: C3H6 and C3H8 ; Ref. 10) over noble metal cata-lysts.

Kinetic parameters

The kinetics of NO reduction with Hz under the static conditions was sUb-jected to empirical laws: rate of NO (or Hz) consumption, V, is generallyexpressed by the following formula,

V = k x p(Hz)m x p(NO)n exp(-llE/RT),where P(H z) and P(NO) is the partial pressure of Hz and NO, respectively, and t.E

is the activation energy. The partial reaction orders, m and n, are determinedfrom the data obtained under the conditions of lower conversion, usually lessthan 30%. The result over a Rh catalyst is shown in Figure 5. With Rh cata-lysts, the reduction rate of NO with Hz was depended to the order of -1.4 withrespect to NO partial pressure and the order of +6.0 with respect to Hz partial

193

pressure. Activities of Pt and Pd catalysts, on the contrary, were independentof NO partial pressure, and dependent to the order of +1.0 (Pt) and +0.7 (Pd)with respect to Hz pressure.

It is known that the dissociation of Hz over noble metal catalysts is con-siderably faster than that of NO, and thus the rate determining step of NOreduction with Hz is the dissociation of NO (Ref. 13). Pirug et al (Ref. 16)have examined NO reduction with Hz over Pt catalysts and found that the reactionrate depended on the ratio P(Hz)/P(NO). For a high P(Hz)/P(NO) ratio, surfaceconcentration of Hz will increase slightly due to the competitive adsorptionbetween Hz and NO. So, the dissociation of NO and the NO-Hz reaction will com-mence at a lower temperature.

-2 Rh(250°C)-o NO-H2u -3I01

c -4E<!J- -50E

-6> P(HZ)var.c

-7

-6.5 - 6.0 - 5.5 - 5.0

In P(Hz) or In P(NO) (rn o l e vs )

Figure 5. Partial pressure dependencies of reaction rate at 250°C for Rh cata-lyst [e.g., P(NO)var.: NO partial pressure is variable and Hz partial pressureis constant].

Information from some studies (Ref. 17) suggests an inverse dependence of therate of NO decomposition with respect to the oxidizing atmosphere over noblemetal catalyst. From the facts mentioned above, it is reasonable that the rateof NO-Hz reaction was positive order with respect to Hz partial pressure butnegative order with respect to NO partial. pressure with Rh catalysts.

194

The negative order, that is NO inhibition, in NO-Hz reactions over noblemetal catalysts is essentially concerned with the periodic operation effect. Theperiodic operation effect is perhaps due to the surface state of the Rh cata-lyst. That is, the catalyst surface under static conditions is easily oxidizedby NO, so that NO chemisorbs to a smaller degree and reaction rates aresuppressed. However, under the optimum cycling feeds, catalyst surface is suf-ficiently reduced and suitable surface compositions of reactants are maintainedin order for reaction to proceed. As a result, the reaction rate reaches themaximum value. The periodic operation effect can be interpreted in terms of thestrong affinity between Rh and oxygen and this concept can be extended tointerpret the behaviour observed in the CO-O z (Ref. 8) and CO-NO (Ref. 9)systems.

Catalytic activities and periodic operation effects in various binary gassystems over noble metal catalysts

In order to clarify the mechanism of the periodic operation effect over noblemetal catalysts, we studied several binary gas systems, which occur in automo-tive exhaust gas: CO-NO, CO-O z, C3H 6-O Z, C3H s-O z, and NO-Hz.

The results obtained from the above-mentioned gas systems are summarized.At first, the order of catalytic activity, under static conditions, in each

binary gas system was as follows:

CO-O z Rh > Pd > PtCO-NO Rh > Pd > PtHz-NO Pt > Pd > Rh

C3H 6-0z Pd > Pt > RhC3H s-O z Pt > Rh >'Pd

It was noted that the activity sequence in NO reduction with CO was thereverse of that for the NO-Hz reaction. This inversion of activity orderdepending upon the reducing agent employed means that the reduction of NO withHz over Pt, Pd, and Rh is faster than NO with CO. The order of catalytic acti-vity in CO-O z and CO-NO is similar. This means that CO is concerned with therate-determining step of respective reactions. It is well known that CO inhibitsthe reactions of CO-O z and CO-NO systems over noble metal catalysts. In theNO-CO-H z reaction, CO inhibits NO reduction (Ref. 11) and strong inhibition byCO is the rate-determining step over three-way catalysts.

As can be seen from the above sequences of catalytic activities, Rh is a goodcatalyst for CO oxidation, Pt is good for NO reduction with Hz and C3H 6oxidation, and Pd is good for C3H6 oxidation.

Under dynamic conditions, periodic operation effects were observed in everybinary gas system. The most efficient catalysts under periodic condition were:

195

Pt and Pd in CO oxidation with Oz or NO (particularly Pt), Pt and Rh in C3H soxidation, and Pt in C3H 8 oxidation. In the NO-Hz reaction, periodic operationwas particularly noticeable for the Rh catalyst.

Comparing the results under static and dynamic conditions, it is clearlyfound that periodic operation effects occurred most noticeably for catalyst oflower catalytic activity and at lower reaction temperatures. These facts indica-te that if catalysts have a poor catalytic activity or they are reacting at alower temperature, their activities can be improved by periodic operation.

Table 1 indicates whether or not catalytic activity was found to be positiveorder with respect to partial pressure in every binary gas system tests overnoble metal catalysts. This table shows that the reactions are promoted or inhi-bited by each reactant. For example, in the CO-O z reaction, CO inhibits, and Ozpromotes the reaction. These values of kinetic parameter were different for eachof the three catalysts and in every binary gas system.

TABLE 1

Partial reaction orders in the binary gas systems over noble metal catalysts

[V=k x P(Reductant)m x P(Oxidant)n exp(-~E/RT)J

CO-O z CO-NO Hz-NO C3Hs-O z C3H 8-0 Z

m n m n m n m n m n

Pt - + - - + + 0 - + + + - -

Pd - + - + + 0 - + + - + +

Rh - + 0 - + + - + - + 0

+ +: highly positive, +: positive, 0: independent,-: negative, - -: highly negative,- +: partly negative(under higher partial pressure of HC).

In all cases where periodic operation effects were noticed, the product oftwo partial reaction orders, m x n, became negative. From these results, itseems that one of the reactants self-poisons a reaction occurring over noblemetal catalysts and that the potential for self-poisoning corresponds to thedegree of periodic operation effect.

196

The periodic operation effect depends on temperature and cycling period andeach catalyst behaves differently. To improve catalytic activity, it is impor-tant that the optimum cycling period for each temperature is used.

CONCLUSIONSCatalytic activity and periodic operation effects in binary gas systems

(CO-O z, CO-NO, NO-Hz, C3H 6-O Z' and C3H g-O z) over three-way catalysts (Pt, Pd,and Rh/a-Al Z03), were compared in terms of conversions and kinetic parameters.

The detailed behaviour of noble metal catalysts in NO reduction with Hz wasinvestigated and the order of catalytic activity was found to be Pt > Pd > Rh.Periodic operation effects were found: that is, the conversion of NO is improvedby the cycling feedstream compared to the static one. This \1aS most noticeablewith Rh catalyst.

For each binary gas system, our findings are as follows:(1) Comparative catalytic activities in binary gas systems under static con-ditions are

CO-0 2 Rh > Pd > Pt,CO-NO Rh > Pd > Pt,NO-H 2 Pt > Pd > Rh,

C3H 6-0 2 Pd > Pt > Rh,C3H g-O z Pt > Rh > Pd.

(2) In binary gas systems, periodic operation effects are observed according tothe catalyst used and the reaction temperature.(3) The optimum period for maximum conversion decreases with increasing tem-perature in all binary gas systems.(4) Periodic operation effects occurred most noticeably for catalysts havinglower activity or when catalysts were operating at lower temperatures.(5) From the results of the partial reaction order, either reactant contained inbinary gas systems can self-poison the surface of the catalyst and suppress thereaction. the potential for self-poisoning corresponds with the degree of theperiodic operation effect.

From these findings, we consider that periodic operation effects arise from adifference of adsorption capability between the two reactants on the catalystsurface, that is, the self-poisoning reactant is the one more strongly adsorbedon the catalyst surface. Accordingly, the catalyst surface under static con-ditions is almost covered by the stronger adspecies, and expected reactions aresuppressed. Conversely, under optimum cycling conditions, these adspecies areeliminated and surface compositions are suitable for reaction to take place.Under these circumstances, the reaction rate reaches the maximum value.

Periodic operation effects can be applied to improve the reactivity of

197

three-way catalysts and, particularly when catalyst bed temperature is lower,catalytic activity can be promoted by a selection of suitable cycling period.

REFERENCES

K.C. Taylor and R.M. Sinkevitch, Ind. Eng. Chem. Prod. Res. Dev., ~, 45,1983.

2 J.C. Schlatter and F.J. Mitchell, Ibid., 19, 288, 1980.3 R.K. Herz, Ibid., 20, 451, 198!. -4 R.K.Herz, J.B. Kie1e and J.A.Sell, Ib i d, , 22, 387, 1983.5 R.K. Herz and J.A. Sell, J. Catal., 94, 166;"" 1985.6 R.K. Herz and LJ. Shinouskis, Ind. Eng. Chem. Prod. Res. Dev.,~, 385,

1985.7 H. Muraki, H. Shinjoh, H. Sobukawa, K. Yokota and Y. Fujitani, Ibid.,~, 43,

1985.8 H. Muraki, H. Sobukawa and Y. Fujitani, Nippon Kagaku Kaishi, 176, 1985.9 H. Muraki and Y. Fujitani, Ind. Eng. Chem. Proc. Res. Dev., submitted 1985.

10 H. Shinjoh, H. Muraki and Y. Fujitani, Appl. Catal. Submitted 1986.11 T.P. Koblinski and B.W. Taylor, J. Catal., 33, 376, 1974.12 H.C. Yao, Y.F. Yu Yao and K. Otto, Ib l d , , 56, 21, 1979.13 K. Otto and H.C. Yao, Ibid., 66, 229, 1980-:-14 H.C. Yao, M. Sieg, H.K. Pl ummer,Jr., Ibid., 59, 365, 1979.15 M. Chen, T. Wang and L.D. Schmidt, Ibid., 60~356, 1979.16 G. Pirug and H.P. Bonzel, Ibid., 50, 64, 1977.17 A. Amirnazmi and M. Boundari, Ibicr:-,11, 383, 1975.


199

The Role of Research in the Development of New Generation Automotive Catalysts

H. S. Gandhi and M. Shelef

Ford Motor Company

Research Staff

Dearborn, Michigan 48121

ABSTRACT

The development of new generation three-way catalysts (TWCs) is based on abroad, fundamental understanding acquired over a number of years. The effortsat Ford Motor Company have been mainly concerned with supported base metal andnoble metal catalysts operating under oxidizing conditions at high-temperatureswhere the extent of the interactions or their absence does have a profoundeffect with an immediate bearing on the practical use. Use of Zr02, aA1 203etc., as a support for Rh offers an opportunity to formulate a durable catalystin which the thermal stability of Rh can be significantly enhanced and therebyit can remain active at temperatures >600°C in an oxidizing environment.

Other work was aimed at the understanding of whether lead originating fromthe combustion of Pb-containing fuel associates preferentially with the noblemetal sites supported on the much larger areas of inert-support materials.Direct association of lead compounds with noble metal sputtered onto Zr02, Ti02and 11.12°3 wafer supports was noted for Pt, Pd and Rh. However, there are majordifferences in the interaction of Pb compounds with different noble metalsurfaces which explain the long-known fact that Pd and Rh are more sensitive toPb poisoning than Pt.

Model reactions were employed as chemical probes to check whether thedesired surface modifications have been achieved and also for determining mode r.of deactivation in used catalysts. For an optimum catalyst each precious metalhas a specific function to perform and must be carried on a specific supportmaterial to maximize activity and durability.

INTRODUCTION

Three-way catalysts (TWC) were conceptually put forward in 1968 [1] and the

first experimental evidence of selective removal of nitric oxide in the

presence of oxygen under conditions close to stoichometry was observed in 1971

[2]. In 1978 Volvo using an Engelhard supplied catalyst was the first automo-

tive company to actually implement a TWC catalyst in conjunction with an

electronically-controlled fuolinjection equipped with closed-loop control.

In the intervening eight years there has been an evolution of the system

and, most importantly, great advances have been made in understanding the often

200

subtle but important chemical phenomena taking place on the catalyst surface.

This understanding has led to the design of much improved catalysts.

Frinciples of the Operation of the TWC

Fig. 1 demonstrates the principle of the operation of a three-way catalyst

in a diagram of extent of conversion vs. the air/fuel ratio in the actual

operating automotive engine. The air/fuel ratio which is usually expressed in

the weight of air per weight of fuel is easily translatable into an equivalence

ratio (>') whose value is unity at stoichiometry (when all the fuel is converted

to water and C02). Values less than unity represent the conditions of fuel

excess (rich mixtures) and values larger than unity represent excess air (lean'

mixtures. While complete conversion of the reducing species is favored under

the conditions of excess air, >.>1, the complete removal of the oxidizing

species, nitric oxide, is favored under reducing conditions (>'<1). It is only

in the region around >.-1, actually somewhat to the rich-side of >., where there

exists a so-called "window" for efficient, simultaneous removal of all the

three main regulated pollutants. The task of the modern, computer-controlled

fuel-metering system is to mantain the A/F ratio as tightly as possible within

this window over all possib:.e variations in driving conditions. The task of

the catalyst designer, on the other hand, is to provide as wide a window of

operation as possible without compromising the activity.

The present embodiment

of an automotive100

90

80

70

80CATO\LYST

EFFICIENCY 50%

40

10

lU 1~5RICH

14.6 14.7UAN

14.8 14.9

catalyst consist of a

monolithic support made

of a high-melting

ceramic material,

cordierite, typically

having 64 square cells

per square centimeter

cross-sectional area,

with the walls between

the cells being 150 ~m

thick. The walls areAIR/FUEL RAnD

Fig. 1 - Conversion of NO, CO, and hydrocarbons

for a TWC as a function of the air-fuel

ratio.

coated with a high

surface "washcoat '1

having a BET area of

80-100 m2/g. Since the

201

weight of the washcoat is -20-30% of the total weight of the catalyst body the

specific BET area for the whole piece is between 16-25 m2/g. The composition

of the washcoat can vary substantially depending on the desired performance,

which will be discussed in the text to follow. Nevertheless, it is known that

the most abundant ingredient of the washcoat is alumina either in its '"I-phase

or in other transitional form such as 8 or o. The alumina may contain a

number of stabilizers usually chosen from the oxides of rare-earth metals

and/or alkaline earth metals. Into this "washcoat" there are incorporated

simultaneously either all three of the precious metals Pt, Pd and Rh or only P:·

and Rh.

It is the Rh that confers on the TWC the ability to selectively reduce

nitric oxide in the presence of oxygen in a stoichiometric gas mixture (A-I).In this process the Rh-catalyzed reduction of nitric oxide is largely directed

to molecular nitrogen. One has to emphasize the scarcity of this metal, which

is mined at a ratio of 1/17 with respect to Pt with which it usually appears as

a by-product. This ratio in the present TWC is usually much higher, between

1/3 to 1/10. This, associated with the much lesser degree of recovery of Rh

from used catalysts emphasizes the utmost desirahility of utilizing the Rh in

an optimal fashion.

The Role of Metal-Support Interactions in TWC

The interactions we are concerned with are not those usually classified

SMSI (Strong-Metal Support Interactions) which are observed after treatment

under reducing conditions and lead to oxygen-deficient forms of the insulator

supports. On the contrary, the interactions we refer to are associated with

oxidation of the active component and its interaction with the support by

sharing oxygens that ultimately bridge the metal ions in the support and the

metal ions of the active component. An extreme example would be, the well-

known formation of a nickel or cobalt aluminate (spinel) if one would support

Ni or Co on 1-A1203 and expose it to high temperatures under oxidizing

conditions. With noble metals more often than not such interactions are

limited to the surface or subsurface region of the insulator support, but not

always. As a rule, the more refractory the support and the more noble the

active metal the less pronounced is the interaction [3].

Of particular interest to the designer of the automotive catalysts are the

interactions with supports of Rh on o~e hand and of Pt on the other, since they

may determine the availability of the active sites of these metals and the

nature of these active sites which in turn determines reactivity.

In general, one may expect that the interactions mediated ,by surface oxygen

ions of the insulator support will be related to the reactivity of these

202

ions. This in turn is related to the stability of the crystallographic form of

the supports. It has been established that Rh begins to penetrate the subsur-

face of ,-A1203 at >600°C by the solid state reaction between Rh203 and ,-

A1203' This is a temperature which is frequently encountered in an operating

catalyst. Minimizing the reactivity of the support will slow down this subsur-

face penetration and loss of active Rh. Fig. 2 shows this behavior [4]. Here

the surface Rh is measured by CO chemisorption. The initial dispersion is

quite similar on the different samples, the higher CO uptake on the Rh ,-A1203being due to geminal adsorption. Treatment at high temperatures under

oxidizing conditions causes a large irreversible loss of site on ,-A1203 a

small loss on ~-A1203 and virtually no loss on ZrC2 .

"~bbe ....\ .........E .03 \ ....

8:~~~,~~800 1000 800 1000 800 1000 1200CALCINATION IN AIR FOR 5 HOURS AT TEMPERATURE, oK

Fig. 2 - The effect of calcination in air on

The consequence of

the disappearance of Rh

from the surface is a

drastic loss of activity

as shown in Fig. 3a [5J.

Using the data shown in

Fig. 2, one can design a

washcoat where the Rh is

protected from direct

contact with ,-A1203'

This is shown in Fig 3b,

where the Rh was

(A) 0.014 wt% Rh/,-A1203' (B) 0.017

wt% Rh/cr-A1203, and (C) 0.010 wt%

Rh/Zr02' ---, Samples reduced at

673°K; ---, samples reduced at 823°K.

From Ref. [4]. supported on zirconia

first and the resulting

powder was incorporated into ,-A1203 washcoat on a monolithic body. The

activity of this catalyst remains virtually intact after calcination in air at

1100°C for one hour [6].

100

A. 130 ppm Rh/y-AI.O.

izoena:...>zou

80

60

40

20

o ~~~~...l.-l....ad~~0.8 1.0 1.2 1.4 1.6 1.8 OS 1.0 1.2 1.4 1.6 1.8 2.0

REDOX RATIO, R REDOX RATIO, R

Fig. 3 - The steady-state activities of (a) Rh/,-A1203 and (b) [Rh/Zr02]/,-

A1203 after thermal treatment at 1100°C.in air for Ih. From Ref. [5].

203

On the other hand in the consideration of interaction of Pt with insulator

supports we are often faced with a completely opposite tas~ i.e. that of trying

to maximize the surface interaction to enhance and maintain high dispersion.

The reason for this is the relative instability of Pt oxide and its tendency to

decompose at temperatures <550°C even under oxidizing conditions. Platinum

oxide does not penetrate into the subsurface of any support material. To

achieve a high dispersion additives are used in which the oxygen ions are more

reactive than in 1-A1203 as for instance Ce02' Thus the addition of 2.6% ceria

to 1-A1203 increases the surface concentration of pta from 2.2 ~mole Pt/m2 to

4.2 ~mole Pt/m2(BET) [3]. As will be shown below other more reactive metal

oxide additives have a similar effect on Pt dispersion. This, in turn affects

the behavior of the catalysts with respect to several structure sensitive

reactions.

Based on the above examples it is quite apparent that the noble metal

interactions with the support under oxidizing conditions playa significant

role in the design of practical and durable catalysts.

Another degree of modification of the catalysts can be achieved by

introduction of components which on one hand affect the dispersion of the

noble metal similarly to the ceria discussed earlier, but also possess cataly-

tic activities of their own. One example of such an additive explored in depth

at Ford Research is molybdenum oxide. Molybdena, similar to ceria, forms a

two-dimensional phase on 1-A1203 and thereby also affects the Pt dispersion ane!

its catalytic properties. Platinum, in turn, affects strongly the reducibility

of molybdena, as shown in Fig. 4, using ESCA to characterize the oxidation

state after reduction in the absence and presence of Pt [7].

A direct confirmation of this behavior is obtained by TPR [8]. One can

deduce that in the presence of Pt the average oxidation state of surface

molybdenum ions will be lower in an operating catalyst. It is possible to

postulate the existence of surface complexes of the type PtMoOx' where below

600"C x may range from a to 2 depending on the reaction temperature. Recent

preliminary EXAFS results seems to corroborate such a picture [9J. Conversely,

one can consider the surface Pt in such complexes as being more oxidized

(electron deficient) than when dispersed in the absence of modifiers such as

molybdena or ceria. This is found to affect the catalytic properties of Pt. A

similar behavior prevails in other systems as well. For instance, it was

recently reported that addition of cer~a to a Pd/A1203 catalyst results in a Pd

surface state which is more difficult to reduce [10].

One can illustrate the reactivity effect of an active modifier such as

molybdenum by the change in behavior with respect to surface processes in which

CO is an important reactant. Table 1 [8] shows that the self-poisoning

204

226.0 230.0

B.E leV)

behavior of Pt with respect to

CO under reducing conditions is

largely obviated by the

presence of molybdenum. Thus,

for the reduction of NO by

hydrogen, the catalyst in which

Mo is absent is much more

reactive. For reactions in

which CO is present the

opposite is true. Note that

the overall conditions are

strongly reducing, causing the

self-poisoning of Pt sites by

CO. The infrared examination

of another sample shown i.n Fig.

5 indicates the availability of

chemisorption sites for CO

associated with reduced surface

Mo oxides [8].

Fig. 4 - Mo(3d) ESCA spectra

after 3 h in H2 at 500'C (a)

3.8% Mo, (b) 3.8% Mo/3.3% Pt,

(c) 3.8% Mo/24.0% Pt. From

Ref. [7].

TABLE 1Temperature ('C) for 50% and 80% Gross NO Conversion in NO-H2, NO-H2-CO and NO-CO Reactions over Pt and Pt-Mo03 Catalysts supported on 1-A1203

CatalystsPt (0.25%)Pt- Mo03(Pt-0.25%, Mo~2%)FeedgasConcentrations

From Ref. [3 ]

NO-H2T50% T80%

60 100175 185

NO 0.1%CO 0H2 1%

NO-H2- COT5O% 180%360 500250 350

NO 0.1%CO 0.75%H2 0.25%

NO-COT50% T80%NR NR345 360

NO 0.1%CO 1%H2 0

A.2180 em-I

2120 em-I

2080 crn"

c.

2080 em-I

2120 crn"

2120em- 1

E.

205

Fig.5 - IR data of CO chemisorbed on a Pt-Mo/(-A1 203 catalyst.reom Ref.(8).(A) Fresh sample. (B)Pre-reduced at 300 C with 60 torr CO.(C) Exposed to air at 25 C.(D)Heated in air at 100 C for 20 min.(E) Heated in air at 300 C for 20 min.IR Peaks assignment: 2180 em-I_CO on PtO;2120 em-I_CO on Pt;

2080 cm-I_CO on M002(

Mo+4 ) .

100

0N

Z 80N

Z

t0 60z

>-'= 40 -<-"'/

,CO l>I- ~O/0w /.....l 20w

~ /({)

~7~~0' I I,O-~

200 300 400 500 600 700TEMPERATURE r-c:

Fig.6 - Selectivity as a function of temperature for NO conversion

over a Pt/~-AI203 and a Pt-Mo03/(-A1203 catalyst.From Ref.(ll).

The effect on selectivity in the reduction of NO by hydrogen, especially

in the presence of CO,is still more dramatic.In this context the selectivity

is defined as that leading to products in which the N-N bond has been formed such

206

as N2 or N20. The formation of such a bond depends on the probability of

forming adjacent pairs of surface entities containing nitrogen. Fig. 6 [11]

shows the enhanced selectivity to the desired product due to incorporation of

Mo. It was shown in another publication that partially reduced surface Mo iuns

adsorb geminal pairs of NO molecules (cis-dimers) [12]. This, and the absence

of poisoning by CO shown in Table I, are the reason for the observed, enhanced

formation of N-N bonds in the Pt-Mo/A1203 catalysts.

insensitive reactions such as the

Fig. 7 shows directly that the

activity for the oxidation of

propane of a catalyst containing

0.07%Pt/y-A1203 is strongly

inhibited by the addition of 3.7%

Ce02. The influence of ceria is to

maintain the high initial dispersion

and to prevent the agglomeration of

Pt into discrete particles. While

this would enhance the structure

oxidation of co and nonsaturated

hydrocarbons it does strongly in-300 400 500

TEMPERATURE (Oe)

200

80;!.

z0(J) 600::W>Z0Uw 40z<t0-00::0- 20

Dispersion of Noble Metals and Structure-Sensitive Reactions

A number of structure-sensitive reactions take place in the catalytic

converter which, as expected, will be influenced by the noble metal dispersion.

These include first of all the very important oxidation of saturated

hydrocarbons. It is an accepted view that whenever the surface reaction

involves the scission of a C-C bond structure-sensitivity is to be expected.

There is ample evidence that oxidation of saturated hydrocarbons, especially

those of short chain length, does not proceed readily on Pt catalysts with very

high dispersion.

100

Fig. 7 - Conversion of propane on a 0.07% hibit the oxidation of saturated

Pt catalyst supported on 7-A1203 with hydrocarbons. Similarly, when

and without 3.7% ceria; 1000 ppm C3H8' molybdena is the additive which

2% 02. stabilizes the dispersion of Pt, the

oxidation of propane is strongly

inhibited. (Table 2) [8J.

207

TABLE 2

Propane Oxidation over Pt, Pt-Mo03 Catalysts

Temperature at Given Conversion

Catalyst

Pt (0.2%)

Pt- Mo03

(Pt~0.25%, Mo~2%)

Feedgas

Concentrations:

S.V. 60,000 hr- l

From Ref. [8]

T25%

230

458

T50 %

267

640

C3H8 0.05%

02 2.25%

S02 20 ppm

N2 Balance

T75%

298

NR

Other noble metals such as Pd or Rh which have more stable oxides than Pt

and therefore tend to remain well-dispersed on ,-A1203 even in the absence of

additives such as ceria or molybdena are usually poor catalysts for the

oxidation of saturated hydrocarbons. In fact, in a mixture of hydrocarbons

containing 2/3 propylene and 1/3 propane the propylene will be easily oxidized

<300'C while the propane remains untouched even at 600'C [13].

Another structure-sensitive reaction which is important in an indirect way

to the behavior of the automotive catalyst is the oxidation of S02. The

average sulfur content of gasoline in the U.S. is 300 ppm by weight which

converts to -20 ppm in the exhaust stream. It may vary in a wide range in the

European countries. Although sulfur compounds fall in the U.S. under the

category of unregulated emissions the oxidation of S02 to S03' or conversely

its reduction under certain conditions to H2S, has environmental and customer

acceptance implications and it might affect the durability of the catalytic

converter by modifying the poisoning resistance, in particular by lead as will

be shown below.

Table 3 shows the suppression of S02 oxidation by stabilizing the Pt

dispersion by ceria. This behavior is almost completely analogous to that

observed in the case of the oxidation of propane by molybdena. Indeed,

stabilization of Pt dispersion by Mo03 similarly inhibits S02 oxidation [8].

208

TABLE 3

S02 Oxidation Over Pt/l-A1203 and Pt-Ce02/1-A1203

% S02 Conversion at TOC

400 450 500 550

Pt (0.07 wt %)

Pt (0.07 wt %) + Ce (3.7 wt%)

80

52

88

54

92

55

82

57

Feedgas: S02 0.02%, 02 1%, S.V. 60,000h- 1

Most importantly, the presence of sulfur oxides has a rather pronounced

influence on the oxidation of saturated hydrocarbons and is intuitively

unexpected. Thus, the presence of 20 ppm of S02 in a stream of 500 ppm propane

in nitrogen carrier gas increases considerably the activity of a Pt/l-A1203

catalyst for propane oxidation.

100

// /

/80 .j Ii /

/• V 70wt%P1

, r ?0 / r

~... f:>.O.06.t%Pt , I /

60 .oO.03wt%Pf Y , P4i /

II / J<3 /r I /

-r 40 I c!I t! /

U' , /

")7 / /

I / .d20 / /' ,t" P

/£0_-0.-0--/0/

00 400 600Temperofllre,'"C surface sulfate group by S02

adsorption and subsequent oxida-

Fig. 8 [14] shows that in the-

presence of S02 a catalyst

containing 0.03% Pt becomes as

active as a catalyst containing 7%

Pt, lowering the temperature of

50% conversion of the 0.03% Pt

catalyst from 500'C to 250°C.

This change of activity is

attributed to the formation of a

Fig. S - Percentage conversion as a·function at 200°C. The IR study [14]

shows that surface sulfates

promote the dissociative adsorp-

tion of C3HS on Pt leading to a

higher propane oxidation

activity.

An important issue of structure sensitivity has to do with the oxidation

tion of temperature for C3HS oxidation

over three Pt/l-A1203 catalysts of

different Pt concentrations.

From Ref. [14].

of methane. Although methane does not have a C-C bond to be cleaved, it is the

hydrocarbon most difficult to oxidize. There are some indications that methane

oxidation may be structure-sensitive which will be studied further. The

oxidation of this rather unreactive molecule is of practical importance, since

there are proposals to lower the allowable hydrocarbon emissions that cannot be

met without at least partial oxidation of methane.

209

Relation between Reactivity and Poison Resistance

It turns out that the reactivity of the catalyst in structure sensitive

reactions may have a large significance in determining the resistance to

poisoning in particular by lead, and therefore can influence the catalyst

durability and the ability to fulfill the regulatory requirements of useful

lifetime.

Although vehicles equipped by catalytic converters are fueled, by law, by

lead-free gasoline the residual amount of lead can have quite a pronounced

deactiving influence. Also the effect of accidental misfueling can be

detrimental. It is well known that the three noble metals used in automotive

converters have a widely disparate resistance to deactivation by lead. The

most sensitive to deactivation is Pd.

Fig. 9 [15] shows the

actual contaminant levels in

extreme sensitivity of Pd

catalyst to the trace lead

levels in the fuel in the

range from 0 to 12

mgPb/gallon (equivalent to 3

mgPb/l). It is worth noting

that the present legal limit

in the u.s. and West Germany

is 50 mgPb/gallon. But it

should also be noted that theo

mq Pb,aI

S.V.-60,000 h·1

T .550·C

0.22 "10 Pd CATALYST

1.2 1.4 1.6REDOX RATIO, R

1.0

80

20

zoill0: 60OJ>ZoUo 40z;!.

R - 1.3. From Ref. [15].

Fig. 9 - Effect of trace Pb levels on the

steady-state NO activity of 0.22% Pd after

-15,000 simulated miles of pulsator aging at

the u.s are considerably

lower, 2-3 mg Pb/gallon, that

is within the range shown for

the data on Fig. 9. Fig 9

shows an extraordinary

sensitivity of the catalytic

activity to the lead levels

and the experiment resolves

clearly between minute

increments of the lead in the fuel. While the data in Fig. 9 refer only to the

loss of activity for NO reduction a similar trend is observed for hydrocarbon

oxidation [15J. The sensitivity of Pd to deactivation by traces of lead is thE

main reason why this relatively abundant and cheap noble metal is generally not

used extensively in place of Pt, in particular in the first converter of a dual

bed system.

210

The experience of automotive catalysis indicates that Rh is only somewhat

less susceptible to poisoning by lead traces than Pd while Pt is by far thc

most resistant.

The use of model systems amenable to detailed surface analysis provides a

means for the direct examination of the association of lead wih the surface of

noble metals [16]. It immediately becomes apparent that in all the three

supported noble metals the lead is directly associated with the noble metal

sites and not with the support material, which in actual catalyst constitutes

over 95% of the exposed BET area. This is shown 0:. Fig. 10 [16J, for Pt

supported on A1203, from the electron probe elemental maps. The Pt and Pb maps

of samples exposed to simulated exhaust generated from combustion of iso-octane

fuel containing 1.5 g Pb/gallon and 0.03 wt%S are exactly superimposed. The

same obtains whether the support is 1-A1203, Ti02 or Zr02 on one hand or

whether the metal is Pt, Rh or Pd.

Fig. 10 - Electron probe elemental map after Pb exposure for 24 h at 700'C for

Pt supported on 1-A1203' From Ref. [16].

211

Nevertheless, Pt is much more resistant than the other noble metals to

lead poisoning and the reason for this is largely indirect. Thus the small

amount of sulfur acts as a scavenger for the lead. To achieve this it is

necessary that the sulfur be in its hexavalent oxidation state to combine with

lead oxide to form a stable lead sulfate which in itself is not a site-specific

poison. Only Pt, among the noble metals is a good catalyst for the oxidation

of S02 to S03 [17J and indeed on a Pt catalyst the lead is present as the

sulfate as shown in Fig. 11. It is clear that large amounts of lead sulfate

present in several overlayers will also act as a non site-specific poison by

obstructing the access of the reactants to the surface. We have established

that in Rh-catalysts the lead is present as an oxide and in the case of Pd

catalysts as an intermetallic compound with the Pd [16].

In all cases the

association of the lead is

...oN

:;;

100

80

specific with the noble metal

because the lead-carrying

molecules, most probably oxy-

halides, decompose on the

noble metals sites leaving

the lead on the surface.60 >-

f--Cf)Zwf--Z

40

<5en ....Q 0a. --S:

0" '"en.Q0.

28

Fig. 11 - X-ray diffraction pattern of

Pt/1-A1203 after Pb exposure for 72 h at

700·C. From Ref. [16].

Table 4 highlights the

specificity of this associa-

tion showing the relative

lead counts in microprobe

analysis when the same

samples of model catalysts of

20 Pt, Pd, Rh supported on

A1203' Ti02 or Zr02 are

exposed to a combustion gas

in which the lead was

originally present either as

"motor mix" i.e. tetraethyl

lead with dibromide or

dichloride scavengers or, in

one case, as Pb02 vapor in

the exhaust. There is more

than two orders of magnitude difference in the amount of lead deposited on the

noble metal as compared with that deposited on the bare support. The

difference when the lead-carrying species is the lead oxide is much smaller and

may be insignificant.

212

TABLE 4

Pb Affinity for Noble Metals (NM) and Various SupportsPb (counts s-l)a

NM

Pt

Pd

Rh

NM/A1203

758/6

344/10

896/6

NM/Ti02

1140/2

740/l(40/2l)b

989/1

NM/Zr02

980/6

895/7

246/8

a

b

Semiquantitative microprobe analysis: average over 10 areas of 100 ~m x

100Mm size; 20 KV beam energy; 20 s counting time; Pb present in iso-

octane as TEL Motor Mix (TEL+EDB+EDC scavengers).

Pb present as Pb0 2 vapor in iso-octane exhaust (EDB and EDC

scavengers absent).

The specificity of the association of lead which derives from the gasoline

with noble metal sites on the surface of the catalyst is the reason that minute

amounts are still quite detrimental as shown most clearly for Pd catalysts in

Fig. 9.

CONCLUDING REMARKS

The foregoing has made it abundantly clear that the automotive catalyst in

itself is a very complex chemical system and becomes even more so when all the

subtle interactions with the exhaust environment are taken into account.

Relatively minor fuel constituents such as the always present sulfur or small

amounts of halides may have a pronounced effect on its overall behavior. By no

means has the preceding been a complete account of all the possible interac-

tions. Thus we have omitted the important effects of possible alloy formation

between the active metals [18, 19J and the various deactivating influences

deriving from automotive lubricants, the most important being the effect of

phosphorus [20]. Further, quite often unexpected contaminants may do severe

harm to the emission hardware [21].

The designer of the automotive catalyst has to take all these into account

as well as the expected physical environment, the most important being the

driving conditions which will determine the temperature of the device.

In an optimal catalyst each precious metal has a specific function to

perform, such as Rh for nitric oxide reduction, Pt for the oxidation of

213

salurated hydrocarbons, etc. In choosing the proper support and its modifiers

for each of the noble metals one has to bear in mind what is the desired

dispersion and one has to balance the utilization of the noble metal, that is

the proportion available for the surface reaction, versus the probability of

the irreversible interaction with the support which results in permanent loss

during use. Further, one has to consider the proper ratios of the noble metals

and the advisability of having them in close contact or separated.

Although the development of modern automotive catalysts started about

twenty years ago and they have been in use for more than 10 years, there still

remains ample room for improvement and better utilization of the scarce noble

metals. This can only be achieved by acquiring more knowledge through well-

directed research.

The driving force for this will be on the one hand more strict

environmental regulations as now witnessed in California, and on the other, the

ever widening environmental concerns in varying parts of the world.

REFERENCES

1 G.P. Gross, W.F. Biller, D.F. Greene and K.K. Kearby, U.S. Patent3,370,914.

2 J.H. Jones, J.T. Kummer, K. Otto, M. Shelef and E.E. Weaver, Env. Sci. &Tech., 2 (1971) 790-98.

3 H.C. Yao, H.S. Gandhi and M. Shelef, "Metal Support and Metal AdditiveEffects in Catalysts", B. Imelik (Ed.), ElseVier, Amsterdam, 1982, pp.159-169.

4 H.C. Yao, H.K. Stepien and H.S. Gandhi, J. of Catalysis, 61 (1980), 547-50.

5 H.K. Stepien, W.B. Williamson and H.S. Gandhi, SAE Paper 800843, Dearborn,MI, 1980.

6 H.S. Gandhi, J.T. Kummer, M. Shelef, H.K. Stepien, and H.C. Yao, U.S.Patent 4,233,189.

7 J.E. deVries, H.C. Yao, R.J. Baird, and H.S. Gandhi, J. of Catalysis, 84(1983), 8-14.

8 H.S. Gandhi, H.C. Yao and H.K. Stepien, Am. Chern. Soc. Symp. Series, No.178, "Catalysis Under Transient Conditions", A.T. Bell and L.L. Hegedus(Eds), 1982 pp. 143-162.

9 S. Sakellson, G.L. Haller and H.S. Gandhi, personal communication.10 A.S. Sass, A.V. Kuznetsov, V.A. Shvets, G.A. Savel'eva, N.M. Popova and

V.B. Kazanskii, Kinetika i Kataliz, 26 (1985) 1411-15.11 H.C. Yao, K.M. Adams and H.S. Gandhi in "Frontiers' in Chemical Reaction

Engineering", L.K. Doraiswamy and R.A. Mashelkar (Eds.), Wiley Eastern,New Delhi, 1984, pp. 129-141.

12 H.C. Yao and W.G. Rothschild, "Proc. 4th. Int. Conf. on the Chemistry ofMolybdenum", H.F. Barry and P.C.H. Mitchell (Eds.), Golden, Colorado,1982.

13 W.B. Williamson, H.K. Stepien and H.S. Gandhi, Env. Sci. & Technology, 14(1980), 319-25.

14 H.C. Yao, H.K. Stepien and H.S. Gandhi, J. of Catalysis, 67 (1981), 231-36.

15 W.B. Williamson, D. Lewis, J. Perry and H.S. Gandhi, Ind. Eng. Chern.,Product R&D, 23 (1984), 531-36.

16 H.S. Gandhi, W.B. Williamson, E.M. Logothetis, J. Tabock, C. Peters, M.D.Hurley and M. Shelef, Surface and Interface Anal., Q (1984) 148-61.

214

17 H.S. Gandhi, H.C. Yao, H.K. Stepien and M. Shelef, SAE Paper 780606,Special Publication (SP43l), 1978.

18 W.B. Williamson, H.S. Gandhi, P. Wynblatt, T.J. Truex and R.C. Ku, AICIIESymposium Series, No. 201, (1980) p. 212.

19 B.M. Joshi, H.S. Gandhi and M. Shelef, Surface Technology, in press, 1986.20 W.B. Williamson, J. Perry, R.L. Goss, H.S. Gandhi and R.E. Beason, SAE

Paper 841406, Baltimore, MD, 1984.21 H.S. Gandhi, W.B. Williamson, R.L. Goss, L.A. Marcotty and D. Lewis, SAE

Paper 860565, Detroit, MI, 1986.

A. Crucq and A. Frennet (Editors), Catalysis and Automotive Pollution Control@) 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

MECHANISMS OF THE CARBON MONOXIDE OXIDATION AND NITRIC OXIDE REDUCTION

REACTIONS OVER SINGLE CRYSTAL AND SUPPORTED RHODIUM CATALYSTS:

HIGH PRESSURE RATES EXPLAINED USING ULTRAHIGH VACUUM SURFACE SCIENCE

+GALEN B. FISHER, SE H. OH, ~OYCE E. CARPENTER, CRAIG L. DiMAGGIO, ANDSTEVEN J. SCHMIEG

Physical Chemistry Department, General Motors Research Laboratories,Warren, Michigan 48090-9055 (U.S.A.)

D. WAYNE GOODMAN

Surface Science Division, Sandia National Laboratories,Albuquer que, New Mexico 87185 (U. s. A.)

THATCHER W. ROOT*, SCOTT B. SCHWARTZ**, AND LANNY D. SCHMIDT

Department of Chemical Engineering and Materials Science,University of Minnesota, Minneapolis, Minnesota 55455 (U.S.A.)

ABSTRACT

21.5

The demonstration that surface parameters obtained in ultrahigh vacuum(UHV) experiments are applicable to high pressure catalytic reactions has longbeen a goal of catalytic surface science studies. This report summarizes aset of work which has successfully shown, for carbon monoxide oxidation andnitric oxide reduction over rhodium, that high pressure rates can be predictedquantitatively using parameters determined solely under ultrahigh vacuum con-ditions. One implication of this work is that, for this important class ofreactions, the strongly-bound surface species present under the condi tions ofUHV studies are the same species reacting at high pressures.

INTRODUCTION

An effort has been made in this work to evaluate the utility of surfaceparameters determined in UHV surface science experiments for understanding the

high pressure kinetics of certain catalytic reactions. We have chosen two

test reactions of considerable significance in automotive exhaust catalysis,CO oxidation (2CO + O2 ~ 2C02) and NO reduction (2CO + 2NO~ 2C02 + N2) over

rhodium. To accomplish this comparison, rate constants for the elementarysteps of both reactions were determined under ultrahigh vacuum conditions.

+Present Address: AC Spark Plug Division, General Motors Corporation,Flint, Michigan 48556.

*Present Address: Chemical Engineering Department, University of Wisconsin,Madison, Wisconsin 53706.

**Present Address: Sherwin-Williams Co. Research Center, 10909 South CottageGrove, Chicago, Illinois 60628.

216

Then, steady state rates for each reaction were measured over both single

crystal and supported catalysts at realistic, high pressures (1-300 Torr).The use of the UHV-determined parameters in kinetic models based on the sur-face chemistry studies is successful in predicting quantitatively the rate

data taken at high pressures for both reactions.

ULTRAHIGH VACUUM AND HIGH PRESSURE SURFACE CHEMISTRY STUDIES

To begin with, the adsorption properties, activation energies for desorp-

tion and dissociation, the orientation, and the binding sites for chemisorbednitric oxide, carbon monoxide, and oxygen were characterized on the single

crystal Rh(111) surface with high resolution electron energy loss spectroscopy

(EELS), UPS, XPS, LEED, and temperature programmed reaction spectroscopy(TPRS) [1-6J. For example, we have found the useful results that the activa-

tion energy for NO dissociation on Rh(111) is 19 ± 1 kcal/mole [~J and onRh(100) is 18 ± 1 kcal/mole [6J. We've also observed that adsorbed NO and CO

form well-mixed surface layers near reaction temperatures [5J, and that theheat of adsorption for CO on Rh(111) is reduced by 8-10 kcal/mole in the

presence of nitrogen atoms [3]. In addition, steady state kinetic studies of-5 -8both reactions on Rh(111) were carried out at low pressures (10 -10 Torr)

[7,8J and high pressures (1-300iorr). The high pressure results have been

compared with results over supported Rh catalysts for the same reactions which

were measured for the same temperatures and pressures [9J. Finally, we havefound that rate expressions based on UHV-determined elementary intermediate

steps using UHV-determined rate constants quantitatively predict the rates at

high pressures for both the CO-02 and NO-CO reactions over single crystal Rhand supported Rh catalysts. This is the first time we are aware that high

pressure catalytic reaction rates have been predicted solely from UHV-deter-

mined experimental parameters. The success of these predictions based on UHVwork shows, for an important class of reactions, that the strongly-boundspecies present under the conditions of UHV studies are the same species

reacting at high pressures.

CARBON MONOXIDE OXIDATION

More particularly for the eo-0 2 reaction, we have measured the reactionrate over Rh(111) for a wide range of pressures around p(eO) ~ P(02) ~ 0.01a t.m , , pressures similar to those found in automoti ve exhaust, and for tempera-

tures between 450 K and 600 K. These data are shown in Fig. 1. The reactionis first order in oxygen and negative first order in CO. From 450 K to 600 K

the reaction rate increases by almost four orders of magnitude and is charac-terized by a single activation energy (29 kcal/mole). We find excellent

agreement between the specific rates and acti vation energies measured for a

217

1000

Q)

100'"-Ca::-,'"Q)

::JUQ)

oE 10No

U

....Q)>oc....::JI-

Pco := P0 2

:= 0.01 atm

• Rh(lll}1------1 Rh/AI203............ Model

..~

\\

\.\.

'Co~.x.

~. ,.,.i··..

2.21.8 2.01OOO/T (K- 1)

0.1 L-_J--_...L-_..J-_-.L._--'-_--"'_

1.6

Fig. 1. Comparisons of the specific rates of the CO-02 reaction measured overRh(lll) and Rh/A120 at P(CO) = P(02) = 0.01 atm. from Ref. 9. The modelprediction fits qUarltitatively with the measured rate data for both catalysts.

Rh(111) crystal and a 0.01 wt% Rh/A1203catalyst, an indication of a struc-

ture-insensitive reaction.The elementary steps which were used to model the CO oxidation reaction basedon the rate constants measured in UHV surface chemistry studies are as fol-

lows:

CO (g) ;::::'

°2 (g)

CO(a)

COra)20(a)

As is shown in Fig. 1. we are able to predict the measured absolute rates andactivation energies using a kinetic model only employing parameters determined

experimentally in UHV studies [9J. In fact. the same rate expression usedsuccessfully at high pressures predicts the CO-02 reaction rate ~t much lower

-8pressures (-10 Torr) and at lower temperatures «400 K) where the CO

218

coverage is approximately the same as at high pressures [7]. Because thereaction rate essentially depends only on reactant surface coverages, our

understanding of CO oxidation clearly bridges the "pressure gap". The picture

of the CO-02 reaction which is confirmed by this work is that the Rh surface

is predominantly covered by adsorbed CO and the reaction is limited by therate of CO desorption (Eq. 1) or, in other words, the rate of creation of a

vacant site, where oxygen adsorption (Eq. 2) and subsequent reaction (Eq. 3)

can occur.

NITRIC OXIDE REDUCTIONFor the NO-CO reaction over Rh(111) at high pressures, we find that the

reaction is positi ve order in NO and surprisingly is zero order in CO. As is

shown in Fig. 2, from 500 K to 650 K the reaction has an activation energyclose to 30 kcal/mole. After reaction the Rh(111) surface is nearly covered

with nitrogen atoms. (The nitrogen atom coverage is also high near the ratemaximum in low pressure studies [8J.) The elementary steps which were used to

model the NO-CO reaction shown below were also chosen based on the UHV mea-

surements of the rate constants of each step.

CO(g) -. CO (a) (4)<-

NO(g) -. NO(a) (5)<-

NO(a) N(a) + Ora) (6)

COra) + Ora) CO 2(g)(7)

NO(a) + N(a) N2(g) + Ora) (8)

2N(a) -. N2(g) (9)

Using a rate expression based on these steps for the NO-CO reaction, we were

again able to predict the observed absolute rates and activation energies athigh pressures over Rh(111) (see Fig. 2) using only UHV-determined elementary

steps and rate parameters [9 J. The explanation of the unexpe cted pr ess uredependence of CO at high pressures is the UHV result [3J that adsorbed nitro-

gen atoms reduce the CO heat of adsorption in Eq. 4 sufficiently to give azero order CO pressure dependence. In fact, the rate of the NO reduction

reaction over Rh(11 1) is controlled by nitrogen atom recombination and desorp-

tion (Eq. 9).In contrast with CO oxidation over the Rh/A1203

catalyst, the NO-CO reaction

over Rh/A1 203is structure-sensitive and has a lower rate and a higher

219

500

• Pea =PNO =001 atm

• Rh (111)u:;100 • Rh/AI 203

ModelQ)-(j)

La:<,(j)Q)

\::J

10 \o IQ)I

0 \E \N \

0 \

~I\

~ \Q) \..Cl \E I::J \Z I •~

\Q)>0c~

::Jl-

0.1 •1.4 1.6 1.8 2.0

1OOO/T (K- 1)

Fig. 2. Comparison of the specific rates of the CO-NO reaction measured overRh(ll 1) and Rh/AI 20 , at p(CO) = P(NO) = 0.01 atm. from Ref. 9. The predictionof the surface chemrstry model follows closely the rate data for Rh(lll).

activation energy at a 1:1 NO:CO ratio (45 kcal/mole) than over the singlecrystal catalyst. This is also clearly seen in Fig. 2. Over Rh/Al 203 weconclude that the NO-CO reaction is limited by the rate of nitric oxide disso-ciation, Which is about 2000 times slower on Rh/Al 203 than on Rh single crys-tals [9J.

CONCLUSIONSThe knowledge from this work of the relative importance of the elementary

steps of the CO-02 and NO-CO reactions clarifies which steps need modificationto increase overall reaction rates. Changes in the supported catalyst whichincrease the NO dissociation rate under NO-CO reaction conditions shOUld pro-vide one path to a better overall reaction rate. Studies which can clarifythe origin of the structure sensitivity of the NO-CO reaction over Rh will

220

also be helpful. A fuller understanding of both the CO oxidation and NO re-

duction reactions will hopefully lead to the more effective use of rhodium inautomoti ve exhaust catalysi s .

REFERENCES

1 G. B. Fisher and S. J. Schmieg, J. Vac. Sci. Tech. A 1 (1983) 1064.2 T. W. Root, L. D. Schmidt and G. B. Fisher, Surface Sci. 134 (1983) 30.3 T. W. Root, L. D. Schmidt and G. B. Fisher, Surface Sci. 150 (1985) 173.4 T. W. Root, G. B. Fisher and L. D. Schmidt, J. Chem. Phys-.--(1986), in

press.5 T. W. Root, G. B. Fisher and L. D. Schmidt, J. Chem. Phys. (1986), in

press.6 G. B. Fisher, C. L. DiMaggio and S. J. Schmieg, in preparation.7 S. B. Schwartz, L. D. Schmidt and G. B. Fisher, J. Phys. Chem , (1986), in

press.8 S. B. Schwartz, G. B. Fisher and L. D. Schmidt, in preparation.9 S. H. Oh, G. B. Fisher, J. E. Carpenter and D. W. Goodman, J. Catalysis 100

(1986) 360.


ELECTRONIC STATE OF CERIUM-BASED CAT AL YSTS STUDIED

BY SPECTROSCOPIC METHODS (XPS, XAS)

F. LE NORMAND 1,2, P. BERNHARDT 1, L. HILAIRE 1, K. KIll 1, G. KRILL 3 and

G. MAIRE 1

1Laboratoire de Catalyse et Chimie des Surfaces, Universi t e Louis Pasteur,

4, rue Blaise Pascal, 67070 Strasbourg Cedex, France

2present address: L.U.R.E., Batiment 209D, Centre Universitaire Paris-Sud,

91405 Orsay Cedex, France

3Laboratoire de Physique des Solides, Univer si t e de Nancy I, BP 239,

54506 Vandoeuvre-Les-Nancy Cedex, France

221

ABSTRACT

X-Ray Photoelectron Spectroscopy (XPS) 0 f the 3d core level of cerium and X-RayAbsorption Spectroscopy (XAS) of the Lm absorption edge of cerium have been used tostudy Pd/Ce02, Pd-Cel y AIZ03 and Ce/y Al z03 catalysts. The oxidation state of ce-rium was Fourid to decrease wlr h decreasing amounts of cerium on the surface. It wasquite close to III for very low contents of cerium (Z-3%). For higher cerium contentsthe oxidation state was nearer to IV but differences between the two methods werefound, owing to the fact that XAS is a volume sensitive probe. The oxidation state ofcerium waiilalso lower for Pd-Cel y AI203 than for Cel y AIZO y suggesting the forma-tion of Ce OCI, chlorine coming from The precursor salt of palladium.

INTRODUCTION

It has been proved that cerium is a very efficient additive in catalysts for the con-

trol of automobile pollutants.

According to literature data (ref. 1,Z), cerium in these catalysts acts as

- a promotor of the catalytic activity for redox cycles; thus cerium can provide

oxygen during the oxidative step of the cycle and remove it during the reductive step;

this is the well- known oxygen storage capacity (O.S.C.) of cerium.

- a stabilizer agent inhibiting the loss of y AI Z0 3 surface area

- a dispersive agent for the transition metal.

In view of the chemical properties of ceria, two questions arise. First, while the stable

ceria CeO Z is well known to be very difficult to reduce (ref. 3), the O.S.c. implies

that it can be easily reduced and oxidized at moderate temperatures. Second, the na-

ture of the interaction between the rare earth and the transition metal is not welles-

tablished : formation of an alloy (ref. 1,4), of an ionic or covalent chemical bond (ref.

5), decoration of a transition metal particle (ref. 6)...

We have applied X-Ray photoelectron spectroscopy (X.P.S.) of the 3d core level of

222

cerium and X-Ray absorption spectroscopy (X.A.S.) 0 f the LIII absorption edge of ce-

rium to Pd/CeO Z' Pd-Ce/y AIZO) and Ce/y AlZO) catalysts. These probes can give us

a description of the cerium electronic configuration, which can be linked to :

- its chemical nature (metallic, ionic, ... )

f .. d"d . (C III C IV)- or an IOniC compoun , Its OXI crron state e or e

The parameters investigated here are :

- influence of the cerium content

- influence of the presence of a transition metal.

EXPERIMENT AL

Catalysts were prepared by coimpregnation of Pd(NH))4 CI 2 and Ce(NO))) on

yAIZO) WOELM (15)m'/g). For the Pd/CeO Z sample, Ce02 support (42m'/g) was pre-

pared by precipitation of Ce (NO))) into hydroxide at pH = 9, followed by calcination

(5 hr s, 550°C). Each catalyst, after wetting, was calcined at ZOO°C, 4 hours, then redu-

ced up to 400°C (4°C/mn, then 1 hour )OO°C), and passivated under nitrogen before

spectroscopic investigations (Table 1).

TABLE 1

Characteristics of the different Pd-Ce/y AI20) with Pd(NH))4CI2 as precursor salt.

Catalysts n? (%Pd)wt (%Ce)wt BET surfacearea (m'/g)

I - 1 9.) 0I - 2 8.1 0.02 110I - 3 6.9 0.3) 126I - 4 7.8 0.52 118I - 5 8.5 1.0 133I - 6 8.5 1.5 82I - 7 7.0 3.2 74I - 8 6.4 12.5 79

II - 1 0.7II - 2 1.5II - 3 2.6II - 4 10.3II - 5 12.3

In series I, the cerium content was varied from 0.3 to 12.4 weight %. Series II inclu-

des Ce/ y AI203 samples from 0.7 to 12.3 weight %. The palladium loading was main-

tained roughly constant at 8.0 ~ 1.5% weight %, for spectroscopic investigations.

X.A.S. experiments were achieved using the synchrotron facility of the EXAFS III

spectrometer at LURE, Orsay, France with a Si(200) monochromator, or on an inlab

spectrometer using a silver Rigaku anode and a Si(311 ) monochromator. Other details

on XPS and XAS experiments have been reported elsewhere (ref. 7).

223

RESUL TS AND DISCUSSION

1) Influence of cerium content

The 3d core level spectra of Pd-Ce/ 'l AIZ0 3 catalysts are compared to -the CeO Zspectrum (see fig. 1). We referred the binding energies to Al Zp at 119.8 eV. For CeO Z'some Al Z0 3 was added mechanically. Apart from the spin-orbit coupling, the 3d 5/Z

transition of cerium exhibits three main contributions for ceria. We observed two tran-

sitions for the cerium based catalysts, as previously reported in the literature (ref. 8,9)

for Ce III spectra. On the other hand we note l) A shift of the v' line of about Z.5 eV

towards lower binding energies (line v"). This line is currently assigned to the 3d94f 1

final state as the major contribution (ref. 10,11). ll ) The disappearance of the well cha-

racterized u'" line, due to the 3d94fO transition. Since there is no possible overlap for

this transition, we could calculate the 3d94fO spectral weight, which is an indication of

the Ce IV oxidation state (see fig. 4). However this value must be considered with some

caution since the probe introduces strong final state effects which results in a transfer

of valence electrons to localized 4f states for the screening of the core hole (ref.

10,11). Finally we observe no shoulder at low binding energy (879-880 eV), characteris-

tic of an alloy. Thus we must conclude that at the surface of the catalyst and under

our treatment conditions, cerium is in an ionic form and preferentially in the +III oxida-

tion state.

We report in fig. Z the XAS LIII edge spectra of a few Pd-Ce/ 'lAlz0 3 and Pd/CeOZ ca-

talysts. Intensity transitions are normalized to transitions in the continuum states. In

addition, we report in fig. 3 the deconvolution of CeO Z and Ce(OH)3 which exhibit the

difference between a CeIII and a Ce IV compound. Ceria exhibits five satellites and

Ce(OH)3' like all CeIII compounds (ref. lZ, 13, 14) only one which corresponds to B1 line.

The B transitions are assigned to Zp5 4f 1 5d final states (Ce III oxidation state) and the

C transitions to Zp5 4fO (and 4fZ?) 5d final states (Ce IV oxidation state). The doublets

of the lines Band C are probably due to crystal field effects. As for XPS, we are able

to calculate the 2p5 4fO 5d spectral weight from the C transitions. Note that in the ca-

se of X-Ray absorption, final state effects are of less importance due to the participa-

tion of the 5d photoelectron to the screening of the core hole. In good agreement with

previously reported XPS and XAS spectra, the two methods give the same value of the

spectral weights for pure CeO Z (ref. 15).

For the Pd-Ce/ y Al Z0 3 catalysts, we observe a continuous evolution from the complex

spectrum of ceria to a strong single transition B1 as the cerium content decreases. Mo-

reover the narrow half width of this transition (co 3.0 eV) strongly suggests that ce-

rium is in a ionic form. Hence, in agreement with our XPS results, cerium is in a stron-

gly reduced state for low cerium content. However for cerium content higher than co

Z-3%, discrepancies appear between the two methods, as evidenced by the respective

spectral weights (see fig. 4).

224

A.U

, ,!d 4f (ivnid

eQ13_o;;2)0 006.BE In eV

1..·I

II

\

i-ig. 1 : XPS 3d core level of Pd-Ce/y AIZ0 3. Influence of the cerium content.

\0 00 ASS

9 00

8 00

7 00

6 00

5 00

... 00

3 00

2 ee

\ 00

Pd-Ce 0.3/ AI20]

Pd-Ce 0.5/ AI20]

Pd-Ce 3.2/ A120]Pd-Ce 12.5/ AI

20]Pd 8.4 ICeD 2

Ce0 2

5733 00

fGIERGY CEV)

5750 88

Figo Z XAS Lm edge of Pd-Ce/ y AIZ0 30 Influence of the cerium content.

225

IIIand Ce (OB)3.

57505110 ~730!OrgV)

Fig.3: Fit of XAS LI I I edge for the CeIV02

• % c 1 .~ l2

• Z v'"

J 0

Cerium Content % 8.47. Pd/Ce02

10

Fig 4:Spectral weight,determinedfinal states,as a function

159 0 5 0froJT,leXPS(3d 4f land. XAS(,2p 4f 5d)

of cerium loading.

226

As XAS is a volume sensitive probe, this suggests that ceria is growing up tridimension-

nally on the support.

Z) Influence of the presence of a transition metal

Some XAS results of Cel y Al Z0 3 are reported in Fig. 5. As for the Pd-Cel y AIZ03catalysts, we note that for very low cerium content the spectra are indicative of a

strongly +111 reduced state of cerium. Probably for these concentrations, Ce3+ cations

occupy vacant cationic sites of y A1203. However for higher cerium content, a careful

examination (compare for example 6.4% Pd - 12.5% Cel y AI Z0 3 (N°I-8) and 1Z.3% Cely Al Z0 3 (nOII-5) suggests a difference between the two series. Cerium is in a more oxi-

dized state for the Cel y AIZ0 3 series. This could be related to the formation of an

oxychloride CeIIlOCI, as evidenced by XRD, in the case of the Pd-Ce/y AI Z0 3 cata-

lysts. The chloride anion comes from the palladium precursor salt. Probably also some

interaction between the transition metal and the rare earth occurs, resulting in a par-

tial electronic transfer from transition metal to the rare earth (ref. 7).

UA

BE In eV

Fig. 5 : XAS on the Ce LIIl edge of Ce/y A1203.

CONCLUSION

XAS and XPS have proved to be powerful probes for the study of the electronic con-

figuration of cerium based catalysts. Thus they can give us useful informations on the

nature of the cerium compounds. In the case of Pd-Cel y AIZ03 (chloride) and Cely AIZ03 catalysts we have evidenced:

- a reduction of the cerium oxidation state as its content decrease on the surface of

the catalyst.

- a different behaviaur for the "surface" and the "volume" cerium.

227

We think that these properties can be related to an occupation of AI3+ vacant lattice

sites for low Ce loading.

- a chemical reaction giving the oxychloride CeIIlOCI.

- an interaction between the transition metal and the rare earth.

- the growing of tridimensionnal Ce0 2 on the surface, for higher Ce loading.

In order to understand better the role of ceria in such catalysts, further experiments

are undertaken, especially in "in situ" conditions (ref. 7).

ACKNOWEDGEMENTS

We are very grateful to Dr. H.. Dexpert and P. Lagarde for their unvaluable help at

LURE.

REFERENCES

1 J.e. Summers and A. Ausen, J. Catal., 58 (1979) 131.2 Y.F.Y. Yeo, J. Catal., 87 (1984) 152.3 G. Brauer and H. Gradinger, J. Inorg. Nucl. Chem., 17 (1955) 1792.4 P. Meriaudeau, J.F. Dutel, M. Dufaux and e. Naccache, Studies in Surface Sci. and

Catal., 11 (1982) 95.5 J.A. Horsley, J. Amer. Chem. Soc., 101 (1979) 2870.6 T.H. Fleisch, R. Hicks and A.T. Bell, J. Catal., 87 (1984) 398 and references therein.7 F. Le Normand, L. Hilaire, K. Kili, G. Krill and G. Maire, in preparation.8 G. Praline, B.E. Koel, R.L. Hance, H.I. Lee and J.M. White, J. Electron Spectra.

and ReI. Phen., 21 (1980) 17.9 P. Burroughs, H. Hammett, A.F. Orchard and G.T. Thornton, J. Chem. Soc., Dalton

Trans. (1976) 1686.10 A. Kotani, H. Mizuta, T. Jo and J.e. Parlebas, Solid State Comm., 53 (1985) 805.11 E. Wuilloud, B. Delley, W.D. Schneider and Y. Baer, Phys. Rev. Letters, 53 (1984)

202.12 E. Beaurepaire, G. Krill and F. Le Normand, Proceedings of the 4th International

Congress on EXAFS and Near Edge Structure, Fontevraud, France, 1986, J. de Phy-sique, submitted.

13 A. Bianconi, A. Marcelli, M. TomeIIini and I. Davoli, J. Magn. Mat., 47 (1985) 209.14 M. Gasgnier, L. Eyring, R.C. Karnatak, H. Dexpert, J.M. Esteva, P. Caro and L.

Albert, Proceedings of the 17th Rare Earth Conference, Hamilton, Canada, 1986.15 E. Beaurepaire, These d'Etat, Universlte Louis Pasteur, Strasbourg, France, 1983.


AN AES INVESTIGATIO~ OF THE REACTIVITY OF Pt, Rh AND VARIOUS Pt-Rh ALLOY

SURFACES TOWARDS O2, NO, CO AND H2

f.C.M.J.M. VAN DELFT G.H.VURENS*, M.C.ANGEVAARE-GRUTERandB.E.NIEUWENHUYS

Gorlaeus Laboratories, State University of Leiden

P.O.Box 9502, 2300 RA Leiden, The Netherlands

* Lawrence Berkeley Laboratory, University of California

Berkeley, California 94720, USA

ABSTRACT

229

The chemisorption and reactivity of oxygen, NO, CO and hydrogen on poly-crystalline Pt, Rh, PtO.55-RhO.45 and PtO.12-RhO.88 alloy surfaces and on aPtO.25-RhO.75 (100) single crystal surface have been studied by AES. For Rhthe results point to the presence of subsurface oxygen, following an exposureto 100 L 02 or NO at 290 K, under which conditions this species was not ob-served for Pt. The Pt-rich alloy behaved as pure Pt. The Rh-rich alloy showedoccasionally a Rh-like or a Pt-like behaviour. The ambivalent behaviour ofthis Pt-Rh alloy appears to be determined by variations in the surface compo-sition, which is strongly dependent on the equilibration temperature in vacuum.At low equilibration temperatures (below 1000 K) a Rh-rich surface is observedfor Rh-rich alloys, whereas at high equilibration temperatures (above 1200 K)Pt surface segregation occurs. These effects are discussed in relation to sur-face segregation theories. The results show that the surface composition andthe catalytic behaviour of Pt-Rh alloys are strongly dependent on the equili-bration temperature, the presence of contaminants and the composition of thegas phase.

1. INTRODUCTION

The catalytic performance of alloys compared to those of the pure, constituent

metals is of both theoretical and practical importance. In the past few de-

cades much research has been concentrated on the catalytic activity, selecti-

vity and durability of binary alloys composed of an active and an inactive

component (1). The effect of alloying on the selectivity can be understood

mainly in terms of geometrical factors (ensemble size effect) (2,3). Much less

information is available concerning the chemical properties of alloys composed

of two catalytically active components, although bimetallic catalysts consis-

ting of two active components are used for a number of very important reac-

tions (1-3). In the present investigation we focus our attention onto the sys-

tem Pt-Rh for the following reasons:I) Many data are already available on the

adsorption properties and reactivity of pure Pt and pure Rh (both polycrystal-

line and single crystal) surfaces (4). II) Both Pt and Rh are components of

the catalytic convertors, which are used for the purification of automotive

230

exhaust gases (removal of NO~, CO and hydrocarbons).

The mechanism of selective NO reduction (both reactant selective (NO vs.02)

and product selective (N2 vs. NH)) has been the focus of many investigations

(5). According to these data, Rh is the component which is most selective in

NO reduction, whereas, Pt is more active in low temperature oxidation of CO

and hydrocarbons. A synergetic effect has been reported by Oh and Carpenter

for the combined Pt and Rh system compared to pure Pt, pure Rh and to a phy-

sical mixture of Pt and Rh as well (6). Su et al. observed enhanced oxygen

storage capacity for the combined Pt and Rh system compared to pure Pt and

pure Rh (7). Apart from selective NO reduction, another important quality of

the Pt-Rh three-way catalysts is their stability during high temperature ex-

cursions in oxidative gas mixtures (5). It appears that alloying of Rh with Pt

stabilizes the metallic phase of Rh, since Rh20 3 decomposes in air at 1100°C,

whereas in the presence of Pt it decomposes already at 800°C upon formation

of a Pt-Rh alloy (8). According to the authors, the lower decomposition tem-

perature is a result of the free energy of formation of the Pt-Rh alloy.

Schmidt and Wang found by TEM, that on a silica support Pt and Rh form alloy

particles, which decompose to Pt and Rh 20 3during oxidation (9). It is likely,

therefore, that the synergetic effect reported by Oh and Carpenter (6) for the

combined system, is due to the presence of alloy particles. These alloy parti-

cles show both a higher activity in CO oxidation and a higher stability in

oxidative gasmixtures than the pure metals (6,9).

In the present investigation the chemisorption and reactivity of 02 and NO

on several Pt-Rh alloy surfaces and pure Pt and pure Rh surfaces have been

studied by AES in order to obtain a better understanding of the behaviour of

Pt-Rh alloys compared to those of pure Pt and Rh. These results will also

serve as central background for our continuing studies on various Pt-Rh alloy

surfaces.

2. EXPERIMENTAL

Polycrystalline wires of Pt, Rh, PtO.S5-RhO.45 and PtO.12-RhO.88 alloys

were wrapped around the curves of a Ta multihairpin filament. The temperature

of the samples was measured by means of a Pt/Pt-Rh thermocouple spotwelded onto

the rear of the specimen. The filament could be heated up to 1500 K by Joule

heating. The experiments were carried out in an all metal UHV system (base

pre~sure - 1.10-9Torr consisting of 90% H2 and 10% CO approximately) equipped

with a single pass CMA with an integral electron gun (Physical Electronics).

Standard experimental conl:itions during Auger analysis were a primary beam of

2 keV electrons and a 5 eV peak-to-peak modulation amplitude. The samples were

cleaned, depending on the nature of the contamination, by either Ar ion bom-

bardment (S.lO-STorr Ar, 1 keV ions) or oxidation (5.10-7Torr 02, 1100 K).

the major contaminants were S, Ca (on Pt) and Fe (on Rh). Finally the samples

were heated around 1000 K for equilibration. The samples were exposed to 100

L of the desired gas (S.10- 7Torr during 200 s) at room temperature. The adsor-

Date Auger signals were measured repeatedly either in vacuum or in a continu-

ous flow of 1.10-sTorr of the desired gas during a temperature program as

shown in fig. J .Tn the following figures a solid line connects the points mea-

sured at the given temperature and a dashed line connects the points measured

in between, following a cooldown to room temperature after heating at the

given temperature.

T(K)t

1500

1000

500

o 10 20 30-.t (min)

Fig. 1 Temperature program used for the experiments in the figs. 2-6.

In addition to the experiments on the polycrystalline surfaces, some AES

experiments were performed on a PtO.2S-RhO.7S (100) single crystal surface.

The cleaning procedures of this surface have been described in detail else-

where (10).

3. RESULTS AND DISCUSSION

3.1 CO exposure

After a 100 L CO exposure on Pt, Rh and the Pt-Rh alloys, the C (272 eV)

signal was monitored during the temperature program described above. The re-

sult for the PtO.12-RhO.ss sample is shown in figure 2. The intensity of the

C (272 eV) signal is normalized to the initial intensity at 290 K. The results

for the other samples are very similar to that shown for PtO.12-RhO.88'

In all cases the 0 (510 eV) signal could not be detected, indicating that CO

232

I272t 1.0

0.5

o 400 800 1200 -+ T(K)

Fig. 2 ~ormalized C signal intensity as a function of the temperature (program-med according to fig.l) for <1 100 L COexposure at room temperature on the PtO.12-RhO.88 alloy. The solid line connects the points measured at the given tempe-rature, the dashed line conneCL~ the points measured in between, following roomtemperature cooldown after heating at the given temperature.

was molecularly adsorbed, since the cross section for Auger excitation of 0 in

molecularly adsorbed CO is small (11). The maximum rate of desorption occurred

around 550 K. Due to the relatively high CO residual pressure some readsorption

of CO occurred during the intermediate cooldown, as shown by the higher level

of the dashed line above 400 K.

~02 exposure

After a 100 L 02 exposure the 0 (510 eV) signal was monitored during the

temperature program and the intensities, normalized to their initial values at

290 K, are shown in figures 3a and 3c for Pt and Rh respectively. As can be

seen from the larger error bars, the initial amount of oxygen on Pt was much

smaller than on Rh. On Pt, the small 0 signal disappeared already at approxi-

mately 400 K. The following processes may contribute to the vanishing 0 signal

intensity: a) desorption; b) reaction with the residual gas (CO and H2);

c) diffusion of 0 to the bulk. According to the literature data (4) any signi-

ficant desorption of oxygen should be unlikely. Hence, we disregard possibility

a). Some contribution of c) cannot be completely ruled out, although no indica-

tions have been found of reappearance of oxygen on the Pt surface at higher

temperatures. However, it is more likely that b) is the dominant process for 0

233

removal since it is known that.on Pt,O reacts very fast with H or CO in the

relevant temperature range (4).lbe relatively low amount of adsorbed 0 follo-

wing exposure at 290 K may also be caused by the rapid reaction with H or CO

from the residual gas. The lower level of the dashed line is also consistent

with b).

On Rh the oxygen Auger signal intensity initially decreased up to 500 K, but

then, it increased beyond the initial value in the temperature range 500 to

800 K. Finally, it decreased above 800 K. The initial decrease can be ascribed

to a reaction with the residual gas or to absorption (see below). Desorption,

is not probable for Rh at these temperatures (12-14). Here again, the dashed

line is lower than the solid line, indicating a reaction with the residual gas

during intermediate cooldown. The increase of the oxygen Auger signal intensi-

ty during heating in vacuum can only be explained by the presence of subsur-

face oxygen, which diffuses to the surface at elevated temperatures (above 500

K). Since the initial Auger intensity is surpassed, this subsurface oxygen must

have been formed already at room temperature (during the exposure). The final

decrease above 800 K is most likely due to desorption, since desorption has

been reported to occur in this temperature range (14).

The result for the Pt-rich alloy, PtO.55-RhO.45' is shown in figure 3b and

displays a Pt like behaviour, which has been observed for all processes stu-

died on this alloy in the present work.

The Rh-rich alloy, PtO.12-RhO.88' showed three types of behaviour as shown

in figures 3 d,e,f and indicated as types I, II and III respectively. Repeated

experiments showed one of these three types of behaviour, although it could not

be predicted a priori which type appeared under the conditions used. Type III

(fig.3f) showed a constantly high oxygen level which did not disappear by hea-

ting up to 1300 K. The oxygen could only be removed by Ar ion bombardment.

The positions of the vague maxima were not reproducible. In our opinion this

type of behaviour is due to small amounts of Si and/or B subsurface impurities,

which were not detectable in our AES analysis. Small amounts of these elements

are known to form stable oxides at the surface (15-19). Type I and type II,

however, were fully reproducible. Type I (fig.3d) is a Pt-like behaviour com-

parable to those of pure Pt (fig.3a) and the Pt-rich alloy (fig.3b). Type II

(fig.3e), which shows a maximum for the oxygen intensity likewise at 800 K, is

a Rh-like behaviour (compare fig.3c). The maximum relative intensity is lower

than that observed for pure Rh. The figures also show that for the Rh-rich al-

loy the dashed line is much lower relative to the solid line than for pure Rh.

This might indicate that the surface oxygen is more easily removed by the resi-

dual gas on the alloy than on the pure Rh. It was shown earlier that on a Pt-

Rh alloy surface oxygen preferentially occupies the Rh sites leaving the Pt

sites initially free (10). If many free Pt sites are present at the surface

234

a b c

1200

1200800

800

400

2

1

1200

It 510 400L-.T(K)

f

800

e

\\

400

400 BOO

800

800

d

400

400

a

a

1.0

1.0

0.5

0.5

Fig. 3 ~0rmalized 0 signal intensity as a function of the temperature for a100 L 02 exposure at room temperature on: a) Pt; b) PtO.SS-RhO.4S; c) Rh;d) PtO.12-RhO.88 type I; e) PtO.12-RhO.88 type II; f) PtO.12-RhO.88 type III.

adjacent to oxygen covered Rh sites, then residual gas (CO, H2) can easily

chemisorb close to the oxygen ada toms resulting in a reaction. Dual selective

chemisorption has been reported for a mixture of CO and NO on Pt-Ru alloys (20)

and on Pt-Ni alloys (21), whereby CO selectively chemisorbs on Pt in both cases

and NO selectively chemisorbs on Ru and on Ni respectively. These effects might

form a basic line of thought for understanding the synergetic effects of alloy-

ing on the CO oxidation activity reported by Oh and Carpenter (6). We are still

left, however, with the question, why the Rh-rich alloy shows reproducibily am-

235

bivalent behaviour. This problem will be discussed in section 3.5.

3.3 NO exposure

The effect of an exposure of 100 L NO on the various samples was studied in

tue S2me way as described for oxygen adsorption. The N (380 eV) signal intensity

W,l» very Iowan Pt and on the Pt-rich alloy, but was easily observed on Rh and

on the Rh-rich alloy,where it decreased around 440 K in both cases.The behaviour

of the 0 (510 eV) signal was identical to that found after exposure to 02 for

ai' samples (see figs.3a-f).This indicates that NO dissociated on all samples,

l~aving oxygen adatoms,which behaved further identically as those formed from

02.The NO dissociation is partly due to intrinsic chemical activity of the

samples (4) and,most probably, partly induced by the primary electron beam.

3.4 Reactions on Rh and PtO.12-RhO.88

The reactions of NO with CO and H2 have been studied on pure Rh and on the

Rh-rich alloy sample. The experiments were carried out by running the tempera--8ture program in a flow of 1.10 Torr of one reactant after a 100 L exposure of

the other reactant.

The Rh results for preadsorbed NO reacting with CO are shown in figs. 4a,b,c

for the N (380 eV), C (272 eV) and 0 (510 eV) signals respectively. As can be

seen nitrogen disappeared at 400 K from the surface, whereas CO readily adsor-

bed on the sites left free by the nitrogen. The oxygen behaved similarly as in

fig.3c, but during intermediate cooldown the oxygen at the surface shows a fast

reaction with CO, as indicated by the low level of the dashed line in fig.4c.

The Rh results for preadsorbed NO reacting with H2 are shown in figs. 5a,b

for the N (380 eV) and the 0 (510 eV) signals respectively. Both the oxygen and

the nitrogen disappeared at low temperatures from the surface. Apparently, hy-

drogen is capable of removing oxygen from the subsurface region whereas CO is

not.

The Rh results for preadsorbed CO reacting with NO are shown in figs. 6a,b,c for

the N (380 eV), C (272 eV) and 0 (510 eV) signals respectively. CO reacted with

NO, which adsorbed on the sites left free by CO but above 400 K nitrogen disap-

peared from the surface, although during intermediate cooldown NO was adsorbed

again. The amount of oxygen on the surface was low but still some subsurface

oxygen was observed to diffuse to the surface at elevated temperatures. The ma-

ximum amount of near-surface oxygen is observed at 1000 K and, hence, at a sig-

nificantly higher temperature than under the conditions of the experiments il-

lustrated by figures 3 and 4. It appears that this shift to higher tempera-

ture is linked up with the lower concentration of adsorbed oxygen.

The ambivalent behaviour of the Rh-rich alloy was also manifested by the

reactivity of preadsorbed NO with CO and hydrogen. The variation of the N, 0

236

and C AES peak intensities with the temperature showed either a Pt like beha-

viour (a small amount of 0 that disappeared upon exposure to CO or hydrogen) or

the Rh like behaviour (with subsurface 0 that reacted easily with H but not

with CO) and, occasionaly, the third type of behaviour. In the last case

the oxygen could not be removed, not even by heating in a hydrogen atmosphere.

800 T(K). 400

0.5

I~o 400

a

32

800 400

b

I510t1.0

0.5

800 1200

Fig.4 Normalized N,C and 0 signal intensities (in a,b and c respectively)as a function of the temperature for Rh in a flow of 1.10-8 Torr CO afterprevious room temperature exposure to 100 L NO.

800 -+ T(K)

b

400

1510t

1.0

0.5

BOO

a

400a

1380t

1.0

0.5

Fig.S Normalized Nand 0 signal intensities (in a and b respectively) as afunction of the temperature for Rh in a flow of 1.10-8 Torr HZ after previuusroom temperature exposure to 100 L NO.

800--+T(K)

c

400800

b

L,OO

1272t

1.0

1200

a

800400

1

2

1380i I

I\

oFi~.6 Normalized N,C and 0 signal intensities (in a,b and c respectively)as a function of the temperature for Rh in a flow of 1.10-8 Torr NO afterprevious room temperature exposure to 100 L CO.

~02 exposure of PtO.25-RhO.75 (100)In our previous work (10) it was shown that the surface composition of a

PtO.25-RhO.75 (100) face is strongly dependent on the equilibration temperature

of the clean alloy in vacuum. At high equilibration temperatures Pt enrichment

was observed. whereas below 1200K Rh enrichment could not be ruled out, as

shown in figure 7. Surface segregation theories, taking into account the diffe-

rence in sublimation enthalpy between Pt and Rh, predict a moderate Rh enrich-

ment. It is very peculiar that the Pt surface enrichment increases with increa-

sing temperature, whereas the existing segregation theories predict that any

surface enrichment should decrease with increasing temperature. A similar tem-

perature dependence was observed by Williams and Nelson for polycrystalline

PtO.10-RhO.90 from ISS studies (22) and by Wolf et al. for the atomically rough

surfaces of a PtO.12-RhO.88 tip with Field Emission Microscopy (23). A theoreti-

cal explanation for the Pt enrichment has recently been given by van Langeveld

and Niemantsverdriet (24). These authors considered the lower Debye temperature

for Pt atoms in the surface. The Debye temperature can be coupled to a vibra-

tional entropy term see e.g. (25). Combination of the enthalpy and entropy ef-

fects yields the following equation for a one layer segregation model:

KX

s(l-x

b)

xb

(l-xs)

-!:£/RTe (eq. 1)

where K is the equilibration constant, X s is the surface molar fraction in Pt , xb is

the buLk molar fraction in Pt, L\G is the free energy 0:':: segregation, LH is the en-

thalpy of segregation, R is the gas constant, T is the absolute temperature and

LSv is the vIbrational entropy term. It can be seen from this equation that the

enthalpy term dominates at low temperatures,yielding a small Rh enrichment.

At higher temperatures the contribution of the enthalpy term diminishes, lea-

ving the entropy term, which yields a strong Pt enrichmen[ as is shown in fig.8

with several literature data (10,22,24,26). The model by van Langeveld and

Niemantsverdriet is qualitatively in good agreement with the experimental re-

sults for the temperature dependence of the surface composition. In a forthco-

ming publication we will further quantify the temperature dependence of the

surface composition of Pt-Rh alloys (27).

1Pt iX1

0.5 I I-II -- -------a 500 1000 1500 -4 T(K)

Fig. 7 Temperature dependence of the surface composition of a PtO.25-RhO.75(100) face. The dashed lines indicates the bulk composition of the alloy.

In order to examine the possible effect of the annealing temperature on the

chemical properties of Rh-rich Pt-Rh alloys, additional measurements were car-

ried out on the PtO.25-RhO.75 (100) surface. On this alloy sample a uniform

temperature could be adjusted, whereas the temperature of the polycrystalline

samples was less uniform over the filament and less constant in time. After e-

quilibration of the PtO.25-RhO.75 (100) surface (fig.7) during 5 minutes at

1425 K (Pt-rich surface) and subsequent 100 L 02 exposure at 290 K, temperature

programmed AES without intermediate cooldown showed no significant amount of

subsurface oxygen, as shown in figure 9a. However, after equilibration at 975 K

during 45 minutes (Rh-rich surface) and subsequent 100 L 02 exposure at 290 K,

substantial amounts of subsurface oxygen segregated to the surface at 1200 K,

as shown in figure 9b. The point at 1240 K marked with a cross was measured af-

ter the measurement at 1300 K, indicating that above the desorption temperature

diffusion to the surface is the rate limiting step for desorption.

1.00.5 ----.o

1.0

xPt <-:-:1 /'

""t /

0.5

Fig. 8 The surface composition of clean Pt-Rh alloys as a function of thebulk composition with several literature data:U van Delft and Nieuwenhuys (10); 6 van Langeveld and Niemantsverdriet (24);• Williams and Nelson (22); x Holloway and Williams (26);The solid line shows the vibrational entropy contribution,the dotted line showsthe enthalpy only contribution at 1000 K and the dashed line shows the theore-tical curve for 1000 K according to the model by van Langeveld and Niemants-verdriet (24)

This subsurface oxygen was only observed on a low temperature equilibrated sur-

face if the subsurfac~ carbon contamination of the alloy was extremely low (see

fig.10), in accordance with the experiments by Salenov and Savchenko (28) for

pure Rh (100). These authors observed an oxygen desorption state at 1230 K only

if the Rh (100) had a very low carbon concentration in the near surface region.

The Auger phenomenon at 1200 K in figure 9b and figure lOb is in our opinion

due to subsurface oxygen and not to the decomposition of a surface impurity

oxide, since this should yield a high oxygen Auger signal from room temperature

up to the decomposition temperature (compare fig. 3f). The background oxygen

signal however, might be due to the presence of small amounts of impurity

oxides. Anyhow, the oxygen behaviour on the Pt-Rh(lOO) surface equilibrated at low

temperatures resembles the behaviour of oxygen on Rh (100).

Returning now to the polycrystalline samples, it may be expected from fig. 8,

240

10

o

975 K

b

500 1000 -. T(K)

321

o

1425 K

a

500 1000 -+ T(K)Fig.9 Normalized 0 signal intensity versus the temperature (without interme-diate cooldown) for PtO.25-RhO.75 (100) after a 100 L 02 exposure at room tem-perature preceeded by equilibration in vacuum a) during 5 minutes at 1425 K;b) during 45 minutes at 975 K.The point marked with a cross in b) was measuredafter the measurement at 1300 K.

that the surface of the PtO.55-RhO.45 alloy sample will be much enriched in

Pt and, hence, exhibit a Pt-like behaviour after equilibration at all tempera-

tures. For the PtO.12-RhO.88 alloy a Pt rich surface and hence. a Pt-like be-

haviour may be expected after equilibration at high temperatures (T > 1200 K).

However, a Rh-rich surface and, hence, a Rh-like behaviour may be expected af-

ter equilibration at lower temperatures (T < 1000 K).The observed ambivalent behaviour is, thus,understood. These surface composi-

tion variations together with the influence of small amounts of impurities have

a profound influence on the adsorption behaviour and catalytic performance of

the Rh-rich alloys as was shown in section 3.2.

4. CONCLUSIONS

The present results indicate that the chemical behaviour of Pt-Rh alloys is

strongly dependent on the bulk composition and the equilibration temperature.

Depending on the bulk composition and the annealing temperature, a Pt-like or

a Rh-like behaviour was observed. On Rh rich alloys both types of behaviour

241

101510 Ii

t 975 K I I

5 1~72Rh <0.01

1302b

0 500 1000 -. T(K)

3 900 K C12722lRh =0.03

1 a302

0 500 1000 -+ T(K)Fig.lO Normalized 0 signal intensity versus the temperature (without interme-diate cooldown) for PtO.2S-RhO.7S (100) after a 100 L 02 exposure at room tem-perature preceeded by low temperature equilibration in vacuum a) with carboncontaminated bulk; b) with very low carbon concentration in the subsurfaceregion.

NONO7',),)77'

290K...

177)} }}o

290K NNo8---=--~~ Inh " ,.+CO 0

- C021290 K

440 K NN<4 };;'7; I;

-N2 0

Fig. 11 Scheme for the reaction between CO and NO on polycrystalline Rh andon a polycrystalline Rh-rich alloy with a Rh-rich surface.

242

could be realized by variation of the annealing temperature. A third type of

behaviour could be attributed to the effect of a small level of impurities.

Based on these results a scheme is given in figure II for the reaction be-

tween CO and NO on Rh and on a Rh-rich alloy with a Rh-rich surface. The sub-

surface oxygen was readily observed on polycrystalline Rh and Rh-rich alloy

surfaces, but was only detected on the (100) surface of such samples if the

contamination level of the subsurface region was below a certain limit. Hence,

the presence of even low concentrations of C prevents the diffusion of 0 into

the bulk (oxide formation),

5. ACKNOWLEDGEMENTS

The authors are indebted to the Royal Shell Laboratories in Amsterdam for

the donation of the UHV AES apparatus used in this study. The stimulating dis-

cussions with dr.A.D.van Langeveld, drs.R.M.Wolf and M.J.Dees are gratefully

acknowledged.

6. REFERENCES

I M.J.Kelley and V.Ponec, Progr.Surf.Sci., 11 (1981) 139 and refs. therein2 V.Ponec, "On the Chemistry and Alchymie of Metallic Catalysts" in: Procee-

dings IXth IntI. Vac.Congr.- Vth Intl.Conf.on Solid Surfaces, J.L.de Sego-via, ed. (ass. Espanole del Vacio, Madrid, 1983)

3 B.E.Nieuwenhuys, "Chemisorption of Gases on Metal Films", P.Wissmann,ed.Chapter 8, Elsevier, Amsterdam(1987)

4 B.E.Nieuwenhuys, Surf.Sci., 126 (1983) 307 and refs. therein5 K.C.Taylor, "Automotive Catalytic Convertors" (Springer, Berlin 1984)

and refs. therein6 S.H.Oh and J.E.Carpenter, J.Catal., 98 (1986) 1787 E.C.Su, C.N.Montreuil and W.G.Rothschild, Appl.Catal., 17 (1985) 758 A.J.S.Chowdhury, A.K.Cheetham and J.A.Cairns, J.Catal., 95 (1985) 3539 L.D.Schmidt and T.Wang, J.Vac.Sci.Technol., 18,2(1981) 52010 F.C.M.J.M.v.Delft and B.E.Nieuwenhuys, Surf.Sci., 162 (1985) 53811 B.E.Nieuwenhuys and G.A.Kok, Thin Sold Films, 106 (1983) L9512 D.G.Castner, B.A.Sexton and G.A.Somorjai, Surf.Sci., 71 (1978) 51913 P.A.Thiel, J.T.Yates and W.H.Weinberg, Surf.Sci., 82 (1979) 2214 D.G.Castner and G.A.Somorjai, Appl. Surf. Sci., 6 (1980) 2915 H.Niehus and G.Comsa, Surf.Sci., 93 (1980) L14716 H.Niehus and G.Comsa, Surf.Sci., 102 (1981) 11417 H.P.Bonzel, A.M.Franken and G.Pjrug, Sur f s Sc i , , 104 (1981) 62518 S.Akther, C.M.Greenlief, H.W.Chen, J.M.White, Appl.Surf.Sci., 25 (1986) 15419 S.Semancik,G.L.Haller, J.T.Yates Jr., Appl.Surf.Sci., 10 (1982) 54620 P.Ramamoorthy and R.D.Gonzalez, J.Catal., 58 (1979) 18821 A.F.M.Wielers, C.J.G. v.d.Grift and J.W.Geus, Appl.Surf.Sci., 25 (1986) 24922 F.L.Williams and G.C.Nelson, Appl.Surf.Sci., 3(1979) 40923 R.M.Wolf, M.J.Dees and B.E.Nieuwenhuys, J.Physique (Paris) in press24 A.D.van Langeveld and J.W.Niemantsverdriet, Proceedings ECOSS-8 (Julich),

Surf.Sci., in press25 K.Hoshino, J.Phys.S0c. Japan, 50, 2 (1981) 57726 P.H.Holloway and F.L.Williams, Appl.Surf.Sci., 10 (1982) 127 F.C.M.J.M.v.Delft, A.D.v.Langeveld and B.E.Nieuwenhuys, to be published28 A.N.Salanov and V.I.Savchenko, React.Kinet.Catal.Lett., 29,,1 (1985) 101


REACTIVITY STUDIES OF AUTOMOBILE EXHAUST CAT ALYSTS IN PRESENCEOF OXIDIZING OR REDUCING CONDITIONS

Guillaume MEUNIER*, Fr ar-cols GARIN*, Jean-Louis SCHMITT*, Gilbert MAIRE*

and Rene ROCHE***Laboratoire de Catalyse et Chimie des Surfaces, U.A. 423 du CNRS, UniversiteLouis Pasteur, 4 rue Blaise Pascal 67070 Strasbourg Cedex FRANCE

**Peugeot S.A., Direction Technique, 18 rue des Fauvelles 92260 La Garenne-Colombes FRANCE

243

ABSTRACTIn this study the catalytic behavior is determined under reducing (hydrogen atmos-

phere) or oxidizing (oxygen atmosphere) conditions. By using the correlations establishedbetween the mechanisms of the skeletal isomerization of hydrocarbons on metalsand the structures (electronic, geometric or crystallographic) of the catalysts we could"characterize" the surface structure of the catalysts used for the CO oxidation reac-tion. The second time we studied I) the influence of the first impregnated salt onAl70 3 on the oxidation reactivity for Pt-Ni and Ni-Pt catalysts Ii) the influence of thecarcination step on the light-off for Pt-Co catalysts; Pt-Ce, Ni-Ce and Ce-Pt-Ni werealso studied. We concluded that no segregation occurs and a random distribution of thesmall metallic particles occurs on AI7D.) and that the Pt-Co/AI20 3 system withoutcalcination is more active; the iiqht-bff" for the CO oxidation being the lowest.

INTRODUCTIONThis paper deals with experimental studies of the reactivities of CO or alkanes

with some platinum-based bimetallic catalysts under oxygen or hydrogen atmospheres,respecti vel y.

Since the nature of the exhaust gases oscillates from oxidizing to reducing agentswith a certain frequency, these catalysts have to be equally active under both environ-nements.

From bibliographic studies several points have to be underlined :a) - Studies of bulk alloys have shown that chemisorption of atoms and molecules(H, 0 and CO) can change the surface composition drastically (1), thereby influencingthe heterogeneous reaction rate. The clean Pt-Ni alloy surfaces in U.H. V. or underhydrogen atmosphere were found to be enriched in platinum in an amount increasingwith increasing platinum concentration in the bulk. On the contrary oxidation treat-ments resulted in a nickel surface segregation which was only slightly offset afterreduction (2).b) - For the CO oxidation reaction the whole elementary steps are not clearly explai-ned.

244

A key question is whether the diatomic molecule in its interaction with metalsurfaces remains molecular or dissociates into carbon and oxygen. Broden et a!. (3)

predicted, by the perturbation of molecular orbitals for CO adsorbed, that only ironcould dissociate CO. However, other metals in Group VIIl such as nickel (4) ruthenium(5) and rhodium (6) can dissociate CO. Recently Ichikawa et al.(7) observed that dispro-portionation of CO to CO2 and carbon occurs on small particles of silica-supported pal-ladium. These results show that carbon deposition phenomena may occur via either dis-sociation of CO on the metals used or disproportionation of CO to CO2 and carbon onsmall platinum particles. Cant and Angove (8) studied the apparent deactivation ofPt/Si02 catalyst for the oxidation of carbon monoxide and they suggested that adsorbedCO forms patches and that oxygen atoms are gradually consumed.c) - The influences of the crystallographic orientations for the CO oxidation reactionmay be important. From the work of Gland et a1.(9) with Pt (321) surface they clearlyshow that oxygen adsorbed on rougher step sites are less reactive than those on smoothterrace sites. A very important work was performed on the adsorption of CO on singlecrystals. For Pt(lll) Hayden and Bradshaw (10) studied this reaction by infrared reflec-tion-absorption spectroscopy (IRAS) and they mentioned that bands due to the C-Ostretch were found in both the linear and bridging regions and that there were distinctbands in the bridging region which could be assigned to CO in two fold bridge and

three folcJ hollow sites. Crossley imrl Kinn (J]) shJrliprl CO on Pt(100) and Pt(I11) andmentioned, from vibrational spectra that formation of islands at low coverages onPt(100) occurs which dimension is about 10 molecules at 400 K for a coverage of 1014

molecules cm -2 (8=0.1). On Pt(I11) at 300 K adsorption initially occurs into isolatedsingletons.

d) - Finally for the CO oxidation reaction at low pressure on Pd(111), Engel and Ertl(12) have shown that this reaction is structure-insensitive as mentionned by Boudart(13). The reactions of CD labeled hydrocarbons on platinum catalysts under hydrogenatmosphere are structure-sensitive (14) and isomerization reactions are very sensitiveto the crystallographic planes as observed on the platinum stepped surfaces where thebond shift mechanism is favored compared with the cyclic mechanism (15).

The aim of this study is to get catalytic performances for two different reactions:one, the isomerization, performed under hydrogen; the other, the CO reaction, involvingoxygen atmosphere. We hope to understand these reactions and the behavior of thecatalysts under these opposite environments.

EXPERIMENTAL

A serIes of ten catalysts 0.2% Pt, 0.7% Pt, Ni-Pt, Pt-Ni, two Pt-Ce, Ni-Ce, Ce-Pt-Ni and two Pt-Co were studied under O2 and/or H2 atmospheres.

245

Catalysts preparationsAll the catalysts were supported on Rhone Poulenc y-A1203 pellets of 1.6-2mm dia-

r'"1eter (ref: CCO 64) with a surface area of 206m'g-\ a density of 1.19 and a pore dia-D

meter of about 100 A; the impurities being (in ppm) : (Na20 40, CaO 70, Fe203 70,SiOt 70, MgO £ 20, 50/- 150). Two impregnation techniques were used:

- the fluidized bed technique (6)- the Ipatieff method.The first one is based on ionic exchange, the second one is a forced impregnation

(17-18). The multimetallic catalysts were prepared by successive impregnations.All the catalysts were calcined at 400DC during 2 h after each impregnation and

finally reduced at 400°C during 2 h under a hydrogen flow of 40cm'min-1. For onePt-Co catalyst two reduction steps were performed: with Co in first. Two Pt-Niwere prepared with Ni or Pt impregnated in first.

Catalysts characterizationThe atomic absorption technique was used to control, during the ionic exchange,

the amount of metal removed from the metal ion species to the active sites on thesupport.

Unfortunately we were not able to check the metallic particle size by T.E.M. sowe used the test reaction of the methylcyclopentane hydrogenolysis (19). We haveshown that the selectivity of the C5 ring opening is correlated to the particle size (20).

ApparatusCatalytic reactions were performed in two differenUal reactor systems under 1

atm.. The laboratory reactor consisted of a glass tube heated by a furnace. A glasspacking was used as preheater. The catalytic charge was always 200mg.

For oxidation reaction the gas mixture was CO 1.5%, 02 1.5% and N2 97% fromAir Liquide cylinders. Tylan Mass flow controllers permitted to vary gas flow between20 to 100 em' min-I. Carbon monoxide analysis was done before and after the reactorby nondispersive IR gas analyzer Binos-Leybold Heraeus.

The programmed heating rate was lODC min-1 until the first reaction temperaturewas reached and then 1°C min-I. Analysis measurements were made after an exposuretime for each temperature to be sure that the catalyst was in a steady state.

The catalytic reactions of hydrocarbons under hydrogen flow were performed inthe classical system already described (19). Prior to hydrocarbon reactions the catalystswere reduced at 350DC overnight. The reaction products were analyzed in a gaschromatographic apparatus.

N ..TA

BLE

la0>

Cat

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enta

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tdi

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oles

mol

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ihE

xten

sive

2C3

C4+

C2

C5+

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3MP

nHM

CP

isom

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Pt

0.7

260

46.

80

1434

5243

3621

33%

Pt

0.2

300

1221

.60

1436

5038

4517

60%

14

Ni-

Pt28

052

88.5

2913

2137

5030

203%

Pt

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ton

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0.5

1.5

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Pt-N

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68.5

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3051

5040

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0.2

0.2

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Pt-

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280

46.

50

1032

5831

4128

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270.

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Ni-

Ce

280

3759

.217

921

5257

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Ce-

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280

53.5

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1427

3450

2525

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5

Pt-

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280

B**

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1563

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3MP

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Pt0.

722

06.

52.

66

1.4

0.8

Pt0.

224

01.

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30

1.7

0.75

Ni

-P

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07.

510

281.

43

Pt-

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240

3562

1.4

4.3

Pt-

Ce

240

21.

83

20.

7

Ni

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9.6

541.

24.

7

Ce

Pt

Ni

240

25.5

1856

1.4

4

Pt-

Ce

240

B14

11.2

51.

90.

7

Pt-

Ce

240

A4

2.8

22.

80.

7

Pt-

Co

240

B0.

30.

20

5.4

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Pt-

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240

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41.

12

2.4

0.6

"" """_1

248

ResultsExperiments performed under reducing conditionIn table La, the distribution of the products obtained from 2-methylpentane are

shown as percentages of reactants having disappeared according to reactions 1 to 6.

C6 -s- iso C6 (1)

C6 -+ IV1CP (2)C6 -+ C5+C1 (3)C6 -+ C4+C2 (4)C6 -+ 2 C 3 (5)

C6 -+ extensive cracking 6 C1 + 3 C2 (6)

The cracking pattern on Pt-Ce is similar to the one observed on Platinum catalysts;on Pt-Co catalysts, the demethylation reaction is more important than on Pt catalyst.When Ni is added, repetitive cracking reactions occur; the ratio of isobutane overn-butane is lower than 1 on these latter catalysts which means that multiple processesoccur on the catalytic surface because n-butane cannot be obtained from 2-methylpen-tane with only one carbon-carbon bond breaking.

About the isomerization distribution On Pt-Ce and on 0.2%Pt the n-hexane is themajor product, which means that the cyclic mechanism is the major process (14). Theamount of methylcyclopentane is very high on Pt-Co catalyst because the conversionis very low, there is desorption of the cyclic product intermediate prior to be hydro-genolysed. On a Ni-containing catalyst, 3-methylpentane is the main isomerized product.

The catalytic reaction of 2-methylpentane is not sensitive to the influence of theimpregnation order.

.!.nD~e.r2c~ 9.f_tl:2eJ~as:tl0.r2 ~0~1{2 _O2_ ~ _CQ2_: An oxidative reaction was perfor-med between two isomerization reactions. The Pt-Ce catalyst has lost its activityafter the carbon monoxide reaction, indicated by the increase of the amount of methyl-cyclopentane and the selectivity to isomers. The same behavior is observed on Pt-Cocatalyst, but its activity, which is always very low, is increased after the CO oxidationreaction; at 280°C the Pt-Co catalyst is inactive before the reaction CO -+ CO2occurs. In both cases the cracking pattern is unchanged.

In table Ib we have mentioned the activity r a F, the reaction temperature and=--w-

the ratio 2MP and 3MP obtained from methylcyclopentane hydrogenolysis.3MP n-H

The ratio 3MP has been correlated to the metallic dispersion (20) and on 0.7% Pt,n-H

0.2% Pt and on Pt-Ce and Pt-Co this ratio is equal to 0.7::0.1 which means that themethylcyclopentane hydrogenolysis is non selective (19) and that the catalysts are welldispersed. The values are equal "before" and "after" the carbon monoxide reaction onPt-Ce and Pt-Co. At the opposite, on catalysts with Ni this ratio is higher (3.9::0.9).

249

The ratio 2MP is near the statistical values which is 2 whatever the catalysts3MP

used, but on Pt-Ce after the carbon monoxide oxidation and on the Pt-Co catalyststhis ratio is very high which could mean that a memory effect occurs; in each case,before the hydrogenolysis reaction there is the 2-methylpentane isomerization.

Repetitive processes occur on catalysts where nickel is incorporated, the amountof these reactions can be measured by the cracking contribution. For the methylcyclo-pentane hydrogenolysis, the experimental conditions are chosen so that no multiplecarbon-carbon bond rupture will occur; which is not the case for Pt-Ni, Ni-Ce andCe-Pt-Ni catalysts.

Experiments performed under oxidizing conditionsIn figures 1 and 2 are plotted the carbon monoxide conversions (a ) as a function

of temperature for various gas flows from 20cm' .min-1 to 100cm' .min -1. In figure 1the Pt-Co catalyst used has been prepared following the classical method: cobalt wasfirst impregnated on y -A1203 and then calcined; the platinum was added later thenalso calcined at 400°C and finally the catalyst was reduced at 400°C. In figure 2 theimpregnation order was Co then Pt and at each step the salt was reduced prior to thesecond impregnation. In the last case the light- off is lower, but is a function of thegas flow. We may observe that:i) on the calcined catalyst the flow rates have no influenceii) on the reduced one, when the flow increases, a bending of the curve at high con-version occurs.

In figure 3 we have plotted the curves a =f(T) for various catalysts mentionedpreviousl y.

For supported nickel catalyst (3wt%), (where no isomers can be formed), the reac-tion rate is very low and it is necessary to increase the temperature to start thereaction.

The bimetallic systems Ni-PtlA1203 have the same activity for CO ...... C02 whateverthe impregnation order is. The light-off is about 210°C at 40cm'.min-1. The 0,2%PtlAl203 catalyst has a half conversion temperature of 195°C; the trimetallic systemCe-pt-NilAI203 where cerium was the last deposited metal did not significantly im-prove the activity. The light-off was around 190°C. Furthermore, the Pt-Co where Cowas first deposited on alumina gives T(a o) IS0oC.

-2-

DiscussionA vast amount of existing catalysis literature is devoted to the interaction of CO

with 02 (21). This seemingly simple catalytic reaction includes the general problemsof heterogeneous catalysis. On the other hand reforming reactions of hydrocarbonson metals and alloys are very well reviewed (22,23,14). But these two types of reaction

250

Temperature(OC)

]50

Convers ion CO

50

100

Temnerature(OC)

150

Convers ion <::0

oL__.:::::::::::::::::;::::::::=::::::~:::::""'--,_100

50

110

Conversion of CO to CO2 versus temperature and flow rate on

Fig. I. Pt-Co catalyst for which FiE. 2. Pt -Co catalyst for

calcination steps occurred. which only reduction stenoccurrea.

Flow rate ~n3 . -Iem m~n a = 20, b 40, c = 60, d 80, e 100.

Conversion (%)

100

50

° ISO 200 250 300 Temperature(OC)

Fig. 3. Conversion of CO to CO2 versus temperature for different3 -Icatalysts. Flow rate = 40 cm mn (a) Ni/A1

20 3-3% in weight

(b) Ni/Ce!A1 20 3- 1.5%-1% : (c) Pt/A1 2 0

3- 0.2% (d) ce/Pt/Ni/A1

20

3-

1%-0.2%-1.5% ; (e) Pt/Co/A1 20 3- 0.2%-0.4% (f) Pt!Ni/A12

03-

0.2%-

1.5% ; (g) Ni/Pt/A1 20 3- 1.5%-0.2%.

are not often studied in parallel and compared.The activity and selectivity of catalysts are determined by the properties of sur-

face complexes formed by chemisorption. In this respect we may compare the ther-modynamics of these two reactions and then analyse the influence of alloying on thechemisorption bond strength.

*Oxidation of Carbon monoxideCO + 112 O2 -e- CO2 Exothermic (6 HO298 = -282.6 kJ mole-I) and practically

irreversible up to 1 500 K (6 GO298 = -256.7 kJ mole-\ 6 So298 = -20.7 e.u.),

On clean metal surfaces CO is adsorbed at a high rate with a sticking probabilityof 0.2 - 0.6, without activation energy.

Adsorbed molecules of CO are located perpendicularly to the metal surface withcarbon atoms facing the metal. The interaction involves an acceptor donor bondwith electron transfer according to Engel and Ertl (24).

*lnteraction of dioxygen with the surface of solid catalystsOxygen chemisorption proceeds very rapidly on clean surfaces of most metals. At

room temperature the sticking probability ranges 0.1 to 1 and is close to unity formany metals. This corresponds to a very small value of the activation energy of che-misorption.

*Hydrocarbons-1Hydrocracking is exothermic 6 HO298 " -50 kJ mole (log K 500°C = 4) and the

rate is slow.The adsorption is more confusing. Owing to Cl3 labeling experiments we obser-

ved that the rate determining step in the reforming reactions is the carbon-carbonbond rupture of a dehydrogenated species a or TT bonded to the surface (14) and notadsorption or desorption steps.

*HydrogenIt absorbs dissociatively on all platinum group metals without any appreciable

activation energy. The initial sticking coefficient is typically of the order of about0.1 but may also reach higher values (0.5 for Pd(lOO) (25)). The adsorption energyranges typically between 60 and lOOk J mor1 corresponding to strengths of the M-Hbond around 250 to 270 kJ mor1.

The thermodynamics of these elements is quite different between the oxygenatedand the hydrogenated compounds. Between CO and O2 a competitive adsorption couldoccur as the sticking coefficients are similar. Furthermore, the catalytic surfacescould be very rapidly equilibrated under H2 or O2 as their activation energies ofchemisorption are small. Not only the thermodynamics controls the reaction, but alsothe structure of the catalyst.

One question may arise concerning segregation of one metal preferentially at thesurface of the catalyst.

251

252

With Pt-Ni catalysts, under reducing atmosphere, clean surfaces of bulk alloysare enriched with platinum when oxygen treatments show that the surface is then enri-ched with nickel (2). On Pt-t'li/Al20 3 catalyst with only 0.2wt% Pt - l.Swt% Ni, thecrystallites may be small enough to prevent any segregation phenomenon.

Arguments in favor of this suggestion are:i) For the CO oxidation reaction, whatever the temperature, the reaction rates arethe same for the two catalysts Ni-Pt or Pt-Ni supported on Al20 y The Pt-Ni (or Ni-Pt) catalvsts are better than Ni/Al 20 3 catalyst. The primary reaction pathway for COoxidation on noble metals is a surface reaction between adsorbed CO and adsorbedatomic oxygen (24). Over the Ni-only catalyst, oxides are readily formed under thenet-oxidizing reaction conditions considered in this study and this oxide may suppressCO oxidation. A random distribution of Pt and Ni on the surface can explain thedecrease in the light-off observed on Pt-Ni and Ni-Pt systems.

ii) For the 2-methylpentane reaction, no great differences are observed in the distri-butions of the catalytic products on these two catalysts. When Ni is the second me-tal to be impregnated the extensive cracking is increased slightly (table 1); at theopposite it is the CS+C1, which is favoured for the second catalyst. It is the only dif-ference to be underlined. The activities, the isomers distributions and the selectivitiesare similar.

iii)For the methylcyclopentane hydrogenolysis, the 3~P/nH ratio is the same for thetwo catalysts (3.6::0.6). The amount of cracked products and the total conversion arehigher on Pt-NilAI20 3 than on Ni-Pt/AI20 3.

At this stage we may suggest that no segregation occurs for Pt-NilAI20 3 catalystsand that the surface structure of the bimetallic catalyst can be visualized as a statisti-cal mixture of Pt and Ni sites. The Pt-ColAl20 3 catalyst may behave in a similar wayas its surface segregation is less important under hydrogen than Pt-Ni catalyst (3l).

In these cases, the majority of the Pt metals would be deposited on to the aluminasurface rather than on the top of nickel or cobalt oxide particles as mentioned by Ohand Carpenter (26).

The results obtained under hydrogen show very clearly that the catalysts withoutnickel do not exhibit repetitive processes and that platinum crystallites are small.From previous works undertaken in the laboratory we were able to correlate, in theisomerization of 2-methylpentane and in the methylcyclopentane hydrogenolysis, thatlarger amounts of n-hexane compared with 3-methylpentane are due to the presence of

osmall platinum crystallites, around 20 A or lower (19,27). The metallic particles ofthe catalysts used in the present study are well dispersed, which is the case for thefollowing catalysts: (0.2% and 0.7% pt, 0.2wt%Pt - 1wt%Ce, 0.2wt%Pt - 0.4wt%Cosupported on A120 3).

At the opposite for catalysts with nickel, the platinum particles could be well dis-persed, but the interaction Pt «» Ni masks the platinum behavior.

,£'.,s the metallic particles are small, we could think that the surface structures willbe modified, as previously mentioned at the beginning of the discussion, either theyare working under oxidizing or reducing atmospheres. These experiments were under-taken with Pt-Ce and Pt-Co supported on Al20y The CO oxidation reaction, in ourexperimental conditions, is not stoechiometric as the gas mixture being 1.5% CO and1.5% O2 in N2. As oxygen and carbon monoxide have similar sticking coefficient, wecan assume that the catalytic surface is in an oxidative state after a CO experiment.

On the Pt-Ce catalyst, after this oxidation reaction, its activity is decreased forthe hydrocarbon reaction. We may think that the oxidation state of cerium is reinfor-ced and the oxide biocks platinum active sites.The activity of Pt-Co catalyst is increased after an oxidation reaction. We may sup-pose that this experiment has favoured either the reducibility of the cobalt or createdPt-Co interactions at the surroundings of the platinum crystallites (28).

It is in agreement with the fact that cobalt oxides are reduced more easily thancerium oxides which may create "metal support interactions" with platinum.

Nevertheless, there are no great differences in the product distribution occur afteran oxidation-reduction cycle. On both catalysts the selectivity in isomers is increasedafter an oxidative reaction. The values of the ratio 3-MP/n-H obtained from themethylcyclopentane hydrogenolysis are not modified by this redox cycle.

These results lead us to conclude that the nature of the catalytic sites is unchan-ged during the redox cycle.

After analyzing the results obtained under hydrogen atmosphere we are going tounderline the results concerning the CO -+ CO2 reaction.

In the figures 1 and 2 we have mentioned the results obtained on the Pt-Co cata-lysts and it is obvious that the light-off is 25°C lower on the catalyst where the twosalts were reduced : i) the cobalt reduced at first, il) then reduction of platinum.

Not only the double on reduction process is important, but also the CO+02 gasflow mixture. When the flow is slow, 20cm'.min-1, we can notice differences betweenthe two types of catalysts, but when it is equal to 80cm' .min -1 no large differencesoccur, only 5°C for the light-off, between the catalyst for which the two salts werereduced and the other one for which the two salts were previously calcined prior toreduction. To try to understand these differences, we may suggest that on the doublyreduced catalyst the cobalt is at the lower oxidation state CO(+II) and on the doublyoxidized catalyst the cobalt is only CO(+I1I). The first one dissociates oxygen moreeasily at slow flow rates so that the light off is lower. Boreskov (21) has mentionedthat CoO shows higher catalytic activity in CO oxidation than co203 which is inagreement with our results.

254

The comparison between the catalysts which were both calcined prior to reduction,except for the Pt-Co previously mentioned, shows that all the Pt-Co catalysts andthe Pt-Ce-Ni system are better than the 0.2% Pt. The catalysts: Pt-Ni, Ni-Ce and3% Ni have higher light-off temperatures (figure 3).

It is remarkable that with Pt-Co catalysts under hydrogen, in isomerization reac-tions (28-29), cobalt behaves as a poison. with Pt-Ni catalysts, in the same conditionsnickel plays the role of an additive except for the 50 atom% in Ni and Pt (30). In theoxidation reaction it is exactly in opposite behavior: Pt-Co is the best system and Niis a poison for our studied systems.

CONCLUSIONOur laboratory reactor experiments have shown that the surface structure of the

bimetallic used are composed with a random mixture of Pt and Ni or Co sites. Thisstructural information has been obtained by using the chemical probes (isomerizationand hydrogenolysis of 2-methylpentane and methylcyclopentane respectively andoxidation) which are more sensitive than the physical techniques.

The cobalt which is a poison when it is added to platinum for the isomerizationreactions is the best additive in our case for the CO oxidation reaction.

The results of this study show the importance of understanding interactions bet-ween the metals and the metal support interactions.

REFERENCES1 - D. Tornmek, S. Mukherjee, V. Kumar and K.H. Bennemann, Surf. Sci., 114 (1982)

lI.2 - J. Sedlacek, L. Hilaire, P. Legare and G. Maire, Surf. Sci., 115 (1982) 54I.3 - G. Broden, T.N. Rhodin, C. Brucker, R. Benbou and Z. Hurych, Surf. Sci., 59 (1976)

593.4 - R.W. Joyner and M.W. Roberts, J. Chem. Soc., Faraday Trans. I 70 (1974) 1819.5 - G.G. Low and A.T. Bell, J. Catal., 57 (1979) 397.6 - F. Solymosi and A. Erdohelhi, Surf. Sci., 110 (1981) 663.7 - S. Ichikawa, H. Poppa and M. Boudart, J. Catal., 91 (1985) I.8 - N.W. Cant and D.E. Angove, J. Catal., 97 (1986) 36.9 - J.L. Gland, M.R. Mc Clellan and F.R. Mc Feely, J. Vac. Sci. Technol. A., 1 (1983)

1070.10 - B.E. Hayden and A.M. Bradshaw, Surf. Sci., 125 (1983) 787.11 - A. Crossley and D.A. King, Surf. Sci., 95 (1980) 13I.12 - T. Engel and G. Ertl, J. Chem. Phys., 69 (1978) 1267.13 - M. Boudart and G. Djega-Mariadassou, in "La cinetique des reactions en catalyse

heteroqene", p. 168 (Masson) 1982.14 - G. Maire and F. Garin, Catalysis: Science and Technology, 6 (1984) 162 - Ed. J.R.

Anderson and M. Boudart (Springer Verlag).15 - F: Garin, S. Aeiyach, P. Legare and G. Maire, J. Catal., 77 (1982) 323.16 - G. Meunier, B. Mocaer, S. Kasztelan, L.R. Le Coustumer, J. Grimblot and J.P.

Bonnelle, Applied Catal., 21 (1986) 329.17 - J.P. Brunelle, Pure Appl. Chem., 50 (1978) 121I.18 - G.J.K. Acres, A,J. Bird, J. W. Jenkins and F. King, Catalysis: A specialist perio-

dical reports, 4 (1981) 1 - Ed. Royal Society of Chemistry.

2SS

19 - J.M. Oartigues, A. Chambellan, S. Corolleur, F.G. Gault, A. Renouprez, B.Moraweck, P. Bosch-Giral and G. Oalmai-Imelik, Nouv. J. Chim., 3 (1979) 591.

20 - F. Garin, O. Zahraa, C. Crouzet, J.L. Schmitt and G. Maire, Surf. ScL, 106 (1981)466.

21 - G.K. Boreskov, Catalysis: Science and Technology, 3 (1982) 39 - Ed. J.R.Anderson and M. Boudart (Springer Verlag).

22 - V. Ponec, Adv. Catal., 32 (1983) 149.23 - F.G. Gault, Adv. Catal., 30 (1981) 1.24 - T. Engel, G. Ertl, Adv. Catal., 28 (1979) 1.25 - R.J. Behm, K. Christmann, G. Ertl, Surf. ScL, 99 (1980) 320.26 - Se H. Oh and J.E. Carpenter, J. Catal., 98 (1986) 178.27 - F.G. Gault, F. Garin, G. Maire, Growth and Properties of metal clusters, 451 (1980)

- Ed. J. Bourdon.28 - S. Zyade, F. Garin, L. Hilaire, M.F. Ravet, G. Maire, Bul. Soc. Chim. F., 3 (1985)

341.29 - S. Zyade, These de speciallt.e 3eme cycle (1984) Strasbourg.30 - S. Aeiyach, F. Garin, L. Hilaire, P. Legare and G. Maire, J. Mol. Catal., 25 (1984)

183.31 - S. Zyade, F. Garin, L. Hilaire and G. Maire, unpublished results.


257

THE EFFECT OF WEIGHT LOADING AND REDUCTION TEMPERATURE ON Rh/SILICA CATALYSTSFOR NO REDUCTION BY CO

W.C. HECKER and R.B. BRENEMANChemical Engineering Department, Brigham Young University, Provo, UT 84602

ABSTRACTRhodium catalysts with very low weight loadings (0.01 to 0.06%) are used to

efficiently reduce the ni trogen oxides in automobil e exhaust. Many academicstudies, however, are done using catalysts with high weight loadings (1 to10%). The study reported herein explored differences in activity and surfaceproperties between high and low weight-loaded Rh catalysts before and duringNO reduction by CO. Ini ti al and steady-state turnover numbers were found toincrease significantly (factor of 7) as weight loading was increased from 0.2%to 12%. At the same time the activation energy decreased from 36 to 24kcal/mole. Power rate laws determined by varying NO and CO partial pressureswere found to be fairly similar for the high and low weight-loadedcatalysts. These results seem to indicate that NO reduction is a structuresensi tive reaction. The effect of varyi ng the reduction temperature of thecatalysts between 200 and 450°C was also explored, but no significant effectwas seen on hydrogen uptakes, infrared spectra, initial rates, or steady staterates.

INTRODUCTIONThe reduction of nitric oxide over supported rhodium is one of the most

important reactions in automobile exhaust catalysis. As such, it has beenstudied quite extensively over the past ten to fifteen years. As one examinesthe literature on this subject, it is found that the studies can be classifiedinto two categories according to rhodium weight loading. The first type arethose done with weight loadings of around 0.01 to 0.06 weight percent, whichare typical of actual weight loadings used in automobil e catalytic converters[1-2]. The rhodium in these catalysts is most probably 100% dispersed. Thesecond type are those done on much higher weight loadings of approximately 1to 10 percent rhodium. These higher loadings are often used for studies of anacademic nature, and also so analytical tools such as infrared spectroscopycan be used with significant sensitivity [3-7]' The rhodium in these higherweight loading catalysts is often much less than 100% dispersed.

The prime objective of the present study was to determine how the kinetics

258

of NO reduction by CO differ over high and low weight-loaded rhodium/silicacatalysts. A secondary objective was to determine if the temperature at whichthese catalysts were reduced affects the kinetics or surface properties. Inorder to determine these effects, rhodi um/s i l ica catalysts of five di fferentweight loadings ranging from 0.04% to 12% rhodium were prepared, andmeasurements of hydrogen uptake, IR absorption, and NO reduction activity weremade.

EXPERIMENTALThe detail s of the experimental apparatus and procedures used can be found

in reference 8. In brief, the five catalysts were made by aqueousimpregnation of RhC1 3.3H 20 onto cabosil silica. The catalysts were calcinedat 450°C for two hours in air. Hydrogen uptake measurements were made us i ngstandard volumetric methods [9]. One variation was that the catalysts weresaturated with hydrogen at 180°C instead of room temperature before measuringthei r isotherms.

Reaction studies were carried out in a specially designed infrared cellwhich doubl ed as a flow reactor [10]. Before a given experiment, the catalystwas pressed into the form of a thin wafer, placed into the reactor cell, andreduced at 200-450°C for twenty hours. The catalyst was then run to steady-state in 3.4% CO and 0.8% NO for sixteen hours before any steady-state datawere obtained. All data were obtained under differential reactor conditionsand analysis of the feed and product gases was accompl ished using an automatedgas chromatographic system [11].

RESULTSHydrogen Uptakes

Table 1 shows hydrogen uptakes for four di fferent weight loaded catalystsand for three di fferent reduction temperatures. As can be seen, the 4.3 and1% catalysts show essentially no effect of reduction temperature for reductiontemperatures between 200 and 450°C. As weight loadi ng decreased, the absol uteuptakes decreased also, and therefore it was more difficult to obtain accurateuptakes. This explains the variance in uptake measurements for the 0.2%catalyst.

Table 2 shows average rhodium dispersions for each of the five weightloaded catalysts. These were calculated froo the hydrogen uptakes in Table 1by a ssumi ng each hydrogen atom represented one rhodi um surface si te and fromthe definition of dispersion which is the fraction of total rhodium atoms that

TABLE 1

Hydrogen uptakes as a function of weight loading and reduction temperature.

259

Weight % Rh HZ Uptake (Ilmoles/gram catalyst)Reduction Temperature (K)

124.31.00.2

TABLE 2

473

1119133

573

8834

9 -13

723

8832

Calculated average rhodium dispersions as a function of weight loading.

Wt% Rh

124.31.00.20.04

aAssumed value based on extrapolation.

H/Rh

0.190.430.671.0

(1. O)a

260

are surface atoms. As can be seen, the dispersion increases significantly asthe weight loading decreases. The value of 100% dispersion for the 0.2%catalyst represents the average of its hydrogen uptake values. Therefore, byextrapolation, the 0.04% catalyst was also assumed to be 100% dispersed,

Ki netic dataFi gure 1 shows turnover numbers for NO reducti on by CO as a functi on of

time for each of the five catalysts of this study. Turnover number is definedas rate per unit site and was determined using the di spersion data in Table2. As can be seen, the activity of each catalyst decreases f'rrm its initialvalue to a steady-state value after approximately 10-15 hours. Veryimportantly, the steady-state and initial activities for the higher weightloading catalysts are significantly greater than those of the lOiler weightloading catalysts. Also significant is the fact that the two lowest weightloading catalysts, the 0.2% and 0.04% catalysts, both of which are apparently100% dispersed, have essentially the same activity. Thus, it would appearthat activity correlates with dispersion and as dispersion increases, activity

T =484 K • 12%Rh/Silicap = .0071 atm1'-0 0 4%Rh/Silica

p = .028atm X l%Rh/SilicaCOFresh Catalysts 0 .2%Rh/Silica

• .04%Rh/Silica

decreases.

50C'?

0T"""

X 40 I

'-(l).0 ...... 30E ,

o::J (l)Z Cf)

20....Q)>0 10C....::Jr-

00 5 10 15

x

20 25

Time(hours)

Figure 1. Rates of NO reduction by CO as a function of time for fiverhodium/silica catalysts of different weight loadings.

261

Figure 2 shows transient activi ty data for NO reduction by CO for two 4%rhodium sil ica/catalysts, one of which was reduced at 200°C and the otherwhich was reduced at 300°C. As can be seen, there is virtually no differenceat all at any point in time in the activity of the two catalysts. Thus, onceagain reduction temperature does not seem to effect the catalyst behavior atleast in this temperature range.

1.0 ,----,----r----,----r-----,

4 % RhiSilica

TurnoverNumber

0.8

0.6

0.4

0.2

T=191CP=.84 atmPNO=.0071 atrnPCO= ..028atm

o 200C Reduction

• 300C Reduction

1084 6Time (hours)

20.0 +-----t-----'-----+----t---~

o

Figure 2. Effect of reduction temperature on rates of NO reduction by CO as afunction of time for a 4% rhodium/silica catalyst.

Figure 3 shows steady-state activities as a function of temperature plottedin an Arrhenius form for the five different weight loadings of this study.Four of the catalysts are al so shown at two di fferent reductiontemperatures. As can be seen, once again the 12% catalyst is more active thanthe 4% catalyst which is more active than the 1% catalyst which is more activethan the 0.2 and 0.04% catalysts. Once again also, the two low weight loadingcatalysts are coincidental in their steady-state activities. Also, in eachcase where two reduction temperatures were used, the data seen to foll ow thesame line, except in the case of the 1% catalyst where there seems to be asl ight variance. Fran the slopes of the 1ines in Figure 3, the apparentactivation energies for NO reduction by CO of each of the catalysts can bedetermined. These activation energies are shown in Tabl e 3. As can be seen,the activation energies decrease from 36 to 24 l<cal/mole as the weight loadingincreases from 0.04% to 12% and as the dispersion decreases from 100% to

262

.....ill>oE 10. 3

:JI-

P _ .0071 atmf'O

P •.028 atmco

o .2%Rh 473 K Reduction

•.2%Rh 573 K Reduction

•.04%Rh 473 K Reduction

o .04%Rh 573 K Reduction

o 12%Rh 473 K ReductionI!II 4%Rh 473 K Reductiono 4%Rh 573 K Reduction

• 1%Rh 473 K Reduction

X l%Rh 573 K Reduction

-410

1.8 1.9 2.3 2.4

Figure 3. Arrhenius plot for steady-state rates of NO reduction by CO forfive rhodium silica/catalysts of different weight loadings and reductiontemperatures.

19%. This trend is consistent with the work of Oh, Fisher, et.al. [12J, whoshowed that as they went from a low dispersion, single crystal catalyst to ahighly dispersed rhodium/alumina catalyst, the activation energy increasedfrom 30 to 45 kcal/mole.

TABLE 3

Apparent activation energies of NO reduction by CO for rhodium/silicacatalysts of fi ve di fferent we i ght 1oadi ngs and two different reductiontemperatures.

Rh loading Reduction Data Apparent Activation(wt.%) Temperature (K) Points Energy (Kcal/mole)

12 473 4 244.3 473 4 334.3 573 5 291.0 473 6 311.0 573 4 300.2 473 4 350.2 573 4 350.04 573 3 36

Power rate 1aw expressi ons were determi ned for a hi gh and a low we ightloaded catalyst to detennine if there was any effect of weight loading on COand NO partial pressure dependency. The partial pressure of CO (Peo) wasvaried between .0084 and .047 atmospheres and the partial pressure of NO (PNO)was varied between .0031 and .013 atmospheres. The resulting power lawexpression for a 0.2% rhodium catalyst shows

NNO = k1 Pe8·2PNOO.4and for a 4.3% rhodium catalyst

NNO = k2Pe8·1PNOO.5where NNO is turnover number for NO reduction and k1 and k2 are ratecoefficients. The very small positive order dependence on PCO and themoderate negative order dependence on PNO is quite consistent with previouswork [3-4J. However, the slight difference in the expressions between the0.2% and the 4% catalysts are probably statistically insignificant. Thus, itappears that there is little or no effect of weight loading (or dispersion) onthe CO and NO partial pressure dependencies, at least in the range of partialpressures studied here.

Infrared dataTwo types of infrared measurements were used in this study. The first type

was made foll owing reduction of the catalys tin hydrogen and then decreasi ngthe temperature of the catalyst to room temperature and exposing it to 3%CO. The resulti ng CO bands all owed us to detennine information regardi ng thestructure of the surface [13J. IR data of this type were obtained for 0.2%and 4% rhodi um/sil ica catalysts. The resulti ng spectra [8J looked qui tesimilar. Each had a large intense band between 2060 and 2080 cm-1 which ischaracteristic of a single CO molecule absorbed on a zero-valent rhodiumsite. The spectra for the 0.2% catalyst did contain a slight bump at 2108cm-1 which is characteri stic of a Rh(I} site. Thus, there was a sl ightdifference between the low weight loaded catalyst and the high weight loadedcatalyst.

The second type of infrared infonnation obtained consi sted of in situspectra obtained under reaction conditions after the catalyst had reachedsteady-state. Taking this type of spectra for the 4% catalyst, it was foundthat the catalyst surface is dominated by adsorbed NO. Thi s result isconsistent with what has been seen before [3,5J and it's also consistent withthe observed negative order dependency on PNO' In situ spectra were much moredifficult to obtain for the 0.2% catalyst because of the low amount ofrhodium. The signal to noise ratio became much greater and hence it wasdifficult to make finn observations. However, it did appear that the dominatespecies on this catalyst was al so adsorbed NO.

264

DISCUSSIONThe question of why the high weight loaded, low dispersion rhodium

catalysts are more active than the low weight loaded, high dispersioncatalysts is an interesting one to consider. One expl anation might be thatthe Rh(I) sites seen fran the infrared spectra to be on the low weight loadedcatalysts, are somewhat less active (or inactive) canpared to the Rh(O)sites. However, the very small number of Rh (I) sites seen on the 0.2%catalyst is not nearly enough to explain the three to four fold difference inactivity between the 0.2% catalyst and the 4% catalyst.

A second possible explanation for the greater activity of the lowerdispersed catalyst might be that the rhodium exists in two different phases, acrystalline phase and a dispersed phase, and that the crystalline phase, whichmore prevalent on the higher weight-loaded catalysts, is much more active thanthe dispersed phase. This is similar to an argument used by Yao, et al. [14Jto describe differences they saw in activity with weight loading on arhodium/alumina catalyst. However, the infrared data tend to discount thisexplanation as well, since according to Rice, et al. [13J, CO adsorbs on adispersed phase in a dicarbonyl structure which has distinct bands at 2030 and2100 cm-1. In thi s work, those bands were not observed.

A third possible explanation for the observed behavior could be a metal-support interaction. Electronic metal-support interactions would result in anoxidized rhodium surface which waul d be detectable in the infrared. However,since very little oxidized rhodium was detected, it appears that one candiscount this possibility also.

A final possible explanation is that of a traditional structure sensitivityas suggested by Boudart several years ago [15J. In his work he showed that asdispersion decreases, crystallite size increases and the average coordinationnumber, or number of nearest neighbors for any site, increases. In previouswork on rhodium/silica for NO reduction by CO [3], it has been shown that therate determining step is probably NOa+S + Na+Oa. Since this is a step thattakes pl ace on two adjacent sites, it stands to reason that the more nearestneighbors a catalyst had, the more readily thi s step, and thus the overall NOreduction reaction, would occur. Also, as the rate detennining step, thisreaction is primarily responsible for the apparent activation energy. Thus,one can postul ate that as weight loading increases, the number of nearestneighbor sites increase which decreases the activation energy of the rate-determining step and increases its rate. In the present work, of course, ithas been seen that the activation energy does decrease as the rate becomesgreater and as the weight loading increases. Thus, there seems to be aconsi stency here that would suggest that NO reduction on rhodi um/sil ica is

265

structure sensitive and that this is the reason that activity increases asweight loading increases.

CONCLUSIONSIn conclusion, rhodium/silica catalysts of low to intermediate dispersions

and high weight loadings are significantly more active than catalysts of highdi spersion and low weight loadings. The high weight loaded catalysts have asignificantly lower activation energy than the low weight loaded catalysts.Power rate law expressions don't seem to be significantly different betweenhigh and low weight-loaded catalysts. These observations along with infraredobservations tend to be consistent with the fact that NO reduction by CO onrhodium/silica is a structure sensitive reaction, and that as the coordinationnumber rhodium crystallites increases, the activity also tends to increase.Further, it has been seen that reduction temperature has essentially no effecton rhodium dispersion or NO reduction activity.

REFERENCES

1. L.L. Hegedus and J .J. Gumbleton, Chemtech, 10 (1980) 630-642.2. K.C. Taylor, Automobil e Catalytic Converters, Berl in, 1984.3. W.C. Hecker and A.T. Bell, J. Catal., 84(1983) 200.4. W.C. Hecker and A.T. Bell, J. Catal., 85(1984) 389.5. W.C. Hecker and A.T. Bell, J. Catal., 92(1985) 247.6. H. Arai and H. Tominaga, J. Catal., 43(1976) 131.7. F. Solymosi and J. Sarkany, Appl. Surf. Sc i . , 3(1979) 68-82.8. R.B. Breneman, M.S. Thesis, Brigham Young University, 1986.9. C.H. Bartholomew and R.B. Pannell, J. Catal., 65(1980) 390.

10. R.F. Hicks, C.S. Kellner, B.J. Savatsky, W.C. Hecker, and A.T. Bell, J.Catal., 71(1981) 216.

11. W.C. Hecker and A.T. Bell, Analytical Chem., 53(1981) 817.12. S.H. Oh, G.B. Fisher, J.E. Carpenter and D.W. Goodman, presented at AIChE

Meeting, Chicago (Nov 1985). See also the paper by Fisher et al.presented at this meeting.

13. C. Rice, S. Worley, C. Curtis, J. Guin and A. Tarrer, J. Chem. Phys . ,74 (1981) 6487.

14. H.C. Yao, Y.Y. Yao, and K. Otto, J. Catal., 56(1979) 21.15. M. Boudart, Adv. in Catalysis, 20(1969) 153.

.\. Crucq and A. Frennet (Editors), Catalysis and Automotive Pollution Control1987 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

REACTIVATION OF LEAD-POISONED pt/A1 203 CATALYSTS BY SULFUR DIOXIDE

J. W. A. SACHTlER, I. ONAl, R. E. MARINANGElIAllied-Signal Engineered Materials Research Center, 50 East Algonquin Road,P.O. Box 5016, Des Plaines, Il 60017-5016, U.S.A.

ABSTRACT

267

The effect of S02 on the oxidation of C3HS by Pb-poisoned and Pb-freePt/Al?03 catalysts was investigated. Transient S02 injection experiments wereperformed on engine-aged catalysts and catalysts aged in the laboratory withPbBr2" In both cases, the increased activity for C3HS oxidation nue to S02exposure is attributed to a reversible promotion of C3HS conversion by S02(independent of Pb content) and a permanent reactivatlon. Catalyst characteri-zation has shown that the permanent reactivation of the Pb-poisoned catalystsinvolves formation of large PbS04 crystals which effectively removes Pb from Ptcrystallites. The C3HS oxidation activity of Pt catalysts is also permanentlyenhanced by S02 regardless of the presence of Pb. In this case, the mechanismmay involve sulfate formed on the alumina.

S02 poisoning of CO oxidation offsets the enhancement due to removal of Pbfrom Pt crystallites. Thus, the net effect of 502 on CO conversion by Pb-poisoned catalysts is small.

INTRODUCTI ONIt is well known that lead and sulfur compounds can poison exhaust gas

catalysts [Ref. 1J. Sulfur dioxide, however, can have beneficial effects.Michalko [Ref. 2] noted that exposure to anhydrous S03 was effective for regen-erating exhaust gas catalysts poisoned by Pb. Hammerle and Graves [Ref. 3]observed that poisoning of a Pt/Al 203 catalyst by Pb is reversed by exposure toS02 below 650°C. On the other hand, Vao, et al. [Ref. 4] observed that S02enhances the rate of C3HS oxidation over a fresh Pt/Al 203 catalyst. Thepurpose of this work was to study the effect of S02 on Pb-free and Pb-poisonedPt/Al 203 catalysts and to gain insight into the mechanism of (re-)activation byS02' Such information is needed for the development of Pb-tolerant oxidationcatalysts. This type of catalyst might be useful in countries where Pb-freegasoline is not readily available.

268

EXPERIMENTAL

The catalyst aging and activity testing were done with a monolithic formu-lation consisting of 0.36 wt.% Pt (10 g/ft 3) on y-A1 203 supported on a 46.5cells/cm2 cordierite substrate. Engine-aging was conducted on a multi-modecycle with a peak temperature of 760°C which has been described previously[Ref. 5J. The aging fuel contained 0.15 g Pb/l and 0.185 g SILo Aging wasconducted for 300 hours at an average fuel rate of 6.9 kg/hour and an air/fuelratio of 15.2. laboratory hydrothermal aging was conducted at 871°C or 927°Cfor 10 hours in 10% H20/90% air. laboratory Pb-poisoning consisted of exposureto PbBr2 evaporated in N2 at 550°C. The flow rate during aging was 3 L/minuteand the exposure time was two hours.

Laboratory activity tests were conducted in a flow reactor on 2.2 cm diam-eter x 5.1 cm length cores taken from the inlet of aged monoliths. The syn-thetic exhaust gas composition is given in Table 1. The space velocity was30,000 hr-1 (STP) based on core volume. Catalyst response to 502 was examinedby testing without 5°2, continuing the test while injecting 20 ppm 5°2, andtesting again without 502'

In order to study the effects of 502 and PbBr2 on CO adsorption and C3H8oxidation in more detail, a series of experiments was conducted using infra-red(IR) spectrometry. The IR spectra were recorded on a Beckman IR-12 spectrome-ter; the samples for these experiments were pressed discs of 1 wt.% Pt/y-A1203with a Pt dispersion of 0.23 as measured by oxygen-hydrogen titration. A sam-ple of the 1 wt.% Pt/y-A1 203 catalyst was poisoned with lead by exposure toPbBr2 vapor in N2, followed by hydrolysis with 3% water in He at 550°C. C3H8oxidation was studied in a static mode, by injecting 1 mL C3H8 (STP) and 10 mL02 (STP) into the 820 mL IR cell. C3H8 conversions were determined from theintensity of the gas phase C3H8 band at 2972 cm- 1 before and after heating 30minutes at 260°C.

Pb-poisoned and reactivated samples were characterized by Scanning Trans-mission Electron Microscopy (STEM), Energy Dispersive X-Ray Analysis (EDX), andX-Ray Diffraction (XRD).

Results and Discussion

Effect of 502 on C~ Oxidation

Figure 1A shows the response of an engine-aged Pt/A1 203 catalyst to 502injection at 300°C from 60 to 130 minutes. The conversion of C3HS increasesfrom 15% to 53%. Following removal of the 5°2, the C3Hg conversion declines to

269

33%. The response to SOZ includes a permanent (long ter~) activation (15% to33%) and a reversible activation (33% to 53%). Similar results were obtainedat 400°C while at 600°C no effect of S02 can be observed (Figure lB).

Figure Z shows the response of a PbBrZ-poisoned Pt/A1 Z03 catalyst to S02injection at 500°C. This sample responds very much like an engine-aged cata-lyst. This result is consistent with reports that Pb-poisoning is due totransport of Pb to the catalyst in the form of halide species [Ref. 6J. The Pbhalides may be hydrolyzed on the catalyst to form lead oxide species. However,catalyst exposure to PbO vapor in the same laboratory apparatus as used forPbBr Z aging did not cause significant deactivation. Since the PbBrZ-poisonedsamples could also be reactivated with SOZ' we judged that the laboratory poi-soning of catalysts with PbBrZ can be used to model the engine-aging of thesesamples with leaded fuel.

The poisoning of the C3HStion of a poisoned catalyst byIR experiments (see Table 2).wt.% Pt/A1 Z03 sample followingof Pb species (Figure 3):

oxidation by PbBrZ' and the permanent reactiva-exposure to S02 and air was also observed in theSTEM and EDX examination of the PbBr Z poisoned 1calcination in air at 500°C revealed three types

i) bimetallic Pt-Pb particles (50-l00A);ii) Pb particles (probably PbO) (lOO-ZOOA); and

iii) amorphous Pb present everywhere on the A1 Z03 in a very thin layer.

Following reactivation by SOZ at 500°C, STEM examination (Figure 4) showedlarge Pb-containing particles with sizes up to 10,000A. Since XRD showed thatthe only crystalline Pb species present was PbS04, these large particles areconcluded to be PbS04• In the IR spectrum of this reactivated disc, a largesulfate band was observed at 1390 cm- l• EDX consistently showed lower Pblevels in the Pt particles after reactivation.

The results obtained with the engine-aged, laboratory PbBrz-aged, andmodel catalysts all clearly indicate that Pb-poisoned Pt/A1 Z03 catalysts can bereactivated by SOZ. The model experiments indicate that the reactivation mech-anism involves the formation of PbS04, which removes Pb from the Pt. Addition-al evidence for this mechanism is found in the CO adsorption experiments, des-cribed in the next section.

The PbS04 formed by S02 reactivation is not stable at high temperaturesand its decomposition again causes poisoning of the Pt, as judged by the almostcomplete disappearance of the sulfate band in the IR spectrum and the low C3HSconversion observed after heating a reactivated disc in vacuum at 750°C.

270

TABLE 1

Synthetic Exhaust Gas Composition

Component Concentrat i on

2190 ppm (C Basis)2%

1520 ppm4.8%10%

11.1%71. 7%

TABLE 2

Sample

Reactivation of Model 1 Wt.% Pt/A1 203 Catalysts Poisoned by PbBr2

Range of C3H8Conversions at 260°C

Fresh Pt/A1 203Pt/A1 203 with SulfatePresent (without Pb)

PbBr2 Poisoned Pt/A1 203PbBr 2 Poisoned Pt/Al 0Reactivated by S02 at 500°C

TARLE 3

53-64%

58-64%

0-7%

55-57%

IR Absorption Band Positions of CO on Pt-Pb/A12~

Sample Position of CO Band (cm-1)

Reduced Pt/A1 203PbBr2 Poisoned Pt/A1 203After Reduction

PbBr2 Poisoned Pt/A1 203Calclned at 500°C

PbBr2 Poisoned Pt/A1 203After C3H8 Oxidation 250°Cand Evacuation 500°C

PbBr2 Poisoned Pt/Al 03Reactivated by S02 at 500°CAfter C3H8 Oxidatlon 260°Cand Evacuation 500°C

2096

2025

2086

2045

2093

271

~ @ 00 00 100 l~l@loo l00~OnO~o

Time (Minutes)

Effect of S02 on C3HSConversion at DifferentTemperatures of an Engine-Aged Pt/A1 203 Catalyst

FIGURE 1

T=300°C

T:600°C

,S02 Off T:4000C

S020nl

S020nl

S020nl

100,----------------

90 r---~--------- g 80e;c 70o

"00 60~

<3 50f 40s 30

2010 '----'------'-----'_-'------'---'-_"'----'------'-----'_-'--o

Effect of S02 on C3HSConversion at 500°C of aPt/Al?03 Catalyst Poisonedby PbBr2

FIGURE 2

50

50 100 150 200 250 300 350Time (Minutes)

FIGURE 3: STEM Micrographs of a Pb-Poisoned 1% Pt on y-A1 203 Catalyst

co"00

~oUcoIC'?U

~ ._. •FIGURE 3A100 kX, ShowingPt and Pb(O) Particles

FIGURE 3B10 kX, Area Scan

272

FIGURE 4: STEM Micrographs of a Reactivated Pt-Pb-y-A1 203 CatalystFIGURE 4A FIGURE 4820 kX, Area Scan Showing Abundance of 150 kX, Showing a Very Thin PbS04PbS04 Crystals, Ranging from 0.05-0.5]J Crystal (a Rare Observation)

FIGURE 5

Effect of S02 on C3HSConversion at 350°C of aPt/A1 203 CatalystHydrothermally Aged at S70°C

350300

/302011

100 150 200 250Time (Minutes)

50

80

§ 70.~

~co060coI(3

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o

90,------------------, FIGURE 6Effect of S02 on COConversion at 350°C of aPt/A1 203 CatalystHydrothermally Aged at 927°C

300 350100 150 200 250Time (Minutes)

50

80~e....70co"00

~60oo 508

40

30 '---_-'--_--'-_---'-_--'-__'--_-'--------'o

As shown in Figure 5, a hydrothermally aged Pt/A1 203 catalyst which doesnot contain Pb also shows a response to 502 in the laboratory activity test.Again, a permanent (4S% to 60%) and a reversible activation (60% to S4%) can bedistinguished. It was noted that in this case the responses to $02 are muchfaster than with the catalysts containing Pb. However, in IR experiments usinga fresh I wt.% Pt/A1 203 sample, treatments with 502 and air that produced sul-fate on the catalyst as detected by IR did not give a significant increase inthe C3HS conversion (see Table 2). It thus appears that our IR experimentscould not replicate the conditions that cause permanent activation of the hy-drothermally agerl catalyst in the activity testing. It may be speculated thatthe sulfate which forms on the alumina support can playa role in the activitytests, in accordance with the report by Yao [Ref. 4J. However, the nature ofthe participation of the sulfate in the C3HS oxidation could not be determinedin our experiments. Because of the above mentioned results for the Pb-freesystem, it is possible that the permanent reactivation of the Pb-poisoned sam-ples observed in the activity test contains a contribution from bothmechanisms.

The reversible activation by 5°2 was observed regardless of the presenceof Pb. We have no data to explain this effect. However, it might be speculat-ed that adsorbed 503 is more effective for C3HS oxidation than adsorbed oxygen.As the equilibrium of 5°2 oxidation to 5°3 becomes unfavorable at high tempera-tures, this could explain the decrease in the magnitude of this effect with in-creasing temperature.

Effect of 502 on CO Oxidation

The engine-aged catalysts were very active for CO oxidation (>99% conver-sion) during the laboratory activity tests. There was no change in activitywhen 5°2 was introduced. Of course, small changes could not be measured atsuch high conversions. Figure 6 shows that 5°2 injection at 350°C causes asharp drop in CO conversion for a hydrothermally aged catalyst but that theinitial activity is recovered after 502 exposure ends. The C3HS conversion forthis sample increased with exposure to 5°2,

The effects of PbBr2 poisoning and reactivation by 5°2 on CO adsorptionwere also followed on the model I wt.% Pt/A1 203 sample. Table 3 shows that ona reduced sample, Pb shifts the IR absorption bands of CO on Pt/A1 203 from 2096to 2025 em-I. Oxidation of the sample causes an increase in band position, ashas been observed in the literature (Ref. 7). Reactivation by $02 at 500°C es-sent i ally restores the ori gi na1 CO absorption band.

274

SUMMARY

Reactivation of Pb-poisoned Pt/AI 203 catalysts for C3HS oxidation by expo-sure to S02 and air has been demonstrated for engine-aged and laboratory PbBr2-aged monolithic catalysts using a flow reactor activity test, and in model ex-periments using PbBr 2-poisoned Pt/A1 203 powder. It has been shown in the modelexperiments that the reactivation by S02 involves the conversion of Pt-Pb spe-cies to Pt and PbS04• In the flow reactor activity tests, the C3HS oxidationactivity of a Pb-free Pt/A1 203 catalyst was also permanently enhanced by S02'This mechanism may involve sulfate formed on the alumina, but the mechanism ofits participation in the C3HS oxidation could not be determined in our experi-ments. It is possible that the permanent C3H8 oxidation activity increase ofthe reactivated Pb-poisoned samples observed in the flow-reactor activity testsmay have included a contribution from this latter mechanism. In addition topermanent effects, S02 also causes a reversible activation in the activitytests. At temperatures above 500°C, the effects of S02 are not observed. Thismay be due to the instability of the sulfates and the unfavorable equilibriumof S02 oxidation at high temperatures.

S02 exposure reversibly poisons CO oxidation on Pt/A1 203• S02 exposurealso converts Pt-Pb species to Pt and PbS04• Thus, the net effect is thatexposure to S02 has little effect on the activity of Pb-poisoned Pt/A1 203 forCO oxidation.

REFERENCES

1. a) G. C. Joy, G. R. Lester and F. S. Molinaro, SAE Paper #790943 (1979).b) J. C. Summers and K. Baron, J. Catalysis, 57, 380 (1979).c) M. Shelef, K. Otto and N. L. Otto, Adv. Ca~ysis, 27, 311 (1978).d) E. C. Su, W. R. H. Watkins and H. S. Gandhi, Appl. <:.italysis, ~' 59

(1984).

2. E. Michalko, U.S. Patent #3,121,694 (1964).

3. R. H. Hammerle and Y. B. Graves, SAE Paper #830270 (1983).

4. H. C. Yao, H. K. Stepien and H. S. Gandhi, J. Catalysis,~, 231 (1981).

5. M. G. Henk, J. J. White, J. F. Skowron and I. Onal, SAE Paper #830271(1983).

6. B. Harrison, J. R. Taylor, A. F. Diwell and A. Solathiel, SAE Paper #830268(1983).

7. A. G. T. M. Bastein, F. J. C. M. Toolenaar, and V. Ponec, J. Chern. Soc.Chem. Commun., (1982) 627.

A. Crucq and A. Frennet (Editors), Catalysis and Automotive Pollution Control1~l87 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

ALUi-lINA CARRIERS FOR AUTOHOTIVE POLLUTION CONTROL

Ii. NORTIER 1 and i-1. SOUSTELLE 2

lRhone-Poulenc Recherches

14, rue des Gardinoux - 93308 AUBERVILLIERS (France)'}

-D~partement de Chimie Physique des Processus industrielsEcole Nationale Sup~rieure des Mines de Saint-Etienne

158, Cours Fauriel - 42023 Saint-Etienne (France)

275

ABSTRACTTransition aluminas help solving the problem of automotive pollution

control. Their intrinsic advantages : chemical inertia, suitable porosity arefound in the forms of pellets or washcoated monoliths, as well as one importantdrawback: thermal aging. This very important feature was studied,experimentally and theoretically; a model is proposed, and an answer found:s t ab i l iz a t i on ,

ALUMINA AS A CATALYST CARRIER

General

Since 1975, CATALYSIS has been the only practical way for automotivemanufacturers to meet the severe regulation of exhaust gas emission in JAPAN

and in the U.S. Similar measures will be applied in Europe in the near future.For numerous economic and technical reasons, automotive emission control

catalysts are supported catalysts. This means that the active phase is

dispersed on the surface of a catalytically almost inert material. Thatmaterial is the subject of this investigation.

The usual constraints for catalyst carriers are- chemical inertia ~n the reacting medium- ability to be impregnated and to carry the active phase.

- to allow a good diffusion of reactants (resp. products) to (resp. from)

the catalytic surface.

This is particularly difficult in the case of exhaust gas control, where

the catalyst is submitted to very harmful conditions i.e.- exposure to poisons (Pb, Zn, P, S from lubricants and gasoline)

vibrations mainly due to the cyclic work of the engine- high temperature (>8OO°C) which is possible but not abnormal

Furthermore, the conversion of the reactants (CO, CnHp, NOx) must be nearly

total even though the residence time of gases in the converter is extremely low

276

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ALUMINIIM

ALKOXIDEPROCESS

ALUMINA CARRIER

Fig. 2 - Industrial processes providing transition alumina carriers

277

'lost of these problems are sat isfac tori 1y resol ved by using t ransi t ion

aluminas as catalyst carrier for the active phases (Pt, Pd, Rh) and promoters

(CeO}, NiO, FeO..• ).Among the numerous transition aluminas (as illustrated in Fig.l),gamma,

delta illld theta AIZ03 are the most used. These disordered spinels can be

~repared by dehydration of boehmite. Fig. 2 shows the main ways of producing

such aluminas.

The reasons for the choice of alumina are numerous :

- Alumina is cheap, partly because the raw materials for special aluminasare GIBBSITE or ALUMINIUM, both of which are available in large amounts at

low cost (especially GIBBSITE) derived from bauxite.

- The Iso Electric Point of alumina is 9 ; its surface can be electrically

charged either positively or negatively and therefore, can selectivelyadsorb ions.

Alumina does not give rise to chemical reaction with the gas feed (except

for some poisons). Moreover, since the diffusion of Platinum is very lowupon alumina this active metal is stabilised as small clusters with a

large surface area.Furthermore, alumina can be shaped with an accurate control of its porosity.

This is very important because the catalytic processes of exhaust gas control

are most often diffusion limited.From the outset, two kinds of shapes -pellets and monoliths- were developed.

They are discussed separately below.

PelletsAt least four processes are known for making pellets from a powder

- PAN PELLETIZING- OIL DROP

- EXTRUSION

- PILLING or TABLETTINGBecause of vibrations, edges have to be avoided and only the two first

processes are still employed, since they provide spherical particles.

The design of the carrier must include the necessity of achieving highefficiency at very low residence times. Consequently the lowest resistance tomass transfer from the gas flow to the catalytic material surface is required,

leading to the following characteristics:- A small radius provides a large contact area between the beads and the

gas, and lowers the intraparticle distance to the active site,

278

Fig. 3 - Cross section of a pellet type converter

-------- -- -------- ----------,pore volume distribution: ..derivation of p.v.d. L, 200 ;::'8cumulat ive s.s.a.

'"8"e>

'"...eQ,

O.

GOOD

10 100 1000PORE DIAMETER (nm)

fig. 4 - Illustration of bimodality

Fig. 5 Shematic showing extremes of micro-macropore distribution

279

TIlis second aim is also the reason for peripheral impregnation of theprecious metals,

- A high level of macropores (diameter more than 0.1 ~m) facilitates theintraparticular diffusion, as micropores (diameter less than 20 nm) are

necessary to develop a high surface area. This double feature is know asbimodality, illustrated in fig. 4.

- Furthermore, poisoning by Zn, Pb and P creates an amorphous, vitreous

surface on the bead that clogs the micropores and only leaves the

macropores open. Thus the distribution of microand macropores must be welldesigned as illustrated in Fig. 5. Photographs 6 to 9 show some details of

a porosity distribution which is exceptionally well adapted to theautomotive exhaust control application.

To increase the porous volume by addition of macropores is also an

advantage for a cold start efficiency, since this decreases the total heatcapacity of the catalytic bed, and enables the catalyst to reach its light-

off temperature more quickly.

Thus, the diffusional properties lead us to design very small and porous

beads. However mechanical considerations limit this tendency since the crush

strength of beads is proportional to the square of their radius and is adecreasing function of their porosity, as illustrated in Fig. 10.

Practical considerations with respect to canning and pressure drop through the

converter prohibit use of beads smaller than 2 mm diameter.

These features as well as the industrial feasibility led to the use of

carriers such as those shown in Fig. 11 (1975-1979) and Fig. 12 (since 1979).

Monoliths

Vibrations can put the beads in motion so that they collide each with otherand their surface are abraded. This is the attrition phenomenon which is absent

from monolithic structures within which no internal shock can occur.Two types of commercial monolithic substrates are available, made from

CERAMICS (fig. 13) or refractory METALS (Fig. 14).The production methods are :

ceramic monolith : mixing components, EXTRUSION and reactive calcination

- metallic monoliths: wrapping two sheets of metal, one of them being

corrugated, and the other flat.Neither of these two materials is suitable for direct impregnation with

precious metals, their specific surface area being much too low (less than

10 m2/g) to allow a good dispersion in a reasonable volume. This explains the

need for an alumina coating on the monolithic substrate, this c~ating being

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Fig. 17 - S.E.M. photograph of afi ller alumina

Fig. 18 - T.E.M. photograph of abinder alumina

285

itself the catalyst carrier.

Although other processes have been developed, the most widely used method iswash-coating, i.e. dipping the substrate into an aqueous suspension of alumina,

blowing out the excess suspension in order to unclog the substrate channels andcalcining.

An alumina coated ceramic monolith is shown on Fig. 15.

A better diffusion of the reactants can be obtained by using a macroporous

coating as shown on Fig. 16. The binder-filler process which involves two kindsof aluminas, one of which being dispersible (Fig. 17), and the other being made

of coarse grains (Fig. 18), is then especially suitable.

Pellets and monolith specific features

Comparison of a classical washcoated monolithic carrier (20 W % Al 203 on400 cell/inch2 cordierite monolith) with a standard pellet support (SCM 129 X,

2.4-4 mm), shows (Fig. 19)

- similar densities and heat capacities

- a greater external (or geometric) surface area for monolith.The intrinsic advantages of the beads are :

- turbulent flow of the gas which facilitates mass transfer- mobility in the converter which lowers poisoning

- as the pellets are small, thermal shocks are not harmful

Their main drawback is : attrition

Advantages of monoliths are :- no attrition

- ease of assembly

- lower pressure dropTheir main drawback is a risk to melt or break under high temperature

(especially in the presence of Pb).

Characteristics of the alumina common to both pellets and monolithic substrates

In each case, alumina must provide a suitable surface to precious metals.

This requires both chemical purity for the nature of the surfaces and thermal

stability of surface area.Chemical purity is not critical in automotive exhaust control, compared with

ego Reforming; Fig.20 illustrates the required level of purity.

Thermal stability is not so easy to obtain. Two mechanisms can account for

the instability of porous transition aluminas:- On the one hand, they consist of small crystals and a decrease in

specific surface area would mean a decrease in the free enthalpy, because

of the surface energy. This is illus~rated in Fig. 21-22.

286

GEOMETRICBULK HEAT SURFACE

CARRIER DENSITY CAPACITY AREAelg / i1ter)

PER LITER PER LITER( J 11.1 ) (m2 /1)

MONOLITH400 2 8CELL / INCH o. 39 343

MONOLITH.O. 47 408 2. 8ALUMINA

LAYER

BEADS o 43 360 1. 1

Fig. 19 - Comparison pellets and monoliths

TOTAL 2 w%

Na20 1500 ppm

Fe203 700 ppm

Si02 8000 ppm

S04~ 6000 ppm

CaO 1500 ppm

MgO 1500 ppm

Fig. 20 - Maximum impurity content in alumina for exhaust control

Fig. 21 - T.E.M. photograph of a fresh carrier, exhibitingmicroporosity

Fig. 22 - T.E.M. photograph of an over calcined carrier(24 h at 982°C),showing collapse of the micropores

1/

287

288

- On the other, the thermodynamically stable phase is corundum

These two phenomena involve solid state transport and are very slow unlessvery high temperatures are reached (above 900°C).

These conditions are abnormal in automotive exhaust systems, but can occurespecially in the case of engine malfunctions, high load, and transientcond i tions.

The consequences can be a decrease in the activity, because sintering of

alumina leads to sintering of precious metals and moreover, mechanical damageof the carrier.

This is the principal reason for Research and Developement into the designof thermally resistant catalyst carriers.

One of the most interesting methods is stabilisation by adding to alumina a

small amount of a foreign oxide.The most recent theoretical developments in this field are reviewed below.

STABILISATION

Alumina is the most commonly used carrier in catalysts for the automotiveexhaust gas control. Aluminas exist in several forms, according to the

temperature, the raw materials and the processing (especially thermal)

conditions. This is illustrated in Fig. 1 and 2. Among all these forms, two

kinds of aluminas can be distinguished besides hydrates :- transition aluminas

- alpha alumina or corundum which is the most stable form at any ~mperaUFe.

The transformation of the transition aluminas into corundum is a seriousdrawbac% to the use of these solids as catalyst carriers,since this irrever-sible transformation has two effects:

- a major decrease in the specific surface area, falling from 100 to10 m2/g, with a disastrous drop in the catalytic activity, as a

consequencea 20% decrease in the specific volume, creating voids in the catalytic bed

which leads to an increase in the attrition phenomenon.The transformation into corundum occurs above 900°C, which temperature can

be reached in a catalytic converter. Consequently it is necessary to stabilizethe transition aluminas to high temperature (1250-13OO°C) in order to improve

the durability of the catalyst.This stabilization can generally be achieved by adding inorganic elements

to the aluminas. These elements can be alkaline earths (ref. 1, 2, 3, 8),zirconium (ref. 3, 8, 11), thorium (ref. 4,7), rare earths (ref. 5, 6, 7, 11),

titanium (ref. 8), boron (ref. 3) and silicon (ref. 10,12).According to these authors, the stabilizing elements are incorporated either

289

just mixing hydrated or transi tion alumina IViLh the corresponding oxides, or!,,' impregnaU.ng alumina with an aqueous so l ut i or; containing the s t a biI i zing

I ern en t as a thermal 1y unsta bI c sal t. The trca t.rnent IVi t h a gas con t a i ning thestabilizing element has also been described in the case of silicon. In eachcase, a calcination step at temperatures betwen 600 and 950°C folloIV8 thisaddition or impregnation step. Amounts of additive in the range of 0.1 to ]S %bv weight of oxides have been reported.

In contrast, other elements destabilize the transition aluminas and increase

the transformation rate to corundum. Iron III (ref. 13, 14), i'langanese Ill,

Vanadium V, Molydenum VI, Cobalt III, Zinc II, chromium III (ref. 13) exhibit

such an effet. Contradictory results are described with magnesium II (r&f. 13,IS) Lanthanium III (r&f. 13, 16) and Zirconium IV (r&f. 13, 14).

TUCKER and HREN (ref. 17) reviewed the effects of several other additives.Other authors, including HARMER (ref. 18) have put f orward qualitative

explanations concerning the mechanism. However none of the proposed models hasa forecasting feature.

Experimental study of the effect of alien cations on the transformation oftransition aluminas into corundum

We studied the transformation rate of transition alumina as a function ofthe added ion nature. RHONE POULENC provided us with the starting material,which is similar to some of the commercial products of that company.

The diameter of these spherical macroporous beads was 2 to 4 mm, the pore

volume 0.92 cm3jg and the specific surface area 118 m2jg. The

crystallographic phases were assigned by X-ray diffraction analysis as gammaand delta Al203 . Chemical analysis indicated a total impurity content

lower than 800 ppm.Addition of foreign cations (doping) was achieved by the method of incipient

wetness using an aqueous. solution of the nitrate salt of the cation. A dryingstep (24 hours at 110°C) and a calcination step (1 hour at 600°C) followed for

each preparation.The following criteria governed the choice of the doping elements :- they were available as water soluble nitrate salts (in order to carry out

all impregnations under the same conditions)

- their ionic radius was either very close or very different from the oneof A1 3+

- they have only one stable oxidation state. Any redox phenomenon in thesolid state was thus avoided and the valency of the alien cation is well

defined. Consequently we can forecast the nature of the point defects

290

100

8

60

40

20

o

% a-A 1203

10

1378K

Zr• -.d__ .!.-.6-~-'=~Ca

-~-"""--- La20 30 40 Tlme Ihours]

Fig. 23 - Transformation ratio as a function of time at 110SoCA3 is the undoped Alumina

120

100

8060

12010

20 40

20

60

30

1378K

40 Time (hour s)

80

Fig. 24 a - Specific surface area versus heating timeFig. 24 b - Specific surface area versus transformation ratio

o : undoped alumina, 6: A (Mg)

291

created by the addition of this cations.

Some features of the added elements are collected in table 1, which alsoincludes the temperature of the top of the exothermic peak which correlateshith the transformation to corundum. This peak was determined by Differential

Thermal Analysis (DTA).

TABLE 1: Some characteristics of the added elements

Element

Al

Mg

CaGa

InLa

Zr

Th

Valency

+3+2

+2

+3+3+3+4

+4

Ionic Cationic Peak Sampleradius ratio temperature reference

0.50 A 0.01 l272°C A(Al)

0.65 0.025 1277 A(Mg)

0.99 0.01 1350 A(Ca)

0.62 0.01 1276 A(Ga)

0.81 0.01 1260 A(In)

1.15 0.01 1384 A(La)

0.80 0.01 1345 A(Zr)

0.95 0.01 1386 A(Th)

A possible influence of the doping method on the behaviour of the differentsamples was eliminated by using an "alumina" doped sample as a reference.

This sample was prepared by impregnation with an aluminium nitrate solution,drying and calcining in the described conditions. It was compared with a sampleimpregnated with aqueous nitric acid at the same nitrate concentrations. These

two samples gave identical results, i.e. same temperature of the DTA exothermic

peak and same rate of transformation into corundum.Transformation ratio as a function of time

Isothermal transformation curves were measured as a function of time.Samples were heated in static air in an electric furnace at a controlled

temperature and were withdrawn at different times (up to 45 hours). The

transformation ratio (into alpha A1 203) was obtained by XRD analysis with

an accuracy of 5 %.Fig. 23 shows curves obtained at 1105°C. Most of them show a sigmoidal

appearance. Comparing to that for pure alumina, two families can be identified:

one is the group of accelerator additives, with an increasing effect followingthe order In, Ga, AI, Mg, and the group of inhibiting additives Zr, Ca, Th, La.

From the position of the A (AI) curve, we can assign an accelerator effect to

292

the nitrate ions.

The inhibiting effect of thorium is substantial, since the transformation

ratio is only 1,5 % after 45 hours at 1105°C and 4,5 %after 160 hours. In thecase of lanthanum, it is still more significant since no alpha AlZ03 wasdetected after 45 hours and only 1.5 % after 150 hours

After calcining new XRD peaks appear that cannot be assigned to corundum.

In order to attribute them, we heated the sample to l450°C for two hours ; atthis temperature the transformation rates are 100 % in all cases. Magnesium

aluminate (MgA1 204) was detected in A(Mg), Ca Al120 l 5 in A(Ca),

In 203 in A(In), Zr0 2 (tetragonal and monoclinic) in A(Zr), and Th02 in

A(Th).No new phase was detected in A(Ga)( Gallium can substitute for aluminium to

produce a solid solution in corundum), and obviously in A(Al), too.Thus transformation of doped transition alumina is accompagnied by

precipitation of a new phase, either the oxyde of the doping element or a

mixed oxide of this element and aluminium.Figures 24 a and 24 b respectively show the dependence of the specific

surface area (according to the BET method) on the heating time and thetransformation ratio for pure alumina and magnesium doped alumina. It is

evident that the faster the transformation to alpha A1 203 the faster the

specific surface area decreases. Furthermore, there is a linear relationship

between the surface area and the extent of transformation provided the latteris more than 10 %. The lines for pure and doped alumina exhibit the same slope.

Influence of the content of the doping elementThe influence of the content in doping element was studied in the case of

thorium with different cationic ratios, namely: 0.001 0.005 and 0.01.DTA curves for the three compositions are shown in Fig. 25. Each exhibits a

peak at l388°C which is assigned to the transformation of thoria-doped alumina.

At the lowest content, a second peak appears at l283°C which corresponds to the

transformation for undoped alumina. Therefore, this sample behaves like a

mixture of thoria doped and undoped alumina.XRD analysis after treatment at temperatures between 1283 and l388°C showed

no thorium oxide, unlike the sample treated above l388°C . Thus, only the dopedpart of the alumina is stabilised by solid solution with thoria. As expected

the curves for the weakly doped products are intermediate between that of pure

alumina and that of A(Th).This indicates only a minor influence of the content of doping element at

least above the minimum amount which is necessary to provide an homogeneous

solid solution and to achieve the full effect of stabilisation.

293

\,

-,

0,01

"

I 1 I I1173 1373 1573

r.

I

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A3

Fig. 25 - Effect of the amount of thorium on the D.T.A. curves(cationic ratio)

+

+

a

/+

/+

//+

J- l n ~

d t

5

'428 '403 1378 13437 750

Fig. 26 - Effect of the temperature on the rate of transformation fora) pure aluminab) : Zr doped alumina

294

A similar result was reported by SCHAPER (ref. 15) for influence of

lanthanum oxide on the stability of gamma alumina carriers.

Influence of temperatureThe transformation rate of pure and zirconium doped alumina was studied at

different temperatures (1070, 1105, 1130 and 1155°C).

Transformation rates are plotted versus temperature in ARRHENIUS coordinatesin Fig. 26. Two straight lines appear from which it can be assumed that the

ARRHENIUS LAW is fulfilled and the apparent activation energies calculated from

thei r slopes.

This energy is not influenced by-1KJ mole for A(Al) and 560 ± 30 KJ

with the existing literature, where

reported.

doping, and the values obtained (533 + 30-1 -

mole for A (Zr) are in good agreementresults between 450 and 650 KJ mole- 1 are

Modelling of the transformationSeveral authors have already proposed kinetic laws to describe this

transformation. Different reaction orders (0,1 and 2) have been suggested but

none of these proposals specified the actual nature of the reacting species.

The notion of order of an heterogeneous reaction, relative to the initialproduct is meaningless if this is considered as pure in its phase. TUCKER and

HREN (ref.17) reviewed different attempts to build models of gamma to alpha

transition. Most of them are summarized in table 2

Table 2 Main at temps at modelling the gamma to alpha transformation

Mechanism

Nucleation-growth""

(with necessary previous sintering)

Stacking faults growth(nucleation at surface and neckregion of particles)

Synchro shear (diffusionlesscooperative atom movement)

Sintering/synchro shear

Volume diffusion

Material

gamma from alungamma, thetagamma thin film

" + La203

gammapowder andthin film

Fe and Cr dopedgamma

theta

gamma

Reference

(6)(7)(8)(9)

(4,5)

(3)

(2)

(10)

295

As none of these models takes into account the chemical reacting species,they cannot explain the influence of ions added in the initial network of thealumina.

TUCKER (ref. 17, 22, 24) demonstrated by transmission electron microscopythat the polycrystalline grains of gamma A1 203 are mainly transformed into

alpha A1 203 monocrystals by a nucleation/growth mechanism, located on thesurface of isolated grains in the neck region of sintering particles.

Our proposed model (ref. 26, 27) is based on the spinel structure of the

transition aluminas, as discussed by WELLS (ref. 28) Furthermore, as proposedby SOLED (ref. 29), these aluminas contain hydroxyl (OH-) ions, substitutingfor some oxygen (0 2-) ions.

Alumina does not contain any divalent cations and in order to comply with

the requirement of the spinel structure, we must assume that all the cationicdivalent positions of the network are empty (vacancies).

Taking into account all these facts, the formula for the transition alumina(ref. 26) can be written as :

A1 2 0 03-v/2 (OH)v< > (1-v/2)where the symbol 0 represents a cationic divalent vacancy and < > an oxygen(anionic) vacancy.

Three levels of differences can exist between the different forms oftransition aluminas :

- the distribution of the A1 3+ ions between the tetrahedral and octahedral

positions of the spinel can be more or less complex

the amount of OH- ions, the presence of which can slightly distort thenetwork of oxygen ions, can be more or less great

the OH- ions can be more or less gathered on the surface of the grains.

These differences do not modify the following model :

Considering the case of impurity or dopant-containing transition alumina,the foreign cation identified as MZ+ may be incorporated in the spinel

lattice either by substitution of A13+ in a trivalent site, or by insertion

in a divalent cationic site. Insertion as interstitial cations can be excludedsince the A1 3+ ionic radius is small.It can be speculated that a cation

having its ionic radius similar to that of A13+ would be preferentially

incorporated by substitution, though a larger one would occupy a divalent site,

the size of which is expected to be larger than that of trivalent ones. Let Nbe equal to the ratio of MZ+ ions substituting A13+ to those incorporated inthe vacant divalent sites, and x the ionic fraction of elements M as regards to

the total amount of cations in the alumina.Then, the general formulation of doped transition aluminas similar to the

296

previously determined formula in the case of pure alumina is

A1 2( l-xt\) ~lx 0 (l-2x(l-~» 0 (4-v-v) (Oll\ < > vThe condition of electroneutrality of the crystal leads to the relation

y + x (3N-z) - v / 2

On heating a transition alumina, dehydration occurs which can be written asa quasi chemical reaction between structure elements, i.e. according to theKROGER notation

2(Oll-0)" • H20 + (02-0)" + (V2-0)" (1)

Although occuring in the homogeneous phase, this reaction creates some

oxygen vacancies in the vicinity of the surface. The higher the temperature,

the greater is the concentration of anionic vacancies. These vacancies are

active in the sintering phenomenon and they can react with the intrinsiccationic vacancies (i.e. structurally present in gamma A1 203), leading to

the destruction of the spinel structure and the transformation into thecorundum form, following the reaction :

(V2- ) .. + (V" )'~O (2)o VAs a consequence of the above mentionned scheme, alpha alumina formation

would proceed by a nucleation and growth mechanism.

We will make some basic assumptions in order to facilitate the quantitative

treatment :

- the particles of transition aluminas are spheres of initial radius roo- the transformation proceeds from the surface at the interface of radius r

(the nucleation is supposed to be homogeneous on the particle surface).

The mechanism is based on three steps :

i) anionic vacancies formation by removal of water according to equation (1)ii) cationic vacancies diffusion towards the particle surfaceiii) reaction between the two kinds of vacancies according to equation (2)

which is the rate determining step.According to this model, the transformation rate versus temperature will

follow this equationA = l-(l-kS (l + x(SN-z-2»t)3 (3)o

Where k is the rate constant of reaction (2) supposed to be simple : k

depends only on the temperature (according to the Arrhenius law), and So is theinitial specific surface area of the transition alumina.

This equation is similar to that derived previously by Vereschagin

(ref. 19), but emphasius the major influences of the initial surface area (So),of the dopant content (x) and of temperature (through k). It obviously includes

the case of pure alumina (x = 0).Comparison with experimental derivation of equation (3) provides

297

/dt = (1-.\ )2/3 3k S (l + x(51\-7~2» (4)o

l'ig. 27 shews a good agreement bct.wccn the theoretical and experimental

lues of d /dt versus (transformation rate), provided the svstr-n is in the-rovt.l: phase, i.e. the value of ,\ is greater than that at the inflexion point

in Fig. 23

At a given value of l, relation (4) becomes:

d l/dt = A (1 + x(SN-z-2» "here A is a constant.We can then compare the influence of the different dopants from the values

of :oJ and 7. "hen' :oJ depends on the cation radius and z is the cation charge.Let N 1 for alumini.um ions and N = ° for the largest cation (lanthanum).

Placing different cations in a radius versus charge space produces Fig. 28. The

straight lines are the isospeed curves A (1 + nx) = C "ith n = ~6, -5, -4,

-1,0, +1.

This provides a classification of these cations according to their influence

on the transformation rate predicted by this model.The fit "ith experimental results is good, and some slight discrepancies

(e.g. Mg 2+ ) can be explained by a difference between the actual ionic radius

in the alumina and that reported in the literature. Thus using 0.62 A instead

of 0.65 makes the (SN-z-2) term become positive (0.005 instead of -0.15).This model predicts that the ARRHENIUS law will be obeyed. A well known

relation is Ea = E1 + L Hi, where Ea is the apparent Activation Energy, Elthe actual Activation Energy(i.e. of the rate determining step, reaction (2»

and L Hi the sum of the enthalpies of the reactions preceeding the rate

limiting step. As dehydroxylation is complete before transformation into alpha

A1 203, and diffusion of cationic vacancies is an athermal process at low

concentrations, the sum in L Hi is equal to zero.It then appears that, according to the model, the apparent activation energy

is not modified by doping. This is experimentally verified in Fig. 5 (ref. 31).Furthermore, experiments on transition aluminas of different initial

specific surface areas (So) verified that the transformation rate is

proportional to So for a given dopant.This model is consistent with the variation of the D.T.A. exothermic

temperature since calculations indicate (ref. 26) that(T - To)/T = Bx (z + I - 4N)

where B is a constant and To and T are the peak temperatures for pure alumina

and doped alumina.The results for Zr, Ca, La and Th are plotted in Fig. 29 and compared with

the model prediction straight line (the values of N are taken from Fig. 28).Relatively good agreement is achieved between actual and predicted values.

298

d'J '00 d t

'1g <,OR <, ,

<,AlCJ'l

Go --01. 1n

pure

02

00 6 8 x

Fig. 27 - Transformation rates for doped aluminas

,

,Zr,

,,II '\"'71 ,0

~, '- ',..>

II \~

"i \, \ \, IAl Go \In \ \ La

1\ '. I'I '\ " , j, " 'i " ",- \j1'0 Hg '- '- Co, I,\\~" '. '\ \ .L'.----\-.~T---~.~·· -1---- - N

1 05 0 r (A)

+2

+3

+4

0.50 0.70 0.90 110 130

Fig. 28 - Isospeed curves in a ionic charge-ionic radius diagram.

70

60

50+

+

40

(Z+l-4N)

Fig. 29 - Changes in the relative temperature of transformation

299

300

CONCLLlSIONFrom this study, we have defined a model of the transformation of transition

aluminas into corundum, and the influence of the addition of alien cations intothe alumina on it.

Fig. 28 is efficient in the prediction of the effect of cations, as weverified, a posteriori, for In and La. It explains the influence of dopantnature, temperature and initial specific surface area.

As the transition alumina stabilisation is more effective when the addedcation is larger or more charged, it is recommanded that silicon, lanthanum andzirconium are used.

REFERENCES

K.K.K. Kearby, N.J. Watchung (to Esso Research and Engineering Company)LIS Patent 291 564, December 1966

2 R.H. Whitman, L.L. Lento (to American Cyanamid Company),US Patent 3 907 964, september 1975

3 Norton Company, French patent 324 361, april 19774 S.E. Voltz, S.W. Weller (to Houdry Process Corporation),

US Patent 2 810 698, October 19575 W.R. Grace et CO, French Patent 2 140 575, January 19736 A.L. Hausberger, E.K, Dienes (to United Catalysts Inc),

US Patent 4 153 580, May 19797 M.Michel, R.Poisson (to Rhone Progil), French Patent 2257335, August 19758 Engelhard Minerals et Chemical Corporation,French Patent 2253560,July 19759 J. Burgin (to Shell Development Co), US Patent 2 422 884, June 194710 F. Buonomo, V. Fattore, B. Notari (to Snam Progetti S.P.A.),

French Patent 2 249 852, May 197511 Tokyo Shibaura Denkikk, Japan Patent 58 183948 A, April 198212 P. Nortier, T. Dupin, B.Latourrette (to Rhone Poulenc Specialites Chimiques)

European Patent 85 402353.8 June 198613 V.J. Vereschagin, V. Yu Zelinskii, T.A. Khabas, N.N. Kolova, Zh Prikl KHim.

(Leningrad), 55 (1982) 1946.14 G.C. Bye, G.T. Simpkin, J. Am. Ceram. Soc.57 (1974) 36715 H. Schaper, L.L. Van Reijen, Mat. Sci. Monogr. ,14 (1982) 17316 H. Schaper, E.B.M. Doesburg, L.L. Van Reijen, Appl. Catal. 7 (1983) 211.17 D.S. Tucker, J.J. Hren, Mat. Res. Soc. Symp. Proc. 31 (1984) 33718 M.H. Harmer, E.W. Roberts, R.J. Brook, Trans. J.Brit.Ceram.Soc. 78(1979) 2219 F.W. Dynis, J.W. Halloran, J. Am. Ceram. Soc. 65 (1982) 44220 H. Yanagida, G. Yamaguchi, J. Kubota, J. Cer. Soc. Jap. 74 (1966) 37121 K.J. Morissey, K.K. Czanderna, C.B. Carter, R.P.Merril, J. Amer. ceram.

Soc.(1984) C-8822 D.S. Tucker, J. Amer. Ceram. Soc. 68 (1985) C-16323 D.S. Tucker, E.J. Jenkins, J.J. Hren, J. Electr. Microsc.Tech., 2 (1985) 2924 J.R. Wynnycktj, C.G. Morris, Met. Trans. B, 16 (1985) 34525 J. Berekta, M.J. Ridge, J. chern. Soc. A 12 (1967) 210626 P. Burtin, These de Docteur Ingenieur, Saint-Etienne, November 1985.27 P. Burtin, M. Pijolat, M. Soustelle, J.P. Brunelle, to be published28 A.F. Wells, "Structural inorganic chemistry", Oxford Press London, 196229 S. Soled, J. Catal. 8(1983) 25230 F.A. Kroger, The chemistry of imperfect crystals, North-Holland publishing

Company, London, 197331 P. Burtin, M. Pijolat, M. Soustelle, J.P. Brunelle, to be published

.\. Crucq and A. Frennet (Editors}, Catalysis and Automotive Pollution Control1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

ADVANCES IN AUTOMOTIVE CATALYSTS SUPPORTS

JOHN S. HOWITTTechnical Manager

Corning Glass Works, MP·8·5·1, Corning, NY 14830U.S.A.

Introduction:

1985 is the eleventh year that catalytic converters have appeared on passenger cars as part of emis-sions control systems. Located between the engine and the muffler, the catalyst chemically transformshydrocarbons (HC) and carbon monoxide (CO) with oxygen into carbon dioxide (C02) and water vapor(H20). Since 1981 it also converts nitrous oxides (NOx) into oxygen (02) and nitrogen (N).

The implementation of this exhaust clean up device on automobiles was actually forced by the CleanAir Act amendment of 1970 in the United States. After ten years of usage, nearly 150 million catalyticconverters have been produced. It is broadly accepted as the most practical way for auto makers tocomply with exhaust emissions regulations.

Although the catalyst is the key ingredient, a vital element in converter design is the manner in whichthe catalyst (normally a combination of precious metals) is supported. The substrate must survive andperform in the very hostile environment of an automobile exhaust.

An automotive catalyst support must provide:

• High geometric surface area• Good catalyst adhesion• Low exhaust back pressure• Resistance to high temperatures• Thermal shock resistance• Corrosion resistance• Mechanical strength• Low cost

When converter systems first appeared on vehicles, manufacturers were divided in their approach tocatalyst supports. (FIGURE 1) But one commonality was the use of porous ceramics. One systemutilizes a large number of small, highly porous alumina pellets on which the precious metal is irn-pregnated. These are bulk packed into a compact container which is typically found under the floorboards of the vehicle. (FIGURE 2)

A second catalyst support type is commonly called a monolith which is a thin walled multi channeledhoneycomb. The ceramic walls between the channels are the base support surfaces for the catalyst.(FIGURE 3) Although they are porous, they are not the direct surface for the precious metal. An in-termediate alumina coating called "washcoat" provides an ultra high surface for the catalyst sights(FIGURE 4 is an illustration of the washcoat precious metal relationship). The catalyzed monolith islikewise assembled into the metal container using a compressible interface material.

Both substrate systems have served well, but in recent years emissions standards have become in-creasingly stringent and passenger car vehicle designs have become smaller and lighter. Monolithicconverters are better adapted to these changing requirements and have become the more dominantdesign. At present, they account for nearly 80% of the world's new-car converters.

However, pelleted units have distinct advantages and continue to play an important role.

A third but less developed catalyst substrate offers some unique advantages. It is a high temperaturemetallic alloy normally produced also as a cellular honeycomb form. To date it has not found generalproduction acceptance.

302

FIGURE 1

FIGURE 2

303

FIGURE 3

ONE CHANNEL OfMONOUTHIC SUBSTRATE

ALUMINAWASHCDAT

FIGURE 4

CERAMICSUBSTRATE

:304

Ceramic Monolith

The ceramic monolith is presently the substrate of choice in the world market because its cellulardesign provides the following benefits:

1 A high degree of geometric surface area.2. Fast catalyst light off3. Low exhaust gas back pressure.4. Compatibility with catalysts and coatings.5. Low cost.

Monolithic ceramic substrates have almost entirely been produced from cordierite which is a phase ofthe 2MgO-2AL 203"5S13 system. It has evolved as the industry standard because it combines the re-quired properties, process capabilities and cost for this application.

More specifically, the benefits of this material are:

1. Adaptability to the extrusion process which is uniquely suited for mass production.2. Thermal shock fracture resistance through an inherently low thermal expansion coeficient (8-12 x

10-7°C).3. A melting point of 1460oC.4. Porosity and pore size distribution for catalyst coating application (30-35% open porosity and 4-15

micron median pore size).5. Sufficient crush strength for assembly into the converter containers and to endure the rigours of

automotive use.6. Raw materials which are (a) economical, (b) readily available, (c) have acceptable firing properties.

Ceramic monoliths have been produced in various cell densities and geometries. Table 1 is a com-parison of the geometric properties of those utilized for automotive application. Automotive industrystandard is mostly 400/6 sq. shaped cells and some 236/12 triangular shapes. The initial designs of1975 were largely 200/12 but process developments over that period of time have allowed for increas-ing number of cells per square inch and thinner walls. These bring with the associated benefits of in-creased GSA, lower density and lower back pressure.

Table I

Cells Per Square Inch 200 300 400 236

Cell Shape Square Square Square Triangle

Web Thickness (ins) 0.0105 0.0105 0.0065 0.0115

Hole Size (ins) 0.060 0.0475 0.0435 0.042

GSA (in2/in 3) 48 57 70 56

Density (GMS/ins 3) 7.90 9.60 6.74 9.63

Open Area (%) 73 68 76 62

As the properties of ceramics monoliths are examined, it's important to note that changes or im-provements in melting temperature, thermal shock resistance, strength, back pressure and/or catalystsurface area each have an effect on other properties. The present ceramic monolith design is the resultof a compromise of these interrelated variables.

Product Development Directions

Auto makers believe present day monolith designs can still be improved upon. The directions of pro-duct development effort are:

~,05

Improved Thermal Shock ResistanceEffective Use of Precious MetalHigh Temperature ResistanceDesign FlexibilityReduced Back PressureFast Catalyst LightoffMounting System

Thermal Shock Resistance

A key property of cordierite monolith is the ability to resist fracture from thermally induced stresses.In an automobile exhaust the temperature and rate of flow of the gases changes very rapidly. At enginestart up the monolith is exposed to a very sharp rise in temperature. In most converter designs the gasflow is not evenly distributed across the cross section and, therefore, the honeycomb does not heatuniformly. The flow most often is concentrated in the center. It heats rapidly while the peripheral areastend to remain relatively cool. Radial thermal gradients are created and the problem is accentuated innon round cross sections. Thermal gradients also occur during the warm up period in the longitudinaldirection.

The cordierite composition minimizes thermal fracture from these stresses by virtue of a low coeffi-cient of thermal expansion. Resistance to thermal fracture has been noticeably improved in recentyears largely the result of:

• Low modulus of the cellular cross section.• Better control of coeff of thermal expansion.• Elimination of filleted corners.• More uniformly straight cells.• Control of the washcoat substrate adhesion relationship.• The use of thicker more resilient mat mount.

Effective Use of Precious Metal

Ceramic monoliths have proven themselves effective as substrates for catalyst washcoat andprecious metal because they provide a relatively uniform porous surface. In the catalyst application pro-cess, the amount of alumina washcoat picked up depends upon the total porosity, as well as, the sizedistribution and shape of the pores within the wall. Likewise, the amount of precious metal picked updepends largely upon the amount of porous washcoat on the substrate. Catalyst coaters, therefore,have learned to optimize their process around typical properties of the substrate. However, throughsubtle variances in raw materials and process steps, variances in porosity occur piece to piece and lotto lot.

The precious metal loading of the finished catalyst performs to strictly specified minimum re-quirements. As a result, variances in substrate porosity can be translated through the coating processsteps to a variance in precious metal loading. Consequently, precious metal loading targets are set wellabove the minimums required to compensate for the fluctuation in actual loadings. To demonstrate thepoint (FIGURE 5) shows the relationship between water absorption (an industry standard measure ofsubstrate porosity) vs. washcoat pick up and likewise washcoat pick up to precious metal.

The history of this product has been a continuing effort to control raw material and process to pro-vide a more consistent product, The progress to date in tightening this value has translated directly intoa saving of expensive catalyst.

High Temperature Resistance

The resistance of the ceramic monolith to melting has been the object of research since it was firstdeveloped. The melting temperature of cordierite monolith (14600C) is well in excess of that reached in

:306

(a) (b)

//0 / /0

'" z":,6// /z '" -

/- ~C5 .... '" 00 x/ /~NO

1( .... ....~NOM / /'" ....

//":,6"",~C5 ....J /...J

/'" ", "~ " u" ~a.. ",

NOM NOMW/C LOADING WATER ABSORPTION

FIG.5 - (a) WASH COAT VS P.M. LOADING.(b) WATER ABSORPTION VS WASHCOAT LOADING .

FIG.6

307

normal operating modes in the exhaust of an automobile. However, engine ignition irregularities andother abnormalities in operation occasionally occur, and they produce a fuel rich exhaust. The catalyticreaction is exothermic, and the potential for dramatic increases in catalyst bed temperatures existswhen the exhaust is heavy with unburned combustion products.

The field results over a 10 year period show that monolith melting is not an unknown experience butis not at the level which is considered serious. However, improvement in this property has been thegoal of emission systems designers for the following reasons:

1. To reduce the number of in-use melt incidents.2 Converter applications which are close coupled to exhaust manifolds and, therefore, have a higher

inlet temperature.3. Truck catalyst applications with higher peak operating exhaust temperatures.4 Certain European vehicles with higher speed operating conditions.

Research results have determined some increase in high temperature resistance over a singlephased cordierite by combining it with a refractory phase such as mullite. However, the cordierite por-tion still melts at temperatures approximately 14600C and only a marginal increase is achieved. The ad-dition of substantially quantities of mullite in cordierite degrades the thermal shock resistance.

The problem of developing higher temperature materials without sacrificing thermal shock resistanceis difficult and has lead researchers on another track, that of examining the control of the micro struc-ture, specifically a micro cracked body. Micro cracking is developed in a ceramic by causing a patternof micro stresses to occur after heat treatment. This can be achieved in a number of ways. One is theresult from a two phase system with substantially different expansion coefficients. Alumina titanate isan example of such a material. Its properties are shown in a Table II. The resistance to thermalstresses is achieved by a large number of micro cracks acting as minute stress relievers.

These materials have been tested in laboratories simulating overtemperature conditions and the ex-pected melt resistance has been demonstrated. They are now in the further evaluation stages to deter-mine their adequacy for strength and long term structural integrity.

Phase

Coeff of Ther. Expan.(250C-1000DC) XlO-7

% Open Porosity

Mean Pore Size

Axial Crushing Strength(PSI)

Melting Temp. °C

Table II

Mullite +Cordierite Alum. Titanate

10 21

33 30

4 13

3000 2200

1450 1700

Design Flexibility

The development of new automotive applications and markets has brought interest in extending thephysical size and shape limitations of the extrusion honeycomb process. Previously limited to round,oval and race track configurations, (FIGURE 6) shows irregular cross section capability used to allowthe positioning of the converter in unusual vehicular locations.

308

Back Pressure

Back pressure produced by the presence of a catalytic converter in the vehicle exhaust system is im-portant because it has a direct and negative effect upon engine volumetric efficiency and fuel economyPressure change as a result of a monolithic converter is a function of, among other things, friction flowresistance of the substrate. The frictional loss across the monolith matrix is a fairly complex measure-ment but can be reduced by monolith designs having larger open frontal area and a larger hydraulicdiameter of the cells. Both of these objectives can be accomplished by the use of thinner cell walls.

Faster Lightoff

The Federal test procedure requires that vehicles demonstrate their ability to pass the standards onassimilated driving cycle (CVS-CH) and important part of that cycle is the "cold start" when the engineis started after being at ambient temperature for a number of hours. The precious metal catalysts donot become active until they reach temperatures of 400-500oF. Heat transfer from the incoming gasesis the only energy source. Therefore, a key to effective performance is shortening the cold start intervalbecause at this time the exhaust is particularly rich in pollutants as a result of carburetor choking.

The contribution of the monolith to a shorter lightoff has been the object of much effort. In short, thethermal response of the monolith increases as its web thickness decreases. This is because substrateswith thinner walls allow a faster heating of the front portion and have a lower front to rear thermal gra-dient. Both of these contribute to faster catalyst lightoff.

EX-21

An extension of the present composition is being developed to address the solution to lower backpressure and faster lightoff namely thin walls. Designated as EX-21, it embodies a reduced total porosi-ty of the ceramic body. The total pore volume as measured by the water absorption method shows areduction of approximately 20%. The result is an increase in wall strength as measured by the industrystandard abc axis crush test.

The strategy for employment of EX-21 is to take advantage of that strengthen increase by producingthe appropriately thinner wall cell walls without sacrificing the overall mechanical integrity of theceramic monolith. Table 3 is a comparison of the new material with the existing composition.

Physical Property Comparison

Property

Thermal Expansion (inches/inch/oC X 10'7)

Crush Strength (PSI)A - AxisB - AxisC - Axis

Water Absorption (EM/inches 3)

Softening Point

% Open Porosity

EX-20

76

416377948

1.23

14400C

.34

EX-21

5.8

61381053

53

103

14400C

.28

Monolith Mounting System

Improvements have been made in the support arrangement of the ceramic monolith within the metalcontainer. A felt like blanket capable of withstanding high temperatures is now used to a great extent. Itis composed of ceramic fiber in a vermiculite base. The material is capable of holding the monolithsecurely in place despite substantial differences in thermal expansion of the ceramic and the metalcan. (FIGURE 7)

It also acts as a gas seal to prevent exhaust gases from bypassing the catalyzed monolith.

FIG. 7

809

CONVERTER SHEll

FIG. 8

UTER WRAPFILL PLUG

:310

PELLETED CONVERTERS

Bulk material catalyst (small extrudates and pellets) have been effectively utilized as catalystsubstrates in other applications such as the petroleum industry well before they were adapted toautomotive use. They are especially well suited for a large volume, low speed engines with relativelylow exhaust gas temperatures. This specific pelleted converter advantages are:

1. High geometric surface area.2 High temperature resistance.3. Catalyst replacement ability.4. Thermal shock resistance.

Pellets are spherical or cylindrical in shape and vary between 1/8 to 1/10 of an inch in diameter.

Since 1975, measures have been taken to improve the design and performance of pelleted con-verters. These were in response to tightened emissions standards and to smaller and lighter vehicledesigns.

1. Improvements and performance durability which are largely the result of resistance to poisonshave been achieved through a unique system of positioning various catalyst metal at differentlevels of subsurface.

2. Improvement in catalyst transient warm up performance through the use of smaller, high surfacearea substrates.

3. Both size and weight of the converter has been reduced through the use of lower density palletswhich achieved be an increase in macro porosity.

Unfortunately, certain models of dual bed pelleted converters have encountered a mechanical designproblem after a period of time in customer use which has resulted in loss of power and drive ability ofthe vehicles.

The upstream (three way catalyst) pellet bed has narrow crevasse areas at the junction of the retain-ing screen and container body. Pellets in these areas were being crushed by the relative motioncaused by the expansion of the metal. The crushed fine particles filtered through the retaining screeneventually plugging the top layer of pellets in the second bed. Over a period of time a restriction in ex-haust flow was built up.

Approximately 1.265 million vehicles have been recalled by the manufacturer to correct this problem.

New pelleted converter designs are being produced specifically for truck applications. They aredesigned for lower back pressure with larger plenums. They also incorporate higher temp. steels.

Metal Monolith

Monolithic catalyst supports of metalilic alloys have been under development for a long period.Although they offer a number of potential advantages for various reasons these metallic units have not,to any large extent, succeeded in being applied to production vehicles.

They are basically of the cellular design of ceramic monoliths but the honeycomb is formed normallyby spirally wound alternate sheets of flat and corrugated metal.

They offer a number of important advantages:

1. An ultra thin wall (.04mm) structure for fast catalyst light off and high geometric surface area.2. Mechanical strength and thermal shock resistance.3. Design flexibility (size and shape).4. Potential for simplified canning assembly.5. Low back pressure.

311

The metal alloys for this purpose are available from a number of sources. Considerable research anddevelopment effort has been directed to the problem of application of the catalyst to the metal surface.The major problems to be overcome generally are considered to be those of cost to be competitive withexisting materials, long term adhesion to catalyst washcoat to the metal surface and warping at hightemps.

S~m':Jl.'l __'LThe catalytic converter has been part of the automotive scene in America for nearly 11 years. Its

use, as a result of government regulation, has spread to a number of other countries. Within the nextdecade, as the result of public concern over air quality, the majority of the free world automobiles willbe so equipped. Its design is still evolving as engine technology and vehicle design changes.

The catlyst support continues to demand research and development effort in the areas of (1) preciousmetal conversion through process consistency, (2) resistance to melting through advanced composi-tions, (3) cost reduction through process and improvements, (4) expanded applications into trucks,motorcycles and adaptation to varying emission certification cycles.

References

1. J. R. Adomaites, J. E. Smith, D. E. Achey, "Improved Pelleted Catalyst Substrates for AutomotiveEmissions Control", SAE Paper 800084, February 1980.

2. J. S. Howitt, "Thin Wall Ceramics as Monolithic Catalyst Supports", SAE Paper 800082, February1980.

3. Spheralite Catalyst Carriers, Rhone-Poulenc.4. I. M. Lachman, R. N. McNally, "High Temperature Monolithic Supports for Automobile Exhaust

Catalyst", American Ceramics Society, May 1981.5. V. D. Rao, "High Temperature Substrate and Catalyst System", SAE Paper 850553, February 1985.6. M. P. Walsh, J. S. Howitt, "The United States Experience with Motor Vehicle Air Pollution Control, A

Regulatory Success Story", 8th International Clean Air Conference, Melbourne, Australia, May 1984.7. S.1. Gulati, "Long Term Durability of Ceramic Honeycombs for Automotive Emissions Control",

SAE Paper 850130, February 1985.8. M. Nonnenmann, "Metal Supports for Exhaust Gas Catalysis", SAE Paper 850131, February 1985

A. Crucq and A. Frennet (Editors), Catalysis and Automotive Pollution Control~) 1987 Elsevier Science Publishers B.Y., Amsterdam -" Printed in The Netherlands

STRUCTURAL CONSIDERATION WITH RESPECT TOTHE THERMAL STABILITY OF A NEW PLATINUMSUPPPORTED LANTHANUM-ALUMINA CATALYST

by F. OUDETl, E. BORDESl, P. COURTINEl, G. MAXANT2,C. LAMBERT2, J.P. GUERLET2

1Universite de Technologie de Compiegne, Dept Genie Chimique,B.P. 233, 60206 Compiegne CedexZComptoirLyon-Alemand Louyot, 75139 Paris Cedex 03.

ABSTRACT

The influence of lanthanum aluminate, LaAI03, on the thermal stability of bothalumina and platinum supported alumina catalysts is investigated.In the case of alumina, the stabilization is interpreted in terms of structural coherencebetween &-A120 3 and a three-fold superstructure of LaAI03.The addition of LaAI03, is shown to increase both the dispersion and the resistance tosintering of the platinum supported alumina catalyst. Moreover, lanthanum hexa-aluminate (La-j3-AIz03) is present in the platinum catalyst fired at 1150°C.These observations are assumed to result for the epitaxial relations between platinumand the lanthanum-alumina support.

1. INTRODUCTION

The catalytic layer of monolithic automotive reactors usually consist ofactive metals (Pt, Pd, Rh) supported on alumina. One of the most important problemsset by these catalysts is the decrease in their activity after thermal exposure to theexhaust gas itself (Ref.1). It is well known that this thermal deactivation is directlyrelated to the sintering of the active components. Moreover, this modification of thesupported metal is drastically enhanced by structural changes of the support. Thususing TEM experiments, Chu et al (Ref. 2) have reported rapid sintering of platinumduring the structural transition y-AIZ03 to a-AIZ03'

It has been clearly established that the addition of lanthanum improves thethermal stability of both active alumina and platinum-supported alumina catalysts(Ref. 3-7). However, the actual cause of the lanthanum effect is still not wellunderstood. We have developed a method of preparation and carried out newinvestigations on the thermal stability of these catalysts.Lanthanum is present in two

31:,

:314

different complex oxides with aluminum, i.e. lanthanum aluminate (LaAI03) andlanthanum hexa-aluminate (La-J3-AI203) . The stabilization of the catalyst is discussedin terms of structural coherence between: i) LaAl03 and o-A1203 and ii) the platinumand the lanthanum-alumina support.

2. EXPERIMENTAL

The lanthanum doped alumina support was prepared by addition oflanthanum and aluminum hydroxides to Boehmite. The resulting slurry was thendried and fired at SOO°C in order to form the perovskite-type compound LaAI03. Thecrystallization of this compound occurs simultaneously with the topotacticdehydration of Boehmite to y-A1203 (ref. 8). Pure LaAl203 is prepared by copre-cipitation of La(OHh and Al(OH)3, followed by calcination of the precipitate at600°C. The platinum catalyst was prepared by the wet impregnation technique, usingH2PtCI6, and reduced at SOO°C for two hours in flowing pure hydrogen. The thermalstability of the support was determined by X-ray diffraction (XRD), BET surfacearea measurements and Transmission Electron Microscopy (TEM) (Jeol 1200 ex,120 KV, L=80cm). Platinum dispersion was measured by the pulse chromatographymethod (H2 adsorption at ambiant temperature) (Ref. 9, 10). Laser diffractionpatterns were recorded on Polaroid film directly from the TEM negative film (A =6328 u, L = 3,77 m).

3. RESULTS

3.1. Thermal stability of the supportThe surface area of the La-containing support, measured for different

molar concentrations of LaAI03 in alumina is shown (Fig. 1). An optimumconcentration is observed at 1% molar lanthanum. The crystalline phases detected byXRD are reported in Table I for each concentration. LaAI03 peaks are absent fromthe XRD patterns at concentrations lower than 2%. Nevertheless, according toSchaper et al (Ref.S), it can be assumed that this phase is present in the product.

Examination of the XRD patterns of y-A1203 and 5% La-A1203 both firedin air at 1150°C (12 hours) shows the transformation of pure y-A1203 to corundum.However, in the latter case y-Al203 is transformed to o-Al203 (Ref.11) and wellcrystallized lanthanum aluminate is also detected (Ref.12).

Table 1Surface area of the support and crystalline phases detected

by XRD for different molar concentrations of LaAI03

LaALO S m2g-1 S m2g-1 XRD XRD(mol %) a b a b

° 3 3 a-A 1203 a-A12030.5 56 25 1'}-AI203 a-A1203

1 63 31 &-A1203 1'}-AI2031.5 57 &-A1203 1'}-AI203

2 45 24 &-A1203 1'}-AI203 + LaAI035 29 19 &-A1203 + LaAI03 1'}-AI203 + LaA103

10 19 &-A1203 + LaAI03 1'}-AI203 + LaAI03

aafter firing 12h at 1150°Cbafter firing 1h at 1300°C

Figure 1:Evolution of the surface area of the La-AlzOz support

for different molar concentrations of LaAI03

70

.-.......'Cl

NE.......oCt 40w0:oCtWoLf0:=>(J) 10

:315

o 5LANTHANUM MOL. %

10

3.2. Thermal stability of the platinum catalystTable 2 summarizes the characteristics of the catalysts investigated and

Fig.2 shows the evolution of the dispertion (DIDo) versus time, at 650°C in purenitrogen.

60%98%60%98%

Sample La(mol%) Pt (wt%) Ta0

C1 0 1 600aC

C2 1 0.8 600aC

C3 0 0.98 800aC

C4 1 1.1 800aC

-_..._-.__ .. - -------_.

Table 2Characteristics of the catalysts and their final dispersion

D}}

'ITo is the calcinationtemperature of the supportprior to impregnationbDois the initial dispersion

Figure 2:Evolution of the dispersion DIDo versus time at 6S0aC in pure N2

---------__ C2

LJ--------_--i"] C4

lk:::===-----------b. C 1

C3

After firing in air at 1150°C, La-j3-Alz03 peaks are observed in the XRDpatterns of the catalyst (for lanthanum contents greater than 2%). Therefore, thesupport is a mixture of two complex oxides (LaAI03 and La-p-Alz03) in alumina.

4. DISCUSSION

The experimental results show that the thermal stability of both alumina andplatinum catalysts is improved by the presence of lanthanum. In the case of thesupport, this improvement can be related to the formation of LaAl03 with 8-Alz03.For the platinum catalyst, lanthanum has additional roles: promoting a betterdispersion of the metal and improving its resistance to sintering.

4.1. Stability of the supportCourtine (Ref. 13, 14) developed a model for solid-solid interactions

between oxides, which takes into account the crystallographic relations between thetwo compounds. These relations can be compared with epitaxial interactions (Ref.15).Such considerations imply that the LaAI03/o-AI203 interface is of major interest.Fig.3 shows an electron diffraction patterns of o-Al203.Three reference axis are welldefined: [110], [001], [111], according to previous observations (Ref. 11, 16, 17). Atypical diffraction pattern of LaAI03 is sh<;:wn in Fig. 4 and is indexed in the cubicstructure. The two main axis are [110] and [110]. The exposed plane of this perovskite-tyEe compound is then statistically a (001) plane. Since o-A1203 preferentially exposes(110) planes, it can be assumed to a first approximation, that the LaAI03/o-A1203interface is composed of these planes. In order to examine the structural analogieswhich could exist between the two oxides, pure LaAl03 was impregnated directly ono-A1203 and fired at 800°C. Fig. 5 shows a typical image of the resulting product.

Comparison of Fig. 6 with the diffraction pattern of pure LaAI03 (FigA)shows that the framework obtained could represent a three-fold superstructure oflanthanum aluminate (3X-LaAl03)' The formation of such a superstructure normallyrequires reaction at high temperature because of the high activation energy of these

:317

Figure 3:Electron <tiffraction pattern

of the [110] zone axis of8-A1203

(scale expansion: 1)

Figure 4:Electron diffraction pattern

of the [001] zone axis ofLaAI0:l

(scale expansion: 1)

:U8

Figure 5:a) TEM image of the

La-AI203 supportb) resulting laser diffraction pattern.

Figure 6:Calculated electron diffraction

from Figure 5.

iI._-

/II 220I--I '/020-I / 110- /1 f--.~

T//1

/ I

The comparison with Figure 4 show thatthis structure could be a three-fold

superstructure of LaAI03(scale expansion: 2)

reactions (Ref. 18, 19).In this investigation a superstructure was obtained at quite lowtemperature (800°C). Thus the activation energy has been lowered by a favourablesituation between the two reactive compounds. Such a situation may occur when twosolids are in structural coherence at least at their interface (Ref. 13, 14). Fig.7demonstrates the possibility of a crystallographic fit between 3 X-LaAI03, and&-AI203·

On this simulated electron diffraction pattern, there is indeed quite a lot ofcorresponding vectors from the two reciprocal lattices as in the case of epitaxialinteraction between two compounds (Ref. 20).

These results are in good agreement with those of Schaper et a!. whoinvoked surface diffusion to account for the sintering mechanism of active alumina,the effect of lanthanum being interpreted as a surface interaction (Ref. 5, 6). Ourwork confirms that lanthanum aluminate LaAI03 is the lanthanum active species, andsuggests a model for the thermal stabilization of alumina which takes into account thestructural interface between lanthanum aluminate and &-alumina.

o

Figure 7 :Superposition of the electron

diffraction pattern of o..AI203and 3X-LaAI03'

The good matching betweenthe two patterns shows the

possibility of a crystallographicfit between the two structures.

(for indexation, see Fig. 3 and 4).(scale expansion: 2)

0-

OIilOIjJOIilO·i

. ~ .......I

I 0 I 0

Figure 8:The superposition of the diffraction

patterns of Pt[OlI] zone axisand La-A1203 simulated diffraction

pattern Shows that the relationsbetween platinum and the support

could be of epitaxial type.(scale expansion : 2)

411.

all ~110 0

• • • • •·O~O~O~O 01110 01110'

• • • • •i 1 a

• • • • •OIilO O~O OIilOIilOfjlO·

• • • • •/8'11 0 0

t>lifall

319

4.2. Stability of the platinum catalystThe results presented in Table 2 and Fig. 2 show that the presence of

lanthanum aluminate increases the initial dispersion and the resistance to sintering ofsupported platinum. It thus confirms that an interfacial interaction proceeds betweenlanthanum aluminate and &-AIZ03 .

The formation of La-~-Alz03 besides LaAI03, &-Alz03 and platinum mustbe carefully examined. The classical synthesis of this compound from LaAI03 andAlZ03 requires very high temperature (1650°C) (Ref. 21). The mechanism of thisreaction, suggested by Ropp and al. (Ref. 22) involves electron migration anddiffusion of lanthanum or aluminium ions between the two solid phases. In our case,the presence of free electrons is undoubtedly connected with the presence of platinum.To confirm this assumption, we have used another compound known to provide freeelectrons: thus introduction of graphitized carbon black in the support yieldsidentically La-~-Alz03 after calcination at 1150°C. Moreover, at temperature higherthan 600°C, the carbon black is oxidized to COz. This implies that the nucleation ofthe ~-phase begins at a temperature lower than 600°C but that La-f3-Alz03 is not

320

sufficiently crystallized to be detected in the XRD patterns.On the other hand, the two solid phases LaAI03 and &-A1203 must be in a

highly favourable relative position at their interface in order to allow the diffusion ofLa3+ and A13+ species. This consideration strengthens the hypothesis of coherentstructural interface between the two oxides which is the only method for transfer ofelectron or species between two solid phases at low temperature (Ref. 13, 14).

The easy electron migration from platinum crystallites to the biphasicsupport clearly indicates that a coherent interface between the metal and the substratemust also be consid.:red. Fig.8 shows the superposition of the electron diffractionpattern from the [OIl] zone axis of platinum with a simulated diffraction pattern forthe support. The good matching between several reciprocal vectors of the twostructures indicates that the relationship between the platinum and the support issimilar to that noted in previous observations on the epitaxial relationship betweenPd[OlI] zone axis and y-A1203 [110] zone axis (Ref. 23). According to Dexpert et a!.(Ref. 23), the resistance to sintering can, therefore, be interpreted as the result of thestrength of the epitaxial interactions which is intermediate between chemical bondsand Van der Waals forces.

s.CONCLUSION

The present work has shown that our method of preparation leads to:

i) the thermal stabilisation of the alumina support,ii) a better dispersion of the supported platinum,iii) an improvement of the resistance to sintering of the platinum catalyst.

Structural characterization of the support indicates that the lanthanumactive component is the perovskite-type compound LaAI03. The experimental resultsare interpreted as the formation of a three-fold superstructure of LaAI03 which is ina good interfacial crystallographic fit with &-A1203 and by the epitaxial relationshipbetween the platinum and the support.

The model of coherent interfaces, already suggested for other catalyticsystems (Ref. 13, 14,23) is considered here as a good working assumption and shouldbe confirmed by further investigations.

Acknowledgements: Thanks are due to Dr A. Vejuxfor the TEM experimentsand helpful discussions.

REFERENCES

1. KA. Dalla-Betta, KC. McCune, J.W. Sprys, Ind. Eng. Chern., Prod. Res. Dev.,15-3 (1976) 169-172.

2. Y.F. Chu, E. Ruckenstein, J. Catal, 55 (1978), 281-298.3. S. Matsuda, A. Kato, M. Mizumoto, H. Yamashita, Proc. 8th ICC; Berlin 1984,

pp.879-889.4. YJ. Ying, W.E. Swartz Jr., Spectroc. Lett., 16(6-7) (1984), 331-343.5. H. Schaper, E.B.M. Doesburg, L.L. Van Reijen, Appl. Catal, 7 (1983), 211-220.6. H. Schaper, OJ. Amesz, E.B.M. Doesburg, L.L. Van Reijen, Appl. Catal. 9

(1984),129-132.7. P. Burtin, These 1985, Ecole des Mines de Saint Etienne, France.8. P.Y. Klevstov, L.P. Sheina, Inorg. Mater., 1 (1965),2006-2012.9. J. Freel, J. Catal., 25 (1972),139-148.10. P.A. Compagnon, C. Hoang-Van, SJ. Teichner, Bull. Soc. Fr. Chirnie, 11

(1974),2311-2316.11. B.C. Lippens, Thesis 1961, Delft, The Netherlands.12. S. Geller, V.B. Bala, ActaCryst., 9 (1956),1019-1025.13. A. Vejux, E. Bordes, P. Courtine, IX Eur. Chern. of Interface Conf. Zakopane,

Poland, May 19-25,1986.14. P. Courtine, ACS Symp. Series, 279, R.K. Grasselli, J.F. Bradzil Eds. (1985),

pp.37-56.15. J.W. Matthews, in Epitaxial Growth, Academic Press, 1975.16. J.P. Beaufils, Y. Barbaux, J. Chim. Phys., 78 (1981), 347-352.17. H. Dexpert, J.F. Larue, I. Mutin, B. Moraweck, Y. Bertaud, A. Renouprez,

J. Metals, Nov. 1985, 17-21.18. M.A. Alario-Franco, MJ. Rodriguez-Henche, 3rd Eur. Conf. Sol. State Chern.,

May 29-31,1986, Regensburg, Vol. 1, pp. 201-202.19. M. Vallet-Regi, J.M. Alonso, J.M. Gonzalez-Calbet, 3rd Eur. Conf. Sol. State

Chern., May 29-31, 1986, Regensburg, Vol. 3, pp. 499-500.20. R. Bonnet, Mat. Res. Bull., 7 (1972), 1045.21. R.C. Ropp, G.c. Libowitz, J. Am. Ceram. Soc., 61 (11-12) (1978), 473-475.22. KC. Ropp, B. Carroll, J. Am. Ceram. Soc., 63(7-8), (1980),416-419.23. H. Dexpert, E. Freund, E. Lesage, J.P. Lynch, in B. Imelik et al. (Eds), Metal-

Support and Metal-Additive Effects in Catalysis, Elsevier (1982), pp. 53-61.

321

Rhone Poulenc Recherches, 14 Hue des

A. Crucq and A. Frennet (Editors), Catalysis and Automotive Pollution Control© 1987 Elsevier Science Publishers B.V .. Amsterdam - Printed in The Netherlands

INFLGENCE OF THE POROUS STRUCTURE OF ALUMINA PELLETS AND THE INTERNAL

CONVECTIVE FLOW ON THE EFFECTIVE DIFFUSIVITY OF EXHAUST GAS CATALYST

S. CHENG 1, A. ZOULALIAN l and J.P. BRUNELLE2

,"Department of Chemical Engineering - Uni versi ty of Technology, BP 233,

60206 Compiegne Cedex (France)

2Hesearch Center of Aubervi lliers

Gardinoux, 93308 Aubervilliers Cedex (France)

ABSTRACT

To analyse the performance of exhaust gas catalyst (actiVity and poiso-ning) the effective diffusivi ty of six Rhone Poulenc alumina supports ismeasured by a physical dynamic method in a single pellet string reactor.

Informa tions on the porous structure of supports give an idea of theeffective diffusivity values, but specifie neither their absolute valuesnor the direction of their variations. Moreover, the experiments demonstratethe necessity of taking the measurements in an external fluid flow so asto determine the influence of internal convective flux on the value of effec-tive diffusivity.

INTRODUCTION

One of the causes of deactivation of the exhaust gas catalyst, in both

pellet and monolith form, is clogging by lead, phosphorus and zinc traces

contained in exhaust gases.

The impurities stick to the periphery of particle pores making the gas

323

flow into the catalyst difficult or impossible. This in turn leads to a

considerable increase in the diffusion resistances during the catalytic

process. One way of fighting this phenomenon is to use double-porosity

alumina. Micropores of about 20 nm are always useful to develop the specific

surface area necessary for a good dispersion and stability of the catalytic

phase. Macropores over 100 nm in diameter help to diffuse the reagents within

the particles. However, the proportion of macropores must not be too great,

as that would diminish the mechanical properties of the support correspon-

dingly. For this reason, Rhone Poulenc has, since 1974, developed and mar-

keted various eXhaust. gas catalyst supports with specific surface areas of

around 100 m2/g whose porosity and porous distribution are extremely variable.

To analyse the performance of a catalyst during a chemical transformation,

324

it is absolutely necessary to know the e f'f e c t i ve diffusivities of the rea-

gents in the catalytic support system. Determining these values and observing

their evolutions as a funcion of certain operating parameters often reflect

their qualities in post combustion reactions. As a matter of fact, merely

determining pore diameter distribution (using, for example, a mercury poro-

simeter) proves to be insufficient to calculate this parameter a priori

(ref. 1).

In this study, we shall deal with the values of e f f e c t i ve d i f fu s iv i ty

of six different transition alumina supports of about 100 m2/g,

in the form

of spheres with an average diameter or 3,2 x 10-3 m. The efrective dirfusi-

vities are found by using a physical dynamic method. A stimulus of tracer

is introduced in the carrier gas crossing with a constant flow rate an open

system containing the alumina particles. The concentration or tracer is

registred at the entrance and at the exit of the system. From these experi-

mental inrormations, an experimental transfer runction is obtained. The

theoretical transfer function can be derived by modeling the system and the

effective d i f f'us i vi ty is one of the parameters or this transfer function.

Identification of experimental and theoretical transfer functions makes it

possible to estimate the effective diffusivity.

EQUIPMENT AND EXPERIMENTAL PROCEDURE

The schematic diagram of the overall experimental set up is given in

Fig. 1.

A fixed bed reactor of small diameter and large length (internal diame-

ter ; 8,5 x 10-3m; total length; 1,30 m) is filled with alumina particles.

A 6-way chromatographic valve is used to mix a tracer gas with the carrier

(nitrogen) before introduction into the fixed bed. The concentration of the

tracer in the carrier is measured at both extremities of the study zone

(length 1 m) wi th a mass spectrometer (Micromass MM 601). The "response"

curves are recorded and stored in a flexy disk of an Apple II microcomputer.

For each flowrate of the carrier gas, at least 10 pairs of response curves

entrance/exit are stored. From these curves, an average entrance (or exit)

curve is defined. Only the curves whose area and first moment do not deviate

from the mean values by more than ± 5% are considered (generally, all the

curves are retained in calculating the average curves).

32.5

8

REACTOR

7

MASS

SPECTROMETER

1

MOLECULARSIEVE

TRACER GAS

CARRIER GAS

ELECTROVALVE

APPLE II

-Legend: I : spherical float flow meter, 2 : needle valve,valv~,4 : sample loop, 5 : gas mixer, 6 : port selector, 78 : exit sampling, 9 : gas meterFig. I . Diagram of the experimental equipmenT.

3 : chromatographicentrance sampling,

Let x(t) and y(t) be the average curves at the entrance and the exit

of the study zone. The experimental transfer function can be expressed by

GE(s) ~ y(s) / XIs)

with

rooxIs) ( 0 x(t) exp(-st/\ll)dt) / Sx

Y(s) ( J:OOy(t) exp(-st/\ll)dt) / Sy

(1)

(2)

(3)

In relations (2) and (3), Sand S represent the respective area un-x yder the average entrance and exit curves, that is :

]:= x(t)dt

(+= Y(t) dtJ0

WI is the rirst moment or the study zone, that is

(I))

(5)

WIJ+OOO ty(t)dt

sx(6 )

The experimental transfer function GE(s) is calculated ror twenty real

values or the reduced Laplace parameter situated between 0 and 4.

If, however, the alumina particles are represented by the pseudohomoge-

neous model, assuming slab geometry, neglecting the axial dispersion and

the f i Irn mass t r-ans rer- resistance (it has been demonstrated (r-e f , 2) that

the e f f'e c t.s or these two variables are negligible in comparison with the

errect or the internal dirfusion resistance), the theoretical transrer runc-

tion ror the study zone is given by

exp [ - osy

x (7)

where 0 is the external porosity or the bed, B is the porosity or the alumina

support, and y is the overall porosity: y 0 + (I-dB. CD designates the

internal d i f'Fue i cn time, which is related to the ef'f'ec t i ve d.i f'f'us i vi ty De

by the expression

B d 2 ! 36 Dep

(8)

Lden t.i f i c a t i on of the theoretical and experimental transfer functions

in order to estimate the effective diffusivity De is obtained by minimizing

a relative error function taken between the two transfer functions. The

Rosenbrock method of optimization has been used. All the measurements have

been made at room temperature and something close to normal atmospheric

pressure. The only parameter that changes is the carrier gas flow rate.

EXPERIMENTAL RESULTS

The main characteristics of the six Rhone Poulenc alumina supports are

given in Table 1. In Figure 2, are presented the porosity distribution obtai-

ned with a mercury porosimeter at the moment the pressure was increased (the

mercury penetration curve).

TABLE 1

Characteristics of the Rh6ne Poulenc support systems and of the fixed bed

Apparent volunic I Particle Fixed bedReference mass of the support Porous volume (em3/g) porosity porosity

(kglm3)total I > IIJlTl[> o,llJlTlf !J E

A 1 62JJ 1,160 0,195 0,395 0,788 O,S05A 2 920 0,790 O,OSO 0,100 0,727 0,538A 3 1170 0,5SO 0,010 0,045 0,644 0,528A4 810 0,800 0,060 O,lSO 0,713 0,519A 5 795 0,960

I0,320 0,410 0,763 0,531

A 6 673 1,190 0,100 0,420 0,801 0,524

327

<110,8E:Jo>04,'":JoLoQ. 100

pore1000

diameter (nrn)

Fig. 2. Porosity distribution of the different supports.

For the various carrier gas flow rates, the experimentally obtained

values of the effective diffusivi ty are given in Table 2 and represented

graphically in Figure 3. Table 2 contains also the values of the effective

diffusivity that were theoretically deduced from the pore diameter distribu-

tion by applying either the Johnson-Stewart model (ref. 3) or the Wakao-Smith

model (ref. 4).

328

TABLE 2Effective diffusivities of the different supports

Reynolds Effective Mean effective Effective Effectivenunber diffusivity slab diffusivity diffusivity diffusivity

Reference pud pseudcharogeneous Johnson~tewart Waka~mith

Re = __.2. rrode1 model rrode1p p

De x 106 (m2/s) - 6 2 De x 106 (m2/s)De x 106 (l/s)De x 10 (rn Is)

20 0,928A 1 35 1,242 1,202 7,86 12,01

CO 1,437

20 0,655A 2 35 o.rss 0,781 5,58 6,16

CO 0,891

20 0,497A 3 35 0,622 0,604 3,32 5,59

CO 0,692

20 0,655A4 35 0,855 0,811 5,27 6,28

CO 0,923

20 0,817A 5 35 1,187 1,101 9,81 9,17

CO r.zss20 0,673

A 6 35 0,923 0,880 9,33 12,9CO 1,043

~ 1.6r------------------------,N'E-00 1,2,

XQIa

0,8

20 30 40

Fig. 3. Effective diffusivities of the different supports.

:J29

For the Johnson-Stewart rnodc l , the effecti vc diffusi vi ty is evaluated

with the relation

of the nitrogen-helium binary system (in

Jl j'~ro fir)drDe 1 11

PDAB DK(r)

DAB j s the molecular diffusivity-6

rn2 / s ,the c3se, 68 x 10 but, in general,

(9 )

D can be evaluated hyI"1B

the kinetic theory of gases, with the relation

3/2 J 10,001858 T -----, r"B

2P (JAB (JAB

(10 )

(11 )Is)

OK is the Knudsen diffusivity of helium in the cylindrical pore of radius r,

DK(r) is given by the relation

9700 r j T["A

f(r)dr is the fraction of internal porous volume of the cylindrical pores

incompassed in the interval from r to r+dr.

In numerical calculations, the tortuosity factor T p has bcen taken a';

3Jl, the value recommfnded by the authors for isotropic porous media.

The Wakao-Smi th model has been found appr-opri a t e- for bidisperse porous

support where the effective diffusi vi ty can be predicted from the porous

structure of the particles. According to this model, the effective diffusi-

vity can be evaluated using the relation:

13 2 ? 1 1 -1De D + ( 1 - 13 i D. + 4 13 (l Ga)(-D- + --u:-) (12 )a a a 1 a a 1

13.where D (_1_ _1_)-1 and D. (_1_ + _1_)-1(__"_)2 (13)

a DAB D 1 DAB DKi 1 - GaKa

DKa and DKi are the respective Knudsen diffusivi ties of the macropores and

the micropores.

Ga and Gi are the respective internal porosities of the macropores and the

micropores

DISCUSSION

In many problems of mass transfer in a solid porous medium with a large

specific surface area (as with catalysts), with or without a chemical reac-

tion, the solutes are considered to be carried only by diffusion (molecular,

superficial or Knudsen diffusion), the molecular barycentric velocity being

330

nul. Therefore, the parameter that expresses the diffusive transport (effec-

tive diffusivity) must be independent of the flow rate of the external fluid.

Our experimental results, however, show a clear increase in effective diffu-

si vi ty as a function of the carrier gas flow, which cannot be attributed

to experimental measurement error alone. It is also worthy of noting that the

effect of the flow grows greater as the relative volume of macropores increa-

ses. This evolution can only be explained by adding an internal convection

flux to the diffusion flux. The hypothesis was first put forward by Nir and

Pismen (ref. 5), who defined the effects of internal convection on a first

order reaction (ref. 5) and on the selectivi ty of a concurrent-consecuti ve

reaction (ref. 6). The internal convection flux can be described by an

internal velocity evaluated by Rodrigues and al. (ref. 7) using the pressure

drop at the extremities of the particles and their permeability coefficient.

If the phenomenon of internal convection flow is ignored, the effective

diffusivity ("apparent" effective diffusivity) increases along with external

fluid flow, and even more so as the permeability coefficient grows larger.

I t is thus observed that in support systems such as AI, A 5 and A 6, the

increase in the "apparent" effective diffusi vi ty is greater than in supports

A 2, A 3 and A 4.

In principle, the "true" effective diffusi vi ty should be calculable for

the preceding measurements when the external carrier gas flow is very low

or zero. In fact, because of the limitations of the experimental measure-

ments, the range of the flow rates studied is insufficient to achieve an

unambiguous result for the "true" effective diffusivi ty, or for the permea-

bility coefficient, which is closely linked to it. It must be conclued, then,

that it will be exceedingly difficult to use the method chosen to measure

the "true" effective diffusivi ty of a porous particle wi th macropores. The

"apparent" effective diffusivity found, however, is surely the best, since,

in actual practice, the particles are to function essentially with an exter-

nal flow (fixed bed, fluidized bed, ... ).

When the experimental values obtained for the effective diffusivity are

compared with the theoretical values deduced from the Johnson-Stewart and

Wakao-Smith models, two points stand out:

The experimental values of the effective diffusivi ties are clearly

lower than the values deduced from the theoretical models, even taking into

consideration the internal convective flow. Of course, the experimental va-

lues depend on the pseudohomogeneous model chosen to represent the alumina

particle, but even if the spherical model were used, the values obtained

(1,8 times those obtained with the slab model by identification of the

variance) would be less than the theoretical values. Thus, the theoretical

models based on the porous structure of the particles cannot be used for

331

an a priori calculation of the effective diffusivi ty of a particle placed

in a flow.

- The variations in the effective diffusivities as a function of the

porous structure of the supports coincide neither for the theoretical values

nor for the experimental values. As the previous discussion has already shown,

the effective diffusivity cannot be estimated from the pore diameter distri-

bution. The arrangement of micropores and macropores must be known. While

it is true that stochastic model of porous structure such as those put

forward by Mann and Golshan (ref. 8) might allow the arrangement of the

different pores to be represented, the resolution of these models in a chemi-

cal reaction appears too complicated.

At the present time, in our opinion, only direct measurements of the

effective diffusivity in a device with a flowing external fluid can diffe-

rentiate porous supports the porous structure of which is known. Thus,

among the six alumina supports, A 1 must be used when the greatest diffusi-

vity is required, in preference to A 6, the theoretical values of which are

nevertheless greater. The difference might be due to the degree of homogenei-

ty of the macropore and micropore distribution inside the beads of the two

alumina supports. In the theoretical model, the porosi ty distribution was

considered to be homogeneous wi thin each alumina bead. If the A 6 support

has more micropores on the periphery of the beads than the mean porosi ty

distribution indicates, it would not be surprising to find that it has less

effective diffusivity than the A 1 support.

CONCLUSION

The whole set of measurements carried out has shown that using a mercury

penetration curve to get information on the porous structure of a support

gives an idea of the probable effective diffusivity, but specifies neither

its absolute value nor the direction of its variation. Moreover, our expe-

riments have demonstrated the necessity of taking the measurements of

effective diffusivi ty in a device with a flowing external fluid so as to

determine the influence of the internal convection flux on the value of the

effective diffusivity.

REFERENCES

1 G. Antonini, A.E. Rodrigues and A. Zoulalian, International ChemicalReaction Engineering Conference, Pune, 1984

2 S. Cheng, A.E. Rodrigues and A. Zoulalian, Proceedings of the IX IberoAmerica Symposium, 1984, pp. 301-309

3 M.F.L. Johnson and W.E. Stewart, Journal of Catalysis, 4, 1965, pp. 248-252

332

4 N. Wakao and J.M. Smith, Chern. Eng. Sci., 17, 1962, pp. 825-8345 A. Nir and L.M. Pismen, Chern. Eng. Sci., 32, 1977, pp. 35-416 A. Nir, Chern. Eng. Sci., 32, 1977, pp. 925-9307 A.E. Rodrigues, B.J. Ahn and A. Zou1a1ian, A.l.Ch.E. Journal, 28, 1982,

pp. 541-5468 R. Mann and H. Golshan, Chern. Eng. Comm., 12, 1981, pp. 377-391

A. Crucq and A. Frennet (Editors), Catalysis and Automotive Pollution Control© 1987 Elsevier Science Publishers B.Y., Amsterdam - Printed in The Netherlands

333

THE EFFECT OF THE CHEMICAL NATURE OF THE WASH-COAT ON THE CAT AL YTlCPERFORMANCE OF CO OXIDA TlON CA TAL YSTS OF MONOLITH TYPE.

Lennart B. Larsson, Lars O. Lowendahl and Jan-Erik OtterstedtDepartment of Chemical Engineering 1, Chalmers University of Technology, 5-412 96Gothenburg (Sweden)

ABSTRACTLight off temperatures and efficiency for Pt on wash-coats of alumina, silica,

aluminosilicate, posivitely charged silica and positively charged aluminosilicate weremeasured. 0.1 wt% Pt, based on the weight of the wash-coat, which is only 10% of theusual amount of Pt in commercial catalysts, on silica and alumina showed as low light offtemperatures and as high efficiency as commercial catalysts.

INTRODUCTION

In the US and Japan automobile exhaust catalysts containing the noble metals platinum,

palladium and rhodium are being used for the control of carbon monoxide, hydrocarbons,

and nitrogen oxides in order to satisfy regulatory emission control requirements and such

catalysts will be introduced in Europe in the near future.

The concentrations of hydrocarbons, CO and NOx can be reduced to the desired level in

a single catalyst unit, a so called three way catalyst, operated in a narrow range around

the stochiometric air/fuel ratio (ref. 1).

A typical three way catalyst consists of a honeycomb monolith structure of a ceramic

material such as cordierite, A14Mg2Si5018 (ref. 2). The ceramic surface is provided with a

layer of high surface area alumina as a washcoat which then will act as a substrate for the

active ingredients. The thickness of the washcoat is usually not uniform but varies in the

range 10-150 Jim (ref. 3). The washcoat may amount to 5-15 wt% of the monolith, and

may provide 15-30 m2/g of surface area (ref. 4).

The converter typically contains 0.15-0.30 g rhodium, which reduces NOx to Nb and 1-2

g platinum, which oxidizes CO and hydrocarbons to C02 (ref. 5). Palladium is sometimes

used in combination with platinum as oxidation catalyst but possible detrimental

interactions between Pd and Pt or Rh when they are used together have been reported(ref. 6).

The reactions taking place on the three way catalyst, that is oxidation of CO and

hydrocarbons to C02 and water and reduction of NOx to N2 interfere with each other

(refs. 5,7). Under reaction conditions strongly chemisorbed CO thus inhibits the oxidation

reactions and chemisorption of NOx also negatively affects the rates of these reactions(ref. 8).

In this investigation the effects of the chemical nature of the substrate and the method

of depositing platinum on the substrate on the efficiency of Pt as an oxidation catalyst

were studied. In subsequent studies the effects of substrate and deposition method

on the efficiency of rhodium as a reduction catalyst and of a complete three way

catalyst will be investigated.

EXPERIMENT AL

1. Materials

Ludox TM: 22 nm silica sol containing 49.5% Si02 from du Pont.

Ludox SM: 7 nrn silica sol containing 30% Si02 from du Pont.

Disperal: Dispersible powder of boehmite (AIO(OH» from Condea Chemie GmbH,

Brunsbuettel, West Germany.

Hydrazine hydrate: N2H50H, 100% "zur Synthese" from Merck, Schuchardt,

Hohenbrunn, West Germany.

Ammonia solution: 25% aqueous NH3, AnalaR, from BDH Chemicals Ltd., Poole, England.

Sodium aluminate: NaAI02 powder technical grade from Kebo Lab AB, Gothenburg,Sweden.

Hydrochloric acid: 37% aqueous HCl"pro analysi" from Merck, Darmstadt, West Germany.

Chlorhydrol Micro-Dry: Aluminum chlorohydrate, from Reheis Chemical Co., New Jersey,

USA.

Calcium chloride: CaCI2'2H20, "pro analysi", from Merck, Darmstadt, West Germany.

Chloroplatinic acid: Prepared by dissolving platinum metal in aqua regia (ref. 9).

Monolith: Honey comb structure of cordierite containing 64' square channels per square

centimeter from Corning Glass GmbH, Wiesbaden-Biebrich, West Germany.

Commercial catalyst: Honey comb structure made by Degussa AG., West Germany, and

obtained from Volvo AB, Gothenburg, Sweden.

Ion exchange resins: Dowex 50 W-X8 from Dow Chemical Co., Midland, Michigan, USA

and Amberlite IRC-50 from BDH Chemicals Ltd., Poole, England.

2. Preparation of colloidal particles

Ludox TM and SM solutions of colloidal silica were decationized using a strong acid

resin, Dowex 50W, in order to reduce the sodium content before they were used as

starting materials for making other colloidal particles.

The modification of the surface of silica sol particles by reacting with aluminum to

form strongly acidic aluminosilicate sites have been described by Alexander (ref. 10) and

Her (ref. 11). The surface of colloidal silica contains 8 Si atoms nm-2 (ref. 12). About 2 of

the Si atoms can be replaced by Al atoms to form negatively charged aluminosilicate

sites, corresponding to a surface coverage of 25% Al atoms. Aluminum in the form of

freshly prepared Na-aluminate solutions and in an amount corresponding to the desired

surface coverage was added in a fine stream to the vortex of a vigorously stirred and

decationized sol of Ludox TM at 250C. The mixture was centrifuged for I hr at 3000 rpm

and the supernatant liquid was aged for 25 hrs at 95 0C and again centrifuged for I hr at

335

3000 rpm. During the two centrifuging steps a small amount of solids, corresponding to

less than 196 of the solids content of the solutions, settled to the bottom of the tube,

whereas Ludox TM centrifuged at the same conditions did not settle, indicating that a

small amount of the sol coagulated during the formation of the aluminosilicate particles.

In this work aluminosilicate sols with a surface coverage of 19.3 96 Al atoms were

prepared.

The preparation of positively charged silica sols by treating them with basic aluminum

chloride, Chlorhydrol, has been described by Alexander (ref. 13). Basic aluminum chloride

consists of extremely small positively charged particles, about 1 nm, with the composition

[A113 04(OH)24(H20)1~ 7+ (ref 14). Assuming that the particles have the shape of

hexagonal prisms with I Al atom at each corner and I Al atom at the center of the prism,

4.5 g Chlorhydrol Micro Dry (containing 46.8 wt 96 Al203) per 25 gram Ludox TM or

aluminosilicate-modified Ludox TM particles will correspond to a I: I ratio of 5i-surface

atoms to Al atoms from ChlorhydroJ. 2396 by weight solution of Ludox T M or

aluminosilicate modified Ludox TM (j 9.396 surface coverage by AI) were run into the

vortex of vigorously stirred solutions of Chlorhydrol, containing 3.0 wt96 A1203, at a rate

of 0.13 g Ludox TM particles per minute. The mixtures were centrifuged for I hr at 4000

rpm and the supernatant liquid contained non-coagulated, non-associated positively

charged particles of silica (+TM), or of aluminosilicate modified silica (+AL5I). About 296

by weight of +TM and about 896 by weight of +AL51 sedimented during centrifugation. In

the case of +AL51, 8596 of the aluminum (AI) from the Chlorhydrol was adsorbed on the

surface of the AL51 particles.

Colloidal solutions of alumina were prepared by adding 300 g Disperal powder to a

solution of 9.5 g 3796 HCl in 690 g H20 under vigorous stirring. The alumina slurry was

stirred for 10 minutes and centrifuged for 1 hr at 2500 rpm. The supernatant liquid,

containing about 30 nm aggregates of about 4 nm primary particles of boehmite, was used

for catalyst preparation.

3. Coating of colloidal particles with Pt

Colloidal particles were coated with Pt by reducing Pt4+ with hydrazine in the same

solution as the colloidal particles. Excess of hydrazine hydrate was added to vigorously

stirred solutions of Ludox TM or aluminosilicate modified TM, containing about 2096 by

weight of 5i02' By using a metering pump a solution of chloroplatinic acid was slowlyadded to the sol solution; typical addition rates were 2'10-5-5'10-6 g Pt min-I. The pH

was maintained at 8.5 for TM and 9.5 for AL51 by adding NH3 solution (2M). The

concentration of the H2PtCI6 solution was adjusted so that the Pt-coated sol contained

14-15 wt 96 5i02' After completed addition of H2PtCl6 the solutions were centrifuged for I

hr at 4000 rpm and the supernatant liquid was used within 18 hrs in catalyst preparation.

Colloidal particles of alumina were coated with Pt in a similar manner except that the

pH was maintained at 4 by adding solutions of HCI (2M) and NH3 (2M).

336

4. Catalyst preparation

Catalyst preparation consisted of the following steps:

a. Preparation of monolith

b. Deposition of wash-coat on monolith

c. Deposition of Pt on the surface of the wash-coat

a. Samples of monolith (length 15 mrn) with a square cross-section containing 81 square

channels were cut from a commercial honeycomb structure of cordierite. The corners

were trimmed off, resulting in a cross-section with 69 channels. In order to ensure that Pt

was deposited on the surface of the wash-coat only, when Pt was applied by direct

impregnation, the coarse porosity of the cordierite samples was eliminated by repeatedly

impregnating them with Ludox SM, containing 30% Si0z, for a total uptake of 17-19%

Si02' Excess Ludox SM was drained from the samples and they were dried at 1l00C for

hr after each impregnation. After the final impregnation the samples were first calcined

for I hr at 1050 0C and then at 550 0C in 100% steam for 3 hrs in order to sinter the 7 nm

Ludox SM particles to density.

b. Wash-coat was deposited on samples of monolith from a. above by repeatedly

immersing them in colloidal solutions containing about 14% by weight of Si02 or A1203'The immersion time in Ludox TM and aluminosilicate modified TM was 120 seconds

whereas it was only I second for solutions of colloidal alumina or positively charged TM

and aluminosilicate modified TM in order to prevent dissolution of already deposited

alumina. Excess colloidal solution was drained from the samples and they were dried at

1200C for I hr. Samples with wash-coats of alumina or positively charged

TM/ alurninosilicate modified TM were heated at 550 0C for 3 hrs after the final

application of wash-coat. In this manner the wash-coat was built up layer by layer to give

a final surface area in the range of 16-26 m2 per gram of monolith + wash-coat.

Depending on the colloidal solution and the preparation of the monolith, it required from 5

to 20 applications to obtain the desired surface area. This corresponds depending on the

sample, to a washcoat weight of 8-25 wt% of the total weight.

c. Deposition of Pt on the surface of the wash-coat was done by I) using Pt-

coated colloidal particles to build up the wash-coat in b. above, 2) directly

impregnating the wash-coat with a solution of chloroplatinic acid and driving off

the solvent (water), or 3) using an adsorption procedure. In the second method the

pore volume of the wash-coat and the volume of the channels were filled by

immersing the wash-coated samples in solutions of chloroplatinic acid. The samples

were dried at 80°C for 4 hrs, In this method monolith samples prepared as in a.

above were used. The method of depositing Pt by adsorption has been described by

van den Berg et al (ref. 15) and can be applied to positively charged surfaces; i.e,

wash-coats of alumina and positively charged TM/aluminosilicate modified TM. In

this method solutions of chloroplatinic acid were circulated through the monolith

:3.37

channels and PtC162- ions were adsorbed on the positively charged wash-coat

surface. However, the adsorption procedure could only be successfully applied to

wash-coats of alumina.

5. Catalyst testing

The apparatus for catalyst testing used in this investigation has been described by

Gandhi et al (ref. 16). The reactor consisted of a vertical stainless steel tube, 900 mm

long and with an inner diameter of 16 mm encased in a tubular furnace. The catalyst was

sealed in the middle of the heated zone with quartz wool. A downflow of the reactant gas

mixture in N2 as carrier gas was led through the reactor and the gas temperature was

measured with a movable vertical thermocouple at the inlet of the catalyst. Reactant and

product gases were analyzed on line using a Beckman OM-14 02 analyzer and two Maihak

Unor 6N lR analyzers for CO and C02'Catalysts containing PtC162- were first oxidized in an air flow of 500 cm 3 min- l at

500 0C for 40 minutes and then reduced in a hydrogen flow of 200 cm 3 rrurr ! at 450 0C for

120 minutes. Prior to testing the catalysts were exposed to a gas flow with a space

velocity of 49000 h- l and containing 3.4 % 02 and 0.6 % by volume of CO in N2 (thecomposition of the reactant gas mixture) at 400 0C for 2 hours.

In order to determine the light off temperature the temperature of the catalyst was

raised from J500C at a rate of 40C per minute and the increase in CO conversion was

recorded. T50 in Table 1 is defined as the temperature at which the CO conversion is 50%.The efficiency of the catalysts was determined by measuring the CO conversion of the

reactant gas mixture at space velocities 196000,245000,291000,317000 and 336000 h- l at

400 0C and at 500 0C for catalysts with low and high light off temperatures respectively.

RESULTS AND DISCUSSION

The platinum content, BET specific surface area, light off temperature (T50), type of sol

and deposition method of Pt for the catalyst samples studied in this investigation are shown

in Table 1.The Pt contents faU in three groups: a high content in the range 0.4-0.6, a medium

content in the range 0.1-0.2, and a low content in the range 0.02-0.05 mg Pt per gram of

catalyst sample. The commercial sample (monolith 26) contained 1.77 mg Pt per gram of

catalyst.

The surface area, measured by a Digisorb 2600 from Micromeritics, varied in the range

16-31 m2g- 1.This corresponds to approximately 100, 20 and 5 Pt atoms per 1000 nm 2 for

the three ranges of Pt contents respectively and to 200 Pt atoms per 1000 nm 2 the

commercial sample.

ALSl, +TM, and +ALSl in column 5 stands for aluminosilicate modified TM, positively

charged TM and positively charged aluminosilicate modified TM respectively. The numbers

338

TABLE 1Pt content, BET surface area and light off temperature (T50) for catalysts with different

wash-coats and deposition methods of Pt.

Monolith

No.

Pt content BET area

°C

Type of

sol

Depositionof PtMethod No.a

[- --------6.44-- - -- - - --i6".5---------itl6----fM ------1-------------2 0.40 22.3 275 TM 13 0.56 19.6 253 TM 24 0.63 16.2 253 Disperal 25 0.53 19.0 257 ALSI 26 0.48 18.7 256 +TM 27 0.53 18.5 272 +ALSI 28 0.61 16.8 255 Disperal 39 0.093 19.3 290 ALSI 110 0.16 21.9 289 TM 111 0.097 18.2 308 TM 212 0.13 16.2 304 Disperal 213 0.11 19.7 289 ALSI 214 0.11 17.3 288 +TM 215 0.1 I 19.3 287 +ALSI 216 0.10 19.2 299 Disperal 317 0.023 19.0 382 ALSI I18 0.033 18.4 333 TM 119 0.053 31.1 418 Disperal I20 0.023 18.5 340 TM 221 0.032 17.5 322 Disperal 222 0.024 20.6 351 ALSI 223 0.024 17.1 324 +TM 224 0.027 19.0 338 +ALSI 225 0.027 18.7 331 Disperal 326 1.77 29.0 274 Commercial catalyst

1,2 and 3 in column 6 refer to deposition of Pt by using Pt-coated sol particles for the

wash-coat, direct impregnation with H2PtCI6, and adsorption of PtCI62-respectively.

I. Effect of Pt concentration and sol type on catalytic performance

Figure 1 shows that the light off temperature increases with decreasing Pt

concentration but is lower than that of the commercial sample for catalyst samples

containing the highest concentrations of Pt. Pt was deposited by direct impregnation for

all the samples in Figure 1 (and in Figure 2). TM gives the lowest, +ALSI the highest light

339

1__.

450

20

400

22

350

20,24

300250200

(". "t(I I 26 I 112 f 24."

4,6 :: 71 I I t.II I I I II I I 13ill :21II I 14' 1;23II : 15/ I

3 II I I I III I I115 I ' iII I I jlI' I 1 ·1I' : I IiII I' I I' I ." 22II i I II :II I I"I' I / ./ '.I I I j / / ••

I , I I I( .:I II ' .'/ V//j :.:'

/ 1.//;/~".- 22. 23.: .?(.rc··· .. '21

~ .. ~/_;..-_.-_/_.~' ---------....L __ .L-

100

80

~

" 60.s"''"~ea 40o

20

Tempera tureJ·C

Flg.L The effect of Pt concentration and sol type on light off temperature. (Pt applied bydirect impregnation.) - - - High: 0.4-0.6, _. _. - Medium: 0.1-0-2,' ..... Low Pt range:0.02-0.05 mg Pt per gram of catalyst. -- Commercial catalyst: 1.77 mg Pt per gramof catalyst.

100

~c'0 90'in

'">c:sau

80

23

.4.12.11,3,5r. --. --

...... 267,2113

15

200 250 300-3 -1

Space velocity xlO. H

Fig. 2. The effect of Pt concentration and sol type on catalyst efficiency (Pt applied bydirect impregnation).

340

off temperature and Disperal, +TM and ALSI give intermediate values for catalysts withthe highest concentrations of Pt. For intermediate concentrations of Pt, ALSI, +TM and+ALSI give the lowest, TM the highest and Disperal intermediate values of the light offtemperature. For the lowest concentrations of Pt, Disperal and +TM give the lowestwhereas ALSI gives the highest and +ALSI and TM intermediate values of the light offtemperature. The increase in CO conversion with temperature becomes more gradual asthe Pt content decreases. This is particularly notable for TM at the intermediate andlowest concentrations of Pt (direct impregnation).

Figure 2 shows that there is no clear cut dependence of catalyst efficiency, expressedas CO conversion as function of space velocity, on Pt concentration. Thus, intermediateconcentrations of Pt on TM and Disperal are as effective as high concentrations of Pt and

100 r ~

l 21;;'- 80 r- 18 : :25 :19

I'"f

20

140

0u b

20 t Q

,I"

300 400 300 400

Temperature, ·C

95 ....... : ....... '.' 21(400·CI........

;;'- 90go

"i3 85ou

80

··.19(500·C)

25 (400·C)

20(500·C)

250

'. "::''8 .

'. (~OO····'. c)

c

300-3 -1

Space velocily x \0. H

Fig. 3. The effect of the method of depositing Pt on catalyst performance. (Silica andalumina as wash-coats.)

341

both concentrations of Pt on these substrates are more effective than the commercial

catalyst. It appears, however, that the Pt concentration cannot be lower than a critical

value without rapid loss of catalyst efficiency. Low concentrations of Pt on Disperal (No.

21) appears to be an exception and gives remarkably high catalytic performance with only6 x 10- 5 Pt atom per A2.

2. Effect of the method of depositing Pt on catalytic performance

Catalysts with low concentrations of Pt were studied in order to bring out differences in

the effect of different methods of application of Pt on catalytic performance.

Figure 3 a shows that wash-coat of Pt-coated TM gives a much steeper increase in CO

conversion with temperature than wash-coat of TM directly impregnated with Pt.

Figure 3 b shows that for wash-coats of Disperal the situation is the reverse; namely

that direct impregnation and adsorption results in lower light off temperature and faster

response to increase in temperature than wash-coat of Pt coated alumina. Figure 3 c also

demonstrates dramatic differences in efficiency of Pt applied by different methods.

Direct impregnation of silica with Pt and wash-coat of Pt-coated alumina result in low

efficiencies whereas Pt-coated silica and direct impregnation of alumina with Pt give

catalysts with high efficiencies.

3. Effect of rate of deposition of Pt on catalytic performance

Figures 4 a and b shows that a faster rate of deposition of Pt on sol particles (0.8 mg Pt

100 r ( 100 ---- -I I -c, <, ::::.'I II 2,,-,

80 I ,"- 2I I ";;' I ;-'

"c I c~ 60 I 0 95 1 -, '"I ~

I I~

-, -,I I \ -,

u 40 I I :3 \ -,a I I a \u I I u \

20 I I 90 b \I I aI II_I

250 300 200 30GIernpercturet C Space velocity x 10-~ H-

1

Fig. 4. The effect of rate of deposition of Pt on catalytic performance. (Pt-coated silicaparticles as wash-coat.),

min-I) gives a lower light off temperature but lower efficiency than a slower deposition

rate (0.02 mg Pt min-I). This probably reflects differences in the degree of dispersion of

Pt caused by differences in the addition rate of Pt to the silica sol in the coating

procedure.

342

Obviously the chemical nature of the substrate and the method of depositing Pt on the

substrate have a profound effect on the properties of Pt as an oxidation catalyst for CO.

In an effort to explain the observed effects, the degree of dispersion of Pt on a number of

catalyst samples were determined by chemisorption of H2 and the results are shown in

Table 2. Samples I through 8 refer to catalysts with Pt amounts in the high range; see

Table 1 for sample identification. Sample 16 refers to a medium range content of Pt on

alumina and has a degree of dispersion of 88% which agrees well with that of sample 8,

90%. The degrees of dispersion of Pt on the catalysts with low range contents of Pt could

not be accurately determined but it is assumed that they show the same trends as those of

the catalysts with high range contents of Pt Fi.gures 3 a and 3 c show that sample 18, Pt-

coated silica sol, has a faster light off response and much more efficient CO conversion

than sample 20, silica wash-coat directly impregnated with Pt, and yet the degrees of

TABLE 2

The degree of dispersions of Pt on selected catalysts.

MonolithNo.

Degree of dispersion%

----1--------.---~f7-·-------------------·------

2 18(22)3 14(30)4 505 116 217 158 9010 43(66)16 88

a The degree of dispersion was measuredby a Chemisorb 2800frOm Micromeriticsat350C and at 4000C (figures within parenthesis).

dispersion of Pt on the two catalysts are about the same; d. samples 1 and 6 in Table 2.

On the other hand, the catalytic behaviour of Pt applied by direct impregnation and

adsorption on alumina, ct. samples 21 and 25 in Figures 3 band 3 c, is almost the same

although the degrees of dispersion of Pt by the two methods of application are quite

different, 50% and 90% respectively for the two samples. Clearly, chemisorption of H2 onthe catalysts of this study cannot be used to explain or predict their catalytic behaviour.

Pt is considered not to disperse well on silica and difficulties have been encountered in

using hydrogen chemisorption to determine the degree of dispersion of Pt on silica (ref.

17). The catalytic behaviour of Pt on a substrate must depend on the interaction between

Pt and substrate, as expressed by dispersion and the structure of the dispersed particles.

Kummer (ref. 4) has proposed that large Pt particles on alumina is more active in the

sense of causing a low light off temperature than highly dispersed, i.e, very small,

particles of Pt, but more sensitive to the inhibiting effect of CO on the oxidation rate.

Comparing samples I and 2 in Figure It then suggests that for Pt-coated silica sols a high

rate of deposition of Pt on the silica may result in larger Pt particles on the silica surface

than a low rate of deposition although the degree of dispersion does not indicate adifference in particle size.

Chemisorption of 02 and CO on samples I (Pt on silica) and 4 (Pt on alumina) were also

measured at 1t00oC. In the pressure range 1-10 torr Pt on silica and Pt on alumina

adsorbed about 0.02 cc (STP) of 02 or CO (corrected for adsorption on support) which

corresponds to about 2 x 1017 molecules of 02 or CO per gram of catalyst sample.

The Pt content of the two samples, about 0.5 mg Pt corresponds to about 2 x 10 18 Pt

atoms per gram of catalyst. If Pt is not completely dispersed but forms clusters

containing 100 or 1000 Pt atoms these clusters would have diameters of 1.4 and 3.1 nm

respectively, and contain 70 and 40% respectively, of the Pt atoms as surface atoms. The

number of surface Pt atoms per gram of catalyst would thus be of the order 10 18•

100% conversion of the reactant mixture (0.6 and 3.1t % by volume of CO and 02respectively in N2) at a space velocity of 200000 h- I corresponds to 6 x 10 18 molecules of

CO reacting per second and gram of catalyst. Assuming the time of adsorption of CO and

02 on Pt is less than I second, say 0.1 second, there are enough surface Pt atoms

available to account for the surprising similarity in catalytic performance of Pt on silica

and alumina for some of the samples studied.

CONCLUSIONS

The catalytic properties of Pt as oxidation catalyst for CO depend strongly on thesubstrate and the method of depositing Pt on the substrate.

The catalytic performance of Pt on silica, applied by direct impregnation or using Pt-

coated silica sol, is as high as that of Pt on alumina, applied by adsorption or direct

impregnation.

The Pt content can be reduced by a factor of at about 15 while maintaining the

catalytic performance at the same level as that of a commercial auto exhaust catalyst.

The catalytic performance of Pt on alumina or silica does not correlate with the degree

of dispersion of Pt determined by chemisorption of H2.

Future work will include studying the effects of substrate and method of deposition on

rhodium as a reduction catalyst for NOx and on platinum + rhodium as a three way

catalyst.

ACKNOWLEDGEMENT

We are indebted to the Swedish Board of Technical Development for their support of

this project. Du Pont, Condea, Corning Glass and Volvo kindly supplied samples of their

products used in this investigation.

344

REFERENCES

C.N. Satterfield, Heterogeneous catalysis in practice, McGraw-Hill, New York, 1980,416 pp.

2 J. Wei, Advances in catalysis, (1975), 57-125.3 Unpublished results.4 J.T. Kummer, Prog. Energy Combust. Sci., 6 (1980) 177-199.5 K.e. Taylor, Catalysis, Science and Technology, 5 (1984) 119-170.6 J.e. Summers and K. Baron, J. Catal., 50 (1977), 407.7 L.L. Hegedus and J.J. Gumbleton, Chem Tech., October (1980) 630-642.8 S.H. Oh, Accepted for publication in J. Catal.9 S. Eo Livingstone, Comprehensive inorganic Chemistry, 3(1975) 1330.10 G. B. Alexander, U.S. Patent No. 2,892,797 (1959).11 R. K. Iler, J. Colloid Interface Scl., 55 (1976) 25-34.12 R. K. Iler, J. Colloid Interface Sci., 43 (1973) 399-408.13 G. B. Alexander, U.S. Patent No. 3,007,878 (1956).14 G. Johansson, Acta Chern. Scand., 14 (1960) 771-773.15 G. H. van den Berg and H. T. Rijnten, Preparation of Catalysts 2, Elsevier,

Amsterdam (1979) 265-277.16 H. S. Gandhi, A. G. Piken, M. Shelef and R. G. Delosh, SAE paper No. 760201.17 G.C. Bond and P.B. Wells, Preparation of Catalysts 4, Louvain-la-Neuve (Belgium)

September, 1986.

A. Crucq and A. Frennet (Editors), Catalysis and Automotive Pollution Control1987 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

THE PROMOTION OF Pt/Si02 CATALYSTS BY W03FOR THE NO-CO REACTION

J.R. REGALBUT02 and E.E.WOLF1

lChemical Engineering DepartmentUniversity of Notre Dame

Notre Dame, Indiana, 46556, USA

ABSTRACT

The activity of a series of Pt/W03/Si02 for the NO-CO reaction has beenstudied using in-situ Fourier Transform Infrared Spectroscopy. It was foundthat high loadings of W03 promote the activity of Pt for this reaction.Catalyst characterization studies, conducted by x-ray diffraction, x-rayphotoelectron spectroscopy, CO chemisorption, and transmission electronmicroscopy, indicate that Pt was decorated by an overlayer of partiallyreduced WO x• The increased activity can be quantitatively related via atwo site mechanism involving Pt sites and adlineation sites formed at theinterface between the overlayers and Pt.

I NTRODUCTI ONElemental tungsten has high NO dissociation activity, but adsorbs

strongly Nand °adatoms and furthermore, under oxidizing conditions, formsW0 3• However if W0 3 is combined with Pt, the noble metal can help to par-tially reduce the oxide via hydrogen spillover to yield hydrogen tungstenbronzes or HTB [1,2]. The combination of a partially reducible oxide witha noble metal can also result in an interaction between the metal and thesupport or SMSI effect, as in the case of Pt, Rh,and Ni supported on Ti02[3] and Pd/La203 [4].

The purpose of the investigations presented in this paper was to studythe promotional mechanism that can arise when a Pt/Si02 catalyst is pro-moted with W0 3 for the NO reduction reaction. This paper summarizes themain results of characterization studies, TEM studies, and FTIR studieswhich has been described in detail elsewhere [5-7].

To whom correspondence should be addressed

2 Present addressDepartment of Chemical EngineeringUniversity of Illinois at ChicagoChicago, IL 60680, USA

:345

346

EXPERIMENTAL

Catalysts

The compositions of the three catalys~s series used in this study areshown in Table 1. The catalystswere supported on a high surface area silica6el (Harshaw, 600m2/g), and were prepared by impregnation of the silicasupport by solutions of chloroplatinic acid, anhydrous ammonium tungstate(AAT) for the low tungsta containing catalysts series, and ammonium meta-tungstate (AMT) for the high tungsta series. The catalysts containedvariable loadings of Pt ranging from 1.2, 2.5, 3.8, and 5.0 wt%. The lowtungsta series contained W03 loadings varying from 6.2 wt.X to 1.7 wt. X,so that the total metal loading adds to 5.0%. In the high tungsta loadingseries, only the Pt loading was varied, the tungsta loading was keptconstant at 25 wt X.

The Pt/Si02 catalysts were prepared by impregnation of the silica sup-port with chloroplatinic acid to incipient wetness, followed by vacuumdrying, calcination in air to 300'C, and reduction in pure H2 at 425'C.The low tungsta series was prepared by first impregnating the support withAAT, followed by a calcination at 700'C and then subsequent Pt impregnationusing the same procedure as for the Pt/Si02 catalysts. The same sequentialimpregnation was used for the high tungsta loading catalysts using insteadan AMT solution to produce 25 wtX W03•

Catalyst characterization

X-ray Diffraction(XRD). Diffraction patterns were obtained in a DianoXPG diffractometer equipped with a Cu-Ka source and a graphite monochro-matOr. The powdered catalysts samples were pressed into wafers and affixedto a glass holder. Instrumental broadening and broadening due to sampledepth, was measured by using a standard consisting of annealed Pt fillingsmixed with Si02• Diffraction patterns were obtained after each preparationstep to identify the crystallographic phase of the catalytic component andto estimate crystallite size using Scherrer's equation.

TABLE 1

Catalysts compositions and designationsa

Tungsta Free Low Tungsta High Tungsta

Pt/Si02b Pt/W03/Si02

b Pt/W03-Si02b

6.2 W0 3/Si02 25W03/Si021.2 Pt/Si02 1.2 Pt/4.7 W03/Si02 1.2 Pt/25W03-Si0 22.5 Pt/Si02 2.5 Pt/3.2 W03/Si02 2.5 Pt/25W03-Si023.8 Pt/Si02 3.8 Pt/1.7 W03/Si02 3.8 Pt/25W03-Si0 25.0 Pt/Si02 5.0 Pt/25W03-Si0 2

a The numbers preceding the catalytic component represents itscomposition in wt%. D generic designation.

Chemisorption. CO chemisorption measurements were carried out byinjecting pulses of 10% CO in He. into an ultrapure He stream flowingthrough a quartz tube containing the catalysts. Prior to chemisorptionmeasurements. the catalysts were pretreated with oxygen at 300·C.reduced inultrahigh purity H2 at 425·C. and then degassed and cooled to room tem-perature. Infrared results indicated that CO adsorbed preferentially on Pt.furthermore. no CO adsorption was detectable on W0 3• Consequently.dispersion was calculated using a 1:1 adsorption stoichiometry between COand Pt. and crystallites sizes were estimated assuming hemisphericalgeometry.

X-ray photoelectron spectroscopy (XPS). These studies were performed atthe Amoco research center at Naperville. Illinois. by Or. Theo Fleisch. AHewlett Packard 5950B ESCA spectrometer was employed using a monochromatedA1 source. Sample wafers were pressed from approximately 50 mg of catalystpowder. and placed in a pretreatment chamber attached to the spectrometer.A detailed description of the apparatus is given elsewhere [8].

Catalysts were scanned for the Pt 4f. (73-71 eV). W4f (33-36 eV). Cl2p (198-200 eV). Si 2p(103-105 eV) and °1s(532-533 eV) transitions. Therelative amounts of Win its various oxidation states were estimated bydeconvolution using a Gaussian peak fitting routine. Surface compositionswere estimated in a conventional manner described elsewhere [ref. 9].

347

348

Transmission electron microscopy (TEM). Conclusions drawn from theabove characterization studies were further corroborated by direct obser-vation of model catalysts. These model catalysts were prepared by depo-siting the active components directly on gold microscope grids coated witha planar silica substrate. After deposition of the precursor salts, thegrids were placed in a quartz flow reactor where in order to mimic the pre-paration of the real catalysts, they were subjected to the same pretreat-ments, as described in the catalyst preparation section.

A JEOL JEM 100C scanning transmission microscope (STEM) was used inthese studies. Most imaging was done in the bright field mode, electrondiffraction patterns were also obtained to differentiate the Pt and W0 3phases. Details of the procedures used in the TEM studies are describedelsewhere [6,10].

Activity and infrared studies.

All activity measurements were conducted in an in-situ infrared reactorcell placed in the sample compartment of a DIGILAB 15C Fourier TransformInfrared (FTIR) Spectrometer. The reactor, described in detail elsewhere[11], consisted of two aluminum flanges with CaF2 IR transparentwindows, a gas inlet and outlet, and two foil fast response thermocoupleswhich were placed in direct contat with the catalyst. The reactor tem-perature was maintained constant by external heaters controlled by atemperature programmed controller. A Teflon coated recycle pump permittedto maintain near isothermal conditions and improve the mixing in the reac-tor. The reactor and associated lines were tested for activity at thehighest temperature used, and it was found to have negligible activity.

The flow rates of all the gases used were metered by a four channelmass flow controller. The electronic flow controller was equipped with aspecially designed circuitry which permitted to increase and/or decreaselinearly the flowrates of a specific reactant to perform "concentrationprogramming reaction" or CPR experiments. The concentration of CO, CO2and NO were measured at the reactor outlet via infrared analyzers.

The catalyst, in the form of a thin (100v) wafer held by the aluminumgasket sealing the reactor, was placed perpendicular to the IR beam. Thereactants flowed along both sides of the catalyst wafer. The wafers wereprepared by pressing approximately 30 mg of the prereduced catalyst powderat a pressure of about 7,000 psi. The wafers, approximately 2.5 cm dia.,were reduced again in the reactor for 12 hours at 200·C.

RESULTS

Catalyst characterization.

X-ray diffraction. Fig. 1 shows the XRD patterns of the 2SW03/Si02(pt free) catalyst after calcination (yellow), and reduction at 42S·C(blue), and of the 1.2Pt/4.7W03/Si02 low tungsta catalyst after reductionat 400·C (black). The chromatic effect has been associated with the for-mation of hydrogen tungsten bronzes [12] which agrees well with the XRDpatterns. For the 2SW0 3/Si02, the dissappearence of the (111) W0 3 peak at28=27.4 degrees, and the conglomeration of the peaks centered at 28=24.Sdegrees into the (110) HTB peak, further corrroborates the phase transfor-mation. from the fully oxidized W0 3• to either the rombic (H.33W03) form orcubic (Ho•SW03) form of the HTB. The diffraction patterns of these twoforms are very similar and their differentiation was hindered by peakbroadening. The XRD pattern of the 1.2Pt/W03/Si02, low tungsta catalyst(black color), is practically identical to the blue HTB plus additionalsmall and broad lines corresponding to Pt. This catalyst was reduced at atemperature of 400·C, wherein the high tungsta catalyst showed no chromaticeffect. This indicates that Pt is indeed helping to reduce W0 3 to aHTB.

The crystallographic composition of the Pt phase was also studied as afunction of pretreatment on the various catalysts. It was found that aftercalcination the only crystalline form present in the Pt/Si02 andPt/W03/Si02 catalysts series was PtC1 2• In addition, W0 3 lines were alsoobserved in the high tungsta series [6]. In each series, PtC1 2 peaksbroadened with increasing W03 loading, i.e. W03 seemed to reducethe size of PtC1 2 crystallites. Upon reduction, peaks correspondingto metallic Pt formed, eXhibiting broadening proportional to the amount ofW03 present. HTBs were detected only in the high tungsta series. Averagecrystallite sizes, calculated for each catalysts from the broadening of thePt(lll) lines, are listed in Table 2 and seem to decrease with W03 loading.

CO Chemisorption. CO uptake was the highest for the Pt/Si02 catalystand decreased with increasing Pt loading. A significant reduction in COuptake was obtained with increasing W03 loading. Average crystallite sizecalculations based on the chemisorption results, listed in Table 2, showthat according to these estimates, crystallite size increases with W0 3loading. This trend is opposite to the one shown by the XRD results.

~49

>- I- en z w I- ZI I . tl

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12

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140

13T

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WO

,

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120

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WO

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III

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WO

,

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TABLE 2.

Average crystallite sizes estimated by XRO and CO chemisorption

Ca ta lyst Crysta 11 i te size [A]

Pt loading Pt/Si02 Pt/WO/Si02 Pt/W03-Si02XRO CHEM XRO CHEM XRO CHEM

1.2P t 600 47. 107 95 394

2.5Pt 500 58. 315 125 95 294

3.8Pt 385 79. 255 154 75 336

5.0Pt 260 133. 115 284

X-ray photoelectron spectroscopy. The XPS spectrum of three catalysts areshown in Fig.2 [5]. The lower spectrum corresponds to the oxidized25W03/Si02 catalysts showing the 14 4f transition corresponding to the +6state. The upper two spectra corresponding to a reduced low tungsta andhigh tungsta Pt containing catalyts, show the appearence of shoulders onthe low energy binding side which corresponded to the 14+5 and 14+4 staterespectively. The relative amounts of 14+6, 14+5, and 14+4 present on eachcatalyst listed in Table 3, were obtained from spectra like the ones shownin Fig.2.

TABLE 3.

Extent of reduction of 14+6 obtained from deconvolution of the 14 4f spectra.

Pt loading Pt/W03/Si02 Pt/W03-Si0214+6 14+5 14+4 14+6 14+5 14+4

0 Pt 100

1.2 Pt 70 30 NO 63 37 NO

2.5 Pt 75 25 NO 67 33 NO

3.8 Pt 85 15 NO 42 33 25

5.0 Pt 40 39 21

NO not detected

352

calcined, reduced T< 4O<fC

>t-CI.)ZWt-Z-

46 42 38 34 30 26

ELECTRON BINDING ENERGY CeV)

Fig. 2 XPS spectrum of W4f transitions of calcined and partiallyreduced low and high tungsta catalysts [5].

Transmission electron microscopy. A representative micrograph of anarea of a calcined model catalyst showing the formation of overlayers isdisplayed in Fig. 3. Such overlayers were observed in 4 of 7 areasstudied, and were formed after calcination on both the W03 crystallites(marked l'l and on the Pt crystallites (marked 2' l. Other resultsdescribed in detail elsewhere [6], indicate that after reduction overlayersspread, and HTBs formed. Overlayers do not form on Pt in the absence ofW03, and they disappeared upon exposure to the electron beam. Possiblecarbon and silica contamination did not affect overlayer formation. Theoverlayers observed on the model catalysts were not detected on the realcatalyst. in part due to lack of constrast. and in part because dif-fusional conditions that facilitate overlayer formation. are greatly dimi-nished in the real catalysts. Consequently the model catalysts represent ahighly exaggerated view of events that occur at a much smaller scale in thereal catalyst.

Fig. 3 TEM micrograph of a region of a model catalyst after calcination,magnification 750,000.

Activity and FTIR results.

The infrared spectrum of CO and NO on the Pt/Si02 catalyst shows bandsat 2100 and and at 1400 cm-1 corresponding to linearly adsorbed CO and NOon Pt. At 220·C, CO-Pt adsorbates quickly displaced NO-Pt adsorbates withoutreaction, and the surface become predominantly covered by CO. In tungstacontaining catalysts, a small shoulder appeared on the left hand of the2100 cm-1 CO-Pt band, and another small broad band at 1400 cm-1• Thesebands were detected only when both NO and CO were present.

Results of a CO-CPR experiment, in which the CO concentration wasincreased linearly from 0 to 0.7% and then decreased likewise, are displayedin Fig. 4 [7]. At this temperature CO2 production became appreciablefor four representative catalysts shown in Fig.4a in terms of mole %CO 2 produced versus time or its equivalent, CO inlet concentration. Fig.4b displays the intensity of the 2100 cm-1 band during the experiment.

354

W.75

"<a:w>0o .50Wo<u.a:::>en .25

(\0 .7

\ COfecdo I\""0 /.6 \

0' /\o .5 I

\l- Iz .. \w / \ca: .3 I \wQ. \W .2...J0~ .I PI

00.0 0.5 1.0 I.S 2.0

TIME (hrs)

1.0

TIME (hrs)

Fig 4 CO-CPR (0.-0.7-0 %CO) into 2% NO at 280·C over representativecatalysts [7].

The results show that as CO concentration increases, the rate ofCO2 production at first increases, and then decreases. The decrease in ratecorrelates with an increase in CO adsorbed during the ramp, i.e. is due toCO inhibition of adsorbed NO species. Furthermore, when the various cata-lysts are compared at a given time, the high tungsta loading catalysts,which exhibit the highest rate of CO2 production, also exhibits thesmallest amount of adsorbed CO. Thus, it is seen that the presence oftungsta removes CO inhibition by decreasing Pt adsorption capacity. Thepromotional effects of W0 3 are also seen in Table 4, which summarizesoverall and specific rates of CO2 production on all the catalysts studied.IR absorbance of the W0 3 promoted catalyst remained higher than that ofPt/Si02 catalysts indicating that the promoter also has an effect on sta-bilizing Pt.

TABLE 4.

C02 Production (rgxl017 mo1ec/sec) and rates (rsxl018 mo1ec/sec/g Pt)

of CO2 Production

355

Pt loading

1.2 Pt

2.5 Pt

3.8 Pt

5.0 Pt

Pt/Si02 Pt/W0 3/Si02 Pt/W03/Si02

rg r s rg r s rg r s0.789 1.44 0.789 2.13

0.747 2.55 0.913 2.27

1.25 3.33 0.664 1.31 1.83 5.06

1.33 3.84 2.82 7.81

DISCUSSION

Catalyst characterization.

The XRD results indicate that the W0 3 phase is present in the reducedcatalysts in the form of a HTB. According to XPS data, the oxidation stateof reduced tungsta varies from w+6 to w+4• If the composition of the HTBare estimated from the relative % of the various oxidation states obtainedfrom the XPS data, it agrees with the XRD only if the HTBs contain onlytungsta in W+6 and r~5 state. It follows that the W+4 state must form a

356

separate phase in order to reconcile the XRD and XPS results. Thisw+4 state can be attributed to WO Z which has a dark brown color which wouldresult in the black coloration observed in the reduced high tungsta cata-lysts.

Pt crystallite sizes shows opposite trends with tungsta loadingdepending if they are estimated from XRD or CO chemisorption data. For thePt/SiOZ catalysts. XRD estimates give larger sizes than chemisorption.This case is commonly encountered when a fraction of the crystallites hassizes below the detection limit of XRD. but they are detectable by chemi-sorption. However with the high tungsta loading catalysts. this trendreverses, with chemisorption estimates being higher than the more conser-vative XRD estimates. This indicates clearly that chemisorption is insuf-ficient to account for the Pt area detected by XRD, and implicates thatchemisorption suppresion occurs with the addition of W0 3•

Chemisorption suppresion can be accounted for if a partially reducedoxide is decorating the surface of the active metal. Such a model is nowwell established in the literature for several SMSI systems. In this case,WOz• which was shown to form a separate phase than the HTB, is likely tobe the species that is decorating the Pt surface. The TEM results obtainedin a model catalyst, clearly show that overlayers can form in some areas ofthese ideal surfaces. Over1ayers could not be detected on the real cata1ytsbut the behaviour of the model catalyst is interpreted as an exaggerated viewof what occurs in the real catalyst.

On the basis of the above information, a model has been proposed [5,6]which consists of small and large crystallites located on the silica supportand on the tungsta phase. Pt on the tungsta phase increased the reducibi-lity of W0 3 and lead to the formation of HTBs and W02• The tungsta phase inturn reduced the Pt crystallite size by decreasing the amount ofPtC1 2 formed after calcination, whereas tungsta suboxide forms a separatephase and decorates the Pt surface thus decreasing CO chemisorption.

The FTIR studies indicate that the presence of W0 3 inhibits CO chemi-sorption thus decreasing in part CO 2 production rates by removing CO inhi-bition. Furthermore the NO-Pt coverage was inversely proportional to CO-Ptcoverage.Additiona1bands at 2100 and 1400 cm-1 accumulated only during thetransition to the CO inhibited regime occurred. Experiments conducted tomeasure the rate of NO dissociation indicate that the high tungsta catalysteXhibited the lowest rate of NO dissociation.

While no information is available in the literature on the nature of2100 and 1400 cm- 1 bands, they can be attributed to CO and NO adsorbed onWO x type sites formed at the interface between the Pt and the decoratingpatches of W0 2 which were determined by the characterization studies. Thistype of sites, termed adlineation sites, have been associated with theincreased activity of several SMSI catalysts [12J.

Using the concept that two sites are responsible for the promotionaleffects observed, it is possible to correlate quantitatively CO2 productionactivity with the surface concentration of both metals provided by thecharacterization results. The contribution of the Pt sites can be calcu-lated by multipling the number of Pt sites Npt, measured by CO chemisorp-tion on each catalyst, with the turnover number of the Pt sites, TON pt'The contribution of the adlineation sites can be assumed to be proportionalto the product of its concentration Npt W' times its turnover number TONpt W•The concentration of the adlineation sites can not be measured, but it canbe estimated to be proportional to the product of NptXW s' where Ws is theatomic surface concentration of W, measured by XPS. One can then write:

A plot of rC02 - TONptN pt versus Nptw=NptxWs' all measured quantities,yielded a straight line with a slope proportional to TON pt W[7J. The turn-over number for the adlineation sites was found to be about 360 Npt' whichconfirms the expectation that the adlineation sites are fewer, but muchmore active than the Pt sites. Kinetic analysis of the reaction has pro-vided further support for the two site mechanism [13J.

In conclusion, this paper summarizes studies on the mechanism of promo-tion of Pt by W03• It is shown via detailed surface characterization that asurface suboxide WO x' decorates the Pt surface. The decorating WO x speciesform special adlineation sites that are assumed to be responsible for thepromoted activity. A correlation based on a two site mechanism, one forthe Pt sites, and the other for the adlineation sites,explains quanti-tatively the results obtained •

357

358

REFERENCES

[1]

[2]

[3]

[4]

[5]

[6]

[7]

Benson. J. E•• Kohn. H. W. and Boudar t , M•• J. Ca ta 1•• 2.. 307 (1966) •

Sermon. P. A. and Bond. G. C•• J. Chern. Soc. Farad. Trans •• !. 72(1976) •

raus ter , S. J •• ACS Syrnp , Ser , 298. 1 (1986) •

Fleisch. T. H. , Hi cks , R. T. and Bell, A. T., J. Catal §l., 398(1987).

Regal buto, J. R. and Wolf, E. E., submitted to J.Catal.

Regalbuto, J. R. and Wolf, E. E., submi tted to J.Catal.

Rega1buto , J. R. and Wolf, E. E. , submi tted to J.Catal.

[8] Fleisch, T. H. and Mains, G. J., J. Chern. Phys., l!(2) , 780 (1982).

[9] Hercules, D. M., Trends in Analytical Chemistry, l(5), 125 (1984).

[10] Sing, A. K., Pande, N. K. and Bell, A. T., J. Catal., 94, 422 (1985).

[11] Kaul, D. K. and Wolf, E. E., J. Catal., 89, 348 (1984).

[12] Boudart, M., Vannice, M. A. and Benson, J. E., Zeitschrift furphysikaliche Chemie Neue Folge, ii, 171 (1969).

[13] Regalbuto, J. R., Ph.D. Thesis University of Notre Dame, 1986.


SURFACE DIFFUSION OF OXYGEN IN Rh/A1203 AND Pt/A1203 CATALYSTS.

H. ABDERRAHIMI and D. DUPREZLaboratoire de Catalyse en Chimie Organique UA CNRS 350Universite de Poitiers, 40, Avenue du Recteur Pineau86022 Poi tiers Cedex, France.1 On leave from the Ecole Normale Superieure, Algiers.

359

ABSTRACTExchange of gaseous 1802 with the 160 of the support was studied at 300-

500°C On precious metal (PM)/alumina catalysts. Rhodium is approximately fourtimes more active than platinum in the exchange reaction. On the other hand,palladium is virtually incapable of promoting oxygen exchange. As regards Pt,the rate of exchange is determined by the rate of adsorption-desorption ofoxygen On the PM particles. The true rate of migration can be measured only Onrhodium catalysts. The coefficient of diffusion appears to have little to dowith the nature of the alumina used as a support. Structural parameters suchas the metal area, the perimeter of the metal/support interface, the degree ofreduction playa determining role in the overall process of exchange. Thesevarious factors are analyzed in the present report.

INTRODUCTI ONOxygen plays an important part in exhaust gas catalytic purification. The

ability of the catalyst to store oxygen for smoothing rapid large oscillationsof oxygen pressure in the gas phase, is generally promoted by the addition ofrare earth oxides, especially cerium oxide [1,2] . Nevertheless. the presenceof precious metals (PM) has been shown to enhance the oxygen storage capacityof three-way catalysts [3,4] . It may thus be inferred that the precious me-tals playa critical role in the transfer of 02 from the gas phase to the pro-moter, via surface diffusion on the support (alumina). We shall nOW report ra-te measurements of this transfer process, deduced from 180- 160 isotopic ex-change On PM/A1203 catalysts.

EXPERIMENTALExchange of gaseous 1802(> 99%, CEA France) with alumina-supported metal

catalysts was carried Out in a recirculatory reactor (ca 50 cm3) coupled witha mass spectrometer allowing the masses 32, 34 and 36 to be mOnitored versustime (Fig 1). The vacuum leak to the mass spectrometer (AEI MS-20)is calibrated

360

vacuum- to MS

o Vacuum valves

PG Pressure gauge

RVP Recirculatory vacuum pump170 cm3 S_1

Te Temperature controller:to.SoC

Fig 1 Recirculatory reactor adapted for exchange experiments

so as to ensure a decrease of less than Z mbar in the reactor within 1 hour(initial pressure: 60 mbar). Preliminary investigations [5] have shown thatoxygen can exchange only via the metal particles : rates of exchange are ne-gligible on the bare alumina support. Moreover, exchange was found to be seve-rely inhibited by certain impurities and, particularly, by chloride ions. Ac-cordingly, the catalysts used in this study were prepared and dechlorinated bymeans of the following procedure. The support (Rhone-Poulenc GFS C palumina,ZOO m2g-1, mean pore radius 45 A, Na, Fe, and Si impurities < 400 ppm) waspretreated either under air flow at 500°C or under a flow of hydrogen at850°C. The resulting materials are referred to as Ao and A, respectively. Thecatalysts were prepared by ion exchange of the supports in aqueous solutionsof rhodium chloride hydrate or chloroplatinic acid, using the low acidity me-dium preparation described in Ref 6. The catalysts were subsequently dried at1Z00C and calcined at 450°C. Dechlorination was performed by treatment under asteam/hydrogen gaseous flow at 450°C for 5h (HZ/HZO molar ratio of 1.35,weight hourly space velocity of steam: Zh-1). The choice of HZ/HZO mixturesarose from previous findings showing that the presence of HZ in steam inhibi-ted the formation of the diffuse oxide phase of rhodium in alumina (Ref 7).The influence of this diffuse oxide phase on the rate of exchange was ascer-tained on catalyst samples calcined at high temperatures. For the sake of com-parison, Rhone Poulenc SCS 79 and Degussa Oxid C aluminas were likewise usedfor the purposes of this study. They are referred to as RPA and DA, respecti-

361

vely. The dispersion of the catalysts was measured by hydrogen chemisorptionand oxygen titration in a pulse flow chromatographic system, as described el-sewhere (Ref 8).

The catalyst sample (0.02 to 0.2g) was reduced in situ in a flow of hydro-gen, and subsequently outgassed at 400°C. A dose of 1802 (0.1 to 0.2 mmole)was then introduced in the recirculatory reactor. After a rapid decrease ofpressure, corresponding to oxygen adsorption (Pt catalysts) or absorption (Rhcatalysts), the partial pressures P32 (1602), P34 (160180) and P36 (1802) wererecorded as a function of elapsed time, whereas total pressure remained vir-tually constant. The rate of exchange rE (in atom of 160 exchanged per min andper g of catalyst) is calculated on the basis of the mass balance of 180 inthe PM particles.

NV d dxr E = RT dt (-2 P36 - P34) - NMdt (1)

where N is the Avogadro numberV the gaseous volume of the reactorR the gas constantT the gas phase temperature (K)t the time (min)NM the number of oxygen atoms in the PM particles

per g of catalystx the fraction of 180 atoms to be found in

these particles.Making abstraction of the pool of oxygen atoms in the PM particles, eq

becomes(2)

50

32

34

t (minIo

p(mbar)

40---------------,

Fig 2 A typical curve of exchange (0.52 % Rh/A1 203, 329°C)

(3 )

362

A typical curve of exchange is shown in Figure 2. Generally, P32 t=O=O, sothat the initial rate of exchange may be computed, in this instance, by a sim-ple equation:

NV l ]r E = RT P34

When equilibrium is attained, the fractions of 180 are equal (,,*) in thegaseous phase and the support; hence

(4 )

(5 )

where Ns is the number of exchangeable oxygen atoms in the support(NG + NM) the initial number of oxygen atoms in the gaseous phase

the PM particlesGenerally, NM « NG and (4) becomes

N N (1 - a*)S = G ~

and in

The value of Ns computed from Eq (5) may be compared to No, the number ofsurface oxygen atoms having been calculated from the saturation coverage ofthe hydroxyl group of aluminas : 6.2 oxygen/100 A2 [9]

The equilibration reaction (eq 6) was performed on certain catalysts.1602 (g) + 1802 (g)-2 160180 (g) (6)

Mixtures of gaseous 1602 and 1802 containing approximately 50% of eachconstituent were prepared and contacted with the catalyst, which was activatedin the same conditions as for exchange reactions. The rates of equilibrationwere calculated on the basis of the rate of appearance of the mass 34 in ga-seous phase.

RESULTS AND DISCUSSIONRhodium Catalysts

The samples used in this study are listed in Table 1.The metal area Am (m2Rhg-1) was calculated using equation 7

Am = 0.0462 Doxm (7)where Do is the dispersion (%) and xm the metal loading (wt %). Eq 7 is basedon the assumpti on that a Rh atom occupi es 7. 9x10-20 m2.

363

TABLE 1Rhodium catalystsSample wt% Rh Support Grain size Dispersion Am

(mm) (%) m2 Rh g-l

1 0.017 Ao 1.2 100 0.0782 0.063 Ao 1.2 100 0.293 0.56 Ao 1.2 75 1.944 0.52 A 0.15 80 1. 925 1. 76 A 0.15 58 4.716 0.6 RPA 0.15 55 1. 527 0.6 DA 0.15 50 1.39

Effect of metal loading (samples 1-5). In this series of cata lys ts jx., wasvaried by two orders of magnitude; our findings allowed us to determine theinitial rates of exchange rE over a wide range of metal areas. The resultsare plotted in Arrhenius coordinates in Fig.3. For this series of catalysts,

'0>

"";.= 46E-'"

o UJ...Clo

...J 45

1.4 1.6

103/ T

.. 1.76 %• 0.56 %

00.52 %D 0.063 %... 0.017 %

300°C

1.8

Fig 3 Arrhenius plot of the initial rates of exchange on Rh/A1 203 catalyst.

364

the curves show a break point in the region To = 300-3800C. Taking account ofprevious results obtained with the 0.52 % Rh/A1203 catalyst in the equilibra-t i on reaction (1602 + 1802- 2 160180) , the break poi nt at To can be exp1ai-ned as follows: (i) at T < To, the limiting step of exchange is theadsorption-desorption process on the rhodium particles; the apparent activa-tion energy of this step is in the range 70-80 kJ mol- 1 throughout this se-ries of catalysts; (ii) above To, the rate of exchange is determined by oxy-gen migration upon the support; in this last instance, the apparent activationenergy is relatively low (19-22 kJ mol-I) in accordance with the very natureof the determining step. If this hypothesis holds, the rate of exchange shouldbe proportional to the metal area in the region in which exchange is control-led by adsorption-desorption of 02 on rhodium particles (T<To)' This is tenta-tively verified by comparing intrinsic rates of exchange at 300°C (Table 2).

TABLE 2Intrinsic rates of exchange at 300°C on rhodium catalysts.

°% Rh rE 300°Cx1019 at min- 1g x1019 at min- 1 m- 2

o (pure alumina) 0.080.017 0.37 4.70.063 1. 35 4.70.56 6.23 3.20.52 6.B3 3.61. 76 7.80 1.7

It is clear that, with the exception of the higly loaded sample, the rate ofexchange is linearly dependent upon metal area. The behaviour of sample 5(1.76 % Rh) could be due to either the presence to residual chlorine, whichproved to severely inhibits oxygen exchange (Ref 5), or to the fact that thebreak·point at To is ill-defined in the case of this catalyst. In the field oflow activation energy, in which the rate determining step is oxygen migrationon the support, exchange contributes to complex kinetics which depend uponboth the coefficient of surface diffusion D, and the specific perimeter of theparticles. According to Kramer and Andre (Ref 10)the quantity of atoms diffu-sed in the early period exchange would amount to :q = IoCe (~Dt)1/2 (8)

where Ce is the concentration of 180 in the metal particles, considered ascircular sources. As concerns rhodium, cataly~ts, assuming a homogeneous dis-tribution of hemispheric particles (Ref 11), 10 (in m of perimeter per g ofcatalyst) is given by

:365

10 = 4.13x108 (A2/x ) (9)m mwhich, combined with (7), yields:10 = 8.81x10 5 D;.Xm (10)

Initial values of dq/dt 1/ 2 at 4000C are recorded in Table 3. Also reportedare the values of the coefficients of diffusion calculated from eq.7, assuming

that Ce is virtually constant and equal to the overall concentration of oxygenon rhodium particles (1.9x10 19 at. 0 m- 2)

TABLE 3Determination of the coefficients of diffusion at 4000C

Sample Rh 10 dq/dtl/2 D(wt %) (l08 m g-l) (1019 at.g-1s-1/2) (l0-18m2 s-l)

1 0.017 1.5 1.48 8.62 0.063 5.5 2.47 1.83 0.56 28 3.98 0.194 0.52 29 3.87 0.165 1. 76 52 3.90 0.05

The samples with the lower rhodium loadings appear to be relatively morecapable of exchanging oxygen of the support. This could be indicative of mar-ked heterogeneity, as the number of particles increases. The mean interparti-cle distances are 57, 29, 15, 13 and 11 nm in samples 1-5, respectively; rho-dium particles act as individual sources interfering much more rapidly in hi-ghly loaded catalysts, particularly if the rhodium particles are not homoge-neously distributed at the alumina surface.

Exchange on various alumina-supported catalysts (samples 3-4-6-7) Exchangewas performed on 0.6% Rh catalysts supported on various types of alumina.Theresults are recorded in Table 4. For the sake of comparison, the rates of equi-libration are also given. It is clear that equilibration, which is exclusively

TABLE 4Exchange and equilibration on 0.6% Rh catalysts supported on various aluminas

Sample Support BET area Pretreatmentm2g- 1

Exchange Equilibrationx 1019 at min-1g-1

3 Ao 210 Air 450 20.8 364 A 180 H2 850 18.8 366 RPA 80 Air 450 13.3 337 DA 100 Air 450 14.0 29

a function of the metal area, is independent of the nature of thesupport; on the other hand, exchange would appear to be slightly ~ensitlve

366

to the alumina used. The coefficients of diffusion calculated from eq.7 are0.12 and 0.34~0-18 m2s- 1 for aluminas RPA and OA, respectively.

These values are similar to the coefficient of diffusion for Ao' measuredat equivalent metal loading, which indicates that the rate of oxygen migrationis virtually independent of the nature of the alumina used.

Influence of the degree of reduction of rhodium. When rhodium-alumina ca-talysts are heated in an(>6000C) there appears, inrhodium which is difficult

oxidizing atmosphere, at elevated temperatur($the alumina matrix, a diffuse oxide phase (OOP) ofto reduce at 5000C (Refs 7,12). Given that three-

way catalysts are exposed to extreme of temperature, it was of signifi-cant interest to study the influence of the OOP on the rates of exchange andequilibration. The results, reported in Table 5, show that rE decreases in pa-rallel with the degree of reduction at 5000C, whereas the rate of equilibra-tion remains unchanged. This result suggests that oxygen included in the DOP

TABLE 5Influence of the temperature of air calcination on the rates of exchange andequilibration at 4000C (sample 5, 1.76 % Rh).

TOC Rhodium reducible Exchange Equil i brati oncalcination at 5000C, % x 1019 at min-1g- 1

450 100 23 37700 80 19 36900 50 11 36

can contribute to the reaction of equilibration. The reason why exchange isrelatively adversely affected by the presence of OOP has yet to be clearlyelucidated. This could be due either to a decrease of the specific perimeter10 of the rhodium particles or to a qualitative modification of the supportfor instance, the coverage of residual hydroxyl groups, which exerts a slightinfluence on the rate of exchange (Ref 5).

Other PM catalystsExchange was performed on platinum and palladium supported on the same

alumina as with rhodium catalysts.The results, recorded in Table 6, demonstrate that rhodium remains the

most active metal in the promotion of oxygen migration on the support. Plati-num is approximately four times less active than rhodium, and palladium cannotpromote, at a measurable rate, the reaction of exchange at 4000C.

TABLE 6Comparison of the rates of exchange at 400°C on various PM catalysts.

PM

PtPdRh

Meta1 loading Dispersion Rate of exchangewt % Do % 1019 at.min- 1g- 1

1. 06 65 4.30.62 33 00.52 80 18.8

The effect of metal loading is shown on Fig 4. For purposes of comparison,46

45

c:E-ttl

o UJ...0144o..J

1.06%PtD

400

<,<,

'O.063%Rh<,

<,"-

"- <,<,

350"C

1.5 1.6

Fig 4 : Rates of exchange on Pt/A1203 catalysts.the curve of the 0.063 % Rh sample is drawn as a dotted line in the figure.The relatively high energy of activation (67 and 59 kJ mol-1 for the 0.17 % Ptand the 0.46 % Pt samples, respectively) implies that the rate determiningstep is the adsorption-desorption of 02 on the platinum particles. This iscompatible with the fact that the rates of exchange on Pt remain low woth res-pect to the rate of migration, which can be measured on Rh catalysts. Surpri-singly, rE no longer increases in the most loaded sample (1.06 % Pt). And yet,the degree of dispersion of this catalyst is very close to that of the less

368

loaded samples. This result could be due to residual chlorine, which has pro-ven to remain strongly held on platinum catalysts. Nevertheless, this questionrequires further investigation.

CONCLUSIONSOn the basis of the results on the isotopic exchange between gaseous 180Z

and 160 of the support in precious metals/alumina catalysts, it can be conclu-ded that :(i) rhodium is the most efficient metal for promoting this exchange. Platinumis approximately four times less active and, palladium is virtually unable topromote the reaction.(ii) for rhodium, the rate determining step at low temperature is the adsorp-ton/desorption process of Oz on the metal. The coefficient of surface diffu-sion can be estimated only at high temperature (> 3500C) in the region inwhich the rate determining step becomes the migration of oxygen at the aluminasurface.(iii) the lowest loaded Rh/A1Z03 catalysts « 0.1% Rh) appear to be relativelymore active in exchange, probably because the particles are homogeneously dis-tributed at the surface. As a rule, the rate of migration depends only to asmall extent on the nature of the alumina used as a support.

REFERENCES1 J.e. Schlatter and P.J. Mi~chell, Ind. Eng. Chem. Prod. Res.

Develop.,19 (1980) 288.Z J.C. Summers and S.A. Ausen, J. Catal., 58 (1979) 131.3 H.C. Yao and Y.F. Yu Yao, J. Catal., 86 (1984) Z54.4 E.C. Su, C.N. Montreuil and W.G. Rothschild, Appl. Catal., 17 (1985) 75.5 H. Abderrahim and D. Duprez, in preparation6 D. Duprez, A. Miloudi. J. Little and J. Bousquet Appl. Catal. ,5 (1983) Z19.7 D. Duprez, G. Delahay, H.Abderrahim and J. Grimblot, J. Chim. Phys.,

in press6 D. Duprez, J. Chim. Phys., 80 (1983) 487.9 B.C. Lippens and J.J. Steggerda, "Physical and Chemical

Aspects of Adsorbents and Catalysts", B.G. Linsen, Ed.,Academic Press, London and New York, 1970 p 171.

10 R. Kramer and M. Andre, J. Catal. 58 (1979) 287.11 D. Duprez, P. Pereira, A. Miloudi and R. Maurel, J. Catal. 75 (1982) 151.12 H.C. Yao, S. Japar and M. Shelef, J. Catal. 50 (1977) 407.


RHODIUM - SUPPORT INTERACTIONS Ii AUTOMOTIVE EXHAUST CATALYSTS

C.z. Van and J.C. Dettling

Research & Develop.ent Dept., Specialty Che.icals Division

Engelhard Corp., Menlo Park, New Jersey 08818-2900 (U.S.A.)

ABSTRACT

369

Within the current TWC catalyst washcoats, rhodium issusceptible to deleterious interactions with various componentsduring a prolonged lean high temperature excursion. To elucidatethe potentially detrimental rhodium compounds formed under suchcircumstances, unsupported rhodium oxides, rare earth metalrhodates, and aluminum rhodate are characterized and measured forcatalytic activity. The intrinsic activities at 673K of NO, COand C3H6 conversions over various unsupported rhodium oxidesspecies are basically structure insensitive. However, theintrinsic activities at the same temperature of both the rareearth metal rhodates and aluminum rhodate appear to be sensitiveto their structure. The interaction between rhodium and the rareearths especially cerium, is found to be much stronger than thatbetween rhodium and aluminum.

INTRODUCTION

The current catalysts controlling automobile exhaust emission

contain noble metals such as Pt, Pd and Rh. Of the three preciousmetals being used, rhodium is the most effective for reducing the

oxides of nitrogen to harmless nitrogen. However, compared to the

other precious metals, rhodium usage in present three-way

catalysts (TWC) technology far exceeds the naturally occurringratio in the mines. Motivated by the high cost and limited

availability of rhodium, it is imperative that methods bedeveloped for its more effective utilization. Conventionally,

rhodium is dispersed on a support such as alumina having

sufficiently high surface area to enhance its durability.Alternative support compositions have been used recently to

promote Rh activity, e.g. Rh on Ti02 (ref. 1,2) • However,

preliminary results indicate that the benefits can only besubstantiated in a net reducing exhaust and no information on the

370

effect of lean exposure and durability is reported.The advent of more fuel efficient vehicles and engine control

strategies in some cases increase the exposure of TWC catalysts tolean conditions. Consequently, it is important to understand how

rhodium interacts with components within the TWC catalyst wash coat

under these conditions. It has been found that rhodium dispersedon a relatively high surface area gamma alumina support interacts

with the support alumina when exposed to a temperature in excess

of 873K in an oxidizing atmosphere (ref. 3,4). We have recentlydemonstrated that rhodium can be rendered less active when it is

in intimate contact with the rare earth oxides or the gamma

alumina support components of the wash coat during a lean hightemperature excursion (ref. 5). Since cerium oxide and other rare

earth oxides are extensively used in current "High Tech" TWCcatalysts to achieve a wide operating window lean and rich of

stoichiometric conditions, an in-depth assessment of the Rhinteraction with rare earth components and alumina was undertaken

in this study.

The present investigation was conducted to identify anddetermine the degree of Rh-base metal oxide interaction, using

unsupported rhodium oxides and bulk aluminum and rare earth metalrhodates. Catalytic activities were determined using monolithic

catalysts containing various bulk rhodium species exposed to a

simulated stoichiometric auto exhaust composition. The activitieswere correlated with information obtained from CO chemisorption

measurements, temperature-programmed reduction,X-ray diffraction, scanning electron microscopy and X-ray

photoelectron spectroscopy.

EXPERIMEIiTALMaterials

Unsupported rhodium (III) oxides were prepared by precipitating

the rhodium ion from a rhodium nitrate solution (5 wt % Rh) with

concentrated NH40H (6N) at pH equal or slightly above 7.0. Theprecipitate was washed with deionized water until the conductiVity

of the washing solution was less than x 10- 5 n -1 cm- 1 and thendried at 398K for a period of 16 hours. The dried precipitate was

heated in air separately at 973K for 24 hours and 1173K for 3

hours to form of. -Rh203 and fJ -Rh203 respectively. UnsupportedRh02, a brownish powder, was obtained from Alfa Products and used

without further treatment. Aluminum rhodate was prepared by co-

371

precipitating a rhodium nitrate solution (2.5 wt % Rh) with an

equimolar sodium aluminate solution. Rare earth metal rhodates

were prepared by precipitating the metallic ions from an equimolar

solution of a rare earth metal nitrate (i.e. lanthanum, neodymium

or cerium) and rhodium nitrate (in case of cerium, half molar

cerium was used) with concentrated NH40H (6N) at pH equal or

slightly above 7.0. The various rhodium containing precipitates

were washed with deionized water until the conductivity of the

washing solution was less than 1 x 10-5 h -1 ~m-1 and then dried

at 398K for a period of 16 hours and further heated in air at

873K for 70 hours to form corresponding rhodates. Engelhard

proprietary rhodium nitrate solution (chloride content < 0.4%,

rhodium purity> 99.5%) and Fisher certified grade chemicals

(except reagent grade ammonium hydroxide and sodium aluminate)

were used for the sample preparation.

Characterization

Surface areas were determined by the BET technique (0.162 nm2

for the cross-sectional area of N2) with a Micromeritics Digisorb

(2500 series) instrument. XRD analyses were obtained with a

computerized Phillips diffractometer using Cu K~ radiation. X-ray

photoelectron spectroscopy (XPS) measurements were carried out

with a PHI 5100 series ESCA spectrometer using Mg 300W X-ray

excitation. Binding energies were referenced to C(1s) at 284.6 eV

for all the measurements. TGA and TPR were studied with a Dupont

(1090 series) thermal analyzer. CO chemisorption was determined

by a dynamic pulse technique according to Freel (ref. 6) using

helium as a carrier gas with automated pulse injection.

Catalvst Preparation and Evaluation

The catalyst samples were prepared on cordierite Corning

monoliths with 62 passageways per square centimeter (400

cells/in2). This ceramic substrate was coated with a thin layer

of a homogeneous mixture of unimpregnated gamma alumina particles

and particles of an unsupported bulk rhodium species of interest

(including various bulk rhodium oxides and various bulk metal

rhodates) and dried at 398K to provide a desired Rh metal loading.

The catalysts were placed in a laboratory flow reactor and

evaluated at 673K inlet temperature at a space velocity of 112,000

hr- 1 under steady state conditions. The reactive gas composition,

simulating an engine exhaust gas near stoichiometry, was 1.54% CO,

372

0.51 % H2, 300 ppm C3H6, 2000 ppm NO, 1.0% 02,10% C02, 10% H20

and balance N2' The analytical section of the reactor system

consisted of detectors for NO (Beckman Model 951A), CO (Beckman

Model 864), 02 (Beckman OM-11EA) and hydrocarbon (Beckman Model

400) with computerized data acquisition.

RESULTS AND DISCUSSIONUnsupported Rhodium Oxides

The physical properties of the three rhodium oxides as prepared

and after reduction at 673K are shown in Table 1. As anticipated,

the d. -Rh203 as prepared shows exclusively hexagonal Rh203

structure and p -Rh203 as prepared demonstrates virtually

orthorhombic Rh203 structure. It is quite surprising that the

Rh02 sample which yields a composition very close to Rh02.0 as

determined by TGA does not fall in the rutile structure as

described by Muller et a L, (ref. 7). Chemical and XPS analyses

confirm that the Rh02 sample was contaminated with about 2.5 wt%

K20 which apparently affected the Rh02 crystal structure.

Examination of the three oxides by SEM at a magnification of

20,000X indicates that the crystals of 0< -Rh203 and fJ -Rh203 are

more or less spheroidal while the Rh02 is polygonal plate-like

with particles having roughly 0.4~ in diameter and 300A in

thickness. After 673K reduction, all three oxides show

exclusively Rh metallic structure as determined by XRD analyses

and demonstrate little change in crystal morphologies. In Table

1, BET surface areas of the reduced oxide samples are found to be

TABLE 1

Properties of Unsupported Rhodium Oxides

Species BET Rh CO Chern CO(S); XRD EMS.A. Content jlmole/g Rh Rh(s) Structurem2;g % (c)

01. -Rh20 3 a) 18.8 81 Hex Rh203 -O.l~ spheroidalb) 11.9 100 121.8 0.46 Rh metal -0.1)' spheroidal

(3 -Rh20 3 a) 6.0 81 Orth Rh203 -0.2/, spheroidalb) 5.2 100 59.0 0.51 Rh metal -0.2,.'l spheroidal

Rh02 a) 31.4 74.5 Unknown 0.03 - 0.4;" plateb) 21.3 96.5 215.5 0.46 Rh metal 0.03 - 0.4~ plate

a) As preparedb) After reduction treatment in hydrog~n at 673K for one hourc) Measured at 298K

:373

proportional to CO chemisorption capacities of the samples.Assuming an area per surface rhodium atom of 7. 6A2 (ref. 8), it is

estimated the reduced unsupported rhodium oxides have a surfaceCO(s)/Rh(s) stoichiometry approximating 0.5 which corresponds wellto a dispersion of a typical heavily-loaded supported Rh species

determined by Yao (ref. 3). The crystallite sizes of the reducedrhodium oxide samples estimated from CO chemisorption capacities

also qualitatively agree with that from SEM analyses. Thus, the

CO chemisorption capacities appear to adequately approximate the

true available adsorption site densities of the three unsupported

rhodium oxides of interest.The results of X-ray photoelectron spectroscopic studies of

three oxides as prepared and their reduced states after exposure

in 7% hydrogen (balance N2) at 673K are summarized in Table 2. Itis interesting to note that similar binding energies of both the

Rh (3d) and 0(1s) electrons for all three oxides are observedirrespective of their distinctive differences in crystallite

structures. The surface Rh of the three unsupported rhodium

oxides apparently exhibit a +3 oxidation state. The undetectable

+4 Rh oxidation state in the Rh02 sample may result from either

thermodynamic equilibration between Rh02 and ~ -Rh203 in thesurface structure as a function of oxygen partial pressure (ref.

7,9), or from the possible potassium impurity. After reduction in

hydrogen at 673K, all three unsupported rhodium oxides show notonly an exclusively metallic Rh surface but also metallic Rh

structure in the bulk. Additional TPR and XRD experiments

TABLE 2

XPS Analyses of Unsupported Rhodium Oxides

Species Binding Energy, eVRh Od) 0(1s)

3d 5/2 3d 3/2

1- lI. - Rh203 308.6 313.6 530.2

2. (J - Rh203 308.6 313.6 530.2

3. Rh02 308.6 313.6 530.2

4. Reduced Oxides· 307.0 311.8

• After reduction in hydrogen at 673K for one hour

374

demonstrate that the three rhodium oxides can be reduced to

metallic Rh even at 523K in a hydrogen atmosphere.Catalytic activities of monolithic catalysts containing various

unsupported rhodium oxide species are shown in Figure 1. Plotting

total conversions of NO, CO and C3H6 respectively of a simulatedstoichiometric gas composition against the total CO chemisorptioncapacities per liter volume of Rh containing catalysts, all three

rhodium species of interest at varying Rh loadings demonstrated a

one-to-one correlation. This suggests that conversions of NO, CO

and C3H6 respectively over the catalysts containing variousunsupported rhodium oxide particles are structure insensitive. As

observed in our own studies and as seen in reference (ref. 10),

non-interactive Rh species can be readily reduced to metallicrhodium in a stoichiometric gas mixture under the reaction

conditions (673K) as well as in a hydrogen environment. Thus, it

100 D~ 100 r:o ,(- Rh203

A (3- Rh203 50

NO o Rh0 2

0100

c0 c 50'Vi .~Q;>

COQ;c >0 c

U 0U

* *

50 r H6C3Hb

20 40

"'" mol CO/liter Catalyst "'" mol CO/liter Catalyst

Fig. 1 (Left). Activity of monolithic catalysts containing

various bulk rhodium oxides.Fig. 2 (Right). Activity of monolithic catalysts containing bulk

aluminum rhodate.

375

is understandable that the Rh activities of these catalystscontaining various unsupported rhodium oxide particles areindependent of their distinctively different crystalline

structures.

UNSUPPORTED METAL RHODATESPhysical Properties of Unsupported Metal Rhodates

Four metal rhodates were studied, i.e. La and Nd rhodates which

may exist in perovskite structure, and Al and Ce rhodates which donot exist in crystalline composite structure according to the

literature (ref. 11). The rhodium contents in the rhodates asprepared and after reduction at 723K as determined by chemicalanalyses and TGA studies are shown in Table 3. The rhodiumcontents in aluminum rhodate, lanthanum rhodate and neodymiumrhodate are about 15% higher than the theoretical stoichiometric

amount while that in cerium rhodate is very close to thetheoretical value.

TABLE 3

Properties of Various Unsupported Metal Rhodates

Species BET Rh CO Chem ~S.A. Content )lmole /g Rh Rh(s)(m2/g) % (c)

1. Aluminum a) 110.0 55rhodate b) 66.0 73.5 442 0.42(Al/Rh=1/1)

2. Lanthanum a) 0.7 37.8rhodate b) 11.0 43 89 0.85(La/Rh=1/1)

3. Neodymium a) 0.6 36.2rhodate b) 11.5 41.1 87.5 0.84(Nd/Rh=1/1)

4. Cerium a) 0.8 44.5rhodate b) 21.2 54.3 11.2 0.04(Ce/Rh=1/2)

a) As prepared (873K/70 hours/air)b) After reduction treatment at 723K for one hourc) Measured at 298K

BET surface area and CO chemisorption capacity of aluminumrhodate are about a factor of 4 to 6 higher than that of the

corresponding unsupported ~ -Rh203. Assuming A1203 and Rh species

:176

in the aluminum rhodate after reduction at 723K are contributingBET surface area in proportion to their measured contents, it is

estimated that the reduced aluminum rhodate has a surfaceCO(s)/Rh(s) stoichiometry of about 0.42 which is similar to the

value obtained with the corresponding reduced ..,( -Rh203' The

aluminum rhodate and the 0< -Rh203 appear to have similar COadsorption properties on the basis of unit surface rhodiumdensity.

In contrast to aluminum rhodate, rare earth metal rhodatesdemonstrate peculiar properties. They exhibit very small BET

surface areas as prepared and appear initially non-porous instructure. However, twenty fold increase in surface areas are

observed when the rhodates have followed a reduction treatment at

723K. This suggests, that the Rh species present in the rareearth metal rhodates as prepared, are quite different from that inthe aluminum rhodate. The rare earth metal ions appear to

strongly associate with the rhodium oxide resulting in a non-

porous interactive structure. Upon hydrogen exposure at 723K, therhodium species in the rhodates migrate out of the structure to

form clusters and leave behind a porous structure. The COchemisorption capacities of lanthanum rhodate and neodymium

rhodate after reduction are similar, and correspond to an

estimated surface CO(s)/Rh(s) stoichiometry of about 0.85 assuming

BET surface area partitions according to the Rh species and rareearth oxide contents in the rhodates. Although small amounts of

CO (about 15%) may have been irreversibly adsorbed on thelanthanum oxide as determined separately, this number is

significantly higher than that obtained with the aluminum rhodate

or d -Rh203' Unexpectedly, cerium rhodate demonstrates extremelylow CO chemisorption capacity after the reduction treatment even

though its BET surface area is about twice the value seen on theother two rare earth metal rhodates. The behavior of cerium

rhodate is similar to an effect of SMSI (strong metal-supportinteraction) phenomenon (ref. 12, 13).Activities of Metal Rhodates

The results of the catalytic activity of monolithic catalystscontaining various amounts of unsupported aluminum rhodateparticles in a simulated stoichiometric gas mixture are shown in

Figure 2. The activity increases with an increase in Rh loadingand levels off in the mass transfer controlled region. Comparing

the non-mass transfer controlled reaction regime results in Figure

2 with those in Figure 1, the aluminum rhodate appears to have

only about 1/3 the activity of the d -Rh203 for NO, CO and C3H6conversions on the basis of equal accessible surface Rh density,

The less active aluminum rhodate species apparently results from

the interaction of the Rh species with the associated aluminum

oxide in the structure,

Table 4 summarizes the actiVity evaluation results of four

monolithic catalysts containing various metal rhodates of the same

Rh metal loading before and after hydrogen reduction at 723K. It

clearly shows that the hydrogen reduction treatment of the

catalysts does not improve catalytic activity. This is not

surprising since it was previously demonstrated that the active

components in the catalysts can be reduced in a simulated

stoichiometric gas mixture as effectively as in a hydrogen

environment. The lanthanum and neodymium rhodates which exhibit

similar physical properties demonstrate similarly low activities.

It is estimated that the lanthanum or neodymium rhodate has about

1/10 the activity of the ~ -Rh203 on the basis of equally

accessible surface Rh density. The cerium rhodate is practically

inactive and does not show appreciable CO chemisorption capacity,

The results of Table 4 suggest that the reactions of NO, CO and

C3H6 conversion over metal rhodate containing catalysts are

sensitive to their structures. It also reveals that different

TABLE 4

Activities of Various Rhodates Containing Monolithic Catalysts

Catalysts: 8.5 x 10- 2 gIl Rhj 2.5 c m(D) x 7.6 cm(L)

Catalyst/Species Conversion %C3 H6 CO NO x

1- Aluminum Rhodate (a) 70 74 89(b) 71 74 90

2, Lanthanum Rhodate (a) 5 8 11(b) 4 7 9

3. Neodymium Rhodate (a) 4 7 10(b) 3 5 6

4, Cerium Rhodate (a) 2 4 5(b) 0 1 2

a) As preparedb) After reduction in hydrogen at 723K for one hour.

378

activities of aluminum rhodate, lanthanum rhodate (including

neodymium rhodate) and cerium rhodate may result from three

distinctive interaction mechanisms. In order to elucidate the

origins of the interactions, thorough characterization of the

aluminum rhodate, lanthanum rhodate and cerium rhodate samples in

comparison with rJ. -Rh203 was carried out.

Temperature - Programmed Reduction

Figure 3 shows the TPR of three rhodates in comparison with

that of rJ. -Rh203' The three r-h o d a t e s were found to be more

difficul t to reduce than the rJ. -Rh203 indicating that the rhodium

species in the rhodates strongly interact with the added metal

cations. In the three rhodates studied, the difficulty of

reduction follows the order

lanthanum rhodate > cerium rhodate 2 aluminum rhodate

In the lanthanum rhodate sample, it appears there is an

intermediate reduction state around 873K. This TPR result is not

totally consistent with that in a recent publication (ref. 14).

Above 1173K all the metal rhodates are completely reduced to

metallic rhodium and refractive oxides as indicated by TGA and XRD

analyses. Thus, after reduction treatment at 723K for one hour in

7% hydrogen (balance nitrogen), the degree of reduction was

determined to be about 93%, 67%, 90% and 100% completion for

aluminum rhodate, lanthanum rhodate, cerium rhodate and ~-Rh203'

respectively. It confirms that a significant portion of the

A

--------------------

'"c.~

Cen

1J D

273 473 673 873 1073 K

Fig. 3. TRP spectra of various bulk rhodium species in hydrogen

(20K min- 1)

(A) aluminum rhodate (B) lanthanum rhodate (C) cerium rhodate

(D) ,j. -Rh203

379

lanthanum rhodate sample is not totally reduced at 723K.X-ray Diffraction

The rhodates as prepared yield only amorphous structures as

shown in Figure 4. SEM analyses confirm the homogeneous non-

crystalline character of the rhodates. The XRD amorphous

structures of the rho dates strongly suggest the rhodium oxide

lattices in the rhodates have been totally obscured by thepresence of the metal cations. It is interesting all the rare

earth metal rhodates as prepared demonstrate nearly identical XRDamorphous structures which are slightly different from that of the

aluminum rhodate. The extremely low BET surface areas of the rare

earth metal rhodates may be correlated to the XRD results. A

LaRh03 perovskite synthesized at 973K and exhibiting quitedifferent XRD patterns was reported recently by Tascon et al.

(ref. 14). Since the preparation method of the lanthanum rhodate

is different between the two laboratories, it is difficult tocompare the experimental results.

20 40 60 802 P (degree)

AC'§OJ

B .£

c

20 40 60 802 e (degree)

A

B

Fig. 4 (Left). XRD patterns of various rhodates as prepared.(A) aluminum rhodate (B) lanthanum rhodate (C) cerium rhodate

Fig. 5 (Right). XRD patterns of the same rhodates after

reduction treatment in hydrogen at 723K.

1. Rh; 2. La203; 3. Ce02

After reduction treatment at 723 0K for one hour in 7% hydrogen(balance nitrogen), X-ray diffraction patterns of the three

380

rhodates are presented in Figure 5. The reduced aluminum rhodate

shows only peaks of metallic Rh having a particle size about 39A

in diameter. The reduced lanthanum rhodate yields a pattern which

can be identified as a mixture of La203 and metallic rhodium

having a Rh particle size about 23A. However, the broad spectrum

indicates that the reduced lanthanum rhodate is somewhat amorphous

in nature. The reduced cerium rhodate gives a clear pattern of a

mixture of Ce02 and metallic rhodium having crystallite sizes

about 50A in diameter for both the Ce02 and Rh particles. The XRD

as well as TPR results confirm that lanthanum rho date is more

difficult to reduce in the bulk structure.

X-ray Photoelectron Spectroscopic Studies

Photoelectron spectroscopic studies of the three rhodates in

comparison with ~ -Rh203 were performed in order to determine

the Rh surface structures. Figure 6 shows XPS spectra of Rh 3d

electrons of the r ho d a t e e as prepared and c1. -Rh203. The three

rhodates exhibit identical Rh XPS spectra with a 3d 5/2 peak at

308.2 eV and a 3d 3/2 peak at 313.0 eV. These spectra indicate

the binding energies of the Rh 3d electrons of the rhodates shift

313.0 308.2Rh[3dl

316 312 308 304

Binding Energy, eV

Fig. 6. XPS Rh(3d) spectra of various r ho d a t e s as prepared a n d

~ - Rh203


(D) c1.-Rh203

381

to a lower number by 0.4 eV for the 3d 5/2 peak and 0.6 eV for the3d 3/2 peak which results in a smaller energy spacing between the

two 3d peaks in comparison to the Rh +3 species of ~ -Rh203' This

suggests the bonding between oxygen and rhodium species in thebase metal rhodate being more covalent in nature with respect to

those of ~ -Rh203'Figure 7(a), 7(b) and 7(c) show 0(1s) XPS spectra for the

aluminum rhodate, lanthanum rhodate and cerium rhodate

respectively. The spectrum of aluminum rhodate indicates thebinding energies broaden toward the lower side while a peak shifts

to a higher number in comparison with a reference alumina. Thespectra of lanthanum rhodate and cerium rhodate appear to be more

complicated and deviate appreciably from those of 01.. -Rh203 (onlysingle peak at 530.2 eV) and the corresponding base metal oxides.Although contribution of each oxygen species in the rhodate to the

O[ls]AI [2pl

70

La[3dJ

c c"'" "§" OJOJ 1; 858 850 842 834 8261;

Binding Energy, eV Binding Energy, eV

Fig. 7 (Left). XPS 0{1s) spectra: base metal rhodates as prepared

(solid line) and reference base metal oxides

(dashed line).Fig. 8 (Right). XPS spectra of base metal: base metal rhodates as

prepared (solid line) and reference base metal

oxides (dashed line).

382

oxygen XPS spectra is difficult to measure, there are at least two

distinguishable peaks of the oxygen binding energies that can be

resolved and related to the interactive structures. One peak is

at 531 eV and the other is around 529 eV which appears on the

shoulder of the spectrum in the case of lanthanum rhodate and

aluminum rhodate. The higher energy peak results from the oxygen

associated with the rhodium. The bonding between oxygen, rhodium

and base metal in the rhodate has more covalent character than

that found in d-Rh203' The lower energy peak may be attributed

to the oxygen associated with the base metals; and it suggests

that the bonding between oxygen and base metals in the rhodates is

more ionic in character with respect to the corresponding base

metal oxides. Of the three rhodates studied, one can see the

interaction most dominant in the cerium rhodate.

Figure 8(a) shows the Al(2p) XPS spectrum of the aluminum

rhodate in comparison with a reference alumina. The binding

energies of the Al(2p) peak of the two samples are identical.

However, the spectrum of the aluminum rhodate appears to broaden

toward the higher binding energy side indicating a disturbance of

electron distribution around the nearby oxygen neighbors which

correlates with the broadening of 0{1s) XPS spectrum observed.

Figure B(b) and 8(c) present the La(3d) and Ce(3d) XPS spectra for

lanthanum rhodate and cerium rhodate respectively. They

demonstrate 3d electron binding energies of the rare earth metal

ions shift to a higher number in comparison with reference La203

and Ce02' The binding energy shift of Ce 3d electrons appear to

be most pronounced. The raising of the binding energy shift of La

or Ce 3d electrons indicate a partial electron transfer from the

La or Ce cations toward nearby oxygen neighbors whioh are not

bridging between the rhodium and the rare earth metals. These

behaviors are consistent with the conclusion learned from 0(1s)

XPS results. It also suggests the interaction characteristics in

the rare earth metal rhodates are different from those in aluminum

rhodate. TheJ also correlate with the different physical

properties previously seen. Although a complete picture as to how

the interactive structures are formed in the rhodates is not

certain, it is speculated that microsphere particles of rhodium

hydroxide may have been produced during the sample preparation.

These particles then were surr~unded by base metal hydroxide

layers. Upon drying and calcining, interactive oxygen bridging

between the rhodium and base metals over the surface of

;J83

microsphere results in the formation of interactive structures.Since the interactive structures alter the lattice parameters ofthe rhodates, it is understandable why the rhodates exhibit no XRDcrystalline structures.

After reduction at 723K for one hour in 7% hydrogen (balance

nitrogen) XPS spectra of Rh (3d) electrons of the rhodates aregiven in Figure 9. All rhodate samples show a major metallic Rh

peak. However, significant portions of surface Rh in aluminumrhodate and cerium rhodate apparently remain in the interactivestate which the 308.2 eV peak on the shoulder of 3d 5/2 spectrum

identifies. The behavior of surface Rh in lanthanum rhodateis comparatively close to that of metallic Rh. XPS spectra of Al

0(15)

316 312 308 304

Binding Energy, eV

311.8 307.0

Rh[3d]

Binding Energy, eV

Fig. 9 (Left). XPS Rh(3d) spectra of rhodium species afterreduction treatment in hydrogen at 723K.


(D) rJ. - Rh 2 0 3Fig. 10 (Right). XPS O(1s} spectra: base metal rhodates after

reduction treatment in hydrogen at 723K (solidline) and reference base metal oxides (dashed

line)

384

(2p), La (3d) and Ce (3d) of the rhodates after the reduction

treatment are similar to patterns from A1203' La203 and Ce02

respectively. Thus, it is quite likely the interactive phase in

the aluminum rhodate and cerium rhodate after the reduction

treatment exist in the vicinity of the Rh particles.

Figure 10 shows the 0(1s) XPS spectra of the rhodates after

reduction. In the reduced aluminum rhodate, only small amounts of

interactive oxygen species can be identified from the spectra.

These oxygen species are apparently located at the interface

between the rhodium particles and the alumina particles. Rh

particles under the influence of such interaction naturally

produce inferior activity when compared to non-interactive Rh

particles of similar metal surface area. 0(1s) spectra of the

reduced cerium rhodate sample reveal even higher concentrations of

interactive oxygen species remain at the interface between rhodium

and cerium oxide. It is very likely that significant portions of

the Rh particle surfaces in the reduced cerium rhodate are in

contact with interactive oxygen species preventing adsorption of

reactive species by Rh. Under such circumstances, most of the Rh

surfaces in the cerium rhodate are not accessible for catalyzing

the redox reactions resulting in extremely low activity. The

reduced lanthanum rhodate gives similar 0(1s) XPS spectra as La203

indicating very little interaction between Rh and La203 on the

reduced rhodate surface. However, examining the ratio of base

metal to rhOdium in the rhodate by XPS intensity measurement

before and after reduction treatment as indicated in Table 5, the

TABLE 5

XPS Estimated Metal Concentration Ratio

Species

AluminumRhodate

CeriumRhodate

LanthanumRhodate

AtomicRatio

Al/Rh

Ce/Rh

La/Rh

As Prepared

1.2

0.6

1.3

After Reductionat 723K

for one hour

1. 25

0.5

1.9

385

surface of reduced lanthanum rhodate appears to be enriched in

lanthanum while the surface composition in aluminum rhodate and

cerium rhodate remain virtually unchanged. Apparently, t h e

observed lanthanum enrichment on the surface of the reduced

lanthanum rhodate and the incomplete rhodium reduction in the bulk

are both effects rendering it less active.

CONCLUSIONConversions of NO, CO and C3H6 are structure insensitive over a

catalyst containing various unsupported rhodium oxide particles.

This is understandable because the original crystalline structures

of the oxides are short-lived under reaction conditions in a gas

composition near stoichiometry at 673K where the oxides are easily

reduced to metallic Rh.

Conversely over the metal rhodate containing catalysts, the NO,

CO and C3H6 conversions appear to be sensitive to their

structures. Unlike the easily reducible rhodium oxides, the

rhodates are harder to reduce under reaction conditions as well as

in a hydrogen atmosphere. XPS spectra reveal that interactive

structures exist between the rhodium ion and the oxygen anion

associated with the metal cations of the rhodates. Even upon

reduction at 723K, a significant portion of surface Rh in aluminum

and cerium rhodates remains in the interactive state while the

interactive structure is apparently destroyed at the surface of

lanthanum rhodate. In the reduced cerium r ho d a t e sample, the

interactive structure dominates the adsorption properties of

surface rhodium resulting in extremely low activity. In the

reduced aluminum rhodate containing catalyst, it is possible the

interactive species present at the interface between the rhodium

and aluminum oxide reduces the catalytic activity. In reduced

lanthanum rhodate species, it is believed that lanthanum

enrichment on the surface and incomplete reduction in the bulk

render the rhodium less active.

REFERENCES

1. Rives-Arnau, V., and Munuera, G., Appl. Surf. ScL, 6(1980),122.

2. Pande, N.K., and Bell, A.T., J. c a t a l , , 98(1986), 7.3. Yao, H.C., Japar, S. and Shelef, M., J. Catal., 50 (1977),

407.4. Yao, H.C., Stepien, H.K., and Gandhi, H.S., J. Catal., 61

(1980), 547.5. Wan, C.Z., and Dettling, J.C., SAE Paper No. 860566 (1986).

386

6. Freel, J., J. Catal., 25 (1972), 139.7. Muller, 0., and Roy, R., J. Less - Common Metals, 16 (1968),

129.8. Yates, D.J.C., and Sinfelt, J.H., J. Catal., 8 (1967),348.9. Bayer, G., and Wiedemann, H.G., Thermochim. Acta, 15 (1976),

213.10. Oh, S.H., and Carpenter, J.E., J. Catal., 80 (1983), 472.11. Wold, A., Arnott, R.J. and Croft, W.J., Inorg. Chem. 2

(1963), 972.12. Tauster, S.J. and Fung, S.C., J. Catal., 55 (1978), 29.13. Meriaudeau, P., Dutel, J.F., Dufaux, M., and Naccache, C.,

"Metal-Support and Metal-Additive Effects in Catalysis",Elsevier Scientific Publishing Co., Amsterdam, 1982, P. 95.

14. Tascon, J.M.D., Olivan, A.M.O., Gonzalez Tejuca, L. and Bell,A.T., J. Phys. Chem., 90 (1986), 791.


DEVELOPMENT OF A COPPER CHROMITE CATALYST FOR CARBON MONOXIDEAUTOMOBILE EMISSION CONTROL

387

J. LAINE, A. ALBORNOZ, J. BRITO, O. CARIAS, G. CASTRO, F. SEVERINOand D. VALERALaboratorio de Catalisis Heterogenea, Centro de QUlmica, InstitutoVenezolano de Investigaciones Cientlficas, I.V.I.C., Apdo. 21827,Caracas 1020-A, Venezuela

ABSTRACTCopper-chromium catalysts employed for CO oxidation were found

to be affected by composition and pretreatment parameters. CuCr204was more active than CuO only if prereduction was carried out andif metal concentration on alumina support was larger than 12 wt%.The presence of Cr with Cu in the oxide limited the extent of cata-lyst reduction leading also to less deactivation as compared to Cuon alumina. The presence of Cr also decreased an activity inhibi-tion effected by water. A supported Cu-Cr catalyst used in an auto-mobile ran with leaded petrol was deactivated by lead deposition.Deposits were mainly lead sulphate located on pellet periphery. Al-so, lead was preferentially distributed on the alumina instead ofon the active metal-rich zones of the catalysts.

INTRODUCTIONUrban atmospheric pollution in countries without strict regula-

tions for automobile exhaust control is currently an important con-sideration in relation to possible threats to health. Many growingcities like Caracas, have increasingly high air pollution levels,CO being one of the most important contaminants.

Among non-precious metals, copper-chromium combinations seem tobe the most effective catalysts for CO exhaust elimination. For ex-ample, monoliths and pellets impregnated with copper-chromite havebeen reported to have activities near those of precious metal-basedauto-emission control catalysts (ref. 1,2). This comparison has al-so been extended to the oxidation of CO with NO (ref. 3,4), anotherimportant auto-emission pollutant.

The present communication reports results regarding CO oxidationover copper-chromium catalysts. The work was started studying un-supported catalysts and then supporting on alumina, looking foroptima catalyst composition and pretreatment. Afterwards, the

388

changes arlslng in a copper-chromium catalyst as a result of usingin an automobile exhaust were examined.

EXPERIMENTALThe catalysts were prepared as described in detail elsewhere

(ref. 5,6). Briefly, unsupported catalysts of various Cu/Cr ratioswere made by re-crystallization of mixed copper and chromium ni-trate hot solutions followed by drying and calcining, and supportedcatalysts by impregnating y-A1 Z0 3 powder (100 mZ/g) with similarsolutions also followed by drying and calcining. Three series ofsupported catalysts were prepared: CuO, CuCrZ04 and Cr Z0 3 with met-al concentrations ranging between Z and 30wt%. The series of chromiacatalysts did not present any detectable activity under the experi-mental conditions employed in this work, thus, we report here re-sults concerning only the Cu and Cu-Cr catalysts.

For the automobile test, a copper-chromite catalyst was preparedsimilarly by impregnation of y-A1 Z0 3 tablets (4.6 mm diameter x 4.6mm length).

Catalyst activities for CO oxidation were measured in a continu-ous flow system. Other characterization techniques employed were:X-ray diffraction, Temperature Programed Reduction, Auger spectros-copy, and Scanning Electron Microscopy.

The automobile test was carried out using a 3.8 liter-V6 enginewith a Z liter exhaust catalytic converter. A non-standard Z,150Km urban driving was carried out in order to compare fresh and usedcatalysts.

RESULTS AND DISCUSSIONThe first part of this work was devoted to an optimization study

of the composition and pretreatment parameters of unsupported andsupported copper and copper-chromium catalysts for the reaction ofoxidation of CO.

The activity of unsupported CuCrZ04 was found to be increasedby pretreating the catalyst with pure CO (ref. 5). Also confirmingearlier findings (ref. 7,8) CuCrZ04 was found to be more activethan CuO for CO oxidation. However, we found (ref. 5) that thiswas the case only if catalyst prereduction was carried out. OptimumCu/Cr ratio found in unsupported catalysts was that correspondingto the stoichiometry of CuCrZ04 (ref. 5). Auger spectra (ref. 9)demonstrated that the activity enhancement achieved by prereducing

389

100--··....- ..- -----

80

~c:0lJ) ,,;~--- ...... ""0Q) /

> ,c: I0 40

,u ,

r,r0

U ~',r

rI

II -_ ... -- ---_.I

I~

// ........-.

0 ~~

0 10 20 30

Metallic Concentration (%)

Figure 1. Effect of metal concentration of supported catalysts onactivity at 400°C .• , CuO/A120s; 0, CuCr204/A1 20s . Catalyst pre-treatment: CO reduction at 400°C for 3h. Sample: 5 mg. Reaction at-mosphere: 5 vol% CO in 540 ems/min air;---, dry air;---,15% H20.

CuCrZ04 was due to an enrichment in copper concentration at the cat-alyst surface, confirming that copper is the active species.

Supporting CuO or CUCr Z04 on alumina produced optima for higheractivities located at different metal concentrations (Fig. 1). CuO-supported catalysts were found to be more active than CUCr Z04-sup-ported catalysts when metal concentration was smaller than approxi-mately 1Zwt%. At larger concentrations supported CuCrZ04 was moreactive. Figure 1 also shows that the oxidation of CO is inhibitedby the presence of water in the gaseous reaction stream, that inhi-bition being significantly more pronounced for CuO than for CUCr Z04.

Figure Z shows the behavior of the activity with time on streamof low (5wt%) and high (30wt%) metal concentration Cu and Cu-Crcatalysts. At low metal concentrations, it can be seen that coppercatalyst is affected by a pronounced deactivation whereas the Cu-Crcatalyst is not. Accordingly, it can be suggested that active cop-per sites in CuCrZ04 are less prone to deactivation, i.e., morestable, than active copper on alumina. In the case of the high con-

:390

80

~60 ......... • .'" • * • ....

0c0 40(J')...

::;;~Q)20>c

0u0u

60CuC'20.1 AI2O,

~

40

eM ......... -..20 _...-__...----...............

o 2 3 4 5Time (h)

6

Figure 2. Activity behavior at 200°C of supported catalysts. Metalconcentration: o e ; 30wt%. 6 .. ; 5wt%. Catalyst pretreatment:open symbols, CO prereduction at 300°C for 3h. Filled symbols, noprereduction. Sample: 10 mg of supported metals. Reaction atmo-sphere: 15% CO in 150 cm 3/min air.

centration Cu samples (i.e. 30wt%), the activity overcame an induc-tion period (Fig. 2). This is probably due to excessive reductionto metallic copper by the CO pretreatment, as confirmed by XRD(ref. 6). Accordingly, copper reoxidation is not easy under the ex-perimental conditions employed in this work.

Figure 3 shows the temperature programmed reduction of supportedsamples, weighted to constant Cu content. It is seen that chromiumsignificantly affects copper reducibility, i.e., Cu supported cata-lysts were more reduced as metal concentration increased, while insupported Cu-Cr reducibility decreased as metal concentration in-creased. Also, the first peak that appears in Cu~Cr samples (about250°C) has an intensity which almost remains constant with varyingmetal concentration. Therefore, that peak could be attributed to asurface CUA1 Z04 phase, as the amount of this phase is probably asaturation value that mainly depends on the alumina surface areaavailable (ref. 10). The second peak can be assigned to CUCr 204phase which is more difficult to reduce as its concentration in-

391

30%

20

10

:::J0

cQ0.. Cu 0 1 AlzO, Cu Crz 041 AlzO,E:::J(f)

C0U

0 5U

100 200 300 400 100 200 300 400Temperature (OC)

Figure 3. Temperature programmed reduction of supported CuO andCuCr Z04 of various concentrations. Reducing atmosphere: 25 vol% COin He. Sample: enough catalyst to provide 25 mg of Cu.

creases, contrary to what was observed above for the Cu catalysts.Accordingly, these results confirm that the catalyst is protectedby Cr against excessive reduction.

The second part of this work consisted in testing a copper-chro-mite catalytic converter in an automobile ran with leaded gasoline(about 0.6 g Pb/l ).

Table 1 shows catalyst composition before and after using for2,150 Km. The amount of lead deposited in the catalytic converteraccounted for about 30% of total lead emitted during driving. XRDanalyses demonstrated that lead on catalyst is mainly as lead sul-phate (ref.11). Also, microscopy examination showed (ref.11) thatlead deposits were almost entirely located at the catalyst pelletperiphery penetrating only about 0.3 mm on average of the totalpellet radius (2.3mm)

Comparing activities of fresh and used catalysts (Table 2), itcan be noticed that the catalyst was deactivated during the run,

392

TABLECharacteristics of catalysts before and after use in automobileexhaust for 2,150 Km.

CatalystComposition (wt%)

Cr Cu Pb BrSurface Area

2m /g

FreshUsed*

3.03.4

2.72.9

o7.3

o0.5

210170

* Used catalyst also contained certain amounts of: S, Fe and Cl.Used catalyst pellets weighted about 8% more than fresh.

TABLE 2Relative activities for CO oxidation of powdered catalyst. Centraland periphery zones were knife-separated before grinding for activ-ity measurement.

Catalyst

Fresh (whole)Used (whole)Used (centre)Used (periphery)

Activity (%)

100788852

that deactivation being significantly more pronounced in the leaded-periphery zone than in the non-leaded-central zone. This indicatesthat lead is a major catalyst poison, as compared with other possi-ble catalyst deactivating elements as for example: sulfur, whoseconcentration is about 0.1% in the gasoline employed.

Energy Dispersion Analysis of X-rays performed during scanningmicroscopy examination of used catalysts, suggested that lead waspreferentially deposited on the alumina support and to a signifi-cant lesser extent on the copper-chromium-rich zones of the cata-lyst (Fig. 4). This suggests that non-alumina-supported copper-chromium (e.g. all-metal catalysts) might be a better lead-tolerantcatalyst.

Work has also been undertaken with lead filtration devices (ref.12), as a part of the present development that looks for betterperformance of the catalyst.

393

6 ••

<l: 4<,.0 •c, •

2 • •• • •

0 02 0.4 0.6CutCr IAI

Figure 4. Relative point concentration of Pb and Cu+Cr by microsco-py analysis of catalyst used in automobile exhaust ran with leadedgasoline.

ACKNOWLEDGEMENTSThe authors thank the Venezuelan Consejo Nacional de Investiga-

ciones Cientfficas y Tecno16gicas (Grant Sl-1184), Ford Motor Co.of Venezuela, Octel Co. and Degussa AG for their valuable assis-tances.

REFERENCES1. G.J. Barnes, Adv. Chem. Series, 143 (1975) 72.2. J.T. Kummer, Adv. Chem. Series, 143 (1975) 178.3. T. Ohara, The Catalytic Chemistry of Nitrogen Oxides, R.L.

Klimisch, J.G. Larson (Ed s . ) , Plenum Press, N.Y., 1975, p. 191.4. R. Hierl, H. Knozinger and H.P. Urbach, J. Catal., 69 (1981)

475.5. F. Severino and J. Laine, Ind. Eng. Chem. Prod. Res. & Dev.,

22 (1983) 396.6. F. Severino, J. Brito, 0 Carfas and J. Laine, J. Catal, in press.7. M. Shelef, K. Otto and H. Gandhi, J. Catal., 12 (1968) 361.8. J.F. Roth and R.C. Doerr, Ind. Eng. Chem., 53 (1961) 293.9. G. Castro, F. Severino, J. Laine, in preparation.10. R.M. Friedman, J.J. Freeman, F.W. Lytle, J. Catal., 55 (1978)

10.11. J. Laine, F. Severino, A. Albornoz, O. Ca r i as , B. Griffe, E.

Marcano and F. Martf, Acta Cientif. Venezolana, in press.12. J. Laine, F. Severino and A. Albornoz, Acta Cientif. Venezolana,

in press.


DEVELOPMENT OF NON-NOBLE METAL CATALYSTS FOR THEPURIFICATION OF AUTOMOTIVE EXHAUST GAS

by Lin Peiyan, Wang Min, Shan Shaochun, Huang Minmin,Rong Jingfang, Yu Shomin, Yang Hengxiang and Wang Qiwu

Modem Chemistry Dept., Science and Technology of China Univ., Hefei(The People's Republic of China)

ABSTRACT

Non-noble metal catalysts A and B were developed in view of the richresources of transition metal oxide and rare earth oxides as well as low cost in China.The catalysts were characterized by high effectiveness for the conversion of CO andHC, high crush strength and high stability to prevent poisoning by S02 and Pb. Thecatalytic converter can also be used in place of exhaust muffler while the consumptionof fuel does not increase.

INTRODUCTION

Since the pollutants from automobiles have greatly increased with the rapidincrease of automobiles in recent years in China, the Ministry of EnvironmentalProtection of China decreed the first regulation controlling pollutants fromautomobiles in 1983. Successes have been scored in some large cities in theimplementation of the regulation in their battle against pollution. The catalystscurrently used for this purpose are mainly those containing noble metals of Pt, Pd, Rhetc. But it would be valuable to develop non-noble metal oxide catalysts in view of therich resources and low cost in China.

The perovskite-type catalysts (ref.1), other non noble metal complex oxidescatalysts (ref.2), and mixed metal oxides catalysts (ref.3) have been studied in ourlaboratory. The various preparation techniques of catalysts (refA and 5), theadsorption and thermal desorption of CO, C2H6 and 02 (ref6 and 7), the reactivityof lattice oxygen (ref.S), the electric conductance of catalysts (ref.9), the pattern ofpoisoning by S02 (ref.10 and 11), the improvement of crushing strength of support(ref.12) and determination of the activated surface of complex metal oxides (ref13)have also been reported.

Based on the above-mentioned work, two kinds of mixed metal oxidecatalysts A and B mainly containing copper oxide as a major compound on y-A1203have been developed. They were made by a special impregnation procedure, dried,calcined and activated. The catalysts were made into a spherical shape (05-7) for

395

396

practical use. The physical characters are shown in Table 1.

i-~_··_------------~~ble~-----_·_----- --

I Physical Characters of Catalysts

Crushing strength(kg/gran)

BET surface area(m2/g)

Bulk density(glml)

Pore volume*(ml/g)

* Benzene replacement method.

Support

19.0

152.0

0.73

0.45

Catalyst A

20.0

90.7

0.83

0.39

Catalyst B

25.0

82.0

0.90

0.35 _J

EVALUATION OF CATALYTIC ACTIVITY

Micro-reactor tests

Catalytic activity experiments were carried out by a micro-reactor.Reactants and products were analyzed by chromatography. Some of the results aregiven in Table 2 and Table 3.

Table 2Percent Conversion of CO on Catalyst A

(0.2 g of sample, 40-60 mesh, SV ::: 4500h-1,

reactants: CO 0.5%,023-5% and balance with NV

lI

No activation treatment

Activation treatment

Reaction Temperature (0C)250 307 380

Percent CO oxidized (%)37 83 96

Reaction Temperature (0C)210 250 300


J

Table 3Effect of La203 or Ce02 in support of Catalyst B

(Reaction conditions are the same as those in Table 2)


Reaction temperature (OC)248.5 292.0 340.5233.0 278.0 318.0168.0 242.0 300.5

I

397

---------------_._--------_._-----I

III

iIII

ITable 3 indicates that the y-A1203 containing La203 or CeOZ could

improve catalytic activity.

75 ml reactor tests

Catalytic activity experiments were carried out by a 75 ml flow reactorsystem, the results are shown in Table 4.

Table 4Conversion of CO for Granular Catalyst

(75 ml of granular catalysts, 0 5-7, SV=4500h-1,reactants: CO 1.1%, O2 6-8% and balance with N2).

Catalyst A

Catalyst B

Reaction Temperature (0C)233.5 288.5 343.0 384.0

Percent CO oxidized (%)45.1 67.8 85.9 95.1

Reaction Temperature (0C)228.0 280.0 323.0 372.0

Percent CO oxidized (%)49.1 64.0 75.3 85.4

Table 5 indicates that the conversion of CO remains as high as 69.3% at436°C after Z hours on catalyst B containing Ce02 when the reactant is CO only(1.1%) diluted in N2.

The results suggest the mobility of lattice oxygen was rather high. Thelattice oxygen took part in oxidation even though there was no oxygen in the gaseous

398

phase during this period. If 6-8% 02 was again added into the mixed reactants, theconversion of CO could be restored to 92% after adding 02 for 0.5 hr at 400°C. Thisproperty is important for practical purposes.

Table 5Conversion of CO in the absence of O2 on Catalyst B

(reaction conditions are the same as those in Table 4)

Reaction Time (min) 0.0Percent CO oxidized (%) 72.3Reaction Temperature (°C)416.0

5.0 15.0 25.0 45.0 70.0 115.0 125.084.4 80.7 78.6 75.1 72.6 69.9 69.3

406.0 412.0 426.0 431.0 434.0 436.0 436.0

Purification of exhaust from gasoline engine

Purification of exhaust from HFG 427A gasoline engine without additionalsecondary air was tested with 6.25 kg of catalyst A. The catalyst was packed in astainless steel converter which was connected to the exhaust outlet. The contents ofCO and HC were measured by an RI503 TH-S CO/HC infrared gas analyzer (Japan).Simulated tests of different rotational speeds of the engine were carried out asindicated in Table 6.

Table 6Purification of CO and HC in exhaust

Rotational speed CO Heof engine Before Percent CO Before Percent He(twins/min) reaction removed reaction removed

(%) (%) (ppm) (%)

600 5.8 95.5 3800 93.7820 1.9 83.7 500 84.0

1230 0.6 88.4 510 80.41640 0.23 78.3 740 89.22050 0.33 85.1 320 97.02460 0.41 88.4 350 94.32850 0.44 83.0 530 98.1

In addition. the bifunction of the catalytic converter was found. Thecatalytic converter could better decrease noise pollution compared with the exhaustmuffler. Therefore. it would be used to replace the exhaust muffler.

STRENGTH AND THERMAL STABILITY

The support was formed by extrusion, cold roll forming, drying andcalcination. The formulation (containing binder, material of pore making, materialfor improving strength and structure stabilizator etc.) and the preparation procedureof support were studied by conducting about one hundred experiments. The X-raydiffraction data of both supports, adding rare earth oxide and pure y-A1203 aftercalcination are given in Table 7.

399

~ ...__ . ------ --- .~--_ .... ---... ----------c

Table 7X-ray diffraction data of support and pure y-A120 3

Support Support y-Alz0 3 y-Alz0 3 e-Alz0 3 LaAlz0 3(calcined (calcined (calcined (from JCPOS card)at 900°C at 1 050°C at 900°C

d IJI d IJI d IJI d IJI d IJI d 1II

4.60 20 4.42 20 4.51 30 4.56 40 4.50 602.39 80 2.82 80 2.83 70 2.39 80 2.85 802.28 70 2.72 90 2.72 80 2.28 50 2.72 801.98 100 2.42 60 2.44 70 1.977 100 2.43 801.51 30 2.33 40 2.29 30 1.520 30 2.31 601.40 100 2.27 60 2.20 30 1.390 100 2.22 60

2.00 70 2.00 60 2.01 801.56 50 1.53 20 1.54 601.41 60 1.40 80 1.40 601.39 100 1.39 100 1.39 100 i3.79 50 3.797 80 I

2.675 100 2.657

lJ2.196 30 2.1881.897 60 1.896 801.698 20 1.696 601.598 40 1.542 801.360 30 1.342 50

d : Spacings ofthe plane nets III: Relative intensity ofdiffraction.

It was found that the temperature of the crystalline phase change fromy-Al203 to e-A1203 increased by about lOO°C-I50°C when adding rare earth oxidesas stabilizator. Even calcined to 1050°C, the a-A1203 phase did not appear.

The effects of rare earth oxides could be concluded from Table 3, 5 and 7 :(a) to improve catalytic activity; (b) to improve mobility of lattice oxygen in catalystB and its activity for CO and HC conversion in a lower ratio of air/fuel; (c) toimprove thermal stability of support to resist crystalline phase change to a phase andmaintain crushing strength and surface area.

400

HIGH STABILITY TO RESIST S02 POISONING

In general, the gasoline in China contains HZS, SOz and other organicsulphides. There is a minimum of 30-40 ppm S02 and other sulphides in the exhaust.It is well known that the sulphides possess strong toxicity for basic metal oxidecatalysts (ref. 14). Therefore, a study on catalytic resistance to S02 poisoning isimportant. The relation between catalytic activity and reaction time when the reactantcontains S02 (50 ppm) was determined. It was found that the conversion of COremained 81.6% for catalyst A after 112 hours and 76.1 % for catalyst B after 64hours in micro-reactor conditions. However S02 poison was reversible forcatalyst A. When S02 concentration decreased and temperature increased, theactivity of the poisoned catalyst could be restored. The data are shown in Table 8 andTable 9.

-----------------------

Reaction of CO(%)

Table 8Relation between activity of poisoned catalyst A

and change of reaction temperature(In micro-reactor, 80250 ppm)

Time (min)

o306090

-120540660840

1800

Reaction temperature(oq

400450450450450450400400400

82.699.899.8

100.0100.0100.096.489.483.2

------------1

I

I

I

401

Table 9Relation between activity of catalyst A

and content of S02 in reactants(Reaction temperature: 400°C in micro-reactor)

888580

Percentage conversion of CO50 ppm 25 ppm 10 ppm(S02) (S02) (S02)

100

Initial percentageI. _<0"""'00 of CO

APPLIED FJELD TEST OF CATALYST A

A field test of 70000 km was made on an EQ-140 type bus with a 6-cylinder gasoline engine (using N° 75 leaded gasoline which contained Pb 0.8-1.0 g/l.) The conversions of CO and HC were measured continually during runningor idle states. The catalytic converter packing (8 kg of catalyst A) was placed in placeof the exhaust muffler. The data obtained are listed in Table 10.

Table 10The change of activity of catalyst A

in the 70 000 km field test ~I

Accumulativetotal km

o104751127804

150811729025175370335105070604

Working state

idleidleidleidlemnning*idleidleidleidlemnning*

Average percentageCO

90.792.780.190.181.881.982.084.888.575.6

Remove ofHC84.688.688.877.081.881.285.677.576.066.8

*Average velocityofvariousvehiclespeedsat 30 km, 40 km, 50 kmand 60 kmper hour.

The average consumption of fuel in 70 000 km process is indicated in Table11. It was proved that the consumption of fuel did not increase while the catalyticconverter was used in place of the exhaust muffler.

The compositions of fresh catalyst A and of catalyst A used after 70 000 kmwere analysed by means of an X-ray fluorescence spectrometer. It was identified thatthe relative content of main components did not change after 70 000 km. Catalyst Aafter the 70000 km test was found to contain 0.4% by weight of Pb. It should havebeen introduced by leaded gasoline.

402

The evaluation of catalytic activity for fresh catalyst A and catalyst A usedfor the 70000 km test were also measured in Table 12.

Table 11Average consumption of fuel in field test

(EQ-140 type bus, 660 type gasoline engine)

Exhaustmuffler

CatalyticConverter

Bus Number

12345678910

11

Consumption offuel for 100 km

(liters)

20.3122.6523.5924.6222.0820.1323.6323.0421.3320.87

20.90

Consumption offuel for 1,000persons per km (liters)

5.045.555.806.285.685.045.785.945.155.77

5.44

Table 12The activity of fresh catalyst A and used catalyst A

after 70 000 km(75 ml reactor, reaction conditions are the same as those in Table 4)

Sample

Fresh catalyst A

Catalyst A used

Reaction temperature(OC)

246.0330.0426.0

271.5331.5421.5

Percent CO oxidized(%)

61.289.596.3

36.663.685.0

Table 12 indicates that the conversion of CO could remain 85% at 421,SOCwhen catalyst A contained 0.4% Ph on the surface after the 70000 km field test. Butthe conversion of CO decreased considerably at low temperatures.

REFERENCES

403

1. Wang Qiwu, Rong Jingfang, Lin Peiyan and Shan Shaochun, KEXUE TONGBAO(Science), 25 (1980), 495-497.

2. Yang Hengxiang, Huang Minmin, Wang Qiwu, Lin Peiyan and Rong Jingfang,HUAN JING HUA XUE (Environmental Chemistry), 2 (1983), 17-22.

3. Wang Min, Yu Shomin, Huang Minmin, Shan Shaochun, Rong Jingfang, YangHengxiang and Lin Peiyan, CUlHUA XUEBAO (1. Catal), 5 (1984), 300-301.

4. Wang Qiwu, Shan Shaochun and Xiao Yujie, ZIRANZA ZHI (Nature), 2 (1979),659.

5. RongJinfang, SHIYOUHUAGONG (Petrochemical Technology), 10 (1981), 310-313.

6. Lin Peiyan and Fu Yilu, SHIYOU HUAGONG (petrochemical Technology), 8(1980),822-828.

7. Fu Yilu and Lin Peiyan, CUlHUA XUEBAO (J. Catal), 3 (1982),205-211.8. Lin Peiyan and Yu Min, HUAXUE TONGBAO (Chemistry), 3 (1985),13-14.9. Lin Peiyan, Yu Min, Shi Wenjun and Wei Zhenyu, J. China Univ. of Sci. & Tech.,

15 (1985) 426-433.10. Huang Minmin, Yang Hengxiang, Wang Qiwu, Lin Peiyan and Rong Jinfang,

CUlHUA XUEBAO (1.Catal), 3 (1982) 277-282.11. Wang Minmin, Yang Hengxiang, Wang Qiwu, Lin Peiyan and Rong Jinfang,

CUlHUA XUEBAO (1.Catal), 4 (1983), 312-314.12. Shan Shaochun and Huang Minmin, Xl'Tl.I (Rare earth), 1 (1984),64-67.13. Lin Peiyan and Fu Yilu, J. China Univ. of Sci. & Tech., 13 (1983),68-73.14. Y.f. Yu Yao, 1. Catal, 28 (1973),124; 28 (1973), 139; 33 (1974),108; 36 (1975),

266; 39 (1975), 104.

A. Crucq and A. Frennet (Editors), Catalysis and Automotive Pollution Control© 1987 Elsevier Science Publishers B.V., Amsterdam _. Printed in The Netherlands

IMPROVING THE S02 RESISTANCE OF PEROVSKITE TYPEOXIDATION CATALYSTby LI WAN* and HUANG QINGl, ZHANG W AN-JING2,LIN BING-XIUNG2, and LU GUANG-LIE2.

1Dept. Chemistry & Environmental Eng., Beijing Polytechnic University, Beijing.zDept. Chemistry, Peking University, Beijing, P.R. China.

ABSTRACT

A properly designed perovskite-type base metal catalyst Lao.6SrOACoi_xMx03 wasfound to have certain SOz resistant properties. Over this catalyst at 56000/hr COcould be oxidized at 100% in the presence of 20, 200, and 400 ppm of SOZ at 2000

,

300°, and 400°C, respectively. Its deactivation by 50 and 400 ppm of SOZ at 200° and300°C respectively was reversible. As revealed by IR spectroscopy, SOZ adsorbed onB site ions deactivate the catalyst by blocking the surface sites that are necessary forCO adsorption and lattice oxygen replenishment. A comparison was made withanother Lao.6SrOACo03, which was found to be more susceptible to SOz poisoning.

INTRODUCTION

To make a perovskite-type base metal oxide capable of being an alternativeto noble metals catalyst for automotive pollution control, its SOZ resistance must beimproved. Some papers reported that perovskite type oxides such as LaCo03'LaMn03, Lao.sSrO.SMn03' and Lao.7Pbo.3Mn03 deactivated irreversibly by severaltens ppm of SOZ at 500°C or lower, and that they become sulfur resistant only whenthe sample contains several tens ppm of Pt (ref. 1-5). One of the authors of this paperhas prepared a Pd-containing perovskite which is highly resistant to SOz (ref.6). Inthis paper studies on a perovskite-type base metal catalyst without any noble metal thatcan stand a certain level of SOZ are presented. Infrared spectroscopy has been used to

identify the nature of the adsorbed species. A discussion on the adverse effect of SOZhas been included.

EXPERIMENTAL

The CatalystCatalyst A, Lao.6SrOACol-xMx03' and catalyst B, Lao.6SrOACo03, were

*To whomcorrespondence shouldbeaddressed.

405

406

prepared by pyrolysis of their amorphous citrate precursers at 850°C (ref.7). Theirstructures were both single phase perovskite type as determined by Rigaku D/Max r.a.X-ray diffractometer. The surface area was determined by gas chromatographicmethod and calculated by BET equation.

Reduction by COAbout 20 mg of the catalyst were placed in a Rigaku TG-DTA

thennobalance. The temperature was raised at a rate of 10°C/min., while highly pureN2 was introduced through the sample at a rate of 60 ml/min. When the desiredtemperature was reached, pulses each containing 3.6 ml CO were introduced and TGand DTA curves were recorded. No air was admitted after each pulse. The continuousweight loss observed was far greater than the amount of oxygen which could bepresent on the surface.

Test ofS02 effectThe susceptibility of the catalyst to poisoning by S02 was tested using

0.5 ml of catalyst placed in a microreactor connected with a nondispersive infraredCO analyzer and a S02 gas analyzer. Firstly, a stream of air containing variousamounts of S02 (from 20 to 400 ppm) was allowed to pass through the catalyst for onehour, then the reactant was introduced and the activity expressed as % conversion ofCO was recorded. Another series of experiments were carried out with the reactantblended with S02' The composition of the gas stream was analyzed at the inlet andoutlet of the reactor.

IR spectroscopyA Nicolet NIC-7199 F.T. infrared spectrometer was used to determine the

nature of the adsorbed species. For CO adsorption, the sample disc was mounted in anlR cell connected to a vacuum rack. It was evacuated at 200°C and 1O-4mm Hg for 4.5hours before 100 Torr of CO was introduced at 120°e. It was allowed to stand for 20minutes and without outgassing further, the IR spectra were recorded. For S02adsorption, disc made by pressing the sample powder with KEr, was mounted in thespectrometer directly.

RESULTS AND DISCUSSION

Activity of the catalystThe activities of the catalysts were tested at 56000/hr, with air stream

containing 2 vol% of CO, and at 38000/hr with 1800 ppm of CH4 in air. The resultsare shown in Table 1.

Table 1 : Activity of the catalyst

% conversion of CO180°C 200°C

Catalyst

AB

Surface area(m 2jg)

13.22.9

Bulk density(g/ml)

1.71.8

9927

100.097.0

407

- -!

% conversion of ICH4 at 460°C I

i

76.6

S02 toleranceThe effect of S02 on catalyst activity has been much less pronounced for

eH4 oxidation than for eo, therefore, only results for eo are presented here. Table 2gives data of S02 tolerance of catalyst A for eo oxidation with a reactant of 2 vol% ofeo in air, at 56000/h. In this Table, "poisoned" means the activity dropped to less than5% of the original value, "reversible" means the activity did not recover after shuttingoff SOz for 30 minutes at otherwise the same condition, resumed some of its activityat 250oe, recovered completely at 280oe, and remained so when the temperature waslowered to 2000 e again. At 2000 e S02 poisoning was very quick and the fatal dosewas 0.02 ml/m2 or 0.14 monolayer on the basis of 30 A2 for the area occupied by oneS02 molecule from Yao. (ref. 3)), but it took 40 minutes at 300oe, and5.2 monolayers.

Table 2 : Effect of S02 on catalyst A

T S02 Exposure to S02 1 h S02 admixed in reactant Note

eC) (ppm) S~~ed Poisoned Poisoning time SO~ p~sed( m) (yes/no) (m m)

200 20 0.05 no not in 1 h 0.0550 0.12 yes 10 min. 0.02 reversible

300 20 - 200 0.05 - 0.49 no not in 1 h 0.05 - 0.49400 0.99 yes 40 min. 0.66 reversible

400 20 - 400 0.05 - 0.99 no not in 1 h 0.05 - 0.99170 5h 2.09 reversible400 2h 1.97 reversible

80 not in 72 h 0.90 s.v.36001h

One point is worth notice; i.e. before the catalyst gets poisoned, S02 wasuptaken completely from the gas phase, and there was no SOz detectable in the outletgas stream. For example, at 3000 e the catalyst was not poisoned in the presence of 200

408

ppm of S02 within one hour, so no S02 was detectable in the product, but altogether0.49 ml/m2 of S02 were passed over which amounted to four monolayers. We do notknow what kind of transformation has been undergone by the retained S02; we failedin looking for a second phase by X-ray diffractometer when a sample poisoned butrecovered and again poisoned by 400 ppm of S02 at 400°C for two hours wasexamined. The result was the same when it was exposed to pure S02 gas at 700°C.Perhaps most of the S02 has reacted by another route over this catalyst.

It was also found that when the catalyst was recovered once from poisoningand was used again, it will get poisoned more readily than the fresh one.

The effect of S02 on catalyst B was shown in Table 3. Catalyst B was muchless resistant to S02 and its poisoning was not reversible. Two types of irreversibilitywere observed and mentioned in Table 3 as IRR I and IRR 2. IRR I means that thecatalyst could recover its activity at high temperature, e.g. at 400°C (300°C is not highenough), but became inactive when the temperature was lowered to 200°C again.IRR 2 means the activity was recovered when S02 was shutted off at otherwise thesame condition, but was lost again when returning to 200° or 300°C.

Mobility of lattice oxygenReducibility of the catalyst by CO or the mobility of lattice oxygen of

catalyst A and B are quite different. The weight loss of the catalysts at differenttemperatures given in Table 4 could be used as a measure of these properties.

,-II Catalyst

Table 4 : Mobility of lattice oxygen

Weight loss temperature (O°C)

409

340 360115 140 208 216

; fresh + ++ +++A

poisoned + +I

freshB

poisoned

266

+++

+ ++

400

++

In this Table one plus sign "+" means "noticeable", three plus "+++" means"very significant", and minus sign "-" means there is no reduction at all. The latticeoxygen of catalyst A was active and mobile at temperatures below 150°C, and becamevery significant at 200°C and higher. After it was poisoned at 200°C by SOz theweight loss began at 208°C and became significant at 266°C. These results areconsistant with the regeneration test described in the previous text. The activity ofsamples poisoned at 200°C could be 100% recovered at 280°C. These facts suggestedthat the activity of the catalyst is closely related with the mobility of its lattice oxygen.

Catalyst B was not so easily reduced. Its weight loss began at 340°C and wassignificant only at 360-400°C. Over this catalyst the redox mechanism of COoxidation is only possible at 400°C or higher, below 400°C reaction probablyproceeds by adsorption mechanism. Over catalyst A redox mechanism is possible at180°C and lower.

Nature 0/adsorbed CO and S02Adsorption o/COBecause of the dark color and the low surface area of the sample, the IR

spectra, especially that of catalyst B, are not very satisfactory, but it gave someinformation as follows. Catalyst A: After the pretreatment described in theexperimental part, water and CO2, free or weakly adsorbed, should be expelled fromthe IR cell. Since the lattice oxygen of this catalyst is mobile at 115°C, there might besome C02 produced when the sample was exposed to 100 Torr of CO at 120°C for 20minutes. The spectra were taken without outgassing after CO was introduced, someCO remained. As shown in Figure 1, besides CO and C02, there are also bands of thefollowing species:

Unidendate carbonate: 1463, 1451, 1289, and 1073 crrr!Bidentate: 1627, 1289, and 1043 crrr!Bridging: 1764, 1738, and 1198 cm-ICarbonyl: 1938, 1985,2005, and 1859 crrr l.

The bands assigned as carbonyl are similar to that of Caz(CO)g. Theassignment of bands are made by reference to ref. 8-9.

410

I-bae. line2-po1eoa.d at 200· C a:J

3-calcied at 400" "; ~in air

2'i-OO 2000 1600 1200 800~AVENUMBERS

Figure 1: Adsorption of CO,I-Catalyst A 2-Catalyst B Fig. 2 : Adsorption of S02 on catalyst A

The spectrum for catalyst B was not good (line 2), but bands can berecognized by enlarging the spectrum and comparing with the data given by thecomputer attached. There existed various types of adsorbed carbonate species such asunidentate at 1483-1411 and 1384 crrr l, bidentate at 1690-1551 crrr l, and bridging at1900-1700 and 1163 cnrl, but no carbonyl band was clear enough.

Adsorptiono!SOZIR spectra for catalyst A after poisoning by 50 ppm of SOz admixed in the

reactant at 200°C was shown in Figure 2. Bands at 1133 and 994 crrr! could beassigned to adsorbed SOZ. They are very close to a coordinated SOz species with abridging structure I, at 1135 and 993 crrr! (ref. 8) :

oII

M-S-M (I)IIo

There were also some carbonate species such as unidentate at 1512, 1202, 1098 and858 em-I, uncoordinated at 1417, 1455, and 875 crrr l , and bridging type at 1710-1760 em-I. Bands at 2857 and 2926 em-I could be assigned to a formate species. Aftercalcining this poisoned sample at 400°C for 2 hours in the air, the bands for SOz didnot vanish (line 3), but more bands due to carbonate species appeared, indicating thatthe amount of SOZ adsorbed was decreased. Some surface sites were set free which inturn benefited the formation of carbonates that would be limited otherwise by latticeoxygen deficiency.

IR spectrum for catalyst B after poisoning by S02 at 200°C was shown inFigure 3, line 2. Bands at 1294 and 1128 cm! could be ascribed to adsorbed S02,similar to a coordinated species II

.. ··0M - s:r (II)·~o

with bands at 1301-1278 and 1100 em-I. There were also unidentate, bidentate, anduncoordinated carbonate. Bands due to carbonate as a whole were noticeably less thanin the same sample not being exposed to S02' After calcination at 400°C for 2 hours inthe air, bands of adsorbed S02 shifted to 1146,1124, and 98cm-I, which are closer tospecies (I) as on catalyst A. There could be a decrease of the concentration of theadsorbed S02, but the vacated sites did not seem to be occupied by C02 at lowertemperature.

La203 : Samples (commercial, 99.9% pure) were exposed to undilutedpure S02 gas at room temperature for IS hours, and then mixed and pressed with KBrto form discs. The discs were mounted directly to the spectrometer and the IR spectrawere taken. No band here could be assigned to S02 adsorption, but strong and sharpbands due to carbonate species were clearly shown as in Figure 4 line 2.

MOx : IR spectra of Max pretreated in the same way as for La203 weremade. There are only very weak bands due to S02 adsorption, such as 1199, 1137,1125, 1099, and 1089 em-I, which are similar to those for S02 coordinated to metalions in the shape as in II at 1198-1185 and 1048 crrr l. There are also bands due tocarbonate species.

411

2:3

1J.JUZ4:~cUl~ l-base line

2-po1soned at 200" C

:3-calcined at 400· C

<XlN

~~oo 2000 1600 1200 ~ooW~"e:NUMBERS

Figure 3:Adsorption of S02 on catalyst B

412

aoo2000 1600 1200WAVENUMBERS

III

i~

2'±OO

t.JuZ<I:CDa:oIf)Q)<r

N~ ~

!~l-base line2-after exposed

to :::(i~

Figure 4:Adsorption of S02 on La203

C0304 : IR spectra for commercial C0304 (AR reagent), treated in thesame way with pure S02 gas were shown in Figure 5. Besides adsorbed carbonates,there are bands due to sulfate species which are more obvious for sample calcined inthe air at 240°C for 2 hours, e.g. 1143, 1098, 1018, and 871 em-Ion line 2 and 1136,1109,1017,983, and 874 em-Ion line 3.

Each number 2 line was taken from Figure 2, 3 and 6 respectively, andplaced together in Figure 6. It is interesting to notice the similarities of the bands dueto S02 adsorption on these three compounds, especially when .they were comparedwith that on La203 (Fig. 4). It is reasonable to regard Co ions as the seats for S02, andLa3+ as the metal coordinated with most of the carbonate species. Sr2+ should behavelike La3+ because of its basicity, but we do not have the experimental data.

From the data given above, it is reasonable to attribute the poisoning effectof S02 on these catalysts to the scramble of surface sites on B site ions. There are twotypes of adsorbed CO species on the surface of the catalyst. One is the carbonyl specieswhich is responsible for lower temperature oxidation of CO, as suggested forCuC0204 by Hertl et al (ref.lO). It is adsorbed on can+. Another is the carbonatesformed by adsorption of CO on lattice oxygen, as suggested for LaCo03 by Tascon etal (ref.l1). It will decompose at 200°C and higher to give C02. For each CO2desorbed one anionic vacancy would be left, which should be filled up by 02adsorption, and this would happen again on the B site ions. S02 is in competition withboth carbonyl and 02 for the same surface sites. The fast and strong adsorption of S02is therefore detrimental for these two reaction intermediates to form, so it deactivates

l-bS.bl8 line2- exposed to SO~, r . t .

3-heated in air,240'C

~ 2~CO ~~\

6 ~V)fl)<t

413

elf00 2000 1600 1200WAvENUMBEF\S

Figure 5 :Adsorption of 802 on C0304

---,800

2

l-CO j 0+ exposed to SOl t r , t.2-Cat.A poisoned at 20cfc3-Cat.E poisoned at 200

QC

wu~ID€X:o(J)co<r.

zsuo 2000 1600 1200wnVENUr1B£F\S

800

Fig.6: Adsorption of 802 on COJ04 (1)Catalyst A (2), Catalyst B (3)

414

the catalyst quickly. It was noticed that there were changes in number and strength ofbands due to carbonate species before and after the catalyst was poisoned. This arosefrom the changes in B site ions available for 0z adsorption thus promoting ordampening the carbonate formation.

At 400°C, although there are still some SOZ adsorbed, the reaction goesfreely if there are enough sites left for lattice oxygen replenishment. As long as thereactants containing SOZ are flowing through, the amount of SOZ adsorbed willaccumulate gradually with time. Whenever an end limit is reached, the catalyst has tounload its SOz burden at 400°C with fresh reactant or at higher temperature. That isthe case for catalyst A, 400 ppm of SOZdid not deactivate it till the end of two hours.

Another feature of catalyst A is when it was poisoned at 200°C, 30 minuteswas too short, or 200°C was too low to remove the adsorbed SOz. But when some ofits active sites were set free from the adsorbed SOZ at 280°C, it could react accordingto the redox mechanism by its mobile lattice oxygen when the temperature waslowered to 200°C again.

By reduction with CO in NZ the reactivation of catalyst A poisoned at 200°Cstarted at 208°C (TG-DTA data), which is 40°C lower than that needed in the airstream containing CO (at 250°C). This fact indicates that the surface could also be setfree of SOz by reducing it with CO in NZ' By passing a stream of 2% CO in NZthrough the poisoned catalyst, a 24% conversion of CO was observed and it remainedso for 1/2 hour.

Deactivation of catalyst B is not reversible, although the concentration ofadsorbed SOZ could decrease upon heating at 400°C, It becomes active at thistemperature. That is a consequence of thermomobilization of its lattice oxygen. Thereaction could run by decomposing the surface carbonate species, a redox reaction,although there is still some SOz on the surface. Lowering the temperature to 200°C,its lattice oxygen are not mobile anymore. Oxidation run by adsorption mechanismcould not proceed without enough surface sites for the reactant, so the reactionstopped.

CONCLUSION

SOz gas deactivates the cobaltate catalyst for CO oxidation by blocking thesurface sites on the B site ions and inhibiting the formation of reaction intermediates.The concentration of the adsorbed SOz species could be partially diminished bycontinuing the reaction with fresh reactant, or by heating to higher temperatures, butcould not be removed completely at temperatures ~ 400°C, At 400°C lattice oxygen ofboth catalyst A and B are mobile enough to carry out the reaction by decomposing thesurface carbonate species. Coming back to 200°C from 400°C, only the lattice oxygenof catalyst A are active, that of catalyst B are not mobile anymore, therefore,catalyst B could not recover its activity at 200°C, The mobility of lattice oxygen andthe ability of forming weaker bonds with sulfur seems to be essential for improving

the S02 resistance of a perovskite type oxidation catalyst.As shown above the adsorptive poisoning of these catalysts by S02 was

highly temperature dependent. Since the warm up period is usually very short, acatalyst which uptakes S02 at that period, but rapidly recovers its high activity whenthe automobile runs normally would be a promising candidate for use in autoexhaustpurification.

REFERENCES

1. R.J.H. Voorhoeve, L.E. Trimble, AND c.r. Khattak, Res. Bull., 9 (1974)6552. P.K. Gallagher, D.W. Johnson, Jr., E.M. Vogel, and F. Schrey, Mater. Res. Bull.,

10 (1975) 623-628.3. Y.F.Y. Yao, 1. Catal., 36 (1975) 266-275.4. Y.F.Y. Yao, J. Catal., 39 (1975) 104-114.5. S. Katz, 1.1. Croat, and J.V. Laukonis, I.E.C. Prod. Res. Dev., 14 (1975) 274.6. Xi Zhen-Sheng and Li Wan, submitted for publication.7. C. Marcilly, P. Couty, and B. Delmon, 1. Amer. Cer. Soc., 53 (1970) 56.8. K. Nakamoto, Infrared and Raman Spectra Of Inorganic and Coordination

Compounds, 3rd Ed. (1978), N.Y. Wiley-Interscience.9. G. Busca and V. Lorenzelli, Mater. Chern. 7 (1982) 89-126.10. W. HertI and R.J. Farranto, 1. Catal., 29 (1973) 352-360.11. 1.M.D. Tascon and L.G. Tejuca, Zeit. Phys. Chern. N.F., 121 (1980) 63-78.

416

A. Crucq and A. Fronnet (Editors), Catalysis and Automotive Pollution Control© 1987 Elsevier Science Publishers B.V .. Amsterdam - Printed in The Netherlands

417

TUNGSTEN CARBIDE AND TUNGSTEN-MOLYBDENUM CARBIDES AS AUTOMOBILE EXHAUSTCATALYSTS

L. LECLERCQ1, M. PRIGENT2, F. DAUBREGE 1, L. GENGEMBRE 1 and G. LECLERCQ1lLaboratoire de Catalyse Heterogene et Homogene, U.A. C.N.R.S. n° 402, Univer-site des Sciences et Techniques de Lille Flandres-Art-u s , 59655 Villeneuved'Ascq Cedex (France).2lnstitut Fran~ais du Petrole, B.P. 311, 92506 Rueil-Malmaison Cedex (France).

ABSTRACTSeveral catalyst samples of tungsten carbide and W,Mo mixed carbides with

different Mo/W atom ratios, have been prepared to test their ability to removecarbon monoxide, nitric oxide and propane from a synthetic exhaust gas simula-ting automobile emissions. Surface characterization of the catalysts has beenperformed by X-ray photoelectron spectroscopy (XPS) and selective chemisorptionof hydrogen and carbon monoxide. Tungsten carbide exhibits good activity forCO and NO conversion, compared to a standard three-way catalyst based on Ptand Rh. However, this Wcarbide is ineffective in the oxidation of propane.The Mo,W mixed carbides are markedly different having only a very low activity.

INTRODUCTIONTransition metal carbides (mainly of Wand Mo) have been shown to be effec-

tive catalysts in some chemical reactions that are usually catalyzed by noblemetals such as Pt and Pd (ref.1). Their remarkable physical properties addedto lower cost and better availability could make them good candidates forsubstitute materials to noble metals in automobile exhaust catalysis. Hence,for this purpose, we have prepared several catalysts of tungsten carbide andW,Mo mixed carbides supported on y alumina with different Mo/W atom ratios.The surface composition has been stUdied by XPS while the quantitative deter-mination of catalytic sites has been obtained by selective chemisorption ofhydrogen and of carbon monoxide. The catalytic performances of these catalystshave been evaluated in the simultaneous conversion of carbon monoxide, nitricoxide and propane from a synthetic exhaust gas.

EXPERIMENTALCatalysts

The tungsten and molybdenum carbides supported on alumina were prepared byUGICARB MORGON (Grenoble, France). The alumina support (grain size < 80 11m,BET surface area: 100 m2.g- 1) is impregnated with ammonium heptamolybdate and

418

paratungstate solutions followed by evaporation of water and further drying at127°e during 10 hours. Then the sample is calcined for 16 hours under flowingnt t roqen to decompose ammonium ions : at 5500e for the catalyst conts iru noonly Wand at 2800e for mixed W. Mo catalysts owing to the possible sublimationof Mo03. The reduction of the supported oxides is then carried out underflowing hydrogen for 6 hours up to 6000e (heating rate 1°e/mn) and the tempe-rature is increased from 6000e to 9000e for 6 hours. At this last temperature.the carburization occurs under flowing carbon monoxide during 40 hrs. Thecatalyst is then cooled down under eo and passivated at room temperature witha gas mixture of 1% oxygen in argon. A commercial catalyst based on platinumand rhodium from PROeATALYSE was used as a reference catalyst (about 0.3 wt %of Pt and Rh. Pt/Rh = 5).

XPS ExperimentsXPS spectra were obta i ned us i ng an AE I ES 200 B spectrometer WI th an Al

cathode (hv = 1487 eV. 300 W). The pressure inside the spectrometer chamber-9was kept lower than 1x10 torr. Severa I references were taken for the

calculations of binding energies (BE) : AU 4f7/ 2 = 84 eV. e1s = 285 eV. AI 2p74.8 eV). Intensity ratios IMo/lW were corrected by taking into account thedIfference of kinetic energies according to EBEL (ref .2).

Adsorption MeasurementsThe BET surface areas and the hydrogen and carbon monoxide adsorption

isotherms were determined by volumetric adsorpti on performed with a TexasInstrument quartz spiral BOURDON gauge in a system already described elsewhere(ref .3) .

Laboratory Bench Te~t

A laboratory dynamic flow reactor system has been specially designed forpowder samples at the Institut Fran~ais du Petrole which allows to observe the

catalyst performances as a function of air to fuel ratio (A/F). For activitytesting, the A/F was set a; different values which involved the flow ratescontrol of NO, CO and propane at a selected space velocity and operatingtemperature. The reactor inlet and outlet 02 concentrations were also measuredwith an oxygen sensor. The light-off performance of the catalyst was evaluatedby varying the temperature for a stoichiometric gas mixture. Three analyzerswere used to determine CO, NO and propane conversions : a flame ionizationdetector (BECKMAN) for detection of remaining propane, an IR detector (COSMA)for CO and a COSMA apparatus for the detection of nitrogen oxides by chemilumi-nescence. 10% of CO2 and 10% of H20 were added to the feed stream. and CO wasmixed with hydrogen in such amount that its concentration by volume is about

419

0.5% of the initial gas mi xture (CO/H2=3). The gas flow rate at the reactoroutlet was about 80 l/h wi th a correspondIng space velocity of 25000 h- 1. Asample of about 0.5g of carbide catalyst was added to 2g of ground cordierite(100 < 0< 160 um) to increase the heat transfer. A same amount of referencecatalyst, about 2.5g, of a ground conmer-c i al catalyst based on Pt and Rh wasused to compared the performance of the catalysts in the same experimentalconditions.

RESULTS AND DISCUSSIONCatalyst compositIons

The composition of the catalysts are reported in Tables 1 and 2.

TABLE 1Weight composition of catalysts from chemical analysis (wt%)

Catalysts CT Cd C Mo W Mo+W

Ko WC/Al 203 0.80 0.64 0.16 0 4.09 4.09K1 (W,Mo)ClAI 203 0.34 0.06 0.28 0.73 4.32 5.05K2 (W,Mo/ClAl 203 0.36 0.10 0.26 1.21 3.08 4.27K3 (W,Mo)ClAI 203 1.07 0.81 0.26 1.82 2.70 4.52K

3_o (W,Mo)C/AI 203 1.99 1.55 0.44 4.36 1. 11 5.47

TABLE 2Atom composition of catalysts and their specific surface areas

Catalysts Mo/Mo+W C/Mo+W Cd/Mo+W BET(atom %) (Atom ratio) (atom.%) Surface area 2 -1)m .g

Ko WC/AI 203 ° 0.60 2.4 87.7K1 (W,Mo)C/Al 203 24.5 0.75 0.16 95K2 (W/Mo)C/Al 203 42.9 0.74 0.28 44.6K3 (W,MO)C/Al 203 56.4 0.77 2.0 85K3_o (W,Mo)C/Al 203 88.3 0.71 2.5 99.2

The molybdenum and tungsten contents were determined by atomic absorptionspectrometry in areducing flame of acetylene-nitrogen protoxid at the ServiceCentral d'Analyse du C.N.R.S. (Lyon).

The metal carbides are always loaded with free carbon the amount of whichdepends on the preparation method. The total amount of carbon was obtained

420

from the combustion of the sample in pure oxygen in a high frequency oven(LECO apparatus). The CO2 formed is then quantitatively detected by a thermalconductivity cell with an oxygen reference flow which directly gives thecarbon percentage. The free carbon content is obta ined by a couIomet ri cmethod. The sample is attacked by a hot mixture of nitric and hydrofluoricacids which dissolve all the components except the non combined carbon. Thiscarbon is then transformed into CO2 in flowing oxygen at 1300°C before thetitration in an electrochemical cell. The determination of the combined carbon(carbidic C) is obtained by difference between the total carbon content (CT)and the free carbon one (carbon deposit Cd)'

From these values, we can calculate : - the stoichiometry of tungsten andmolybdenum carbides (C/Mo+W) i.e. the number of combined carbon atom per metalelement ; - the atom ratio of free carbon deposit Cd/Mo+W ; - the atom percen-tage of molybdenum in the mixed carbide compounds (Mo/Mo+W%). The results aresummarized in Table 2.

XPS ExperimentsXPS spectra of W4f and M0 3d for alumina supported carbides are very different

from those of bulk carbides (ref .4). Besides Wand Mo carbides, W or Mooxidized are present in large amounts, probably as W(Mo)+4, Mo+5 and W(Mo)+6(Fig.1 and 2).

Such oxides are probably partly formed during the passivation treatment.However, while with bulk carbides these oxides are easily reduced by a treat-ment under hydrogen at 300°C or 500°C (ref .4,5), with supported carbidessimilar reducing treatments do not seem to markedly influence the propor-tion of oxide phases. Such a stability of these oxides could be related to theformation of combinations with the alumina support (Ref.6). The presence ofcarbidic surface phase was also checked by the C1s peak at a binding energy of282.5 eV assigned to carbidic carbon (Fig. 3a). It can be mentioned thatcarburization can be increased by a treatment in CO and H2 at 300°C as can beseen in figure 3b.

Using the intensities of Mo 3d, W4f photopeaks corrected according to EBEL(ref.2) we have ~stimated the surface composition of the various catalysts asreported in table 3.

From table 3, it can be seen that most of Wand Mo are in an oxidized statein the layer analyzed by XPS. In this respect K2 seems to be an exceptionsince the proportion of carbide is much higher than in the other catalysts.

Compared to the bulk composition from chemical analysis, the surface compo-sition from XPS data seems to indicate a slight molybdenum surface enrichmentwhich can explain that molybdenum is more affected by oxidation as shown intable 3.

421

Mo3d

Binding Energy IeV] 230t I I I I23'

Binding En."SI)' leV]

Fig. 1 and Fig. 2. Spectra of W4f (Fig.1) and M03d (Fig.2) from the mixedcarbide catalyst (W,Mo)C/A1 203 (K2).

TABLE 3Surface compositions from XPS data

Catalysts Mo/Mo+W bulk Mo/Mo+W XPS WOxide WCarbide Mo Oxide(atom %) (atom %) (%) (%) (%)

Mo Carbide(%)

KoKlK2K3K3-0

o24.542.956.488.3

o30.549.657.6

100

95884596

51255

4

10059

10089

o41o

11

Adsorption MeasurementsBefore adsorption measurements, the catalysts were pretreated at 400°C

under flowing hydrogen for 7 hours and then outgassed at 400°C for 10 hoursat a pressure of 10-6 torr.

Hydrogen chemisorQtion. The amount of adsorbed hydrogen, derived fromthe adsorption isotherms at room temperature is zero. But if the adsorptiontemperature is increased the hydrogen uptake also increases as seen in table 4.

Since hydrogen adsorption is generally exothermic, the hydrogen uptake atequilibrium must decrease when the temperature increases (ref .7). The oppositeresult obtained leads to the conclusion that the hydrogen adsorption is anactivated process. A similar result was reported by BENZIGER et a1. (ref.8)for a tungsten single crystal carburized by ethylene. In order to compare thevarious catalysts, the hydrogen adsorption isotherms have been determined at400°C (Table 5).

422

Fig.3. C( 1s) electron spectrum from WC/AI Z03 (Ko)' (a) as obtained after cata-lyst preparatIon (b) after further carburlzatlon wIth CO and HZ at 300°C.

TABLE 4Hydrogen uptake on the sample K3(W,Mo)C/ AIZ03 as a function of the adsorptiontemperature. Values obtained by extrapolation at zero pressure.

Adsorption Temperature (OC)

23300400500

Hydrogen Uptake (]J mol.g- 1)

°13.218.334.0

The relative number of potential hydrogen adsorption sites (H/Mo+W) incr-eases with the atom percent of molybdenum, however it is rather low and signi-ficantly lower than 1. When the ratio H/Mo+W is plotted versus the bulk %Mo/Mo+Wa straight line is obtained (F~9. 4) which could indicate that the slightsurface enrichment in molybdenum suggested by the XPS results after the passi-vation treatment, disappears in hydrogen at 400°C leading to Mo and Wsurfacecompositions very close to the bulk compositions. In addition, such a linearfunction clearly shows that the hydrogen uptake is not significantly influen-ced by the amount of free carbon which is the lowest for the K1 sample andmaximum for K3_o, their corresponding experimental points fitting very well onthe I inear plot. The only exception is for the K2 sample which adsorbs alarger amount of hydrogen. This result can neither be explained by the amountof free carbon (higher than in K1) nor by the surface enrichment in molybdenum(which is the lowest for K2). The larger hydrogen uptake for this K2 sample

423

could be a consequence of the largest extent of the carbided phase as seen intable 3.

TABLE 5

Chemisorption of H2 and CO on Wand Mo carbides

Catalysts Mo/Mo+W a) H2 uptake H/Mo+W(atom%) (IlmOl.g-1)

b) CO uptake Co/Mo+W(Il mol.g- 1)

H/CO

KO 0 4.1 3.7 10-2 8.6 3.9 10-2 0.95K1 24.5 10.0 6.4 10-2 19.2 6.2 10-2 1.04K2 42.9 18.0 12.3 10-2 28.2 9.6 10-2 1.28K3 56.4 18.3 10.9 10-2 31.2 9.3 10-2 1. 19K3_o 88.3 39.6 15.4 10-2 30.5 5.9 10-2 2.60

aAdsorotion temoerature 400°C bAt room temperature

Carbon Monoxide Chemisorption. As for hydrogen adsorption, the number ofactive sites for CO adsorption is obtained by extrapolation at zero pressureof the isotherms recorded at room temperature (Table 5). The pretreatment ofthe catalysts was exactly identical.

"

10

, ,

jo L- --~-~~

Mo/Mo ..W [At.";I.0

0113·0

100Mo/Mo. w [At', ]

Fig.4 and Fig.5. Variations of the number of hydrogen adsorption sites (H/Mo+W)(Fig.4) and of the number of CO adsorption sites (CO/Mo+W) (Fig.5) .as afunction of W, Mo bulk composition.

424

The variation of the number of CO adsorption sites as a function of W, Mobulk composition is similar to that found in hydrogen adsorption including theK2 exception (Fig .5). For all the samples the atom ratio of the adsorbedamounts H/eO is close to 1 except for the K3_ 0

sample H/eO is 2.6.This ratio H/eO equal to 1 for tungsten and mixed carbides seems to indicate

that on these catalysts CO is adsorbed non dissociatively and linearly. Thisis in agreement with the results of BENZIGER et al. (ref.8) which, by comparingthe CO adsorption on W metal and carbide, have shown that the di ssociati veadsorption is strongly inhibited by the formation of surface carbide leavingCO molecules weakly bound in the linear form. Bui: for molybdenumcarbide, the adsorption of one molecule of CO needs two (or more) potentialsites, either as dissociative or bridged species.

eata lyti c TestsTwo types of experiments were carried out in this study :

- The steady state conversion of CO, NO and propane was determined as afunction of the equivalence ratio R which is related to the air to fuel ratio(A/F) and to the stoichiometry of the reaction: R = (A/F) stoichiometric/(A/F)actual.R = corresponds to a stoichiometric gas mixture where A/F = 14.68.R> 1 represents an overall reducing gas mixture (rich of stoichiometry)R< 1 represents an overall oxidi z i nq gas mixture (lean of stoichiometry).

R was varied from 0.95 to 1.05 to reproduce the narrow operating window ofa three-way catalyst.- The conversion was studied as a function of the temperature at R=l todetermi ne the 1ight-off temperatures where at 1east 50% convers ion of thereactants occurs, but a1so to test the res istance of the cata lysts to hightemperatures.

The activity of the carbide catalysts was compared to that of a referencecatalyst based on platinum and rhodium strictly in the same experimentalconditions. Typical curves of the steady state CO, NO, propane conversions as afunction of temperature or as a function of the equivalence ratio are represen-ted respectively in Figures 6 and 7. The light-off temperatures of the CO, NO,propane are respectively 160°C, 180°C and 250°C. They are indicative of usualcommercial catalysts. By comparison, for the tungsten carbide Ko the light-offtemperatures of the CO and NO conversions are respectively 300°C and 360°C(Fig.8). They are higher than for the usual Pt-Rh catalysts but they are stilllow enough to be interesting (ref.9). Concerning the variation of the conver-sion with the equivalence ratio at 465°C, the tungsten carbide catalyst seemsvery stable with a good conversion for CO and NO (Fig.9). However in no casethe conversion of propane is significant on carbides.

425

REFeRrNcr C.ATALYST450 d.g C Yl'H 25hDO /HOUR

"" Q" D'n '.DD 'Dt '''2 'vJfaJl~AHIO fUI11)

20 :tn;rgMC.kllON-r-- NO,

VYH; 25000 /HOUR

Fig.6 and Fig.? Diagrams of catalyst conversion efficiency in CO, NO andpropane removal plotted against temperature (R=1) (Fig.6) or againts equivalenceratio R at 450°C (Fig.?) on the reference catalyst (Pt-Rh PROCATALYSE, France)(- CO, 0 NO, * C3HS)

¥YB: 25000 /HOUR Ko

o ~_;~~H(,~,~~~#~~~~#~~#~,~~~,~~~

TFMPFRAT1!RF

465 dl!g C Ko YVH 25000 /HOUR

->,~ loi+- ----,--------+-------i-- ,-{,

~ 7~;;;~ 60

~ S/1

'"'-' «o

lot fin '."4HkllVAlflU ~AfIO

Fig.S and Fig.9. Diagrams of catalyst conversion efficiency in CO, NO andpropane removal plotted against temperature (R=1) (Fig.S) or against equivalenceratio at 465°C (Fig.9) on the tungsten carbide catalyst (WC/AI 203, K ).(_CO,O NO, * C3HS) 0

For the mixed carbides (K1, K2 samples) the conversions are surprisinglyvery low as shown in figure 10 for K1. K2 is almost inactive at any temperature.

Hence tungsten carbide seems to be a much better catalyst for postcombus-tion catalysts than molybdenum carbide. The unexpectedly low activity of K1and K2 which contains rather high tungsten contents could be related to amolybdenum surface enrichment in an oxidizing atmosphere, which seems to berevealed by XPS analysis.

426

VYR: 25000 /HOUfl

i Ii 1i TTlT.-T i

i I . I j? IJg • l.I.I...;... L_~ I_ t ~ j l ji I ! I i! I ' i i

1I!

Fig.10. Diagram of catalyst conversion efficiency in CO, NO and propane removalplotted against temperature (R=1) on the mixed carbides catalyst (W,Mo)C/AI 203,K1. (.CO, o NO, *C 3H8).

After a catalytic test at 600°C, the XPS spectra show that the carbidicspecies of Wand Mo have totally disappeared. Only the oxide surface phasesare present.

CONCLUSIONOf course, the oxidability of these carbides is a handicap for the moment.

However one might envisage that some support effects, or (and) the addition ofelements exhibiting oxygen storage capacity could improve their resistance tooxidation. In that case, owing to its reasonably good activity for CO and NOconversions, tungsten carbide might be considered as a possible component ofthe catalysts for automobile emission control, probably in association withsome other elements active for hydrocarbon oxidation.

REFERENCES1 R.B. Levy and M. Boudart, Science, 181 (1973) 547.2 M.F. Ebel, Surf. Interface Anal., 2 (1980) 173.3 G. Leclercq and M. Boudart, J. Catal., 71 (1981) 21.4 M. Provost, Thesis: "Etude des propriAtAs catalytiques d1alliages du molyb-

dene et du tungstane avec Ie carbone"; University of Poitiers (France) 1984.5 B. Vidick, J. Lemaftre and B. Delmon, J. Catal., 99 (1986) 428.6 I.E. Wachs, C.C. Chersich and J.H. Hardenbergh, Applied Catal., 13 (1985)

335.7 S.E. Wanke and N.A. Dougharty, J. Catal., 24 (1972) 367.8 J.B. Benziger, E.I. Ko and R.J. Madix, J. Catal., 54 (1978) 414.9 J.T. Kummer, Progress in Energy and Combustion Sci., 6 (1980) 177.


DYNAMIC BEHAVIOR OF AUTOMOTIVE THREE-IvAY EMISSION CONTROL SYSTEMS

RICHARD K. HERZ

Dept. A.M.E.S./Chemical Engineering, University of California at San Diego,La Jolla, CA 92093, U.S.A.

427

ABSTRACTThe operation of warmed-up automotive three-way catalysts is considered.

Special emphasis is given to the observation that significant fractions of CO,hydrocarbon, and NO emissions in urban driving tests occur during vehicleacceleration. The increased emissions during acceleration occur as a result ofincreased exhaust flow rates and rich air-fuel ratio excursions of the air-fuelratio control system. A method is presented for displaying and analyzingcatalyst response to dynamic changes in operating conditions. The inclusion ofa rich air-fuel ratio excursion test in catalyst evaluation procedures isrecommended.

INTRODUCTIONThree-way automotive catalysts never operate under steady-state conditions:

catalyst temperature increases rapidly after engine starting, and the exhaustflow rate and composition fluctuate rapidly under all modes of operation.

Numerous studies have shown that the performance of catalysts under dynamicconditions differs greatly from their performance under steady-state conditions

(e.g., ref.1-4). Thus, it is manditory to evaluate and compare the performance

of three-way catalysts on the basis of tests that involve dynamic conditions.The dynamic behavior of three-way emission control systems involves several

aspects: catalyst behavior during warm-up from a cold start, air-fuel ratio

control following catalyst warm-up, and catalyst behavior following warm-up.

This discussion considers only the last aspect. The report consists of two main

results and discussion sections. In the next section, an analysis of dynamic

conditions and catalyst performance during variable speed driving is presented.In the following section, a method analysis of dynamic response experiments ispresented. All of the experimental work described in the report was performedunder the direction of the author at the General Motors Research Laboratories.The mathematical modeling work that is presented was performed since the authorhas been a member of the faculty of the University of California at San Diego.

428

DYNAMIC CONDITIONS DURING DRIVING

A good way to get a feel for the dynamic nature of automotive catalyst

operation is to look at a plot of speed versus time during the U.S. Federal Test

Procedure (FTP) as shown in Fig. 1. Clearly, steady-state conditions do notexist in this test, which simulates the acceleration and deceleration cycles of

urban driving and which is similar to the European test procedure.

The first step in analyzing the performance of a catalyst in an emission

control system is to determine "what the catalyst sees" in terms of temperature,

exhaust composition, and exhaust flow rate variations during the driving cycle.

The nature of the conditions that a catalyst is exposed to is not only a

function of the driving cycle and the vehicle type, but also is dependent upon

the air-fuel control system. Tests which record the dynamic conditions have to

be repeated and evaluated statistically since the detailed results of each test

will vary as a result of random test-to-test variations.

The second step in the analysis is to classify the different conditions of

catalyst operation and determine the distribution of emissions between the

different types of operating conditions. This step in the analysis allows one

to identify the most significant conditions of operation where catalyst

performance can be improved. In catalyst aging tests, one can identify

condi tions of operation where catalyst performance has deteriorated most and,

thus, were stability should be improved.

The changes in operating conditions that a catalyst sees during driving can

be separated into two time scales. Fast oscillations (ca. 0.5 to 4 Hz) in air-

fuel ratio (A/F) about the A/F control point occur as a result of the response

characteristics of the A/F control system. Slower changes in exhaust

Nt~ Warmed.UP-...

~Operation

, ~I~ !~

100

80

:2 60Co~"0Q)Q)Co 40

Ul

20

aa 500 1000

Time(s)1500 2000

Fig. 1. Plot of vehicle speed during the urban driving cycle of theU.S. Federal Test Procedure (FTP).

429

composition, flow rate, and temperature occur as a result of acceleration and

deceleration. These slower changes have characteristic frequencies of 1 Hz and

less. In this section, we focus on the relatively slow time-scale transients

associated with acceleration and deceleration.

The results presented in this section were obtained with a carbureted vehicle

that was mounted on a chassis dynamometer and that was driven through the U.S.

PTP. All of the data presented are for operation following warm-up of the

catalyst (i .e., accel-dece1 cycles number four and greater). Two complete sets

of emission analyzers allowed simultaneous measurement of exhaust composition at

the inlet and outlet of the pellet-type catalytic converter. Here we present a

brief review of the results of these tests. A more detailed description is

given in (ref.4).

Concentrations of exhaust components were digitized and recorded by computer

every 0.5 seconds during each driving test. In addition, other data such as

vehicle speed, throt tIe position, and fuel consumption rate also were recorded

every 0.5 seconds. All data recorded were electronically loaded into a

computerized relational data base program. This program allowed computations

(e. g., of AlP and exhaust flow rate), logical searches and classification of

data (e.g., selection of all periods where the Alp and the acceleration were

within specified limits), and plotting of results to be performed qu i ckl y and

easily.

The impact of accelerations on emissions can be seen in Pig. 2. The left

column of plots were obtained when the automobile was operated at constant speed

(not during the PTP urban driving cycle). The right column of plots show

results obtained during driving through several of the accel-decel cycles of the

U.S. PTP. The top row of plots is driving speed. The second row of plots shows

the AlP during. The bottom two rows of plots show the rate of emission of CO

and NO in terms of grams per second at the outlet of the converter. Emissions

of these species are low during constant speed operation but significant burst

or spikes of emissions occur during variable speed dri ving. The emission of

increased amounts of pollutants during variable speed driving results from (a)

perturbations to the AlP control system during acceleration (see third row of

plots), (b) perturbations to the exhaust gas recirculation (EGR) control system

during acceleration (not shown), and (c) increased exhaust flow during

acceleration (see fourth row of plots), which causes increased engine-out

emission rates (even at constant exhaust composition) and decreased reactant

residence times in the converter.

The A/F control system on the automobile used in these tests was set to

control at a mean Alp of 14.7. The stoichiometrically balanced AlP for the fuel

used was 14.6. We arbitrarily divided the A/F scale into three regions for

430

analysis of these tests: "lean excursions" where the A/F was greater than 14.9

(i.e., the control ratio of 14.7 + 0.2 A/F units, a control region where the A/F

was between 14.9 and 14.5, and "rich excursions" where the A/F was less than

14.5.

Table summarizes the performance of the vehicle and emission control system

during the U.S. urban driving cycle following warm-up. The distribution of the

results between rich and lean excursions and the control region will of course

be different for different engines and, especially, for different A/F control

FTP DRIVING

15

14

1320

50

40

30

20

10

o

10

o0.4

02

o

O"'~0.000,

500 550 600 650 700

CONSTANTSPEED

50

40Z~ 30"tl 20""c,C/) 10

0

2 15;;;a:0;:Ju, 14';::<i:

13

"~<;)-,..." 10;;;a:s0u: 0

"L<;)'<,

02rn0u

0O"'L<;)-,C>

0Z

00000 10

Time(s)

lEe Prod.Res.s Dev.

Fig. 2. Comparision of air-fuel ratio control, exhaust flow rate, andCO and NO emission rates during constant speed driving and variablespeed driving through a selected segment of a U.S. FTP test (ref.4).

systems.

however.

431

We expect that the results apply qualitatively to most vehicles,

Note that whi Le the A/F was in a rich excursion only 14% of the time, a

disproportionate share of the total exhaust flow, 20%, occurred during rich

excursions. This is because rich excursions occur as a result of acceleration

This result is significant but not

and exhaust flow rate increases with rate of acceleration (ref.4). Most

significantly, disproportionate shares of the emissions of CO, NO, and

hydrocarbons occur during rich excursions.

surprising for CO and hydrocarbon emissions.

The resul t that NO emissions are significant during rich excursions and

negligible during lean excursions may be surprising to many people. On the

basis of measurements of NO conversion during steady-state or "cycled-A/F"tests, we expect NO conversions to be high under rich conditions and low under

lean conditions (in cycled-A/F tests, average conversion is measured during

cycling of the A/F, typically at a frequency of 1 Hz and an amplitude of ± 0.5

A/F units). This is an excellent example of how steady-state and even cycled-

A/F tests can be misleading.

There are several reasons why one cannot use steady-state or cycled-A/F

conversion measurements to directly predict emission rates versus A/F during

driving. The tail-pipe emission rate, for one species at a given A/F, is equal

to the mutual product of (a) inlet concentration, (b) exhaust flow rate, and (c)

fractional conversion. Each of these factors differ between the different

cond i tions of opera t ion. Relati vely high NO concentrations can occur during

initial periods of acceleration as a result of the response characteristics of

the EGR control system. High exhaust flow rates during acceleration contribute

to high engine-out emission rates, even at constant composition, and result in

decreased reactant residence time in the converter. Finally, different

fractional conversions are obtained during A/F excursions while driving because

slowly responding transient chemical processes cause the catalyst to be in a

Table 1.

A/F CONTROL AND EMISSIONS DURINGWARMED-UP PORTION OF FTP

"Rich Excursion"A/F<14.5

"Control"145< A/F< 149

"Lean Excursion"A/F> 149

% of Total Time 14 76 10% of Total Flow 20 73 7% of Total CO 48 39 2% of Total He 56 41 4

% of Total NO 34 64 2 lEe Prod. Res.& Dey.(rol.4)

432

different chemical state than it is at the same AIF during steady-state or

cycled-A/F tests.Fig. 3 shows the average conversion obtained versus A/F during the FTP. The

A/F was always fluctuating during these tests and stayed only briefly at each

value of A/F. The total mass of component (e.g., CO) entering the converter andthe total mass of component exiting the converter, whenever the A/F was at the

given value, were used to calculate the conversion.

Fig. 3 looks very different from a plot of conversion versus A/F obtainedduring steady-state tests or cycled-A/F tests. First, note that NO conversionis high at lean A/F's, whereas steady-state and cycled-A/F tests would show low

conversion. Transient chemical processes, such as reaction of NO with reduced"oxygen storage" components in the catalyst, serve to maintain high NO

conversions during transient lean AIF excursions.Second, note that CO and hydrocarbon conversions remain relatively high even

at very rich A/F's. The oxygen storage function of the catalyst serves to

oxidize CO and hydrocarbons during rich excursions (through stoichiometric

reactions with a limited capacity and, thus, limited duration) in the absence of

sufficient gaseous oxygen.The fact that NO conversions drop-off at rich A/F's in Fig. 3 may be due to

inhibition of NO conversion by high CO concentrations. On the other hand, the

fact that the conversions of all three components are about the same over the

100

90

c0

·Vi80Q; ---co

>C ----NO0

U -'-'-HHCCQl 70~Ql

0..

60

50+-----....----....-----,-------,

135 14 145

Air/Fuel Ratio15 155

lEe Prod. _. & Dev.

Fig. 3. Average conversions obtained over a pelleted catalyst duringthe warmed-up portion of the U.S. FTP (ref.4).

433

entire A/F range suggests that the conversion of each reactant may have beenlimited by mass transfer rates during this test. The inverse correlation ofexhaust flow rate and A/F that is obtained during driving, and the resultingpositive correlation of reactant residence time in the converter with A/F, may

explain why lower conversions are obtained for all three components at richA/F's.

At this stage, we have discovered some of the conditions that a catalyst cansee during driving. We have found that rich excursions associated with

acceleration are responsible for a disproportionate share of emission of CO,

hydrocarbons, and NO. And finally, we have seen that steady-state and cycled-A/F tests do not accurately reflect the performance of a catalyst duringdriving. In the next section, we will discuss studies that try to identify the

transient chemical processes in a catalyst that determine reactant conversionsunder dynamic conditions.

ANALYSIS OF DYNAMIC TESTS

Introduction

The goal of the analyses discussed in this section is to identify anddetermine the mechanism and kinetics of transient chemical processes that canaffect the dynamic performance of an automotive catalyst. By "transient II we

mean that the conversion due to the process changes at a rate that is somewhatslower than the rate of change in conditions during driving. That is, theconversion due to the process does not change either instantaneously or very

slowly. Some of the the transient processes that have been identified in

previous work are listed below:

1. Adsorption and accumulation of CO on the surface of the precious metalsin the catalyst. Stoichiometric reaction of the accumulated CO during

rich-to-lean transients (ref.S).

Z. Accumulation of reactive oxygen atoms by adsorption and/or reaction of

0z and NO with the precious metals and base metal oxides.Stoichiometric reaction of the accumulated reactive oxygen atoms with

CO, HZ' and hydrocarbons during lean-to-rich transients (ref.Z,6,7).3. Transient catalytic reaction of H20 with CO (water-gas shift) during

rich conditions over Rh oxidized under lean conditions (ref.8,9).

4. Oxidation and partial deactivation of the catalytic activity of one ormore of the precious metals under lean conditions. Reduction and

reactivation under rich conditions (ref.lO).

434

[n addition to the transient processes listed above, there may be others thatare important and that have not been identified yet. Although the processeslisted have been identified, there is much to be learned about their mechanisms

and kinetics.In addition to transient chemical processes, transient thermal processes may

also be important to determining catalyst response to changes in operating

conditions, even following warm-up. We do not consider the participation of

transient thermal processes in this report. These processes should not be

neglected in experimental work, and converters and laboratory reactors should beinstrumented with thermocouples at several different locations in the catalyst

pellet bed or monolith.We can introduce a simplification in our analysis of catalyst response by

classifying each of the possible transient chemical processes as one of two

types.The first two processes listed above involve accumulation of reactive species

during some periods of operation, followed by reaction of these species duringsubsequent periods of operation. I will call these "accumulation-reaction"

processes. The presence of this type of transient process can be identified bythe presence of a transient discrepancy in the mass balance of one or more

chemical elements across the converter. For example, more oxygen atoms might be

coming out of the converter in the exhaust at a particular instant in time thanare entering in the exhaust (when correction is made for the residence time of

exhaust in the plug-flow converter).The last two processes listed above involve only minor accumulation of

reacti ve oxygen atoms during oxidation of the precious metals. Instead, theyinvolve primarily a change in the catalytic activity of the catalyst. Thechange in catalytic activity occurs sufficiently slowly such that the dynamic

response of conversion over the catalyst is affected. I will call these"activity change" processes. The presence of this type of transient process canbe identified by the presence of a complex dynamic response that is not

accompanied by a discrepancy in an elemental mass balance across the converter.

One type of experiment that can be performed to study the dynamic response ofa catalytic converter is to make rapid changes in conditions - composition, flow

rate, temperature while continuously measuring exhaust compositionsimultaneously at the inlet and outlet of the converter. Ideally, one would

like to make fast measurements of all important exhaust species. Although thisis not possible for all species at present, we have made fast measurements of COin exhaust at the inlet and outlet of a converter. The apparatus used is

described in detail elsewhere (ref.ll,12,13).Fig. 4 and 5 show the results of measurements made during A/F cycling and

following a step-change in A/F setting. How does one go about analyzing the

20r-----------------------~

435

~c.9~C 1.0~couau

0.5 Hz 10Hz 167 Hz

O'--.,- -,_-,- ,--,-- ---,_.....J

t----45--........-1 1-----45-----<"" 1----45 --~..,

lEGProd.Res. & Dev.

Fig. 4. CO concentrations measured by infrared absorption spectroscopyat the inlet of a converter as the A/F setting was switched in a square-wave between a rich setting of 14.1 and a lean setting of 15.1. Equalperiods of time were spent at each A/F setting (ref. 11).

1.5

~z 1.0a>=~a:>-ZwUza 05u0u

10 20TIME(5)

30

Fig. 5. CO concentrations measured by infrared absorption spectroscopyat the inlet (top) and outlet (bottom) of a catalytic converter during alean-to-rich transient. The A/F setting was switched from a leansetting of 15.1 to a rich setting of 14.1.

436

results of such experiments? A common procedure is to compare experimental data

to the predictions of a detailed mathematical model of the physical system.

Such a model is not currently available.

We found a way out of this difficulty by going back and asking what it was

that we wanted to find out. First, we want to know whether the dynamic response

of a catalyst is "complex," that is, affected by changes in a transient chemical

process, as defined above. Second, If the response is complex, we would like to

be able to determine whether "accumulation-reaction" processes are present and

whether "activity change" processes are present. Only after these first two

questions are answered do we need more detailed information that would require a

detailed mathematical model.

In order to determine whether the response of a catalyst is complex, we only

need to compare the measured response to that predicted for a model catalyst

that has a "simple" response. Such a model catalyst with a simple response

would have (a) the same steady-state performance as the real catalyst, (b) no

accumulation of reactive species or reaction of accumulated species during

transients, and (c) only instantaneous changes in catalyst activity during

transients. We call the response of a model catalyst with this behavior

"instantaneous response. " Essentially, the model catalyst exhibits

instantaneous response to steady-state conditions.

Calculation of the instantaneous response that corresponds to the actual

measured response of a real catalyst is simple and is described in detail in

(reLll). At each instant in time, one takes the measured composition of

exhaust entering the converter, goes to a table of steady-state measurements and

finds the corresponding outlet composition, accounts for the residence time of

exhaust in the converter, and plots the "instantaneous" outlet concentrations

determined in this way along with the measured concentrations. The presence of

a discrepancy between the instantaneous response and the measured response

clearly indicates that the dynamic response of the catalyst is complex. In

addition, the discrepancy between the two response curves for an exhaust species

can be integrated to give a quantitative measurement of the discrepancy.

This procedure can also be applied to "cycled-A IF" experiments in which only

time-averaged concentration measurements are recorded. Note that one must first

average the inlet concentrations over the A/F cycle and then average the

appropriate steady-state outlet concentrations over the A/F cycle before

calculating an average "instantaneous response" conversion. One can not average

the steady-state conversion levels themselves in order to get an average

conversion.

Fig. 6 shows the instantaneous and measured CO response curves determined

for a catalyst following a step-change in A/F setting from lean to rich

conditions.

437

The notation next to the curve indicates that the action of the

transient chemical processes in the catalyst resul ted in the "extra" conversion

of 44 micro-mol of CO per gram of catalyst following the step-change .in A/F

setting.

The power of this method of analysis is shown, for example, in (ref.S) where

the presence of the transient enhancement of water gas shift, an "activity

change" process, was demonstrated. Although we were able to separate the two

types of transient processes to some extent using CO measurements alone, in

general, one requires measurement of more than just CO. Especially critical is

measurement of 02'

Numerical Simulation of CO Oxidation Response

In order to more fully explore the powers of this method of analysis, we

introduce a simple mathematical simulation of CO oxidation in a plug-flow

catalytic reactor. The purpose of the simulation is to demonstrate the method

of analysis, not to accurately simulate an automotive catalytic converter. The

advantage of using the simulation here is that we can look at the 02 and CO2response as well as the CO response.

The equations used in the mathematical simulation are described in the

Appendix. The only species considered are CO, 02' and CO2' For convenience, werefer to the calculated outlet concentrations as the "measured" concentrations

or responses. The "instantaneous" responses shown were determined from the

inlet signals and the steady-state performance of the simulated converter by the

procedure described above.

First, consider the response of the simulated converter to a lean-to-rich A/F

transition. The inlet 02 and CO signals are shown in Fig. 7, where the unit of

LEAN---...----RJCH_10

~I-w....~ 05

00

Jo

"0 10

44,.,mol/g

20

TIME (5)30

J. Catalysis

Fig. 6. Solid line: CO response measured by infrared absorptionspectroscopy at the outlet of a converter containing a Pt/Rh/Al °pelleted catalyst. Dashed line: computed instantaneous respon~e~ Thearea between the two curves shows that transient chemical processes inthe catalyst resulted in enhanced conversion of 44 micro-mol of CO onaverage, per gram of catalyst (ref.S). '

438

time is reactor residence time. The measured and instantaneous 02' CO, and CO2responses are shown in Fig. 8 through 10. The reasonableness of the shape of

the CO response can be seen by comparing Fig. 9 to Fig. 6. The existance of

complex dynamic behavior is clearly seen in the CO and CO2 responses, but is not

readily noticable in the 02 response.Performance of a mass balance on oxygen results in the top curve in Fig. 11

which shows that there is a discrepancy in the direction of excess oxygen atoms

appearing in the outlet of the converter during and following the AIFtransition. Performance of a complete mass balance in the real situation of

exhaust gas is difficult, of course, since several other reactant and product

species are involved.

We now use the mass balance discrepancy to subtract the contribution of any

"accumulation-reaction" processes present from the measured responses. For

example, the corrected response curve for CO, shown in Fig. 12, was obtained at

each instant in time by adding to the measured outlet CO concentration the

amount of CO that was converted to CO2 by reaction with oxygen atoms stored in

the catalyst, as determined from the oxygen balance. The fact that the

corrected CO response and the corrected CO2 response, shown in Fig. 13, do not

match the instantaneous response demonstrates the action of an "activity change"

type of transient chemical process. The process resulted in lower-than-expected

CO conversion, since the activity change process included in the simulation was

partial deactivation of the catalyst in lean exhaust (e.g., by oxidation of the

precious metal).

Note that the particular activity change process included here tends to

lessen the difference between the measured and instantaneous response curves.

Theoretically, one could get identical measured and instantaneous response

curves in the unlikely event that two transient chemical processes exactly

offset each other. However, discrepancies would probably still appear in the

mass balances if any accumulation-reaction process were present, and

discrepancies between the measured and instantaneous response curves would

probably appear in other types of transient response experiments on the same

catalyst.

Next we consider the response of the simulated converter to a rich-to-lean

transition. The inlet signals are not shown but are just an inversion of Fig.

7. In contrast to the case of the lean-to-rich transition, we find that only

minimal differences exist between the measured and instantaneous response curves

for CO and CO2 (Fig. 14 and 15). However, now there is a substantial difference

between the two responses for 02' as seen in Fig. 16.

The discrepancy in the oxygen balance is shown by the lower curve in Fig. 11.

The integrated area below the positive-going lean-to-rich curve is equal to the

439

1.0

0.8

~ 0.60c:8 0.4

0.2

0.00 2 4 6 8 10 12 14

Time

1.0

0.8

Z 0.6

'"0 0.4

0.2

0.00 2 4 6 8 10 12 14

Time

Fig. 7. Inlet signals to converter in simulatedlean-to-rich transition from AlF=15.1 to AlF=14.1.Time is in units of converter residence time inthis and all successive plots.

Fig. 8. Oxygen response at converter outlet,lean-to-rich transient.

0.8,.---------------,

1412108642

J0:====--t"\. instantaneous

10.0 L.-_.l-_....L._-'-_.......l__L-_.l-_.J

o

10.4

10.6

10.2

10.8

11.0 ~--------------.,

z'"8

1412108

lit instantaneous

6420.0 l..-_.....""--'-_...L..._...L-_....L._........_...J

o

0.6

~ 0.400

0.2

Time Time

Fig. 9. CO response at converter outlet,lean-to-rich transient.

Fig. 10. Carbon dioxide response at converteroutlet, lean-to-rich transient.

0.4

0.2

~

t1 0.0Q)

1il>'5 -0.2~0<'<

-0.4

-0.60 2 4 6 8 10 12 14

Time

Fig. 11. Oxygen atom discrepancy resulting frommass balance over converter during transients.

1412108642

10.8.----------------...,

10.6

10.0 L-_.L-_-'-_-'-_--'_---''-_-'-_-'o

102

,".~'-~', measured· 0 discrap

'"oo

14121086420.0 L--i~_1.--_L---JL---JL---JL---'

o

440

08

0.6

~ 0.4"--0o

0.2

TIme TIme

Fig. 12. CO response corrected to show contri-bution of "activity change" process and comparedto instantaneous response, lean-to-rich transient.

Fig. 13. Carbon dioxide response corrected toshow contribution from "activity change" processand compared to instantaneous response, lean-to-rich transient.

lit'" measured slighijy higher

141210864

0.8 11.0

10.80.6

l 10.6e:.0 0.4 No 8 10.4

0.210.2

10.02 4 6 8 10 12 14 0 2

TIme TIme

Fig. 14. Measured and instantaneous COresponse at converter outlet, simulated rich-to-lean transient from AJF= 14.1 to AJF= 15.1.

Fig. 15. Measured and instantaneous carbondioxide response, rich-to-Iean transient.

1.0

0.8

e 0.6 measured

d" 0.4

area closely approximates0.2 oxygen uptake by catalyst

0.00 2 4 6 8 10 12 14

TIme

Fig. 16. Measured and instantaneous oxygenresponse, rich-ta-Iean transient:

441

area above the negative-going rich-to-1ean curve. The earlier peak and lesser

tail of the rich-to-lean curve reflects the behavior of real three-way

ca ta1ysts, where re-oxidation of the oxygen storage component is fas ter than

reduction (ref.6).

When the measured 02 response curve is corrected for the loss of oxygen from

the outlet gas by re-oxidation of the oxygen storage component, we find that the

corrected curve and the instantaneous response curves are almost identical (not

shown). Thus, even in the presence of an activi t y change process, the area

between the measured and instantaneous response curves for 02 closely

approximates the oxygen storage capacity of the oxygen storage agent.

This result suggests that a good estimate of the oxygen storage capacity of a

three-way catalyst can be obtained from dynamic measurements of 02 during rich-

to-lean transitions, even in the presence of other transient chemical processes.

Of course, measurement of NO and other exhaust constituents would add to the

reliability of the estimate. In (ref. 6) we made measurements of changes in

reactive oxygen content, however, those measurements involved flushing the

catalyst with dry N2 between exhaust exposures. Dynamic measurement of 02 would

provide this information without such drastic perturbation of the catalyst.

RECOM!1ENDATrONS

Current test methods do not provide an accurate indication of catalyst

performance following warm-up. Whereas steady-state conversion measurements are

valuable for research purposes, they have no relevance to catalyst performance

during driving. Cycled-A/F tests are of limited value since they only provide

an accurate measure of performance during constant speed operation. (In partial

defense of cycled-A/F tests, we have found that they usually give the right

qualitative ranking of CO conversion performance during driving. However, they

are not very sensitive: differences measured between catalysts in cycled-A/F

tests are much smaller than differences measured during driving.)

Clearly, there is a need for development of additional tests that provide

measurements of catalyst response to the types of transients that occur during

variable ~peed driving. Our work indicates that a reasonable test would be one

in which a catalyst is stabilized at constant speed conditions (i.e., cycling

about the A/F control point) and then is subjected to a rich transient,

preferably with an increase in gas flow rate.

A reliable estimate of catalyst performance during driving might be obtained

by averaging emissions measured during A/F cycling about the control point with

emissions measured in a rich transient test, using weighting factors appropriate

to the A/F control system under consideration. The weighting factors could be

determined by (a) taking a variety of catalysts of different formulation, (b)

measuring their performance in the two simplified tests as well as in the

442

complete driving cycle, and (c) determining weighting factors for the two

simplified tests that give the best correlation with the driving cycle results.The rich transient test would receive lower weighting with A/F control systemsthat maintain tighter control of A/F during acceleration.

Even with perfect A/F control during acceleration, however, there will stillbe a large increase in exhaust flow rate. Catalyst response to this increase in

flow rate will be affected by the transient chemical processes in the catalyst,

resulting in a complex change in emissions during acceleration that should be

measured in the process of catalyst testing.In conclusion, we hope that our work has demonstrated the need for further

studies of catalyst response during variable speed driving and for furtherstudies of the transient chemical processes that affect the dynamic behavior of

automotive catalysts.

REFERENCES

L. L. Hegedus, J. C. Summers, J. C. Schlatter and K. Baron, J. Catal. 54(1979) 321.

2 H. S. Gandhi, A. G. Piken, M. Shelef and R. G. Delosh, Soc. Auto. Eng.Paper No. 760201 (1976).

3 Y. Kaneko, , H. Kobayashi, R. Komagome, O. Hirako and O. Nakayama, Soc.Auto. Eng. Paper No. 780607, SAE Trans. 87 (1978) 225.

4 R. K. Herz and E. J. Shinouskis, Ind. Eng. Chern. Prod. Res. Dev. , 24(1985) 385.

5 Y. Barshad and E. Gulari, Am. Inst. Chern. Eng. J., 31 (1985) 649.6 R. K. Herz, Ind. Eng. Chern. Prod. Res. Dev., 20 (1981) 451-457.7 M. A. Shulman, D. R. Hamburg and M. J. Throop, Soc. Auto. Eng. Paper No.

820276 (1982).8 R. K. Herz and J. A. Sell, J. Catal., 94 (1985) 166-174.9 J. C. Schlatter and P. J. Mitchell, Ind. Eng. Chem. Prod. Res. Dev. , 19

(1980) 288.10 E. Koberstein, Soc. Auto. Eng. Paper No. 770366 (1977).11 R. K. Herz, J. B. Kiela and J. A. Sell, Ind. Eng. Chern. Prod. Res. Dev.,

22 (1983) 387-396.12 J. A. Sell, R. K. Herz and D. R. Monroe, Soc. Auto. Eng. Paper No. 800463,

SAE Trans. 89 (1980) 1833.13 J. A. Sell, R. K. Herz and E. C. Perry, Soc. Auto. Eng. Paper No. 820388

(1982) .

APPENDIXThe mathematical simulation of CO oxidation in a catalytic converter that is

discussed above includes the following processes:

Reaction of CO with O2 with inhibition by CO.

Oxygen storage and reaction of stored oxygen.- Partial deactivation of CO oxidation activity by metal oxidation under

lean conditions.

443

The differential equations describing reaction over the simulated catalyst are:

d[CO]dt

k l (1- 8 )[02 ] [CO](1 + K [CO])2 kred (\jI-\jIe)

I Izero when \jIe > \jI

d[02]dt

d8dt

\jIe

-0.5 k l (l-8 )[02] [CO] ()(1 + K [CO])2 - 0.5 k ox \jIe - \jI

I Izero when \jIe< \jI

0.0165 k Oll 2 [02]k red (0.01 + [CO] )

d\jl

dt

Where,

k red- - (\jI-'I':)'I'cap e

I Izero when \jIe > 'I'

+k--.2! ('If, - '1')'I'cap e

I Izero when \jIe< \jI

[CO] CO concentration (%)[Oz] Oz concentration (%)

8 fractional coverage, by deactivating oxide, of surface active for CO oxidation8m maximum value allowed for 8 (= 0.65)

'I' fractional extent of oxidation of "oxygen storage component"'l'e "equilibrium" or steady-state extent of oxidation of oxygen storage component'I'cap capacity of oxygen storage component (= 2 %)kl rate constant for CO oxidation reaction (= 15 %-1 time-I)

K CO inhibition parameter (= 1.7 %-1)kax rate constant for oxidation of oxygen storage component (= 3 % time-I)lcred rate constant for reduction of oxygen storage component (= 0.9 % time-l)kp-on rate constant for oxidation and deactivation ofCa oxidation activity (= 1 %-1 time-I)kp-off rate constant for reduction of CO oxidation activity (= 0.8 %-1 time-I)

The values given in parentheses are the values of the parameters used in the

solution of the equations for the results presented in this paper.The equations describing the action of the oxygen storage component are

written so that they show the experimentally observed behavior that the oxygen

storage component does not contribute to CO conversion after steady-state

444

conditions are reached. The mechanism and kinetics of oxygen storage component

action in automotive catalysts is not well understood at the present time.

The local rate equations given above were incorporated in conservation

equations for a plug-flow reactor and integrated to give the results plotted in

the text. The units of time on the abscissa of all of the plots is reactor

residence time.

Fig. 17 below shows the predic ted steady-state CO conversion versus simulated

A/F. Fig. 18 shows the predicted variation in steady-state reactive oxygen

content of the simulated converter with simulated A/F.

0 1.0o0c; 0.80.§Ql>c 0.60o

"iiic0

0.4~LL

0.2 L_...L...._....L._--L__L...-_..I.-_....L.....l14.0 14.2 14.4 14.6 14.8 15.0 15.2

AJF

Fig. 17. Steady-state fractional conversion ofCO versus simulated NF predicted by numericalsimulation.

100 .--------------,

80

60Oxygen Content

(% of max.) 40

20

14.15 14.35 14.55 14.75 14.95 15.15NF

Fig. 18. Reactive oxygen content of simulatedconverter at steady-state conditions.

A. Crucq and A. Frennet (Editors), Catalysis and Automotive Pollution Control© 1987 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

EFFECT OF LEAD ON VEHICLE CATALYST SYSTEMS IN THE EUROPEAN ENVIRONMENT

445

M A Kilpin' A Deakin' H S Gandhi'

'Engine Engineering, Product Development, Ford of Europe

'Research Staff, Ford of USA

ABSTRACT

There are two catalyst operating parameters that could be significantly

different in Europe to the USA. These are the average operating temperature

and the lead levels in fuel.

A test programme was initiated to investigate the effect of lead on Three

Way Catalyst (TWC) with high temperature excursions to simulate autobahn

running. The programme was completed in three stages: Laboratory,

Dynamometer and Vehicle tests.

Testing showed that, depending on owner usage, the effect of permissible

levels of lead, according to DIN standard, in the fuel could significantly

affect the efficiency of the catalyst with extended usage.

INTRODUCTION

Background:-

The maximum lead level in unleaded fuel has been set at 13 mg/l Pb in the

DIN standard applicable in EEC territories, with a waiver to 20 mg!l

applicable for 6 months after introduction. These levels are anticipated to

give a concern of catalyst poisoning if they appear in the field. TWC's are

particularly affected by lead oxide compound covering the Rhodium sites, (2).

Typical Pb level in fuel generally available in U.S.A. is 0,8 mg/l. TWC's

can contain this level without concern. However if unique conditions in

European market such as non-dedicated tankage, or octane boosting using Pb

results in Pb levels reaching the legal maximum, then as shown by the

following data, there will be a high risk of contamination.

446

It was the possibility of high lead levels in the pump fuels which led to

the initiation of the extensive test programme described in this paper.

TEST PROGRAM

The test stages were:-.

1. Laboratory

2. Dynamometer

3. Vehicles

Pulsator Tests

Simulated 80K km Ageing

80K km ageing on AMA City Driving

Schedule

Two lead levels were used during the test programme.

Trace lead (up to 3mg/l) similar to that found currently in U.S.A. pump

fuel.

10mg/1 was chosen as it was anticipated that early supplies of unleaded

fuel in Europe could be close to the legal limit.

Each stage contributed data from a different

advantage of Laboratory and Dynamometer data was that

much quicker than by using 80K km vehicle tests.

Laboratory Pulsator Tests

aspect of

it could

ageing. The

be generated

Catalyst samples were aged in a pulse flame reactor (1).

The test cycle and activity measurements were as shown in Fig. 1. It

included a high temperature mode (1000 deg C) for 25% of the cycle time to

take account of autobahn driving. The test procedure was developed to

simulate the conditions found on the AMA City Driving Cycle, with

modification for Europe, and has a nominal space velocity of 40000/hr which

duplicates 48km/h steady state vehicle operation. To be able to evaluate

temperature effects on Pb retention another catalyst was aged on a modified

cycle that used only 730 deg C for 6% of the cycle instead of 1000 deg C for

25% of the cycle.

447

Catalyst Temperature Cycle:

25% Time: 1000°C max. with 3% CO excess

75% Time: 500°C 14.45:1 AFR

Activity Measurements:

Pulsator Modulation: 500°C; 40000/hr (Nominal);

+ 1 AFR at 0.5 Hz;

Final Steady-State: 550°C; 60000/hr

Fig. 1 Pulsator Test Cycle and Activity Measurement Conditions

Reprinted with permission c 1985 Society of Automotive Engineers, Inc

The ageing fuels consisted of isooctane with 0.2 mg/l P and either 3 or 10

mg/l.Pb added. The source of Pb was "TEL Motor Mix" containing

tetraethyllead (TEL), ethylene dichloride (EDC) and ethylene dibromide (EDB)

in an atomic ratio of Pb:Cl:Br of 1:2:1 The Pb containing isooctane was

injected with a nebulizer directly into the hot portion of the pulsator

furnace for combustion. Steady state activities of the pulsator aged

catalysts were measured at 550 deg C and 40000/hr. A diagram of the

apparatus and the synthetic gas mixture used is described in reference (3).

Dynamometer Tests

To maximise lead deposition, and to simulate the vehicle that spends its

life doing city driving, two catalysts were aged on a dynamometer engine to a

predominantly low temperature, low load cycle. The cycle is summarized in

Fig. 2, the ageing duration is 300 hrs. This represents 80K km on the road.

The two catalysts, one aged with 3mg/l fuel and the other with 10mg/l were,

in turn, fitted to an emission test data vehicle, that had a known emission

performance using a 6,4K km aged catalyst. A series of 83US emission tests

were undertaken with both catalysts.

448

TWC Ageing

Time

Inlet Temp

AIF Ratio

RPM

Fuel

Condition 1

8%

885°C

14,65 + 0.10

3000 - 3500 RPM

Condi tion II

8%

815 -c14,95 + 0.10

Ageing Time

Condition III

84%

465°C

14,65 + 0.10

Lead:

Phosphorus:

Sulphur:

0.003 or 0.010

0.001

0.225

gil

gil

gil

300 hrs 80,000 km

Fig 2 Ageing Cycle for Dynamometer Tests


Vehicle Durability

A fleet of 5 vehicles were prepared to each complete 80K km to the AMA City

Driving Schedule. Two vehicle types and engine capacities were chosen to

widen the database generated. Vehicles 4 and 5 were 49 State Federal models

from a european competitor. They were 1985 model year production vehicles

purchased from a franchised dealer in the USA. Vehicles 1 and 2, 4 and 5 were

paired, one running on trace lead fuel the other on 10 mg/l. They were

assigned as shown in Fig. 3. Vehicle 3 was tested at 0 mile, 6,4K km and

then run straight through to 80K km on 10 mg/1. This was to generate

information as quickly as possible. Knowing data from this car, modifications

to the test method, and emission test interval, for the other vehicles could

be incorporated if desired. Vehicles 1,2,4, and 5 have been emission tested

to the 83 U.S. test procedure according to the schedule shown on Fig. 4.

Fig 3

Vehicle Identification Engine Size Fuel Lead Level

1 2.0L 10 mg/l

2 2.0L Trace Lead

3 2.0L 10 mg/l

4 1.8L 10 mg/l

5 1.8L Trace Lead

Engine Size & Lead Levels for 80K km Vehicles

449

Vehicle Ident

1

2

3

4

5

o

X

X

X

X

X

6.4

X

X

X

X

X

10

xX

X

X

30

X

X

X

X

50

X

X

X

X

80 K km

X

X

Fig 4 Test Schedule for 80K km Vehicles

The vehicles were all multi point EFI equipped with HEGO control and full

engine management suitable for 83 U.S. markets. Servicing was carried out to

the routine specified for the vehicle plus any non scheduled maintenance

required.

DISCUSSION & RESULTS

Laboratory Pulsator Tests

Increasing residual Pb levels in the fuel from 3 to 10 mg/l for pulsator

ageing at a maximum temperature of 1000 deg C substantially decreased TWC

performance during pulsator modulation and steady state conditions. See Fig 5.

450

% Conversion

Pulsator (500°C) Steady State (550°C)

Simulated 14.5 AFR 14.6 AFR 14.3 AFR

Fuel Mileage Km

mg Pb/l (OOO'sl HC CO NOx HC CO NOx HC CO NOx

3 24 63 67 67 95 98 98 66 41 82

10 24 37 33 22 92 95 92 52 45 67

Fig 5 Effect of Fuel Pb levels on Activity of Pulsator-Aged Catalysts


Evaluated at 500 deg C at an air fuel ratio (AFR) of 14.5:1 + 1 A/F at 0.5

Hz the Nox performance was the most affected dropping from 67% conversion at

24K for 3 mg/l to 22% conversion for 10 mg/l. CO suffered a 34% decrease in

efficiency and HC was least affected with a 26% drop to a 37% conversion

rate. Analysis of the catalysts after ageing with 3 mg/l showed no Pb

retention on the catalyst surface. Therefore the threshold for retention

occurs above 3 mg/1 but is already highly deleterious by 10 mg/l.

Steady state conditions measured at 550 deg C at AFR 14.5:1 and 14.3:1

respectively shows that at stoichiometry the conversion efficiency loss for

HC, CO and Nox is 3%, 3% and 6% respectively when comparing 10 mg/1 and 3

mg/1 aged catalysts. However at AFR rich of stoichiometry the performance

deterioration is very significant for HC and Nox at 14%, and 15% respectively.

% Conversion (550°C)

Max

Temp C

Fuel

mg/l

Simulated

km (OOO's) HC

14.6 AFR

CO Nox HC

14.3 AFR

CO Nox

1000

730

3

3

24 95

24 96

98

98

98

98

66

32

41

60

82

69

Fig 6 Steady State Activity for Catalysts Aged with 730 & 1000 deg C Maxima

451

The cycle was modified to include 6% of the time at 730 deg C instead of

25% at 1000 deg C. The results are shown in Fig. 6 for the same Pb level of

3 mg/l, at 550 deg C 14.3 AFR steady state, the HC conversion was 32%

instead of 66%. The surface area of the catalyst at 730 deg C was twice that

at 1000 deg C. However as stated previously the Pb retention at 1000 deg C

with 3 mg/l was zero. Therefore the poisoning effect of Pb deposition at 500

and 730 deg C is more significant than the loss of 50% surface area to

catalyst efficiency.

Dynamometer Ageing

A pair of catalysts, one dynamometer aged to 80K km on 3 mg/l and the other

~n 10 mg/l Pb was tested in turn on a 1.6L Ford Escort with a known emission

history. 83 U.S. tests were conducted. The results obtained are shown in

Fig 7.

o km

80K km

80K km

Legal Limit

HC

0.18

0.26

0.80

0.32

co

1.18

1.85

4.52

2.62

NOx

0.11

0.12 3 mg Pb/l

0.26 10 mg Pb/l

0.77 Assumes 1.3 D.F.

Fig 7 Emission Results with Catalyst Dynamometer Aged

(Values in grams/mile)

The maximum temperature reached during the ageing cycle was 885 deg C.

This temperature was achieved for only 8% of the cycle. 84% of the cycle was

at 475 deg C which was low enough to maintain high surface area but it also

meant lead deposition was high. This accounts for the deactivation, but

indicates that a typical vehicle is able to travel 80K km of urban,

relatively low temperature, driving and still remain inside legal levels if

the Pb level is 3 mg/l. whereas 10 mg/l deactivates the catalyst

significantly and produces HC and CO figures above the legal level.

Interpolating between these points, assuming linear deactivation against lead

level, up to 5 mg/l could be tolerated before the catalyst would be too

deactivated to remain inside the legal limits.

452

To demonstrate this, catalysts were tested on the pulsator rig and results

showed that efficiencies had decreased to 50%, 61% and 47% for HC,CO and Nox

respectively. These results compare with those at 14.5 AFR shown in Fig 5.

and broadly substantiate the assumption that increasing lead levels reduce

catalyst activity linearly in this range.

This test sequence clearly indicates that a vehicle that has good

conformity at zero mile and 80K km with 3mg/l fuel deteriorates significantly

with lOmg/l Pb fuel.

Vehicle Durability

The vehicles used during this stage of testing are shown in Fig 3. and the

emission test schedule undertaken is shown in Fig. 4.

A summary of the 83 U.S. emission test data, and the corresponding catalyst

conversion efficiencies is shown in Fig 8.

Vehicle ,000 Emissions % Conversion RemarksNo km (gms/mile)

HC CO Nox HC CO Nox

0.32 2.26 0.77 Legal level assuming1.3 D.F

0 0.285 2.24 0.26 86.4 80.9 91.36.5 0.509 4.32 0.38 79.9 67.7 89.3

1 50 1.012 7.66 0.41 71.4 55.6 86.3 Aged Hego 10 mg Pb/l80 1.260 6.83 0.56 68.6 54.1 81.8 Aged Hego80 0.748 4.14 0.87 75.1 66.3 73.8 Fresh Hego

0 0.248 1.07 0.61 89.4 89.3 80.72 6.5 0.418 2.52 0.58 83.8 77.5 84.6 Trace Pb

50 0.479 3.92 0.45 83.03 71.93 84.8 Aged Hego

6.5 0.152 1.36 0.62 89.2 88.7 85.43 80 0.607 6.00 0.70 65.0 69.4 83.6 Aged Hego 10 mg Pb/1

80 0.358 2.86 1.03 76.7 79.1 76.2 Fresh Hego

0 0.156 1.01 0.26 90.5 89.6 88.6 10 mg Pb/14 6.5 0.358 2.24 0.63 78.3 82.0 84.2

50 0.675 3.32 1.18 71.8 69.0 59.7 Aged Hego

0 0.175 0.85 0.44 88.85 6.5 0.184 1.16 0.70 89.8 86.9 77.0 Trace Pb

50 0.216 1.47 1.37 90.0 80.4 52.5 Aged Hego

Fig 8 Summary of Emission Results for 80K km Durability Vehicles

453

Vehicles 1 and 2 were fitted with an early, partly developed, hence the

higher than ideal emission levels at zero mile. Vehicle 1 used 10 mg/l

whilst vehicle 2 ran trace Pb fuel. Fig. 8 shows the emission performance of

the two vehicles and the catalyst efficiency throughout the test. Vehicle 2

was damaged before 80K km had been reached resulting in the 50K km test being

the last data point. Sufficient distance had been covered to demonstrate the

catalyst performance characteristic. Fig 9 illustrates the large differences

in catalyst efficiencies that developed as the test progressed. The vehicle

1 catalyst demonstrates a significant loss for HC and CO by 10K km after

which HC and CO conversions were never above 72% and 65% respectively. On

vehicle 2 with trace lead however the HC performance remained constant over

50K km with conversions always above 80%. For CO some deterioration did

occur from 90% at start of test to 72% at completion, but its performance was

superior to the 10 mg/l catalyst.

1

1

80

2

50

-------------2

~-------l

'-.) ':rJ:

><

60

c:2; 100wH'-.)H(,..(,..w

02; '-.)0 6H[fJ0::W> 1002;0'-.)

x 8002;

60

0 10

Fig 9. Catalyst Efficiencies for Vehicles 1 and 2

The Nox conversion performance of both catalysts was satisfactory but the

deterioration factor generated by the trace lead catalyst is 32% better than

that of the 10 mg/l catalyst. Although vehicle 2 had to be stopped after 50K

km the superior performance of the catalyst at this point relative to vehicle

1 is demonstrated by the HC figures of 0.48 g/m against 1.01 g/m and the CO

of 3.92 g/m against 7.66 g/m.

454

The catalyst that had been subjected to the 10 mg!l fuel has clearly

suffered 10% to 15% performance loss due to lead. After this data had been

generated a fresh HEGO sensor was fitted to vehicle 1 and the test repeated.

The results show a 7% conversion efficiency improvement for HC, a 12%

improvement for CO and a 8% deterioration of Nox. This indicates that it was

controlling the engine leaner than the 80K aged HEGO. Therefore with ageing

there had been a rich drift, and maximum catalyst conversion potential was

not being used.

The result of vehicle 3 at 6,4K km shows the HC, CO and Nox levels

significantly inside the legal limit, with conversion efficiencies ranging

from 85% to 89% on the three gases. At 80K km the conversion efficiency for

HC and CO had dropped to 65% and 69.4% respectively which results in tailpipe

levels of 0.61 g/m and 6.0 g/m. Both these are above the legal level. Nox

conversion however was retained at 84% giving a 0.7 g/m result.

The results from vehicle 3 show that the catalyst activity is almost

sufficient to achieve legal levels at 80K km. Fitting a fresh HEGO sensor

to vehicle 3 showed the same trend as vehicle 1. HC and CO efficiencies

increased whilst Nox efficiencies decreased indicating the HEGO sensor had

drifted rich during ageing. The changes observed for vehicle 3 were 10% for

HC 10% for CO and 7% for Nox. This is of similar order to the changes on

vehicle 1.

Vehicles 4 and 5 were the competitor vehicles as described in Fig. 3.

Vehicle 4 was fuelled with 10 mg/l, vehicle 5 with trace Pb. Data for these

two vehicles is available to 50K km. At the 6,4K km test point for vehicle 5

the engine settings were found to be away from specification significantly,

and so emission data generated at 0 mile was discarded. The engine was reset

to specification and retested. The subsequent poor Nox performance of this

vehicle has not been explained but is subject to further investigation.

Fig 10 shows the catalyst efficiencies over 50K km and comparing the two

vehicles for HC and CO only, it can be seen that the trace lead catalyst

retains a constant performance for HC, and only exhibits 7% CO

deterioration. Catalyst conversion remained between 80% and 90% throughout

the test. The 10mg/l catalyst however has suffered a 12% loss in HC

conversion efficiency and a 13% loss for CO, bringing the conversion

efficiency between 70% and 80%.

455

The Nox conversion efficiencies of the catalyst on vehicle 4 shows a 29%

deterioration over 50K km, indicating a severe effect from the Pb in the

fuel. This deterioration results in the tailpipe Nox levels increasing by

almost 19/m to 1.18g/m. Vehicle 5 emission data shows it to be within legal

limits for HC and CO at 50K km, but Nox must be disregarded as explained

earlier. The mileage accumulation for vehicles 4 and 5 is still in

progress. Consequently data is not yet available for the BOK km stage, or

for the fitting of a fresh HEGO sensor.

>< 100oZr::IH 90 0_0 0 o 5t.lH x-x~

X o~x HC~ 80 x 5 0r::I oX'

x ____ : ==---0 x COZ 700 4H Xt/.I~

~60

0t.l 50

10 30 50 ,000km

Fig 10 HC & CO Catalyst Efficiencies for Vehicles 4 and 5

CONCLUSIONS

The programme described was intended to be wide ranging in the simulation of

service conditions.

The laboratory pulsator test simulated mixed urban and autobahn driving. The

results indicate that for any vehicle subjected to this mix, lead levels up

to 3 mg/l will not cause concern, due to lead being returned to metallic Pb

and removed from the catalyst. However, 10 mg/l fuelling together with the

catalyst surface reduction caused by, high temperature excursions will result

in unacceptable catalyst efficiency deterioration.

The dynamometer ageing test results demonstrated that fuel with 10 mg/l is

unacceptable. However, if the effect of lead deposition is assumed to be

linear then maximum Pb levels of approx 5 mg/l can result in legal emission

levels being achieved at BOK km when catalysts experience a modest duty cycle

as described in this paper.

456

The vehicle durability tests have consistently shown that substantial

catalyst deactivation takes place with fuel at 10 mg/l Pb. Lead levels of 3

mg/l and below allow catalyst systems to function satisfactorily during 80K

km of AMA drive cycle which would indicate that even if a customer

continually drives at low speed (which gives max lead deposition condition)

then catalyst deterioration due to lead will be minimal. The test results

indicate that the effect of lead on the HEGO sensor is more critical than its

effect on the catalyst.

Since the test schedule started the lead levels in unleaded fuel available

at the pumps in Europe (Germany, Switzerland, Austria) has been monitored.

Against expectation lead levels have dropped rapidly to an average of 2mg/l.

This level, if maintained, will ensure that the effect of lead on catalyst

systems will be negligible up to 80K km.

This test programme has also illustrated that if lead levels do rise in

future to 10 mg/l or above, catalysts and HEGO sensor systems would be

deactivated such that compliance with 83 US legal levels at 80K km would not

be possible. This may arise in a territory that introduces lead free fuel

with less control than has been exercised in Germany/Switzerland and Austria

to date.

REFERENCES

1. K Otto, R A Dalla Betta, and H C Yao, "Laboratory Method for the

Simulation of Automobile Exhaust and Studies of Catalyst Poisoning"APeA J 1974 24, 596

2. H S Gandhi, W B Williamson et al "Affinity of Lead for Noble Metals on

Different Supports".

3. H S Gandhi, A G Piken, M Shelef, R Delosh "Laboratory Evaluation of

Three Way Catalysts" SAE Transactions 1976.

4. W B Williamson, H S Gandhi, M E Szpilka, A Deakin "Durability of

Automotive Catalysts for European Applications". SAE paper 852097.

A. Crucq and A. Frennet (Editors), Catalysis and Automotive Pollution Control(D 1987 Elsevier Science Publishers B.Y. Amsterdam - Printed in The Netherlands

A LABORATORY METHOD FOR DETERMINING THEACTIVITY OF DIESEL PARTICULATE COMBUSTION CATALYSTS

R. E. MARINANGELI, E. H. HOMEIER and F. S. MOLINAROAllied-Signal Engineered Materials Research Center, 50 East Algonquin Road,P.O. Box 5016, Des Plaines, Illinois 60017-5016

ABSTRACT

457

Diesel particulates are a health hazard and legislation has been estab-lished (in the U.S.A.) to reduce diesel particulate emissions. Particulatetraps have been developed which can filter (up to 90%) of these particulates[Ref. IJ, but require some external means to burn the collected particulates.One way to ignite these particulates effectively at low temperatures is to usetraps which initiate soot combustion catalytically.

In order to determine the relative activity of various catalytic composi-tions, a two stage method to collect diesel particulates and accurately deter-mine the activity of the catalytic materials has been developed.

In addition to the description of the two-stage method, the activity ofselected base metal and noble metal catalysts are compared. The mechanism forsoot combustion is also discussed in light of the combustion rates found.

INTRODUCTION

Particulate emissions from diesel engines are implicated in health prob-lems (e.g., cancer, respiratory stress, etc.) and contribute to lowered visi-bility in densely populated urban areas [Ref. 2J. Owing to these factors, theU.S. Environmental Protection Agency plans to implement strict standards aimedat controlling diesel emissions. Since the most severe standards cannot bereadily met by engine modifications, work has focused on trapping the particu-lates. Numerous trap designs have been tested including fibrous filters [Ref.3J, woven filters [Ref. 4J, metal mesh filters [Ref. 5J, ceramic foam filters[Ref. 6J, and wall-flow monolith filters [Ref. IJ. Of these systems, only cer-amic foam and wall-flow monolith filters have shown promise as effective dieselparticulate traps.

158

Once the particulates are trapped, the next problem is conversion of theseparticulates into innocuous substances. Since the combustion temperature ofdiesel particulates is about 650°C in the absence of catalysts and since theexhaust temperature in diesel passenger vehicles is often no higher than 300-450°C, the particulates will not spontaneously ignite. Alternate ways areneeded to burn the particulates so that excessive back-pressure due to trapplugging does not occur. Two ways have been proposed to burn the particulates:1) external means of heat generation to increase the exhaust temperature, and2) use of catalytic materials which will lower the combustion temperature ofthe particulates.

This paper will focus on the catalytic combustion of diesel particulates.The majority of the work on catalyzed diesel traps has focused on engine or ve-hicle measurements. However, some work has been done to quantify the catalyticcombustion of diesel particulates. For example Otto, et al. [Ref. 7J collectedparticulates and then burned them in the laboratory. They determined the ef-fect of temperature, oxygen pressure, and step-wise combustion on reactivity.No catalyst was involved in this study. Hillenbrand and Trayser [Ref. 8J tooksoot collected from an engine, mixed it with metal salts (Cu, Na, Co, and Mn),and burned it in a laboratory reactor. A substantial lowering of the combus-tion temperature was observed with the use of such salts. McCabe andSinkevitch [Ref. 9J also looked at mixing base metal additives either with thesoot or the fuel and then determined the effect on soot combustion temperature.Finally, Goldenberg, et al. [Ref. 10J looked at soot oxidation either alone oron a catalytic material.

Most of the work cited above has dealt with treating the soot in some waybefore doing the combustion experiments. We wish to report experiments con-ducted on soot from a diesel vehicle which has been deposited onto catalyticmonolithic substrates. This sooted substrate is then placed in a laboratoryapparatus where a synthetic gas mixture flows over the sample, and the sootcombustion is monitored as a function of temperature. The laboratory set upsimulates regeneration conditions on a vehicle. Using this technique we havebeen able to obtain kinetic information about the oxidation of soot and gaseousproducts. Comparisons of base metal and noble metal catalysts were also con-duct~d and are reported. It is intended that this work will help elucidate themechanism involved in the catalytic combustion of soot which should help in de-veloping improved catalytic materials.

459

EXPERIMENTAL

Catalyst Preparation

Catalysts were prepared on Corning EX20 cordierite, open channel monolith-ic substrates (nominally 62 square channels per square centimeter). High sur-face area supports were activated with base or noble metal components. Thefinal composition of the fresh catalysts are shown in Table 1, where the metalcontent is expressed as grams of metal per liter of catalyst (including sub-strate).

The monolithic substrates were cut lengthwise into quarter sections priorto preparation of the catalyst. Once the four catalytic samples were prepared,they were combined to yield a complete monolith by cementing the quartered sec·tions together with Sauereisen Number 8, a ceramic adhesive, as indicated inFigure 1.

Deposition of Diesel Soot

Once an open channel monolith was reassembled it was sealed into a de-mountable catalyst holder and placed in the exhaust of a diesel vehicle whichwas driven over a prescribed cycle on a chassis dynamometer. The vehicle was a1977 International Harvester diesel Scout equipped with an indirect injected3.2L, six cylinder engine. Commercial number two diesel fuel was used for allthe vehicle experiments.

The diesel soot deposition cycle which was used is described in Table 2.The maximum temperature at the inlet of the catalyst was maintained at 288°C(550°F) by adjustment of the load. Generally, 48 hours of soot collection wassufficient to permit evaluation of the catalysts.

No catalyst durability experiments will be reported here. However, forsome catalysts an accelerated aging was used which involved eight consecutivesootings at an inlet temperature of 370°C for three hours each followed by aregeneration during which the inlet temperature was increased to 650°C for 15minutes. The regeneration and sootings were performed at constant engine speedand load. Following the accelerated aging, the diesel soot was applied for sixhours using the previously described soot deposition cycle.

Laboratory Activity Test

The soot containing cores were tested for conversion of C3Hg, a modelhydrocarbon, and the retained carbonaceous soot using an automated laboratory

460

TABLE 1

Composition of Experimental Diesel Catalysts

Support

NS/A1 203Al 203Al 203

Catalytic Metal Content,Grams/Lite.~r _

Pt , 0.53Pt, 0.53

None

NonePt/Pd/Cu/Cr, 0.53/0.53/3.53/1.77

Pt/Pd, 0.53/0.53Cu/Cr,3.53/1.77

aNovel support.

TABLE 2

Diesel Soot Deposition Cycle

Time Inlet Temp-Mode Speed (MPH) (Seconds) erature (OC)

1 Idl e 15 149 Average2 Idle-24 14 193 Peak3 24 Cruise 13 182 Average

4 24-20 11 171 Minimum5 20-35 21 254 Peak6 35 44 240 Average

7 35-20 17 177 Minimum8 20 10 1779 20-Idle 8 149 Minimum

10 Idle 10 14911 Idle-40 17 288 Peak12 40 40 28813 4O-Idle 20 149 Minimum

461

Cemented FIGURE I

Schematic Design of the ReassembledMonolith

l~j

CatalystA .__----7"-----.. CatalystB

00: 0 0Cemented - - - - - - T - - - - Cementedo 0 I 6-0

Catalyst0 CatalystCIIIIIIIIIII

Cemented

METERED GASES

SELECTORVALVE

FIGURE 2

Experimental Apparatus

Effects of Support and Noble Metal onCO2 Prorluction During DieselParticulate Oxidation

FIGURE 3

200

o PtlNS{:, AIZ030n1yo PtlAl203o NSonly

E Z400~, -~---~~ ~-- ·,--c .-

§:ZZOOf

fJ ZooofC? 1800fz 1600f81400f~ 1zoofaJ l000[o 8005 600~ 400f~ ZOO~ 0l'o'\on~~>'--<l:;t=::::'~;;--'o 100

o PtlNS{:, AI2030nlyo PtlAI203o NSonly

200 300 400 500 600 700 800INLETTEMPERATURE(DEG.C)

FIGURE 4

Effects of Support and Noble Metal onCO Production During Diesel ParticulateOxidation

462

test as follows. Pieces of the cylindrical cores (2.22 cm in diameter by 1.27cm high) were subjected to a temperature programmed oxidation in the apparatusshown schematically in Figure Z. The activity testing equipment has been des-cribed previously [Ref. 11], and was modified for these experiments by additionof a low level (0-Z500 ppm) COZ analyzer.

The feed gas composition was selected to simulate a highly oxidizing(lean) diesel exhaust gas except that COZ was absent. This gas composition issummarized in Table 3. With this simulated exhaust flowing over the catalyst,the temperature at the catalyst inlet position was increased from 120°C to750°C at 15°C/minute with 15 minute holds at 300, 350, and 400°C. The analysisof the product gas for CO, COZ' C3HS and 02 permitted determination of propaneand soot-carbon (i.e., carbon and adsorbed hydrocarbon) burning rates versusthe inlet temperature. The CO 2 formation rate minus the C3HS and CO disappear-ance rates equals the soot combustion rate. When a temperature of 750°C wasreached the catalyst was immediately cooled to 1Z0°C to determine if residualsoot remained and to permit comparison of C3HS burning rates without the sootpresent.

RESULTS

Effects of Noble Metal Addition

Table 4 shows the average carbon content after soot deposition for mono-liths wash coated with A1 Z03 and monoliths wash coated with Pt/A1 Z03• The aver-age carbon content after soot deposition is the same for both types of mono-lith. The Pt/A1 Z03 catalyst does not initiate soot combustion during the stan-dard soot deposition cycle. Figure 3 compares the CO Z production, a measure ofsoot combustion rate, as a function of temperature for A1 Z03 and Pt/A1 Z03• ThePt/A1 Z03 catalyst not only initiates soot combustion at a lower temperature«300°C) but reaches a maximum rate at a lower temperature (500°C versus550°C). Since soot oxidation occurs over a wide temperature range, local hotspots may be minimized. Figure 4 shows that no CO is produced during soot oxi-dation by Pt/A1 Z03, while A1 Z03 produces a substantial amount of CO. Figure 5shows that the rate of hydrocarbon oxidation, as measured by C3HS conversion,is substantially higher for Pt/A1 Z03•

We believe that Pt initiates oxidation of the easily combusted, adsorbedhydrocarbons on the particulate (T <300°C). This provides a local exotherm

TABLE 3

Simulated Laboratory Exhaust Gas a

463

Component

aDry basis.

Concentration

300 ppmooo

10%100 ppm50 ppm

Balance

Added 10% steam at the reactor.

TABLE 4

Average Carbon Content on Catalysts After Sooting(Estimated Standard Deviation = 0.4%)

Catalyst

A1 Z03*Pt/A1 Z03*

Novel Support*Pt/Novel Support*

Pt/Pd/Cu/Cr/A1Z0 3**Pt/Pd/A1 Z03**Cu/Cr/A1 Z03**

Carbon Content(Weight Percent)

3.53.5Z.6Z.O

Z.63.65.8

*Six measurements; six hours sootingafter eight sootings and regenerations.

**Three measurements only; 48 hours sooting.

464

which lights-off oxidation of the relatively less reactive carbonaceous partic-ulate. Pt activates oxygen effectively. This enhances the soot oxidation rateat all temperatures and minimizes formation of partially oxidized particlessuch as CO.

Effects of the Support Material

The influence of the support material was determined by examining mono-liths coated with a novel support. Table 4 shows that this support was lesseffective than A1 203 for particulate trapping during the standard sootingcycle. This support material may also catalyze some particulate burning duringthe soot trapping cycle. The Pt/novel support (NS) catalyst did catalyze par-ticulate oxidation during the sooting cycle as determined by the lower carboncontent after sooting. Figure 3 shows that the novel support catalyzes partic-ulate oxidation at a lower temperature than A1 203 and that Pt/NS is more effec-tive at initiating and completing particulate oxidation than Pt/A1 203• Figure4 shows that Pt/NS is effective at minimizing CO emissions. Figure 5 showsthat Pt/NS may be less effective than Pt/A1 203 for C3H8 oxidation. The Pt/A1 203 results, however, may be influenced by a substantial local exotherm dueto the high rate of soot oxidation from 400°C to 500°C. The plot of C3H8 out-let concentration shows a deviation at an inlet temperature of 450°C which maybe explained by the temperature rise in the catalyst bed.

The support obviously influences the performance of particulate trap-oxidizers. First, the novel support is apparently less effective at trappingparticulates. This support obviously has some catalytic activity itself as thecomparison of the novel support to A1 203 shows. The activity of Pt/NS is stillhigher than can be explained by the activity of the novel support itself. Wecan speculate that the support affects the overall activity of the system byinfluencing the activity of the Pt itself or by influencing the deactivationrate of the catalyst.

Effects of Base Metals

Both CuO and Cr203 are known to catalyze graphite oxidation [Ref. 12J atrelatively low temperatures. Particulate oxidation catalysts were preparedwith Cu/Cr/A1 203, Pt/Pd/A1203 and Pt/Pd/Cu/Cr/A1 203 to determine the effects ofthese base metals. Table 4 shows that both Pt/Pd/A1203 and Pt/Pd/Cu/Cr/A1 203accumulated significantly less carbon than Cu/Cr/A1 203 during sooting indicat-ing that these catalysts are active for particulate oxidation during the soot-ing cycle. Pt/Pd/Cu/Cr/A1 203 may be more active than Pt/Pd/A1203 during

465

200 300 400 500 600 700INLETTEMPERATURE(DEGC.)

FIGURE 5

Effects of Support and Noble Metal onC3Hg Oxidation During DieselParticulate Combustion

o PtJPd/Cu/Cr6 PtJPdOCu/Cr

'[ 10000~-~---Q.N 90008 8000is 7000~ 6000a: 5000!2:ui 4000ois 3000u 2000~ 1000~ O~~=c;;:c;;---:;;~-~cc--~~=-=:':::Jo 200

FIGURE 6

Comparison of Catalysts ContainingCulCr for CO2 Production During DieselParticulate Oxidation

o PtJPd/Cu/Cr6 PtJPdOCu/Cr

In300 400 500 600INLETTEMPERATURE- DEG. C

700

FIGURE 7

Comparison of Catalysts Including CulCrfor CO Production During DieselParticulate Oxidation

FIGURE g

Comparison of Catalysts Including CulCrfor C3HS Oxidation During DieselParticulate Oxidation

700

o PtJPdlCu/Cr6 PtJPdOCu/Cr

300 400 500 600INLETTEMPERATURE- DEG. C

E350,---~---,--~--~-~Q.gU 300h-""""~-&o-XD--~I

Z 250o~ 200a:e-m150ois 100otiJ 50--'~ 0o 20SCO------c~-----.7vc--~~-~oc---d

466

sooting, but this conclusion is not statistically significant at the 90% confi-dence level. Figure 6 shows that during the laboratory soot oxidation testthese two catalysts have the same activity. The only difference between theCO 2 evolution curves is due to the higher carbon content on Pt/Pd/A1 203• TheCu/Cr/A1 203 "lights-off" for particulate oxidation at about 400°C. Figure 7shows that this catalyst produces a significant amount of CO during particulateoxidation. Figure S shows that the C3HS oxidation rate is enhanced by the exo-therm produced by soot oxidation. The local temperature increase is about100°C.

Base metals such as Cu/Cr/A1 203 may initially catalyze soot oxidation, butthey are probably rapidly poisoned by sulfate formation. Fishel et al. [Ref.13J have shown that sulfate poisoning deactivates Cu/Cr/A1 203 base metal auto-motive catalysts. These sulfates decompose at temperatures of ~460°C

[Cr2(S04)3J [Ref. 14J to 5S0°C (CuS04) [Ref. 13J thereby reactivating the cata-lyst. The light-off of the Cu/Cr/A1 203 catalyst at about 400°C in the lab testis consistent with a mechanism involving generation of local hot spots leadingto sulfate decomposition and a rapid increase in catalyst activity.

Base metal oxides such as CuO and Cr203 are, therefore, not effective forinitiating low temperature particulate oxidation compared with noble metalssuch as Pt or Pd. Base metal catalysts, however, could effectively propagatethe soot oxidation reaction after initiation by noble metals if the local cata-lyst temperature exceeds the sulfate decomposition temperature. The data forcarbon content during sooting are consistent with some enhanced performance forPt/Pd/Cu/Cr/A1 203• This catalyst, however, would be in the less active sulfat-ed state at the beginning of lab testing.

CONCLUSIONS

The study which we have conducted shows that the support material has aneffect on the soot combustion characteristics of a supported catalyst, with thebest support being a novel support. Comparing base metal to noble metal cata-lysts ~e have found that certain base metal catalysts (i.e, CuO or Cr203) arepoisoned by sulfur, do not promote complete combustion of the soot, and initi-ate soot combustion at a higher temperature than noble metal catalysts. Theseresults are consistent with a soot combustion mechanism in which the metal (es-pecially the noble metals) initiates oxidation of easily combusted adsorbedhydrocarbons and thereby provides local exotherms which initiate the oxidationof the soot.

467

REFERENCES

1. N. Higuchi, S. Mochida, and M. Kojima, Society of Automotive Engineers,Paper No. 830073 (1983).

2. M. P. Walsh, Society of Automotive Engineers, Paper No. 840177 (1984).

3. J. S. MacDonald and G. L. Vaneman, Society of Automotive Engineers, PaperNo. 810956 (1981).

4. H. F. Sullivan, G. M. Bragg and C. E. Hermance, Society of AutomotiveEngineers, Paper No. 800338 (1980).

5. B. E. Enga, M. F. Buchman and I. E. Lichtenstein, Society of AutomotiveEngineers, Paper No. 820184 (1982).

6. Y. Watable, K. Irako, T. Miyajimor, T. Yoshimoto and Y. Murakami, Societyof Automotive Engineers, Paper No. 830082 (1983).

7. K. Otto, M. H. Sieg, M. Zinbo and L. ~artosiewicz, Society of AutomotiveEngineers, Paper No. 800336 (1980).

8. L. J. Hillenbrand and D. A. Trayser, Society of Automotive Engineers,Paper No. 811236 (1981).

9. R. W. McCabe and R. M. Sinkevitch, Society of Automotive Engineering,Paper No. 860011 (1986).

10. E. Goldenberg, M. Prigent and J. Caillod, Revue De 1'Institute Francais duPHrole, ~(6), pp, 793-805 (1983).

11. G. C. .loy , G. R. Lester and F. S. Molinaro, Society of AutomotiveEngineers, Paper No. 790943.

12. D. W. McKee, Carbon !, 623 (1970).

13. N. A. Fishel, R. K. Lee and F. C. Wilhelm, Environmental Science andTechnology !(3), 260 (1974).

14. P. 5. Lowell, K. Schwitzgebel, T. B. Parsons and K. J. Sladek, Ind. Eng.Chern. Process Des Develop. ~(3), 384 (1971).


SYNTHESIS OF HIGHER ALCOHOLS ON LOW-TEMPERATURE METHANOL CATALYSTS

G. FORNASARI, S. GUSI, T.M.G. LA TORRETTA, F. TRIFIRO' and A. VACCARI

Istituto di Tecnologie Chimiche Speciali, Facolta di Chimica Industriale,

Viale del Risorgimento 4, 40136 BOLOGNA (Italy).

ABSTRACT

469

The synthesis of higher molecular weight alcohols together with methanol fromsynthesis gas was investigated using low temperature methanol catalysts obtainedfrom homogeneous precursors with hydrotalcite-like structures. The addition ofpotassium up to about 0.4% by weight favoured the synthesis of higher molecularweight alcohols, a formation which, however, was observed also with the undopedcatalysts, as a function of the catalyst composition and the reduction conditionadopted. The catalyst composition influenced also the selectivity in the diffe-rent alcohols. The formation of higher molecular weight alcohols was favoured bythe pressure and low values of the HZ/CO ratio in the gas mixture and of the in-let space velocity, with a range of operating temperature limited to 5Z3-573K.

INTRODUCTION

Despite the actual low price of oil, the development of new syngas-based pro-

cesses is one of the objectives of the near future, especially in light of the

fact that syngas can be produced from various carbonaceous sources (natural or

associated gas, coal, heavy residual, a.s.o.) (1-3). One of these short-term

objectives is the production of motor-fuel substitutes from non-petroleum sour-ces. Furthermore, there is a tendency above all in the Western world towards the

elimination of lead alkyl additives from gasoline (4). To compensate for the con-siderable loss in fuel octane rating caused by the elimination of lead, it is

possible to modify the refinery process and/or to add non-hydrocarbons havingthemselves a high octane rating.

There is general agreement that methanol blends in gasoline will be a suitable

way to improve the octane number.Methanol is facing serious overcapacity and isavailable at low cost(5-9);however,it needsa cosolvent in qasrl tne.mai nly CZ-C 6 al co-

hols,to avoid problems such as phase separation,high volatility (lO-lZ).

The higher the molecular weight of the alcohol, the more effective is its

470

influence as a cosolvent. Higher molecular weight alcohols (H.M.A.) can be pro-

duced at a high cost from olefins, or in a cheaper way together with methanolfrom syngas. It was already known that it was possible to produce H.M.A. together

with methanol by addition of potassium to the classical Zn/Cr catalysts operatingat high temperature and pressure (13). However, the last few years have seen a

great development of studies on catalysts operating at low temperature and pres-sure (14-19), and many processes have been claimed by different companies (20-

25). The main features arising from the patent and scientific literature are

summarized in Table I.

TABLE 1Range of operating conditions for the synthesis of methanol and higher molecular

weight alcohols.

CAT~lYST TEMPER~TURE PRESSURE H/CO R~TIO I NlE T sr~CE VELOC I TY RFFF REflCF

( K ) ( MPA ) ( wI )

Cu/Co/CR(~L )/AcKAL I 493-623 5-15 0.5-4. a 3000-6000 20

Cu/TH/CR/~LKAL r 553-603 5.3-7 1.0-2.0 27- laO 21

CU/ZN/~L!~LKAll 523-673 8-15 1.7-3.0 1000-10000 22

CullNICR/AcKALI 603-703 9-18 0.5- 3. a 3000-15000 23

MOS/~LKAL r 513-598 10-27 0.7- 3. a 300-5000 2/IA

523-673 0.3-1.0 > 3000 211B

Cu/b';~L!~lKAll 473-593 5-15 0.3-1.9 3000-15000 25

ZNICR/K 710 25.3 5.0 20000 13

CU/ZN/K 560 7.5 0.45 2500-5000 16

Cu/TI/N~ 620 6.0 2.0 11000 18

CUIlN/~L!K 555 13.0 0.5 3200 33

In our recent studies, a characterization of title propert i es and of the cat alyt i c

behaviour in the low temperature methanol synthesis of Cu:Zn:Me (Me= Al and/or

Cr) catalysts have been reported as a function of the composition (26-28). Theaim of this paper was to investigate the possible parameters which influence the

selectivity of these catalysts towards the synthesis of H.M.A., with a particu-lar emphasis on reaction conditions. Thus we tested catalysts chosen among the

471

most active and selective in the methanol synthesis, focusing our attention onthose obtained from homogeneous hydrotalcite-like precursors (26-28). As pre-viously reported, these phases are characterized by the presence of all the ca-tions in positively charged brucite-like layers (29), thus favouring the inte-

ractions among the elements.

EXPERIMENTAL

The precursors with different composition (see below, Table 2) were obtained

by coprecipitation from an aqueous solution of the nitrates of the elements withsodium bicarbonate at constant pH and 333K, under continuous stirring. The re-

sulting precipitates were filtered and washed in vacuo until the complete elimi-nation of the nitrates and until the residual amount of sodium, determined witha Mark II EEL photometer, was less than 0.05% (as Na 20). The precipitates were

dried at 363K for 12h, calcined at 623K for 24h and crushedto a particle size of 0.250-0.420 mm. The catalysts were impregnated with diffe-rent percentages of potassium (w/w) using solutions of CH 3COOK and calcined at623K. K-doped alumina was prepared in the same way using a Y-A1 203 (Akzo-Chemie,

grade E) with a surface area of 125 m2/ g, and the absence of surface acid centerswas verified by titration (30).

XRO powder patterns were collected with Ni-filtered CUK u radiation (A=0.15418 nm) using a Philips goniometer equipped with stepping motor and automa-ted by means of a General Automation 16/240 computer. The phase compositions and

crystal sizes were determined by a profile fitting method, comparing the obser-ved profiles with the computed ones, calculated according to Allegra and Ronca

(31). A Carlo Erba Sorptomatic 1826 apparatus with N2 adsorption was used to

measure the surface area and pore volume.The calcined precursors were reduced in the reactor by hydrogen diluted in

nitrogen, with the hydrogen content and temperature being progressively increa-

sed (14,23,32). The catalytic tests were performed in a copper plug flow reac-tor, operating up to 2.0 MPa and 623K, using 0.3-0.5 g of catalyst, differentspace velocities and reaction gas mixtures.

The reaction products were analyzed on-line without condensation using a Car-lo Erba 4300 gas chromatograph equipped with FlO and two columns (1/8-in. diam.x 2.0-m long) fitted with 80-120 Poropack OS. After cooling at 263K, the gaseswere analyzed by a Carlo Erba 4300 gas chromatograph equipped with TCO and two

472

columns (1/8-in. diam. x 2.0-m long) fitted with Carbosieve 100-120. The chro-matographic data were collected and processed by a Perkin-Elmer Sigma 15 Data

Station.

RESULTS AND DISCUSSIONIn Table 2, the compositions and the characteristic data of the catalysts exa-

mined, after both drying at 363K and calcination at 623K, are summarized, while

the XRD powder patterns are reported in Figures la and b, respectively.

TABLE 2Catalyst compositions and characteristic data after drying at 363K and calcina-

tion at 623K for 24h.

SAnPLE Co,~pos I T I ON ArOMIC RATros

(%)

SURFACE ARF.A* SURFACE AREA:t::t:

CuO

(RYST III SIZE (rm)

ZNO SPlflEl-lTKE PHASE

CAT 2 Cu,ZN,AL,CR 38,0,38,0,12,0,12,0

CAT 3 Cu:ZN:AL 38,0,38,0:24,0

~. a QlJEoi?~ -AlIORPHOUS

11,5 2..!:!~5! MlORPHOUS

CAT 1 CU:ZN:CR 38,0:38,0:24,0 106 119

138

72

6,5

5,0

3,0

5,0 3,0

* AFTER DRYING AT 363K FOR 12H,** AFTER CALCINATION AT 623K FOR 24H,

In all the precipitates only a hydrotalcite-like phase was present, with lo-

wer crystal size for the chromium containing compounds. After calcination, astrong increase of the surface area was observed for all the samples. They alsoshowed pore size distribution curves with a narrow peak centered around the mostfrequently occurring pore radius (28) and low crystal sizes.

Role of the potassium concentration and of catalyst compositionThe relationship between the catalyst characteristics and the amount of po-

tassium added are shown in Figures 2 and 3. It is possible to observe a decreaseof surface area by increasing the amount of promoter added, with this effect

being more marked for the chromium containing sample. However, the decrease of

surface area did not exceed the 40% of the original values even for the highestamounts of potassium examined.

473

b

7040 50 60 7028-

2010

Cat 2

Cat 2C")00

<0Cat3

0 Cat 30

Fig. 1. XRD powder patterns of the samples after drying at 363K (a) and aftercalcination at 623K for 24h (b).

""ClN...... 140

E....(il 100Ql...(il

Ql 600(il....... 20::J

(J) 00

.~ .--.-.-.------- --.-. .-2 3

K percentage (w/w)

Fig. 2. Potassium concentration effect on BET surface area for Cat 1 (4t) andCat 3 (.).

The doping gave also rise to an increase of crystal size, particularly in the

0-2% range of potassium added, that was more evident for the aluminum-containingsample (Cat 3) which, without potassiu~ showed a quasi-amorphous pattern (Fig. 1).

474

Furthermore, as reported in Figure 3, the chromium containing catalysts show the

presence of a new phase (marked with an asterisk) that may be identified as a

K2Cr207-type compound, even though its diffraction pattern did not correspond

exactly to that reported for Lopezite (NBS 12-300).

Figure 4 reports the catalytic activity for the Cat 1. It is worth noting

that H.M.A. were obtained also with the undoped catalyst, even if an increase of

activity both in methanol and H.M.A. synthesis was observed by doping with a low

amount of potassium (up to 0.4% ca.). These data are in good agreement with tho-

se reported in the literature (16,33), taking into account the different ways to

express the amount of potassium added. Furthermore, a decrease of the hydrocar-bon formation was observed in this range.

10 20 30 40 50 6029-

70

Fig. 3. XRD powder patterns of Cat 1, undoped and doped with different potassiumpercentages (w/w). (. KiCrzOrtype phase )

475

C'l0

co x.....U -5 10 <'JCl U::s::: Cl

.J: ...~ ::s:::-,4 8 .J:en <,

Q) en0 Q)

E 3 6 0E

>-

i ->:... 2 4 >-> ...... >U ...:::l 2 o"0 :::l0 "0"- e, 0 0a. 0 "-

0 0.2 0.4 0.6 0.8 1.0 a.

K percentage (w/w)

Fig. 4. Productivity in methanol (II), H,M.A. (~) and hydrocarbons (.-) forCat 1 as a function of the amount of potassium added (T= 553K; P= 1.5 MPa;H2/CO= 2; GHSV= 1700 h-l).

..... .....:J I I I I :JU U

Cl

!~Cl<, <,

N 125 - 30 NE Eal al100 I- + - 24Q) <, Q)'- '-al

.~-t'O

Q) 75 '-/"'. 1 8 Q)c Ual al... ..."- "-:::l 50 "-,:: 12 :::len

~ "" ent.. • "-Q) 25 -

I \.

- 6 Q)a. a.a. a.0 0U 0 I I I 0 U

0 0.2 0.4 0.6 0.8 1.0

K percentage (w/w)

Fig. 5. Copper surface area for Cat 1 as a function of the amount of potassiumadded: (.-) after reduction; (II) after reaction.

476

By increasing the amount of potassium, we observed a deactivation of the ca-

talyst which was practically complete for percentages higher than 1%. The decrea-

se in the activity is more significant than that of BET surface area, and may beattributed to a specific interaction with the active phase (16,33). Furthermore,

its trend is similar to that observed for the copper surface area after both re-

duction and reaction ( Fig. 5) , even if this parameter did not alone justify

the differences observed in the catalytic activity. Worthy of note are the lowervalues from all the samples after reaction; this fact may be attributed to the

surface adsorption of higher molecular weight compounds (34). However, the XRD

powder patterns evidence a strong interaction of the potassium with the spinel-

like phase present after calcination (28,35), as was observed for the Zn/Cr ca-

talysts (34,36).

The behaviour of the catalyst containing aluminum was similar (Fig. 6): howe-

ver, in this case the potassium did not show any activating role on the methanol

synthesis and the H.M.A. were not obtained with the undoped catalyst, with this

latter characteristic being strictly related to the reduction condition adopted.

Furthermore, this catalysts showed a selectivity towards the C4-alcohol fraction

....N...

t'il

~.~U »c

OJ ....~ 1.2 6 ...

t'ilJ: U<, ------. OJ(J) ~QJ ---------. ~

J:0 0.8 <,

E 4 (J)&--- QJ

--. +~& 0>- E...

"> 0.4 --- '+--- 2... >-o .- ...:::l >"0 ...0 U... 0 0 :::la. "00 0.2 0.4 0.6 0.8 1.0 0...

K percentage (w/W) a.

Fig. 6. Productivity in methanol (.), H.M.A. (.) and hydrocarbons (e) forCat 3 as a function of the amount of the potassium added (T= 553K; P= 1.5 MPa;H2/CO= 2; GHSV= 1700 h-1).

477

(mainly isobutyl alcohol), in agreement with that reported in literature (15,16),which was higher than that observed for the chromium containing catalyst. Cat Z,containing both elements, presented an activity similar to that of Cat 1, whilethe H.M.A. distribution was more similar to that observed for Cat 3.

On the other hand, the tests with K-A1 Z03/Cat 3 mixtures showed that the ac-tivity depends only on the amount of methanol catalyst present and not on thatof the alumina-supported potassium (Fig. 7), in disagreement with the Morgan

et al. hypothesis of an aldolic condensation (37).

Effect of the gas mixture composition

With the hydrogen-rich feed typical of the recycling loop in an industrial

plant for the low temperature methanol synthesis, only methanol was observed.Appreciable productivities of H.M.A. were obtained for HZ/CO ratios ~ Z, with

the maximum for every alcohol progressively displacec towards the lower valuesof the H /CO ratio when che chain length lncreases. At the same time a li-Znear decrease of the methanol productivity was observed. Therefore, by increa-

sing the CO concentration, the relative rate of hydrogenation of the methanolprecursor decreases. Furthermore, H.M.A. formation appears to be a slow stepin comparison to the rate of,hydrogenation of the methanol precursor on the sur-face.

It is worth noting that at HZ/CO ratios < 1, a strong increase of hydrocarbonformation, mainly methane, was observed especially for the undoped catalysts. Onthe other hand, it is also necessary to consider the water gas shift reaction(in particular when the synthesis is performed continuously with recycle of the

gas) that increases the H2/CO ratio. Therefore a HZ/CO ratio = 2 was employedin the following tests, in line also with the data reported by some authors (14,20, 38).

Role of the reaction conditions

The influence of the reaction conditions (pressure, temperature and inlet spa-ce velocity) are illustrated in Figures 8, 9 and 10, respectively. The pressurefavours the synthesis of alcohols (methanol and H.M.A.) much more than that of

hydrocarbons, irrespectively of reaction temperature.The H.M.A. synthesis was limited to the 523-573K range, with similar trends

for all the catalysts and the different potassium percentages being examined. At

478

1.0

-

-

-

I

I

I

I

1

I

1

/~

/:1.0 ~ •

;- 0.5- ./ee

0 1/ 1

o 0.2 0.4 0.6 0.8

Cat 3/Cat3+K.AI203 (w/wJ

,....-mU

Ol~ 2.0 l-.e<,(JlQ) 1.5 I-

aE

....U:Jl::laL-a.

Fig. 7. Total productivity for Cat 3 mixed with different amounts of 3%K-A1 203(4t) and 6%K-A1 203 (II) (T= 568K; P= 1.5 MPa; H2/CO= 2; GHSV= 2800 h- l).

-

I

2.0I

1.8

I

1.6

I

1.4

I

....g 2.5 I-l::laa: Ol.--....L--...l-----'----'----'---'

~ 5.0 I-

>

C'l

52x....-012.5 -

Ol~

'E. 10.0 f-(JlQl

aE 7.5 t-'

Pressure (MPaJ

Fig. 8. Pressure effect on productivity in H.M.A. (~,L1) and hydrocarbons (4t,())for Cat 1 doped with 0.2% of potassium (temperature: 543K (closed symbols), 563K(open symbols); P= 1.5 MPa; H2/CO= 2; GHSV= 1700 h- l).

479

N,... 0-ril xu

e/

,...-Ol \ ril~ - 12 U..c 5.00 \ Ol<, . / ~(j) \ II + ..c<1l <,

0 3.75 ~/9 (j)

E ,\e <1l

'" 0

>- 2.50 '.~ 6 E- / ~."> ,"-.. >.3

...... 1.25 '" --- >0 e \:J _./ ..."'0 00 0 0 :J"- "'0a. 530 550 570 590 0

"-Temperature (K) a.

Fig. 9. Temperature effect on productivity in methanol (.), H.M.A. (A) andhydrocarbons ("J for Cat 1 doped with 0.2% of potassium (P= 1.5 MPa; H2/CO= 2;GHSV= 1700 h-l).

2 4 6 8Space velocity (h-1) x10-3

,...-rilU

Ol 10~..c<,III 8<1loE 6

>

No

2.5 U:J"'0o"-a.

5.0 >-...

o

x,...12.5 ;;;

otn

~10.0..c

<,(j)

<1l7.5 0

E

I I I / I

/-f- • -

/",---"'j+-"'-

f- I t;--+--t;- -'" / // -I(yt;/ 0""-

f- ,.0 ./ -1t;.~0 /

I ;;-:--+-0-l- I- /0 -

,00

f- /.'e_e__+-e-I I I I

4

2

oo

>-o:J"'0o"-a.

Fig. 10. Inlet space velocity effect on productivity in methanol (.,[]), H.M.A.(~,L» and hydrocarbons (",()) for Cat 1 doped with 0.2% of potassium (tempera-ture: 543K (closed symbols), 563K (open symbols); P= 1.5 MPa; H2/CO= 2 )

480

low temperatures only methanol was observed (selectivity ~ 99.5%), while at

the higher values the methanation reaction became marked, together with a deac-

tivation of the catalyst. It must be noted that, in the temperature range repor-

ted above, the methanol synthesis reaction was at the thermodynamic equilibrium,

if we take into account the pressure employed (39,40).

The decrease of the inlet space velocity, strongly increased the selectivity

to H.M.A. while decreasing that in methanol. However, it must be taken into ac-

count that in lowering the inlet space velocities an increase of the hydrocarbon

formation was also observed, particularly at the highest temperatures examined.

Our data confirm the Smith-Anderson pattern for alcohols (15,33) and agree also

with Klier (41).

On the other hand, it must be noted that both the industrial and the scien-

tific papers (see Table 1) report inlet space velocities lower than about 5000-1h for the system operating at low temperature.

CONCLUSION

H.M.A. together with methanol was obtained with low temperature methanol ca-

talysts, without and with the addition of potassium. In this latter case the

productivity in H.M.A. increased up to about 0.4% of the added potassium, after

which a deactivation was observed with a trend similar to that observed for the

copper surface area. It is noteworthy that all the catalysts showed lowest va-

lues after reaction, attributable to the presence of high molecular weight com-

pounds adsorbed on the surface. In all cases the deactivation must be attribu-

ted to an interaction of the potassium with the active phase.

The reaction parameter played an important role: the H.M.A. synthesis was fa-

voured by low HZ/CO ratios in the feed and low inlet space velocities, with a

limited range of operating temperature and a positive effect from pressure in-crease.

The reported data are in agreement with the mechanism of H.M.A. formation

reported in the literature (15,16,19) and can be understood on the basis of a

slow initial growth step and a rate of growth favoured by the increase of the

CO partial pressure.

481

ACKNOWLEDGMENT

The authors wish to thank SNAMPROGETTI (S. Donato Milanese, Italy) for finan-

cial and scientific support.

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30 M. Ballkova and L. Beranek, Collect. Czech. Chem. Commun., 40 (1975) 3108-3113.31 G. Allegra and G. Ronca, Acta Crystallogr., A34 (1978) 1006-1013.32 P. Courty, D. Durand, A. Sugier and E. Freund (I.F.P.), U.K. Pat. 2118061A

(1983).33 K.J. Smith and R.B. Anderson, Can. J. Chem. Eng., 61 (1983) 40-45.34 M. Di Conca, A. Riva, F. Trifir6, A. Vaccari, G. Del Piero, V. Fattore and

F. Pincolini, Proceedings 8th Intern. Congress on Catalysis, Vol. II, DECHEMA,Frankfurt am Main, 1984, pp. 173-183.

35 S. Gusi, F. Trifir6 and A. Vaccari, Reactivity of Solids, in press.36 A. Riva, F. Trifir6, A. Vaccari, G. Busca, L. Mintchev, D. Sanfilippo and

W. Manzatti, Faraday Symposium 21 Promotion in Heterogeneous Catalysis, Bath(U.K.), September 23-25 1986.

37 G.T. Morgan, D.V.N. Hardy and R.H. Procter, J. Soc. Chem. Ind., 51 (1932) 1-7T.38 M.E. Frank and A. Hernandez-Robinson, AIChE 1986 Spring National Meeting,

Fuels and Petrolchemical Division, New Drleans (USA), April 6-1~ 1986.39 R.H. Ewell, Ind. Eng. Chern., 32 (1940) 149-152.40 W.J. Thomas and S. Portalski, Ind. Eng. Chern., 50 (1958) 967-970.41 K. Klier, in S. Kaliaguine and A. Mahay (Eds.), Catalysis on the Energy Scene,

Elsevier, Amsterdam, 1984, pp. 439-454.


AN ALKENE ISOMERIZATION CATALYST FOR MOTOR FUEL SYNTHESIS

B.G. BAKER and N.J. CLARKSchool of Physical Sciences, Flinders University, South Australia 5042

483

SUMMARYA method for the preparation, conditioning and operation of a catalyst

containing tungsten oxide is described. The active catalyst contains tungstenin an intermediate valence state maintained by the alkene in a carrier gascontaining hydrogen. Results for the isomerization of 1-butene, 1-pentene and1-hexene show that the ratio of branched to straight chain alkenes in theproduct approach the equilibrium ratios, that double bond shift and single chainbranching adjacent to the double bond are the major reactions and thathydrogenation of the alkene is negligible.

The application of the method for the synthesis of MTBE, TAME and other octaneimprovers for gasoline is proposed. An application of the method to branchalkenes synthesised by a selective Fischer-Tropsch catalyst is demonstrated.

INTRODUCTI ONThe widespread introduction of unleaded gasoline to fuel engines fitted with

catalytic converters has generated problems for the petroleum refining industry.Octane ratings and octane distribution requirements now have to be met by highoctane blend stocks of appropriate boiling range. Generally these are notavailable in sufficient quantity without major refinery changes and theintroduction of new processes.

The light alkenes (propene, butene and pentene) are important feedstocks foralkylation, oligomerization and the synthesis of ethers (refs. 1,2). MTBE(methyl tert-butyl ether) and TAME (tert-amyl methyl ether) have research octanenumbers of 118 and 112 respectively. These premium blend stocks are synthesisedby reaction of methanol with isobutene or isopentene (refs. 3,4). The reactionwith methanol is selective towards the branched alkenes so that a mixture may betreated and the straight chain alkenes recovered for other processing such asalkylation.

The supply of branched alkenes from cracking processes is limited and there isa need for catalytic isomerization processes to increase the supply. Onemethod is to isomerize butane and then to dehydrogenate the isobutane (ref. 5).

An alternative strategy is to catalyse the skeletal isomerization of an alkenewithout destroying the double bond.

The present work describes the procedure for preparing, conditioning andoperating a catalyst containing tungsten oxide to achieve this objective.

Other catalysts containing W03 have been shown to have activity in double bondshift and in metathesis (ref. 6,7,8). The conditions for skeletal isomerizationdeveloped in the present work do not favour the metathesis reaction and it makesno apparent contribution to the product. However double bond shift doesaccompany skeletal isomerization.

484

CATALYST PREPARATIONThe active constituent of the catalyst is an oxide of tungsten prepared by

partial reduction of W0 3. Unsupported W0 3 was found to have activity butbetter specific activity is obtained by depositing the oxide on a support. Anumber of grades of alumina and silica were tested. Gamma alumina was found toreact with W0 3 at 400°C to a considerable extent to form aluminium tungstate.This was identified by XPS and tests on A1 2(W04)3 showed it to be inactive as acatalyst for the isomerization reaction.

A heat treated y-alumina, partially converted to a-alumina by heating to1200°C for 30 min. and having a surface area of -20 m2g-1 was preferred. Thepreparation and structure of this support is described elsewhere (ref. 9). ThisHT-alumina is relatively unreactive to W0 3 and satisfactory catalysts have beenprepared with loadings of 1% by weight. Better catalyst life is achieved with6% W0 3/HT-alumina and the tests described here were made on such catalysts.

A catalyst was prepared by impregnating a sample of alumina with a solution ofsodium tungstate and mixing with the aid of ultrasonic agitation. The samplewas dried in a vacuum dessicator (-4 hour) and in air at 90°C (1 hour).Concentrated nitric acid (10 mL) was added and the beaker warmed on a hot platefor 5 minutes and the solid was then washed with -1M nitric acid by decantation.To remove sodium nitrate, 1M nitric acid (250 mL) was added, the catalystdigested for 1 hour on a hot plate and the nitric acid decanted. This washingwas repeated three times and the catalyst dried at 150-160°C.

CATALYST CONDITIONING AND TESTING

6% W03/HT-aluminaIg10 mL min-l (i.e. 600 hr- l)

1 atmos.Reaction pressure

The activity for skeletal isomerization of alkenes was only achieved after apartial reduction of the yellow, W0 3, component of the catalyst. This reductionwas made by flowing a mixture of hydrogen and water vapour in a ratio about 40:1over the catalyst at -400°C for 16 hours. Under these conditions W0 3 (yellow)is partially reduced to a dark blue oxide, suggesting that W20058 (blue) andW18049 are formed.

The reactivity of the catalyst was tested by flowing alkene vapour in asuitable carrier gas at temperatures 260-360°C. Product was sampled by a gassampling valve and analysed by gas chromatography. The column, n-octane onporosil C, was operated at 42°C to analyse butenes and at 92°C for the pentenes.Product distributions are reported in weight percent.

The following reaction parameters apply to the tests unless otherwisespecified.Catalyst compositionCatalyst sampleFlow

~drocarbon reactant

Jater content of reactant stream

-1I-butene 25 mg hr-1I-pentene 25 mg hr

-0.025 atmos.

485

Two fresh samples offor I-pentene reaction

The composition of the reactant gas stream needed to optimize branchinglctivity was determined as follows. The catalyst were preconditioned in wetlydrogen for 16 hours at 380·C. These samples were then cooled to operatingtemperature and hydrocarbon admitted along with a variety of different carrierJases. The results for I-butene and I-pentene are in figures 1 and 2.Isomerization for I-pentene is best with a H2 carrier stream. For I-buteneHZ/H20 promotes the longest useful isomerization life. It appears that in orderto sustain isomerization the hydrocarbon must be accompanied by some hydrogen asrunning in argon resulted in rapid deactivation for both I-pentene and I-buteneisomerization.

The product distributions (tables 1 and Z) show that double bond shift andsingle chain branching adjacent to the double bond are the major reactions. Themaximum yields are limited mainly by equilibrium rather than kinetic factors.

Irrespective of the carrier gas the catalyst initially displays a widerproduct spectrum including some cracked and hydrogenated hydrocarbons. Howeverwith time on stream these reactions diminish. The product distribution forI-pentene under the various carrier gases after extended reaction times are intable 3. In all cases hydrogenated product is minor.

Throughout these tests no substantial occurrence of metathesis was observed.Only minor disproportionation activity is evident upon initial contact ofhydrocarbon with the catalyst and this behaviour is short lived.

Other preconditioning treatments were investigated. Samples of catalysts wereheated at 380'C for 16 hours in high purity dry HZ (10 mL min-I). After treat-ment the catalysts were cooled to operating temperature and isomerizationactivity for both I-pentene and I-butene investigated. The results in figures3 and 4 are compared with Hz/HzO treatments. It appears that I-pentene is moresensitive than I-butene to this treatment and low isomerization activity result~

However hydrogenated product represents only Z.6% of the product after 75 mineven though the catalyst has been exposed to quite severe reduction treatment.

The effects of argon and pentene on the catalyst in the absence of hydrogenwere tested. The yellow catalyst containing W03 treated in a glass reactor inargon at 450'C and cooled to 300·C. No visible change in colour or morphologyof the catalyst was noticed during this preconditioning. However immediatelyI-pentene vapour came in contact with the catalyst a colour change to greyoccurred indicating some reduction.

Reaction in the absence of hydrogen was investigated.catalyst were treated in the following ways then tested

486

TABLE 1

TABLE 2

Reaction of I-pentene/H 2/H 20 (300°C) at stated reaction times

Product distribution (%)10 mins 75 mins

propene 0.72-methyl propane 0.5I-butene trace2-methyl propene 9.6 3.72-methyl butane 8.7 1.2pentane trace2-methyl I-butene 1.3 1.6I-pentene 1.4 1.92-pentene 31. 4 37.12-methyl 2-butene 46.3 54.5

Reaction of I-butene/H 2/H 20 (360°C) at stated reaction times

Product distribution (%)10 mins 75 mins

propene 11. 82-methyl propane 5.8butane traceI-butene 8.62-butene 37.62-methyl propene 36.1

45.-------------,

-o'"-5 25co

as20

15L-__-'----__--:-__-'------'o 1 2

Time (hours)

4.21.60.4

12.145.236.4

70.--------------------,

4

Time

Fig. 1. Catalytic isomerizationof I-butene at 360°C in variouscarrier gases.Catalyst preconditioned in H2/H20at 380aC

Fig. 2.I-pentenegases.Catalyst380°C.

Catalytic isomerization ofat 380°C in various carrier

TABLE 3 Reaction of I-pentene at 300°C after extended operation invarious carrier gases

Product distribution ( )HZ/HZO (Z3 hr) HZ (Z3 hr) Argon (4 hr)

487

Z-methyl propeneZ-methyl butanepentaneZ-methyl I-buteneI-penteneZ-penteneZ-methyl Z-butene

0.3O.Z

1.73.6

46.647.6

Z.10.4

0.61.4

38.057.4

0.74.6

54.440.3

UQ>s:oc 30o

OJ

50,--------------,

•

~ 45 ~\.40 ~

~ .-o ~o 0....::.. 35 """""0

~o_

25 l-

70.------------,

65

_ 60u:J-o::a. 55UQ>s:oc 502

OJ

45 e_e_

oI I1 2

Time (hours)

Io 4

(hours I

Fig. 3. Isomerization ofI-butene/H? at 360°C. Catalystpreconditioned.at 380°C in HZonly, • ; and ln HZ/HZO, o.

Fig. 4. IsomerizationI-pentene/H at 300°C.preconditio~ed at 380°C• ; and in HZ/HZO, o.

ofCatalystin HZ only,

in an argon carrier at 300°C. Low temperature treatment; heated at 300°C inargon (5 min) then I-pentene/argon admitted. High temperature treatment; heatedat 450°C in argon, (10 min) cooled to 300°C in argon then I-pentene/argonadmitted. The results in table 4 show the initial distributions. In bothcatalysts activity fell rapidly with time. It is important to note that neithercatalyst has been exposed to HZ or HZO in its pretreatment or during thereaction test.

The alternative procedure of conditioning in argon then running in HZ/HZO wasinvestigated. In this experiment I-butene was used as the test hydrocarbon andthe following pretreatment undertaken on fresh sample of catalyst. Lowtemperature; air at room temperature then argon at 360°C then I-butene/H Z/H ZOadmitted. High temperature; argon heated to 450°C, 15 mins in air at 450°C then

488

cooling to 360°C in argon then I-butene/H Z/H ZO admitted. Results are in table5. After low temperature treatment the major activity is double bond shift,while after high temperature treatment hydrogenation activity predominates.

The effect of the temperature of preconditioning was investigated to testwhether shorter times at higher temperatures are effective. Fresh catalyst washeated in HZ/HZO to 450°C for 30 mins and cooled to 300°C then tested forreaction of I-pentene in HZ/HZO at 300°C. The results (table 6) are comparedwith the previous results obtained after catalyst conditioning at 380°C for Z8hours. This high temperature activation treatment results initially in rathermore disproportionation and hydrogenation. Catalyst life and ultimate productdistributions were not adversely affected.

Exposure of a conditioned catalyst to air was found to be detrimental,particularly for the butene isomerization. Fresh catalyst was heated in aglass tube at 400°C for 4 hours in HZ/HZO. Upon cooling the catalyst wastransferred in air to a reactor tube, heated under HZ/HZO to 360°C and I-buteneadmitted. Results (table 7) indicate that decreased skeletal isomer is formedand that increased hydrogenation occurs.

ISOMERIZATION OF HIGHER ALKENESReaction of I-hexene (table 8) occurs at lower temperatures and yields higher

ratio of branched than pentene. Very high activity for the isomerization ofI-hexene was observed at higher temperatures. At 400°C the loading of hexenewas increased to 670 mg/g of catalyst with conversion to 50% branched product.

With higher molecular weight alkenes a competitive reaction occurs whichbecomes more dominant as the molecular weight of the alkene increases. Thisreaction involves cracking the alkene to produce mainly propene, Z-methylpropeneor Z-methyl 2-butene. Table 9 shows the product distribution, by carbon number,from the cracking of l-octene: greater than 95% of the products were branched.At Z80°C only 35% of the octene was cracked, mainly to Z-methylpropene andpropene. All of the alkenes produced by cracking show very high branched/straight chain ratios e.g. Z-methylpropene/Z-butene = 4.4 and Z-methyl Z-butene/Z-pentene = 2.0. At Z80°C the l-octene which was not cracked was highlyisomerized but identification of the isomers was not made.

When I-dodecene was passed over the catalyst at 300°C the product distributionshown in Fig. 5 was obtained. Within anyone carbon number the ratio ofbranched chain/straight chain molecules was very high, being about 4:1 for C4'sand 3:1 for C5's. The lifetime of catalysts was substantially reduced bycracking but as the cracking activity decreased the ability of the catalyst toskeletally isomerize without cracking became apparent. Thus, after 21 hourscracking of I-dodecene at 300°C the products from the catalyst consisted almostentirely of branched (but unidentified) dodecenes.

TABLE 4 Effect of preconditioning in argon; reaction of1-pentene/argon at 300°C

Product distribution (%)preconditioned preconditioned300°C, 5 min 450°C, 10 min

489

2-methyl propene2-methyl butanepentane2-methyl I-butene1-pentene2-pentene2-methyl 2-butene

10.077 .013.0

2.31.3

1.94.5

39.750.3

TABLE 5

TABLE 6

Effect of preconditioning in air/argon; reaction of1-butene/H2/H20 at 360°C

Product distribution (%)preconditioned preconditionedair at 25°C air at 450°Cthen Ar at 360°C then Ar at 360°C

propene 1.32-methyl propane 0.35 0.3butane 0.35 42.0I-butene 16.8 6.42-butene 70.5 28.82-methyl propene 11. 9 22.0

Effect of precondition conditions with H2/H 20;reaction of 1-pentene/H2/H 20Product distribution (%)

preconditioned preconditioned450°C, 30 min. 380°C, 28 hr.

2-methyl propene2-methyl butanepentane2-methyl I-butene1-pentene2-pentene2-methyl 2-butene

Total branched isomers

6.83.93.11.61.6

30.452.4

64.7

3.71.2

1.61.9

37.154.5

61.0

ISOMERIZATION OF FISCHER-TROPSCH PRODUCTThe conditions for operating the tungsten isomerization catalyst are

compatible with the composition of the exit streGm from a Fischer-Tropschreactor. The presence of unreacted hydrogen and water vapour together with COand CO 2 provides an effective oxygen partial pressure equivalent to thatrequired by the isomerization catalyst.

490

TABLE 7 Effect of exposure of conditioned catalyst to air;reaction of 1-butene/H2/H 20 at 360°C

Product distribution ( )catalyst reduced catalyst reducedthen exposed to air on line

propene 6.1 6.82-methyl propane 1.4 2.6butane 19.7 0.3I-butene 8.1 10.02-butene 32.7 43.72-methyl propene 31.9 36.6

TABLE 8 Isomerization of 1-hexene/H /H20,Catalyst preconditioned H2/~20 at 380°CProduct distribution (%)

250°C 320°C

2-methyl pentane 3.3 5.6hexane 2.3 3.62-methyl 1-pentene 1.6 2.01-hexene 7.5 8.22-hexene 20.6 15.73-methyl 2-pentene 58.3 58.22,3-dimethyl 2-butene 6.3 6.8

TABLE 9 Products from the cracking and isomerization of1-octene/H2/H20 at 300°C

Products numbers

Weight %C1 C2 C3 C4 C5 C6 C7 C8

4.1 39.8 8.0 3.6 2.9 41.6C9

f- -

f- -

~o2 3 4 5 6 7 8 9 10 11 12

PRODUCT CARBON NUMBER

Product distribution for reaction of 1-octene/HZ/HZO on6% WOx/HT-alumina, 300°C.

20

25

5

t-ZWu 150::w0..

t- 10:I:C>W~

FIG. 5.

25

!z 20LlJ'-'a:~ '5

o Product from F-T catalyst

[1i:l Straight chain} Products from• Branched WOx catalyst

491

5

23456 7PRODUCT CARBON " 12

FIG. 6. Product distribution from an alkene selective Fischer-Tropschcatalyst before and after passage over 6% WOx/HT-aluminaisomerization catalyst.

A Fischer-Tropsch catalyst with high selectivity to alkenes has been developed(ref. 9). Product from this reaction was passed over the 6% W03/HT-aluminacatalyst contained in a separate reactor tube and preconditioned in H2/H 20. TheF-T product before and after the isomerization catalyst shown in fig. 6.Alkenes above C6 were cracked to branched alkenes and C4-C 6 alkenes werebranched. The resulting ratios of branched/straight chain alkenes were close toequilibrium values. Over a period of Z hours the tungsten catalyst lost itsbranching activity and produced mainly straight chain Z-alkenes. However it wasregenerated when treated with air at 450°C for 5 minutes and resumed its initialactivity.

The apparatus was later modified so that the products from reactor 1 passedthrough a cooling coil to trap hydrocarbons greater than CS' The remainingproducts were passed over the tungsten oxide catalyst in reactor Z. The productdistribution was that which would be expected from isomerization alone, withlittle cracking, and the lifetime of the catalyst was much greater.

CATAL1ST REGENERATIONThe specific conditions for catalyst activity depended in the al kene and on

the operating temperature. Diminished activity was observed after variousreaction times (see figs. I,Z). Treatments to regenerate isomerization activitywere investigated. These involved oxidation followed by reduction.

The reaction of I-pentene/HZ/H20 at 300°C was followed after each of thefollowing treatments on catalysts which had lost activity.

A. Heat 450°C in air, 15 min; cool in argonB. Heat 450°C in air, 15 min; cool in HZ/HZO

492

C. Heat 380°C in air, 60 min; cool in argonD. Heat 380°C in air, 60 min; cool in HZ/HZO

The results in table 10 refer to initial catalytic activity 10 mins afteraddition of the hydrocarbon to the stream. The trend in all cases is forhydrogenation activity to subside with time. The final column contains datafrom an optimally conditioned catalyst for comparison. It is concluded that inorder to minimize pentane production and maximize branched product the catalystshould be exposed to HZ/HZO prior to admission of 1-pentene.

Reaction of 1-butene/H Z/H ZO at 360°C was tested after the followingregenerations.(a) Increase temperature to 450°C; 15 mins air; decrease to 380°C in argon

then Z8 hours in HZ/HZO.(b) Increase temperature to 450°C; 15 mins air; decrease to 360°C in argon

and immediately introduce 1-butene/H Z/H ZO.(c) Increase temperature to 450°C; 15 mins air then 30 mins HZ/HZO at 450°C

prior to cooling 360°C.The results are shown in figure 7. In case (b) significant quantitites of

butane are present in the product stream. It appears that the most effectiveregeneration is (c) as it is quick and returns the catalyst to excellentisomerization activity. This activity is prolonged as 33% total branchedproduct was observed after ZZ hours.

The data in figures 1 and Z indicate that the effective catalyst lifetime forI-butene isomerization is ~ubstantially shorter than that of I-pentene.

Experiments were designed to investigate whether a catalyst inactive forI-butene isomerization could effectively isomerize 1-pentene and also todetermine the level of I-butene isomerization for a catalyst that has been

Fiq.7. Reaction ofI-butene/Hz/HzO at 360°C.Comparison of threeregeneration procedures(see text).

~5 r-

- o~;l. ·,,"0 lc~·-- ~o r- .'\..___________

~ \ 101 0-o 0_5. 35

"0 __0-

1 0"'1 blg30-/(IJ

25

I I Io I 2 3Time afler regeneration I hr s I

493

TABLE 10 Reaction of I-pentene/HZ/H ZO at 300°C on regenerated catalyst

Treatment as in textA B C D Fresh catalyst

conditionedHz/HZO 380°C

<C4 3.1 1.ZZ-methyl propene 0.3 11.6 1.0 3.0 9.6Z-methyl butane 9.Z 0.3 0.8 8.7pentane ZZ 10.Z 15. Z lZ.8Z-methyl I-butene 1.0 0.9 1.3 1.9 1.3I-pentene 1.9 1.Z 3.3 Z.1 1.4Z-pentene 3Z.7 Z4.0 38.0 30.8 31.4Z-methyl Z-butene 4Z.0 39.8 40.9 48.7 46.3

Total: Branched isomers 43.3 61.5 43.5 54.4 65.9

isomerizing I-pentene efficiently for an extended period.In the first instance a catalyst running in I-butene/H Z/H ZO at 360°C for -4

hrs and giving <30% branched product was exposed to I-pentene/HZ/H ZO at 360°C.The resultant branching of I-pentene was excellent -63%. Other runs in whichthe reaction temperature was lowered to 300°C prior to admission of I-penteneagreed with the above result. It is concluded that a catalyst inactive forI-butene isomerization may still have activity for I-pentene isomerization.

In the second case, a catalyst that had been effectively isomerizing I-penteneat 300°C in HZ for -Z3 hours was heated to 360°C and I-butene/HZ admitted. Thelevel of I-butene isomerization to branched product under these circumstanceswas very poor <16%.

DISCUSSIONS AND CONCLUSIONSThe activity for skeletal isomerization exhibited by these tungsten catalysts

is developed only under specific conditions of treatment and operation. Thesource of tungsten oxide and the method of support are not critical. The HT-alumina favoured in these tests had the advantage of good dispersion of W03 andlimited loss by reaction with the support to form aluminium tungstate.

To summarize the best conditions for isomerization activity, it must be notedthat the differences between pentene and butene is also a difference in reactiontemperature; probably the determining parameter.

For I-pentene, 300°C; best carrier gas hydrogen. For I-butene, 360°C; bestcarrier gas HZ/HZO in the ratio 40:1.

The favoured preconditioning treatment of the catalyst is 380°C in HZ/HZO for16 hours. Also 450°C in HZ/HZO 30 min is effective but the initial productdistribution is not optimum.

The best regeneration technique is to heat to 450°C in air for 15 min then toconvert to HZ/HZO before cooling and admitting the alkene.

494

The skeletal isomerization was associated with double bond shift l-ene to2-ene. The equilibrium favours the 2-ene at these reaction temperatures so thatit is not obvious that a shift of the double bond is an essential step in thebranching reaction. The 2-enes undergo the same branching reaction.

The nature of the active catalyst is not easily defined. The conditionsemployed, the blue colour and XPS analysis all indicate the presence of mixedvalence states of tungsten, wax where 2.65< x >2.95 approximately. Within thisrange W20058 and W18049 have defined structures (fig. S). The structure W0 20058(light blue) has W(V) at sites along the shear planes: WlS049 (dark blue-purple) has more W(V) ions. It is possible that these are reaction sites.

Excessive reduction, particularly after oxidation treatments favourshydrogenation probably due to the production of zero valent tungsten. Theoxidized catalyst on the other hand lacks skeletal isomerization activity butshifts the double bond. This is a reaction characteristic of acidic catalysts.

The role of water in the hydrogen carrier gas is likely to be that ofestablishing a controlled oxygen partial pressure to prevent excessivereduction. It might also produce a surface concentration of OH groups whichparticipate in the reaction.

The practical application of a skeletal isomerization catalyst for alkenesare numerous. There is an increasing interest in conventional petroleumrefining in optimizing the use of light alkenes both to increase liquid yieldsand at the same time improve octane quality.

a

FIG. 8. Structures of (a) W20058 and (b) W18049projected on to the (010) plane.

495

A particularly favourable application would be in the synthesis of MTBE andTAME. The selective reaction of methanol with the branched alkene would enablethe straight chain alkenes to be recycled through the isomerization catalyst.Since the methanol for such a process would likely be synthesised from CO and HZit would be possible to run this process in parallel with an alkene selectiveFischer-Tropsch process to achieve a self contained conversion of CO and H2 to ahigh octane fuel blend stock.

The isomerization catalyst described here is the subject of patent applicatlon(ref. 10).

ACKNOWLEDGEMENTSThis work was supported by the Australian National Energy Research Development

and Demonstration Council. The authors wish to thank H. McArthur for assistancewith the experiments.

REFERENCES1. V.E. Pierce and A.K. Logwinuk, Hydrocarbon Processing, 64 Sept (1985)

75-79.2. R.M. Heck, R.G. McClung, M.P. Witt and O. Webb, ibid, 59, April (1980)

185-191.3. J.D. Chase and B.B. Galvez, ibid, 60, March (1981) 89-94.4. L.S. Bitar, E.A. Hazbun and W.J. Piel, ibid, 63, Oct. (1984) 63-66.5. G.R. Muddaris and M.J. Pettman, ibid, 59, Oct. (1980) 91-95.6. F.P.J.M. Kerkhof, R. Thomas and J.A. Moulijn, Rec.Trav.Chim.Pay-Bas 96

(11) (1977) M121-126.7. A.J. Van Roosemalen and J.C. Mol, J. Catalysis 78 (1982) 17-23.8. J.J. Rooney and A. Stewart, in "Catalysis" (special ist periodical reports)

(C. Kemball, ed.) The Chemical Society, London, Vol. 1 (1977) 277.9. B.G. Baker and N.J. Clark, IV International Symposium on the Scientific

Bases for the Preparation of Heterogeneous Catalysts, Elsevier (1986).10. B.G. Baker, N.J. Clark, H. McArthur and E. Summerville, International

Patent Application PCT/AU83/00110.

catalysis and automotive pollution control, proceedings of the first international symposium (capoc...

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