isotopic characterization of no adsorption, dissociation and coadsorption with co on pt(100): an...

SURFACE AND INTERFACE ANALYSIS, VOL. 25, 81È87 (1997)

Isotopic Characterization of NO Adsorption,Dissociation and Coadsorption with CO onPt(100) : an Infrared ReÑection–Absorption Study¤

Noel P. Magtoto and Hugh H. Richardson*Department of Chemistry, and Condensed Matter and Surface Science Program, Clippinger Laboratories, Ohio University,Athens, OH 45701, USA

We present the results of the in situ infrared study of the behavior of NO with CO on Pt(100) carried out in acontinuous Ñow reactor at 470 K and 500 K. The frequency of the solitary IR band observed at 470 K duringadsorption of nitric oxide shifted by 68–73 cm—1 when we used 15N18O instead of NO. This shift correspondsclosely to the square root of the ratio of reduced masses, [l(NO)/l15N18O) ]1¿2, indicating that NO is molecularlyadsorbed on the platinum surface even at temperatures as high as 470 K. We did not observe any molecularadsorption of NO at 500 K, though. However, we observed the appearance of an infrared band at ¿1630 cm—1

during the reaction of CO and NO both at 470 K and 500 K. The peak absorbance of this band appears tocorrelate with high rates of production. On the other hand, at low reaction rates, this band completelyCO

2disappears and is replaced by infrared bands that correspond to a growth of CO adlayer. Isotopic studies utilizingC18O showed that the ¿1630 cm—1 wavenumber band is due to molecularly adsorbed NO, indicating that nitricoxide exists largely in undissociated state during its interaction with CO on Pt(100) at elevated temperatures.

1997 by John Wiley & Sons, Ltd.(

Surf. Interface Anal. 25, 81È87 (1997)No. of Figures : 7 No. of Tables : 0 No. of Refs : 44

KEYWORDS: infrared ; absorption ; reÑection ; platinum; adsorption ; carbon monoxide

INTRODUCTION

Considerable attention has been given to the interactionof NO on the Pt surface not only because of its useful-ness in industrial and automotive emission control butalso because of its ability to provide information onmolecular processes occurring on the surface. Using avariety of surface science tools, studies on NO adsorp-tion, dissociation and coadsorption with CO have beendone on supported catalysts,1h3 on polycrystallinesurfaces4h6 and on single-crystal platinum surfaces.7h19

Because of the amphoteric nature of NO, its behavioris a study in complexity. Added to that complexity isthe hex to (1 ] 1) phase transition that occurs inPt(100).20 Nitrous oxide causes the immediate trans-formation of the Pt(100)-hex into the metastable (1 ] 1)at temperatures above 210 K.21 It is found to adsorbnon-dissociatively on the hex and molecularly and dis-sociatively on the (1] 1) phase at 300 K.5,7,11,12 In astudy that involved a transient exposure of NO gas tothe Pt(100) surface, Bonzel et al. found that adsorptionof NO on the (1 ] 1) phase is both molecular and disso-ciative in the temperature range 400È420 K, but onlydissociative beyond 420 K.22 The same authors foundno evidence for molecular adsorption of NO on the hexat temperatures above 380 K. Adsorption and disso-ciation of NO on Pt(100) studied by AES, UPS andXPS showed that the presence of a relatively largeamount of NO in dissociative state belies the claim thatonly the surface steps and defects promote NO disso-

¤ Presented at Surface Analysis “96, Ann Arbor, MI, USA, 12È14June 1996

* Correspondence to : H. H. Richardson.

ciation at room temperature.23 A more recent studyinvolving high-resolution electron energy-loss spectros-copy (HREELS) of NO adsorption on reconstructedand unreconstructed Pt(100) reveals that dissociativeadsorption takes place on the Pt(100)-(1 ] 1) phaseeven at 300 K, and when the saturation adsorptionlayer is heated, dissociation of NO “proceeds similarlyon both structural phasesÏ.24 Coadsorption studies haveuncovered particularly interesting properties of thecatalytic reaction between NO and CO. Various groupshave demonstrated that the interaction of NO and COon Pt(100) exhibits non-linear behavior such as surfaceexplosion,11,25h28 oscillations11,29h33 and chaos.34,35

In Ref. 36 we reported the possibility of molecularadsorption of NO on Pt(100) at temperatures as high as470 K under continuous Ñow conditions. We alsoshowed the likelihood that the formation of on theCO2oxygen-covered surface occurs via an intermediatespecies that absorbs at D1630 cm~1. At 470 K, in situinfrared reÑectionÈabsorption spectra (IRAS) showed aband that grew and shifted from 1640 to 1656 cm~1.This band Ðnally disappeared as we continuouslyexposed the surface to NO gas. We assigned this bandto NO being adsorbed molecularly on Pt(100). TheNOÈCO coadsorption experiment that followed showedthe appearance of a band at 1632 cm~1 in the IRspectra immediately after admitting CO gas into thereaction chamber. The appearance of this bandÈreferred to as the complex bandÈis accompanied by theproduction of Higher partial pressures of COCO2 .(pCO) led to higher rates of production which cor-CO2related with the growth of the complex band. Increasingthe pCO to a certain critical value triggered a dramaticdecline in the rate of production and the completeCO2disappearance of the complex band. We tentatively

CCC 0142È2421/97/020081È07 $17.50 Received 12 June 1996( 1997 by John Wiley & Sons, Ltd. Accepted 19 September 1996

82 N. P. MAGTOTO AND H. H. RICHARDSON

assigned this band to a complex that is a direct result ofthe interaction between and The resultsCO(ad) O(ad) .obtained at 500 K appeared to conÐrm this assignment.Infrared spectra obtained at 500 K showed no charac-teristic bands that can be attributed to molecularlyadsorbed NO or to any surface-dipole active chemicalspecies. When CO was admitted into the reactor, aband at 1629 cm~1 was observed. This band exhibitsthe same characteristics as the complex band observedat 470 K, i.e. it appears during the high rate of CO2formation and disappears at the onset of the transitionto the low reaction branch. Because there were no IRbands attributed to molecular NO before admittingCO, we ruled out re-adsorption of NO.

To further understand the behavior of NO on Pt(100)under Ñow conditions at elevated surface temperatures,we performed isotope labeling studies. In particular wewant to ascertain NO adsorption on the hex surface ofPt(100) at elevated temperatures and resolve the molec-ular identity of the D1630 wavenumber band observedduring coadsorption of NO and CO. We present in thispaper the results of the IRAS study of 15N18O adsorp-tion, dissociation and coadsorption with CO and thecoadsorption of NO and C18O at 470 and 500 K.

EXPERIMENTAL

Experiments were performed in an ultrahigh vacuum(UHV) Ñow reactor equipped with facilities for IRAS. Adetailed description of our instrument can be found inRef. 37. The UHV system consists of two chambers, i.e.the sample chamber (reactor) that contains the platinummetal and the main chamber that houses the quadrupo-le mass spectrometer (QMS). A gate valve regulates theÑow of gases between the two chambers. We probed themolecular nature of the platinum adsorbates in realtime by continuously leaking the reactant gases into thesample chamber with the IR beam being directed ontothe surface at an angle of 80¡. The platinum sample washeated to the desired temperature prior to admittingany gas. We used QMS to analyze the composition ofthe gas mixture in the reactor.

RESULTS AND DISCUSSION

Figure 1 summarizes the results obtained when theplatinum surface was exposed to 7.80 ] 10~5 Torr of15N18O gas under Ñow conditions at 470 K. Spectrum 1in Fig. 1(a) shows a single vibrational band at 1572cm~1. This band appeared after exposing the Pt surfaceto 15N18O for D54 s. Continuous exposure of thesurface to 15N18O caused the increase in the intensityand shift in the frequency of this band to 1583 cm~1.However, after an exposure time of 216 s, the intensityof the 1572È1583 cm~1 IR band began to decrease andthen Ðnally dropped to a value close to the noise level.Figure 1(b) shows that gas is being formed through-N2out the 15N18O adsorption experiment. The partialpressure of increased, continued to do so up to15N2spectrum 4 and then began to decline at spectrum 5.

The IR spectra in Fig. 2 were collected using NOunder similar conditions described above.36 The behav-

Figure 1. (a) Infrared reflection–absorption spectra of 15N18O onPt(100) taken during continuous exposure of the Pt(100) surfaceto 7.8 Ã10É5 Torr of 15N18O gas. The Pt(100) surface temperatureis 470 K and the spectra are displaced for clarity. (b) The partialpressure of in the flow reactor when the spectra in (a) were15N

2collected.

ior of NO shown in this Ðgure resembles that observedin Fig. 1, but with one notable di†erence. The fre-quencies of the solitary IR band in all the spectra of Fig.2 were all shifted when NO was replaced by 15N18O.This band experienced a shift of 68È73 cm~1. This shiftcorresponds closely to the square root of the ratio ofreduced masses, [k(NO)/k15N18O)]1@2, which strongly

( 1997 by John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 25, 81È87 (1997)

NO ADSORPTION, DISSOCIATION AND COADSORPTION WITH CO ON Pt(100) 83

Figure 2. Infrared reflection–absorption spectra of NO onPt(100) taken during continuous exposure of the Pt(100) surfaceto 7.8 Ã10É5 Torr of NO gas. The Pt(100) surface temperature is470 K and the spectra are displaced for clarity.

suggests that the species that gives rise to the IR bandat 1640È1656 cm~1 (Fig. 2) is molecular NO adsorbedon the Pt surface. The frequency of the band that weobserved agrees well with that observed by Gardner etal.21,38,39 which was conÐrmed to be due to molecularNO adsorbed on the (1 ] 1) surface. This result indi-cates that adsorbed NO can still initiate the lifting ofthe reconstruction even at 470 K, contrary to Fink etal.Ïs observation that NO can induce the hex\ (1 ] 1)phase transition only by cooling the surface below 410K.28 However, we did not observe the band at D1680cm~1 associated with adsorbed NO on defect sites thatare created during the hex \ (1] 1) transition.21,24 It ispossible that there are a few of these sites at elevatedtemperatures, hence the NO adsorbed on these sites iswell below the detection limit of IRAS.

We therefore conclude from the data given in Figs 1and 2 that NO initially adsorbs molecularly on the hexsurface and then triggers the transformation of hex intothe (1] 1) phase. The phase transition proceeds via theformation of (1] 1) islands27 that trap NO molecules.The trapped NO near the center of the cluster exists inmolecular form, causing the appearance of the infraredband observed at 1572È1583 cm~1 in Fig. 1(a) (1640È1656 cm~1 in Fig. 2). On the other hand, NO moleculesaround the perimeter of the islands are dissociated intoN and O atoms. The N atoms recombine forming N2 ,which readily desorbs leaving O atoms on the surface.Growth of (1 ] 1) islands continues with increasingexposure time until the surface is completely trans-formed into the (1 ] 1) phase (spectrum 4). Because the(1 ] 1) surface dissociatively adsorbs NO at this tem-

perature, the IR band due to molecular NO is no longerobserved. Dissociative adsorption under this conditionis evidenced by the continued formation of gas. TheN2decline in the production of (spectra 5 and 6 in Fig.N21(b)) is caused by the presence of adsorbed atomicoxygen, which is known to inhibit the decomposition ofNO.40

Figure 3 gives an account of the e†ect of increasingthe partial pressure of CO in the reaction chamber withthe platinum surface previously exposed to 15N18O.Each spectrum corresponds to a particular pCO, whichincreases from spectrum 1 to 8. Carbon monoxide gaswas introduced into the system when the IR band inFig. 1(a) Ðnally went away. Figure 3(a) shows that theadmittance of CO caused the appearance of an absorp-tion band at 1556 cm~1 (spectrum 1). This event isaccompanied by the formation of and gases.CO2 N2When the pCO was increased in the region betweenspectrum 1 and 7, this IR band grew in intensity andshifted to a lower frequency at 1540 cm~1 ; the rate of

production increased as well (Fig. 3(b)). The 1540ÈCO21556 cm~1 band is at its maximum absorbance at spec-trum 7, which corresponds to the maximum rate of CO2production. When the pCO reached its value at spec-trum 8, the IR bands associated with adsorbed CO (atD2080 cm~1 for on-top site and 1900 cm~1 for bridgedsite) suddenly appeared, the band at 1540È1556 cm~1disappeared completely and the rate of productionCO2went down markedly. The spectra in Fig. 4 were col-lected under similar conditions but with NO as thereactant gas. The complex band is observed at 1630È1623 cm~1. The isotopic change from NO to 15N18O isaccompanied by a shift of 76È83 cm~1 in the frequencyof the complex band. Infrared bands associated withCO are not manifested in spectra 1È7. The bands thatcorrespond to adsorbed CO are observed only after thepCO has reached its value at spectrum 8 of Fig. 3(a). Atthis point, the rate of production enters the lowCO2reaction branch. The data shown in Figs 3 and 4 clearlydemonstrate that the growth of the complex band cor-relates with the high rate of formation, while itsCO2disappearance signals the transition to the low reactionrate branch.

Previously we considered three possibilities to explainthe appearance of the complex band :36 CO is adsorbingon threefold or fourfold Pt sites ; NO is re-adsorbingmolecularly when a large number of sites have beenvacated during the reaction between andCO(ad) O(ad) ;and the complex band is a direct result of the inter-action between and presumably forming aCO(ad) O(ad) ,complex between them. If neither the oxygen nor thenitrogen is involved in the formation of the complexband, as in the case where CO merely adsorbs on high-order adsorption sites, then the absorption frequency ofthis band should remain at D1630 cm~1 once 15N18Ois used in the reaction. The fact that we observed a shiftof 68È73 cm~1 in the frequency of this band deÐnitelyrules out the possibility that the complex band is due toCO adsorbed on threefold or fourfold sites. However,re-adsorption of NO is not evident even from the iso-topic studies done with 15N18O because the changes inthe frequency experienced by NO and the complexband are not identical. These changes di†er by as muchas 10 cm~1, with the complex band experiencing thelarger shift. Furthermore, results obtained by Gardner



Figure 3. (a) Infrared reflection–absorption spectra taken duringthe coadsorption of 15N18O and CO at surface temperature of 470K. The partial pressure of 15N18O remained constant at 7.8 Ã10É5

Torr while the partial pressure of CO was gradually increased to1.3 Ã10É4 Torr. Each spectrum corresponds to a particular value ofpCO. (b) The steady-state rate of CO18O formation in the flowreactor when the spectra in (a) were collected.

et al.39 and Banholzer et al.,41 showing that an absorp-tion band attributed to molecular NO disappears whencoadsorbed with CO, reject such interpretation. More-over, results obtained at 500 K can be invoked to ruleout the possibility of the re-adsorption of NO.

Figures 5 and 6 present results before and after intro-

Figure 4. Infrared reflection–absorption spectra taken during thecoadsorption of NO and CO. The partial pressure of NO remainedconstant at 7.8 Ã10É5 Torr while the partial pressure of CO wasgradually increased to 1.3 Ã10É4 Torr. Each spectrum correspondsto a particular value of pCO. The surface temperature is held at 470K.

ducing CO into the reaction chamber previously Ðlledwith 15N18O with the platinum sample preheated at500 K. The spectra in Fig. 5(a) show no spectral signa-ture above the noise level. Using NO in our previousstudy, the same unremarkable spectra were obtained.36Despite the absence of infrared bands, though, we knowthat NO is dissociating and that oxygen atoms arepresent on the surface, as evidenced by the formation of

(see Fig. 5(b)). But the moment CO is allowed intoN2the system, a band that shows similar behavior to thecomplex band obtained at 470 K is observed (Fig. 6(a)).This band appears during the high rate of forma-CO2tion and disappears at the onset of the transition to thelow reaction branch (Fig. 6(b)). Because there were noIR bands attributable to molecular NO before admit-ting CO (spectrum 6, Fig. 5(a)), the complex bandappears to be due to an adsorbate other than NO.

However, any categorical statement about the molec-ular nature of the complex band necessitates the use ofone of the isotopic forms of CO. We performed thesame set of experiments described above except thistime we used NO and C18O. Only after conducting thecoadsorption experiment between C18O and NO did weÐnally resolved the molecular identity of the complexband. Figure 7 presents results before and after intro-ducing C18O into the reaction chamber previously Ðlledwith NO with the platinum sample preheated at 470 K.We recognize in spectra 1È7 the familiar NO stretchthat grows and goes away as we continue exposing theplatinum surface to NO alone. When C18O was bledinto the reactor, an IR band at 1630 cm~1 was observed



Figure 5. (a) Infrared reflection–absorption spectra taken duringcontinuous exposure of the Pt(100) surface at 500 K to 7.8 Ã10É5

Torr of 15N18O gas. (b) The partial pressure of in the flow15N2

reactor when the spectra in (a) were collected.

(spectrum 8). We see IR spectra in Fig. 7 (spectra 8È13)that are almost identical to the spectra shown in Fig. 4(spectra 1È7). The complex band did not experience anychange in frequency in changing the reactant gas fromCO to C18O, but a shift of D50 cm~1 in the CO stretchwas observed, as expected from the ratio [k(CO)/k(C18O)]1@2. The rate of production was mappedCO2out by monitoring the mass corresponding to C18O16O.

Figure 6. (a) Infrared reflection–absorption spectra taken duringthe coadsorption of 15N18O and CO. The partial pressure of15N18O remained constant at 7.8 Ã10É5 Torr while the partialpressure of CO was gradually increased to 1.3 Ã10É4 Torr. Thesurface temperature is held constant at 500 K. Each spectrum cor-responds to a particular value of pCO. (b) The steady-state rate ofCO18O formation in the flow reactor when the spectra in (a) werecollected.

The same two branches of reaction rates previouslyrevealed in Fig. 3(b) were observed during the inter-action of NO and C18O on Pt(100) at 470 K. At 500 K,we noted the same observation. The frequency of thecomplex band did not change. These data lead only to



Figure 7. Infrared reflection–absorption spectra taken before and after introducing C18O into the reactor previously filled with NO gas. Thepartial pressure of NO remained constant at 7.8 Ã10É5 Torr. The surface temperature is held at 470 K. Spectra 1–7 were collected duringadsorption of NO alone, while spectra 8–13 were collected during coadsorption of C18O and NO. Each of the spectra in the coadsorptionexperiment corresponds to a particular value of pC18O.

one conclusion, i.e. CO is not involved in the formationof the complex band ; we shall continue to refer to thisband as such for consistency. This result appears to bein conÑict with that obtained in our earlier studyinvolving the system.42 In this work,Pt(100)/CO] O2no nitrogen-containing compounds were introduced.Yet the band at 1630 cm~1 was observed during theoxidation of CO at high production. This bandCO2was subsequently replaced by absorption bands due tochemisorbed CO when the reaction rate dropped to anegligible value. Although we recognize similarities inthe behavior of this band in the two systems, which sug-gests an NO origin, we are not certain as yet whetherthey are purely coincidental. One reason is that thisband was detected at two di†erent temperature regions.Under the same pressure regimes the band was notdetected beyond 430 K during the reaction between COand but was observed in the NO ] CO system atO2 ,temperatures as high as 500 K. In addition, experimentson the NO ] CO system conducted at lower tem-peratures (to be presented elsewhere) do not show theexistence of the 1630 cm~1 band even at temperatureswhere this band was observed in the Pt(100)CO/O2system. Because we are aware of the complexity ofsurface processes, even with seemingly simple moleculessuch as NO and CO, the existence of di†erent speciesthat give rise to the 1630 cm~1 band in the two systemsis also a possibility. However, we are not in a positionto o†er resolution at this time because of the absence ofevidence that supports either possibility. We are cur-rently pursuing ideas that will enable us to state cate-gorically any di†erence or similarity of the 1630 cm~1band observed in both systems.

In the meantime, whether or not the origin of thecomplex band in the system is NO,Pt(100)/CO] O2the results reported here suggest that the D1630 cm~1band observed during NO ] CO coadsorption iscaused by chemisorbed NO. This leaves us with onepossible scenario : NO is re-adsorbing molecularly afterCO has reacted away the adsorbed oxygen atoms. Thisis quite a surprising development, particularly because

of the observation that molecular adsorption of NO at500 K is not taking place prior to admitting CO intothe reactor. It has been shown that both adsorbedO28,29,40,43 and CO1,44 stabilize the (1 ] 1) structure,which means that NO should be re-adsorbing on the(1] 1) phase of the Pt(100). Because NO dissociativelyadsorbed on the Pt(100)-(1 ] 1) phase beyond 420 K,22we do not expect to detect molecular NO on thesurface ; hence, no IR band associated with this speciesshould be observed. But the results from the isotopestudies deÐnitively show that NO is actually re-adsorbed and stays in molecular form in the presence ofCO. Because molecular adsorption of NO can takeplace only on the hex phase of Pt(100), the surface mustbe relaxing back to the hex phase once begins toCO(ad)react with But the complex band that is observedO(ad) .at D1630 cm~1 after NO re-adsorption is characteristicof molecular NO adsorbed on a (1] 1) surface.21,38,39These results indicate a rapid interconversion betweenthe two Pt(100) phases in the presence of both CO andNO.

In the absence of CO, we know that NO eventuallydisappears as long as we continue to leak in NO gasinto the chamber, which is attributed to the reactivity ofthe (1 ] 1) phase towards NO dissociation. But in thepresence of CO, molecular NO can exist on the surfaceindeÐnitely, which is detected by IRAS to absorb at afrequency of D1630 cm~1 despite the fact that it isadsorbed on the (1 ] 1) phase, indicating that duringthe NOÈCO reaction NO exists largely in an undis-sociated state. As shown in Figs 3, 4, 6 and 7, this bandgoes away only when the pCO reaches a certain criticalvalue, which occurs when the pCO/pNO ratio is closeto a value of 2. Cutting o† the supply of CO is anotherinstance in which the complete disappearance of the1630 cm~1 wavenumber band can take place. It appearsthat CO is blocking the required sites for NO disso-ciation to proceed, which is quite unexpected becausewe anticipate that the surface coverage of CO should bevery low prior to the onset of the low reaction branch.Notice that there is no IR band that can be assigned to



CO before the drastic decline in the reaction rate, whichsigniÐes that the surface coverage of CO is so small thatit lies well below the detection limit of IRAS under thiscondition. One may argue that the cause of the limiteddissociation of NO is the absence of the stoichiometricratio during the catalytic reaction between NO and CO.This condition is true only at the beginning of the reac-tion where the partial pressure of NO is larger than thatof CO. But at the time before the disappearance of theD1630 cm~1 wavenumber band, the partial pressuresof the reactant gases are close to the stoichiometricratio. Yet the peak absorbance of the complex bandduring this time is at its maximum. An alternative viewinvolves the oxygen atoms serving as site blockingagents. This appears to be a reasonable assumptionbecause we expect a relatively large oxygen surfacecoverage, particularly at the early stages of thecoadsorption experiment where the partial pressure ofCO is relatively small. But then again this coverageshould decrease at maximal production. Perhaps aCO2complete picture can be drawn from the involvement ofboth adsorbed CO and O. It is possible that blockage ofsites is directed by adsorbed oxygen atoms at the earlystages of the reaction, while adsorbed CO assumes thesame role at the later stages.

The same picture may also help to explain the inverserelation between the peak absorbance and the D1630cm~1 wavenumber band. Note that peak absorbance ofthe complex band increases while the frequency is shift-ing to lower values (Figs 3, 4, 6 and 7). Clearly this rela-tion cannot be explained in terms of the changes in

coverages of NO. Another interesting feature noted inour study is the shift in the frequency experienced bythe NO stretch from 1640 cm~1 (1656 cm~1) to 1630cm~1 (1623 cm~1) upon the adsorption of CO. Yet theintegrated absorbance (*R/R) of the complex band islarger than that of the single band (1640È1656 cm~1)observed during adsorption of NO along (Fig. 2). Againchanges in NO surface coverage cannot account for thisbehavior. However, reduction of dipoleÈdipole couplingdue to dilution by adsorbed CO and O at di†erentstages of the reaction may account for this frequencyshift.

CONCLUSION

In this paper we provided strong evidence that NOadsorbs molecularly on Pt(100)-hex and that theremoval of the reconstruction induced by NO adsorp-tion can take place even at temperatures as high as 470K. We also showed that steady-state catalytic reactionof NO and CO on Pt(100) proceeds in terms of tworeaction rate branches, i.e. low and high. Results fromisotopic studies showed further that NO re-adsorbsmolecularly once adsorbed CO reacts with pre-adsorbed oxygen atoms, which indicates that thePt(100) surface reconstruct back to the hex phase. Wewere able to show also that NO exists largely in anundissociated state during NOÈCO reaction on Pt(100)under Ñow conditions and at elevated temperatures.

REFERENCES

1. W. F. Egelhoff, in The Chemical Physics of Solid Surfaces andheterogeneous Catalysis , Vol. 4, edited by D. A. King and D.P. Woodruff, Chap. 9. Elsevier, New York (1982).

2. D. Lorimer and A. Bell, J . Catal . 44, 223–240 (1979).3. B. A. Morrow, J. Chevrier and L. E. Moran, J. Catal . 91, 208

(1985).4. D. T. Wickam, B. A. Banse and B. E. Koel, Surf . Sci . 223, 82

(1989).5. D. T. Wickam, B. A. Banse and B. E. Koel, J . Catal . 119, 238

(1989).6. H. Miki, T. Nagase, T. Kioka, S. Sugai and K. Kawasaki, Surf .

Sci . 225, 1 (1990).7. G. Pirug, H. P. Bonzel, H. Hopster and H. Ibach, J. Chem.

Phys. 71, 593 (1979).8. D. G. Dunn, M. W. Severson, J. L. Hylden and J. Overend, J.

Catal . 78, 225 (1982).9. B. E. Hayden, Surf . Sci . 131, 419 (1983).

10. J. Gohndrone and R. I. Masel, Surf . Sci . 209, 44 (1989).11. M. W. Lesley and L. D. Schmidt, Surf . Sci . 215, 215 (1985).12. H. P. Bonzel and G. Pirug, Surf . Sci . 62, 45 (1977).13. R. Lambert and C. Comrie, Surf . Sci . 46, 61 (1974).14. W. Adloch, H. Lintz and T. Wiesker, Surf . Sci . 103, 576

(1981).15. S. Schwartz and L. Schmidt, Surf . Sci . 183, L269 (1987).16. H. Bolten, T. Hah, J. Le Foux and H. Lintz, Surf . Sci . 160,

L529 (1985).17. Y. Park, W. Banholzer and R. Masel, Surf . Sci . 155, 341

(1985).18. R. Klein, L. Schwartz and L. Schmidt, J . Phys. Chem. 89, 4900

(1985).19. E. Scharf and J. Benziger, J . Catal . 136, 342 (1992).20. H. P. Bonzel, C. R. Helms and S. Keleman, Phys. Rev. Lett . 35,

1237 (1975).21. P. Gardner, R. Martin, M. Tushaus and A. M. Bradshaw, Surf .

Sci . 240, 112 (1990).22. H. Bonzel, G. Borden and G. Pirug, J.Catal . 53, 96 (1978).

23. S. Sugai, K. Takeuchi, B. Takahisa, M. Hirofumi and K. Kawa-saki, Surf . Sci . 282, 67 (1993).

24. D. Yu. Zemlyanov, M. Yu. Smirnov and V. V. Gorodotsky,React . Catal . Lett . 53, 87 (1995).

25. J. M. Gohndrome and R. I. Masel, Surf . Sci . 209, 261 (1989).26. T. E. Fischer and S. R. Keleman, J.Catal . 53, 24 (1978).27. Th. Fink, J. Dath, R. Imbihl and G. Ertl, Vacuum 41, 301

(1990).28. Th. Fink, J. Dath, M. Bassett, R. Imbihl and G. Ertl, Surf . Sci .

245, 96 (1991).29. Th. Fink, J. Dath, R. Imbihl and G. Ertl, J . Chem. Phys. 95,

2109 (1991).30. S. Lombardo, M. Slinko, Th. Fink, T. Loher, H. Madden, R.

Imbihl and G. Ertl, Surf . Sci . 265/270, 481 (1992).31. G. Vesser and R. Imbihl. Surf . Sci . 265/270, 465 (1992).32. A. Hopkinson and D. A. King, Chem Phys. 177, 433 (1993).33. Y. Uchida, R. Imbihl and G. Lehmpful, Surf . Sci . 275, 253

(1992).34. G. Vesser and R. Imbihl, J . Chem.Phys. 100, 8483 (1994).35. G. Vesser and R. Imbihl, J . Chem.Phys. 100, 8492 (1994).36. N. P. Magtoto and H. H. Richardson, J. Phys. Chem. 100,

8482 (1996).37. S. Hong and H. H. Richardson, J. Vac. Sci . Technol . A 11,

1951 (1993).38. P. Gardner, M. Tushaus, R. Martin and A. M. Bradshaw,

Vacuum 41, 304 (1990).39. P. Gardner, R. Martin, M. Tushaus and A. M. Bradshaw, Surf .

Sci . 269/270, 405 (1992).40. R. J. Gorte and L. D. Schmidt, Surf . Sci . 109, 367 (1981).41. W. W. Banholzer and R. I. Masel, Surf . Sci . 137, 339 (1984).42. S. Hong and H. H. Richardson, J. Chem. Phys. 97, 1258

(1993).43. J. Radnick, K. Oster and K. Wandelt, Surf . Sci . 287, 330

(1993).44. R. J. Behm, P. A. Thiel, P. R. Norton and G. Ertl, J . Chem.

Phys. 78, 7437 (Part I) ; 7448 (Part II) (1983).


isotopic characterization of no adsorption, dissociation and coadsorption with co on pt(100): an...

Documents