Journal of Catalysis 183, 131154 (1999)

Article ID jcat.1999.2390, available online at http://www.idealibrary.com on

Carbon-Supported CoppI. Characterizatio

i

8

Cu crystivated carpared andX-ray diffrprogramminfrared spof these caducibility ogroups onment priora higher dihigher oxidcarbon. Asupport coa wet imprtion of Cusurface. Cuthe graphitgates of gloand smalleplanes. Ththese partiof these cry

Carbonbecause obon and tsp2 hybriof graphitlike activin diamonbon possebe altered

1 Curren2 Curren

Boston, MA3 To who

,epeaA. Dandekar,1 R. T. K. Baker,2 anDepartment of Chemical Engineering, Pennsylvania State Univers

Received October 23, 1998; revised December 17, 199

tallites dispersed on different forms of carbon, i.e., ac-bon, graphitized carbon fibers, and diamond, were pre-characterized by CO chemisorption, N2O decomposition,action, transmission electron microscopy, temperature-ed reduction, and diffuse reflectance Fourier transform

decadesheterogcarbonthe threity, surfectroscopy (DRIFTS). Both surface and bulk propertiesrbons had an impact on the dispersion as well as the re-f Cu. Increasing the concentration of oxygen-containing

the surface of the activated carbon by a nitric acid treat-to Cu impregnation was beneficial in terms of renderingspersion of Cu; however, Cu was better stabilized in theation states over a high-temperature-treated activated

higher dispersion of Cu was obtained with the diamondmpared with the graphitized fibers when prepared viaegnation technique, and it is attributed to the stabiliza-through interactive dangling bonds on the diamonddispersed by an ion-exchange method was stabilized on

ized fibers in two morphologically different forms: aggre-bular particles deposited on top of graphitic basal planesr crystallites at the edges of and at defects within theseese morphological differences changed the reducibility ofcles and also may have altered the electronic propertiesstallites. c 1999 Academic Press

INTRODUCTION

materials represent a unique family of supportsf the diverse nature of the different forms of car-he complex functions they can perform. From thedized chemical bonding in the orderly structureized carbons and disordered turbostratic carbonsated carbon, to the sp3 hybridized configurationd, the different allotropic forms of elemental car-ss distinct bulk and surface properties which canto modify their characteristics. In the last three

t address: Mobil Technology Company, Paulsboro, NJ 08066.t address: Chemistry Department, Northeastern University,

02115.m correspondence should be addressed.

chanicalreviewsbeen pucharacteface betinfluencsorptionticles havoluminadditionallotropreceivedHoweveerties ofvestigateand cata

Furthtoelectrotronic st(1113),effects oCu/carbcarbon mnot onlyparticlesducibilitof Cu diFor coppphases ha mixturthe dispcontrol tpriate. Cfrom thetive to r

131er Catalystsn

d M. A. Vannice3

ty, University Park, Pennsylvania 16802

; accepted December 22, 1998

the use of these different forms of carbons as aneous catalyst support has grown, with activatederhaps being the most studied catalyst support ofbecause of the versatility of properties like poros-

ce area, and chemical nature, in addition to its me-resistance, stability, and inertness. Comprehensiveand critical analyses of these aspects have alreadyblished (110). The optimization of carbon surfaceristics to induce desirable interactions at the inter-ween the metal precursor and the support and thee of these modifications on the morphological, ad-, and catalytic properties of dispersed metal par-ve received much attention, as indicated by theous literature cited in these review articles. Into activated carbon, the application of two other

es of carbon, i.e., graphite and diamond, has alsosome attention in the past few years (1, 1146).

r, whereas the morphological and electronic prop-metals dispersed on these substrates have been in-d, the consequent effects on adsorption behaviorlytic activity have received very limited attention.ermore, despite recent theoretical and X-ray pho-n (XPS) studies that suggest changes in the elec-

ructure of copper deposited on carbon substratesthe number of literature reports addressing thef modified carbon chemistry on the properties of

on catalysts is small (4755). The different forms ofight be anticipated to have a significant influenceon the dispersion and sintering propensity of Cu

dispersed on their surfaces, but also on their re-

y; however, no systematic study of the propertiesspersed on these forms of carbon could be found.er hydrogenation catalysts, three different active

ave been proposed in the literature: CuC, Cu0, ande of CuC and Cu0 species (56). A study in which

ersion and reducibility of Cu are varied to therebyhe distribution of Cu surface sites seemed appro-oupled with this interest to study Cu/C catalystsstandpoint of adsorption and catalysis is an incen-

eplace the commercial copper chromite catalysts

0021-9517/99 $30.00Copyright c 1999 by Academic Press

All rights of reproduction in any form reserved.

132 DANDEKAR, BAKER, AND VANNICE

which have been extensively used for industrial hydrogena-tion reactions (56). Consequently, Cu crystallites dispersedon different forms of carbon, i.e., activated carbon, graphi-tized cacterizof thewas elysts idifferezationbeencharacchemisition,and tefuse rtroscostate oto idekinetia follo

Carbo

TheNorita specjectedchemiundergen ansamplgloveheatining andrying(GF) u(P-25,and mface awas obof 25 m

Cataly

A sto prethreeamondlutionter, alCu/ACoven o

ple was prepared by an ion-exchange (IE) technique usingan aqueous solution of Cu(NO3)2 and a concentrated am-monia solution according to procedures outlined elsewhere

oi

hcnp

ly

yA0mn

theo3

osa

Numluycmpsairr

K

vtui

eoarbon fibers, and diamond, were prepared and char-ed, the effect of different surface and bulk propertiescarbons on properties of the resulting Cu catalysts

xamined, and the catalytic behavior of these cata-n crotonaldehyde hydrogenation was studied afternt reduction pretreatments. A detailed characteri-of the pure carbon supports used in this study has

presented elsewhere (57). This paper describes theterization of Cu supported on these carbons by COsorption, oxygen chemisorption via N2O decompo-X-ray diffraction, transmission electron microscopy,mperature-programmed reduction. In addition, dif-eflectance Fourier transform infrared (FTIR) spec-py (DRIFTS) was employed to probe the chemicalf surface Cu sites using subambient CO adsorption

ntify oxidation states. The consequent effect on thecs of crotonaldehyde hydrogenation are described inwing paper (58).

EXPERIMENTAL

n Supports

activated carbon used in this study was obtained fromCorp (Norit A8933), designated AC-ASIS, and it hadified surface area of 800 m2/g. This sample was sub-to two different pretreatments to modify its surface

stry (59, 60). A high-temperature treatment (HTT)flowing H2 for 6 h at 1223 K was used to remove oxy-d any sulfur impurities from the carbon surface. This

e, labeled AC-HTT-H2, was stored in a N2-purgedbox. The third sample, AC-HNO3, was prepared byg AC-ASIS in 12 N nitric acid for 12 h at 363 K, wash-d filtering with water to achieve a pH of 7, and thenovernight at 393 K. The highly graphitic carbon fiberssed in this study were pitch-based commercial fibersAmoco Performance Products). The fibers were cutanually ground to yield a fine powder with a BET sur-rea of 6 m2/g. The synthetic diamond powder (DM)tained from Alfa Aesar and had a BET surface area2/g.

st Preparation

tandard wet impregnation technique (WI) was usedpare approximately 5 wt% Cu catalysts with all

activated carbons, the graphitized fibers, and the di-powder. After impregnation with an aqueous so-

of Cu(NO3)2, using deionized doubly distilled wa-l five catalysts, i.e., Cu/AC-ASIS, Cu/AC-HTT-H2,-HNO3, Cu/DM, and Cu/GF-WI, were dried in anvernight at 393 K. In addition, a 1.8 wt% Cu/GF sam-

(23, 6appracter1 atmat eit40 sccontetion s

Cata

COsuredtion s(63).at 30deterMG Iflow(Allttionas 14uptakmeth

Dito meetryis a sgravisampin sitpurittroduit asythenreverfinalrequ

X-usinga CuEachpassiHe ameaslinewusinginstrualsocroscwere1, 62). The pH of the suspension was maintained atximately 10 during stirring. Prior to any further char-zation, the catalyst samples were pretreated in situ atby flowing 40 sccm He (GHSVD 100 h1) for 1 her 423, 473, 573, or 673 K, followed by reduction inm H2 at the same temperature for 4 h. The exact Cut in each catalyst was determined by atomic absorp-ectroscopy.

st Characterization

uptakes on the pretreated catalyst samples were mea-at 300 K using a stainless-steel volumetric adsorp-stem giving a vacuum below 106 Torr at the samplefter the initial isotherm, the sample was evacuatedK for 1 h and a second isotherm was measured toine reversible adsorption. The H2 (99.999% purity,d.) and CO (99.99% purity, Matheson) were made torough molecular sieve traps (Supelco) and Oxytraps

ch Assoc.) for additional purification. O2 chemisorp-n these samples at subambient temperatures as lowK yielded nonreproducible results because of large

es on the carbon support itself; consequently thisd could not be used to estimate Cu disper-sions (62).

sociative N2O adsorption at 363 K was therefore usedsure metallic Cu surface sites based on the stoichiom-2O(g)C 2 Cu0s!CusOCusCN2(g) (64), where Cusrface atom. The measurements were carried out in aetric apparatus with a sensitivity of 0.1 g. After the

e was loaded in the analyzer pan, it was pretreatedbefore being cooled to 363 K, 10% N2O (99.99%

, Matheson) in Ar (99.999% purity, MG Ind.) was in-ed, and the increase in weight was monitored untilptotically reached a steady value. The sample was

urged with Ar (99.999% purity, MG Ind.) to desorbibly adsorbed N2O, and the difference between thend initial weights represented the amount of oxygened to oxidize the surface Cu0 atoms to CuC.ay diffraction (XRD) spectra were obtained ex situa Rigaku Geigerflex diffractometer equipped withfi radiation source and a graphite monochromator.

sample was given the desired pretreatment and thenated by exposure to a flowing mixture of 1% O2 in300 K for 1 h prior to handling in air during XRDrements. Cu crystallite sizes were calculated from thedth at half-height of the primary Cu0 and Cu2O peaksthe Scherrer equation with Warrens correction formental line broadening. The passivated samples werexamined in a Philips 420T transmission electron mi-pe at an accelerating voltage of 120 kV. The samples

dispersed ultrasonically in acetone and placed on a

CHARACTERIZATION OF C-SUPPORTED Cu CATALYSTS 133

400-mesh carbon-coated copper grid, and Cu particle sizeswere obtained by counting 120200 particles in each cata-lyst.

The tewere obtthe samp50 K/minwere anaquadrupthe amouas a func

The inFTIR sy(HVC-Dmodifiedflowingdesignedto subama soleno(99.999%ing themAssoc.)gas flow(Tylan Cto-noisethree acting a CaFreactor cCaF2 usicases, anoise raresolutioout any d

Afterwas purgnal dampterferogrscans obting of 5cedure wcatalyststreated sthen subously disogram. Tthat of treferencsurface.also usedspectra oheatingdevice auid N2. TCO/90%

Interferograms were collected under 75 Torr CO as well asafter purging the catalyst sample.

o

i1rirHmdt

,

.umperature-programmed reduction (TPR) spectraained by using a tubular quartz reactor and heatingle from room temperature to 1173 K at the rate ofin a 10% H2/90% He mixture. The effluent gaseslyzed with an on-line, computer-controlled, UTI

ole mass spectrometer, which was used to monitornts of NO and NO2 evolved from the nitrate aniontion of temperature.frared studies were conducted with a Sirius 100stem (Mattson Instr., Inc.) using a DRIFTS cellPR, Harrick Sci. Corp.) that has been extensivelyto allow in situ treatments up to 800 K under

gases (65, 66). A simple inexpensive attachmentearlier also allowed the sample cell to be cooledbient temperatures using liquid nitrogen and

id valve (65). Ultrahigh-purity H2, Ar, and COpurity, MG Ind.) were further purified by pass-through separate indicating Oxytraps (Alltech

and molecular sieve traps (Supelco), and preciserates were maintained using mass flow controllersorp., Model FC-260). To enhance the low signal-ratios typically obtained with carbon samples, allivated carbon samples were diluted with CaF2 us-2 : C ratio of 10 : 1 before loading into the DRIFTS

ell. The graphitized fiber samples were diluted withng a CaF2 : C ratio of 5 : 1 prior to loading. In bothlower dilution ratio yielded a very low signal-to-tio, whereas a higher ratio lowered the spectrumn (57). The diamond samples were analyzed with-iluent.

a sample was loaded into the DRIFTS cell, the celled overnight with Ar to minimize additional sig-ing caused by ambient moisture. The recorded in-ams typically consisted of 10,000 signal-averagedtained using a postamplifier gain of 8, an iris set-0, and a resolution of 4 cm1. A standard pro-as used to collect the interferograms for all the

. The first interferogram was that of the initial un-ample under flowing Ar (20 sccm). The sample wasjected to one of the desired pretreatments previ-cussed and cooled to 300 K for the second interfer-his interferogram, when Fourier transformed with

he corresponding pretreated unloaded support ase, yielded the spectrum of the pretreated catalystThe interferogram of the pretreated catalyst was

as the background reference for the subsequentf adsorbed CO for that sample. Following this, the

cartridge was replaced by the subambient coolingnd the DRIFTS cell was cooled to 173 K with liq-he catalyst was then exposed to a mixture of 10%Ar for 30 min and purged with pure Ar for 30 min.

Chemis

A typin Fig.elsewheirreversafter pCu/AC-ture frototal anboth toan increCu/DM

FIG. 1and (B) pRESULTS

rption

cal set of isotherms for CO adsorption is shownfor the Cu/DM catalyst; others are provided

e (62). Table 1 lists the corresponding total andble CO uptakes at 100 Torr on the six catalystsetreatment at different temperatures. For the

NO3 catalyst, increasing the reduction tempera-423 to 573 K resulted in a decrease in both the

reversible uptakes. However, for Cu/AC-HTT-H2,al and irreversible uptakes increased followingase in the reduction temperature. In the case ofboth the total and irreversible uptakes passed

CO adsorption isotherms for: (A) Cu/DM reduced at 573 K,re DM treated at 573 K in H2.

134 DANDEKAR, BAKER, AND VANNICE

TABLE 1

CO Uptakes, Apparent Cu Dispersions, and Cu Particle Sizes

d (nm)D 1.2 (CO/Cu).c

d

e

f

through a maxcreased from 4the irreversiblenot vary muchcontrast, largeIE catalyst, alafter reductionCOads : Cus, whCu dispersionsirreversible Cshould be noteof catalyst wasthe pure suppto provide therationale discuat 300 K can bCuC sites, whion the pretreabe attributed tface Cu0 andon the total COthe reversible

Oascee

esst

rt,asInde

re

a-2.e

toto

-d

is-dBased on total CO uptake.Based on irreversible CO uptake.Cleaned in H2 at 573 K for 1 h.No measurable uptake.

imum as the reduction temperature was in-23 to 673 K. In comparison, both the total andCO uptakes were low on Cu/GF-WI and didas a function of reduction temperature. Intotal uptakes were obtained on the Cu/GF-

though no irreversible uptake was detectedabove 473 K. Using a 1 : 1 stoichiometry forere Cus represents a surface Cu atom (67),were estimated based on both the total and

O uptakes and are reported in Table 1. Itd here that the reversible uptake per gramalways higher than the reversible uptake on

ort, which was subtracted from the uptaketotal uptake values in Table 1. Based on thessed earlier (67), the irreversible adsorptione associated with the stabilization of CO onle the difference in the reversible uptakested catalysts and the respective supports cano weak reversible adsorption of CO on sur-

residual Cu2C species. The dispersion baseduptake on each catalyst, after correcting for

uptake on the pure support, could provide

an approximate value for total Cu surface atoms if the Ccoverage on metallic Cu is high at these pressures, wherethe irreversible uptake would relate only to CuC surfasites. As seen in Table 1, with AC-supported catalysts thapparent dispersions based on either of these uptakwere highest for Cu dispersed on AC-HNO3 and lowefor Cu on AC-HTT-H2. Cu dispersion on the DM suppoalthough lower than that on any of the AC supports, wstill much higher than that obtained for Cu/GF-WI.contrast, apparent dispersions with the ion-exchangeCu/GF-IE catalyst based on total CO uptake were thhighest of all the catalysts, and no surface CuC species wedetectable after reduction at 473 K or higher.

Data obtained from N2O decomposition on these catlysts after different pretreatments are reported in TableFigure 2 represents the initial increase in the weight of thCu/AC-HNO3 sample (reduced at 423 K) after exposure75 Torr N2O in Ar at 363 K, followed by the decreasethe final weight gain after purging with Ar at the same temperature. From the corresponding uptakes of oxygen anthe assumption of a 1 : 2 stoichiometry for Oad : Cus, dpersions of metallic Cu were calculated and are reporteTredCO uptake (mol/g)a

Catalyst (K) Total Irreversible

4.8% Cu/AC-HNO3 423 183 52473 163 25573 140 10

4.6% Cu/AC-ASIS 423 142 89573 130 26

4.9% Cu/AC-HTT-H2 423 96 22573 102 20

5.1% Cu/DM 423 67 55473 74 45573 84 35673 60 10

5.1% Cu/GF-WI 423 10 5473 10 7573 9 6673 10 5

1.8% Cu/GF-IE 423 122 25473 141 0573 113 0673 68 0

Cu Powdere 9.6 f

a At 100 Torr.bCOIrr/CuTdcrys (nm)

COT/CuT .Cu1Cs /CuT/ d1b;c d2b;d

0.24 0.069 4.6 190.22 0.033 5.4 350.19 0.013 6.2 88

0.20 0.12 6.2 100.18 0.036 7.0 34

0.12 0.029 9.4 410.13 0.026 8.6 45

0.083 0.069 13.2 160.092 0.057 12 200.11 0.044 11 260.076 0.012 15 91

0.013 0.0065 88 1760.013 0.009 88 1260.012 0.0077 98 1470.013 0.0065 88 176

0.44 0.082 2.7 130.49 0 2.3 0.39 0 2.9 0.24 0 4.7

CHARACTERIZATION OF C-SUPPORTED Cu CATALYSTS 135

TABLE 2

Oxygen Adsorption via N2O Decomposition

Cata

4.8% Cu/AC

4.6% Cu/AC

4.9% Cu/AC

5.0% Cu/DM

5.1% Cu/GF

1.8% Cu/GF

Cu powderb

a d (nm)b Cleane

FIG. 2 3AC-HNO

in Table 2. Once again, the Cu0 dispersions were depen-dent on both the support and the reduction temperature.The highest dispersion with any activated carbon was ob-. Increase in weight during O chemisorption via N2O decomposition at3 after treatment in H2 at 423 K.on Cu Catalysts at 363 K

Tred O uptake dcrysalyst (K) (mol/g) Cu0/CuT (nm)

-HNO3 423 110 0.280 4.0473 77 0.20 5.7573 56 0.15 7.4

-ASIS 423 0 0 573 55 0.15 7.4

-HTT-H2 423 0 0 573 42 0.11 10.5

423 0 0 473 19 0.048 23573 25 0.064 18673 35 0.09 13

-WI 423 473 573 673

-IE 423 41 0.29 3.7473 55 0.39 2.8573 64 0.45 2.4673 31 0.22 5.0 6

D 1.1/(2Oad/CuT).d in H2 at 573 K for 1 h.

tained for Cu/AC-HNO3 reduced at 423 K. In contrast, theCu/AC-HTT-H2 showed no O uptake after reduction at423 K, although a significant uptake was observed after re-duction at 573 K. For the Cu/AC-ASIS catalyst, the uptakeafter reduction at 423 K was also nil, but after reduction at573 K it was similar to that for Cu/AC-HNO3 and higherthan that for Cu/AC-HTT-H2. In the case of Cu/DM, no sig-nificant change in the uptake was observed when reducedat or above 473 K, and the dispersions were lower thanthose obtained with any of the AC catalysts. No O uptakewas detected on the Cu/GF-WI catalyst after any pretreat-ment, presumably because the dispersion was so low thatany weight gain was within the sensitivity of the gravimet-ric apparatus used for the measurement. In contrast, withCu/GF-IE, significant uptakes were observed that did notvary significantly with reduction temperatures of 473 K andabove. Exposure of the pure supports to N2O showed nosignificant uptakes, indicating that the increase in weightof the catalysts was due to the reaction of N2O with sur-face Cu0 species. The measurement of Cu0 surface atomsby N2O decomposition, therefore, should be more accuratethan that obtained from the reversible CO uptake, whichinvolved correction for uptake on the support as well as un-certain adsorption stoichiometries. However, as discussed63 K over (A) Cu/AC-HNO3 reduced at 423 K and (B) pure

136 DANDEKAR, BAKER, AND VANNICE

in detoxygen uptakes alone represent only Cu0 sites and do notaccousurfatechn

FIG. 4. X

erRD patterns for Cu/GF-WI after (A) He treatment at 423 K, (B) reduction at 423 K, (C) reduction at 573 K, and (D) reduction at 673 K.nt for any unreduced surface CuC species; thus totalce Cu atoms can be underestimated using only thisique.

Thter ptivelyFIG. 3. XRD patterns for Cu/AC-HNO3 after (A) He treatment at 423 K, (B) reduction at 423 K, and (C) reduction at 573 K.

ail elsewhere (67), dispersions of Cu based on these X-ray DiffractionXRD patterns for Cu/AC-HNO3 and Cu/GF-IE af-etreatment at different temperatures, shown respec-in Figs. 3 and 4, are typical and others are provided

CHARACTERIZATION OF C-SUPPORTED Cu CATALYSTS 137

TABLE 3

Average Cu Crystallite Sizes (nm) from Adsorption, XRD, and TEM Methods

elsewherports havreductionat 35.3,flectionsthese spefrom 423ties that are indicative ofCu oxidation state. The cobased on both cuprous oxiTable 3.

Transmission and Scannin

Representative transmiCu/AC-HNO3 and Cu/DMin Figs. 5A and 6A, and thdistributions based on appeach are shown in Figs. 5Blysts are provided elsewheameter, dnD

Pni di=

Pni

diameter, dsDP

ni d3i =P

erage diameter, dv DP

niin each catalyst after a gTable 3. Fairly narrow Cuobtained with any of theafter reduction at 573 K.observed on the AC-HNOwere seen on AC-HTT-H2

o

w

ep

surface

ed car-the ob-

issionly fea-could

exam-esenta-GF-WIown ingraphse com-sed onFig. 8.

ger Cug fromu par-5 and

s wered to ex-ing thea progressive transition of therresponding Cu crystallite sizes,de and metallic Cu, are listed in

g Electron Microscopy

ssion electron micrographs forreduced at 573 K are shown

e corresponding Cu particle sizeroximately 120150 particles for

and 6B. Those for other cata-re (62). The number-average di-, surface area-weighted averageni d2i , and volume-weighted av-d4i =

Pni d3i , of the Cu particles

iven pretreatment are listed inparticle size distributions were

three activated carbon supportsThe smallest Cu particles were

3 support, whereas the largest, thus suggesting a favorable in-

ing forces between the precursor and the support(40).

Because of the nature and texture of the graphitizbon fibers used, instrumental limitations preventedservation of any discernible features in the transmelectron micrographs of these Cu/GF catalysts. Ontures present at the edges of the fiber fragmentsbe observed; consequently, the samples were alsoined by scanning electron microscopy (SEM). Reprtive transmission electron micrographs of the Cu/and Cu/GF-IE catalysts reduced at 573 K are shFig. 7, while corresponding scanning electron microof these two samples are provided elsewhere (62). Thbined particle size distributions for these samples, baboth the SEM and TEM image analyses, are shown inAs seen in Fig. 8A, a broad distribution of much larparticles occurred in Cu/GF-WI, with sizes rangin30 to 240 nm, whereas in Cu/GF-IE, a bimodal Cticle size distribution existed, with maxima around45 nm. With the latter catalyst, the larger particleobserved in the SEM images and could be attributecess Cu deposited on the graphitic basal planes durTredAdsorption

Catalyst (K) (COIrrC 2Oad)/CuT dsurf4.8% Cu/AC-HNO3 423 0.36 3.2

473 0.24 4.9573 0.16 7.2

4.6% Cu/AC-ASIS 423 0.12 6.2573 0.19 7.5

4.9% Cu/AC-HTT-H2 423 0.03 9.2573 0.13 8.5

5.0% Cu/DM 423 0.07 12473 0.11 13573 0.11 9673 0.10 13

5.1% Cu/GF-WI 423 0.012a 88a

473 0.012a 88a

573 0.011a 98a

673 0.012a 88a

1.8% Cu/GF-IE 423 0.31 2.9473 0.42 2.8573 0.48 2.4673 0.25 5.0

a Based on total CO uptake.

e (62). The peaks corresponding to the carbon sup-e been analyzed elsewhere (57). Depending on the

temperature, broad peaks of varying intensities36.5, and 42.9, corresponding to the primary re-of the CuO, Cu2O, and Cu0 phases, are seen inctra. As the reduction temperature is increasedK, gradual changes occur in the peak intensi-

fluencepersionticles. Incle sizecomparagiven thtwo supXRD TEM

Cu2O Cu dnum dsurf dvol

1.3 2.9 1.1 4.9 5.6 6.4 6.7

9.2 5.86.8 7.0 8 10 11

11 9.810 23 14 29 36

13 17 13 15 1912 14 10 13 23 18

19 51

21 78 94 136 154 100

1.5 39 48 30 81 100

f the oxygen-containing surface groups on the dis-and sintering resistance of the supported Cu par-the case of Cu/DM, a wider distribution of parti-as seen, although the average particle sizes were

ble to those on AC-HTT-H2. This was surprisinghuge difference in BET surface areas between theorts, and it suggests a role of interactive stabiliz-

138 DANDEKAR, BAKER, AND VANNICE

FIG. 5. (A) Transmission electron micrograph of Cu/AC-HNO3 after a 573 K reduction. (B) Corresponding Cu particle size distribution.

CHARACTERIZATION OF C-SUPPORTED Cu CATALYSTS 139

FIG.

ion-exchanthe range 3basal planeble that the6. (A) Transmission electron micrograph of Cu/DM after a 573 K reduction. (B) Corresponding Cu particle size distribution.

ge and/or drying procedure. Smaller particles in6 nm can be seen located near the edges of these

s in the TEM images of this catalyst. It is possi-se may be decorating steps and discontinuities

along the graphitic adlayers, which would be consistent withthe results of Richard et al., who observed a similar mor-phology of Pt particles deposited on a graphitized carbonby ion exchange (2325).

140 DANDEKAR, BAKER, AND VANNICE

FIG. 7. Transmission electron micrographs of (A) Cu/GF-WI and (B) Cu/GF-IE after a 573 K reduction.

CHARACTERIZATION OF C-SUPPORTED Cu CATALYSTS 141

FIG. 8.(B) Cu/GF

DRIFTS

The Fprior to aspectra upure suptra therenated Cubroad ov1800 cm

1740 cm2029, andabsorbanmentionis that ofply a lossinal carbThe specbands in1582 cm

lar to that of Cu/AC-ASIS, with weaker peaks at 960 and1582 cm1, while the intensity of the 1355 and 1419 cm1

peaks changes little. The effect of H2 reduction at higher

Oa,hd

it

0ep

e

et3t

ed

t3

1

a

aofidr

a

nxwrc

keParticle size distributions for Cu in (A) Cu/GF-WI and-IE after a 573 K reduction.

TIR spectra of the three AC-supported catalystsny pretreatment are shown in Fig. 9. The referencesed here are those of the corresponding untreatedports (diluted with CaF2). The bands in these spec-fore essentially represent the nature of the impreg-

species. The spectrum of Cu/AC-HNO3 exhibitserlapping bands in the region between 1200 and1, with peaks at 1285, 1355, 1419, 1596, 1669, and1, along with weaker absorbance bands at 967,

2918 cm1. In addition, a significant drop in thece intensity is seen around 31003500 cm1. As

ed earlier, since the reference spectrum in this casepure AC-HNO3, this drop in intensity would im-of surface functional groups present on the orig-

on after interaction with Cu precursor solution.trum of the Cu/AC-ASIS shows fewer absorptionthe same region, i.e., at 960, 1245, 1355, 1419, and1. The spectrum of the Cu/AC-HTT-H2 is simi-

temperaified in FAC-HN423 K lefeaturesexcept tleads totrum ofalong wnegative310035peratur

The oresultedallowedsupportDM as ware showthe untrbands aaroundpeaks aweak p2467, anbe seenreducesand 142leads tothat at673 K. Treductio

Similtinct peawith weand a brtained ato the d2924 anas wellperhapsspeciesthe redu

DRIFASIS, a573 K, epurgingare refetreatedafter redtion peasharp ptures on the groups found on Cu/AC-HNO3 is typ-ig. 10. Each reference spectrum is that of the pure

3 after a similar treatment in H2. Reduction atds to a modification in the overall shape of thesewith a significant drop in intensities of all the peaksose at 1355 and 1419 cm1, and reduction at 573 Kisappearance of all the bands present in the spec-

the original catalyst. It should be noted here thath these changes in the region 12001800 cm1, theband representing loss of intensity in the region0 cm1 slowly disappears as the reduction tem-is increased to 573 K.tically transparent nature of the diamond powderin significantly higher signal-to-noise ratios andthe collection of very clean spectra of diamond-d Cu with no dilution. The spectra of untreated Cu/ell as those after reduction at higher temperaturesn in Fig. 11. Similar to the AC-supported catalysts,ated Cu/DM spectrum exhibits strong absorption1341 and 1423 cm1 along with a broad band500 cm1 consisting of identifiable overlapping3361, 3450, 3532, and 3582 cm1. In addition,

aks are seen at 869, 910, 1047, 1179, 2087, 2343,2740 cm1, and a loss in absorption intensity can

centered around 1744 cm1. Reduction at 423 Khe intensity of all bands, with those at 1047, 1341,cm1 remaining prominent. Reduction at 573 K

disappearance of all the significant peaks, except341 cm1, which disappears after reduction at

he negative band at 1744 cm1 remains even aftern at 673 K.r spectra for Cu/GF-IE are shown in Fig. 12. Dis-ks are seen at 1049, 1358, 1422, and 3548 cm1 alongker ones at 791, 871, 1110, 2924, and 2964 cm1

ad band around 3472 cm1. All these peaks are re-ter reduction at 423 K, but reduction at 573 K leadssappearance of virtually all bands except those at

2964 cm1. The spectra for Cu/GF-WI were notesolved and informative as those for Cu/GF-IE,due to the very low concentration of surface Cus implied by the low dispersion of Cu observed inced catalyst in addition to dilution with CaF2.T spectra are shown for Cu/AC-HNO3, Cu/AC-d Cu/AC-HTT-H2 after reduction at either 423 orposure to a 10% CO/90% Ar mixture at 173 K, andith Ar in Figs. 13 and 14, respectively. All spectra

enced to the spectrum of the corresponding pre-atalyst prior to CO admission. As shown in Fig. 13,uction at 423 K for Cu/AC-HNO3 a distinct absorp-

developed at 2124 cm1 along with a weaker butak at 2107 cm1. After reduction at 573 K, only a

142 DANDEKAR, BAKER, AND VANNICE

FIGthe cor

FIGspectra. 9. DRIFT spectra of untreated Cu/AC catalysts: (A) Cu/AC-HNO3, (B) Cu/AC-ASIS, (C) Cu/AC-HTT-H2. Reference spectra are those ofresponding pure carbons.

. 10. DRIFT spectra of Cu/AC-HNO3 (A) before any pretreatment, (B) after reduction at 423 K, and (C) after reduction at 573 K. Referenceare those of the corresponding pretreated pure AC-HNO3.

CHARACTERIZATION OF C-SUPPORTED Cu CATALYSTS 143

FIG. 11.at 673 K. R

FIG. 12.spectra areDRIFT spectra of Cu/DM (A) before any pretreatment, (B) after reduction at 423 K, (C) after reduction at 573 K, and (D) after reductioneference spectra are those of the corresponding pretreated DM.

DRIFT spectra of Cu/GF-IE (A) before any pretreatment, (B) after reduction at 423 K, and (C) after reduction at 573 K. Referencethose of the corresponding pretreated pure GF.

144 DANDEKAR, BAKER, AND VANNICE

F

FIG. 13. DRIFT spectra of CO adsorbed on Cu/AC catalysts after reduction at 423 K; TadsnD 173 K.

IG. 14. DRIFT spectra of CO adsorbed on Cu/AC catalysts after reduction at 573 K; TadsnD 173 K.

CHARACTERIZATION OF C-SUPPORTED Cu CATALYSTS 145

FIG. 15 t(E) 673 K;

more inttrast, the spectrum of Cu/AC-HTT-H2 exhibits a strong ab-sorptiontreatmeductionnear 2122109 cmate featuThe abssorptionassignedfrequensurface

The Dafter dispectrumhibits a2120 cmtensifiesis reduchigher-f2130 cmintensitypeak at2130 cmsifies.

ethe DRIFpeak around 2158 cm1 after the low-temperaturent along with a weaker peak at 2126 cm1. After re-at 573 K, the 2158 cm1 peak disappears while that2 cm1 intensifies and a new sharp peak appears at1. The spectra for Cu/AC-ASIS exhibit intermedi-res compared with those of the other AC catalysts.

orption band at 2158 cm1 corresponds to CO ad-on Cu2C sites, those from 2122 to 2127 cm1 can beto CO adsorbed on CuC sites, while adsorption at

cies of 2110 cm1 and below can be associated withCu0 sites (6769).RIFT spectra of CO adsorbed on Cu/DM at 173 K

fferent pretreatments are shown in Fig. 15. Theof the catalyst treated in Ar at 423 K for 1 h ex-

broad band at 2158 cm1 along with another one at1. After reduction at 423 K, the 2120 cm1 peak in-and shifts to 2122 cm1 while the 2158 cm1 peak

ed significantly. Reduction at 473 K removes therequency band and shifts the peak at 2122 cm1 to1. Reduction at 573 K results in a decrease in the

of this latter peak and the appearance of a new2110 cm1. Finally, after reduction at 673 K, the1 peak disappears and the 2110 cm1 peak inten-

samples. For Cu/GF-WI the results were similar to those forCu/DM, in that treatment in Ar gives a peak at 2155 cm1

with a shoulder at 2122 cm1, as shown in Fig. 16a. Afterreduction at 423 K, the former peak intensity drops con-siderably while that of the latter is enhanced. Reduction at573 K leads to the disappearance of the 2155 cm1 peakand the development of a new peak at 2092 cm1. Finally,the spectrum of the catalyst reduced at 673 K exhibits onlyone significant band at 2093 cm1. Corresponding spectrafor Cu/GF-IE after different pretreatments are shown inFig. 17b. The spectrum of the catalyst treated in Ar at 423 Kfor 1 h exhibits a strong peak at 2156 cm1 and a shoulder at2120 cm1. After reduction at 423 K, the higher-frequencypeak completely disappears while that near 2120 cm1 in-tensifies, and a new strong peak develops at 2066 cm1.Reduction at 473 K results in complete removal of the2120 cm1 band and a shift in the low-frequency peak to2060 cm1 with a significant enhancement in its intensity.

DISCUSSION

Catalyst Surface Chemistry

Characterization of the pure carbon supports used in thisstudy using temperature-programmed desorption, XRD,. DRIFT spectra of CO adsorbed on Cu/DM after (A) treatment in Ar aTadsnD 173 K.

ense band at 2108 cm1 remains (Fig. 14). In con- Figur423 K and reduction at (B) 423 K, (C) 473 K, (D) 573 K, and

16 shows the effect of reduction temperature onT spectra for CO adsorbed at 173 K in the Cu/GF

146 DANDEKAR, BAKER, AND VANNICE

FIG.TadsnD 1

FIG. 17.16. DRIFT spectra of CO adsorbed on Cu/GF-WI after (A) treatment in Ar at 423 K and reduction at (B) 423 K, (C) 573 K, and (D) 673 K;73 K.

DRIFT spectra of CO adsorbed on Cu/GF-IE after (A) treatment in Ar at 423 K and reduction at (B) 423 K and (C) 473 K; TadsnD 173 K.

CHARACTERIZATION OF C-SUPPORTED Cu CATALYSTS 147

and DRIFTS has been discussed in detail earlier (57).Whereas all three activated carbons exhibited high BETsurface areas, those of GF and DM were significantly lower.XRD patized carturbostrand COthat thethe mosoxygen fbon follcarbon pevidencpresumaavailablat tempthe otheobservemond poof pureconsiderever, simin the phave shotion usepregnaton the cpresentcur (70to identof the mthe unlocatalysts

The pAC-HNcopper cbon by ifunctionand COtwo equasymme1675 cming vibrathese baacetate,ments hcarbonschangeFurtherXPS specarbon wand metgroups oment co

form the observed Cu carboxylates. The assignment of the1596 and 1285 cm1 peaks to the asymmetric and symmet-ric CO2 vibrations of surface Cu carboxylate-type species

5

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itterns indicated that the structure of the graphi-bon fibers is much more ordered compared with theatic activated carbon. TPD evolution profiles of CO2 as well as DRIFT spectra of the carbons indicatednitric acid-treated carbon surface (AC-HNO3) wast acidic of all, and both strongly and weakly acidicunctional groups were created on the AC-ASIS car-owing the nitric acid treatment. The spectra of theretreated in H2 at 1223 K (AC-HTT-H2) provided

e for substantial amounts of chemisorbed hydrogen,bly on highly reactive edge carbon atoms made

e by the decomposition of CO-yielding complexeseratures above 873 K during the H2 treatment. Onr hand, only weakly acidic or nonacidic groups wered on the surfaces of the graphitized fiber and dia-wder. As mentioned earlier (57), characterization

carbon supports using IR techniques has receivedable attention during the past decade or two; how-ilar studies of metal-impregnated carbon samples

ublished literature are very limited. XPS studieswn that, depending on the metalprecursor solu-

d and the preparation technique applied, the im-ed metal species can be stabilized in different formsarbon surface and changes in the functional groupson the surface of the original carbon can also oc-74). One obvious way to detect these species andify these changes is to study the DRIFT spectraetal-impregnated samples using the spectrum of

aded carbon as the reference. Such spectra for thein this study are shown in Figs. 912.

rominent band in the spectrum of untreated Cu/O3 (Fig. 9) can be tentatively assigned to unidentatearboxylate species formed on the surface of this car-

nteraction of the precursor ions with surface oxygenalities (75, 76). In such carboxylate salts, the C==O

bonds are strongly coupled and are replaced byivalent carbonoxygen bonds, resulting in a strongtric CO2 stretching vibration in the region 15751 and a somewhat weaker symmetric CO2 stretch-tion around 12601420 cm1. Weaker overtones ofnds have been observed at lower wavenumbers forformate, and oxalate salts (75). Oxidative treat-ave been reported to produce ion-exchangeable

, in which the protons of carboxylic acids can ex-to form complex organometallic cations (77, 78).more, binding energy shifts have been identified inctra of Cu and other metals dispersed on activatedhich have been attributed to metal carboxylic salts

al carbonates (70). In this study, the acidic surfacen AC-HNO3, introduced by the nitric acid treat-

uld behave in a similar fashion to anchor Cu and

thereforand 124well as Clate specthe prec

Bandbuted tostretch o1380 cmterminastrong aweakerBased othe specto nitratsurface.spectradue to asition ineither tocarbon sexistingthe precdue to Ction perfurtherthis time

The esurfaceter redutween 1overallCu carbindicatintent witthat theatures bpeaks coafter thpletelythat comcurs betbands cosupportactivelyon AC-groups.Cu/AC-cursor ssufficienspecies,e seems to be reasonable. The weaker bands at 1582cm1 observed in the spectra of Cu/AC-ASIS as

u/AC-HTT-H2 can also be assigned to Cu carboxy-ies on these two surfaces formed via interaction ofursor with residual surface acidic groups.

in the region 13001500 cm1 can also be attrithe precursor nitrate species. The asymmetric NO3f inorganic nitrate salts absorbs strongly at 13501 and weakly below 1000 cm1 (76). Similarly, thenitro group, with two identical NO bonds, gives aymmetric vibration around 13701470 cm1 and aymmetric one between 1315 and 1340 cm1 (75).

n this, the distinct peaks at 1355 and 1419 cm1 inra of all three untreated AC catalysts are attributed

e (NO3) and nitrite (NO2) species on the carbonThe additional peaks at 1669 and 1740 cm1 in theor Cu/AC-HNO3 cannot be assigned to absorptiony precursor species, so based on their general po-

the IR spectrum (57), they are tentatively assignednewly generated carbonyl (C==O) groups on the

urface or to enhanced absorption intensity of pre-C==O species due to an electronic interaction withursor ions. The weak peak at 2918 cm1 could beH stretches in these species. Because no informa-aining to this issue could be found in the literature,ustification of these assignments is not possible at.fect of pretreatment in flowing H2 on the variouspecies in Cu/AC-HNO3 is shown in Fig. 10. Af-

ction at 423 K, the general shape of the bands be-00 and 1800 cm1 is modified, with a drop in the

ntensity of absorption. The peaks associated withoxylate species on the surface are much weaker,g a decrease in their concentration. This is consis-the TPD of CO2 from AC-HNO3 which showed

acidic groups start decomposing in H2 at temper-elow 423 K. No significant effect on the strongrresponding to NO2 and NO3 species is apparents treatment. Reduction at 573 K, however, com-emoves these latter two strong peaks, indicatingplete decomposition of the nitrate precursor oc-een 423 and 573 K, along with the removal of all

rresponding to Cu carboxylate species. This furtherthe proposal that CO2-yielding acidic groups are

involved in anchoring Cu in the carboxylate formNO3, as compared with less acidic CO-yieldinghe TPR spectra for NO and NO2 desorption fromNO3, representing the thermal stability of the pre-

ecies (62), also indicated that reduction at 573 K ist to remove most of the surface nitrate and nitriten agreement with the DRIFT spectra.

148 DANDEKAR, BAKER, AND VANNICE

Similar DRIFT spectra of DM-supported Cu are shownin Fig. 11. The broad band between 3300 and 3600 cm1,with overlapping peaks at 3361, 3450, 3532, and 3582 cm1,can btroduaqueoby thtion aattrib(nitraAs inat 423This idesormosting re2343,of unrelatetentabondbe du1047at thito losunloathe prface tsomespeciepeak

Thestronter retributpeakbe assand afiberssolutiexchacan b3472assignat 292of sur1110 cof othat thi

Cu D

In asuch

was provided that N2O decomposition at 363 K measuresonly the metallic Cu surface atoms, whereas irreversibleCO adsorption at 300 K is proportional to the surface con-

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a

ne assigned to OH stretching vibrations of H2O in-ced on the DM surface during impregnation of theus Cu(NO3)2 solution. This assignment is supported

e fact that these bands nearly disappear after reduc-t 423 K. Prominent peaks at 1341 and 1423 cm1 areuted to asymmetric and symmetric vibrations of NO3te) and/or NO2 (nitrite) species, as discussed earlier.the case of Cu/AC-HNO3, these peaks are stable in H2

K, but decompose on reduction at 573 K or higher.s also verified by the TPR spectra for NO and NO2ption from Cu/DM (not shown), which indicate al-complete decomposition of the precursor species dur-duction at 573 K (62). Additional peaks at 2740, 2467,1179, 1047, 910, and 869 cm1, seen in the spectrum

treated Cu/DM, cannot be assigned to any precursor-d species. Based on their wavenumbers, the former is

tively assigned to enhanced CH stretches of stronglyed surface hydrogen (76). The 1179 cm1 peak coulde to the wagging motion of these CH groups, but thecm1 peak is not assigned to any particular vibrations time. The negative band at 1744 cm1 is clearly dues of certain functional groups originally present on theded DM surface. The spectra of pure DM indicatedesence of various ketonic carbonyl groups on the sur-hat absorb at 1750 cm1 (57); thus it is possible thatof these carbonyl species interact with Cu precursors to anchor Cu on the surface and create a negative

in this region.spectrum of untreated Cu/GF-IE in Fig. 12 exhibits

g bands at 1422 and 1358 cm1 which disappear af-duction at 573 K. As in other cases, these are at-ed to NO2 and NO3 precursor species. The sharpat 3548 cm1 and the broad one at 3472 cm1 canigned to NH and NH2 stretching vibrations of aminemide groups formed on the surface of the graphitized(75, 76) by interaction with the ammonium hydroxideon used to create a basic suspension for the cationicnge of Cu. The wagging vibrations of these NH2 bondse observed at 791 and 871 cm1. The retention of thecm1 band after reduction at 423 K argues against itsment to OH groups. Similar to Cu/DM, the bands4 and 2964 cm1 can be assigned to stretch vibrationsface CH groups whose wagging modes are visible atm1. The peak at 1046 cm1, also witnessed in the caseer catalysts, is not assigned to any particular species

s time.

ispersion

parallel study of Cu dispersed on a variety of supportsas DM, GF, SiO2, Al2O3, TiO2, and ZrO2, evidence

centrCu shof theirrevetion acompestiment ca

ThactivaN2OTablebasedtransamplthe pthreeall thchainlogicawhercrystaport pwith CH2. Sthe Acal prthe inface otreatmcan bcursothe imacidicas evhydroa surfvent.the waccesternadistri

Ththe pbeenoxygevisionformthe exanchotrastition of cuprous ions (67). The actual dispersion ofould, therefore, be closer to that based on the sumamount of N2O decomposed and the amount of COrsibly chemisorbed. That based on N2O decomposi-lone would be applicable only if all Cu species wereetely reduced to Cu0. This rationale was used here tote and compare the dispersion of Cu on these differ-

rbon supports.effect on Cu dispersion due to pretreatment of the

ted carbon prior to Cu impregnation, based on bothecomposition and CO adsorption, is reported in

3, which lists the corresponding Cu crystallite sizeson adsorption, XRD, and TEM analyses. From theission electron micrographs of these catalysts (ex-

s of which are shown in Figs. 57), it can be seen thatrticles tend to be relatively globular in outline on all

C supports. The particles exist as discrete entities incases, with no tendency to agglomerate to form eitheror clusters being evident. A more thorough morpho-

l characterization of these catalysts is provided else-(79). The particle size distribution and the average

llite size, however, are strongly dependent on the sup-retreatment employed. The best dispersion, obtainedu/AC-HNO3, is more than twice that of Cu/AC-HTT-nce the same AC-ASIS carbon was used to prepareC-HNO3 and AC-HTT-H2 supports and an identi-cedure was employed to impregnate them with Cu,

crease in dispersion can be attributed mainly to sur-xygen-containing groups introduced by the nitric acident. The promotional effect of surface functionalitiesexplained by the interaction between the metal pre-and the chemical state of the carbon surface during

pregnation stage (1). The removal of residual surfacegroups during the high-temperature treatment in H2,enced by the TPD spectra (57), should increase the

phobicity of the original activated carbon, leading toce less favorable for interactions with an aqueous sol-n the other hand, the nitric acid treatment enhances

ttability of the AC-HNO3 carbon surface, facilitatesof the aqueous precursor solution through the in-pore structure, and results in a more homogenousution, giving a higher dispersion of copper (77, 80).maintenance of the initial metal distribution during

ecursor decomposition and reduction stage has alsoeported to be affected by the concentration of surfacen groups (77, 81). The surface oxygen groups are en-ed to act as chemical anchorage centers through thetion of some type of MO complex on the surface,tent of this anchoring depending on the stability of thering sites and pretreatment conditions (1). The con-g behavior of the CO2- and CO-yielding groups on

CHARACTERIZATION OF C-SUPPORTED Cu CATALYSTS 149

the carbon surface has been contemplated in this context.Although no unanimously agreed on theory exists, the moreacidic CO2-yielding groups (e.g., carboxyl) are thought to bemore efCO-yiemore ecatalysthigherthe hydtional nature olation oet al. (8Pt dispgreaterthe TPstrongeColomaoxygenarguedters duthe highPt, whispeciesclarify tAntonution tecof oxygplatinucontradof limiting sitetemperducingare beybe of litchoringreductisired, thoxygenbe beneoptimucrotonaevidentcomplecomposan incrwhich cCO2-yithat coutotal amwhich ction ofbetwee

and DRIFT spectra indicate that AC-HNO3 has the highestconcentration of surface anchorage sites, while AC-HTT-H2 has the lowest. This, coupled with the dispersion data

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csnhfective catalyst dispersing sites, while the less acidiclding groups (e.g., carbonyl) are considered to beffective for dispersion maintenance (1). For Pd/Cs, a higher initial dispersion was attributed to the

acidity of the CO2-yielding groups, which enhancedrophilicity of the carbon surface and provided addi-ucleation sites (77); however, the reduction temper-f 373 K was lower than that required for decarboxy-f the carbon surface. In the work by Prado-Burguete0), a linear correlation was reported between initialersion and the amount of CO-yielding groups. Thestability of the CO-yielding groups, evident from

D spectra, was considered to be responsible for ar anchorage which hindered sintering. In contrast,et al. (82) reported a detrimental effect of surface

groups on the final dispersion of Pt/C catalysts. Theythat the carboxylic groups that act as anchoring cen-ring the impregnation step are decomposed during-temperature treatment at 773 K required to reduce

ch favors the surface mobility of the Pt precursorand leads to a lower Pt dispersion. In an attempt tohe role of these surface functional groups on carbon,cci et al. (83) used XPS and potentiometric titra-hniques to confirm that an increase in the amountsenated species on the carbon surface decreases them surface area in Pt/C catalysts. These apparentlyicting observations suggest a nonintuitive paradoxed thermal and chemical stability of metal anchor-s on carbon surfaces which limit the metal reductionature. Indeed, if the temperatures required for re-the metal precursor and obtaining catalytic activityond the stability limits of the surface groups, it wouldtle consequence to use such unstable groups for an-the metal. In the case of copper, since complete

on of the copper species to Cu0 is not always de-e effect of increasing the concentration of surface

-containing groups by HNO3 treatment appears toficial in terms of rendering higher dispersion. The

m reduction temperature for the highest activity inldehyde hydrogenation is 423 K (58) which, as isfrom the TPD data, is lower than that required for

te decomposition of CO2-yielding groups. N2O de-ition and CO adsorption data, however, do indicate

ease in Cu crystallite size after reduction at 573 K,ould be due to partial removal of the anchoring

elding groups, thus leading to mobile Cu particlesld agglomerate to yield larger crystallites. From theounts of CO2 and CO desorbed from the supports,

an be used as a measure of the surface concentra-these groups, a qualitative correlation can be maden the surface acidity and Cu dispersion. Both TPD

obtainefor thepersiontotal amdistinctiof sites

The mof Cu/Gtent witNo disttransmiScanninindicatetical paglomeragraphitilarge Cu30 to 24on GF btron micof an enon Cu/Gthe sizesion eleindicatetallites amuch natrast waflat moredges oserved blizationthe ion-anchoripreferreserved,start viasites (33the pecCu/GF-chemisothe cheparticleCu0 in tlarger Cmethodphologiparticlesignificawithin thave a dfrom adsorption, XRD, and TEM analyses, speaksvorable effect of these surface groups on Cu dis-

However, since the dispersions correlated with theunts of both CO2- and CO-yielding groups, a clearn between the relative importance of the two types

annot be made.uch larger Cu crystallite sizes observed in the case-WI, compared with Cu/AC catalysts, are consis-the much smaller surface area of the GF support.guishable Cu particles could be observed in the

sion electron micrograph of this catalyst (Fig. 7).electron micrographs of the Cu/GF-WI samplethe presence of predominantly circular or ellip-

ticles, either as remote isolated islands or as ag-ed labyrinth-type clusters, located on top of thebasal planes (62). A broad size distribution ofparticles is observed, with particles ranging fromnm. In contrast, the morphology of Cu dispersed

y ion exchange is quite different. A scanning elec-ograph of this catalyst (62) indicated the presence

semble of globular particles similar to those seenF-WI, with particles being relatively smaller andistribution ranging from 15 to 120 nm. Transmis-

tron micrographs of Cu/GF-IE (Fig. 7), however,the presence of much smaller individual Cu crys-

long the edges of the fiber fragments, which have arrower size distribution. A low, fairly uniform con-

seen at all locations, implying a predominantlyhology of these particles. Similar decoration of

graphite basal planes by Pt particles has been ob-fore and has been attributed to preferential stabi-f Pt along steps and edges of these planes duringxchange process (2325). In a recent study of Ptg on graphite, platelets of Pt metal with a highly

orientation on the graphite substrate were ob-nd the genesis of this structure was presumed tothe molecular anchoring of the precursor at defect34). A similar interaction could be responsible forliar particle size distribution of Cu in the case ofE. This bimodal distribution is consistent with theption data and XRD patterns of this catalyst; i.e.,isorption data predict the presence of small Cuwhereas the sharp narrow peaks corresponding toe XRD patterns indicate the existence of muchparticles. Thus Cu dispersed by an ion-exchange

is stabilized on the graphitized fibers in two mor-ally different forms: aggregates of large globulardeposited on top of graphitic basal planes andtly smaller crystallites at the edges and defectsese planes. These morphological differences couldistinct effect on the electronic properties of these

150 DANDEKAR, BAKER, AND VANNICE

crystallites and affect their reducibility, as discussed in thenext section.

The chemistry of Cu deposited on the diamond support isuniquthis caogy (to thaof Cunatiobeen(40, 4dangbeendangland ccrysta(Fig.carboreticadiamotwo bin a fthe fobondthe Cparticinterawas pa mothe abe mhighethat otheseelectrbasaledges

It scrystamuchXRD

Distri

Asportsa signper spthe pternsamouresenrespeCu sp

(Cu2C), cuprous (CuC), or metallic (Cu0) form, and it wasobserved in most cases that Cu existed in more than oneoxidation state. This distribution of surface Cu oxidation

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eeire in its own way. Transmission electron micrographs oftalyst reveal Cu particles of a fairly uniform morphol-

Fig. 6). Despite the surface area of DM being similart of the graphitized fibers, the average crystallite sizeon DM is much smaller than that obtained by impreg-

n of GF. Enhanced stability of Cu particles on DM hasshown before, both experimentally and theoretically1), to be related to the interaction of Cu with surfaceling bonds. The surface chemistry of diamond hasdiscussed in some detail (57), and free valences withing bonds have been proposed to exist on the edgesorners of diamond crystals as well as (100) and (111)l planes. The loss of intensity for Cu/DM at 1750 cm1

11) perhaps indicates the stabilization of Cu throughnyl group functionalities on the DM surface. A theo-l investigation of the electronic structure of coppernd interfaces indicates that Cu can be anchored in

asic geometries: Cu on top of a surface C atom and Cuourfold coordinated position above C atoms (41). Inrmer case, when Cu is positioned on top of a dangling

, a reasonably strong covalent bond is formed betweenu 4s orbital and the C dangling bond, with minimalipation of the Cu 3d band. In the latter case, a weakerction with a greater participation of Cu 3d electronsredicted with neighboring dangling bonds, leading tore effective metallic type of bonding. In either case,dhesion between Cu and diamond was predicted touch higher than that between Cu and graphite. Ther dispersion of Cu observed on DM, compared withn GF prepared by wet impregnation, is consistent withtheoretical predictions. Similar adhesion owing to anonic interaction is observed between Cu and graphiticplanes when Cu is deposited via cationic exchange on, adlayers, and defects of these planes.hould be noted here that in all the above cases, Cullite sizes based on both O and CO uptakes are inbetter agreement with those obtained from TEM andthan those based on O uptake alone.

bution of Cu Oxidation States

indicated earlier, the use of different carbon sup-with varying surface and bulk properties can haveificant impact on the reducibility of dispersed cop-ecies. Both the DRIFT spectra of CO adsorption on

retreated catalysts (Figs. 1317) and the XRD pat-clearly illustrated this effect. Furthermore, the relativents of irreversibly adsorbed CO and O, which rep-t the surface concentrations of CuC and Cu0 species,ctively, also illustrate the same influence. The surfaceecies in these catalysts can exist in either the cupric

statesand ttimeeffecfor eFor tthat edecontion swerepositof COtotaltivelyat 30versibconceconceing threspothe thfor al

ThHNOencewhichappewhichter p(Cu0)AC-Htempcorretion tat 42erageA simtion oof thof thpresetechnand sCu oobserAC-HH2, preducducibdispeaturewas dependent on both the reduction temperaturee carbon support used to disperse Cu, as reductionas held constant at 4 h. In an attempt to quantify thisthe fraction of Cu in each valence state was estimatedch catalyst as a function of reduction temperature.e DRIFT spectra of adsorbed CO, it was assumedach fraction was proportional to the area under thevoluted peak corresponding to each individual oxida-ate. With the XRD patterns, the absolute intensitiessed for these fractions, which represent bulk com-ns. Regarding the chemisorption data, the amountsand O irreversibly adsorbed directly yielded the

urface concentrations of CuC and Cu0 sites, respec-If it is then assumed that the total CO uptake on CuK and 75 Torr (obtained after correcting for the re-

le uptake on pure supports) approximates the surfacentration of Cu in all three oxidation states, the surfacentration of Cu2C species can be estimated by subtract-ose of CuC and Cu0 from the total uptake. The cor-ding fractionsCu2C ; CuC ; and Cu0 obtained from

ree characterization techniques are listed in Table 4catalysts.strongly adsorbed CO species 2124 cm1 on Cu/AC-reduced at 423 K provides evidence for the pres-

f CuC species on the surface along with metallic Cu,adsorbs CO giving a peak at 2107 cm1. The dis-

rance of the former peak after reduction at 573 K,parallels an enhancement in the intensity of the lat-ak, clearly indicates complete reduction to metalliccopper. In contrast, the copper species dispersed onTT-H2 seem to be more difficult to reduce at lowerratures, as testified by the strong peak at 2158 cm1,ponding to CO on unreduced Cu2C species, in addi-

that at 2126 cm1 for CuC species after reductionK. After reduction at 573 K, a considerable cov-

of CuC species remains along with metallic copper.ilar transition in the Cu oxidation states as a func-f reduction temperature as well as the pretreatmentactivated carbon is observed in the XRD patterns

se catalysts. As seen in Table 4, the fractions of Cut in different valence states, as estimated by all threeques, are typically quite consistent with each otherow that Cu on AC-HNO3 reduces more easily thanAC-HTT-H2 at lower reduction temperatures. Thisation, coupled with the fact that Cu dispersion onNO3 is more than twice that obtained on AC-HTT-rhaps implies that smaller Cu particles are easier to. This is in agreement with a previous study on Cu re-lity by Robertson et al. (84) which showed that highlysed CuO/SiO2 was reduced at a much lower temper-than unsupported CuO. The effect observed in the

CHARACTERIZATION OF C-SUPPORTED Cu CATALYSTS 151

TABLE 4

Distribution of Cu Oxidation States in Cu/Carbon Catalysts

S

Cat

4.8% Cu/A

4.6% Cu/A

4.9% Cu/A

5.0% Cu/D

5.1% Cu/G

1.8% Cu/G

a Based

present sAC-HNOlate specsamples.(78), theproton ecarbon ssition anwas agaidepositement priat aboutCu carbofor greatCu0 on ACu/AC-Htially higCu oxide

Alternbe rationat the suare surroClose proxide lapulsive fforce thalattices ocould be

pupport was found to be effective for the decompo-d easy reduction to metallic Pd. Recently, the effectn observed for a similar catalyst (77) in which Pdd on a carbon surface subjected to HNO3 treat-or to Pd impregnation was found to reduce easily373 K. Similar to this behavior, the formation ofxylate species in this study might be responsible

er dispersion and consequently easier reduction toC-HNO3. The absence of these surface species onTT-H2 results in a lower dispersion, and substan-

her temperatures are required to reduce the largerparticles.

atively, one can speculate that the effect can alsoalized on the basis of electrostatic repulsive forcesrface of AC-HNO3. Cu crystallites on AC-HNO3unded by electron-rich oxygen-containing groups.oximity between these functional groups and Cuttice oxygens could give rise to electrostatic re-orces on the carbon surface, constituting a drivingt could facilitate easier removal of O from Cun AC-HNO3. On the other hand, no such forcesenvisioned to exist on the surface of AC-HTT-H2,

stantiate this explanation, it does provide a rationale thatcan help differentiate between surface and bulk reducibili-ties.

Similar to the AC-supported samples, the data obtainedwith Cu/DM and Cu/GF-WI from the three different tech-niques are quite consistent with each other and support theearlier argument that Cu reducibility is dependent on thecrystallite size to some extent. Based on the infrared peakassignments discussed elsewhere (67), those at 2120 and2158 cm1 in the DRIFT spectra of CO adsorbed at 173 Kon Cu/DM (Fig. 15) can be assigned to CO on CuC and Cu2C

species, respectively. Thus there is a mixture of Cu2C andCuC surface species on DM-supported Cu after the calcina-tion step. Autoreduction during the partial decompositionof the nitrate precursor at this low calcination temperaturecould be responsible for the generation of these cuprousspecies, as observed previously (8589). After reduction at423 K, a small fraction of the cupric form is still retained,with the rest of the Cu surface being in the cuprous state. Re-duction at 473 K yields a uniform surface covered with CuC

species. Increasing the reduction temperature to 573 K con-verts a significant fraction of these CuC species to metallicTredChemisorption DRIFT

alyst (C) Cu2C CuC Cu0 Cu2C

C-HNO3 423 0 0.19 0.81 0473 0 0.14 0.86 0573 0 0.08 0.92 0

C-ASIS 423 0.37 0.63 0 0.40573 0 0.19 0.81 0

C-HTT-H2 423 0.77 0.23 0 0.70573 0 0.19 0.81 0

M 423 0.32 0.68 0 0.15473 0 0.54 0.46 0573 0 0.41 0.59 0673 0 0.13 0.87 0

F-WI 423 0.5a 0.5a 0a 0.65473 0.3a 0.7a 0a 0.32573 0 0.66a 0.33a 0673 0 0.5a 0.5a 0

F-IE 423 0.13 0.21 0.66 0473 0 0 1 0573 0 0 1 0673 0 0 1 0

on total CO uptake.

tudy is attributed to the greater dispersion of Cu on3, presumably via the formation of Cu carboxy-

ies, as indicated by the DRIFT spectra of theseIn a study of Pd/C catalysts by Morikawa et al.formation of palladiumamine complex cations via

xchange with carboxylic acid groups on an oxidized

resultingto that owho comCuO disof similaAlthoug: CO adsorption at 173 K XRD

CuC Cu0 Cu2C CuC Cu0

0.60 0.40 0 0.24 0.760.11 0.89 0 1.0 0 0.11 0.89

0.60 0 0.22 0.31 0.470.28 0.72 0 0.20 0.80

0.30 0 0.36 0.30 0.340.39 0.61 0 0.18 0.82

0.85 0 0.63 0.37 00.78 0.22 0.19 0.31 0.600.38 0.62 0 0.27 0.730 1.0 0 0.12 0.88

0.35 0 0 0.50 0.500.68 0 0.44 0.56 0 0.36 0.640 1.0 0 0 1

0.24 0.76 0 0.05 0.950 1.0 0 0 1.00 1.0 0 0 1.00 1.0 0 0 1.0

in CuO crystallites with a reduction profile similarf bulk CuO. The observations by Robertson et al.,pared the reducibility of bulk CuO with that ofersed on SiO2, could also be interpreted in terms

rly enhanced O removal from supported CuO.h no additional evidence is available to further sub-

152 DANDEKAR, BAKER, AND VANNICE

Cu, as evident by the appearance of the new strong band at2110 cm1; however, complete reduction of all surface Cuspecies to metallic Cu appears to occur only after reductionat 673The pcouldthe sureducin thethesedeposgavecatalythe obe indoptimfurtheistry i(1). AtheseCu0 sstabildesirawill acuprochoicesor sobut wimpreacetatcatalyas theto bewhen

Admodifities oviatiolustraindicachemit is ptuallyassumthe eportsbefor(111on meof allin theassocienhanlence

by considering the nature of CO bonding with transitionmetals in terms of a bonding model (68). Accordingto this model, CO bonding with Cu involves the delocaliza-

af

To

pri

a

e

cg5zaro

d

eal9

tudeifi

i

c

No

DiK, as depicted by the single strong band at 2111 cm1.ossible weak shoulder on the high wavenumber sidebe due to stabilization of CO on isolated Cu0 sites onpport. Thus, the degree of difficulty encountered ining Cu dispersed on DM is similar to that observedcase of Cu/AC-HTT-H2, and Table 3 indicates thattwo catalysts have comparable Cu dispersions. Cuited on GF via a wet impregnation technique, whicha much lower dispersion compared with the othersts, is much more stable and difficult to reduce thanther catalysts. This behavior exhibited by Cu couldicative of a unique complication encountered whenizing carbon surfaces for supported Cu catalysts andr endorses the importance of carbon surface chem-

n determining the behavior of metal/carbon catalystscomprehensive study of the catalytic properties of

catalysts indicated that the presence of both CuC andpecies is required to achieve the highest activity andity (58). Whereas a higher Cu dispersion would beble to enhance Cu surface area, a lower dispersionpparently lead to a better stabilization of Cu in theus form on a relatively inert support. An optimum

of the support, the pretreatment, and the precur-lution could lead to a catalyst with higher dispersionith unaccelerated reducibility of Cu. For example,gnation of the hydrophobic AC-HTT-H2 with coppere in ethanol could perhaps render such an optimumst formulation. Although this behavior might changeCu loading is varied, all these factors would have

taken into account from a commercial point of viewoptimizing the pretreatment conditions.ditional interaction with the support can also lead tocations in the morphological and electronic proper-

f very small Cu crystallites which can result in de-ns from the general behavior depicted above, as il-ted by the Cu/GF-IE catalyst. With XRD patternsting the presence of 40- to 50-nm Cu0 particles and

isorption indicating an average particle size of 23 nm,ossible that a large fraction of these particles are ac-smaller than the surface average size predicted bying a uniform particle morphology. Perturbations in

lectronic structure of Cu on graphitic carbon sup-as the cluster size is decreased have been observede by various spectroscopic studies of this system5). Whereas the peak corresponding to CO stabilizedtallic Cu is around 2100 cm1 in the DRIFT spectraother catalysts, it is red-shifted to around 2060 cm1

case of Cu/GF-IE. This lower-wavenumber IR bandated with stabilization of CO on Cu0 may indicate ancement in the electron density of the hybrid dC s va-bands of these Cu0 particles, which can be explained

tion oorbittranssymming).CO c andadsoring isbondbratiovacanincreof COfurththe abto thelowerof thetronias su(232localithe grCu paelectrof thebe inindexpositcur husualtent (strucsioneadlayexpermodichangsorpt(58).

Cui.e., amondtion,electrandcontaf the CO 5 -electron pair to unoccupied hybrid dC sls of the adsorption sites ( bonding) and electroner from the occupied d orbitals of the correspondingetry to the 2 antibonding orbitals of CO ( bond-he stretching vibration frequency in the adsorbedmplexes will depend on the relative contributions of bonding which, in turn, depend on the state of thetion site. For Cu2C ions, the contribution of bond-elatively small and CO adsorption involves mainly

ng, which leads to an increase in the frequency of vi-n. On the other hand, for metallic Cu with no valencecies, a significant role is played by bonding, whichses the electron density in the antibonding orbitalsand results in a strong shift to lower frequency. A

r increase in the charge density of Cu, as proposed inove model, should then enhance the contribution due bonding and shift the vibration frequency to stillvalues. Such an enhancement in the electron densityse smaller crystallites might be indicative of an elec-interaction between Cu and the graphitized fibers,gested by Richard and co-workers for Pt/graphite). This interaction could be due to an electron de-ation from the center of the polyaromatic rings inphite basal planes toward the periphery where theseticles are located, with or without the participation ofn-donor O groups located at the edges and steps

se basal planes. The low IR band frequency may alsoicative of the presence of a large fraction of Cu(111)planes in these crystallites (69). Flat Pt particles de-d on supports like graphite on which epitaxy can oc-ve also been shown by electron microscopy to be

y represented by the (111) crystal plane to a large ex-0). Since both Cu and Pt have fcc-centered latticeres, by analogy an identical situation could be envi-for small Cu particles deposited along the edges andrs of the graphitic basal planes. Although no othermental evidence is available to further support thesecations proposed for Cu deposited on GF by ion ex-e, they do provide one explanation for the unique ad-on and catalytic properties exhibited by this catalyst

SUMMARY

crystallites dispersed on different forms of carbon,tivated carbon, graphitized carbon fibers, and dia-

, were prepared and characterized by CO chemisorp-2O decomposition, X-ray diffraction, transmissionn microscopy, temperature-programmed reduction,RIFTS. Increasing the concentration of oxygen-

ning groups on the surface of the activated carbon

CHARACTERIZATION OF C-SUPPORTED Cu CATALYSTS 153

by a nitric acid treatment prior to Cu impregnation wasbeneficial in terms of rendering a higher dispersion of Cu.A higher dispersion of Cu was obtained with the diamondsupportpared viato the stbonds oexchangin two mular parand relawithin thhanced ttered the

FinanciFoundatio

1. Radov243 (1

2. Stiles,worths

3. CamerJenkin

4. RadovTechnoEds.).

5. Bird, AStiles,

6. AugusChemi

7. RodrigApplic

8. Ehrbu9. Juntge

10. Leon y(1996)

11. Baetzo12. Carley

Chem.13. Jirka,14. De Cr

Battist15. Sriniva16. Noron

J. Cata17. Guerr

J. Chem18. Phillip

(1980)19. Medin

Catal.20. Nedor

Babkin21. Self, V

(1990)

22. Takasu, Y., Sakuma, T., and Matsuda, Y., Chem. Lett., 1179 (1985).23. Richard, D., and Gallezot, P., in Preparation of Catalysts IV

(B. Delmon, P. Grange, P. A. Jacobs, and G. Poncelet, Eds.), p. 71.ier-yier

gich)i

J

8a8r,)nrj

)gms)gorr

inl.inilinks,ae,eAs

ZjiTr.s

.s

R0e)ea

nl

2compared with the graphitized fibers when pre-a wet impregnation technique, and it is attributed

abilization of Cu through interactive danglingn the diamond surface. Cu dispersed by an ion-e method was stabilized on the graphitized fibersorphologically different forms: aggregates of glob-ticles deposited on top of graphitic basal planestively smaller crystallites at the edges and defectsese planes. These morphological differences en-

he reducibility of these particles and may have al-electronic properties of these crystallites.

ACKNOWLEDGMENT

al support for this study was provided by the National Sciencen via Grant CTS-9415335.

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INTRODUCTIONEXPERIMENTALRESULTSFIG. 1.TABLE 1TABLE 2FIG. 2.FIG. 3.FIG. 4.TABLE 3FIG. 5.FIG. 6.FIG. 7.FIG. 8.FIG. 9.FIG. 10.FIG. 11.FIG. 12.FIG. 13.FIG. 14.FIG. 15.FIG. 16.FIG. 17.

DISCUSSIONTABLE 4

SUMMARYACKNOWLEDGMENTREFERENCES