RESEARCH NOTES

Coke Formation on Pt/ZrO2/Al2O3 Catalysts during CH4 Reformingwith CO2

Mariana M. V. M. Souza,† Donato A. G. Aranda,†,‡ and Martin Schmal*,†,‡

NUCAT/PEQ/COPPE, Universidade Federal do Rio de Janeiro, C.P. 68502,21945-970 Rio de Janeiro, Brazil, and Escola de Quimica, Universidade Federal do Rio de Janeiro,C.P. 68542, 21940-900 Rio de Janeiro, Brazil

The CO2 reforming of methane was studied over Pt supported on Al2O3, ZrO2, and x % ZrO2/Al2O3 (1 e x e 20 wt %). The Pt/Al2O3 deactivated very quickly during 20 h onstream at 1073K and a CH4/CO2 ratio of 1:1, while the catalysts with a ZrO2 content above 5 wt % presentedimproved stability during 60 h. Temperature-programmed oxidation studies showed that theamount of carbon on Pt/Al2O3 is much larger than that on zirconia-containing catalysts.Deactivation is attributed to carbon formation surrounding the metal-support perimeter. Thehigh stability of zirconia-based catalysts is probably due to strong Pt-Zrn+ interactions, whichreduce carbon formation during the reaction by promoting CO2 dissociation. High ratios of CH4/CO2 were also used in an attempt to accelerate deactivation, but even under these severedeactivation conditions, the catalyst with 10 wt % ZrO2 exhibited excellent stability.

1. Introduction

The catalytic reforming of CH4 with CO2, rather thanH2O, for the production of synthesis gas, i.e., a mixtureof CO and H2, has attracted substantial interest.1-4 Thereaction is well suited to a lower H2/CO product ratio,which is preferable as feed for Fischer-Tropsch plants5

and for the synthesis of acetic acid, dimethyl ether, andoxoalcohols.6 Moreover, dry reforming is of interest forenvironmental reasons because it reduces both CO2 andCH4 emissions, which are so-called greenhouse gases.7

The major obstacle for larger diffusion of this processin industry is the high thermodynamic potential to formcoke under elevated temperatures.5 Temperatures around1073 K are required to reach high conversions becauseof the high endothermic nature of the process. Thus, thecatalyst deactivation is a serious challenge and mustbe overcome by effective catalysts.

Although the most commonly used support for CO2/CH4 reforming is alumina,2,8,9 it has been found thatPt reaches much higher conversions and better stabilitywhen supported on ZrO2.10-12 Van Keulen et al.13

showed that Pt/ZrO2 is very stable for a period of over1000 h, at 923-973 K and a feed ratio of CO2/CH4 ) 2.The coking resistivity of Pt/ZrO2 is associated withstrong Pt-Zrn+ interactions, which result in the forma-tion of ZrOx species in close contact with the Pt

surface.11 Therefore, the reforming reaction proceedsbasically on the metal-support perimeter.14

When zirconia is dispersed on alumina, it provides abetter dispersion for Pt particles because these systemscombine the unique chemical properties of ZrO2 withthe high surface area and mechanical stability of Al2O3.We showed earlier that Pt/ZrO2/Al2O3 catalysts exhibithigh activity and suffer less deactivation than Pt/Al2O3or even Pt/ZrO2.15 In the present work we tried todevelop a better understanding of the nature of cokingin CO2 reforming of CH4, by doing temperature-programmed oxidation (TPO) analysis of carbon depositsand varying the CH4/CO2 ratio in the feed. We inves-tigated the relation between coking and catalyst per-formance at a temperature typically used for commercialoperations (1073 K).

2. Experimental Section

2.1. Catalyst Preparation and Characterization.Al2O3 (Harshaw) and ZrO2 were used as supports. Al2O3was calcined in air at 823 K for 16 h (BET area ) 200m2/g), and ZrO2 was prepared by calcination of zirco-nium hydroxide (MEL Products) in air at 823 K for 2 h(BET area ) 62 m2/g).

ZrO2/Al2O3 samples were prepared by impregnationover an alumina powder with a nitric acid solution (50%)of zirconium hydroxide, as described elsewhere.15 Zir-conia loading was varied between 1 and 20 wt %. Thecatalysts were prepared by an incipient wetness tech-nique, using an aqueous solution of H2PtCl6‚6H2O(Aldrich), followed by drying at 393 K for 16 h andcalcination in air at 823 K for 2 h. For all catalysts, theplatinum content was around 1 wt %. The preparedcatalysts will be referred to as PtAl for Pt/Al2O3, PtZrfor Pt/ZrO2, and PtxZr for Pt/x % ZrO2/Al2O3.

* To whom correspondence should be addressed. E-mail:schmal@peq.coppe.ufij.br. Fax: (5521) 2562-8300.

† NUCAT/PEQ/COPPE.‡ Escola de Quimica.

CH4 + CO2 T 2CO + 2H2 ∆H298K ) 247.3 kJ/mol(I)

4681Ind. Eng. Chem. Res. 2002, 41, 4681-4685

10.1021/ie010970a CCC: $22.00 © 2002 American Chemical SocietyPublished on Web 08/06/2002

H2 and CO chemisorptions were measured on allcatalysts at room temperature after reduction by 10%H2/Ar at 773 K using an ASAP 2000 apparatus (Mi-cromeritics). Because of the metal-support interac-tion,14 chemisorption on the PtZr catalyst was alsocarried out after reduction at 573 K to better estimatethe Pt dispersions.15

2.2. Catalytic Test. The reaction was carried out ina fixed-bed flow-type quartz reactor, loaded with 20 mgof catalyst. A thermocouple was placed on top of thecatalyst bed to measure the catalyst temperature. Thecatalysts were dried in situ with flowing nitrogen at 423K, before reduction with 10% H2/N2 for 1 h at 773 K.After reduction, the sample was purged with nitrogenfor 30 min at the same temperature. All catalytic testswere performed under atmospheric pressure, and thetotal feed flow rate was 200 cm3/min (WHSV ) 160 h-1),over the temperature range 723-1173 K. Stability testswere carried out at 1073 K with stoichiometric condi-tions (CH4:CO2 ) 1:1) as well as with an excess of CO2for PtAl and an excess of CH4 for the Pt10Zr catalyst,maintaining the total feed flow rate of 200 cm3/min withhelium. The reaction products were analyzed by anonline gas chromatograph (CHROMPACK CP9001),equipped with a Hayesep D column and a thermalconductivity detector.

Temperature-programmed surface reaction (TPSR)was also performed to investigate CO2 reforming of CH4over platinum catalysts, using a dynamic mode ap-paratus. After reduction at 773 K, the catalyst waspurged with He at this same temperature during 1 hand cooled to room temperature. The amount of catalystand total feed flow rate were the same as those used instability tests, with a CH4/CO2/He ratio of 1:1:18. TPSRwas performed by heating the catalyst at 10 K/min upto 823 K, maintained for 30 min, and subsequentlyramped to 973 K, remaining at this temperature for 1h. The effluent gas composition was monitored onlineby a quadrupole mass spectrometer (Dycor MA100MAmetek).

TPO of carbonaceous deposits was carried out in thesame dynamic mode apparatus as that used for TPSR.After reaction, the samples were cooled to room tem-perature under a helium flow and then heated to 1073K at a rate of 10 K/min in a 5% O2/He mixture (30 cm3/min). Integration of the CO2 evolution spectra allowedthe quantification of carbon deposition.

3. Results and DiscussionThe amounts of irreversibly adsorbed H2 and CO, at

room temperature, were reported in a previous paper.15

Alumina-supported catalysts with zirconia loading upto 10% presented high H/Pt values (around 0.85). Thelower dispersion of the Pt20Zr catalyst (H/Pt ) 0.60)can be attributed to the presence of large ZrO2 crystal-lites on the support. After high-temperature reduction,the H2 chemisorption on PtZr was markedly decreasedcompared to that on PtZr reduced at 573 K (H/Pt variedfrom 0.34 to 0.57). This type of behavior suggests themigration of partially reduced zirconia onto the plati-num surface (a SMSI type state). However, the presenceof ZrOx moieties did not cause any decrease in COchemisorption on the PtZr catalyst. The high values ofthe CO/H2 ratio on zirconia-containing catalysts and inparticular on the PtZr catalyst reduced at 773 K (CO/H2 ) 6.9) predict an interaction of CO with the Pt-ZrOxinterface, shown by IR of CO adsorbed on PtZr systems,as reported in ref 15.

The activity of the catalysts was evaluated underreforming conditions, with a CH4/CO2 ratio of 1:1, overa temperature range of 723-1173 K. The activity isinfluenced by the nature of the support.15 At highertemperatures, PtAl and Pt1Zr catalysts are less activethan Pt10Zr, the most active catalyst over the wholetemperature range investigated: the CH4 conversionranged from 5.5% at 723 K to 93.5% at 1173 K.

TPSR measurements were carried out under similarconditions. Figure 1 presents the TPSR profiles of COproduction on PtAl, PtZr, and Pt10Zr catalysts. Asshown, the PtZr catalyst exhibits higher initial activity,mainly at lower temperatures, but deactivates very fast,while PtAl and Pt10Zr catalysts present good stabilityat 823 K during the first 30 min onstream. On the otherhand, the Pt10Zr catalyst showed the best performanceat 973 K.

The support influences strongly the stability of thecatalysts, as reported previously.15 The order of activitymaintenance at 1073 K was Pt10Zr > Pt5Zr > Pt20Zrand PtZr . PtlZr > PtAl. As reported, PtAl and Pt1Zrcatalysts exhibited high linear deactivation rates of 4.0+ 0.5%/h and 3.3 + 0.4%/h, respectively, during the first20 h onstream at 1073 K because of the rapid depositionof inactive carbon, which will be discussed later. ThePt10Zr catalyst deactivated only at a rate of 0.1%/hduring 60 h onstream at this temperature.15

There are two potential causes of deactivation: cokedeposition and sintering of metal particles. Most authorsagree that the coke formation is the main source ofdeactivation.10,16-18 Bitter et al.10,17 showed that sinter-ing of Pt particles during reforming conditions can beexcluded based on EXAFS results of fresh and used Pt/Al2O3 and Pt/ZrO2. Thus, the fast deactivation of PtAlshould be associated with carbon deposition.

Figure 2 shows the oxidation profiles of carbondeposited on the PtAl, PtZr, and Pt10Zr catalysts after21 h of reaction at 1073 K and a CH4/CO2 ratio of 1:1.For the PtAl catalyst, the oxidation started at 520 K,exhibiting a major peak around 700 K and a smallerone at 830 K. For zirconia-containing catalysts, theonset of the oxidation was at lower temperature, around373 K. The amount of carbon deposited at 1073 K, asquantified by integration of the CO2 formation duringTPO runs, was normalized to surface Pt, and the resultsare displayed in Table 1. It shows that after 21 honstream the Pt surface is not entirely covered bycarbon; thus, the high values of C/Pt, mainly for the PtAlcatalyst, must be due to accumulation of carbon on thesupport.

Figure 1. CO formation during TPSR over Pt catalysts. Reactionconditions: CH4:CO2:He ) 1:1:18, total feed flow rate ) 200 cm3/min.

4682 Ind. Eng. Chem. Res., Vol. 41, No. 18, 2002

Noronha et al.19 have suggested that the various TPOpeaks are not due to different forms of carbon but ratherto different locations on the catalyst surface, for Pt/Al2O3and Pt/ZrO2. The low-temperature peaks observed inTPO of coked Pt/Al2O3 have been typically ascribed tocarbon surrounding the metal particles, while those athigh temperatures are ascribed to the carbon depositionover the support.20 Deactivation is attributed to carbonformation surrounding the metal-support perimeter.On zirconia-containing catalysts, coke has a higherreactivity and does not cause any blockage of Pt-ZrOxinterfacial sites.

The higher stability and coking resistivity of Pt/ZrO2have been related to strong Pt-Zrn+ interactions, whichresult in the formation of ZrOx species on the Ptsurface.11 Indeed, our TPR results15 indicated thatzirconia can be reduced at lower temperatures than 500K, resulting in ZrOx species that may decorate the Ptsurface, diminishing the hydrogen chemisorption capac-ity. The interfacial sites on Pt-ZrOx are active for COadsorption and CO2 dissociation, providing active spe-cies of oxygen that may react with carbon formed byCH4 decomposition on the metal particle, suppressingcarbon accumulation.11,15,16 Moreover, zirconia is a well-known oxygen supplier, and its oxygen mobility is about3 times higher than that of alumina,21 which helps tokeep the metal surface free of carbon. When zirconia isdispersed over alumina, the Pt surface is not extensivelyrecovered by ZrOx, as shown by chemisorption measure-ments.15 Thus, the Pt-ZrOx interface in ZrO2/Al2O3systems appears to be more active and stable for CO2reforming of methane.

Carbon deposition during methane reforming can beoriginated from either methane decomposition (reactionII) or CO disproportionation (Boudouard reaction III),which are thermodynamically favorable below 1173K.5,22

Kinetically, both the methane decomposition reactionand the Boudouard reaction, which give undesirablecarbon, are known to be exceptionally slow in theabsence of a catalyst, but both can be readily catalyzedby many transition metals.23 Thermodynamic calcula-tions5 showed that the extent of carbon depositionduring reforming decreases at higher reaction temper-atures, in agreement with several experimental obser-vations.2,13,24 These results suggest that CO dispropor-tionation is the main contributor to carbon depositionbecause it is exothermic and the equilibrium constantdecreases with increasing temperature. Despite theseevidences, there are disagreements concerning thesource of carbon deposition. Swaan et al.25 and Ef-stathiou et al.26 showed by TPO with isotopic mixturesthat most of the carbon accumulated during reformingreaction over Ni/SiO2 and Rh/Al2O3, respectively, isderived from a CO2 molecule. Other authors claimedthat carbon is formed from methane, over a large varietyof catalysts, including Pt/Al2O3 and Pt/ZrO2.17,18,27,28 So,there is not a consensus in the literature about theorigin of the coke formation during reforming conditions.

We tried to elucidate this question by investigatingthe PtAl catalyst, which exhibited the fastest deactiva-tion under stoichiometric conditions, in a more detailedway. Stability tests were performed with an excess ofCO2 in the feed, as shown in Figure 3. It shows that asurplus of CO2 improves the performance of the catalystand an excess of 50% is enough to reach good stability.On the other hand, when methane flowed alone overthe catalyst at 1073 K, the maximum conversion wasonly 0.8%, which suggests that PtAl is not active for CH4decomposition. Therefore, it is possible to ascribe thecoke deposition to CO disproportionation because anexcess of CO2 favors the reverse reaction, which mini-mizes carbon formation.

We also examined the behavior of Pt10Zr, whichshowed the best stability under stoichiometric condi-tions,15 with higher ratios of CH4/CO2, to accelerate thecatalyst deactivation. Stability tests under severe condi-tions were carried out with CH4/CO2 ratios of 2:1 and3:1, and the results are shown in Figures 4 and 5,respectively. The Pt10Zr catalyst exhibited excellentstability even under severe deactivation conditions.With the CH4/CO2 ratio of 2:1, the rate of deactivationwas 0.15%/h, and with the ratio of 3:l, this rateincreased to 0.4%/h, based on CO2 consumption. Thesedeactivation rates are much lower than those reportedby Stagg-Williams et al.29 for Pt/ZrO2. In this case the

Figure 2. TPO profiles for PtAl, PtZr, and Pt10Zr catalysts, afterexposure to a CH4:CO2:He ) 1:1:18 mixture at 1073 K for 21 h.

Table 1. Amount of Coke As Quantified by TPO

catalystmg of coke/

(g of catalyst)‚h C/Pt catalystmg of coke/

(g of catalyst)‚h C/Pt

PtAl 6.3 10.2 Pt10Zr 0.3 0.5PtZr 0.6 0.9

Figure 3. Effect of the ratio CO2/CH4 on the stability of PtAl at1073 K. Reaction conditions: flow rates of CO2 ) 10 cm3/min andCH4 + He ) 190 cm3/min.

CH4 T C + 2H2 ∆H298K ) 75 kJ/mol (II)

2CO T C + CO2 ∆H298K ) -172 kJ/mol(III)

Ind. Eng. Chem. Res., Vol. 41, No. 18, 2002 4683

rate of deactivation was 1.5%/h during 23 h of reformingreaction at 1073 K and the CH4/CO2 ratio of 2:1, whenthe X(CH4)/X(CO2) conversion ratio was greater than0.5. On the basis of the conversion rate, Stagg et al.16

proposed that the CH4 decomposition rate is initiallygreater than the CO2 dissociation rate and that the CH4decomposition is responsible for the deactivation of thecatalyst. In the present study, the X(CH4)/X(CO2) con-version ratio was 0.47, only slightly lower than 0.5 dueto the reverse water gas shift reaction, whose influenceis small at high temperatures. It reinforces our previousstatement that CO disproportionation is determinantfor coke deposition because the presence of CO2 belowthe stoichiometric value favors this reaction (2CO T C+ CO2).

4. Conclusions

CO2 reforming of methane is strongly affected by thenature of support oxides. The Pt/ZrO2/Al2O3 and Pt/ZrO2catalysts presented very high stability compared to thePt/Al2O3 at 1073 K. The deactivation is primarily causedby coke deposition surrounding the metal-supportperimeter, although carbon deposits are located basi-cally on the support. The principal route of carbondeposition is the CO disproportionation. The catalystwith 10 wt % ZrO2 exhibited excellent stability evenunder severe deactivation conditions.

Acknowledgment

M.M.V.M.S. and D.A.G.A. are grateful to FAPERJ(Fundacao de Amparo a Pesquisa do Estado do Rio deJaneiro) and CNPq (Conselho Nacional de Pesquisa e

Desenvolvimento Cientifico, Brazil) for financial supportduring this work.

Literature Cited

(1) Ashcroft, A. T.; Cheetham, A. K.; Green, M. L. H.; Vernon,P. D. F. Partial Oxidation of Methane to Synthesis Gas UsingCarbon Dioxide. Nature 1991, 352, 225.

(2) Richardson, J. T.; Paripatyadar, S. A. Carbon DioxideReforming of Methane with Supported Rhodium. Appl. Catal.1990, 61, 293.

(3) Edwards, J. H.; Maitra, A. M. The Chemistry of MethaneReforming with Carbon Dioxide and its Current and PotentialApplications. Fuel Process. Technol. 1995, 42, 269.

(4) Bradford, M. C. J.; Vannice, M. A. CO2 Reforming of CH4.Catal. Rev., Sci. Eng. 1999, 41, 1.

(5) Gadalla, A. M.; Bower, B. The Role of Catalyst Support onthe Activity of Nickel for Reforming Methane with CO2. Chem.Eng. Sci. 1988, 43, 3049.

(6) Aparicio, L. M. Transient Isotopic Studies and MicrokineticModeling of Methane Reforming over Nickel Catalysts. J. Catal.1997, 165, 262.

(7) Denlmon, B. How to Reduce the Greenhouse Effect and aFew Other Questions Concerning Catalysis. Appl. Catal. B 1992,1, 139.

(8) Vernon, P. D. F.; Green, M. L. H.; Cheetham, A. K.; Ashcroft,A. T. Partial Oxidation of Methane to Synthesis Gas, and CarbonDioxide as an Oxidising Agent for Methane Conversion. Catal.Today 1992, 13, 417.

(9) Masai, M.; Kado, H.; Miyake, A.; Nishiyama, S.; Tsuruya,S. Methane Reforming by Carbon Dioxide and Steam over Sup-ported Pd, Pt and Rh Catalysts. Stud. Surf. Sci. Catal. 1988, 36,67.

(10) Bitter, J. H.; Hally, W.; Seshan, K.; van Ommen, J. G.;Lercher, J. A. The Role of the Oxidic Support on the Deactivationof Pt Catalysts during CO2 Reforming of Methane. Catal. Today1996, 29, 349.

(11) Bradford, M. C. J.; Vannice, M. A. CO2 Reforming of CH4over Supported Pt Catalysts. J. Catal. 1998, 173, 157.

(12) Seshan, K.; Mercera, P. D. L.; Xue, E.; Ross, J. R. H.Process for the Production of Synthesis Gas. U.S. Patent 5,989,-457, 1999.

(13) Van Keulen, A. N. J.; Hegarty, M. E. S.; Ross, J. R. H.;van den Oosterkamp, P. F. The Development of Platinum-ZirconiaCatalysts for the CO2 Reforming of Methane. Stud. Surf. Sci.Catal. 1997, 107, 537.

(14) Bitter, J. H.; Seshan, K.; Lercher, J. A. The State ofZirconia Supported Platinum Catalysts for CO2/CH4 Reforming.J. Catal. 1997, 171, 279.

(15) Souza, M. M. V. M.; Aranda, D. A. G.; Schmal, M.Reforming of Methane with Carbon Dioxide over Pt/ZrO2/Al2O3Catalysts. J. Catal. 2001, 204, 498.

(16) Stagg, S. M.; Romeo, E.; Padro, C.; Resasco, D. E. Effectof Promotion with Sn on Supported Pt Catalysts for CO2 Reformingof CH4. J. Catal. 1998, 178, 137.

(17) Bitter, J. H.; Seshan, K.; Lercher, J. A. Deactivation andCoke Accumulation during CO2/CH4 Reforming over Pt Catalysts.J. Catal. 1999, 183, 336.

(18) Nagaoka, K.; Seshan, K.; Aika, K.; Lercher, J. A. CarbonDeposition during Carbon Dioxide Reforming of MethanesComparison between Pt/Al2O3 and Pt/ZrO2. J. Catal. 2001, 197,34.

(19) Noronha, F. B.; Fendley, E. C.; Soares, R. R.; Alvarez, W.E.; Resasco, D. E. Correlation between Catalytic Activity andSupport Reducibility in the CO2 Reforming of Methane over Pt/CexZrl-xO2 Catalysts. Chem. Eng. J. 2001, 82, 21.

(20) Bacaud, R.; Charcosset, H.; Guemin, M.; Torresea-Hidalgo,H.; Tournayan, L. Study of the Formation and Removal ofCarbonaceous Deposits on Platinum-Ruthenium Alumina Sup-ported Bimetallic Catalysts. Appl. Catal. 1981, 1, 81.

(21) Yang, S.; Maroto-Valiente, A.; Benito-Gonzalez, M.; Rod-riguez-Ramos, I.; Guerrero-Ruiz, A. Methane Conversion overSupported Palladium Catalysts. I. Reactivity and Active Phase.Appl. Catal. B 2000, 28, 223.

(22) Tsang, S. C.; Claridge, J. B.; Green, M. L. H. RecentAdvances in the Conversion of Methane to Synthesis Gas. Catal.Today 1995, 23, 3.

Figure 4. Stability test with Pt10Zr at 1073 K and a feed ratioof CH4:CO2 ) 2:1. Reaction conditions: flow rates of CH4 ) 10cm3/min, CO2 ) 5 cm3/min, and He ) 185 cm3/min.

Figure 5. Stability test with Pt10Zr at 1073 K and a feed ratioof CH4:CO2 ) 3:1. Reaction conditions: flow rates of CH4 ) 10cm3/min, CO2 ) 3.3 cm3/min, and He ) 187 cm3/min.

4684 Ind. Eng. Chem. Res., Vol. 41, No. 18, 2002

(23) Nakagawa, K.; Anzai, K.; Matsui, N.; Ikenaga, N.; Suzuki,T.; Teng, Y.; Kobayashi, T.; Haruta, M. Effect of Support on theConversion of Methane to Synthesis Gas over Supported IridiumCatalysts. Catal. Lett. 1998, 51, 163.

(24) Bradford, M. C. J.; Vannice, M. A. Catalytic Reforming ofMethane with Carbon Dioxide over Nickel Catalysts: II. ReactionKinetics. Appl. Catal. A 1996, 142, 97.

(25) Swaan, H. M.; Kroll, V. C. H.; Martin, G. A.; Mirodatos,C. Deactivation of Supported Nickel Catalysts during the Reform-ing of Methane by Carbon Dioxide. Catal. Today 1994, 21, 571.

(26) Efstathiou, A. M.; Kladi, A.; Tsipouriari, V. A.; Verykios,X. E. Reforming of Methane with Carbon Dioxide to Synthesis Gasover Supported Rhodium Catalysts. II. A Steady-State TracingAnalysis: Mechanistic Aspects of the Carbon and Oxygen Reaction

Pathways to Form CO. J. Catal. 1996, 158, 64.(27) Rostrup-Nielsen, J. R.; Bak Hansen, J. H. CO2-Reforming

of Methane over Transition Metals. J. Catal. 1993, 144, 38.(28) O’Connor, A. M. Carbon Dioxide Reforming of Methane

over Pt/ZrO2 Catalysts. Ph.D. Thesis, University of Limerick,Limerick, Ireland, 1998.

(29) Stagg-Williams, S.; Noronha, F. B.; Fendley, G.; Resasco,D. E. CO2 Reforming of CH4 over Pt/ZrO2 Catalysts Promoted withLa and Ce Oxides. J. Catal. 2000, 194, 240.

Received for review November 30, 2001Revised manuscript received May 29, 2002

Accepted June 13, 2002

IE010970A

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