Influence of Mutual Platinum-Dispersed Ceria Interactions on the

J. Phys. Chem. B 1998, 102, 4357-4365

4357

Influence of Mutual Platinum-Dispersed Ceria Interactions on the Promoting Effect of Ceria for the CO Oxidation Reaction in a Pt/CeO2/Al2O3 Catalyst A. Martı´nez-Arias, J. M. Coronado, R. Catalun˜ a,† J. C. Conesa, and J. Soria* Instituto de Cata´ lisis y Petroleoquı´mica (CSIC), Campus UniVersitario de Cantoblanco, 28049 Madrid, Spain ReceiVed: December 16, 1997; In Final Form: February 23, 1998

Catalytic activity tests in combination with characterization studies (using CO temperature programmed reduction, electron paramagnetic resonance, and Fourier transform infrared) have been performed with the aim of establishing which are the main factors influencing the reactivity of an oxidized Pt/CeO2/Al2O3 catalyst for the CO-O2 reaction. Several effects are induced by the presence of both platinum and dispersed ceria in the catalyst. The results show that the low-temperature reducibility of both platinum and dispersed ceria is enhanced when the two components are present in the catalyst. Analysis of the characteristics of the centers responsible of the latter effect, carried out by means of EPR using oxygen as the probe molecule, indicates that in this sample the most reducible sites are formed by platinum located on bidimensional ceria patches present at the alumina surface. The enhancement by ceria of the low-temperature reduction of platinum at these sites under a CO + O2 reacting mixture, observed even when starting from oxidized catalysts, leads us to propose these sites as highly active for the CO oxidation reaction.

Introduction Cerium oxide is widely used as a promoter in the so-called “three-way catalysts” for the elimination of toxic exhaust gases in automobiles, composed mainly by platinum group metals supported on CeO2/Al2O3.1 The promoting effect of cerium oxide was originally attributed to the enhancement of the metal dispersion and the stabilization of the γ-Al2O3 support toward thermal sintering.2 It was later shown that ceria can be a chemically active component, working as an oxygen store by release of oxygen in the presence of reductive gases and removal of it by interaction with oxidizing gases3 and participating in the water-gas shift (WGS) reaction4 or the decomposition of nitrogen oxides.5 More recent efforts are devoted to elucidation of the participation of ceria in important metal/support interactions that can substantially affect catalyst properties.6 Models attempting to explain these effects attribute an important role to the formation and/or reoxidation of oxygen vacancies, which are believed to be formed at the ceria component or at the metal/ceria interface of the catalysts.1,6 These models are mainly based on observations of ceria promoted production of CO2 by interaction of CO with ceria supported Pt7 or Rh8 and on reactivity studies of CO-NO or CO-O2 reactions which show a great enhancement in reactivity upon addition of ceria to the catalysts.9,10 To explain these results, it has been proposed that ceria can induce C-O bond weakening for CO adsorbed on platinum particles, platinum thus acting as an adsorption activator of CO as in the spillover phenomena, or that the Ce-O bond strength is decreased for ceria localized near platinum.10 The latter point of view is also involved in proposals suggesting that interface oxygen can migrate on top of metallic Rh particles where it can react with adsorbed CO.8 Recent reports have focused on the influence * Corresponding author. E-mail: [email protected]. † Present address: Escola de Engenharia, Departamento de Engenharia Quı´mica, UFRGS, Brazil.

of the ceria structure on the adsorption and reaction properties of ceria supported rhodium, showing that carbon monoxide interaction with support oxygens depends critically on ceria surface structure.11 Studies by Hardacre et al.6 have shown that ceria coverage of Pt(111) strongly promotes CO oxidation, suggesting that new sites at the metal/oxide interface become available for the reaction. A Pt(111) wafer assumed to be fully encapsulated with ceria has even been shown to be a much more effective catalyst, thus showing that reactive sites on the ceria surface could play an important role in the reactivity of the system. It is proposed that formation of oxygen vacancies on ceria by CO interaction with surface oxygen ions, producing CO2, is promoted by an electronic interaction. The subsequent reaction step would involve reoxidation of the vacancies by consumption of gas-phase oxygen.6 A similar mechanism has been invoked by Golunski et al.12 to explain the enhancement of CO conversion by submitting Pt/CeO2 catalysts to consecutive reactivity tests. Results on oxygen adsorption on a Pt/CeO2/ Al2O3 catalyst have shown the formation of different superoxide species upon interaction with reduced cerium centers of the support.13 EPR monitoring of these superoxide species allows important information to be obtained at a structural level on the defects generated on ceria by reduction treatments. In this work we study the redox processes induced by mutual platinumsupport interactions correlating the results with the catalytic activity for CO oxidation shown by these systems. The possibility that the results obtained were related to interactions of CO with ceria related paramagnetic oxygen species formed on the support14 has also been examined. Experimental Section Materials. The CeO2/Al2O3 support was prepared by incipient wetness impregnation of γ-Al2O3 (as 1.8 mm diameter spheres supplied by Condea; SBET ) 200 m2g-1) with an aqueous solution of Ce(NO3)3‚6H2O. The resulting material

S1089-5647(98)00530-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/08/1998

4358 J. Phys. Chem. B, Vol. 102, No. 22, 1998 was dried overnight at 353 K and subsequently calcined at 773 K under a dry air flow for 6 h. The final CeO2 content was 10 wt %. Platinum (to a final loading of 0.5 wt %) was incorporated by impregnation of the support with a solution of [Pt(NH3)4](OH)2 (Johnson Matthey) previously neutralized with HNO3. To decompose the deposited complex, the sample was dried and calcined in the same conditions described above for the support. A chlorine-free platinum precursor has been chosen in order to avoid chlorine induced interactions due to the formation of cerium oxychloride microphases at the support.15 A Pt/Al2O3 sample (0.5 wt % Pt) was prepared using the same methods and materials described above. With the exception of the catalytic tests, the catalyst spheres were ground into powder with an agate mortar. For the catalytic tests, the catalyst spheres were ground and sieved to 0.4-0.5 mm particles, a size allowing internal diffusion effects to be avoided.16 All of the gases employed were of commercial purity and, for adsorption experiments, were further purified by vacuum distillation methods before storage. Techniques. EPR spectra were recorded at 77 K with a Bruker ER 200 D spectrometer operating in the X-band and calibrated with a DPPH (R,R′-diphenyl-β-picrylhydrazyl) standard (g ) 2.0036). Computer simulations were used to check spectral parameters. Portions of about 40 mg of sample were placed inside a special quartz probe cell with greaseless stopcocks using a conventional high-vacuum line for the different treatments. In all cases, the samples were pretreated in the EPR cell under oxygen (300 Torr) at 673 K for 2 h, cooled to room temperature in the same atmosphere, and finally extensively outgassed at room temperature. For the oxygen adsorption experiments at Ta ) 77 K, O2 doses of about 70 µmol/g were used, the excess gaseous oxygen being desorbed subsequently at the same temperature until a stationary pressure (≈8 × 10-5 mbar) was attained. Fourier transform infrared (FTIR) spectra were recorded at room temperature with a Nicolet 5ZDX Fourier transform spectrometer, with a resolution of 4 cm-1 and with accumulation of 128 scans for every spectrum. Thin self-supporting disks (ca. 10 mg cm-2) were prepared by pressing the powders at 2500 N/cm2 and handled in standard greaseless cells, where they could be subjected to thermal or adsorption treatments. Catalytic tests were carried out using a glass gas flow reactor system. The analysis of the feed and outlet gas streams was performed using a Perkin-Elmer FTIR spectrometer model 1725X, coupled to a multiple reflection transmission cell (Infrared Analysis Inc. “long path gas minicell”, 2.4 m path length, ca. 130 cm3 internal volume); O2 was determined with a paramagnetic analyzer (Servomex 540 A). Before the tests, the catalysts were subjected in the system to a standard pretreatment consisting of heating under a 3% O2:N2 flow at 673 K during 1 h, cooling in the same flow to room temperature, and then purging briefly (5 min) with N2. For the CO temperature programmed reduction (CO-TPR) experiments, about 300 mg of sample as placed in a U-shape quartz microreactor and subjected to an initial precalcination treatment in a flow of 20% O2/Ar at 673 K for 2 h. After being cooled to room temperature in this flow, the mixture was switched to a 1% CO/Ar mixture, flowing at a rate of 50 cm3 min-1, and, after a short time, a programmed temperature ramp was initiated at a rate of 8 K min-1. Analysis of the evolved gases was made by means of a VG 100-D quadrupolar mass spectrometer.

Martı´nez-Arias et al.

Figure 1. CO-TPR profiles of (A) CeO2/Al2O3, (B) Pt/Al2O3, and (C) Pt/CeO2/Al2O3. Solid lines correspond to the QMS signal of AMU 28 (CO), dashed lines to AMU 44 (CO2), and dotted lines to AMU 2 (H2). Vertical units (arbitrary for the three gas molecules) are the same in the three graphs.

Results Reactivity toward CO. CO-TPR. The profiles of the temperature programmed reduction under CO of the CeO2/ Al2O3, Pt/Al2O3, and Pt/CeO2/Al2O3 samples are shown in Figure 1. In the case of CeO2/Al2O3, CO consumption begins at about 500 K, the corresponding reduction peak being centered around 600 K. A second CO consumption step is observed for T > ca. 650 K. CO2 is first released around 400 K. Since no simultaneous CO consumption seems to be produced at temperatures below 500 K, this low-temperature CO2 production is probably due to decomposition of carbonate-type species (like hydrogencarbonate complexes adsorbed on alumina, which are known to decompose at these relatively low temperatures17) that could have been formed during equilibration of the sample. Electron paramagnetic resonance (EPR) experiments in the next subsection support this hypothesis, since no important support reduction seems to be produced in this relatively low-temperature range. A small emission of H2 is also detected for T > 600 K, indicating the onset of the WGS reaction. In the case of Pt/Al2O3 an initial weak CO consumption peak is centered at ca. 420 K, while a more important consumption is observed for T > 550 K. CO2 production occurs simultaneously with CO consumption. The second CO consumption step (onsetting at ca. 550 K) coincides with the detection of H2. This suggests that the WGS reaction, involving CO interaction with surface hydroxyl groups of the sample (CO(ads) + 2OH-(support) f CO2(g) + H2(g) + O2-)4,18 and most likely produced at the metal/support interface,18 can, at least partially,

Pt/CeO2/Al2O3 Catalyst Reactivity for the CO-O2 Reaction be involved in the features observed at T > ca. 550 K. To check that this reaction does not involve undesired leak water, a second TPR was performed subsequently, after the sample was cooled in pure Ar to room temperature. This experiment showed a significant decrease in the H2 signal, thus indicating that the WGS reaction observed in the first TPR involves interactions with hydroxyl groups of the sample. For Pt/CeO2/Al2O3, a first low-temperature CO uptake is observed around 400 K, while the most important consumption occurs from ca. 450 K producing a maximum centered at ca. 550 K. Almost simultaneous CO2 production is detected while the peaks centered at ca. 550 K coincide with H2 production, higher than for the Pt/Al2O3 sample and shifted to lower temperature, thus indicating that the WGS reaction is contributing in a more important way to CO consumption/CO2 production in this range. It is noteworthy that the CO consumption/CO2 production peaks at T < 500 K are stronger in this case than for the Pt/Al2O3 sample. The low-temperature CO consumption/CO2 production profiles observed in these experiments, when no H2 production is detected, must be ascribed to processes involving reduction of the samples, since no CO disproportionation is expected at the relatively low temperatures used for these experiments.10 The peak observed in the 300-550 K range for Pt/Al2O3 must be ascribed to platinum oxide reduction, but when ceria is present, ceria reduction can also be involved in the CO uptake. In this way, CO consumption for T < ca. 450 K for the Pt/CeO2/Al2O3 sample might involve both platinum oxide and ceria reduction, consistent with the stronger intensity of these features in comparison with Pt/Al2O3. On the other hand, in cases where H2 is produced, the WGS reaction might be partially involved in the CO consumption/CO2 production, as already commented on. In the case of CeO2/Al2O3, the two CO consumptions are probably related to surface and bulk reduction of cerium oxide aggregates, while the WGS reaction can contribute to the observed profiles for higher temperatures. In summary, comparison of the Pt/Al2O3 and Pt/CeO2/Al2O3 reduction profiles shows that a higher temperature is required to reduce platinum oxide in the absence of cerium, which indicates that the presence of ceria promotes platinum reduction, while comparison of CeO2/Al2O3 and Pt/CeO2/Al2O3 profiles indicate that ceria reduction is also promoted by the presence of platinum. EPR. Reduction of cerium oxide leads to the formation of oxygen vacancies along with reduced cerium ions.19 As shown in previous reports, a detailed study of the characteristics of the oxygen vacancies formed upon CO interaction can be gained using EPR by monitoring superoxide radical formation when using oxygen as a probe molecule.13,19,20 To determine to what extent support reduction might be involved in the reduction by CO of these systems, complementary EPR experiments have been performed on CeO2/Al2O3 and Pt/CeO2/Al2O3 submitted to reduction treatments in CO and subsequent dosing with small amounts of oxygen. The reduction treatments in CO consisted of heating the sample in the EPR cell under 100 Torr of CO at a certain reduction temperature (Tr) during 1 h, followed by outgassing at the same temperature during 0.5 h. Following every oxygen adsorption test and prior to the subsequent reduction treatment at higher Tr, the samples were regenerated by outgassing for 0.5 h at the corresponding Tr of the test, to avoid any effects resulting from the interaction with the small amount of adsorbed oxygen molecules. Several minor signals are observed after submitting CeO2/ Al2O3 to reduction treatments at Tr ) 373-773 K. A hyperfine structure of six narrow lines with symmetric shape, centered at

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Figure 2. EPR spectra of CeO2/Al2O3 after oxygen adsorption at 77 K on the sample reduced in CO at (a) 373, (b) 473, (c) 573, (d) 673, and (e) 773 K. Computer simulations are overlapped as thinner dotted lines.

g ) 2.005 and with a splitting A ) 92 G corresponds to Mn2+ impurities.13 This signal decreases somewhat for Tr ) 573 K and disappears for Tr > 573 K, most likely due to reduction of the manganese cations. A narrow axial signal at g⊥ ) 1.967 and g| ) 1.938, signal A, due to electrons trapped in oxygen vacancies adjacent to cerium ions, probably stabilized by the presence of impurities,20 is observed for Tr ) 373 K, decreasing its intensity for Tr ) 473 K, and being no longer observed for Tr > 473 K. A symmetric signal at g ) 2.003 with ∆Hpp ≈ 3 G, signal B, due to electrons trapped at oxygen vacancies or carbonaceous impurities, is observed for Tr ) 773 K. Similar minor signals are observed for Pt/CeO2/Al2O3; the signal due to Mn2+ impurities and signal A show here somewhat higher stability against reduction than in the absence of platinum, being observed in all the examined Tr ranges, while signal B is observed for Tr g 573 K in this sample. Very slight changes are produced in these signals upon O2 adsorption, which together with their very small intensity in comparison with the oxygen signals produced by this interaction, as seen below, indicates that these species are not involved in the oxygen adsorption process in an important way and they will not be considered any further. The spectra obtained after oxygen adsorption at 77 K on CeO2/Al2O3 reduced in CO at Tr ) 373-773 K are shown in Figure 2. The overlapping signals forming the spectra are classified by considering their g values, obtained by computer simulation methods, and summarized in Table 1. The overall contribution of these signals along with their partial contributions to the spectra are shown in Figure 3A and in Figure 3B,C, respectively. For Tr ) 373 K a weak signal OI is formed upon oxygen adsorption (Figure 2a). A certain contribution of signal OI is required to simulate the spectrum obtained for Tr ) 473

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TABLE 1: Characteristics of the EPR Signals Observed upon Oxygen Adsorptiona signal

g tensor parameters

proposed assignment

OI OI′ OII OII′ OIII

g⊥ ) 2.028, g| ) 2.012 g⊥ ) 2.026, g| ) 2.012 gz ) 2.028-2.027, gx ) 2.020-2.017, gy ) 2.011 gz ) 2.027, gx ) 2.015, gy ) 2.011 gz ) 2.037-2.032, gx ) 2.014-2.013, gy ) 2.011

O2- -Ce4+ on highly dispersed Ce4+ or at the edge of bidimensional ceria patches on alumina O2- -Ce4+ at the surface of bidimensional ceria patches on alumina O2- -Ce4+, affected by the presence of platinum cations O2- -Ce4+ at large CeO2 particles

a For the axis assignment of the superoxide radicals,19,20 the z axis is considered to lie along the O-O bond and x is perpendicular to the [O Ce O] plane of the O2--Ce4+ radical.

Figure 4. EPR spectra of Pt/CeO2/Al2O3 after oxygen adsorption at 77 K on the sample reduced in CO at (a) 373, (b) 473, (c) 573, (d) 673, and (e) 773 K. Computer simulations are overlapped as thinner dotted lines. (f) Following outgassing at room temperature of sample given in spectrum c.

Figure 3. (A) Overall integrated intensities of oxygen-derived radicals, extracted from Figures 5 and 6: squares, CeO2/Al2O3; triangles, Pt/ CeO2/Al2O3. Contributions of the signals, extracted from computer simulations of Figures 2 and 4, to the EPR spectra (OI type is referred to OI or OI′ signals) for CeO2/Al2O3 (B) and Pt/CeO2/Al2O3 (C).

K (Figure 2b), but the largest contribution to this spectrum comes from a new signal OII, the gx component of which is broadened and unresolved in this spectrum but can always be assessed, as it influences the overall spectrum shape. Signal OII is present in all of the spectra obtained for Tr > 473 K; the apparent narrowing of its components occurring when the

sample is treated at these higher Tr allows resolution of the central gx feature and thus clearer discernment of its presence (Figure 2c-e). For all of these cases, however, it is necessary to include a certain amount of a broadened signal OII, similar to that detected for Tr ) 473 K, to simulate satisfactorily the experimental spectra. Signal OI is no longer observed for Tr > 473 K, a new signal OI′ appearing in these cases. Additionally, new small signals type OIII, characterized by having higher gz and lower gx values than signals OI or OII, are detected for Tr > 473 K, as indicated by the appearance of shoulders at the low- and high-field parts of the spectra. Signals OIII are similar to those obtained by O2 adsorption on CeO2 reduced by outgassing treatments.19 The spectra obtained after O2 adsorption at 77 K on Pt/CeO2/ Al2O3 reduced with CO at Tr ) 373-773 K are shown in Figure 4. As in the case of CeO2/Al2O3 an increase of the overall intensity is produced with increasing Tr (Figure 3A). However, for Tr e 573 K the overall intensity of oxygen signals is greater in the presence of platinum. This increase of the intensity is mainly due to a larger contribution of signal OI and to the presence of a new signal OII′ (with a gx value slightly lower than signal OII), as deduced by comparison of data obtained by computer simulations; the magnitudes of these contributions are

Pt/CeO2/Al2O3 Catalyst Reactivity for the CO-O2 Reaction shown in Figure 3B,C. For Tr > 473 K, signal OI′ and a narrower signal OII are observed, while signals OI and OII′ disappear. As in the case of CeO2/Al2O3, we designate generically as signal OII the contribution of two signals with similar parameters but different line widths. The evolution of the small signals OIII with the increasing sample reduction temperature shows a behavior not too different from that observed for CeO2/Al2O3; they appear at Tr ) 573 K and increase in intensity for higher Tr. For CeO2/Al2O3, the EPR spectra of the oxygen-derived signals show no important modifications when the samples are warmed to room temperature. However, in the case of Pt/CeO2/ Al2O3, an apparent decrease of the signals, along with a certain narrowing of them (possibly due to reoxidation of some oxygen vacancies under platinum influence), is produced for Tr e 573 K. This might be due to reoxidation of the oxygen adsorption sites beyond the O2- formation step, i.e., to further electron transfer to the adsorbed oxygen molecules following the sequence O2 f O2- f O22- f 2O- f 2O2-. Increased progress along these steps may be favored by deep reduction of ceria or by the influence of close metallic platinum.14 The intensity decrease produced upon warming to room temperature affects to the relative contribution of all of the signals observed, particularly for 473 e Tr e 573 K, signals type OII being most affected by this warming. The presence of the signals OIII, whose contribution increases with Tr, is revealed more clearly by subjecting the samples to a short outgassing at room temperature, which leads to the disappearance of signals OI, OI′, and OII or OII′ (Figure 4f). These latter signals are recovered by subsequent oxygen adsorption at 77 K. The assignment of the signals is based on previous studies in which adsorption of 17O-enriched mixtures was performed.19,20 In all cases, the signals can be assigned to superoxide species adsorbed on cerium ions, i.e., formally O2- Ce4+. These species are formed by interaction of molecular oxygen with a surface site formed by a doubly ionized oxygen vacancy and a reduced cerium center (formally Vo-Ce3+), leading, as mentioned above, to electron transfer from the latter toward the oxygen molecule.19 Superoxide formation can therefore be used as a method for evaluating support reduction in these materials. Differences between the parameters of these signals reflect changes in the chemical environments of the cerium ions on which the corresponding species are formed. More details on the reasoning behind the different assignments can be found elsewhere.19,20 Briefly, signals OIII, which are similar to those observed in unsupported CeO2,19 are assigned to superoxide species formed on relatively large tridimensionallike CeO2 particles (3D-Ce), known to be present in these samples as revealed by X-ray diffraction (XRD) measurements,16 while for signals OI/OI′ and OII/OII′, not formed on pure CeO2, the higher gx value indicates an influence of a more or less alumina-like environment on the Ce ion at the adsorption site, leading to an increased covalence in the Ce4+-O2adsorption bond20 and to a different stability of these species toward outgassing and thermal treatments.19,20 From these results, and on the basis of previous work correlating ceria dispersion with the properties of the different superoxide signals,20 the structure of cerium oxide in these samples might be envisaged as follows: a small amount would involve 3DCe particles, responsible for the smaller signals OIII, while most of the cerium oxide would be present as bidimensional patches (2D-Ce) dispersed on the alumina surface. Signals type OI and OII would then be related to different positions within these bidimensional patches, type OII signals being due to superoxide

J. Phys. Chem. B, Vol. 102, No. 22, 1998 4361 radicals located at the surface of the patches while type OI signals (on the basis of their higher gx values, indicating a higher effect of alumina) would be located at the edge of the patches.20 Concerning signal OII′, which is observed exclusively in Pt/ CeO2/Al2O3 reduced at Tr e 473 K, its gx value is a little lower than that of signal OII observed for CeO2/Al2O3 reduced at Tr g 473; this might be due to the influence of platinum cations on sites at the 2D-Ce patches surface that otherwise should produce signal OII. Thus, when platinum cations, probably forming platinum oxide-like clusters, are located on 2D-Ce patches, they favor the formation of oxygen vacancies at the surface of those patches, leading to signal OII′. Once they are reduced to Pt0, and agglomerate into metallic particles, they cease to modify the crystal field around the Ce ions, and signal OII′ is no further observed. One point to take into account is that the stability of O2- species near metallic platinum should be very low; an easy reoxidation of the corresponding oxygen vacancies should be expected. If the nuclearity of the Pt metallic particles (i.e. sintering level) is higher than that of the initial oxidized Pt species, the fraction of 2D-Ce patches surface directly affected by Pt may be lowered upon full Pt reduction; thus some OII species may be detected after adsorption of oxygen at room temperature, undisturbed by Pt, even if the latter promotes easy reoxidation of the most closely located sites. Although no strict quantitative conclusion can be extracted from the evaluation of EPR intensities of superoxide radicals with respect to the extent of ceria reduction in these samples, due to the lack of information on specific magnetic interactions and on the possible formation of diamagnetic oxygen species after O2 adsorption on the reduced samples, a good qualitative correlation is observed between the CO-TPR experiment and the overall intensity of superoxide radicals for CeO2/Al2O3 (Figures 1A and 3A). These results show that ceria reduction in this sample remains at a low level for Tr e 473 K, presumably below the detection limit of the CO-TPR technique, while a most important reduction is produced for Tr higher than ca. 500 K. For Pt/CeO2/Al2O3, the higher intensity of superoxide signals for Tr e 573 K shows that, as already suggested by the TPR results, the presence of platinum promotes the reduction by CO of the 2D-Ce patches. For higher Tr, the results obtained by EPR do not offer evidence of a promoting effect of platinum on the reduction of cerium oxide, the total intensity of the oxygen signals being actually lower than in the absence of platinum; since ceria sites near metallic platinum are probably most easily oxidized, no clear picture on the global ceria reduction can be extracted from the EPR spectra of superoxide radicals in that case. Reactivity toward CO + O2. Light-Off Tests. The two platinum catalysts have been tested for their reactivity in a gas mixture containing 1% CO and 0.5% O2 in the N2 carrier (i.e. of stoichiometric redox composition) at a rate equivalent to a space velocity of 30 000 h-1. After a short time on stream (in order to ensure a steady gas flow on the catalyst), heating of the oven at a 5 K/min rate up to 773 K was started. CO and O2 are consumed together with simultaneous production of CO2. Figure 5 shows the CO conversion profiles for both catalysts. Characteristic points of these profiles along with the apparent activation energy calculated with points around T20 are collected in Table 2. On the other hand, catalytic tests performed on the platinum-free Al2O3 or CeO2/Al2O3 supports showed a significantly lower reactivity (results not shown) than the corresponding platinum containing catalysts. For Al2O3 the reaction begins at ca. 573 K and T50 is at T > 773 K, while for CeO2/Al2O3 the onset is observed at ca. 523 K and T50 ≈ 673 K.

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Figure 5. CO conversion profiles for the CO-O2 reaction on Pt/Al2O3 (circles) and Pt/CeO2/Al2O3 (triangles).

TABLE 2: Isoconversion Temperatures (K) Extracted from the Reaction Profiles (Figure 5) at 10, 50, and 90% CO Conversion and Apparent Activation Energy Values (kJ mol-1) catalyst

T10

T50

T90

Ea

Pt/Al2O3 Pt/CeO2/Al2O3

483 363

563 443

588 483

134 43

These results show that in the present conditions the presence of cerium oxide significantly enhances the catalytic performance of the system, lowering the isoconversion temperatures by ca. 100 K in all of the ranges while the apparent activation energy is greatly diminished. It is worth considering the differences between these results and those of Serre et al.,10b where no particular enhancement in reactivity was observed when preoxidized Pt/CeO2/Al2O3 and Pt/Al2O3 were compared (in contrast with prereduced catalysts), even working at a lower space velocity. They attribute this result to the stabilization of oxidized platinum at the platinum/ceria interface, leading to deactivation of metal/support interactions. There may be several reasons that could account for the differences between that work and the present one. One of them could be the fact that the platinum precursors used for the catalyst preparation were different; in their case, hexachloroplatinum acid was used and it is known that chlorine is not readily eliminated when cerium oxide is present in the catalyst.15 The formation of Pt-Cl-Ce instead of Pt-O-Ce bonds, if it occurred, would most likely produce differences in the redox behavior of the system. FTIR. To determine the changes produced by the reaction mixture on the platinum state, both catalysts have been subjected at different temperatures to a CO + O2 flow in conditions similar to those employed for the catalytic test (1% CO, 0.5% O2 in Ar), after which FTIR measurements, using CO as probe molecule, were made. Thus, after a standard calcination pretreatment in 3% O2:Ar, the catalysts were treated in the CO/ O2 flow during 10 min at the desired temperature in the IR cell, cooled in the same flow to room temperature, and, after being outgassed until equilibrium, contacted with about 40 Torr of CO at room temperature. Spectra for the Pt/Al2O3 catalyst in the C-O stretching range are shown in Figure 6. No band becomes apparent following treatment at 373 K, suggesting that platinum reduction is not achieved by this treatment, but when the treatment is performed at 473 K, the spectrum shows a band at 2066 cm-1. Increasing the temperature to 573 K produces a slight shift of this band to 2069 cm-1, without modifying its intensity significantly. This band can be assigned to CO adsorbed linearly at open sites, i.e., low coordination atoms, of metallic platinum particles.21 The fact that no band appears at 2080-2090 cm-1, which is typical of carbonyl species adsorbed at terrace sites,21 nor at ca. 1850 cm-1, as expected for bridge bonded CO,22 suggests

Figure 6. FTIR spectra of Pt/Al2O3 after adsorption of 40 Torr of CO at room temperature on the sample treated in a flow of CO + O2 at (a) 373, (b) 473, and (c) 573 K.

Figure 7. FTIR spectra of Pt/CeO2/Al2O3 after adsorption of 40 Torr of CO at room temperature on the following: (a) a sample calcined in oxygen at 673 K; samples treated in a flow of CO + O2 at (b) room temperature, (c) 373 K, (d) 473 K, and (e) 573 K.

that small size metallic platinum particles are formed in these conditions.23 For the Pt/CeO2/Al2O3 sample, as appreciated in Figure 7, the situation is more complex; this may be ascribed to the existence of different locations of platinum in the sample since, in addition to platinum on alumina, platinum can be interacting with 2D-Ce patches or 3D-Ce particles in this sample. After the calcination pretreatment in diluted oxygen at 673 K, CO adsorption results in a band at 2090 cm-1. This band is still present, with a slight blue shift, when the Pt/CeO2/Al2O3 is treated under the CO/O2 flow at T e 473 K. Its frequency is close to that of CO linearly adsorbed (at saturation coverages) on terraces of metallic platinum clusters.21 However, this assignment is not consistent with the observed behavior of this band since separate experiments have shown no significant wavenumber shift upon outgassing at room temperature, when certain decrease is observed in the intensity of this band. An alternative assignment for the latter would be that it belongs to carbonyl species adsorbed on partially oxidized platinum entities or on metallic platinum clusters containing adsorbed oxygen.7b,24 This attribution seems most likely in view of the presence of this band after the oxidative pretreatment. These species must be stabilized by interaction with dispersed ceria since they are absent in the Pt/Al2O3 catalyst. On the other hand, contact of the sample with the reaction mixture, even at room temperature, induces the formation of a band at 2050-2060 cm-1, whose intensity rises upon increasing reaction temperature. As indicated for Pt/Al2O3, this band is due to carbonyl species adsorbed linearly at open sites (e.g. steps, corners) of metallic platinum,21

Pt/CeO2/Al2O3 Catalyst Reactivity for the CO-O2 Reaction indicating that small platinum particles are formed in these conditions. Comparison with the Pt/Al2O3 sample treated in similar conditions, Figure 6, certainly shows a higher platinum reducibility upon interaction with the reaction mixture when ceria is present in the catalyst that must be attributed to some aspect of platinum/ceria interactions. Following the treatment at 573 K, the FTIR spectrum is constituted by two intense bands at 2081 and 2061 cm-1, along with a weak band centered at 1834 cm-1; a considerable increase of the overall intensity is observed in this case. The band at 2081 cm-1 is already present as a strong shoulder after treatment at 473 K (Figure 4d). It must correspond to CO linearly adsorbed on platinum terraces, while the band at 2061 cm-1 is assigned as in the previous cases to CO linearly adsorbed on open sites of the platinum particles.21 On the other hand, the weak band at 1834 cm-1 can be attributed to bridge-bonded CO adsorbed on metallic platinum.22,25 Finally, a broad shoulder appears to form at 2020 cm-1 for the sample treated in CO + O2 at 473 or 573 K, Figures 7d,e. This might be attributed to linear carbonyls formed on small platinum particles formed on 3D-Ce particles; similar “low-wavenumber” carbonyls have been observed on different M/CeO2 samples (M ) Pd, Cu, Rh), the red-shift being attributed to metal/support interactions.26 These results indicate that, under reaction conditions, the formation of metallic platinum begins at lower temperatures in the case of the Pt/CeO2/Al2O3 catalyst, although a more complete reduction requires temperatures of at least 573 K since the higher overall intensity is observed after this experiment. Additionally, differences in the frequencies of the bands of CO linearly adsorbed on Pt0 between both catalysts reflect that the morphology of the platinum particles is influenced by the support;23 this might be a consequence of the different reduction degrees attained by both systems. Indeed, larger particles seem to be formed for the Pt/CeO2/Al2O3 catalyst, as indicated by the formation of carbonyl species adsorbed at terrace sites and bridge bonded CO in the sample treated at 573 K. On the other hand, platinum cations affected by support interaction or metallic platinum clusters containing adsorbed oxygen can be available for CO adsorption at low reaction temperature, giving an IR band at 2090 cm-1, for the Pt/CeO2/Al2O3 catalyst. The marked differences between both samples indicate that for Pt/CeO2/Al2O3 an important contribution of platinum interacting with cerium is observed which points in turn toward a preferential location of platinum on the cerium oxide part of the support, rather than on the alumina free surface. EPR. Figure 8a shows the spectrum obtained after submitting the calcined Pt/CeO2/Al2O3 sample to CO + O2 at 423 K (a temperature above the light-off temperature for CO oxidation), followed by room temperature outgassing. Subsequent oxygen adsorption at 77 K produces signal OI (as obtained by computer simulation, Figure 8b), with lower intensity than in Figure 4a, but does not show indications of signals OII or OII′, easily obtained by O2 adsorption at 77-298 K after treatments with CO alone. The spectrum is not significantly changed upon warming at room temperature (and recording it at 77 K). The reactivity of the superoxide radicals giving this signal OI toward CO was then studied. Previous works13,14 had shown that superoxide radicals obtained after oxygen adsorption on samples previously reduced at relatively high temperatures decreased upon interaction with CO subsequently added at room temperature. To avoid a possible route for superoxide elimination via migration of these adsorbed species, which are rather labile (as shown by their elimination upon room temperature outgassing), to the platinum particles and further adsorption and

J. Phys. Chem. B, Vol. 102, No. 22, 1998 4363

Figure 8. EPR spectra of Pt/CeO2/Al2O3 precalcined in O2 at 673 K and submitted to 700 µmol/g of stoichiometric (CO + 1/2O2) at 423 K: (a) after room temperature outgassing; (b) with subsequent oxygen adsorption at 77 K (computer simulation of signal OI is overlapped as dashed line).

reaction on these latter, an experiment has been designed in order to check if these particular superoxide species can be eliminated in conditions where the amount of free adsorption sites on the platinum particles is very low, a condition which can be achieved by CO preadsorption. Thus, the sample in the state reflected in Figure 8b was outgassed at room temperature, which leads to the elimination of signal OI; subsequently, CO was adsorbed on it at room temperature (followed by room temperature outgassing), to achieve coverage of the platinum particles by CO.27-29 Then oxygen was adsorbed at 77 K, yielding signal OI, and the evolution of the signal with time upon bringing the sample at room temperature was followed. Only a slight intensity decrease of the superoxide signal, within experimental error, was detected under these conditions. Subsequent adsorption of further CO at room temperature did not lead to significant changes in the EPR spectra either. Subsequently, in a second experiment, the sample was outgassed at 423 K prior to oxygen adsorption in order to deplete CO coverage of the platinum particles.27 Then, oxygen is adsorbed at 77 K, yielding signal OI with a higher intensity than in the previous case. Warming at room temperature does not produce any significant change in the EPR spectrum. Subsequent CO adsorption at room temperature did not result in a decrease in signal OI in this case either. The absence of signals OII and OII′ and the lower amounts of signal OI observed in these experiments, in comparison with the spectra of oxygen adsorbed on the sample treated in CO alone (Figure 4a), may indicate that the corresponding oxygen vacancies have not been formed during the pretreatment in COO2. However, considering that the pretreatment temperature is well above the light-off temperature and that, on the basis of the FTIR experiments of Figure 7, the presence of metallic platinum is expected, the observed difference could be due also to the (Pt-catalyzed) dissociation of the adsorbed oxygen molecules and reoxidation of the oxygen vacancies. This would imply that during the cycle of catalyst reduction/reoxidation by interaction with CO/O2, the second part (reoxidation) is faster than the first one (reduction), leading to a very low amount of the corresponding vacancies in the stationary state. Discussion It is generally assumed that the CO-O2 reaction on platinum single crystals28 or alumina supported platinum10b,29 proceeds

4364 J. Phys. Chem. B, Vol. 102, No. 22, 1998 via a Langmuir-Hinshelwood mechanism under which, below a certain temperature (which depends on reactant partial pressures29), large CO coverage of the platinum particles greatly reduces the sticking probability for O2.28 Desorption of CO from the metallic particles, leaving behind vacant sites allowing oxygen adsorption, dissociation, and further reaction with coadsorbed CO, is thus considered to be the limiting step of the reaction. Thus, when similar partial pressures of both reactants and prereduced samples are used (to ensure that platinum is in the metallic state), the onset of the reaction is observed at temperatures around 423 K for Pt/Al2O3.10b A similar interpretation can be given here to explain the catalytic results on our Pt/Al2O3 sample, since notwithstanding that a preoxidized sample is used, the metallic platinum state can be attained by reaction of the catalyst with the CO-O2 mixture at T > 373 K, as indicated by the results shown in Figure 6. The reactivity of this system can thus be explained by reaction between coadsorbed CO and dissociated oxygen on the surface of supported metallic platinum particles, as previously reported.10b,28,29 A considerable enhancement in reactivity is exhibited by Pt/ CeO2/Al2O3 since noticeable CO conversion is observed at temperatures as low as 333 K, suggesting that important interactions between platinum and dispersed cerium oxide are involved in this system. FTIR experiments (Figure 7) show that, for preoxidized Pt/CeO2/Al2O3, a part of the platinum might achieve the metallic state upon interaction with the CO:O2 reactant mixture at a relatively low temperature (already at 300 K), indicating that at the reaction onset temperature (≈333 K, Figure 1) a part of the platinum might be in the most active oxidation state.10 However, according to the arguments of the previous paragraph, formation of metallic platinum particles is not sufficient to account for the significant low-temperature activity since the CO inhibition effect should avoid observation of a significant conversion.10b,28,29 The results of catalytic activity suggest that the simultaneous presence of platinum and ceria can provide a new reaction pathway different from that operating for pure metallic platinum or platinum supported on nonreactive oxides. In this respect, a survey of the literature offers, as the main differential feature of metals supported on ceria systems, their apparent reactivity toward CO alone, yielding CO2.6-8,10 This result is due to the reactivity of the ceria itself, which in turn is promoted by the presence of the precious metals. Those works have then proposed that this could be a key initial reaction step in the catalytic cycle in the presence of ceria, and consequently that the reactivity might be further enhanced by improving the reducibility of ceria in the catalyst.30 TPR and EPR experiments reveal enhancement in the ease of ceria reduction by CO in the presence of platinum at relatively low temperatures (Tr < 500 K). EPR studies using oxygen as probe molecule allow a more specific determination of the centers involved in these effects. Results in Figures 2-4 show that the presence of platinum promotes mainly the reduction of ceria sites located at 2D-Ce patches, according to the higher intensity of signals type OI and OII′ (and, to a smaller extent, OII) in that Tr range for Pt/CeO2/Al2O3. In view of the EPR results, a possible interpretation of the promoting effect in the presence of platinum would be that the reduced ceria entities at the 2D-Ce patches assist in the reduction of platinum located in their environment; similar ceria-metal electron-transfer processes have been recently proposed to occur in Pd/CeO2 systems.31 Formation of metallic platinum by this process would in turn increase further reduction of ceria by the cooperation of

Martı´nez-Arias et al. metal-induced CO activation and metallic platinum/ceria interactions.7,10 Connecting with the arguments of the previous paragraph, it can then be proposed that there are two factors that would lead to the achievement of an optimum catalyst of this kind: first, obtention of a highly dispersed ceria support, in which ceria is forming small bidimensional patches on the alumina surface, which, in the presence of platinum, are more easily reducible by CO (Figures 2-4), and second, location of the (initially oxidized) platinum entities in positions close to those most reducible sites at the 2D-Ce patches. FTIR results on the Pt/CeO2/Al2O3 sample subjected to CO + O2 corroborate this hypothesis. The results of Figures 6 and 7 indicate that platinum reduction is promoted in the presence of ceria, thus indicating that metallic platinum will be present even at room temperature and thus it is most likely involved in the reactivity of the system in those conditions. Interpretation of the EPR results on the sample submitted to CO + O2 is not so straightforward. Thus, formation of only signal OI after O2 adsorption on the sample submitted to CO + O2 (Figure 8b) might suggest that only the sites at the edges of 2D-Ce become reduced by interaction with the reactant mixture. This result could then be taken as an indication that those most labile20 O2- anions at the edges of bidimensional ceria patches are the only ones able to react with CO under the reaction conditions (yielding reduced cerium centers able to form signal OI upon subsequent oxygen adsorption). However, the absence of reaction (at room temperature) between these superoxide radicals and CO suggests that in fact these radicals might not be directly involved in an important way in the low-temperature promotion of the reactivity. Previous reports have indicated that such O2- + CO reaction can occur on this type of catalyst13,14 and have thus led to the presumption that O2- /CO interactions could play an important role in the low-temperature activity of such systems.14 According to the model given in those previous works,14 the reaction between O2- -Ce4+ and CO would involve ceria sites located at or close to the metal/ceria interface; in those experiments, the initial condition of the catalyst presumably included platinum in a metallic state (playing the role of electron reservoir able to provide additional electrons necessary for completing the reaction besides acting as adsorption centers for CO) and reduced ceria (where oxygen would be activated by formation of superoxide species). The absence of reaction between the O2- radicals shown in Figure 8b and CO in the experiments reported here suggests that some of these conditions is not fulfilled after submitting the catalyst to CO + O2 at 423 K (temperature at which significant CO conversion is already attained, as shown in Figure 5). The most likely explanation would then be that the superoxide radicals observed in Figure 8b (located at edge sites in the 2D-Ce patches) are relatively far from the Pt entities. This would not completely explain, however, the absence of reaction of these O2- toward CO since such a process has been observed (both for OI and OII superoxide radicals) also on Pt-free CeO2/Al2O3.13 The degree of reduction of ceria is likely to be more important: in the experiments reported in refs 14 the samples were preconditioned by reduction treatments in H2 at 673 K (+ vacuum at 873 K), while the sample yielding the nonreactive O2- species in Figure 8b has been handled only at 423 K. Thus, the severity of the treatments given previously to the catalyst may be important in determining the possibility of the CO-O2- reaction. This would mean that a different process (reaction between Pt-adsorbed CO and diamagnetic O2- at nearby ceria surface sites) could be responsible for the low-temperature catalytic activity when the

Pt/CeO2/Al2O3 Catalyst Reactivity for the CO-O2 Reaction degree of reduction of the ceria surface under steady state conditions is only moderate. Other observations in this work agree with this model, as discussed below. Infrared experiments (Figure 7) show that metallic platinum is easily formed in Pt/CeO2/Al2O3 by interaction with CO + O2, and comparison with Pt/Al2O3 (Figure 6) suggests that the corresponding platinum particles are closely interacting with ceria. On the other hand, sites forming OII-type species are not detected after treatment in catalytic cycle conditions, and those forming OI species show a significantly lower intensity, with respect to the corresponding experiments in CO alone. Thus, it can be proposed that the cerium ions at these sites have lost the ability of stabilizing O2- species upon contact with the CO + O2 mixture, due to reoxidation during the reaction (leading to occupation of the corresponding vacancies by diamagnetic O22- or O2- species) facilitated by the presence of nearby Pt. The catalytic cycle would thus occur between these diamagnetic species and CO adsorbed on the nearby platinum particles, rather than between CO and adsorbed O2-. This would probably constitute the rate determining step, which, according to the data obtained (Table 2), would have substantially lower activation energy than that dominating in the reaction on the ceria-free system. The sites giving the superoxide radicals in Figure 8b would correspond then to locations far from Pt/ceria interface positions, where platinum-promoted reoxidation effects would be lower, and would have a minor role in the catalytic reaction once the steady state is reached after light-off. It is worth noting that while the effects just discussed are related to the reduction of Pt to the metallic state, some of the observations on the Pt/CeO2/Al2O3 catalyst can be attributed to the presence of oxidized platinum entities. Thus, the modification in the g tensor observed for signal OII′ (in comparison to signal OII formed on CeO2/Al2O3) may be related to an electrostatic or polarization effect of platinum cations in the coordinative environment of the cerium ions on which the corresponding superoxide species are formed. These platinum cations, probably in the Pt2+ state since the signal is detected in a Pt/CeO2/Al2O3 sample which has been moderately reduced (Tr e 473 K, Figure 3C), while no paramagnetic platinum species is detected, can be present either as isolated ions, stabilized by ceria, or in an aggregated state formed upon partial reoxidation by O2 of Pt0 clusters. On the other hand, the IR band appearing at 2090 cm-1 after CO adsorption during the first stages of the reduction can be related to the presence of such Pt2+ centers. In any case, this redox state of Pt is not likely to be involved to a great extent in the observed enhancement in CO + O2 reactivity, since extensive reduction of the catalyst, leading to elimination of these partially oxidized states, is known to lead to more active materials.10 Finally, it should be remarked that, as the TPR experiments evidence, the presence of ceria also promotes the WGS reaction involving interaction of CO with surface hydroxyl groups of the samples; a shift of ca. 100 K is detected for the onset of this reaction in comparison with the Pt/Al2O3 case. This is therefore an additional effect of Pt/ceria interaction in this system. However, due to the relatively higher temperatures at which this process is observed, it is not likely that the latter contributes significantly to the enhancement of the activity for the CO-O2 reaction at lower temperatures. In conclusion, there are several differential features that can lead to the enhancement of CO oxidation in an (initially oxidized) Pt/CeO2/Al2O3 system. Among them, according to the temperature range at which the different phenomena are

J. Phys. Chem. B, Vol. 102, No. 22, 1998 4365 observed, it seems that mutual interactions between Pt and 2DCe lead to an enhancement of the reducibility of both components, providing sites for the activation of both CO and O2. It is proposed that by maximizing both the amount of these 2DCe species and their proximity to the supported platinum entities, the best initial conditions for the catalyst would be reached. Acknowledgment. Technical assistance by F. Sa´nchez Constenla is acknowledged for the recording of some of the EPR spectra. Thanks are also due to Dr. J. A. Anderson for revision of the manuscript. Financial support of this work under CICYT Projects No. MAT94-0835-C03-02 and MAT97-0696C02-01 is gratefully acknowledged. References and Notes (1) (a) Harrison, B.; Diwell, A. F.; Hallett, C. Platinum Met. ReV. 1988, 32, 73. (b) Leclercq, G.; Dathy, C.; Mabilon, C.; Leclercq, L. Stud. Surf. Sci. Catal. 1991, 71, 181. (2) (a) Dictor, R.; Roberts, S. J. Phys. Chem. 1989, 93, 5846. (b) Su, E. C.; Rothschild, W. G. J. Catal. 1986, 99, 506. (3) (a) Yao, H. C.; Yu Yao, Y. F. J. Catal. 1984, 86, 254. (b) Engler, B.; Koberstein, E.; Schubert, P. Appl. Catal. 1989, 48, 71. (c) Miki, T.; Ogawa, T.; Haneda, M.; Kakuta, N.; Ueno, A.; Tateishi, S.; Matsuura, S.; Sato, M. J. Phys. Chem. 1990, 94, 6464. (4) Shido, T.; Iwasawa, Y. J. Catal. 1993, 141, 71. (5) Martı´nez-Arias, A.; Soria, J.; Conesa, J. C.; Seoane, X. L.; Arcoya, A.; Catalun˜a, R. J. Chem. Soc., Faraday Trans. 1995, 91, 1679. (6) Hardacre, C.; Ormerod, R. M.; Lambert, R. M. J. Phys. Chem. 1994, 98, 10901. (7) (a) Jin, T.; Okuhara, T.; Mains, G. J.; White, J. M. J. Phys. Chem. 1987, 91, 3310. (b) Jin, T.; Zhou, Y.; Mains, G. J.; White, J. M. J. Phys. Chem. 1987, 91, 5931. (8) Zafiris, G. S.; Gorte, R. J. J. Catal. 1993, 139, 561. (9) (a) Oh, S. H. J. Catal. 1990, 124, 477. (b) Oh, S. H.; Eickel, C. C. J. Catal. 1988, 112, 543. (10) (a) Serre, C.; Garin, F.; Belot, G.; Maire, G. J. Catal. 1993, 141, 1. (b) Serre, C.; Garin, F.; Belot, G.; Maire, G. J. Catal. 1993, 141, 9. (11) (a) Bunluesin, T.; Cordatos, H.; Gorte, R. J. J. Catal. 1995, 157, 222. (b) Cordatos, H.; Bunluesin, T.; Stubenrauch, J.; Vohs, M.; Gorte, R. J. J. Phys. Chem. 1996, 100, 785. (12) Golunski, S. E.; Hatcher, H. A.; Rajaram, R. R.; Truex, T. J. Appl. Catal. B 1995, 5, 367. (13) Soria, J.; Martı´nez-Arias, A.; Coronado, J. M.; Conesa, J. C. Colloids Surf. A 1996, 115, 215. (14) (a) Sass, A. S.; Shvets, V. A.; Savel’eva, G. A.; Popova, N. M.; Kazanskii, V. B. Kinet. Katal. 1987, 27, 777. (b) Tarasov, A. L.; Przheval’skaya, L. K.; Shvets, V. A.; Kazanskii, V. B. Kinet. Katal. 1989, 29, 1020. (15) El Fallah, J.; Boujana, S.; Dexpert, H.; Kiennemann, A.; Majerus, J.; Touret O.; Villain, F.; Le Normand, F. J. Phys. Chem. 1994, 98, 5522. (16) R. Catalun˜a. Ph.D. Thesis, Universidad Polite´cnica de Madrid, 1995. (17) Padley, M. B.; Rochester, C. H.; Hutchings, G. J.; King, F. J. Catal. 1994, 148, 438. (18) Jackson, S. D.; Glanville, B. M.; Willis, J.; McLellan, G. D.; Webb, G.; Moyes, R. B.; Simpson, S.; Wells, P. B.; Whyman, R. J. Catal. 1993, 139, 207. (19) Soria, J.; Martı´nez-Arias, A.; Conesa, J. C. J. Chem. Soc., Faraday Trans. 1995, 91, 1669. (20) Soria, J.; Coronado, J. M.; Conesa, J. C. J. Chem. Soc., Faraday Trans. 1996, 92, 1619. (21) Hollins, P. Surf. Sci. Rep. 1992, 16, 51. (22) Gland, J. L.; Kollin, E. B. Surf. Sci. 1985, 151, 260. (23) Kappers, M. J.; Van der Maas, J. H. Catal. Lett. 1991, 10, 365. (24) Barshad, Y.; Zhou, X.; Gulari, E. J. Catal. 1985, 94, 128. (25) Haaland, D. M. Surf. Sci. 1987, 185, 1. (26) (a) Bensalem, A.; Muller, J.-C.; Tessier, D.; Bozon-Verduraz, F. J. Chem. Soc., Faraday Trans. 1996, 92, 3233. (b) Martı´nez-Arias, A.; Catalun˜a, R.; Conesa, J. C.; Soria, J. J. Phys. Chem. B 1998, 102, 809. (c) Soria, J.; Martı´nez-Arias, A.; Fierro, J. L. G.; Conesa, J. C. Vacuum 1995, 46, 1201. (27) Barth, R.; Pitchai, R.; Anderson, R. L.; Verykios, X. E. J. Catal. 1989, 116, 61. (28) Engel, T.; Ertl, G. AdV. Catal. 1979, 28, 1. (29) Anderson, J. A. J. Chem. Soc., Faraday Trans. 1992, 88, 1197. (30) Cordatos, H.; Ford, D.; Gorte, R. J. J. Phys. Chem. 1996, 100, 18128. (31) Filotti, L.; Bensalem, A.; Bozon-Verduraz, F.; Shafeev, G. A.; Voronov, V. V. Appl. Surf. Sci. 1997, 109, 249.