CeO2 Catalyst

Mar 20, 2007 - Institute of Isotopes, Hungarian Academy of Sciences, P.O. Box 77, ... is a key requirement for a future hydrogen economy, in general, ...
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J. Phys. Chem. C 2007, 111, 5426-5431

Surface Water-Assisted Preferential CO Oxidation on Pt/CeO2 Catalyst Olga Pozdnyakova-Tellinger,† Detre Teschner,*,†,‡ Jutta Kro1 hnert,‡ Friederike C. Jentoft,‡ Axel Knop-Gericke,‡ Robert Schlo1 gl,‡ and Attila Wootsch† Institute of Isotopes, Hungarian Academy of Sciences, P.O. Box 77, Budapest H-1525, Hungary, and Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany ReceiVed: October 25, 2006; In Final Form: January 28, 2007

The production of clean hydrogen is a key requirement for a future hydrogen economy, in general, and, specifically, for the application of proton exchange membrane fuel cells (PEMFC). Here, we focus on one of the essential purification methods, the so-called “PROX” reaction, the preferential oxidation of traces of CO in a large hydrogen excess. Small platinum particles on a reducible support like ceria are effective to remove CO from hydrogen feed. The paper specifically addresses the mechanism of the PROX reaction on a Pt/CeO2 catalyst using in situ experimentation with time-resolved and temperature-programmed diffuse reflectance infrared spectroscopy. Surface species (carbonates, formates, carbonyls, hydroxyls, and adsorbed water) present under reaction conditions are identified, and correlations of their abundance with catalytic performance allow the discrimination between mechanistically relevant species (intermediates) and spectator species. The following scenario is proposed: hydrogen initially adsorbed on platinum spills over to the support, leading to ordered vacancy formation in the ceria bulk as well as hydroxylation and hydration of the surface. CO is mainly adsorbed in on-top orientation on metallic platinum. The linear relationship between the amount of adsorbed water (H2Oads) and the CO2 production indicates that the hydrated ceria supplies an oxidizing agent at the metal/support interface reacting with the nearby surface carbonyls on the Pt particles yielding CO2. Moreover, adsorbed water also blocks hydrogen oxidation because of desorption hindrance. From the correlations in the presented results, an intelligent PROX catalyst can be formulated, providing a guideline for future developments.

Introduction The aim of preferential oxidation (PROX reaction) is to oxidize CO selectively in the presence of a large hydrogen excess.1-6 The PROX process is one key step in the development of an economically feasible technology to produce hydrogen for proton exchange membrane fuel cells (PEMFCs).7 Hydrogen can be liberated from various resources such as natural gas, biogas, hydrocarbons, and alcohols by steamreforming or partial oxidation reactions.8,9 Unfortunately, about 5-15% of CO is formed along with H2, H2O, CO2, and CH4. The CO concentration in the hydrogen feed must be kept below 1-100 ppm for proper operation of PEMFCs.7 Thus, first a water-gas-shift reaction, WGS (CO + H2O f CO2 + H2), is carried out to reduce the amount of CO to 0.5-1%,8 which is then followed by a PROX process. For the development of PROX catalysts, it is highly beneficial to understand the reaction mechanism and the entire interplay between feed mixture and surface sites. In this paper, we examined platinum supported on ceria, one of the catalytic formulations showing promising behavior in the PROX reaction,5,6,10-12 using time-resolved and temperatureprogrammed in situ diffuse reflectance Fourier transform IR spectroscopy (DRIFTS) combined with on-line mass spectrometric (MS) analysis. Spectroscopy under reaction conditions combined with on-line activity monitoring is the most powerful tool of catalysis researchers. The goal of in-situ spectroscopic * Corresponding author. Tel.: +49 30 8413 5408. Fax: +49 30 8413 4676. E-mail: [email protected]. † Hungarian Academy of Sciences. ‡ Fritz-Haber-Institut der Max-Planck-Gesellschaft.

experiments is to establish correlations between surface state, surface species, and catalytic activity. The obtained information can immediately be implemented in the rational design of new catalysts. Here, we monitor the evolution of different surface species (carbonates, formates, carbonyls, hydroxyls, and adsorbed water) and search for correlations between their concentration and the catalytic performance to identify which species play a role in the PROX reaction and which are merely spectator species. Experimental Section The catalyst with a nominal metal loading of 1 wt % was prepared by impregnation of ceria (Rhodia Catalysts & Electronics, France, BET ) 96 m2 g-1) with an aqueous solution of Pt(NH3)4(OH)2.5 The impregnated sample was dried at 393 K overnight, calcined for 4 h at 773 K in flowing air (30 mL/ min), and reduced at 673 K for 4 h in flowing H2 (30 mL/min). The metal dispersion, determined by low temperature (223 K) H2 adsorption after reduction, is D ) 62%. As a reference, CeO2 was submitted to an equivalent procedure including all steps but using doubly ionized water as the impregnating solution. DRIFT spectra were acquired with a Bruker IFS 66 FTIR spectrometer, equipped with an MCT detector and purged with purified air. A “Selector” diffuse reflectance mirror accessory was combined with the “Environmental Chamber” reaction cell (both Graseby Specac) and placed in the spectrometer sample compartment. The reaction cell features a heatable sample holder, several ports for gas flow-through, and a single ZnSe window for the IR beam. The samples were contained in a 2.5 mm high gold cup with 8.5 mm o.d. and 7.2 mm i.d. A spectrum of KBr recorded in N2 served as background. The feed stream

10.1021/jp0669862 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/20/2007

CO Oxidation on Pt/CeO2 Catalyst

Figure 1. In situ DRIFT spectra (in reflectance) of 1% Pt/CeO2 and CeO2 at T ) 383 K in steady state in the presence of PROX reaction mixture (1% CO, 1% O2 in H2). Spectra are offset for clarity.

was mixed from analytical grade gases using mass flow controllers. The total gas inlet flow was 50 N mL/min, containing 1% CO and 1% O2 (λ ) 2) in H2. The measurements were carried out using ca. 100 mg of fresh sample (corresponding to the volume of the gold cup) pretreated in situ in flowing air (30 mL/min) at 573 K. The catalyst was purged in N2 while cooling to the reaction temperature of 383 K, which is slightly above the optimum operating conditions of Pt/ceria catalysts (340-360 K) but limits accumulation of water.5 A spectrum of the activated sample before reaction was collected in N2 at the reaction temperature, and then the PROX reaction mixture was admitted to the reaction cell. Spectra were collected with a temporal resolution of 17 s until steady-state conditions were reached and during in situ temperatureprogrammed heating to 523 K (10 K/min). The IR bands used for the quantification of the concentrations of the surface species of interest were as follows: formates, 3000-2675 cm-1; carbonyls, 2100-1880 cm-1; carbon dioxide (gas phase), 24002275 cm-1; carbonates, 1150-970 cm-1; water, 3750-2600 cm-1 (here formates subtracted). These bands are characteristic for the respective species; that is, in the regions selected for integration, no bands of other species interfere. The spectra were transformed into the Kubelka-Munk function, and the area of the given bands was integrated with the OPUS software using a five-point baseline. The effluent gas composition was analyzed continuously by a Pfeiffer Omnistar mass spectrometer. Results and Discussion Evolution of Surface Species under Constant Reaction Conditions. Different formates, carbonates, carboxylates, and carbonyls were detected on ceria supported Pt catalysts by IR spectroscopy upon adsorption of CO13-15 as well as during the PROX reaction.6 The detailed identification of all surface species can be found in ref 6. In Figure 1, steady-state DRIFT spectra of the pure ceria support and of Pt/CeO2 in a PROX mixture of 1% CO and 1% O2 in H2 at 383 K are compared. CO2 was not detectable in case of the ceria (Figure 1), indicating low activity of the support in the PROX reaction at T ) 383 K. Pt/CeO2 oxidized about 44% of the CO under the chosen conditions in the DRIFT cell.6 In the region of OCO vibrations, intense bands of different types of carbonates are present in the spectra of the ceria support. On the Pt/CeO2 surface, formate species were additionally identified by bands of OCO vibrations between 1800 and 1000 cm-1 (Figure 1) and the according CH vibrations. A role of formates in the water-gas-shift and in the PROX reaction has been

J. Phys. Chem. C, Vol. 111, No. 14, 2007 5427 discussed controversially.16-20 The strong band at 2054 cm-1 and the weak band at ca. 1970-1960 cm-1 are assigned to linearly bonded CO (ν(CO)L) on platinum6,21 and to CO adsorbed on the platinum-ceria interface (ν(CO)I).22 Wellresolved and intense ν(OH) bands of hydroxyl groups can be identified for ceria itself, while a broad and intense feature between 3000 and 3800 cm-1 indicates the presence of hydrogen-bonded adsorbed water on Pt/CeO2, as reported earlier.6,10 To follow the evolution of different species and to relate their abundance to the catalytic data, the Kubelka-Munk function was applied, which for most of the considered reflectance values (F > 0.25) should be proportional to the concentration of a given species and thus to the surface coverage. Baselines were subtracted according to the profile of the spectral background, and bands were integrated. The concentration of different species as a function of time is compared to the results of activity measurements in Figure 2. Analysis of the gas-phase CO2 contribution by IR spectroscopy agreed perfectly with the MS data. For CO2, only DRIFTS data are shown, while for CO, MS data will be shown. As the reaction mixture is introduced to the oxygen-pretreated catalyst, the CO level in the effluent stream declines rapidly and reaches a steady state; however, there is no corresponding immediate evolution of CO2 (Figure 2a). Instead, additional carbonates accumulate quickly on the ceria support followed by the emergence of formates (Figure 2b). The coverage of the CO species (θCO) (Figure 2a and Figure 3a) on the metalsupport boundary (1950-1975 cm-1)22,23 develops faster as compared to the coverage of the linear CO species (2054 cm-1). In fact, linear CO species have the highest heat of adsorption among different carbonyls on the Pt surface;24 consequently, it is reasonable to assume that CO favors first terraces and step platinum sites. The time lag in the population of terraces and steps with linearly bound CO under PROX conditions is in strong contrast to the situation in a CO/N2 mixture (Figure 3b), where a strong ν(CO)L band (2063 cm-1) develops immediately upon introduction of the gas mixture. Thus, the delay in the build up of θCOL is unambiguously induced by the presence of O2 and/or H2. Considering the high activity of the platinum metals for hydrogen oxidation,25 we assume that Pt is able to dissociate H2 rapidly at PROX temperatures, with hydrogen then spilling over to the oxidized ceria surface to form OH groups and H2Oads.26 Indeed, H2Oads is readily formed on Pt/CeO2 (Figure 2c), while it is practically absent on pure ceria (Figure 1a), which is in accordance with earlier findings27 showing that H2 does not chemisorb on ceria at temperatures lower than 473 K. The readsorption of the gas-phase water produced on Pt particles could also contribute to a highly hydroxylated state of the ceria surface. The slight “overshoot” of the surface water concentration indicates some over-saturation and subsequent desorption from the surface. CO2 appears with a time delay, only after an appreciable amount of carbonates and formates has been formed on the ceria surface (Figure 2b). The maximum CO2 concentration is reached after several minutes on stream (Figure 2a). These time-resolved data indicate that the buildup of carbonates, formates, and adsorbed water on the ceria surface is favored over the release of CO2 in the effluent gas; CO2 is observed only after the ceria surface is equilibrated with the reactants. This equilibration not only involves the accumulation of the above-mentioned surface species, but a significant modification of the catalyst structure, too. Previously, we clearly demonstrated that CeO2 transforms

5428 J. Phys. Chem. C, Vol. 111, No. 14, 2007

Pozdnyakova-Tellinger et al.

Figure 3. (A) Dynamic changes of in situ DRIFT spectra of carbonyl species as a function of time at T ) 383 K upon admittance of the PROX reaction mixture (1% CO, 1% O2 in H2). (B) Dynamic changes of in situ DRIFT spectra of carbonyl species as a function of time at T ) 383 K upon admittance of CO (1% CO in N2).

Figure 2. (A) Time dependence of the integrated areas of the bands in the range of 2100-1880 cm-1 (carbonyl species) and 2400-2280 cm-1 (gas-phase CO2) and of MS m/z ) 28 signal (gas-phase CO). (B) Time dependence of the integrated areas of the bands in the range of 3000-2675 cm-1 (formate species) and 1150-970 cm-1 (carbonate species). (C) Time dependence of the integrated areas of the bands in the range of 3750-2600 cm-1 (OH and surface water; formate excluded) and 2400-2280 cm-1 (gas-phase CO2).

into an oxygen-deficient structure with ordered vacancies in its bulk (CeO1.695).6,11 On the other hand, the surface, terminated with adsorbed water, is less reduced than one might expect in an excess of H2. High-pressure XPS experiments showed rapid reoxidation of the top surface layer from Ce3+ to a nonperfect, vacancy-rich Ce+4 state when hydrogen was replaced with the PROX reaction mixture.6 The slight broadening of the Ce 3d V hybridization state28 (at 882.4 eV) toward the higher binding energy side indicated that this vacancy structure extended into the terminating layers of the crystal. As a catalyst consisting of Pt on a reducible support (like ceria) is much more active at low temperature (340-360 K) than a combination of Pt with a non-reducible oxide (e.g., Pt/alumina), the support, although itself not active, must play a significant role in the PROX reaction mechanism.5

Figure 4. Dynamic changes of in situ DRIFT spectra of carbonyl species as a function of temperature in the presence of PROX reaction mixture (1% CO, 1% O2 in H2).

Evolution of Surface Species with Increasing Temperature. To unravel the way reducible supports enhance activity in the CO2 production, we applied temperature-programmed DRIFT spectroscopy. As demonstrated previously,5,6 both the selectivity toward CO oxidation and the CO conversion decrease strongly for Pt/CeO2 above the optimum temperature of 340360 K. Between 380 and 420 K, the carbonyl region displays no modification (Figure 4), although the CO2 production is significantly lower (Figure 5a). Above 420 K, the intensity decrease and typical red-shift of ν(CO)L reflect weakened repulsive CO-CO adsorbate interactions due to slightly decreasing CO coverage.29 Furthermore, a continuous increase of the interface CO- ν(CO)I band intensity toward a stationary

CO Oxidation on Pt/CeO2 Catalyst

Figure 5. (A) Temperature dependence of the integrated areas of the bands in the range of 3750-2600 cm-1 (OH/surface water) and 24002280 cm-1 (gas-phase CO2) and of MS m/z ) 28 signal (gas-phase CO). Inset: integrated area of the bands in the range of 3750-2600 cm-1 (OH/surface water) and 2400-2280 cm-1 (gas-phase CO2) as a function of (CO+O2) pressure (in kPa units) in reaction mixture, at constant λ ) 1. (B) Temperature dependence of the integrated areas of the bands in the range of 3000-2675 cm-1 (formate species) and 1150970 cm-1 (carbonate species).

level occurs. An increased contribution of the low-frequency band to the carbonyl envelope might serve as an indicator of an increased ion-dipole interaction between O atoms of the (possibly tilted) CO species with charged cations from the support as dehydroxylation of the ceria takes place with increasing temperature. Kappers et al.23 showed that adsorbed water can preferentially shield this interaction. The amount of carbonates (Figure 5b) first increased slightly and from ∼420 K strongly decreased. On the contrary, the concentration of formates increased linearly up to 490 K (Figure 5b), almost mirror-imaging the CO2 evolution. A remarkably good correlation to the diminishing CO2 production was observed when the spectral region representative of surface water was integrated and plotted versus temperature, together with the CO and CO2 gas-phase concentrations (Figure 5a). Hence, the concentration of hydroxyls/H2Oads correlates positively with the CO2 production. Beside the temperature, the partial pressures of the reactants (CO and O2) also determine the catalytic performance.3,5 In the inset of Figure 5a, the steady-state production of CO2 and the amount of surface water are shown for different reaction mixtures with the same oxygen excess λ ) 1. The catalyst produces more CO2 as the partial pressures of CO and O2 are increased, and this correlates positively with the abundance of

J. Phys. Chem. C, Vol. 111, No. 14, 2007 5429 surface water. Thus, two independent experiments demonstrate that the presence of OH/H2Oads enhances the rate of CO oxidation. Mechanistic Considerations. Most of the effective PROX catalysts contain a reducible additive or a reducible support. The reasons given in the literature can be summarized as follows: under operating conditions, oxygen does not adsorb on metals, either because the surface is fully covered by CO (on Pt at T < 440 K3) or because of the very low sticking coefficient (on Au30). Therefore, oxygen should be activated on different sites: either on the reducible support2,4,5 or, in the case of modified Pt catalysts, on partially oxidized patches, such as Ru(ox),31 Sn(ox),32 Ce(ox),5,12 or Fe(ox).33 According to this model, oxygen diffuses to interface sites and reacts at the perimeter with adsorbed CO. Without rejecting this scenario, the results presented here point to another very important role of the reducible component, facilitation of a state of high hydroxylation and hydration of the surface. H2Oads/OHads, produced after hydrogen activated on the metal spills over to the support, is stabilized on coordinatevely unsaturated Cex+ (oxygen defect sites) and might act as a selective oxidant of CO linearly adsorbed on the metal. Provided water is an oxidizing agent, it has to be activated homolytically to supply active O species in a non-O2- state. The indications that water is strongly involved in the oxidation of CO can be found in the literature. The saturation of a Pt/ Al2O3 sample with water before the PROX reaction resulted in a significantly higher selectivity toward CO oxidation than without this pretreatment.34 For a pure platinum surface, unity bond index quadratic exponential potential (UBIQEP) calculations35 indicated that hydroxyls or adsorbed water are necessary in the PROX as well as in the WGS reaction mechanism to account for a reasonable agreement with experimental data.3,36 Besides the possible direct reaction of surface water with adsorbed CO, the presence of H2Oads could also slow further water production by self-poisoning of H oxidation sites. The desorption of water adsorbed on the support of Pt/ceria (or produced on ceria during PROX) is hindered at lower temperature because oxygen vacancies stabilize the hydrated state, while there is certainly no limitation for the desorption of CO237 from the surface of platinum immediately after formation. Taking into account that in most of the experiments presented here oxygen conversion was high (ca. 70-80%; higher conversion is limited by the cell design), the observed positive correlation between CO2 production and surface water concentration can be rationalized as follows. With increasing temperature, the rate of surface water desorption increases and sites active for H2 oxidation are liberated; as a consequence, more H2 and less CO can be oxidized, with the oxygen conversion remaining almost unchanged at a high level. Hence, the declining selectivity and CO2 production as a function of temperature could be explained by the widely different desorption energies of the main products of the competing reactions. Moreover, the density of oxygen vacancies decreases at higher temperature as revealed by our recent high-pressure XPS experiment,6 thus facilitating water desorption. At increasing CO and O2 partial pressures in the feed, both CO2 and H2O production are enhanced, however, the CO2 production more so. Because of the equilibrium between surface and gas-phase H2O, higher concentrations will lead to a higher surface coverage until saturation is reached. Hence, variation of the CO and O2 content in the feed results in concurrent trends for CO2 production and water coverage, but this parallel behavior is not automatically synonymous to a correlated chemistry.

5430 J. Phys. Chem. C, Vol. 111, No. 14, 2007 Similarly to the proposed “formate mechanism” of the lowtemperature water-gas-shift (LT-WGS) reaction over Pt/ceria catalysts,16-20 a (negative) correlation between the concentration of the surface formates and CO2 production could imply the involvement of formate species in the preferential CO oxidation. According to this mechanism, the active site is the bridging OH group (Type IIB) on partially reduced ceria, which reacts with CO to surface formates. The rate-limiting step is assumed to be the water-assisted decomposition of the formate to H2 and unidentate carbonate. The carbonate decomposes to CO2 while regenerating the OH group as active site. This model may also be adapted to our experimental data: a decreased H2Oads/OHads concentration (Figure 5a) would decelerate the decomposition of formates (Figure 5b), leading to increased formate and decreased carbonate levels and thus lower CO2 production. (Note the not systematically decaying carbonate curve could be caused by the overlap of different types of carbonates in the region of integration.) The optimal reaction temperature of PROX on Pt/ CeO2 is, however, significantly lower than the temperature window of LT-WGS (340-360 K vs 423-573 K,16,20,38 respectively); hence, the two reaction mechanisms can be rather different. As an effective catalyst in its as-synthesized form is meta-stable and will be modified by the interaction with the reactant feed, it is not surprising that the surface of ceria is considerably different under PROX and WGS conditions. While under PROX reaction ceria is generally in the Ce4+ state6 with some oxygen vacancies, it is clearly Ce3+ under WGS conditions.39 Obviously, the presence of oxygen enables the reoxidation of cerium oxide to a certain degree, even in the high excess of reducing gases, and thus can control the relative population of different species on its surface. On the other hand, in the WGS reaction, the reduction power of CO seems to be more effective than the oxidizing potential of water. Water adsorption was shown to strongly depend on the redox state of ceria. A perfect CeO2 surface will not be hydrated,40 whereas a slight surface reduction (oxygen vacancy formation) facilitates strong hydroxylation and hydration.6 On the other hand, on a surface entirely reduced to Ce3+, water preferentially dissociates (filling the vacancies), and its mobility is thus severely reduced.41-43 Therefore, a major difference between WGS and PROX reaction lies in the interaction of water with the surface, which depends on the redox state of ceria and hence on the overall redox potential of the gas mixture. As no significant role of conventional supports (like alumina) was proposed (as opposed to ceria),5 there is a good reason to believe that the surface carbonyls are oxidized, and the presence of formates only marks a state of the ceria surface that is not beneficial for the reaction. Therefore, we consider that formates are not directly involved in the PROX reaction. The negative correlation of their concentration with the CO oxidizing activity as a function of temperature may result from the fact that water desorbs and OH groups necessary for formate formation are liberated. The key feature in the PROX mechanism appears to be the linearly bound CO near the Pt/ceria interface in a close interaction with the hydrated ceria, which supplies an oxidizing agent to the reaction. Whether or not activated H2Oads/OHads species are directly involved in the PROX process is still unclear. However, it seems to be comprehensible from the present and previous results that activated surface water on oxygen-deficient ceria helps to produce CO2 and improve selectivity. Conditioning of an Effective PROX Catalyst with a Reducible Component. A material that activates CO and oxygen but does not adsorb hydrogen would represent the ideal

Pozdnyakova-Tellinger et al. solution for the PROX process. Unfortunately, this is almost never the case. By using in-situ spectroscopic methodology on a catalyst with a reducible support, we demonstrated that enabling a pathway for the undesired product water to be retained on the surface or to be consumed by a reaction with adsorbed CO to reproduce H2 and form CO2 can be effective in the PROX process. The directions for suitable reaction conditions can be formulated as follows: (1) Initially, hydrogen activation should be enabled to create oxygen vacancies on the support, which will stabilize the produced water on the surface. (2) During steady operation, hydrogen activation should be suppressed relative to CO activation to decrease the undesired hydrogen oxidation, that is, to maintain high selectivity. (3) Water should be retained to block Hads oxidation sites and to ensure a constant water supply at the Pt-support interface. This can be achieved through proper adjustment of the surface oxidation state by creating or maintaining vacancy sites. (4) Conditions favoring formate accumulation should be avoided as they appear to be characteristic of a surface state with poor PROX performance. (5) The classical reverse water-gas-shift (RWGS) reaction should be suppressed. Conclusion By using in-situ FTIR spectroscopy with online mass spectrometric analysis, we could verify the important role of surface water in the preferential oxidation of CO on a Pt/ceria catalyst. The positive correlation between CO2 production and surface water concentration suggests that either the hydrated ceria surface supplies an oxidizing agent to the Pt/ceria interface to convert linear carbonyls or the presence of surface water suppresses hydrogen oxidation, thus shifting the PROX process in the desired direction. Retention of water on the surface is facilitated by oxygen vacancies in ceria, indicating the possibility to fine-tune the properties of ceria by introducing structural promoters to stabilize vacancy sites. Acknowledgment. Financial support from the Hungarian National Science Foundation (Grant OTKA F046216), the Athena Consortium, the cooperation between the Fritz Haber Institute and the Institute of Isotopes, and the Hungarian Academy of Sciences (a Bolyai Grant to A.W.) is gratefully acknowledged. References and Notes (1) (a) Oh, S. H.; Sinkevitch, R. M. J. Catal. 1993, 142, 254. (b) Dudfield, C. D.; Chen, R.; Adock, P. L. Int. J. Hydrogen Energy 2001, 26, 763. (c) Lee, S. H.; Han, J.; Lee, K.-Y. J. Power Sources 2002, 109, 394. (2) Kahlich, M. J.; Gasteiger, H.; Behm, R. J. J. Catal. 1999, 182, 430. (3) Kahlich, M. J.; Gasteiger, H. A.; Behm, R. J. J. Catal. 1997, 171, 93. (4) Avgouropoulos, G.; Ioannides, T.; Papadopoulou, Ch.; Batita, J.; Hocevar, S.; Martalis, H. K. Catal. Today 2002, 75, 157. (5) Wootsch, A.; Descorme, C.; Duprez, D. J. Catal. 2004, 225, 259. (6) Pozdnyakova, O.; Teschner, D.; Wootsch, A.; Kro¨hnert, J.; Steinhauer, B.; Sauer, H.; Toth, L.; Jentoft, F. C.; Knop-Gericke, A.; Paa´l, Z.; Schlo¨gl, R. J. Catal. 2006, 237, 1. (7) (a) Appleby, A. J.; Foulkes, F. R. Fuel Cell Handbook; Van Nostrand Reinhold: New York, 1989. (b) Lemons, R. A. J. Power Sources 1990, 29, 251. (8) Armor, J. N. Appl. Catal. 1999, 176, 159. (9) Aupretre, F.; Descorme, C.; Duprez, D. Catal. Commun. 2002, 3, 263. (10) Pozdnyakova, O.; Teschner, D.; Wootsch, A.; Kro¨hnert, J.; Steinhauer, B.; Sauer, H.; Toth, L.; Jentoft, F. C.; Knop-Gericke, A.; Paa´l, Z.; Schlo¨gl, R. J. Catal. 2006, 237, 17. (11) Teschner, D.; Wootsch, A.; Pozdnyakova, O.; Sauer, H.; KnopGericke, A.; Schlo¨gl, R. React. Kinet. Catal. Lett. 2006, 87, 235.

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