DFT Study on CO Oxidation Catalyzed by PtmAun (m + n = 4) Clusters

Aug 3, 2010 - By performing density functional theory calculations, we show the mechanism details of CO oxidation catalyzed by several PtmAun (m + n =...
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DFT Study on CO Oxidation Catalyzed by PtmAun (m + n ) 4) Clusters: Catalytic Mechanism, Active Component, and the Configuration of Ideal Catalysts Fang Wang, Dongju Zhang,* and Yi Ding* Key Lab of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan, 250100, P. R. China ReceiVed: February 17, 2010; ReVised Manuscript ReceiVed: July 25, 2010

By performing density functional theory calculations, we show the mechanism details of CO oxidation catalyzed by several PtmAun (m + n ) 4) clusters. It is found that in all situations, the reaction prefers to proceed via the single-center pathway to the two-center pathway according to a two-step mechanism: the initial activation of O2 molecule to form a peroxide-like intermediate followed by the rupture of the peroxide bond to complete the reaction. In Pt-Au bimetallic clusters, Pt sites are the catalytically active centers, whereas Au sites are “formally spectators” for CO oxidation. The calculated barriers for the reactions mediated by bimetallic clusters Pt3Au, Pt2Au2, PtAu3, are comparable with that catalyzed by monometallic Pt4 cluster, implying that the catalytic activity of Pt centers in the bimetallic clusters seems not to be dependent on its surroundings. On the basis of the present results, we propose an ideal configuration of Pt-Au bimetallic catalysts, where each active Pt atom is suitably spaced (stabilized) by Au atoms. Such catalysts are “less expensive and more efficient” compared to the corresponding pure Pt catalysts for CO oxidation at room temperature. 1. Introduction The preferential oxidation (PROX) of CO is an important step to purify H2 in fuel reforming. During the past several decades, various noble metal-based catalysts have been developed for the PROX reaction of CO.1-3 Among them, platinumbased catalysts are the most frequently used catalyst materials due to their high CO conversion and minimal H2 loss at high temperature.4,5 However, there are two challenging issues we must face for platinum-based catalyst materials: (i) they are very prone to be poisoned by trace amounts of CO, and (ii) their supply is currently limited owing to the scarcity and high costs of platinum. A problem-solving strategy is making use of platinum alloy catalysts. It is highly desirable to find the suitable catalysts that are less expensive and more efficient for the PROX of CO. To this end, much effort has been made and great progress has been achieved in recent years. It is found that the platinum catalysts alloyed or modified by a range of transition metals, such as Ru, Mo, and Pd, meets this aim.6-8 Recently, Pt-Au bimetallic catalysts, promising alternatives to exceedingly scarce and expensive pure Pt catalysts,9 have attracted special interest owing to their unique activity and selectivity toward low-temperature CO oxidation.10-12 With the technical development of nanotechnology, various new methods are now available to economically prepare highly efficient Pt-Au catalysts, including vapor13 and electrochemical14,15 depositions, and the laser vaporization/controlled condensation (LVCC) technique.16 These methods uses commercial metallic Pt and Au as targets and do not need any expensive precursors or solvents, so they provide economic, simple, and effective synthetic routes for preparing Pt-Au bimetallic catalysts. Several groups consistently observed the higher catalytic activity of Pt-Au bimetallic nanoparticles compared to the corresponding monometallic catalysts; and different explanations have been * To whom correspondence should be addressed. E-mail: zhangdj@ sdu.edu.cn (D. Z.), [email protected]. (Y. D.). Phone: +86-531-88365833. Fax: +86-531-88564464.

offered in the literature. For example, Auten et al.17 proposed that the higher catalytic activity of Pt-Au bimetallic catalysts likely arises from the synergy of Pt and Au, that is, the catalytically active site likely involves a bimetallic Pt-Au ensemble. In contrast, Ortiz-Soto et al.11 emphasized that the support may play a crucial role for the improved catalytic activity of Pt-Au catalysts toward low-temperature CO oxidation. These discrepant explanations indicate that our understanding for the catalytic mechanism of Pt-Au bimetallic catalysts is still in its initial stage. Some fundamental issues, such as how the structure, composition, and size of the bimetallic catalysts influence their catalytic activity, remain unclear. In particular, to obtain a “less expensive and more efficient” Pt-Au catalyst for CO oxidation, it is important to know what the desired structure and composition of the bimetallic catalysts are, which can provide a clue to experimenters for improving the catalyst activity. In recent years, density functional theory (DFT) has become a valuable tool for studying the properties of molecules and materials18,19 and for identifying reaction mechanisms.20,21 Herein, employing DFT calculations, we try to address the important issues concerned above via comparing the reactivity of several bimetallic PtmAun (m + n ) 4, and m, n ) 1-3) clusters to monometallic Pt4 and Au4 clusters toward CO oxidation. These smallest bimetallic clusters represent the simplest prototypes of Pt-Au bimetallic catalysts with different compositions. It is well-known that gas-phase clusters are valuable model systems for investigating the reaction taking place on catalytic surfaces.22 So we expect that the conclusion drawn out from the present study will provide assistance to some extent for understanding the catalytic mechanisms of Pt-Au bimetallic catalysts involved in the realistic and complicated catalytic systems. In addition, it is known that the theoretical studies are also of crucial importance in guiding relevant experiments, as verified in many occasions. For example, a new ruthenium core-platinum shell nanoparticle catalyst with low light-off temperatures for

10.1021/jp101470c  2010 American Chemical Society Published on Web 08/03/2010

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both CO and H2 has been synthesized recently by Alayoglu et al.23 under the direction of the first principles. In the present work, based on the theoretical results, we also proposed an ideal configuration of Pt-Au bimetallic catalysts, expecting to show a useful clue for the future synthesis of “less expensive and more efficient” Pt-based catalysts. 2. Computational Details The calculations are carried out in the framework of DFT with the generalized gradient approximation (GGA) implemented in Gaussian 03 program.24 Perdew-Wang’s 91 exchange and correlation functional (commonly referred to as PW91)25-27 was chosen to describe the reaction systems. Considering the strong relativistic effect of Au and Pt, the Los Alamos LANL2DZ effective core potentials (ECP) and valence double-ζ basis sets28,29 were used for the heavy Au and Pt atoms. The standard 6-311+G(d) basis set, which includes polarization and diffuse functions, was used for oxygen and carbon. No symmetric constraints were imposed during geometrical optimizations. The synchronous transit-guided quasi-Newton method30 was adopted for locating the transition states. The nature (minima or first-order saddle points) of optimized structures is identified by the subsequent frequency calculations that also provide zero-point vibrational energy (ZPE) corrections. Intrinsic reaction coordinates (IRC)31 calculations have been performed to verify that each saddle point links two desired minima. Stability tests of wave functions for all identified stationary points have been carried out to ensure that the lowest energy solutions in the SCF procedures are found. The accuracy and reliability of the chosen functional and ECP for describing Pt-Au bimetallic clusters and the catalytic CO oxidation have been confirmed in our previous studies.32,33 3. Results and Discussion 3.1. O2 and CO Binding on PtmAun (m + n ) 4) Clusters. The bindings of O2 and CO molecules on catalysts are considered as a necessary step for the sequent CO oxidation. We first study their adsorption behavior on PtmAun (m + n ) 4) clusters. After the isolated clusters and molecules are optimized, we attached the molecules on the clusters to build initial geometries of cluster-molecule complexes. Various possible binding sites of the molecules on the clusters and the different relative orientations between the cluster and molecule have been taken into account for initially designed geometries. The most stable geometries optimized at the chosen level of theory are shown in Figure 1. We first consider the binding of an O2 molecule on clusters. As can be seen in Figure 1, the calculated binding energies (Eb) of the O2 molecule on Pt sites of bimetallic clusters Pt3Au, Pt2Au2, and PtAu3 are 32.22, 28.33, and 27.71 kcal mol-1, which are comparable with that on monometallic Pt4 cluster, 31.60 kcal mol-1. In contrast, the corresponding Eb values on Au sites of bimetallic clusters are 13.83, 14.25, and 12.14 kcal mol-1, which are slightly larger than that on monometallic Au4 cluster, 8.10 kcal mol-1. From these Eb values, it is clear that the O2 molecule prefers to bind on Pt sites to Au sites of bimetallic clusters. Similarly, for the CO binding on bimetallic clusters, calculated Eb values of CO on Pt sites in Pt3Au, Pt2Au2, and PtAu3 are 64.12, 65.18, and 64.38 kcal mol-1, which are in contrast to the corresponding those on Au sites, 41.94, 40.72, and 40.29 kcal mol-1. These values are slightly larger than the corresponding those on monometallic Pt4 and Au4 clusters, 61.32 and 36.75 kcal mol-1, respectively. Much larger Eb values of

Figure 1. Optimized structures for the complexes between PtmAun (m + n ) 4) and O2 or CO molecule with selected geometrical parameters (in Å). The symbols and values in square brackets denote the electronic states and the binding energies (in kcal mol-1), respectively. The blue, golden, gray, and red balls denote Pt, Au, C, and O atoms, respectively.

Figure 2. The HOMO and LUMO isosurfaces for Pt3Au, Pt2Au2, and PtAu3 clusters.

CO on Pt sites than those on Au sites demonstrate that CO also prefers to bind on Pt sites when both Pt and Au sites are available. These results can be understood by analyzing the frontier molecular orbitals of bimetallic Pt-Au clusters. Figure 2 shows that both the highest occupied molecule orbitals (HOMOs) and the lowest unoccupied molecule orbitals (LUMOs) of the bimetallic clusters mostly concentrate on Pt sites, implying these positions are the active centers whatever the coming guest molecules are electronic-deficient or electronic-rich. This well

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Figure 3. Potential energy surfaces for CO oxidation promoted by Pt4 along path IPt (red line) and path IIPt (black line). The corresponding intermediates and transition states related to the two pathways are also presented. The sum of energies of free Pt4, O2, and CO is taken as the zero-point energy, which is in kcal mol-1. The blue, gray, and red balls denote Pt, C, and O atoms, respectively.

explains the stronger binding of CO and O2 on Pt sites in bimetallic clusters. Although both O2 and CO molecules prefer to bind on Pt sites of bimetallic clusters, it is reasonable to draw out the conclusion that the active sites in bimetallic clusters would be first occupied by the coming CO rather than O2, in view of its much larger Eb values than those of O2. 3.2. CO Oxidation Promoted by PtmAun (m + n ) 4) Clusters. Generally, the most accepted explanation for the improved catalytic activity of a bimetallic catalyst, as compared to monometallic catalysts, is based on the bifunctional mechanism34,35 and/or electronic structure effects.36 According to the bifunctional mechanism, for example, we can understand the superior activity of Pt-Ru catalysts toward methanol oxidation, where the dehydrogenation process occurs on Pt site, and the Ru site provides oxygenated species, which is necessary to oxidize the residual CO-like products, that is, each metal in the bimetallic catalyst play a separate role for the reaction. Does the bifunctional mechanism also apply to the CO oxidation promoted by Pt-Au bimetallic catalysts? Our primary studies indicate that the CO oxidation prefers to proceed at Pt sites, and Au is formally a spectator. In other words, the bifunctional mechanism seems to fail to explain the improved catalytic activity of Pt-Au bimetallic catalysts in comparison with the corresponding monometallic catalysts. In this case, how do we explain the improved activity of Pt-Au catalysts? Furthermore, what is the effect of the composition of Au on the activity of catalysts? To address these concerns, we studied the detailed catalytic mechanisms of three bimetallic clusters Pt3Au, Pt2Au2, and PtAu3 toward CO oxidation, and compared their catalytic activity with monometallic Pt4 and Au4 clusters. For all these five clusters, we considered two possible catalytic mechanisms: single-center mechanism (denoted as path I) and two-center mechanism (denoted as path II). In the former, the adsorption of CO and O2 as well as the sequent CO oxidation occur on the single Pt site. This path was inspired of the stronger adsorption

of O2 and CO on Pt sites in Pt-Au bimetallic clusters as described in Section 3.1. In the latter, the reactions occur at two metal centers, which is designed to confirm whether the bifunctional mechanism work or not. In the following sections, these two pathways will be denoted as IPt and IIPt for Pt4 mediated reaction, IAu and IIAu for Au4 promoted reaction (path IAu can not be identified in the present work), IPt3Au and IIPt3Au for Pt3Au mediated reaction, IPt2Au2and IIPt2Au2for Pt2Au2 promoted reaction, IPtAu3and IIPtAu3for PtAu3 promoted reaction, respectively. Figures 3-7 show calculated potential energy surface profiles along two pathways, where all of the geometries of minima and transition states along each path have also been presented to understand the reaction process clearly. 3.2.1. Pt4- and Au4-Mediated CO Oxidation. To compare the reactivity of bimetallic Pt-Au clusters with corresponding monometallic clusters, we first consider Pt4- and Au4-mediated CO oxidations along both the single-center and two-center pathways. As shown in Figure 3 for Pt4-mediated reaction, along path IPt, CO and O2 initially coadsorbed on the single Pt atom to form intermediate IM1, which lies below the entrance by 74.61 kcal mol-1. This intermediate is converted into IM2 via transition state TS1-2 with a barrier of 19.79 kcal mol-1. In TS1-2, the O2 molecule is being activated, as indicated by the calculated geometrical parameters and the transition vector corresponding to the imaginary frequency of 409i cm-1. IM2 is a peroxide-like species, where the O-O distance has been elongated to 1.526 Å. Along the reaction coordinate, IM2 further evolves into IM3, a complex of Pt4O with CO2, which lies below the entrance by 114.95 kcal mol-1, and serves as the precursor forming CO2. The saddle point connecting IM2 and IM3 is TS2-3 with an imaginary frequency of 389i cm-1. The barrier to be surmounted from IM2 to TS2-3 is 4.91 kcal mol-1. The direct dissociation of IM3 to Pt4O and CO2, indicating the accomplishment of the reaction, requires an energy of 11.26 kcal mol-1. And the overall reaction is calculated to be exothermic by 106.21 kcal mol-1.

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Figure 4. Potential energy surface for CO oxidation promoted by Au4 along path IIAu. The corresponding intermediates and transition states related to the pathway are also presented. The sum of energies of free Au4, O2 and CO is taken as the zero point energy, which is in kcal mol-1. The golden, gray, and red balls denote Au, C, and O atoms, respectively.

As far as path IIPt is concerned, the reaction starts with the formation of IM4, where CO and O2 adsorbed on separate Pt atoms. As can be seen in Figure 3, the two-center pathway also involves two elementary steps similar to those of the single center pathway: the initial activation of O2 molecule to form the peroxide-like species IM5 followed by the rupture of the peroxide bond to result in the product-like intermediate IM3. Transition states involved in these two processes are TS4-5 and TS5-3, respectively. It should be noted that TS2-3 and TS5-3 connect the same product-like intermediate IM3. It is worthy to note that the potential energy surface profiles along pathways IPt and IIPt have common features: the overall reaction is highly exothermal, and the initial activation of O2 is the rate-determining step. However, the calculated energy barrier for the rate-determining step along pathway IPt is lower by ∼10 kcal mol-1 than that along pathway IIPt, implying that the CO oxidation catalyzed by monometallic Pt catalysts may prefer the single-center pathway to the two-center pathway. For the Au4-mediated reaction, we also try to search for both the single- and two-center pathways, IAu and IIAu. However, as shown in Figure 4, only pathway IIAu is located from the present calculations. It should be in mind that along path IIAu there is a surface crossing from triplet to singlet before the formation of TS24-25 owing to the strong spin-orbit interactions. By comparing Figure 4 with Figure 3, we find that the barriers for the Au4-mediated reaction are comparable than those for the Pt4-mediated reaction, however reaction profiles for the former lie above those for the latter. These facts explain unique catalytic activity of Au nanoparticles for CO oxidation at low temperature and the easy poisoning of Pt catalysts resulting from the excess adsorption of CO on Pt. However, it is believed that the CO oxidation at room temperature on Au may be less effective than that on Pt, owing to the weak adsorption (low surface coverage) of CO on Au catalysts. So, Pt-based catalysts toward realistic application of CO oxidation at room temperature are still highly desired. 3.2.2. CO Oxidation Promoted by Bimetallic Clusters. Auten et al.17 employing in situ FT-IR investigation have found that CO adsorbed on both Pt sites and Au sites of Pt-Au bimetallic catalysts, however CO desorbed from Au sites at higher temperature owing to its weak interaction with Au. Thus, they inferred that Au sites possibly did not take part in the catalysis. Our calculations have also demonstrated that the interaction of

CO with Pt in bimetallic Pt-Au clusters is stronger than that with Au (see Figure 1). On the basis of these facts, it seems reasonable to suppose that both the single-center pathway and two-center pathway mediated by the bimetallic clusters start from the CO adsorption on Pt site. Figures 5-7 show the calculated PES profiles with the optimized geometries of the stationary points for CO oxidation promoted by three bimetallic clusters, Pt3Au, Pt2Au2, and PtAu3. We find that the geometrical parameters for the stationary points located along the reaction coordinates and the mechanistic details regarding the CO oxidation are very similar to those discussed above in Figures 3-4. For simplification, we just summarize the main conclusions drawn out from present calculations. In all three situations along both the single-center pathway and two-center pathway, the reactions proceed according to a two-step mechanism: the first step, the rate-determining step, involves the formation of peroxide-like intermediates, and the second step relates the rupture of the peroxide bond to complete the reactions. The calculated barriers of the rate-determining steps along the single-center pathways are 20.55, 18.85, and 19.17 kcal mol-1 for Pt3Au-, Pt2Au2-, and PtAu3-mediated reactions, while those corresponding to the two-center pathways are 28.90, 33.05, and 34.85 kcal mol-1, respectively. Clearly, the single-center mechanism is more favorable than the twocenter mechanism. This seems to imply that the general bifunctional mechanism proposed in literature for a bimetallic catalyst does not apply to the present systems. As shown in Figures 5-7, according to the single-center mechanism, Au atoms are “formally spectators”, implying that the Au constituent in PtmAun may influence the activity of the bimetallic catalyst in an indirect way. An important experimental work by Zhang et al.37 supports our statement of the “formally spectator” for Au in Pt-Au bimetallic catalysts. They found that the Au in Pt-Au bimetallic catalysts does not acts as a catalytic active center, and its crucial role is to stabilize Pt by enhancing the oxidation potential of Pt, thus making the Pt catalysts retain continuous catalytic activity. Comparing the barrier of the rate-determining step along single-center pathway for Pt4-mediated reaction with corresponding those mediated by bimetallic-clusters, it is seen that with the decrease of Pt composition in the clusters, no remarkable change is observed for the barrier of the ratedetermining step. For example, the barrier is 19.79 kcal mol-1

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Figure 5. Potential energy surfaces for CO oxidation promoted by Pt3Au along path IPt3Au (red line) and path IIPt3Au (black line). The corresponding intermediates and transition states related to the two pathways are also presented. The sum of energies of free Pt3Au, O2, and CO is taken as the zero-point energy, which is in kcal mol-1. The blue, golden, gray, and red balls denote Pt, Au, C, and O atoms, respectively.

Figure 6. Potential energy surfaces for CO oxidation promoted by Pt2Au2 along path IPt2Au2 (red line) and path IIPt2Au2 (black line). The corresponding intermediates and transition states related to the two pathways are also presented. The sum of energies of free Pt2Au2, O2, and CO is taken as the zero-point energy, which is in kcal mol-1. The blue, golden, gray, and red balls denote Pt, Au, C, and O atoms, respectively.

in Pt4-invloved reaction, whereas it is 19.17 kcal mol-1 in the PtAu3-involved reaction. This fact implies that the catalytic activity of Pt center is almost not related to its surroundings. In other words, it will not make much difference for the catalytic activity of Pt center whether it connects to Pt atoms or Au atoms. Thus, we can imagine an ideal configuration of Pt-Au bimetallic catalysts for room-temperature CO oxidation, as schematically shown in Figure 8, where several Pt-Au nanostructures with different shapes show that each single Pt atom serving as a catalytic activity center is surrounded by Au atoms that support

and effectively stabilize the Pt centers. Such catalysts are obviously less expensive than pure Pt catalysts since the Pt concentration in it is significantly reduced. More importantly, such a catalyst is more efficient for CO oxidation than the pure Pt catalysts, as observed in recent experiments.10-13 This is because that replacing the Pt atoms surrounding the active center with Au atoms will decrease the coverage of CO on catalyst surfaces and leave room for incoming O2 molecules, thus reducing the catalyst poisoning and hence making CO oxidation more effective. In such a desirable catalyst, all Pt sites act as

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Figure 7. Potential energy surfaces for CO oxidation promoted by PtAu3 along path IPtAu3 (red line) and path IIPtAu3 (black line). The corresponding intermediates and transition states related to the two pathways are also presented. The sum of energies of free PtAu3, O2, and CO is taken as the zero-point energy, which is in kcal mol-1. The blue, golden, gray, and red balls denote Pt, Au, C, and O atoms, respectively.

Figure 8. Schematic drawings of the ideal models for Pt-Au bimetallic catalysts. The blue and golden balls denote Pt and Au atoms, respectively.

the catalytically active centers, unlike pure Pt catalysts where part even most of Pt sites are occupied by CO, O2 molecules are blocked, and so the reaction will be stopped after some time. From our calculated results, Au atoms in Pt-Au bimetallic clusters do not participate in the reaction directly, so we conjecture that their primary roles are to avoid excess adsorption of CO around the catalytically active centers and to stabilize these active centers. It is worthy to note that the ideal catalyst has an experimental basis, as reported by Goodman et al.38 recently. By comparing the catalytic activity of Pd/Au(100) with that of Pd/Au(111) for the vinyl acetate reaction, Goodman et al. has shown that the critical reaction site consists of two noncontiguous, suitably spaced Pd monomers, and the role of Au is to isolate single Pd sites. The improved catalytic activity of Pt-Au catalysts may have similar origin with Goodman’s Pd-Au alloy catalysts. It should be noted that a very recent investigation33 has indicated that the Pt-core-Au-shell configurations are the energetically most stable structures of Pt-Au bimetallic clusters, which seems to be discrepant with the ideal catalyst models shown in Figure 8. However, it is well-known that nanosized materials exhibit diversity in structures, where diverse metastable structures generally coexist with ground state structures. In particular, with the development of various experimental techniques in nanoscience and nanotechnology, the synthesis

of composite nanomaterials with strictly controlled size, shape, and composition is now easily achieved. So we feel that there is a real possibility for synthesizing the “less expensive and more efficient” Pt-Au bimetallic catalysts. Such catalysts are also expected to show high activity toward other important catalytic reactions commonly promoted by Pt-based catalysts, such as methanol oxidation,39 formic acid oxidation,40 and oxygen reduction.37 Once the synthetic challenge is mastered successfully, the ideal catalyst is expected to find extensive applications in the fields of heterogeneous catalysis and surface science. As mentioned in the introduction, we pay special attention for purifying H2 in fuel reforming using Pt-based catalysts. Recent experiments show that (i) at a large temperature range the adsorption of H2 on Pt-Au catalysts is much weaker than that on pure Pt catalysts;41 (ii) Pt-Au catalysts have higher selectivity toward CO oxidation than pure Pt catalysts;42 and (iii) the CO oxidation over Pt-Au catalysts is generally preferred at ∼90 °C,42 whereas H2 oxidation favors at low temperature.43 From these experimental findings, we conjecture that Pt-Au clusters may be a promising catalyst to remove CO from H2 fuels. However, the most actual concern for purifying H2 in fuel reforming may be the issue about the loss of H2 during the remove of CO. So it is sure to be valuable for further study

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about H2 oxidation over Pt-Au clusters. Nevertheless, this is beyond the scope of this work and will be pursued in our future work. 4. Conclusion In summary, we have presented a theoretical exploration for the reactivity of PtmAun (m + n ) 4) clusters toward CO oxidation, aiming at understanding the improved catalytic activity of Pt-Au bimetallic catalysts. In all situations, the reaction proceeds according to the single-center mechanism except the Au4-involved reaction. The Pt sites in the bimetallic clusters are the active centers, while Au sites are “formally spectators” and their role is to avoid the excess adsorption of CO around the active centers and to stabilize the Pt atoms. The activity of Pt active centers in the bimetallic clusters seems not to be dependent on its surroundings. Based on the calculated results, we show a picture of the ideal “less expensive and more effective” Pt-Au catalyst for CO oxidation, where Pt atoms (active center) are suitably spaced (stabilized) by Au atoms, which more weakly adsorb CO than Pt atoms and thus leave room for the coming O2 necessary to CO oxidation. Acknowledgment. This work was sponsored by the National Science Foundation of China (20873076, 20773078), the National 863 (2006AA03Z222), and the 973 Program Projects of China (2007CB936602). Y. D. is a Tai-Shan Scholar supported by the State Education Ministry’s NCET Program (NCET-06-0580). References and Notes (1) Oh, S. H.; Sinkevitch, R. M. J. Catal. 1993, 142, 254–262. (2) Rosso, I.; Galetti, C.; Saracco, G.; Garrone, E.; Specchia, V. Appl. Catal. B: EnViron. 2004, 48, 195–203. (3) Kahlich, M. J.; Gasteiger, H. A.; Behm, R. J. J. Catal. 1997, 171, 93–105. (4) Marin˜o, F.; Descorme, C.; Duprez, D. Appl. Catal. B: EnViron. 2004, 54, 59–66. (5) Suh, D. J.; Kwak, C.; Kim, J. H.; Kwon, S. M.; Park, T. J. J. Power Sources 2005, 142, 70–74. (6) Samjeske´, G.; Wang, H.; Lo¨ffler, T.; Baltruschat, H. Electrochim. Acta 2002, 47, 3681–3692. (7) Brankovic, S. R.; Wang, J. X.; Zhu, Y.; Sabatini, R.; McBreen, J.; Adzˇic, R. R. J. Electroanal. Chem. 2002, 231, 524–525. (8) Todoroki, N.; Osano, H.; Maeyama, T.; Yoshida, H.; Wadayama, T. Appl. Surf. Sci. 2009, 256, 943–947. (9) Mirdamadi-Esfahani, M.; Mostafavi, M.; Keita, B.; Nadjo, L.; Kooyman, P.; Remita, H. Gold Bull. 2010, 43, 49–56. (10) Wang, Y. H.; Zhu, J. L.; Zhang, J. C.; Song, L. F.; Hu, J. Y.; Ong, S. L.; Ng, W. J. J. Power Sources 2006, 155, 440–446. (11) Ortiz-Soto, L. B.; Alexeev, O. S.; Amiridis, M. D. Langmuir 2006, 22, 3112–3117. (12) Skelton, D. C.; Tobin, R. G.; Lambert, D. K.; DiMaggio, C. L.; Fisher, G. B. J. Phys. Chem. B 1999, 103, 964–971. (13) Park, J. B.; Conner, S. F.; Chen, D. A. J. Phys. Chem. C 2008, 112, 5490–5500. (14) Wang, R.; Wang, C.; Cai, W. B.; Ding, Y. AdV. Mater. 2010, 22, 1845–1848. (15) Ge, X.; Yan, X.; Wang, R.; Tian, F.; Ding, Y. J. Phys. Chem. C 2009, 113, 7379–7384.

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