Structures and Energetics of Pt Clusters on TiO2: Interplay between

Article pubs.acs.org/JPCC

Structures and Energetics of Pt Clusters on TiO2: Interplay between Metal−Metal Bonds and Metal−Oxygen Bonds De-en Jiang,*,† Steven H. Overbury,†,‡ and Sheng Dai†,‡,§ †

Chemical Sciences Division and ‡Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States S Supporting Information *

ABSTRACT: Depositing size-selected nanoclusters on a welldefined support surface provides a way to probe the metal− support interaction and the size dependence of the catalytic activity; however, the detailed structural information at such interface is often missing. Here we examine from density functional theory the interfacial structure of Pt4 to Pt8 clusters on rutile TiO2(110). We find that Pt4 prefers a flat, nearly square structure on TiO2(110), while larger clusters such as Pt5, Pt6, Pt7, and Pt8 have a two-layer structure with the top layer not interacting with the support directly. The interaction strength generally increases with the contact area between Ptn and TiO2(110). The interfacial structure is a result of optimizing the Pt−Pt, Pt−O, and Pt−Ti interactions: Pt4 prefers the square planar configuration on TiO2(110) with more Pt−Ti interaction over a two-layer, bi-triangle configuration of more Pt−Pt bonds; Pt8 prefers a hut-like two-layer structure over an edge-sharing bi-pyramid structure of greater internal strain. Our findings will be useful for understanding the interface of size-selected clusters on a typical reducible support such as TiO2 and its catalytic activity for reactions such as CO oxidation.

1. INTRODUCTION

Depositing size-selected metal clusters onto a well-defined support surface provides a desirable way to control the cluster size and to investigate the size effect and the metal−support interaction. Tremendous progress has been made in this direction. Here we discuss several typical examples. Yoon et al. deposited mass-selected Au8 on MgO(001) and investigated its CO-oxidation activity; they found that charging of the supported Au8 cluster is important to its catalytic activity.12 Lee et al. soft-landed Au clusters up to Au7 on rutile TiO2(110) and found that Au7/TiO2(110) is over 50 times more active for CO oxidation than smaller clusters, such as Au2/TiO2(110).13 Tong et al.14 deposited size-selected Aun (n = 1−8) on TiO2(110) and concluded that the 2D-to-3D transition happens at n = 5. Kaden et al. found that electronic structure controls CO oxidation activity of size-selected Pd clusters (up to Pd25) on rutile TiO2(110)15 and that the efficiency of oxygen activation is also size-dependent.16 Vajda et al. deposited sizeselected Pt8−10 clusters on a porous aluminum oxide support and found that they are over 40 times more active for the oxidative dehydrogenation of propane than conventional catalysts, while retaining the high selectivity to propylene.17 We are especially interested in the interaction between a noble metal such as Pt and a reducible support, such as TiO2, for which the strong metal−support interaction was first

Metal−support interaction is of great importance in heterogeneous catalysis. It helps stabilize finely dispersed active metal nanoparticles on a support for a long lifetime to avoid sintering and deactivation. The support needs to provide a robust, stable, and often porous structural framework that can sustain catalystactivation processes, as metal nanoparticles are usually deposited from a solution of their salt precursors, followed by heat treatment and reduction. Hence, strong metal−support interaction is key to preventing sintering of metal nanoparticles.1 Another important factor in catalysis by metal nanoparticles is the size effect. This has been nicely demonstrated for catalytic CO oxidation by gold nanoparticles supported on TiO2 and extensively discussed.2−6 In addition, morphology of the supported metal nanoparticles also plays a role; for example, the bilayer gold on TiO2 has been found to be quite active for CO oxidation.7 Regarding the size effect, polydispersity of metal nanoparticles by conventional preparation methods, however, limits one to discuss the dependence of activity on an ensemble average of the particle sizes. It would be desirable to control all metal nanoparticles on the support to be a single size of precise composition. Although such monodispersity can be achieved through ligand-protected, atomically precise metal clusters,8−11 the very presence of ligands such as sulfur, phosphorus, and halogens further complicates the interface with the support. © 2012 American Chemical Society

Received: July 20, 2012 Revised: September 20, 2012 Published: September 24, 2012 21880

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discovered.18 Recently, Isomura et al. investigated Ptn (n up to 15) deposited on rutile TiO2(110) by soft landing; their scanning tunneling microscopy (STM) data suggested that Ptn prefers a flat structure on TiO2(110) for n < 8, whereas larger clusters (Pt 8 to Pt 15 ) have a two-layer structure on TiO2(110).19 However, a recent density functional theory (DFT) study suggested that Pt4−Pt7 all have a two-layer structure on TiO2(110).20 Watanabe et al. found that the activation energy for CO oxidation on Ptn/TiO2(110) decreases greatly at n = 8 and attributed it to the transition to a 3D structure at n = 8.21 Rashkeev et al. compared TiO2supported Au and Pt clusters for their CO oxidation activity by using DFT and assumed that Pt clusters adopted structures similar to those of Au.22 More recently, Bonanni et al. deposited Pt7 on TiO2(110) and examined its catalytic activity for CO oxidation.23 These reports on Ptn/TiO2(110) prompted us to examine the structure of the small Ptn clusters on TiO2(110) from first-principles DFT, with a goal to find answers to the following three questions in particular: (1) is there a transition from a planar to a 3D structure and where does it occur; (2) how does the metal−support interaction depend upon size; (3) how do the Pt clusters bond to the TiO2(110) surface? The rest of the article is organized as follows: in Section 2, we explain the DFT method used to explore the Ptn/TiO2(110) interface; in Section 3, we present our detailed results and analysis of Ptn (n = 4−8) on TiO2(110), in comparison with previous literature; Section 4 summarizes our main findings.

where E(Ptn/TiO2), E(TiO2), and E(Ptn) are the energies of the Ptn/TiO2(110) system, the TiO2(110) surface, and the gasphase Ptn cluster, respectively. The energy of an isolated gasphase Ptn cluster was computed by placing the cluster inside a 12.0 × 12.0 × 12.0 Å3 cubic box and with Γ-point only for the k-point sampling. The most stable configurations of the clusters in the gas phase were used; Figure S1 in the Supporting Information shows their structures and spin states. The partial atomic charges were obtained by the Bader’s atom-in-molecule analysis, as implemented by Henkelman et al.31 The transition state between different isomers of a Ptn cluster on TiO2(110) was located by the climbing-image nudged elastic band method.32

3. RESULTS AND DISCUSSION Recent experimental progress in depositing site-selected group 10 metals such as Pd and Pt on TiO2(110) showed interesting structural and electronic transformation with the cluster size and the resultant impact on the catalytic activity.15,16,19,21,23,33,34 However, detailed structural information at the interface is still missing, given the difficulty in pinpointing the location of each atom in the cluster. Here we focus on Ptn for n = 4−8 on TiO2(110), which is manageable for state-of-the-art DFT codes on a supercomputer and at the same time has direct relevance to recent experimental progress.19,21,23,34 By exploring different initial configurations and surface basin-hopping technique, we obtained the putative global minima for Ptn (n = 4−8) on rutile TiO2(110), and their structures are shown in Figure 1. One can see from Figure 1 that only Pt4 prefers a flat, nearly square structure on TiO2(110), whereas larger clusters all have a two-layer structure. Pt5 has a pyramid structure built upon the square base of Pt4, whereas Pt6 extends Pt5’s pyramid base with five atoms at the interface. Pt7’s structure can be viewed as a further extension of the base or the interfacial layer of the pyramid to an edge-sharing bi-square; still only one Pt atom stays in the second layer. Hence, one can see a common thread of the structural transition from Pt4 to Pt7, that is, the square interface and the pyramid structure. This transition is most clearly viewed in Figure 2, where the TiO2(110) surface is omitted for clarity. For Pt8 on TiO2(110), we found a rather different structure. Figure 3 shows a perspective view of the Pt8/TiO2(110) interface. The interfacial Pt4 square in Ptn (n = 4−7) has now been split into two chains at the interface: one with two Pt atoms, the other three. The remaining three Pt atoms form the second layer atop the two chains. The other interesting feature is that the interfacial Ti atom bonding to two Pt atoms is pulled out by almost 1 Å by the strong Pt−Ti interactions (at a length of 2.57 Å). The structure looks like a hut supported by two neighboring O rows. The interfacial structures in Figure 1 for Pt4−Pt8 on TiO2(110) now allow us to relate them to recent experiments.19,21 The STM experiments19 indicate that the height distributions of Pt4 and Pt7 clusters were narrow, suggesting that both types of clusters had “only planar structure and lay flat on the surfaces”. (They did not report heights for Pt5 or Pt6.) We found that indeed Pt4 has a planar structure lying down on the surface. The STM height distributions also indicate a larger mean height for the Pt7 cluster compared with Pt4 by ∼0.5 Å, in reasonable agreement with our observation of 0.4 Å. In addition, the lateral image of Pt7 from the STM experiments also appears to be very similar to the computed structure in Figure 1d. Furthermore, the experimental observation of

2. METHOD We used the Vienna ab initio simulation package (VASP)24,25 to perform DFT calculations with planewave bases and periodic boundary conditions. The Perdew−Burke−Erzonhoff (PBE) form of the generalized-gradient approximation (GGA) was chosen for electron exchange and correlation.26 The electron− core interaction was described by the projector-augmented wave (PAW) method27,28 within the frozen-core approximation; standard PAW-PBE potentials for Pt, Ti, and O were used. A converged kinetic energy cutoff of 400 eV was employed. Lattice parameters of a = 4.60 Å and c = 2.96 Å from DFT were used for rutile TiO2 and are in good agreement with the experiment.29 The supercell approach was employed to model the TiO2(110) surface, which includes four O−Ti−O triple layers and 12-Å vacuum along the surface normal. During structural optimization, the bottom two O−Ti−O triple layers were fixed at their bulk positions, whereas the top two layers were allowed to relax together with the Ptn cluster; dipole correction of the total energy along the surface normal was applied to the asymmetric slab. Force convergence criterion for geometry optimization was set at 0.025 eV/Å. A (4 × 2) lateral cell of the TiO2(110) surface was used, corresponding to dimensions of 11.84 Å × 13.00 Å. A (4 × 4 × 1) MonkhorstPack k-mesh was used to sample the Brillouin zone. The structures of the Ptn clusters on TiO2(110) were explored in two ways: (a) geometry optimization of different initial configurations and (b) a surface basin-hopping approach starting with a couple of initial configurations for ∼200 Monte Carlo steps.30 The most stable configuration for each cluster size was selected for comparison; the adsorption energy between the cluster and TiO2(110) is defined as EAd(Pt n) = E(Pt n/TiO2 ) − E(TiO2 ) − E(Pt n) 21881

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Figure 3. Perspective view of the most stable configuration of Pt8/ TiO2(110).

assumes both planar and nonplanar structures, indicating nearly iso-energetic structures. We explored the planar configurations of Pt5, Pt6, and Pt7 on TiO2(110). For Pt5, the flat configuration with a capped square geometry is 1.00 eV higher in energy. In the case of Pt6, the flat geometry is only ∼0.12 eV higher in energy; this is due to the additional Pt−O bonds brought about by the edge-shared bi-square (Figure 4), balancing the energetic

Figure 1. Most stable structures of Ptn/TiO2(110): (a) Pt4, (b) Pt5, (c) Pt6, (d) Pt7, and (e) Pt8. Left, side view; right, top view. Pt, blue; O, red; Ti, green. Same color scheme is used in subsequent Figures. The criteria for bond formation are: 2.78 Å for Pt−Pt, 2.61 Å for Pt− O, and 2.75 Å for Pt−Ti.

Figure 4. Second most stable structure of Pt6 on TiO2(110), featuring a flat, edge-sharing bi-square on the surface. This structure is ∼0.12 eV higher in energy than that of Figure 1c.

cost of breaking the Pt−Pt bonds in comparison with the pyramid. For Pt7 and Pt8, we found that the flat configuration is at least 1.0 eV higher in energy. Evidently the transition from planar to nonplanar configurations is subtle and depends on the positioning of the adsorption site relative to the TiO2 lattice. In the STM experiments, the clusters were “randomly positioned”,19 which complicates the comparison between the experiment and the present results, but, in general, the simulations are in very good agreement with experiment, as discussed above. Next, we look at the energetics of cluster adsorption as a function of the cluster size. Figure 5 shows the energetic trend. One can see that the interaction is quite strong for such small Pt clusters when compared with metals such as gold. For example, similarly sized gold clusters on the stoichiometric rutile TiO2(110) surface have an adsorption energy of −0.45 to −1.76 eV.35 The increase in the interaction from Pt4 to Pt8 correlates roughly with the increase in the contact area at the interface. The general trend, as shown in Figure 5 for Ptn on the stoichiometric rutile TiO2 (110), is similar to that found on the partially reduced surface.36 The strong interaction between Ptn and TiO2(110) will affect the preferred geometry of the cluster at the interface, as

Figure 2. Structural transition from Pt4 to Pt7 on TiO2(110). The TiO2(110) surface below the square interface is hidden for a clear view.

greater average height change from Pt7 to Pt8 is consistent with our finding that starting with Pt8 the cluster begins to adopt a structure where significantly more atoms stay in the second layer, whereas Pt7 has only one atom in the second layer. Interestingly, the experiments also show that Pt8 apparently 21882

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easily surmountable at room temperature. Assuming a prefactor of 1013 s−1, the barrier of 0.40 eV of changing from the square to the bi-triangle shape will happen at a rate of 106 s−1 at 298 K; namely, the structure will be constantly oscillating between the square and the bi-triangle shapes at a period of microseconds. This indicates that Ptn clusters manifest fluxionality on TiO2(110). The occurrence of multiple minima close in energy also agrees with the experimental observation of a height distribution of 1 Å of Pt4 on TiO2(110),19 as the top atom in the bi-triangle configuration is only >1.4 Å higher than the interfacial layer. The similar stability between the square and the bi-triangle in Pt4 provides a good example to analyze the factors determining the interfacial structure. Here we focus on local bonding by examining the numbers of Pt−Pt, Pt−Ti, and Pt−O bonds among which Pt−Ti is expected to be much weaker than Pt−Pt and Pt−O. One can see from Table 1 that the two

Figure 5. Adsorption energy (EAd) of Ptn on TiO2(110) as a function of n from 4 to 8.

Table 1. Number (N) and Average Length (R in Å) of Pt−Pt, Pt−Ti, and Pt−O Bonds in the Square and Bi-triangle Configurations of Pt4 on TiO2(110)a

dictated by the structure of the rutile TiO2(110) with its exposed O rows and the five-coordinate Ti atoms in the troughs between the O rows. Here we give a detailed analysis in the case of Pt4. In the gas phase, the cluster prefers a tetrahedron structure with two unpaired electrons (Figure S1 in the SI). After landing on the surface, we found that the cluster prefers a flat structure with close to zero spin moments. In their study of the Ptn/TiO2(110) interface (n = 1−8) under water− gas shift reaction conditions by using constrained ab initio thermodynamics, Ammal and Heyden found the most stable structure of Pt4 to be an edge-sharing bi-triangle of a two-layer configuration.20 We compared this structure (Figure 6a) with

Nsquare Nbi‑triangle Rsquare Rbi‑triangle

Pt−Pt

Pt−O

Pt−Ti

4 5 2.51 2.55

4 4 2.16 2.12

4 2 2.60 2.50

a Criteria for bond formation are: 2.78 Å for Pt−Pt, 2.61 Å for Pt−O, and 2.75 Å for Pt−Ti.

configurations have the same number of Pt−O bonds, whereas the square configuration has two more Pt−Ti bonds to offset the loss of one Pt−Pt bond compared with the bi-triangle configuration. As a result, the two configurations are relatively comparable in energy, with the square slightly favored. The Pt− Ti interaction is clearly visible in the local density of states shown in Figure 7, where significant hybridization takes place

Figure 6. Transition between two configurations of Pt4/TiO2(110): from the bi-triangle (a) to the square (c) via the transition state (b).

our square configuration (Figure 6c) and found that the square is ∼0.12 eV more stable than the bi-triangle. Because the energy difference between the square and bi-triangle shapes of Pt4 on TiO2(110) is small, we further checked how this difference could be affected by the input parameters, such as choice of exchange-correlation functional, k-point mesh, energy cutoff for planewaves, and the supercell size. (See Table S1 in the SI.) We found that although the magnitude varies, the qualitative conclusion remains the same: the square configuration is more stable. Furthermore, we found that the bi-triangle can transform to the square through a barrier of 0.28 eV (Figure 6b), which is

Figure 7. Site-projected, orbital-resolved density of states for Pt 5d and Ti 3p of Pt4/TiO2(110) of the square, flat configuration. The Ti atom is directly bonded to Pt at a distance of 2.60 Å.

between Pt 5d and Ti 3p states. This interaction is also visible in the real-space plot of the electron-density difference (Figure 8), where one can see that electrons move away from Pt and settle at regions along the Pt−Ti bonds. On the whole, we found that the Pt4 cluster is slightly positively charged or oxidized, with 0.30 e of charge being transferred to the support. 21883

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interesting question would be at what size Pt clusters will start to grow across the oxygen rows, instead of along them. Another question would be whether other metals would grow the same way as does Pt. Pd is especially interesting to compare, given that size-dependent activity for CO oxidation and oxygen activation has been found for size-selected Pd clusters on TiO2(110).15,16 In their study of CO oxidation activity on Ptn/TiO2(110) for n up to 15, Watanabe et al. found that the reaction rate per Pt atom increases linearly and reaches the maximum at Pt8 for reaction temperatures of 572 and 598 K and at Pt7 for 625 K.21 To understand these interesting experimental trends from first principles, a detailed study of reaction mechanism, as done in a recent report of single-atom catalysis of CO oxidation by Pt1/ FeOx,37 would be necessary for each cluster size. Our present structural models provide a good starting point. One further point of interest here is whether adsorption of CO and high temperature would induce structural change to the Pt clusters at the interface, thereby causing a different maximum-rate cluster size at different temperatures. If one supposes that such changes would be less likely at low temperatures, the maximum-rate at n = 8 for 572 and 598 K may be related to more second-layer atoms in the case of Pt8. Of course, this hypothesis warrants further investigation of the reaction mechanisms and structures of larger Pt clusters on TiO2(110).

Figure 8. Electron density difference between Pt4/TiO2(110) of the square, flat configuration and the sum of Pt4 and TiO2(110). Cyan isosurface, electron accumulation; purple electron depletion; isovalue at 0.075 e·Å−3.

Similar magnitude of charge transfer is also found for Pt5 (0.36 e), Pt6 (0.28 e), Pt7 (0.42 e), and Pt8 (0.45 e) on TiO2(110). If Pt4 can be viewed as a typical case of a planar geometry on TiO2(110), then Pt8 can be viewed as an important transition to a 3D geometry. This is because Pt5, Pt6, and Pt7 grow upon the square interface and have only one atom in the second layer, whereas Pt8 has a rather different interface and more atoms on the second layers. Here we note that the structures we found for Pt5, Pt6, and Pt7 are the same as those found by Ammal and Heyden.20 However, they found the edge-sharing bi-pyramid to be the most stable structure for Pt8 on TiO2(110). We compared this bi-pyramid structure (see Figure S2 in the SI) with our hut structure and found our structure to be 0.35 eV more stable. As we previously pointed out, the relative energetics is a result of balancing the interfacial interaction among Pt−Pt, Pt−O, and Pt−Ti bonds. In the case of Pt8, we found that just counting the total number of bonds does not seem to explain the energetic difference between the hut and the bi-pyramid structures. Instead, we focus our analysis on the average Pt−Pt length within the cluster. For the hut structure, the average of 11 Pt−Pt bonds yields a length of 2.543 Å, close to that of the average Pt−Pt length of the gasphase Pt8 cluster (at 2.564 Å); for the bi-pyramid structure, the average of 16 Pt−Pt bonds is 2.659 Å. This significantly longer Pt−Pt bond length indicates that the bi-pyramid structure is under a greater tensile strain and that forming the hut structure helps release the strain and arrives at shorter Pt−Pt bonds. The tensile strain is a result of the lattice mismatch between the oxygen rows of TiO2(110) and the Pt−Pt framework of the cluster: the O−O distance of the oxygen rows is at 2.969 Å, whereas the Pt−Pt cluster has an average Pt−Pt length of 2.564 Å. The structural and energetic trends we presented here for Pt4−Pt8 on TiO2(110) call for the study of larger Pt and other metal clusters on TiO2(110). High computational cost, however, prevents routine exploration of much larger clusters such as 2 to 3 nm in size on a nontrivial support surface such as TiO2(110), especially if one intends to find the global minimum of the metal/support interface. With great efforts, exploration of 1 nm-sized clusters (40−60 atoms) on the support seems to be within reach in the near future. One

4. CONCLUSIONS We have explored the interfacial structure of Ptn clusters (n = 4 to 8) adsorbed on rutile TiO2(110) by using DFT. We found that Pt4 prefers a nearly square, flat structure on TiO2(110), in agreement with what the STM experiment suggested19,21 and slightly more stable than the two-layer, bi-triangle structure computed previously.20 The flat structure has a strong interaction with the support through not only the Pt−O bonds but also the Pt−Ti bonds. Pt5, Pt6, and Pt7 grow from Pt4 by forming a pyramid and extending the square base with increased interaction strength through Pt−Ti bonding. Although Pt5−Pt8 all prefer a two-layer structure, Pt6 has a planar configuration, which is only slightly higher in energy by 0.12 eV. Pt8 prefers a hut-like structure with three atoms in the second layer, ∼0.36 eV more stable than the bi-pyramid structure previously found.20 Smaller internal strain is suggested to be the reason for the hut structure’s higher stability. These structures now provide a basis to explore their size-dependent catalytic activity such as CO oxidation and also prompt the exploration of larger clusters of Pt and other metals on a typical reducible support such as TiO2.

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ASSOCIATED CONTENT

S Supporting Information *

Structures for gas-phase Ptn (n = 4−8) clusters, the bi-pyramid structure of Pt8/TiO2(110), and effect of input parameters on the two isomers of Pt4/TiO2(110). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 21884

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(31) Henkelman, G.; Arnaldsson, A.; Jonsson, H. Comput. Mater. Sci. 2006, 36, 354−360. (32) Henkelman, G.; Uberuaga, B. P.; Jonsson, H. J. Chem. Phys. 2000, 113, 9901−9904. (33) Kaden, W. E.; Kunkel, W. A.; Roberts, F. S.; Kane, M.; Anderson, S. L. J. Chem. Phys. 2012, 136, 204705. (34) Isomura, N.; Wu, X. Y.; Hirata, H.; Watanabe, Y. J. Vac. Sci. Technol., A 2010, 28, 1141−1144. (35) Chretien, S.; Metiu, H. J. Chem. Phys. 2007, 127, 084704− 084709. (36) Ç akır, D.; Gülseren, O. J. Phys. Chem. C 2012, 116, 5735−5746. (37) Qiao, B. T.; Wang, A. Q.; Yang, X. F.; Allard, L. F.; Jiang, Z.; Cui, Y. T.; Liu, J. Y.; Li, J.; Zhang, T. Nat. Chem. 2011, 3, 634−641.

ACKNOWLEDGMENTS This work was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy. This research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under contract no. D.-AC02-05CH11231.

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REFERENCES

(1) Campbell, C. T.; Parker, S. C.; Starr, D. E. Science 2002, 298, 811−814. (2) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647− 1650. (3) Kung, H. H.; Kung, M. C.; Costello, C. K. J. Catal. 2003, 216, 425−432. (4) Overbury, S. H.; Schwartz, V.; Mullim, D. R.; Yan, W. F.; Dai, S. J. Catal. 2006, 241, 56−65. (5) Kung, M. C.; Davis, R. J.; Kung, H. H. J. Phys. Chem. C 2007, 111, 11767−11775. (6) Haruta, M. Faraday Discuss 2011, 152, 11−32. (7) Chen, M. S.; Goodman, D. W. Science 2004, 306, 252−255. (8) Turner, M.; Golovko, V. B.; Vaughan, O. P. H.; Abdulkin, P.; Berenguer-Murcia, A.; Tikhov, M. S.; Johnson, B. F. G.; Lambert, R. M. Nature 2008, 454, 981−983. (9) Liu, Y. M.; Tsunoyama, H.; Akita, T.; Xie, S. H.; Tsukuda, T. ACS Catal. 2011, 1, 2−6. (10) Lopez-Sanchez, J. A.; Dimitratos, N.; Hammond, C.; Brett, G. L.; Kesavan, L.; White, S.; Miedziak, P.; Tiruvalam, R.; Jenkins, R. L.; Carley, A. F.; et al. Nat. Chem. 2011, 3, 551−556. (11) Nie, X.; Qian, H.; Ge, Q.; Xu, H.; Jin, R. ACS Nano 2012, 6, 6014−6022. (12) Yoon, B.; Hakkinen, H.; Landman, U.; Worz, A. S.; Antonietti, J. M.; Abbet, S.; Judai, K.; Heiz, U. Science 2005, 307, 403−407. (13) Lee, S. S.; Fan, C. Y.; Wu, T. P.; Anderson, S. L. J. Am. Chem. Soc. 2004, 126, 5682−5683. (14) Tong, X.; Benz, L.; Kemper, P.; Metiu, H.; Bowers, M. T.; Buratto, S. K. J. Am. Chem. Soc. 2005, 127, 13516−13518. (15) Kaden, W. E.; Wu, T. P.; Kunkel, W. A.; Anderson, S. L. Science 2009, 326, 826−829. (16) Kaden, W. E.; Kunkel, W. A.; Kane, M. D.; Roberts, F. S.; Anderson, S. L. J. Am. Chem. Soc. 2010, 132, 13097−13099. (17) Vajda, S.; Pellin, M. J.; Greeley, J. P.; Marshall, C. L.; Curtiss, L. A.; Ballentine, G. A.; Elam, J. W.; Catillon-Mucherie, S.; Redfern, P. C.; Mehmood, F.; et al. Nat. Mater. 2009, 8, 213−216. (18) Tauster, S. J.; Fung, S. C.; Garten, R. L. J. Am. Chem. Soc. 1978, 100, 170−175. (19) Isomura, N.; Wu, X. Y.; Watanabe, Y. J. Chem. Phys. 2009, 131, 164707. (20) Ammal, S. C.; Heyden, A. J. Phys. Chem. C 2011, 115, 19246− 19259. (21) Watanabe, Y.; Wu, X. Y.; Hirata, H.; Isomura, N. Catal. Sci. Technol. 2011, 1, 1490−1495. (22) Rashkeev, S. N.; Lupini, A. R.; Overbury, S. H.; Pennycook, S. J.; Pantelides, S. T. Phys. Rev. B 2007, 76, 035438. (23) Bonanni, S.; Ait-Mansour, K.; Harbich, W.; Brune, H. J. Am. Chem. Soc. 2012, 134, 3445−3450. (24) Kresse, G.; Furthmüller, J. Phys. Rev. B 1996, 54, 11169−11186. (25) Kresse, G.; Furthmüller, J. Comput. Mater. Sci. 1996, 6, 15−50. (26) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865−3868. (27) Blöchl, P. E. Phys. Rev. B 1994, 50, 17953−17979. (28) Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1758−1775. (29) Kuznetsov, A. Y.; Machado, R.; Gomes, L. S.; Achete, C. A.; Swamy, V.; Muddle, B. C.; Prakapenka, V. Appl. Phys. Lett. 2009, 94, 193117−193113. (30) Jiang, D. E.; Overbury, S. H.; Dai, S. J. Phys. Chem. Lett. 2011, 2, 1211−1215. 21885

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