A Comparative Study of Hydrogen Spillover on Pd and Pt Decorated

Jan 28, 2010 - The deposition of Pt and Pd on MoO3 was first carefully investigated. .... Byung Hoon Kim , Han Young Yu , Won G. Hong , Jonghyurk Park...
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J. Phys. Chem. C 2010, 114, 3052–3058

A Comparative Study of Hydrogen Spillover on Pd and Pt Decorated MoO3(010) Surfaces from First Principles Baihai Li,†,‡ Wai-Leung Yim,§ Qiuju Zhang,† and Liang Chen*,† Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, P. R. China, Department of Materials Science and Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109, and Institute of High Performance Computing, 1 Fusionopolis Way, #16-16 Connexis, Singapore 138632 ReceiVed: October 13, 2009; ReVised Manuscript ReceiVed: January 5, 2010

Hydrogen chemisorption and diffusion on Pt and Pd decorated MoO3(010) surfaces were examined using periodic density functional methods. The deposition of Pt and Pd on MoO3 was first carefully investigated. The strong metal-support interactions were found to greatly reduce the catalytic activity of Pt and Pd atoms anchored at their most favorable binding sites. On the other hand, the energies and activation barriers along selected diffusion pathways indicate that hydrogen dissociation and diffusion on the supported Pt5 and Pd5 clusters are feasible, whereas Pt clusters exhibit better catalytic activity than Pd clusters. Subsequently, the dissociated hydrogen atoms tend to directly diffuse onto the sublayer oxygen atoms instead of the surface oxygen atoms. 1. Introduction It is known that hydrogen bronze materials (e.g., HxMoO3) have great application potentials as hydrogenation/dehydration catalysts, fuel cell electrodes, and electrochromic devices. A vast number of experiments have been performed to study their formation, catalytic activity, and other chemical properties.1-11 Hydrogen bronze materials can be prepared by chemical or electrochemical methods.8 For example, an atomic force microscopy study by Smith and co-workers demonstrated that MoO3 undergoes topotactic reduction in H2-N2 mixtures at 700 K and subsequently forms hydrogen molybdenum bronzes, HxMoO3 (0 < x < 2).9,10 Alternatively, these materials can also be prepared efficiently via the hydrogen spillover technique, in the presence of palladium or platinum catalysts.11,12 In the spillover process, H2 molecules are first dissociatively chemisorbed on the dispersed catalyst particles; then, the dissociated H atoms migrate onto the substrate and further diffuse into the oxide lattice. It is essential that the atomic hydrogen should be able to move freely on the catalyst-substrate interface and from the vicinity of the catalysts to other adsorption sites in order to achieve the maximum spillover efficiency. Furthermore, in addition to the preparation of hydrogen bronze, the spillover technique has also been successfully utilized to achieve high hydrogen storage capacity in carbon-based materials.13-17 The detailed mechanism of hydrogen spillover has been widely investigated by means of experimental and theoretical techniques. However, most published studies only addressed the hydrogen adsorption and diffusion on the substrate (e.g., MoO3) surfaces and into the bulk.18-23 For example, a semiempirical method study on the HxMoO3 formation, stability, and proton transport pathways has been reported by Mehandru and co-workers.19 By comparing the binding energies, they found that the hydrogen molecules heterolytically adsorbed on the edge * To whom correspondence should be addressed. E-mail: chenliang@ nimte.ac.cn. † Chinese Academy of Sciences. ‡ University of Michigan. § Institute of High Performance Computing.

sites of MoO3 with H+ bonded to O2- and H- bonded to MoVI are more stable than two H• homolyticallly adsorbed on the basal plane. In addition, they obtained a 1.1 eV energy barrier for H• migration between two layer terminal oxygen sites. More recently, we have studied the H diffusion within the MoO3 lattice and concluded that H can move nearly freely due to the abundant H-bond existing in the special structure.23 In the present paper, we first performed systematical and comparative calculations to investigate the deposition of Pt and Pd on the MoO3(010) surface. Using the optimized Pt/MoO3 and Pd/MoO3 structures, we subsequently studied the hydrogen spillover from the gas phase into the MoO3 bulk. In particular, we primarily focused on hydrogen adsorption and diffusion on the catalyst-substrate interface in order to unravel the detailed spillover mechanisms and compare the catalytic activity of supported Pt or Pd clusters. 2. Computational Methods All calculations were performed using the spin polarized generalized gradient approximation (GGA) as implemented in the Vienna ab initio simulation package (VASP).24,25 The Perdew-Wang (PW91)26 exchange-correlation functional was employed with the electron-ion interactions described by ultrasoft pseudopotentials.27 For a single Pt (Pd) atom adsorbed on the substrate, a supercell containing one (2 × 2) MoO3(010) slab and 18 Å vacuum space was used. The Brillouin zone was sampled within a 3 × 3 × 1 Monkhorst-Pack mesh.28 For the Pt5 and Pd5 clusters deposited on the substrate, a larger supercell of (3 × 3) slab was chosen to avoid lateral interactions of the clusters between images. Accordingly, the Brillouin zone was sampled within a 2 × 2 × 1 mesh. The validity of the onelayer slab model is supported by the fact that the MoO3 interlayer interactions are dominated by the weak van der Waals forces. The presence of a thicker layer has negligible influence on the interaction between the overlayer and the surface. The energy cutoff was set to 400 eV in all calculations. All atoms were fully relaxed with the forces converged to less than 0.03 eV/Å. Electron smearing was employed using the Methfessel-

10.1021/jp9098396  2010 American Chemical Society Published on Web 01/28/2010

Pd and Pt Decorated MoO3(010) Surfaces

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Figure 1. Side view (left and middle) and top view (right) of the MoO3(010) surface. The red and olive spheres are O and Mo atoms, respectively. The larger red spheres shown in the right figure represent the surface O atoms. The second layer O atom under the OH site is highlighted as a small yellow sphere.

Paxton technique,29 with a width of 0.1 eV, in order to minimize the errors in the Hellmann-Feynman forces. The climbing image nudged elastic band (cNEB)30 method was used to examine the energy profile along prescribed diffusion or dissociation pathways. The identified transition states were further checked by the vibrational analysis to ensure that they are truly the saddle points. The binding energy of Pt or Pd atoms deposited on MoO3 is defined as

EBE ) [E(MoO3) + n × E(CMatom) E(CMn/MoO3)]/n (CM ) Pt, Pd) (1) where E(MoO3), E(CMatom), and E(CMn/MoO3) are the energy of the bare MoO3 slab, the energy of a single metal atom in the gas phase, and the total energy of the CMn/MoO3 complex, respectively. The calculated adsorption energy for atomic hydrogen is defined as

Eb ) [E(CMn/MoO3) + 2 × E(H) - E(2HCMn/MoO3)]/2 (CM ) Pt, Pd) (2) where E(H) is the energy of an isolated H atom in the gas phase. Our calculated binding strength of the H2 molecule is 4.60 eV. Hence, the dissociation adsorption on the CMn/MoO3 complex is thermodynamically favorable only if the calculated Eb is greater than 2.3 eV (the binding energy of 1/2 H2). 3. Results and Discussion 3.1. Adhesion of Single Pt and Pd Atoms on (2 × 2) MoO3(010). MoO3 is an orthorhombic crystal containing layers of distorted MoO6 octahedra, which share edges and form chains with oxygen atoms. As shown in Figure 1, four different binding sites are identified on the MoO3(010) surface: one-fold on-top site (OT), 2-fold symmetric site (OS), 2-fold asymmetric site (OA), and 4-fold hollow site (OH). The calculated binding energies (EBE) for the single Pt and Pd atoms are summarized in Table 1. The OS site is identified as the most favorable binding site with binding energies of 3.90 and 2.79 eV for Pt and Pd, respectively. This is mainly because the MoO3(010) surface is very open. The distance between two adjacent OT atoms is large enough (∼3.7 Å), so that Pt and Pd atoms can be perfectly embedded onto the OT-OT bridge. The two concerted Pt-OT or Pd-OT bond lengths are calculated to be 1.98 and 2.03 Å, respectively. Consequently, the two lobes of Pt-d and Pd-d orbitals can maximize overlap with the two adjacent O-2p orbitals. The significant hybridization is further confirmed by

TABLE 1: Binding Energies (EBE) and Bond Lengths of the CM1/MoO3(010) and CM5/MoO3(010) Systemsa Pt1/MoO3(010) site EBE (eV) dPt-O (Å) dPt-Pt (Å)

OT 1.81 1.86

OS 3.90 1.98

OA 3.14 1.97

Pd1/MoO3(010)

Pt5/MoO3(010) OH 3.42 2.08

3.57 2.02-2.06 2.66-2.68b, 2.99-3.0c Pd5/MoO3(010)

OS OA OH site OT EBE (eV) 1.56 2.79 2.03 2.57 2.43 dPd-O (Å) 1.97 2.02 2.06 2.16-2.22 2.04-2.09 dPd-Pd (Å) 2.70-2.73b, 2.97-2.99c a The superscripts b and c denote the distances of CMtopCMbottom and CMbottom-CMbottom in the CM5 clusters, respectively (CM ) Pt, Pd).

the Bader charge analysis.31 Pt and Pd donate around 0.15 and 0.10 electrons to each of the two nearest OT and Mo atoms, respectively. For comparison, on other close-packed surfaces [e.g., MgO(100) and MgO(111)],32,33 it is difficult for Pd to maximize orbital overlap with two O atoms simultaneously. Furthermore, the repulsive interactions from adjacent Mg ions force the metal adatoms to be adsorbed right above the surface O atoms. In the vicinity of the Pt and Pd atoms, the surface structure of MoO3 is only slightly distorted, while the OS oxygen atom below metal adatoms is pushed inward by 0.28 and 0.23 Å, respectively. In fact, the electrostatically attractive interactions between Pt (Pd) and OS atoms further stabilize the deposition of Pt and Pd atoms. When Pt is deposited on the 4-fold OH site, the diagonals of the surface oxygen rectangle are shortened by 1.24 Å due to the attractive interactions between Pt and OT atoms. The four-coordinated Pt atom leads to Pt-OT bond lengths of 2.08 Å, which are 0.1 Å longer than those on the 2-fold OS and OA sites. As for Pd deposited on the OH site, the diagonals of the outmost oxygen square are shortened by 1.07 Å. The Pd-OT bond lengths are in a range of 2.16-2.22 Å, which are about 0.2 Å longer than those on other binding sites. To ensure that our (2 × 2) slab model is sufficiently large, we have also calculated the binding energies of single Pt and Pd atoms on the OS site of a (3 × 3) MoO3(010) slab. The calculated EBE values are 3.91 and 2.81 eV, respectively, which are in good agreement with the values presented above. The density of states (DOS) for the Pt1 (Pd1)/MoO3 complexes with Pt (Pd) on the OS site as well as the clean MoO3 slab are displayed in Figure 2. The band gaps of Pt1/MoO3 (Figure 2a) and Pd1/MoO3 (Figure 2b) become narrower compared to that of the clean MoO3 slab (Figure 2c), even though they still exhibit some insulating features. The peaks below the Fermi level for

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Figure 2. Density of states for (a) Pt1/MoO3, (b) Pd1/MoO3, and (c) clean MoO3(010) surface. The Fermi level is shifted to 0 eV.

Pt1 (Pd1)/MoO3 are mainly composed of Pt-dxy and Pt-dx2-y2 states hybridized with the O-px and O-py states. Moreover, the orbital overlap in the xy plane between Pt and OT appears to be more pronounced than Pd-OT, which rationalizes the stronger binding strength of Pt1/MoO3 than that of Pd1/MoO3. 3.2. Hydrogen Adsorption on Pd1/MoO3(010) and Pt1/ MoO3(010). We next investigated the catalytic activity of Pt1/ MoO3 and Pd1/MoO3 with Pt and Pd on the most favorable OS site. Initially, we placed two hydrogen atoms that are separated by 1.6 Å above the Pt and Pd atoms. Upon structural optimizations, the two separated H atoms would either combine to be a H2 molecule or move onto the nearest oxygen atoms. In the former case, the calculated distances from the bond center of H2 to Pt and Pd are 1.9 and 2.0 Å, respectively, which are significantly longer than the bond lengths of H-Pt and H-Pd for single H atom adsorption. Correspondingly, the interactions between H2 and Pt (Pd) atom are relatively weak, with small adsorption energies of 0.42 and 0.30 eV for H2-Pt1/MoO3 and H2-Pd1/MoO3, respectively. As expected, the weak H2-metal interactions only induce slight elongation of the H-H bond. The H-H bond lengths are calculated to be 0.82 and 0.78 Å in the H2-Pt1/MoO3 and H2-Pd1/MoO3 systems, respectively, which are slightly longer than that of the free H2 molecule (0.74 Å) in the gas phase. In the latter case, the deposited Pt and Pd atoms cannot accommodate two H atoms simultaneously. At least one of the H atoms has to leave and migrate onto the nearest oxygen atom. In order to understand the nature of interactions between hydrogen atoms and the CM1/MoO3 (CM ) Pt, Pd) complex, the charge density differences between the optimized 2H-CM1/ MoO3 (CM ) Pt, Pd) complex and the superposition of atomic charge densities are displayed in Figure 3, where electron excess is represented as solid lines, while electron deficiency is represented as dashed lines. It is clearly illustrated in the plots that electrons are transferred from Pt and Pd atoms to the H atom and neighboring OT atoms. The positively charged Pt and Pd ions are unable to donate enough electrons to the antibonding orbitals of H2 and thus fail to dissociate H2. Furthermore, due to the shape and strong directivity of the d-orbital, only one lobe of the d-orbital remains available for upcoming H atoms after metal atoms deposited on MoO3. It can bond with only one spherical H-1s orbital, while it cannot maximize the overlap with two H-1s orbitals simultaneously. In consequence, only one hydrogen atom can be absorbed on the supported Pt and Pd atoms with bond lengths of 1.55 Å for Pt-H and 1.58 Å for

Li et al. Pd-H, respectively, whereas, the other H atom bonds to the nearest O atom with a length of 0.978 Å. In other words, the single Pt and Pd atoms deposited on MoO3 would lose their catalytic activity toward hydrogen dissociative chemisorption. In contrast, H2 molecules have been shown to readily dissociate and chemisorb on the unsupported Pt and Pd clusters in our previous reports.34-37 3.3. Adhesion of Pt5 and Pd5 Clusters on (3 × 3) MoO3(010). In a realistic heterogeneous catalytic system, the catalyst particles usually contain thousands of atoms and are dispersed on the support materials with various sizes, shapes, and orientations. Therefore, modeling such realistic particles is computationally difficult. In the present study, we instead built the smallest three-dimensional clusters (square pyramid Pt5 and Pd5) on a larger supercell of (3 × 3) slab with two atoms on the OS sites, two atoms on the OA sites, and the fifth atom on the top of the square to represent the catalyst particles. The optimized structures of Pt5/MoO3 and Pd5/MoO3 are displayed in Figure 4. These supported three-dimensional cluster models possess sharp corners and edges, which are often deemed to be the most active catalytic sites in experimental and computational studies.38 More importantly, we will show that the top atom of the Pt5 and Pd5 clusters is the active site for adsorption and dissociation of hydrogen molecules. Although such models are much smaller than the realistic particles, we believe that it can still provide an opportunity to examine chemistry at the catalyst-substrate interface. In particular, it can help distinguish the roles of the noble-metal atoms, whether they are in direct contact with the oxide support. Indeed, similar subnano cluster-substrate models have been widely and successfully utilized in many other theoretical studies.39-44 An exhaustive study of various Ptn/MoO3 and Pdn/MoO3 (n > 5) structures will not be focused on in the present study. The binding energies and bond distances between metal atoms and the substrate of CM5/MoO3 (CM ) Pt, Pd) are shown in Table 2. Compared to the bare Pt5 and Pd5 structures, the bond lengths between the top and bottom atoms are slightly elongated by 0.06 Å, while the sides of bottom squares are significantly elongated by about 0.44 Å for both Pt5/MoO3 and Pd5/MoO3. The expansion implies that the interactions between metal atoms are considerably weakened upon deposition on MoO3. As shown in Figure 4, the four bottom metal atoms bond with their nearest oxygen atoms. The Pt-OT bond lengths are calculated to be 2.02-2.06 Å, and the Pd-OT bond lengths range from 2.04 to 2.09 Å. The binding energies of Pt5 and Pd5 on MoO3(010) are calculated to be 3.57 and 2.43 eV/atom, respectively. These values include two contributions: (i) the metal-substrate interactions of 0.84 eV for Pt5/MoO3 and 0.77 eV for Pd5/MoO3; (ii) the metal-metal interactions of 2.73 eV for Pt5/MoO3 and 1.66 eV for Pd5/MoO3, of which both are 0.16 eV lower than those values of the bare Pt5 and Pd5 clusters due to the structural expansion. The former term can also be regarded as the binding energy with respect to the unsupported Pt5/Pd5 clusters and the clean oxide slab, which can be calculated using

Ef ) [E(MoO3) + E(CM5) E(CM5/MoO3)]/n

(CM ) Pt, Pd)

(3)

Therefore, the metal-metal interaction term is defined as

Emm ) EBE - Ef

(4)

The strong metal-oxygen and metal-metal interactions sustain the stability of the structures of Pt5/MoO3 and Pd5/MoO3.

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Figure 3. (010) cuts of the charge density differences for (a) 2H-Pt1/MoO3 and (b) 2H-Pd1/MoO3. The (010) facet is chosen.

Figure 4. Side view (a) and top view (b) of Pt5/MoO3 and side view (c) and top view (d) of Pd5/MoO3.

TABLE 2: Adsorption Energies (Eb) and H-CM Bond Lengths of 2H-CM5/MoO3(010) Complexes (CM ) Pt, Pd) 2H-Pt5/MoO3(010) states Eb (eV) dPt-H (Å)

(b) 3.01 1.57

(c) 3.18 1.58

(d) 3.27 1.57

(e) 2.99 1.57

(f) 3.18 1.57

(g) 3.44 1.57

2H-Pd5/MoO3(010) states Eb (eV) dPd-H (Å)

(b) 2.69 1.79

(c) 2.97 (Pdb) 1.66; (Pdt) 1.82

(d) 2.98 (Pdb) 1.67; (Pdt) 1.76

The DOS in Figure 5a and b indicate that the deposited Pt5 and Pd5 clusters introduce metallic features to the interfaces, which is not observed in either Pt1/MoO3 or Pd1/MoO3. This feature arises from the fact that the wave functions of Pt5 and

(e) 3.00 (Pdb) 1.66; (Pdt) 1.77

(f) 3.28 (Pdb) 1.66; (Pdt) 1.78

(g)

Pd5 penetrate into MoO3. Hence, new electronic states are formed and fully fill the band gap of MoO3. This is clearly reflected in the Bader charge analysis: each of the nearest neighbor OT and Mo ions obtains about 0.20 and 0.25e in both

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Figure 5. Density of states for (a) Pt5/MoO3, (b) Pd5/MoO3, and (c) clean MoO3(010) surface. The Fermi level is shifted to 0 eV.

Figure 6. Energy landscape for hydrogen spillover on Pt5/MoO3. The bottom O layers are removed to avoid crowdedness of the figure. The bottom oxygen layers in the states a-f and TS1-TS4 are removed to avoid overloading the figure.

the Pt5/MoO3 and Pd5/MoO3 systems, respectively. In fact, such a phenomenon is usually referred to as the metal induced gap states (MIGS),45,46 which has also been observed in other metal/ oxide or insulator/metal systems such as Pd/MgO(100)33 and LiCl/CM(001) (CM ) Cu, Ag) interfaces.47,48 Moreover, the MIGS also induces some electrons to propagate further into the subsurface oxygen atoms. 3.4. Hydrogen Adsorption and Diffusion on Pd5/ MoO3(010) and Pt5/MoO3(010). With the optimized Pt5/MoO3 and Pd5/MoO3 structures, we were able to investigate the hydrogen adsorption and diffusion on them. Initially, the H2 molecule was placed approximately 3.8 Å above the top of Pt5 and Pd5 clusters to ensure no chemical bonding between H2 and Pt5 (Pd5) clusters (state a in Figures 6 and 7). As shown in the energy profiles (Figure 6), the H2 molecule can readily approach the Pt5 cluster without any barrier, and then be dissociatively chemisorbed onto the top of Pt5 with two equivalent Pt-H bond lengths (∼1.57 Å). The two dissociated H atoms are well separated by 1.79 Å (state b). This clearly highlights the extraordinary catalytic activity of the supported three-dimensional Pt clusters compared to the single Pt atom. The adsorption energy of state b in Figure 6 is 2.70 eV per H atom. Each hydrogen atom gains 0.02 electrons from Pt. In contrast, hydrogen is found to be adsorbed on the top of Pd5 in the form of a dihydrogen molecule with the elongated H-H bond length of 0.83 Å (state b in Figure 7). The distance between H2 and the top Pd-5 atom is calculated to be 1.79 Å. This configuration is similar to that of the hydrogen molecule above Pd1/MoO3. However, the H-Pd bond is about 0.2 Å shorter

Li et al.

Figure 7. Energy landscape for hydrogen spillover on Pd5/MoO3. The bottom oxygen layers in states a-e and TS1-TS4 are removed to avoid overloading the figure.

than that of H2-Pd1/MoO3, yielding a slightly higher adsorption energy of 0.90 eV for the H2 molecule. This is consistent with our previous conclusion that the Pt clusters have considerably better catalytic activity toward hydrogen dissociative chemisorption than Pd clusters.35-37 Moreover, the MoO3 support greatly deactivates the base atoms of both Pd5 and Pt5 clusters. Only the top atom that is not in direct contact with the support retains the original high reactivity toward hydrogen adsorption and dissociation. Subsequently, hydrogen diffusion will proceed along different pathways on Pt5 and Pd5 clusters. As labeled in Figure 3, there are two distinct types of sites (Pt-1 and Pt-3 on the OA sites and Pt-2 and Pt-4 on the OS sites) for hydrogen adsorbed on the bottom of the clusters. Extensive pathway searching shows that the two dissociatively adsorbed hydrogen atoms on the top of Pt5/MoO3 prefer to move down to the bottom Pt atoms (Pt-1 and Pt-3) sequentially. At the first stage, one hydrogen atom diffuses down to the Pt-1 (or Pt-3) atom, while the other hydrogen atom stays on the top Pt-5 atom. As depicted in Figure 6 (from state b to c), this step has to overcome an activation barrier of 0.26 eV via a transition state (TS1), where one hydrogen atom stays on Pt-5 with a Pt-H bond length of 1.57 Å while the other H locates on the bridge of Pt-5 and Pt-1 with distances of 1.63 Å to Pt-5 and of 2.34 Å to Pt-1, respectively. The migrated hydrogen atom subsequently lands on Pt-1 with a bond length of 1.57 Å and obtains 0.05 electrons from this Pt atom, while the other hydrogen atom still stays above the top Pt-5 atom with the H-Pt bond pointing toward Pt-3 (state c). The adsorption energy for this asymmetric configuration is calculated to be 3.18 eV per hydrogen atom. Alternatively, we considered the case that one hydrogen atom first diffuses onto the Pt-2 atom, while the other hydrogen atom stays above the top Pt-5 atom with the H-Pt pointing toward Pt-4 on the other OS site. Upon optimization, the two hydrogen atoms symmetrically converge to the bridge of the Pt-5 and Pt-2 as well as the bridge of Pt-5 and Pt-4. The corresponding adsorption energy is calculated to be 2.87 eV per hydrogen atom, which is 0.31 eV lower than that of the configuration presented above. Namely, such a configuration is energetically metastable. We have demonstrated that the Pt or Pd atoms deposited on the OS site would lose catalytic activity due to the orbital directivity and the depletion of electron density. Therefore, it is not surprising that they play only a minor role in the whole spillover process. At the second stage (state c to d in Figure 6), no activation barrier is detected for the migration of the other hydrogen atom

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Figure 8. (010) cuts of the charge density differences for (a) 2H-Pt5/MoO3 and (b) 2H-Pd5/MoO3.

from Pt-5 to Pt-3. The adsorption energy is calculated to be 3.27 eV per hydrogen atom. Combining the two sequential steps from state b to d, the rather low energy barrier implies that hydrogen dissociation and migration on the Pt5 cluster is quite facile. The moderate barrier (0.26 eV) of the first step is expected to be readily overcome under ambient conditions. For the Pd5/MoO3 complex, we first suppose that there is a local stable structure similar to state c for 2H-Pt5/MoO3, in which one hydrogen atom is adsorbed on a bottom Pd atom on the OA or OS sites, and the other hydrogen atom stays above the top Pd-5 atom. However, upon optimization, the hydrogen atom initially placed above Pd-5 migrates down, while the other hydrogen placed on a bottom Pd atom climbs up. Eventually, the two hydrogen atoms reach the bridges between the top and bottom Pd atoms, forming a symmetrical structure: either on the bridges of Pd-5 and Pd-1, 3 (state c in Figure 7) or the bridges of Pd-5 and Pd-2, 4. The adsorption energy (2.97 eV) of the former configuration is 0.50 eV greater than that of the latter configuration. Each hydrogen atom of the former structure gains 0.15 electrons, which are slightly smaller than that of the latter structure (∼0.20 electrons). The transferred electrons fill in the antibonding states of H-Pd bonds and weaken the H-Pd bond strength in return. Therefore, the weakened hydrogen molecule above Pd5/MoO3 prefers to simultaneously migrate down and be adsorbed symmetrically on the two bottom Pd atoms (Pd-1 and Pd-3) at the OA site. As shown in Figure 7, hydrogen atoms have to overcome an energy barrier of 0.95 eV to proceed from state b to c. Compared to the facile activation barrier for Pt, this higher energy barrier is mainly due to the work required for breaking the weakened H-H bond, since H2 is not completely dissociatively adsorbed on Pd5/MoO3. The charge density differences of state d of 2H-Pt5/MoO3 and state c of 2H-Pd5/MoO3 are shown in Figure 8. The contour plots clearly illustrate the interactions between hydrogen atoms and the Pt5/MoO3 and Pd5/MoO3 complexes. Electron perturbation is localized among chemisorbed hydrogen atoms and metal atoms, and does not propagate further into the substrate. Again, it is shown that both Pt and Pd atoms serve as an electron reservoir and donate electrons to the O and H atoms. Note that each hydrogen atom can form a strong H-Pt bond with one single Pt atom. In contrast, each hydrogen atom has to bond with two Pd atoms simultaneously. A second Pd atom is required to stabilize the adsorption. Indeed, the superior catalytic activity of Pt clusters over Pd clusters is mainly due to the more reactive 5d96s1 electron configuration of Pt, while Pd possesses a closedshell electron configuration (4d105s0). The relatively inert nature prevents Pd clusters from donating enough electrons to antibonding orbitals of H2 to weaken the H-H bond.

Finally, we address the key step of hydrogen diffusion from Pt5 and Pd5 clusters to the MoO3 substrate. Ideally, it is anticipated that the adsorbed hydrogen is able to readily migrate onto or penetrate into the substrate at moderate temperatures. However, cNEB calculations indicate that it is difficult for hydrogen atoms to directly migrate from Pt5 and Pd5 clusters to the nearest surface oxygen atoms. As shown in Figures 6 and 7, the hydrogen migration from Pt-1 and Pd-1 to the nearest OT atom has to overcome significant energy barriers of 1.45 and 1.18 eV, respectively. The structures of Pt5 and Pd5 clusters undergo some distortion during the diffusion, which may partly account for these high barriers. More importantly, the hydrogen atom adsorbed on Pt-1(or Pd-1) atoms is actually pointing toward the direction of OA and OF atoms instead of OT atoms (see Figure 4b and d). From a geometrical point of view, the diffusion from Pt-1 to OT is virtually a detour. In contrast, the alternative diffusion onto sublayer OA or OF atoms is a shortcut and is kinetically more likely to occur. Indeed, the calculated activation barriers are considerably reduced if hydrogen directly diffuses into the MoO3 lattice via the hollow channel (namely, the OH site in Figure 1). On Pt5/MoO3, an activation energy of only 0.88 eV is required for the hydrogen atom to migrate from Pt-1 to the subsurface OA and OF site of the lower oxygen sheet. For 2H-Pd5/MoO3, the activation barrier is also reduced to 1.10 eV. According to the Bader charge analysis, electrons entirely transfer from the H-1s orbital to the O-2p orbital during the diffusion, which thus makes the hydrogen atom become virtually a proton. Accordingly, both the 2H-Pt5/MoO3 and 2H-Pd5/ MoO3 systems undergo a transition from electrostatically repulsive O-H interactions to attractive proton-oxygen interactions. Fortunately, the extensive H-bonding network in the MoO3 lattice and surface helps to reduce the migration energy barriers and increase the H mobility.23,49 In particular, the highly attractive nature of the hollow channel facilitates hydrogen migration (TS3 and TS4 in Figures 6 and 7). After migration to the MoO3, the hydrogen atoms bond to the neighboring oxygen atom by forming strong covalent O-H bonds in both the 2H-Pt5/MoO3 and 2H-Pd5/MoO3 systems. However, the subsurface OT and OA atoms do not show an energetic preference over the bottom Pt or Pd atoms for hydrogen adsorption, as shown in Figures 6 and 7. Instead, the sublayer OF site is identified as the most favorable for hydrogen adsorption, which may be the destination of hydrogen atoms in the whole spillover process. 4. Conclusions In summary, we performed first principles calculations to study hydrogen spillover on Pt and Pd decorated MoO3(010)

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surfaces. The most favorable binding site for single Pt and Pd atoms is identified as the OS site. However, upon deposition on the OS site, both Pt and Pd would lose catalytic activity toward hydrogen dissociation due to the unique shape of the d-orbital. Instead, we built three-dimensional Pt5 and Pd5 clusters supported on MoO3 to represent the catalysts. Pt5/MoO3 exhibits much better catalytic performance than Pd5/MoO3. H2 can be readily dissociated onto the supported Pt5 as two separated hydrogen atoms while only chemisorbed on the Pd5 cluster as a weakened molecule. Subsequently, the dissociatively chemisorbed hydrogen atoms can diffuse to the bottom atoms of the cluster by passing relatively low energy barriers and would directly diffuse into the MoO3 lattice to form strong O-H bonds. However, the energy profiles reveal that it is difficult for hydrogen atoms to escape from the Pt or Pd clusters and freely diffuse onto the MoO3 surface under low pressures. We expect that hydrogen diffusion from larger metal clusters may be slightly facilitated because of the weaker H-metal interactions. Nevertheless, the increased hydrogen partial pressures as well as some bridge building are required to enable the spillover process. Acknowledgment. This work is supported by the National Natural Science Foundation of China and Zhejiang Provincial Natural Science Foundation. References and Notes (1) Braida, B.; Adams, S.; Canadell, E. Chem. Mater. 2005, 17, 5957. (2) Sakagami, H.; Asano, Y.; Takahashi, N.; Matsuda, T. Appl. Catal., A 2005, 284, 12. (3) Sakagami, H.; Ohno, T.; Takahashi, N.; Matsuda, T. J. Catal. 2006, 241, 296. (4) Matsuda, T.; Uozumi, S.; Takahashi, N. Phys. Chem. Chem. Phys. 2004, 6, 665. (5) Noh, H.; Wang, D.; Luo, S.; Flanagan, T. B.; Balasubramaniam, R.; Sakamoto, Y. J. Phys. Chem. B 2004, 108, 310. (6) Rousseau, R.; Canadell, E.; Alemany, P.; Galvan, H.; Hoffmann, R. Inorg. Chem. 1997, 36, 4627. (7) Hoang-Van, C.; Zegaoui, O. Appl. Catal., A 1997, 164, 91. (8) Schollhorn, R.; Kuhlmann, R.; Resenhard, J. O. Mater. Res. Bull. 1976, 11, 83. (9) Smith, R. L.; Rohrere, G. S. J. Catal. 1996, 163, 12. (10) Smith, R. L.; Rohrere, G. S. J. Catal. 1998, 173, 219. (11) Khoobiar, S. J. J. Phys. Chem. 1964, 68, 411. (12) Kim, J. G.; Regalbuto, J. R. J. Catal. 1993, 139, 153. (13) Wang, L. F.; Yang, R. T. J. Phys. Chem. C 2008, 112, 12486. (14) Chen, L.; Cooper, A.; Pez, G.; Cheng, H. J. Phys. Chem. C 2007, 111, 18995. (15) Li, Y. W.; Yang, R. T. J. Am. Chem. Soc. 2006, 128, 726.

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