Origin of Oxidation and Support-Induced Structural Changes in Pd

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Origin of Oxidation and Support-Induced Structural Changes in Pd4 Clusters Supported on TiO2 S. Vincent Ong and S. N. Khanna* Department of Physics, Virginia Commonwealth University, Richmond, Virginia 23284-2000, United States

bS Supporting Information ABSTRACT: Theoretical first-principles studies of the effect of oxidation on the atomic and electronic structure of a Pd4 cluster and the nature of cluster support interactions have been carried out by investigating a Pd4 cluster deposited on a rutile TiO2(110) surface. Our studies based on a gradient-corrected density functional approach indicate that the deposited Pd4 has a compact ground state with an energetically close pseudoplanar structure that is expected to coexist with the ground state. The compact ground state undergoes a transition to a planar structure upon the absorption of a single O atom. The addition of a second O, however, generates two different structures: (1) a pseudoplanar structure where a Pd site mediates the interaction between the two O atoms, and (2) an energetically more stable species with a planar Pd4 structure and O in a “spillover” mode. Experimental evidence supporting this spillover O is discussed. Detailed analysis of the electronic states and charge densities reveals that the interaction between the Pd and the lattice O sites is mainly responsible for the variations in structure of the deposited species as well as the relaxation of the underlying lattice. Specifically, variations in the charge state of the lattice Ti and O sites are shown to lead to large displacements of Ti ions providing a microscopic mechanism of observed strong metal support interactions.

’ INTRODUCTION Small clusters/nanoparticles of late transition elements palladium, platinum, or rhodium supported on oxide supports constitute an important class of heterogeneous catalysts employed in a variety of industrially relevant processes.1,2 For example, supported palladium catalysts are vital for methane combustion as they lower the combustion temperature, reducing production of pollutants such as CO and NOx, while providing a high turnover rate. Palladium-based catalysts are also important for CO oxidation and are widely used in catalytic convertors. This has stimulated considerable interest in palladium clusters, and numerous experimental and theoretical studies of the catalysis of free and supported clusters have been carried out. One of the important findings is that, for small clusters, the activity can change with size and that the activity of free clusters can be radically different from those of the supported species. These variations are partially rooted in the fact that the support can alter the geometrical structure of the cluster. Since the electronic structure is intimately linked to the geometry, the changed geometry can alter the reactivity. Further, the support can enhance the activity by modulating the electronic structure through cluster support interactions. At small sizes, these effects are complemented by the fact that most oxidation reactions proceed via initial adsorption of oxygen that often draws charge from the supported species. The charging as well as the metal oxygen bond can also affect the shape and electronic structure of the cluster. Consequently, understanding the role of support and oxidation on the electronic and geometric structure is vital to understanding r 2011 American Chemical Society

the catalytic process and in designing effective catalysts. The purpose of this paper is to carry out a comprehensive study of these effects through studies of a Pd4 cluster supported on a rutile TiO2(110) surface. In a recent work, Kaden et al. investigated the CO oxidation of O2 by Pdn clusters containing between 2 and 25 atoms and supported on the rutile TiO2(110) surface.3 In these experiments, size-selected cluster cations were first deposited on the TiO2(110) surface and an oxidation of CO in the presence of O2 was carried out. To relate the cluster activity to the electronic features of the cluster, Kaden et al. probed the 3d core states of the Pdn clusters through X-ray photoemission spectroscopy. Their results showed that the catalytic activity could be directly correlated with changes in the Pd 3d binding energy. In particular, clusters where the 3d level was higher than the value corresponding to a simple scaling with size showed enhanced activity. They also probed the atomic structure of the deposited species through ion scattering. Their results suggested that the clusters containing up to 10 Pd atoms are forming quasi-planar structures and that a second layer develops as more Pd atoms are added. The planar structure for smaller species was a bit puzzling as the ground state atomic configurations of free Pdn clusters are all compact structures. Recent studies in our group indicate that the Pdn clusters continue to have compact structures on alumina Received: June 10, 2011 Revised: September 5, 2011 Published: September 08, 2011 20217

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The Journal of Physical Chemistry C surface.4 All this indicated that the palladium clusters undergo strong interactions with the TiO2(110) surface. This conclusion is also supported by the earlier studies that indicated a strong metal support interaction (SMSI). In fact, Pd particles have been found to be either encapsulated by or alloyed with the substrate atoms after high-temperature reduction.5
7 A microscopic understanding of the atomic and electronic structure of the deposited clusters is needed to understand these findings. In this paper, we present a comprehensive study of the atomic and electronic structure of Pd4 on a TiO2 surface. Our studies focus on three questions: (1) What is the ground state structure of unoxidized, supported Pd4, and are there competing geometries that could affect the observed behavior at finite temperatures? (2) What is the nature of cluster support interactions, and do the Pd atoms interact with the lattice O or the Ti sites? (3) How does the atomic structure change as successive O atoms are added to the cluster, and what electronic features control the change? We believe that an answer to these questions is critical to a microscopic understanding of the catalytic phenomenon as it involves addition and removal of O atoms. Our emphasis on Pd4 stems from the studies on free clusters that indicate that Pd4 is a highly stable species with a tetrahedral structure. Consequently, understanding the microscopic mechanism that results in structural changes for such a stable species is an important problem. All of the studies are first-principles and are carried out within the methods described below.

’ COMPUTATIONAL DETAILS AND SURFACE MODEL First-principles theoretical studies were performed within the density functional theory framework using the generalized gradient approximation (GGA) as proposed by Perdew et al.8 The calculations were carried out using the Vienna Ab initio Simulation Package (VASP).9 The projector-augmented wave method was used to model electron
ion interaction, and the valence states of Ti, O, and Pd were described by [Ar] 3d34s1, [He] 2s22p4, and [Kr] 5s14d9 electron configurations, respectively.10,11 Geometry optimizations were performed using a conjugate gradient algorithm, and atoms were only considered relaxed when all atomic force components were smaller than 0.01 eV/Å. For all geometry optimizations and total energy calculations, the Brillouin zone was sampled at the Γ point, which was found to give convergent results. The plane-wave basis set with a kinetic energy cutoff value of 400 eV was used and was determined to give convergent results. In addition to the calculations on the bulk surface, we have carried out supplementary calculations on representative clusters to identify the electronic phenomenon associated with local chemical bonds. These studies were carried out using the Amsterdam Density Functional (ADF) code.12 The exchange correlation effects were incorporated via a PBE gradient-corrected exchange correlation with a QZ4P basis set and the ZORA (Zeroth Order Regular Approximation) for relativistic effects.13 To simulate the rutile TiO2(110) surface, a periodic slab with a thickness of four TiO2 trilayers (12 atomic layers) was placed in a supercell, where each slab was separated by vacuum layer of ∼18 Å in the [110] direction. The large vacuum layer serves to ensure no interaction between periodic images, particularly accounting for the additional height added by the Pd cluster. To ensure no lateral Pd cluster interaction, a 4  2 surface supercell (four unit cells in the [001] and two in the [110] direction) was used. This results in a supercell with lengths of 13.1  11.9 Å and

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Figure 1. Top view of the 4  2 surface supercell of rutile TiO2(110). Large (blue) and small (red) spheres represent titanium and oxygen atoms, respectively. The two types of titanium atoms, six-fold-coordinated [Ti(6c)] and five-fold-coordinated [Ti(5c)] are labeled. The two types of oxygen atoms, bridging [O(2c)] and in-plane [O(3c)], are also labeled. The dashed lines indicate the 1  1 unit cell. For clarity, only the top layer of the slab is shown here.

effectively creates a separation of at least 9 Å between Pd4 clusters. Throughout all calculations, we allowed the top two trilayers to fully relax during the geometry optimization, while freezing the bottom two trilayers in the calculated bulk coordinates. Accounting for the relaxation of surface atoms has previously been shown to be necessary in predicting the adsorption site of even a single Pd atom.14 For free cluster calculations, the clusters were placed in the center of a supercell identical in size to the supercell used for the slabs. Unlike bulk Pd, small Pdn clusters have nonzero spin moments.15,16 Our search for lowest energy structures, therefore, included a search over different spin multiplicities. In many cases, the ground state was found to be marked by the presence of energetically close isomers. In fact, the isomeric form for bare Pd4 is stabilized as an O atom is added. They may also be accessible at finite temperatures in the experiments. Consequently, our results include information on these isomers. Our investigations of the deposited cluster and its oxidation included effects associated with charge transfers. In this work, we have used a Bader charge analysis as implemented by Henkelmen et al.17,18

’ RESULTS The 4  2 surface supercell used to model the TiO2(110) surface is shown in Figure 1. The surface consists of two types of Ti and two types of O atoms, which are labeled by their atomic symbol and coordination number (e.g., Ti(5c) and Ti(6c) for the five- and six-fold-coordinated Ti atoms, respectively). Our optimized bond lengths for the TiO2 surface are all within 5% of previously reported experimental19 and theoretical20 values (Table S1, Supporting Information). We begin by considering the ground state of a Pd4 deposited on the TiO2 surface. A Pd4 cluster was placed on the surface in various initial structures and at different locations on the surface. This included starting from the compact, tetrahedral ground state of the free Pd4 cluster as well as planar structures. To attain the lowest energy configurations, the positions of both the Pd atoms and top two trilayers of the surface were allowed to relax 20218

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Figure 2. Pd4, Pd4O, and pseudoplanar Pd4O2 ball-and-stick drawings. For all figures, the positive and negative numbers in the top portion of the figure indicate the Bader charge on the Pd and O atoms, while the negative number below the surface indicates the total charge on the TiO2 surface. The numbers on the bottom portion of the figure are bond lengths. Coloring scheme: silver (Pd), blue (Ti), and red (O).

until the forces dropped below the threshold value. The results indicate a tetrahedral ground state for the adsorbed Pd4 cluster (Figure 2a). The structure (including bond lengths) is nearly unchanged from the calculated free cluster geometry. The Pd4 cluster sits in the surface channel made by the bridging O(2c) atoms that protrude out of the TiO2(110) surface. Three Pd atoms are coordinated to the surface, and the base of the tetrahedral cluster is centered over a Ti(5c) atom. The surface undergoes a relaxation in which several surface atoms are displaced from their initial positions. For example, by comparison with the unreconstructed surface, the Ti(5c) atom protrudes out of the [110] plane by a distance of 0.35 Å and shifts 0.11 Å in the [110] direction, centering directly beneath the base of the Pd4 tetrahedral. The bridging O(2c) atom nearest to the Pd4 cluster, depicted with a drawn bond in Figure 2a, is tilted toward the channel by 0.11 Å from its initial position. This leaves a Pd
O bond length of 2.15 Å on this side of the cluster, while the two Pd atoms on the other side of the cluster are each about 2.90 Å away from the O(2c) atoms nearest to them. As the surface undergoes substantial relaxation, we calculated the energy required for the surface deformation in the following way. We calculated the total energy of the bare surface frozen in the atomic positions when the cluster is supported and then compared this energy to that of the relaxed, bare surface. Our results yielded a surface deformation energy of 1.27 eV for the deposited Pd4 cluster. The ground state has a spin magnetic moment of 2.0 μB that is localized entirely on the Pd d-states. This indicates that the cluster maintains its free cluster geometry and spin configuration when deposited on TiO2(110). The ground state is followed by a pseudoplanar structure, which is only 0.10 eV above the ground state. Within the accuracy of the density functional approach, this energy difference is small, and the pseudoplanar structure can be considered as an isomer that coexists with the tetrahedral structure. This pseudoplanar geometry also sits inside the surface channel; however, this

isomer sits over two protruding Ti atoms. A ball-and-stick model of the resulting atomic positions is shown in the Supporting Information Figure S1a. The cluster is partially flattened out on the surface in comparison to the tetrahedral ground state. This effectively leads to shorter bond lengths of around 2.23 Å between three of the Pd and neighboring O(2c) atom pairs. The higher coordination between Pd atoms and surface atoms leads to the quenching of the magnetic moment of the cluster. The small energy difference between pseudoplanar and compact structure is quite striking for two reasons. First, our calculations on a free Pd4 cluster show that a planar structure is almost 0.85 eV less stable than the compact structure (Figure S2a). Second, the Pd4 cluster maintains its compact structure when deposited on an alumina surface.4 What features then stabilize the planar structure in the present case? What role does the planar structure play in oxidation reactions? To answer these questions, we investigated the effect of adding an O atom to the cluster. To determine the ground state structure, we performed geometry optimizations for 13 different conformations of an O atom and supported Pd4 cluster. We found the optimum binding site by relaxing the O atom, the Pd4 cluster, and the top two trilayers of the support for all initial binding sites and then comparing the total energies. Initial conformations included O adsorption sites both on and off the Pd4 cluster, and O adsorption on the Pd4 cluster was always found to be more stable. In the free Pd4 cluster, an O atom occupies a two-fold bridging site, leaving the structure compact (see Figure S2b). For the supported cluster, our studies indicate a planar ground state structure with the O atom occupying a threefold hollow site, shown in Figure 2b. The adsorbed O atom is bonded to three Pd atoms with an average Pd
O bond length of 2.01 Å. Previous work by Todorova et al. indicates that this is also the case for the Pd(111) surface, where an O atom from a dissociated O2 molecule sits at a three-fold hollow site with an average Pd
O bond length of 1.99 Å.21 As seen in Figure 2b, the 20219

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the O atoms is bound to three Pd sites, while the other occupies a two-fold bridge site. In this case, there is very little surface deformation that takes place on the TiO2 support. The calculated surface deformation energy is 0.92 eV, the smallest deformation energy among the cases considered here. Since the ground state Pd4 structure changes with a varying degree of oxidation, one of the central issues is what controls the geometry and the associated electronic structure changes.

Figure 3. Pd4O2 (oxygen spillover) structure depicted by ball-and-stick drawing.

Pd4 cluster sits in the surface channel centered over two protruding Ti surface atoms. In comparison to the tetrahedral Pd4 structure, the bond lengths between Pd atoms and the bridging O(2c) atoms of the surface are shortened. That is, each of the four Pd atoms is coordinated to a single bridging oxygen atom, which is canted toward the center of the channel and thus the Pd4 cluster. The calculated surface deformation energy of 2.18 eV indicates a much stronger surface deformation by the Pd4O cluster compared to Pd4. In the ground state of the supported Pd4O structure, the adsorbed O is bound to three Pd atoms with an average Pd
O bond length of 2.01 Å. Does the planar structure survive as more O atoms are added? To determine the possibility of another structural change, an additional O atom was added to the cluster. To determine the ground state structure, we performed geometry optimizations by the same methodology as mentioned above for the single O atom, but by sampling 23 different conformations of two O atoms and a Pd4 cluster. Conformations where one or both O atoms were adsorbed off the Pd4 cluster were also sampled, and the results indicate a preference for at least one O atom to remain bound exclusively to the Pd4 cluster. It should be noted here that two supported Pd4O2 structures will be discussed. Both structures represent local minima in the potential energy surface, but one is 0.60 eV higher in energy than the ground state. The higher energy (less stable) structure is the lowest energy configuration in which both O atoms remain bound exclusively to the Pd4 cluster, while in the other case, one of the two O atoms “spills over” onto the TiO2 surface. The latter is the calculated absolute ground state geometry, but the possibility of a high potential energy barrier for oxygen spillover leads us to consider both structures in detail. In the ground state structure, the Pd4 motif remains planar and adsorbs a single O atom at a three-fold hollow site, as in the case of the supported Pd4O cluster; however, the second O atom spills over to the neighboring, bare Ti(5c) atom and binds with a Ti
O bond length of 1.72 Å and a Pd
O bond length of 2.18 Å. The structure is depicted in Figure 3. This Ti(5c) atom is displaced out of the surface by 0.81 Å. The resulting surface deformation energy is 3.56 eV. The case where both O atoms remain bound to the Pd4 cluster is shown in Figure 2c, where the Pd4 cluster possesses a pseudoplanar geometry. One of

’ DISCUSSION To examine the nature of bonding and the origin of cluster and surface deformations, we undertook a detailed analysis of the charge transfer, binding energies, and electronic structure of the clusters and substrate. For the sake of clarity, we discuss the Pd4, Pd4O, and pseudoplanar Pd4O2 results first and discuss the case of oxygen spillover separately. We first address the compact and pseudoplanar configurations of a Pd4 cluster. For a free Pd4 cluster, a planar structure is almost 0.85 eV higher than the compact geometry (Figure S2a). In bonding to the surface, however, only the Pd atoms forming the base of the tetrahedron are bound to the surface O sites. On the other hand, the pseudoplanar structure allows better bonding with the surface sites. This stabilizes the planar form, making it only 0.10 eV higher than the compact ground state. We also carried out an analysis of the charge redistribution upon adsorption. Figure 2 shows the charges at various atomic sites with the number below the figure indicating the net charge on the TiO2 surface (positive numbers indicate charge depletion, while negative numbers indicate charge accumulation). Figures 2a and S1a show that, while the compact Pd4 transfers 0.48 e
to the surface, the pseudoplanar cluster transfers 0.57 e
. The planar form is really stabilized via the addition of an O atom. In fact, the cluster reconstructions are driven by the accumulation of charge by adsorbed oxygen. In the case of Pd4O, by adsorbing at a three-fold hollow site, the O atom maximizes Pd
O bonds and accumulates a Bader charge of 0.82 e
(Figure 2b). This is the maximum amount of charge gained by the adsorbed O atom compared to all other sampled Pd4O geometries. In free clusters, the ground state Pd4O is a tetrahedral Pd4 with O bound at a two-fold bridging site. Such a structure for the deposited cluster is 0.41 eV higher in energy and allows only 0.76 e
to be accumulated by the O atom (Figure S1b, Supporting Information). In general, charge accumulation is maximized with a maximum number of Pd
O bonds and directly leads to the ground state structure, despite a substantial rearrangement of Pd atoms from their free cluster geometry. We observe the same trend for pseudoplanar Pd4O2 species. Here, one of the O atoms accumulates 0.86 e
of charge at a three-fold hollow site, while at the opposite side of the cluster, the second O atom accumulates 0.80 e
at a two-fold bridging site. We describe the stability of this particular structure by the following two mechanisms. First, compared to the cases where both O atoms are bound to the Pd4 cluster, we see that the pseudoplanar Pd4O2 results in the maximum charge accumulation by oxygen. Second, the electron accumulation by the O atoms effectively creates two point charges interacting via a Coulomb potential. By adsorbing at opposite sides of the cluster with a mediating Pd site, these point charges are separated by 3.79 Å, and the Coulombic repulsion is minimized. Further, the Pd
O bonds stabilize the structure through polar, covalent interactions. We believe that this O
O separation plays an important role in minimizing the 20220

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total energy of the system and is the mechanism that unfolds the Pd4 structure into a pseudoplanar geometry. Our studies further indicate that a configuration where both the O atoms occupy spillover Ti(5c) sites on opposite sides of the cluster is 2.00 eV less stable than the above ground state where one of the O atoms is on the Pd4 cluster and the other is in the spillover mode. The reduced stability of this configuration could be reconciled by the fact that the Pd site mediating the two O atoms is more positively charged when one of the O atoms is located on the deposited cluster. A resultant attractive interaction between Pd and O sites then drives the system to a lower (more stable) total energy. It is well-known that the metal
O bond strength plays an important role in the catalytic oxidation of CO.22 Metal
O binding energies of 3.3 to 4.0 eV are optimal for the highest CO oxidation activity.22 We calculated the binding energy of the O atom to the Pd cluster by taking the difference between the total energy of the ground state oxidized cluster and the sum of total energies of the unoxidized cluster and free oxygen atom. Our results indicate that, in the case of Pd4O, the O atom binds with an energy of 4.98 eV. For pseudoplanar Pd4O2, the second O atom binds with an energy of 4.33 eV. In the latter case, the reduced oxygen binding energy is due to the competition of charge accumulation of the two O atoms, effectively weakening each Pd
O bond. In this respect, it may be interesting to Table 1. Comparison of the Cluster Binding Energy (eq 1), Charge Transferred from the Cluster to the Surface, and the Surface Deformation Energy for Pd4, Pd4O, and Pseudoplanar Pd4O2 Supported Clusters Pd4

Pd4O

Pd4O2

Pd4Ox binding energy (eV)

2.40

3.21

3.03

charge from Pd4Ox to surface (e
)

0.48

0.64

0.56

surface deformation energy (eV)

1.27

2.18

0.92

examine if this binding can be further weakened by additional O2 molecules. Cluster support interactions play an important role in catalysis, and we next analyze this interaction. To estimate the strength of bonding, we compared the total energy of the deposited species to the total energies of the free Pd4Ox motifs and the free TiO2 surface by the following equation: BEðPd4 Ox =TiO2 Þ ¼ EðPd4 Ox Þ þ EðTiO2 Þ
EðPd4 Ox =TiO2 Þ

ð1Þ

where (x = 0, 1, 2), and we consider the Pd4Ox cluster to bind as a single motif. The results are reported in Table 1. There is a clear correlation between the number of Pd atoms directly coordinated to the surface and the binding energy. We find a rough binding energy of 0.8 eV per directly coordinated Pd atom. That is, for Pd4 where three atoms directly coordinate to the surface, the cluster binds with an energy of 2.40 eV, while Pd4O coordinates with four Pd atoms and binds with 3.21 eV. The pseudoplanar Pd4O2 structure represents an intermediate case where between three and four atoms are directly coordinated to the surface, yielding a binding energy of 3.03 eV. This correlation of binding energy and number of directly coordinated Pd atoms agrees with results previously shown for Pd6 and Pd13 clusters supported on TiO2 as reported by Murugan et al.23 and Pd12 supported on TiO2 as reported by San-Miguel et al.24 To further probe the nature of interactions marking the Pd4, adsorbed O, and the surface sites, we examined the total density of states (DOS) and the projected density of electronic states (PDOS) at various sites of the TiO2 and the deposited cluster. Figure 4 shows the total and various local densities of states for Pd4 on TiO2, where the Fermi level has been set to E = 0. The sharp peak of O states at about 1.3 eV below the Fermi level originally makes up the top of the valence states for the TiO2 surface. The insertion of Pd states between this peak and the bottom of the conduction states reduces the gap. In Figure 5,

Figure 4. Total density of states (DOS) and projected DOS of the bare Pd4 cluster supported on TiO2. The Fermi level is shifted to E = 0. The states above the horizontal line at density = 0 are majority states, while below are the minority states. 20221

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Figure 5. Total and projected DOS for (a) Pd4O and (b) pseudoplanar Pd4O2 cluster supported on TiO2.

we show the DOS for (a) Pd4O and pseudoplanar (b) Pd4O2 supported on TiO2. Near the Fermi level, we see now the mixing of Pd d-states with the states of adsorbed O and a metallic character. To determine the role of Pd clusters in the deformation of the surface, we return to our results from the Bader analyses shown in Figure 2. We find that the more a cluster coordinates to the surface, the more charge it transfers to the surface. For all three cases, Pd atoms readily donate charge, which is transferred almost entirely to the Ti and O atoms in the top trilayer of the surface. These atoms are resilient to a change from their bare surface charge state. This is indicated by a change in charge state of less than 0.06 e
for all atoms except the Ti(5c) atoms directly beneath the Pd cluster. Effectively, the Pd cluster is donating a small amount of charge to all of the neighboring bridging O(2c) and in-plane O(3c) atoms, resulting in the accumulation of charge by Ti atoms, as well. We find that the amount of charge accumulated by these Ti atoms governs the extent of Ti atom displacement and thus governs the surface deformation energy. That is, for Pd4, Pd4O, and pseudoplanar Pd4O2, the two Ti atoms beneath the Pd cluster accumulate a combined 0.15, 0.21, and 0.07 e
, respectively, and are displaced from their original position a combined distance of 0.42, 0.44, and 0.20 Å, respectively. By comparing the displacements for all atoms before and after Pd cluster deposition, the protrusion of these Ti atoms accounts for almost all of the surface deformation. The mechanism for surface reconstruction can be understood in the following way. The Ti
O bonds are partially ionic, and the accumulation of charge by Ti results in the weakening of this ionic bond between the Ti atoms and nearest-neighbor O atoms. Our conclusion is reinforced by our calculations on a free TiO2 cluster. We monitored the change in Ti
O bond length as charge was added to the system. It was found that the Ti
O bond length increases by 3% upon the addition of an electron to the TiO2 cluster.

We now consider the microscopic mechanism of the so-called “strong metal support interaction” (SMSI) observed for some transition metals supported on reducible oxides.25 As indicated above, the Ti sites are displaced from their positions in the lattice by the charge transfer from lattice O sites, which depends on the degree of coordination between cluster species and support. In addition, we found that there is minimal interaction between the Pd and Ti sites. This can be seen from the local density of electronic states at the Pd, Ti, and O sites shown in Figure 6. There is no appreciable mixing between Ti and Pd states. As a further confirmation to this, we carried out model electronic structure calculations on a free Pd
Ti dimer using the ADF code. The molecule is bound with a binding energy of 3.17 eV and a bond length of 2.20 Å. Figure 7 shows the one-electron energy levels and molecular orbitals, and one can clearly identify the bonding orbitals. We then examined the progression in bonding as O atoms were added. Figure 7 shows that, in the oxidized structure, the Pd
Ti bond stretches to 2.66 Å, and the molecular orbitals do not show any significant Pd
Ti bonds. In regards to supported clusters, this suggests that the Pd
Ti interactions are weak and have no direct relation to the large displacement of Ti out of the surface. Instead, the weakened Ti
O ionic interactions, due to Pd-to-O charge transfer, allow Ti ions to protrude and become mobile. This Pd-to-O charge transfer represents the first of two mechanisms in which the Ti
O ionic interactions are weakened and Ti atom surface protrusions occur. We describe the second mechanism in the following paragraph. We would like to add that, in the prototypical case of Pt nanoparticles supported on TiO2(110), the SMSI state is observed as either encapsulation of Pt or the formation of an intermetallic alloy by diffusive Ti ions.26,27 We believe that an appreciable charge transfer between the nearsurface Pt atoms and the surface O(2c) and O(3c) atoms again results in similar weakening of the Ti
O bond. 20222

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Figure 6. Projected DOS showing the local bonding character of a single Ti (blue), O (green), and Pd (red) atom. The ball-and-stick models are colored according to the DOS plot.

Figure 7. Electronic states and free cluster geometries of PdTi (left) and PdTiO2 (right). The electronic energy levels and corresponding molecular orbital plots are shown. The Fermi energy has been shifted to E = 0. White spheres represent Pd atoms, blue spheres represent Ti atoms, and red spheres represent O atoms.

We now return to Pd4O2 where oxygen spillover is considered. The O atom that remains bound to the Pd4 cluster accumulates 0.78 e
of charge, while the spilled O atom accumulates 0.88 e
. It is visible in Figure 3 that the Pd atom between the two adsorbed O atoms is coordinated to both and hence donates more charge than the other three Pd atoms. Following the binding energy equation stated earlier, our calculated Pd4O2 binding energy is 3.64 eV. The Ti(5c) surface atom, that the O atom is spilled onto, is displaced from its original position by 0.81 Å. The mechanism behind such a large displacement is due to the Ti(5c) atom’s tendency to weaken Ti
O ionic interactions in order to donate charge to the spilled O atom. Considering the Bader charge of 0.88 e
on the spilled O atom, one might expect the Ti(5c) directly beneath it to undergo a dramatic change in charge state. However, the Bader analysis indicates the Ti atom

depletes only 0.07 e
more than it would in the bare surface. By undergoing such a large displacement, the Ti atom effectively decoordinates from neighboring O atoms, which accumulate charge from the Pd atoms, and this results in a charge transfer of 0.88 e
to the spilled O atom. Thus, the decoordination of Ti to neighboring O atoms, in order to donate charge to the spilled O atom, represents a second mechanism of weakening Ti
O ionic interactions. The existence of oxygen spillover in Pd4O2 could also account for recent experimental findings in the group of Anderson on the catalytic oxidation of CO by TiO2(110)supported Pdn clusters.28 Oxidation of CO at 550 K by the oxidized Pdn species generates CO2, but the amount of CO2 is much less than to account for the removal of all O atoms. Yet, the experiments do not see desorption of O2 up until 600 K. In fact, there are no adsorbates left on the Pdn surface, suggesting that the remaining O atoms have spilled over onto the TiO2 surface. While we have not considered the CO oxidation in this paper; we believe that the O adsorbed on the Ti(5c) sites could have a higher barrier for oxidation than those on the Pd sites. This is because our preliminary studies indicate that the preferred CO adsorption site on a Pd4 cluster is a hollow site where the C can easily bind to the neighboring O atom on a Pd site. On the other hand, recent studies indicate that a CO molecule binds on top of a Ti(5c) site on the TiO2 surface.29 Since the Ti sites are separated by 3.1 Å, the oxidation of CO by the adsorbed O on another Ti site is more difficult. We are currently carrying out detailed studies to investigate these possibilities.

’ CONCLUSIONS The present work demonstrates that the supported Pd4 clusters on the TiO2 surface undergo significant geometrical and electronic changes as O atoms are added to the cluster. For the bare Pd4, our studies indicate both compact and pseudoplanar configurations. We show that Pd4O presents a planar structure while Pd4O2 presents two different possibilities, namely, a pseudoplanar structure and oxygen spillover structure. For Pd4O2, our studies indicate that the most stable configuration 20223

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The Journal of Physical Chemistry C corresponds to a spillover O that binds to a surface Ti site, and that these findings could account for some of the recent experimental observations. The studies also indicate a locally stable pseudoplanar Pd4O2 structure where both O atoms are bound to the Pd4 cluster. We show that there is significant interaction between the deposited clusters and the substrate, and that structural changes in the deposited clusters can be related to the charging of the O atoms. Finally, our studies suggest that the microscopic origin of the strong metal support interaction may lie in two different mechanisms. In both cases, Ti
lattice O interactions are weakened and Ti atoms are displaced out of the surface. In the first weakening mechanism, the deposited Pd atoms donate charge to the surrounding lattice oxygen atoms, while in the second mechanism, the Ti donates its charge to spilled over oxygen. Palladium clusters are important oxidation catalysts, and we believe that the ability of the clusters to adapt to the deposited atoms could play an important role in controlling the reaction barriers.

’ ASSOCIATED CONTENT

bS

Supporting Information. Ball-and-stick models of isomer and free cluster geometries and a table listing the bond lengths of the optimized TiO2(110) geometry. 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].

’ ACKNOWLEDGMENT We gratefully acknowledge support from the Air Force Office of Scientific Research (AFOSR) through a MURI Grant No. FA955008-1-0400. In addition, we thank Prof. Scott L. Anderson and Prof. H. Wang for stimulating discussions. ’ REFERENCES (1) Ciuparu, D.; Lyubovsky, M. R.; Altman, E.; Pfefferle, L. D.; Datye, A. Catal. Rev.
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