Adsorption of Pd Atoms and Dimers on the TiO2 (110) Surface: A First

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J. Phys. Chem. C 2007, 111, 3949-3955

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Adsorption of Pd Atoms and Dimers on the TiO2 (110) Surface: A First Principles Study Javier Fdez. Sanz* and A. Ma´ rquez Departamento de Quı´mica Fı´sica, Facultad de Quı´mica, UniVersidad de SeVilla, E-41012 SeVilla, Spain ReceiVed: June 26, 2006; In Final Form: NoVember 28, 2006

The adsorption of Pd-isolated atoms and dimers on the (110) surface of rutile TiO2 has been investigated in a range of coverage using a periodic supercell approach within density functional theory using a generalized gradient approach and plane-wave basis sets. Pd atoms prefer to adsorb on the surface channels near to the 5-fold-coordinated Ti atoms but tilted toward the protruded oxygen atoms. The adsorption energies along the channel appear to be close to each other, suggesting easy mobility of Pd atoms on the surface. Adsorption at surface oxygen vacancies is stronger than that on the defect-free surface, indicating that defects would act as nucleation sites. Pd dimers on the surface are also located on the channels and exhibit a noticeable metal-metal bond. The analysis of the metal-surface chemical bond shows that the main contribution to the interaction energy is due to the Pd polarization. A detailed study of the electronic structure of the system in terms of the density of states and electron density maps is also reported.

I. Introduction Transition metal deposition on metal oxides plays a crucial role in such diverse industrial areas as microelectronics, catalysts, photovoltaic cells, and protective coatings for metals.1-3 Understanding the nature of such interfaces constitutes one of the most appealing challenges nowadays for material scientists. Among a wide variety of metal oxides used as supports, probably the most studied model oxide surface is the TiO2 (110) surface.4-6 On the other hand, palladium-supported catalysts are used in important processes as hydrocarbon combustion and hydrogenation reactions.7,8 The structural and electronic properties of the Pd/TiO2 (110) system have been extensively studied using a wide variety of experimental techniques. These involve photoelectron spectroscopy and resonant photoemission,9 coaxial impact-collision ion scattering spectroscopy (CAICISS),10 infrared absorption spectroscopy (FT-RAIRS),11 high-resolution electron energy-loss spectroscopy (HREELS),12 and especially scanning tunneling microscopy (STM).13-18 In particular, Goodman and co-workers have carefully studied the adsorption of Pd on this surface.13 From the STM images obtained at low coverage, these authors reported that Pd dimers and tetramers are present on the surface, adsorbed on the 5-fold titanium rows (see Figure 1 for a general view of the surface). Also, and significantly, images corresponding to isolated Pd atoms adsorbed on the surface were not observed. From a theoretical point of view the number of theoretical studies devoted to Pd adsorption on metal oxides is relatively scarce. Adsorption on the MgO (100) surface has been analyzed both from quantum mechanical calculation approaches at different levels of theory19-22 and molecular dynamics simulations.23 The Pd/alumina interface has also been investigated from a theoretical point of view using modern quantum mechanical methods,24-28 as well as MD simulations.29 However, the number of works addressing the Pd/TiO2 (110) system is surprisingly low. Bredow and Pacchioni reported a quantum mechanical study of the interaction of Pd atoms and dimers on the TiO2 (110) surface.30 Using embedded cluster models and * To whom correspondence should be addressed. E-mail: [email protected].

B3LYP density functional calculations, these authors found that the preferred sites for Pd atoms were the protruding oxygen atoms, with adsorption energies of about 1.0 eV. Yet, test periodic supercell calculations carried out in the same work seemed to show a slight preference for the 5-fold titanium rows. A drawback of these calculations is that the surface was kept fixed, and therefore the effects of a possible induced relaxation of the surface were not accounted for. The adsorption of isolated Pd atoms on the (110) surface of rutile TiO2 was also investigated by Sanz et al.31 through ab initio embedded cluster calculations performed at the Hartree-Fock, MP2, and B3LYP levels, also using a frozen surface approach. In these models, the role played by the value of the surrounding charges used in the embedding procedure was carefully analyzed. The most stable site for adsorption consisted of a 4-fold hollow site in which the Pd atom was coordinated to a 5-fold Ti atom, two in-plane oxygen atoms, and a protruded oxygen atom. However, the adsorption energies computed after correcting the basis set superposition error (BSSE) seemed to favor a bridge site in which the Pd atom binds two protruded oxygen atoms. In the same work, preliminary periodic slab calculations performed within the density functional theory (DFT) and the gradient corrected approach (GGA) were also reported. The results showed that, for full coverage, the hollow site was more stable, although Pd displacements along the 5-fold Ti channels should be almost free. After surface relaxation, the adsorption energy computed was found to be 1.88 eV. These contradictory results make it clear that the question is far from being well-understood and that more theoretical work on this system is compulsory. In the present work a theoretical analysis of the adsorption of Pd atoms and clusters on the TiO2 (110) surface based on DFT calculations performed under periodic boundary conditions is reported. Several sites and surface models sampling a range of Pd coverage have been considered, and both stoichiometric and reduced surfaces have been modeled. The paper is arranged as follows. In section 2 we describe the surface models used in the calculations. Section 3 deals with the results, and the main conclusions are summarized in section 4.

10.1021/jp0639952 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/21/2007

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Figure 1. TiO2 general view (left) and adsorption sites view from the top (right).

TABLE 1: Pd-TiO2 (110) Binding Energies (eV) Obtained from Different Slab Models

O(top) O(br) Ti(5) h(Ti) O(in) h(O)

1×1 5 layer

2×1 5 layer

2×1 4 layer

4×1 4 layer

2×1 3 layer

4×1 3 layer

4×2 3 layer

1.60 1.51 1.55 1.70 1.79 1.90

1.10 1.48 1.27 1.46 1.51 1.62

0.91 1.37 1.21 1.41 1.43 1.51

0.88 1.36 1.25 1.44 1.46 1.52

1.25 1.61 1.38 1.53 1.61 1.83

1.38 1.74 1.45 1.58 1.71 1.93

1.46 1.71 1.41 1.62 1.83 1.83

II. Computational Details and Surface Models To simulate the TiO2 (110) surface, a periodic slab of finite thickness has been used, each slab separated by a vacuum of 10 Å width. As reported by Bates et al.,32 the ideal slab thickness for most purposes is four or more layers, where a layer means a (110) plane that contains both Ti and O atoms. In the present work three-, four-, and five-layer slabs have been used, depending on the size of the surface cell. The dimensions of the smallest surface unit cell are cx2a, (along [11h0] and [001] directions, respectively), a and c being the two parameters of the tetragonal lattice of rutile TiO2. Multiples of the unit cell along the [001] direction give cells of the n × 1 type, which correspond to a coverage of θ ) 1/n for one Pd atom. Doubling along the [11h0] direction gives n × 2 cells, which for one Pd atom gives coverage of θ ) 1/2n. The surface cells that we have used in the present work have been the following: 1 × 1 and 2 × 1 five-layer thick (30 and 60 atoms, respectively); 2 × 1 and 4 × 1 four-layer thick (48 and 96 atoms); 2 × 1, 4 × 1, and 4 × 2 three-layer thick (36, 72, and 144 atoms). To represent the extended nature of the TiO2 (110) rutile surface, DFT calculations under periodic conditions along the three directions were performed using the VASP code.33-35 In these calculations the energy was obtained using the GGA implementation of DFT proposed by Perdew et al.36 Ultrasoft pseudopotentials37 were employed to remove the core electrons from the calculations, and a plane-wave basis set was used to span the valence electronic states. The cutoff energy for the plane waves was 396 eV. For 1 × 1, 2 × 1, and 4 × 1 cells, the structural optimizations were performed using the lowest order Monkhorst-Pack set of 2 × 2 × 1 k-points. The Γ point was used for the large 4 × 2 cell. In the structural optimizations full relaxation of the slab was performed. Forces on the ions were calculated through the Hellmann-Feyman theorem as the partial derivatives of free energy with respect to the atomic position, including the Harris-Foulkes38 correction to forces. This calculation of the forces allows a geometry optimization using the conjugategradient scheme. Iterative relaxation of atomic positions was stopped when the change in total energy between successive steps was less than 0.001 eV. With this criterion, forces on the atoms were generally less than 0.1 eV/Å. Binding energies were

calculated as the difference between the sum of the free surface and isolated Pd energies and the surface-Pd system energy:

BE ) E(surf) + E(Pd) - E(surf-Pd) Most of the recent work about chemisorption on TiO2 has been carried out using a GGA representation of the exchange correlation functional. However, Mattsson and Jennison39 have pointed out that for defect-free metal oxide surfaces, GGA binding energies tend to be underestimated, and then, a local density approximation (LDA) seems to be more reliable. Although the Pd-surface interaction is not, in general, that low, to analyze this aspect appears to be convenient at this point. The binding energy for the Pd/TiO2 (110) system reported from GGA periodic supercell calculations is 1.88 eV.31 Compared with the values obtained from embedded cluster calculations of 1 eV (B3LYP)30 and 2.3 eV (HF-MP2),31 as well as with that computed from periodic slab calculations, 0.95 eV (BLYP), the GGA functional does not seem to underestimate the interaction, although, of course, the comparison is not meaningful because of the dispersion of the data and, mainly, the lack of experimental results. A further test can be drawn by computing the adsorption energy for a well-known system involving Pd: the Pd/MgO interface, for which both experiment40 and high-level quantum mechanical calculations19 agree that the interaction energy is about 1 eV. The calculations were setup using a MgO surface model consisting of a 2 × 2 atom cell five layers thick to which periodic boundary conditions were imposed as described above. The adsorption energies computed at the oxygen site are 2.18 eV (LDA) and 1.41 eV (GGA). However, in these energies the lateral interaction between adsorbed Pd atoms is included (Pd atoms are only 4 Å far). Computing the interaction energy not with respect to the Pd isolated atom but with respect to the Pd layer gives 1.83 eV (LDA) and 1.14 eV (GGA), the latter in quite good agreement with experiment and theoretical predictions. These results show that at least as far as the Pd-metal oxide interface is concerned, GGA gives rise to interaction energies quite reasonably. III. Results and Discussion Structure and Bonding Effects. Using the above-described supercell models, six different sites on the TiO2 (110) surface

Pd Adsorption on the TiO2 (110) Surface

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Figure 2. Optimized structures of adsorbed Pd on the perfect TiO2 (110) surface (left) and at an oxygen vacancy (right).

were considered for Pd, as depicted in Figure 1. Position O(top) corresponds to adsorption on top of a protruded oxygen atom. In position O(br) Pd interacts with the surface bridging two protruded oxygen atoms. Position Ti(5) corresponds to adsorption on top of 5-fold coordinated titanium atoms, while O(in) is on top of in-plane oxygen atoms. There are two more sites, which correspond to hollow sites: h(Ti) and h(O). In h(Ti) Pd atom sits on the channels between two Ti(5) sites, while, in h(O), the Pd atom binds two in-plane oxygens, a Ti(5), and a protruded oxygen atom. The results obtained after the structural minimizations for the different models and sites are reported in Table 1. Using the 1 × 1 five-layer thick model, the preferred site is the hollow h(O), the interaction energy being 1.90 eV close to that previously obtained from calculation that only included a partial surface relaxation.31 The O(in) position is only 0.11 eV less favored and is close to the h(Ti) hollow site. In general it appears that the sites on the channels are more stable than those interacting only with the protruded oxygen atoms. To check the dependence of the adsorption energy on the coverage, we can compare these results with those obtained using the 2 × 1 five-layer thick model slab. As can be seen in Table 1, there is a general lowering of the interaction energies by roughly 0.25-0.30 eV for the channel sites, while, for the atop and bridge sites, the behavior seems to be contradictory: large reduction for the atop O(top) site, and almost negligible for the bridge O(br) position. As for the Pd/MgO system referred to above, the larger adsorption energies for the 1 × 1 cell are due to the Pd-Pd intercell interactions that are only 4.78 Å far. The energy of the Pd layer in the 1 × 1 cell is 0.75 eV larger than that of free Pd (computed using a cubic cell of side 10 Å). When the Pd layer is adsorbed on the surface, the Pd electronic clouds are polarized toward the surface, and therefore the lateral interaction is lower than in the isolated Pd layer. Moreover, Pd polarization is larger when it sits on the channels than when it only binds the protruded oxygen atoms. There are three scenarios for this adsorbate-adsorbate interaction which also rely on the metal polarization. When Pd atoms are lying on the channels, the lateral interaction is weak whatever the site is, and the decrease of the energies is low. At variance, when Pd is on top of protruded oxygen atoms, because of the lower Pd polarization, the lateral interaction is larger, and

enlarging the cell dimension gives rise to a significant lowering of the interaction energy (0.49 eV). The third case is when Pd atom is at the O(br) position, where the protruded oxygen atoms partially screen the lateral interaction in the 1 × 1 cell, and therefore the diminution in the adsorption energy when the cell is enlarged is small. We can now analyze the effects produced when the thickness of the layer changes. In Table 1 one can see that the adsorption energies obtained with the 2 × 1 five-layer and 2 × 1 fourlayer slabs are practically identical, maybe slightly lower, showing that both model slabs behave similarly in agreement with previous findings.32 The changes in the adsorption energies observed for the 2 × 1 three-layer thick slabs are slightly larger. With respect to the 2 × 1 five-layer thick models there is a regular increment in the interaction energies of at most 0.11 eV, depending on the sites. This kind of oscillation with the slab thickness has already been observed for the surface energy,32 as well as for the energy associated with vacancies formation.41 If we assume the interaction energies obtained with the five-layer thick slab as converged, the four-layer slabs provide a quite good alternative at lower computational cost, while the three-layer slabs appear to be somewhat overestimated. Doubling the cell along the [001] direction again increases the adsorption energy, and the same effect is observed when the cell is doubled again, but now in the [11h0] direction. The regular increment of the interaction energies found when the three-layer thick cell is enlarged is likely due to the increasing contribution of the surface relaxation to the metalsurface bond. Upon adsorption, there is some surface relaxation induced by Pd atoms that mainly involves distortion of the geometry around the protruded and in-plane oxygen atoms, as shown in Figure 2, where the optimized structure for the h(O) site is depicted. However, this perturbation also affects the neighboring atoms, and the extent of the relaxation grows with the cell dimensions. To analyze the contribution of the relaxation to the interaction energy, a set of unrelaxed calculations in which the surface atoms were kept fixed at the positions obtained from the surface optimization was performed. Some results of these calculations in which only the Pd position is optimized are reported in Table 2. As expected these energies are systematically lower and show that surface relaxation roughly accounts

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TABLE 2: Pd-TiO2 (110) Binding Energies (eV) Obtained Using Unrelaxed Slab Models

O(top) O(br) Ti(5) h(Ti) O(in) h(O)

2×1 5 layer

2×1 4 layer

4×1 4 layer

2×1 3 layer

4×1 3 layer

4×2 3 layer

1.05 1.33 1.19 1.43 1.40 1.38

1.07 1.31 1.18 1.42 1.39 1.35

1.02 1.33 1.22 1.41 1.38 1.35

1.08 1.37 1.23 1.40 1.38 1.39

1.11 1.41 1.23 1.41 1.40 1.40

1.27 1.50 1.27 1.54 1.59 1.52

TABLE 3: Main Structural Parameters from Unrelaxed and Relaxed Calculations 4×1 3 layer O(top) O(br) Ti(5) h(Ti) O(in) h(O)

2×1 4 layer

distance/Å

relaxed

unrelaxed

relaxed

unrelaxed

Pd-O(top) Pd-O(br) Pd-Ti(6) Pd-Ti(5) Pd-Ti(5) Pd-O(in) Pd-Ti(5) Pd-O(in) Pd-O(br) Pd-Ti(5) Pd-O(in) Pd-O(br)

2.036 2.119 2.639 2.264 2.649 2.317 2.839 2.096 2.665 2.335 2.640 2.226

2.068 2.175 2.672 2.349 2.719 2.310 3.059 2.092 2.800 2.540 2.483 2.395

2.043 2.134 2.548 2.278 2.675 2.318 2.825 2.107 2.828 2.381 2.580 2.287

2.033 2.159 2.637 2.359 2.721 2.309 2.975 2.088 2.956 2.555 2.491 2.399

for 0.2-0.3 eV, depending on the model and the site we are considering. Regardless the number of layers, the 2 × 1 and 4 × 1 models give virtually the same binding energies and order preference, while larger values are found for the 4 × 2 threelayer cell, suggesting again that this model is not converged. It is worth noting that the relaxation contributions for the 4 × 2 model slab is similar to that estimated for smaller cells, indicating that the larger interaction energy observed is likely due to the larger surface polarization. It is also worth highlighting that the site preference sequence that we found for Pd remarkably agrees with that reported for Pt using the 2 × 1 four-layer thick model and a similar theoretical setup for the DFT calculations. The exception concerns only the two less stable sites Ti(5) and O(top) whose order is inversed. With respect to the adsorption energies Pt atom was found to bind more strongly to the surface (by ∼0.5 eV).42 Main structural parameters for two case examples have been reported in Table 3 for both relaxed and unrelaxed calculations. With respect to the slab thickness, the Pd-surface atoms distances are found to be similar and agree with previous periodic DFT calculations.31 In general the distances for relaxed calculations are found to be shorter than those for unrelaxed surfaces, although the differences are not as large as those observed when Pd atoms are deposited on the R-Al2O3 (0001) surface.24 Disregarding the slight differences found for the different thicknesses used to model the surface, the analysis of the data reported in Tables 1 and 2 allows one to conclude that Pd atoms prefer to lie on the channels of the TiO2 (110) surface although they are not sitting atop of 5-fold coordinated Ti atoms but instead they tilt toward the protruded oxygen atoms. Although these results disagree with those reported by Bredow and Pacchioni,30 they confirm our previous work carried out using both periodic cells and embedded cluster models with fractional charges.31 The preference for the channels also agrees with the experimental STM data obtained at low coverage reported by Xu et al.13 which indicate that nucleation of Pd clusters would be undertaken at the 5-fold Ti atom channels although our

computations show that they prefer to be tilted toward the protruded oxygen atoms. One of the most important issues of the TiO2 (110) surface concerns the presence of defects that clearly appear as spots in both STM and AFM images. These defects have been assigned to missing bridge oxygen atoms with a density of 7-10% per surface unit cell.5 Vacancies also involve subsurface oxygen atoms though in a lower density. To analyze the Pd adsorption on these defective surfaces, we have considered slab models with oxygen vacancies either at the bridge rows or at the subsurface. According to recent first principles calculations,41 we have modeled these slabs by means of a 4 × 1 four-layer thick supercell, which gives a vacancy density of 1/4. Adsorbed Pd atoms readily occupy the surface vacancies although the binding energy increases only by ∼0.3 eV. This moderate reinforcement of the interaction is in contrast with that reported for Pt, which was estimated to be 3.5-3.8 eV.42 This noticeable disparity can be understood by taking into account the different electronic configuration of these two metals. In a localized description, the metal-reduced surface bond involves pairing with the electrons left by the oxygen, which can be mostly described as Ti 3d1. Such a pairing is almost costless for Pt, whose ground-state configuration is 5d9 6s1. However this is not the case for Pd, 4d10, for which the 4d9 5s1 configuration appears to lie more than 1 eV above. Actually the experimental difference of energy between the ground state of Pd atom, 1S, and the center of the 2D manifold arising from the 4d9 5s1 configuration is ∼9500 cm-1.43 When this quantity is taken into account, it turns out that the difference Pt-Pd on the reduced surface is of the same order as that found with the defect-free one. In any case, the larger affinity of Pd atoms for oxygen vacancies and their noticeable mobility along the channels indicate that the surface vacancies would be filled from the initial steps of the deposition and would act as nucleation centers for further growth. Moreover, our calculations show that the presence of subsurface vacancies does not improve the Pdsurface interaction as the binding energies remain practically unaltered. On the other hand, one of the more intriguing features observed in the STM images relies on the absence of isolated Pd atoms, while the presence of dimers, trimers, and tetramers is clearly observed at low coverage. In particular, Pd dimers aligned along the [001] direction are clearly observed in the images obtained under atomic resolution. To explore further these features, we considered a model consisting of two Pd atoms deposited on a 4 × 1 four-layer thick slab. These atoms might be together, forming a Pd2 dimer, or fall apart, which is equivalent to having one Pd atom on the 2 × 1 cell. After structural relaxation we found the dimer to be more stable by 0.49 eV, indicating a strong tendency of Pd atoms to bind each other. This result is in contrast with that reported by Bredow and Pacchioni,30 who, on the basis of embedded cluster calculations, concluded that Pd dimers adsorbed on the TiO2 surface lose most of the Pd-Pd interaction due to the relatively strong bond with the substrate. Instead our periodic DFT

Pd Adsorption on the TiO2 (110) Surface

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Figure 3. Plots of electron density isosurfaces for a Pd2 dimer deposited on the TiO2 (110) surface for isovalues of F ) 0.3 e/Å3, and colored according to the magnitude of the gradient of the total density (left, side view; right, top view).

calculations, whatever the slab model is, show a noticeable interaction between Pd atoms, as can be seen in the density isosurfaces depicted in Figure 3. In these plots, an isosurface of the total electron density corresponding to F ) 0.30 e-/A3 is depicted on top and side views. The selected isosurface has been colored according to the magnitude of the gradient of the total density to emphasize the regions associated with bonding interactions. These regions appear as cyan, and the examination allows one to distinguish the formation of a bonding interaction between the Pd atoms and both the titanium and protruding oxygen on the side view, but also the existence of a Pd-Pd bond clearly seen on the top view. The computed Pd-Pd distance is 2.71 Å, close to that measured in the more open Pd(110) surface (2.75 Å), but noticeably shorter than that estimated from the STM images (∼3.5 Å).13 However, as stated previously, STM experiments can lead to significant overestimation of cluster sizes due to the convolution of the STM tip and the fact that the density of states is the quantity actually measured.44,45 Besides this fact, the present results agree with both initial nucleation sites and low coverage growth mode reported by Xu et al.13 Indeed, the interaction energies reported in Table 1 suggest that channel Pd atoms can move quite easily along the [001] direction provided that they avoid the center of the channels. Moreover, they could also move between channels through a moderate barrier across the bridge positions. This mobility would favor interatomic encounters and further growth according to the classical nucleation model which assumes that only monomers are mobile whereas dimers are stable nuclei. Electronic Structure Analysis. Photoelectron spectroscopy and resonant photoemission experiments at the sub-monolayer regime have shown that Pd deposition on the TiO2(110) surface induces new features from the early stages of the deposition. In particular, even at low coverage, a shoulder in the valence band is clearly observed.9 In Figure 4 we have reported the density of states, DOS, plots computed using the 2 × 1 fourlayer thick model slab. The top panel corresponds to the clean surface, while the bottom panel is that after adding Pd. The valence band of the clean TiO2 surface is dominated by O 2p states, while the conduction band arises from the empty Ti 4d states. The band gap appears clearly underestimated with respect to the experimental value (3 eV) as is well-known from GGA calculations. As can be seen, after addition of Pd there is a new set of peaks falling in the band gap which are due to Pd 4d sates in agreement with the experimental data.

Figure 4. Density of states of the TiO2 (110) surface (bottom) and after Pd deposition (top). The Pd contribution is represented in red color.

A key aspect in the study of the metal-surface interaction relies on the extension of the reduction of the substrate after metal deposition. The analysis of resonant photoemission data indicates that the electronic structure of the substrate is not affected by the palladium deposition and that there is a very weak or no reaction between the transition metal and the TiO2 surface.9 However, these conclusions are in contrast with those based on Auger data which seem to indicate that some reduction of the substrate takes place.46 To gain an insight into the bonding mechanism between the Pd atoms and the surface, we have performed a constrained space orbital variation, CSOV, analysis which allows one to carry out an energy partition according to different physical contributions.47-49 Due to technical reasons, this CSOV analysis has been performed using an embedded cluster model approach using the routines implemented in the HONDO program50,51 and the B3LYP exchange-correlation functional. Also for the same reason only the high-symmetry O(br) site has been considered. Details concerning basis set and cluster models can be found out in ref 31. In particular we have used a model labeled as B in this reference, for which B3LYP calculations gives an interaction energy of 2.07 eV. This analysis

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Figure 5. Plots of the isosurface of the difference on the electron density computed as ∆F ) F(Pd/TiO2) - F(TiO2) - F(Pd) and corresponding to isovalues of ∆F ) +0.04 e (blue) and ∆F ) -0.04 e (red).

shows that after a moderately large initial repulsion of the electron clouds (1.3 eV), all remaining terms are attractive. Among these bonding contributions, and discarding the surface to Pd charge-transfer term that is unphysical and corresponds to BSSE, the main term is the Pd polarization that accounts for 75% of the bonding interaction. The remaining terms are of similar magnitude, with an 11% contribution being assigned to the Pd to surface charge transfer and 7% to surface polarization. The magnitude of these contributions is close to that found for the related Pd/R-Al2O3 system24 and is in agreement with previous analysis of the chemisorption bond for similar systems.52 These findings reveal that Pd deposition is accompanied by a strong polarization of the adsorbed atoms which eventually would transfer some electron density toward the surface. Since there is not a direct relationship between the extension of the charge transfer and the contribution to energy, to quantify the substrate reduction is not straightforward. However, the variation of electron density that takes place upon adsorption may be easily observed in the drawings reported in Figure 5 in which plots of the difference in the electron density, ∆F, estimated using the 2 × 1 four-layer thick supercell, are reported. These differences are computed according to ∆F ) F(Pd/TiO2) - F(TiO2) - F(Pd) and are plotted as surfaces of values of ∆F ) +0.04 e (blue) and ∆F ) -0.04 e (red). One can see that upon adsorption a significant repolarization of the electron density around the Pd atom takes place together with an increase on the surface, in particular on top of the 5-fold Ti atom of the channel. Finally, a further point of interest in the CSOV analysis is the existence of a small, but not negligible, contribution from surface polarization that, keeping in mind the limited nature of this cluster model, will agree with the behavior observed in the binding energies obtained from different slab models shown in Table 1, where, as discussed previously, a small variation on the binding energies is observed and interpreted in terms of a contribution from surface reorganization. IV. Conclusions We report in this paper a theoretical analysis of the interaction between Pd atoms and the (110) surface of rutile TiO2. Our calculations, based on state-of-the-art periodic DFT methods and fully relaxed slab models, unambiguously show that Pd single atoms and dimers prefer to adsorb on the surface channels in agreement with the STM images reported by Xu et al. The computed binding energies are fairly dependent on the thickness of the slab, and we have observed an oscillation similar to that

reported for surface energies. Using a four-layer thick slab, our best estimate for the interaction energy is 1.52 eV and corresponds to the interaction of Pd with two in-plane oxygen atoms, a Ti 5-fold coordinated center, and a protruded oxygen atom. However the interaction energies with the in-channel sites are quite close to each other, suggesting an easy diffusion of Pd atoms along the channels. Because of this mobility, Pd encounters appear to be quite likely, giving rise to the formation of Pd dimers that obviously would slow the diffusion of Pd atoms and thus Pd cluster growth. Such diffusion would also be hindered by surface defects as Pd atoms preferentially occupy oxygen vacancies, which indeed could act as efficient nucleation sites. We have performed an analysis of the metal-surface chemical bond using the CSOV approach to obtain a partition of the interaction energy. We have found that the main contribution to the energy comes from the Pd polarization. Both this analysis and the study of DOS curves and electronic density maps show that upon deposition almost no Pd oxidation occurs and support the resonant photoemission experimental data according to which the substrate is not affected by the Pd deposition. Acknowledgment. This work was funded by the Spanish Ministerio de Educacio´n y Ciencia and CEE FEDER, Project MAT2005-01872. References and Notes (1) Goodman, D. W. Chem. ReV. 1995, 95, 523. (2) Gates, B. C. Chem. ReV. 1995, 95, 511. (3) Chemisorption and ReactiVity on Supported Clusters and Thin Films: Towards an Understanding of Microscopic Processes in Catalysis; Lambert, R. M., Pacchioni, G., Eds.; Kluwer: Dordrecht, The Netherlands, 1997. (4) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, U.K., 1994. (5) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (6) Henry, C. R. Surf. Sci. Rep. 1998, 31, 231. (7) Qi, C.; An, L.; Wang, H. Appl. Catal., A 1996, 140, 17. (8) Sarkany, A.; Weiss, A. H.; Guczi, L. J. Catal. 1986, 98, 550. (9) Della Negra, M.; Nicolaisena, N. M.; Lib, Z.; Møller, P. J. Surf. Sci. 2003, 540, 117. (10) Suzuki, T.; Souda, R. Surf. Sci. 2000, 488, 33. (11) Evans, J.; Hayden, B. E.; Lu, G. Surf. Sci. 1996, 360, 61. (12) Chang, Z.; Thornton, G. Surf. Sci. 2000, 459, 303. (13) Xu, C.; Lai, X.; Zajac, G. W.; Goodman, D. W. Phys. ReV. B 1997, 56, 13464. (14) Jak, M. J. J.; Konstapel, C.; van Kreuningen, A.; Verhoeven, J.; Frenken, J. W. M. Surf. Sci. 2000, 457, 295. (15) Jak, M. J. J.; Konstapel, C.; van Kreuningen, A.; Chrost, J.; Verhoeven, J.; Frenken, J. W. M. Surf. Sci. 2001, 474, 28.

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