Adsorption of Small Palladium Clusters on the Î±-Al2O3(0001) Surface

Article pubs.acs.org/JPCC

Adsorption of Small Palladium Clusters on the α-Al2O3(0001) Surface: A First Principles Study Sandeep Nigam* and Chiranjib Majumder* Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400 085, India S Supporting Information *

ABSTRACT: In this work, we report the growth behavior of small Pdn clusters (n = 1−7) on the α−Al2O3 (0001) surface using a first principle approach based on the plane wave-pseudopotential method. The results reveal that in general the interaction of Pdn clusters with Al2O3 surface deforms the equilibrium geometry of the isolated clusters. For Pd atom, the most preferred adsorption site is found to be on top of the oxygen atom with an interaction energy of 1.40 eV. For the dimer, binding between two Pd atoms is more favorable than atomic adsorption at a distance. The competition between Pd−Pd and Pd−surface interactions further governs the growth motif of larger clusters. As the size of the Pd cluster increases, it prefers an open structure to maximize the Pd−surface interaction. In case of Pd7, the compact pentagonal bipyramidal structure of the isolated cluster reorganizes into a hexagon with one central atom. Further, a comparison of chemical bonding analysis through electronic density of state (EDOS) between the gas phase and deposited clusters shows that the EDOS of the deposited Pdn cluster is significantly broader, which has been ascribed to the enhanced spd hybridization.

1. INTRODUCTION Deposition of a transition metal cluster on metal oxide surfaces has become a major interdisciplinary area of research over the past decade. Oxide-supported metal cluster plays a crucial role in diverse technological applications, namely microelectronic devices, oxide-supported transition metal catalysts, and metal− ceramic-based gas sensors.1−7 Deposition of cluster on oxide supports leads to modified cluster properties depending on the cluster/metal−oxide interaction. Identifying the nature and bonding in the complex system is of extreme significance for heterogeneous catalysis and coating of materials. Therefore, understanding and prediction of metal cluster growth on oxide supports is of great importance.1,2 Extensive experimental8−34 and theoretical35−62 investigations have been performed on transition metal atom/clusters deposited on various oxide surfaces. In particular, palladium cluster supported on various oxide surfaces has been extensively used for various catalytic applications.23−32 Especially aluminasupported palladium metal cluster have attracted a great deal of attention due to their special relevance in heterogeneous catalysis.28−32 Maillet et al.29 studied the catalytic behavior of a Pd/Al2O3 catalyst for oxidation of carbon monoxide, propene, propane, and methane. They found that activity of the Pd/ Al2O3 catalyst depends on chemical state of Pd. Prasad and coworkers30,31 investigated the catalytic properties of aluminasupported Pd catalysts for the hydrodechlorination of chlorobenzene. It is generally accepted that the activity of deposited metal cluster in chemical reactions on various supports is strongly dependent on cluster shape and size. Other than the growth mode, the charge transfer between metal cluster and oxides are also very important in catalytic reactions. © 2012 American Chemical Society

Thus, it is worth investigating the structural and electronic properties of Pd cluster on alumina surface. Many reports are available for palladium supported on alumina using different experimental33,34 and theoretical55−62 tools. The adsorption of palladium on the α-Al2O3 surface has been studied by means of secondary ion mass spectrometry in static mode (SSIMS) and by thermal programmed desorption (TPD).33 From this study, they predicted that at low coverage Pd−Al bonds are formed first and that Pd−O bonds appear to be formed in a subsequent step. Cai et al.34 reported the size dependence of the electronic structure of Pd clusters deposited on an ordered Al2O3 film using angle-resolved photoemission in conjunction with synchrotron radiation. Rivanenkov et al.56 applied a density functional method with a consistent cluster embedding in an elastic polarizable environment to study adsorption complexes of single Pd atom on the relaxed polar Al-terminated surface α-Al2O3(0001) and top position on the oxygen atom was found to be the most favorable adsorption site. In another ab initio study, Illas and co-workers57,58 investigated the interaction of Pd atom with the Al-terminated α-Al2O3(0001) surface using an embedded cluster model and periodic-supercell approaches within density functional formalism and found that there was a slight preference for adsorption above surface sites sitting directly above oxygen atoms. Lodziana performed a DFT study on the interaction of Pd atom with surface of α-alumina and showed that the interaction very strongly depends on the surface stoichiometry.59 Xiao et Received: August 24, 2011 Revised: December 26, 2011 Published: January 23, 2012 2863

dx.doi.org/10.1021/jp2081512 | J. Phys. Chem. C 2012, 116, 2863−2871

The Journal of Physical Chemistry C

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al.60 examined the effect of surface termination on adsorption of late transition metal atom (Pt, Pd, Au, and Ag) and found that OH-termination diminished the strength of binding due to unavailability of charge accepting lewis acidic Al sites. Other than the adsorption of single Pd atom on the α-Al2O3 (0001) surface, there are also few reports available on the adsorption of Pd clusters. The interaction of small Pdn clusters (n = 3, 4) with the relaxed Al-terminated α-Al2O3(0001) surface was investigated by Illas and co-workers61 using embedded cluster and periodic slab models within a first principles density functional approach and they concluded that the structure of supported Pd3 is largely distorted from the gasphase equilibrium geometry, whereas the structure of supported Pd4 is less distorted in comparison to most stable gas-phase isomer. Sanz and co-workers62 have performed the molecular dynamics simulations of Pd deposition on the α-Al2O3 (0001) surface and showed that the adsorbed Pd particles prefer 3D structures. Nasluzov et al.63 applied relativistic density functional elastic polarizable environment embedded cluster approach to study adsorption of Pd3 clusters on the Alterminated α-Al2O3(0001) surface and reported that these cluster get adsorbed in a perpendicular fashion rather than parallel fashion. Recently, Khanna and co-workers64 investigated Pdn (n = 1−7, 10) clusters deposited on alumina/ NiAl(110) using a gradient-corrected density functional approach. Their studies indicated that the free Pdn clusters maintain their compact nature when deposited on the alumina/ NiAl(110), undergoing only small relaxations of the Pd−Pd distance. Although palladium clusters are important for various technological and catalysis applications, to the best of our knowledge there is no report available on systemic growth pattern of palladium cluster Pdn (n = 1−7) on a clean αAl2O3(0001) surface. Further, to date no studies have addressed the interaction of Pdn cluster with a clean α-Al2O3(0001) surface having a cluster size more than four. Moreover, Pd is of special interest due to its special position in the periodic table with a d10s0 electronic configuration, which changes in various situations depending on its chemical bonding and determines its specific chemical behavior. In the present work, we present a detailed and systematic theoretical investigation of Pdn cluster deposited on a relaxed Al-terminated α-Al2O3(0001) surface under the density functional formalism. The current study aims to (i) identify preferred adsorption sites and structure of deposited Pd cluster, in particular analyze the relaxation in comparison to most stable gas-phase isomer, and (ii) to explore the nature of Pd/Al2O3 bonding and to quantify adsorption energy to obtain relevant insights for heterogeneous catalysis.

total energy convergence was tested with respect to the planewave basis set size and simulation cell size, and the total energy was found to be accurate to within 1 meV. The spin polarized calculations of gas-phase clusters were carried out by placing the clusters in a 15 × 15 × 15 Å cubic box to prevent interaction between clusters. The Brillouin zone integrations for the isolated clusters are carried out at the Γ point only. To obtain an accurate description of the Al2O3 surface, test calculations were carried out for the bulk Al2O3. The calculated lattice parameter (using 5 × 5 × 5 K-points, a = b = 4.766 Å and c = 12.999 Å) are found to be in good agreement with the experimental value.69 Alumina has a direct band gap of 8.8 eV.70 Our calculated band gap was found to be 6.2 eV, exhibiting the usual underestimation of band gaps by GGA and agreeing with results reported in the literature.71 The cohesive energy has been calculated to be 32.8 eV per mole of alumina, which compares well to the experimental value of 32 eV/mol.72 After bulk calculation, we model the α-Al2O3(0001) surface in a 18-atomic layer slab obtained by truncating the bulk α -Al2O3 structure (a = b = 4.766 Å and c = 12.999 Å). To minimize the interaction between periodic images, we have used vacuum layer of 13 Å between slabs. Because the main objective of the present study is to extract details of cluster/ surface interactions, in the present study the 2 × 2 supercell containing 120 atoms (30 atom/perunit cell) is considered to ensure the large lateral surface (surface area ≈ 91 Å2). Overall supercell dimensions used were 9.53 × 9.53 × 25.99 Å containing 120 atoms (Figure 1). The minimum distance

Figure 1. (Color online only) Top and side vies of Al2O3 surface.

between lateral images for the largest size cluster (i.e., Pd7) is found to more than 5 Å.73 A Monkhorst−Pack set of 5 × 5 × 1 K-points was used. For structural relaxation, all 18 atomic layers were allowed to relax and relaxation was performed including the dipole correction to avoid any error due to interaction of the slab images due to dipole moments.74 The geometry optimization of modeled 18-atomic layer Al-terminated slab shows large structural relaxation (compression) in which the Al atoms of the first layer move inward by −0.70 Å (83%), ending up almost coplanar with oxygen atoms. The large inward relaxation reduced the charge on the aluminum atom compared to the bulk, as the calculated Al−O distance, which is 1.86 Å in the bulk, decreases to 1.69 Å at the surface. The relaxation of second, third, and fourth layer with respect to unrelaxed geometry was found to be −6%, 46%, and 21%, respectively. Calculated surface relaxations are consistent with previous reports.50,62,75,76 The surface energy of the relaxed surface is calculated to be 1.41 J/m2. This value agrees well with the previous reports.76,77 The average adsorption energy of Pdn cluster on α-Al2O3 surface was calculated as ΔE = [E(Pdn/ Al2O3) − E(Al2O3) − E(Pdn)]/n. Further, a comparison of chemical bonding analysis through EDOS between gas phase

2. COMPUTATIONAL DETAILS All of the calculations were performed using plane wave-based pseudopotential approach as implemented in the Vienna ab initio Simulation Package (VASP).65 The electron−ion interaction was described by the full-potential all-electron projector augmented wave (PAW) method,66 as implemented in VASP by Kresse and Joubert.67 The spin polarized generalized gradient approximation68 has been used to calculate the exchange-correlation energy. The cut off energy for the plane wave basis set was fixed at 400 eV for all calculations performed in this study. The geometry optimization was performed by ionic relaxation, using a conjugate gradient minimization. The geometries are considered to be converged when the force on each ion becomes 0.01 eV/Å or less. The 2864



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and bonding at the interface. To identify the most favored location of Pd atom on the Al2O3 substrate, a large number of adsorption sites are considered. We name each site according to the atoms lying directly beneath the site. In Figure1, we display five atop adsorption sites, Al(1), O(2), Al(3), Al(4), and O(5), in which the number inside the parentheses refers to the layer on which the atom resides. Other than positioning the Pd atom, atop on these sites, some other bridge positions, like Al(1)− O(5), Al(1)−O(2), Al(4)−O(5), Al(3)−O(2), and so forth, have also considered. On the basis of the results, it is found that Pd atom prefers to occupy the O(2) atop site (Figure 3) with

and deposited clusters has been carried out. For this analysis, we use Gaussian smearing with a width of 0.05 eV.

3. RESULT AND DISCUSSION 3.1. Gas Phase Clusters. To compare the geometrical deformations between the isolated and deposited Pd clusters, which are the main purposes of this article, we have optimized several initial configurations of Pd clusters in the gas phase. The optimized geometries and relative stabilities of few low-lying isomers are shown in Figure 2. The Pd−Pd distance in the Pd2

Figure 3. (Color online only) (A) Side and top view of Pd atom deposited on Al2O3 surface. (B) Isosurface density and charge density difference for deposited Pd atom (blue and red colors indicate increase and decrease of charge, respectively).

an adsorption energy of −1.40 eV. This is in good agreement with previously reported values.57,59 The shortest Pd−Al(1) and Pd−O(2) bond distances are found to be 2.40 and 2.17 Å, respectively. Another configuration, where Pd binds directly with surface aluminum Al(1), is 0.5 eV higher in energy. From the charge density difference plot (Figure 3), it is seen that the electron density is localized along the Pd−Al(1) bond. To understand the nature of bonding, we have analyzed the orbital decomposed density of states, as shown in Figure 4. The ground state of Pd is spin singlet with electronic configuration of Pd is 4d10 5s0. Upon deposition on the surface, a finite contribution of s-electrons is observed at the fermi energy. The adhesion of a metal on oxide surface depends on strength of metal−metal and metal−surface interaction. The adsorption of dimer give us the opportunity to compare the metal−metal and metal−surface interaction. Hence, for the adsorption of Pd dimer, we have followed two approaches: (i) The Pd2 dimer is placed on the surface in parallel and perpendicular orientations through various angles, and (ii) two Pd atoms are placed at far apart on the surface. The geometry optimization of all these configurations suggest that Pd2 dimer prefers to orient in parallel to the surface plane and the Pd−Pd bond length is elongated from 2.48 to 2.56 Å (viz. Figure 5). In the second case, when two Pd atoms are placed at far apart (4− 5 Å away), on relaxation they are adsorbed on two different O(2) sites as two separate Pd atoms (viz. Figure 5). Further, it is seen that the configuration having Pd−Pd bond is more stable on both the surface in comparison to the case where two Pd atoms are far apart. On the basis of these results, we infer that the strong metal−metal interaction is preferred over a metal−surface interaction. It is worth mentioning here that,

Figure 2. (Color online only) Gas phase low lying structure of Pdn cluster.

dimer is found to be 2.48 Å with bond energy of 1.28 eV. The most stable isomer of the Pd3 cluster forms a regular triangle with Pd−Pd distance of 2.52 Å. The linear isomer is found to be 1.18 eV higher in energy. From n = 4 onward, Pdn clusters adopt 3D geometries. The lowest energy isomers of the tetramer, pentamer, hexamer, and heptamer form a tetrahedron, triangular bipyramid, octahedron, and pentagonal bipyramid, respectively. In this context, it is appropriate to mention that all Pdn (n = 2−7) clusters are spin polarized in the ground state. For n = 6, the singlet and triplet isomers are found to be degenerate with a small difference of 0.03 eV, the later being the lowest energy isomer. The ground-state structures obtained in this study are in good agreement with previously published results.64,78−83 The comparison of average binding energy values with the earlier reported data78,79,82 is shown in Table S1 of the Supporting Information. 3.2. Palladium Atom and Dimer Adsorption on the Al2O3 Surface. In this section, we briefly describe the atomic and dimer adsorption on the Al2O3 surface because these interactions reflect the most fundamental aspects of structure 2865



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Figure 4. (Color online only) Representative density of states of up spin (black) and down spin (red) spectra of gas phase and deposited Pd atom. The dashed line indicates the location of the HOMO level.

Sanz et al.35 have investigated Pd@TiO2 (110) system and found that adsorption of Pd2 dimer (Pd−Pd distance as 2.71 Å) is more stable in comparison to isolated atomic adsorption. However, in the present case the bond length of Pd dimer on Al2O3 surface was found to be 2.56 Å only. To further probe the nature of metal−metal and metal− surface interactions, we have compared the Pd−Pd(111)surface and Pd−Al2O3 interactions. For this purpose, we have studied the Pd2 adsorption on the Pd(111) surface using similar approach that has been used for Al2O3 surface. After relaxation of both configurations, it is seen that the configuration having Pd−Pd bond is more stable on the Pd(111) surface in comparison to the case where two Pd atoms are far apart. The details of the results are summarized in Table 1. From the table, it is clear the Pd adsorption on Pd surface is stronger than on alumina surface. 3.3. Pdn (n = 3−7) Cluster Adsorption on Al 2O 3 Surface. For clusters having more than two Pd atoms, we have optimized various gas phase isomers on the Al2O3 surface.

Figure 5. (Color online only) (A) Side and top view of Pd2 dimer deposited on Al2O3 surface (B) Adsorption of two Pd atoms on Al2O3 surface.

Table 1. Comparison of Adsorption of Pd/Pd2 on α-Al2O3 and Pd(111) Surfaces Cluster/Surface

Binding Energy (eV) (gas phase)

Pd atom 2 Pd atoms adsorbed separately as atomic palladium placed far apart (4−5 Ǻ away) on surface Pd2 dimer (The two pd atom adsorbed on surface with a Pd−Pd bond)

Adsorption energy (eV) α-Al2O3

Pd(111)

−1.40 −1.55b −2.27b

−2.68c −0.07d −4.54d

a

−1.26 eV

Adsorption energy for single Pd atom adsorption on alumina surface ΔE = E (Pd/Al2O3) − E(Al2O3) − E(Pd atom). bAdsorption energy for Pd2 adsorption on alumina surface ΔE = E (Pd2/Al2O3) − E (Al2O3) − E(Pd2 dimer). cAdsorption energy for single Pd atom adsorption on Palladium surface ΔE = E [Pd/Pd(111)] − E[Pd(111)] − E(Pd atom). dAdsorption energy for Pd2 adsorption on Palladium surface ΔE = E[Pd2/Pd(111)] − E[Pd(111)] − E[Pd2 dimer)]. a

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A large number of adsorption sites have been identified by considering following two criteria (i) maximum number of Pd atoms should be at the atop positions (Al(1), O(2), Al(3), Al(4), and O(5)) of the surface: atop mode (ii) maximum number of Pd atoms should be at the bridge positions (Al(1)− O(5), Al(1)−O(2), Al(4)−O(5), Al(3)−O(2)) of the surface: bridge mode. In the following section, we present results for four low energy structures of the deposited clusters. The equilibrium geometry of the isolated Pd3 cluster is a regular triangle. Upon deposition on the Al2O3 surface one of the angles widens from 60° to 66° and therefore the opposite Pd−Pd distance increases up to 2.74 Å (Figure 6). The

Figure 7. (Color online only) Side (A) and top (B) view of lower energy structure of Pd4 cluster on Al2O3 surface. Relative stabilities (ΔE, eV) is also presented.

Al(1) and Pd−O(2) bond distances are found to be 2.51 Å and 2.21 Å, respectively. The adsorption energy is calculated to be −0.63 eV/atom, which is comparable to the adsorption energy (−0.69 eV/atom) reported by Gomes et al.61 The interaction of a planar structure of Pd4 with alumina, where three of the Pd atoms are connected to the bridge position, shows 0.90 eV higher in energy. Adsorption of tetrahedron isomer is 0.65 eV higher in energy. Interestingly, Gomes et al.61 in the slab model calculation have also found that 2D adsorption of Pd4 cluster is more favorable than 3D mode. The Pd5 cluster shows trigonal bipyramid structure in the gas phase. This reorients into a square pyramidal geometry after the adsorption on Al2O3 surface (viz. Figure 8). The shortest Pd− Al(1) and Pd−O(2) bond distances are found to be 2.50 and 2.18 Å, respectively. The adsorption energy is calculated to be −0.53 eV/atom. It is worth mentioning here that square pyramidal Pd5 geometry is the second lower energy isomer (0.05 eV higher in energy) in the gas phase. The interaction of Pd5 with capped rhombus and trigonal bipyramidal geometry are found to be 0.05 eV and 0.32 eV higher in energy. The Pd6 cluster, which forms octahedron geometry in the gas phase, deforms into a capped pentagon structure on the alumina surface (viz. Figure 9). The adsorption energy is


adsorption energy of Pd3 cluster is found to be −0.75 eV/atom. The aluminum atoms participating in the bonding with the Pd cluster move 0.5 Å upward. Other low lying structures with their relative stabilities are shown in Figure 6. The Pd4 prefers a tetrahedral configuration in the ground state. Upon interaction with the Al2O3 surface, it is distorted to a bent rhombus, where alternate Pd atoms are placed close and away from the surface atoms (viz. Figure 7). The shortest Pd− 2867



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On the basis of these results, we infer that there is a significant change in the growth motif of the Pd7 cluster in comparison to other smaller clusters. Pd−Al(1) bond distances were found to be 2.44 Å, whereas the nearest Pd−O(2) bond distance is 2.34 Å. On the basis of the above discussion, it is clear that the equilibrium geometries of isolated Pd clusters are distorted significantly after deposition on a substrate. There is interplay between the strength of Pd−Pd and Pd−surface interaction, which primarily decides the interface geometry. The deposited clusters reorient in such a way that it can maximize the Pd− surface interaction with minimum sacrifice in the Pd−Pd bonds. For Pd5 and Pd6, not all atoms are interacting with the surface, as this will lead to rupturing some Pd−Pd bonds. However, in the case of the Pd7 cluster, the cluster could interact with all seven atoms because simultaneously it is able to retain the Pd−Pd bonds. Further, to explain the growth motif, we have tabulated the interaction energy vis-à-vis average bond distances, cluster deformations, and surface deformations in Table 2. Whereas the interaction energy gives an account of energy released due

estimated to be −0.49 eV/atom. The aluminum atoms participating in the bonding with the cluster move 0.5 Å upward. The shortest Pd−Al(1) bond distances is found to be 2.48 Å, whereas the nearest Pd−O(2) bond length is found to be 2.18 Å. When the planar isomers of Pd6 cluster (tricapped triangle or regular hexagon) are optimized on the alumina surface, the relaxed structure becomes nonplanar and form zigzag arrangement. These structures are 0.01−0.5 eV higher in comparison to previous structure. The Pd7 cluster forms pentagonal bipyramid structure as the lowest energy isomer in the gas phase. Remarkably, this 3D geometry becomes planar upon interaction with alumina surface (viz. Figure 10). The lowest energy structure of the Pd7 on Al2O3 forms a zigzag patterned hexagonal structure with three Pd atoms binding atop the aluminum (Al1) site and other three atoms occupying the top sites of Al4 and the seventh atom (i.e., the central atom) resides over the Al3 atom.73 The adsorption energy is calculated to be −0.55 eV/atom. Other 3D structures of the Pd7 cluster are 0.4−0.8 eV higher in energy. 2868



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Table 2. Interaction Energy, Deformation Energy and Average Bond Distances of Pdn Cluster; for Calculation of Average Bond Distance Only Those Pd−Al, Pd−O, and Pd−Pd Bonds Are Considered, Which Are within 25% Limit of Dimer Pd−Al, Pd−O, and Pd−Pd Bond Distances cluster

no. of atoms interacting with surface

interaction energya

ΔE − clusterc

ΔE − surfaced

net adsorption energyb

Pd−Al

no. of bonds

Pd−O

no. of bonds

Pd−Pd

no. of bonds

Pd Pd2 Pd3 Pd4 Pd5 Pd6 Pd7

1 2 3 4 4 5 7

−1.77 −3.34 −4.34 −5.08 −4.14 −5.48 −8.25

0.03 0.11 0.49 0.10 0.66 1.51

0.38 1.04 1.98 2.07 1.38 1.86 2.92

−1.40 −2.27 −2.25 −2.52 −2.66 −2.96 −3.82

2.40 2.40 2.58 2.52 2.58 2.63 2.44

1 1 2 2 3 5 3

2.17 2.11 2.23 2.26 2.26 2.26 2.35

1 1 3 4 2 4 6

2.56 2.60 2.61 2.68 2.68 2.69

0 1 3 5 9 10 12

E = E[Pdn@Al2O3]opt − E[Pdn]having deposited geometry − E[Al2O3]having geometry after Pdn deposition. bEads = E[Pdn@Al2O3]opt − E[Pdn]gas phase opt geometry − E[Al2O3]optimized geometry. cΔE = E[Pdn]gas phase opt − E[Pdn]having deposited geometry. dΔE = E[Al2O3]opt − E[Al2O3]having geometry after Pdn deposition. a

to the interaction of the cluster and the surface, the deformation energies provide an idea about the changes occurring in the cluster as well as the surface during the interaction. The interaction energy and the energy loss due to the deformation finally give rise to the net adsorption energy. From the table, it is clear that Pd5 cluster interacts weakly in comparison to other clusters, which results in lower interaction energy. The adsorption of Pd4 and Pd7 clusters on the alumina surface involves a significant change in the cluster structure as well as in the surface. 3.4. Charge Distribution Analysis. To understand the nature of chemical bonding between Pdn cluster and Al2O3 substrate, the difference in charge density (Δρ) of the adsorbate−substrate complex has been calculated, which is expressed as

Δρ = ρ(M n/Al2O3) − ρ(Al2O3)fix − (M n) The first-term ρ(Mn/Al2O3) is the charge density of the total system, ρ(Al2O3)fix is the charge density of the alumina surface fixed at the adsorbed geometry, and ρ(Mn) is the charge density of a metal atom/cluster isolated in the same periodic box. In general, it is found that when Pd atoms are occupying a hole site, (around Al(4) or nearby bridge position) charge is transferred to the nearby oxygen atoms and when the Pd atom is directly connected to the Al of the top layer Al(1), a finite amount of charge transfer takes place from the surface to the adsorbate atom. In Figure 11, we show a representative isosurface density and charge density difference contours for the lowest energy isomer of Pd7 cluster deposited on the Al2O3 surface. It is clear from the figure that, in the case of Pd7, while charges are depleted from the down atoms, charge is accumulated along the Pd−Al(1) bond. Isodensity surface and charge density difference contour for the lowest energy structure of all Pdn cluster are shown in the Supporting Information. 3.5. Electronic Structure. The electronic structure of any material is related to its chemical properties like reactivity or chemical bonding. Here, to understand the effect of Pd cluster deposition on the alumina surface toward chemical properties, we have analyzed the local density of states (LDOS) spectrum as shown in Figure 12. Instead of showing the LDOS picture for all clusters, we have used Pd7 cluster as a representative case. From Figure 12, it is clear that in the gas phase Pd7 is spin polarized and d-orbital contribution is significantly higher than s- and p-orbital contributions. From this figure, two effect are very clear; (i) the electronic states of Pd7 cluster have been pushed toward higher energy (less negative energy values), and

Figure 10. (Color online only) Side (A) and top (B) view of lower energy structure of Pd7 cluster on Al2O3 surface. Relative stabilities (ΔE, eV) are also presented.

Figure 11. (Color online only) Isosurface density and charge density difference for Pd7 cluster. 2869



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adsorption it is found that configuration having Pd−Pd bond is energetically more stable in comparison to the case where two Pd atoms adsorbs separately as atomic palladium. The gas phase tetrahedron geometry of Pd4 cluster open up to bent rhombus structure after deposition on the alumina surface. The Pd5 and Pd6 clusters prefer partially open structures as these clusters do not make use of its all atoms to interact with surface. In case of Pd7, the gas phase geometry with pentagonal symmetry changes to hexagonal zigzag structure to take advantage of the maximum interaction with the substrate. A detailed electronic structure analysis of the deposited Pdn cluster vis-à-vis gas phase cluster shows considerable change in the electronic states. After deposition, the electronic states of Pdn cluster become broader and amount of spd hybridization increases.

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Structures, density of spin graphs. This material is available free of charge via the Internet at http://pubs.acs.org.

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(ii) the peak width becomes broader after deposition. Moreover, the amount of peak broadening and the contribution from s- and p-orbitals of Pd increases as the cluster size increases. The LDOS spectrum for the lowest energy structure of all studied cluster Pdn (n = 1−7) has been shown in the Supporting Information. This higher s and p contributions are attributed to the enhanced spd hybridization as can be verified from the electron distribution chart listed in Table 3. From this

Pd Pd2 Pd3 Pd4 Pd5 Pd6 Pd7

%-p

* E-mail: [email protected]. * E-mail: [email protected].

■

ACKNOWLEDGMENTS We are thankful to the members of the Computer Division, BARC, for their kind cooperation during this work.

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%-d

gas phase

deposited

gas phase

deposited

gas phase

deposited

0.00 2.34 3.39 3.03 2.91 2.87 2.91

1.98 2.79 3.47 3.50 3.09 3.14 3.21

0.00 0.36 0.65 0.82 0.93 1.03 1.09

0.64 0.97 1.29 1.43 1.30 1.43 1.44

100 97.31 95.95 96.16 96.15 96.11 95.99

97.38 96.23 95.28 95.07 95.60 95.42 95.35

AUTHOR INFORMATION

Corresponding Authors

Table 3. Distribution of Charge in s-, p-, and d-Orbital of Pd Atoms of Pdn Cluster %-s

ASSOCIATED CONTENT

S Supporting Information *

Figure 12. (Color online only) Representative density of states of up spin (black) and down spin (red) spectra of gas phase and deposited Pd7 cluster. The dashed line indicates the location of the HOMO level.

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table, it is clear that the contribution of s and p orbitals has been significantly increased with respect to the gas phase as well as the cluster size.

4. CONCLUSIONS We have calculated the structure and electronic properties of Pdn cluster deposited on the α-Al2O3 surface. We have used the plane wave-based pseudopotential method under the framework of density functional theory. The electron-ion interaction was described using PAW and the exchange correlation was approximated using spin polarized GGA scheme. The equilibrium geometries of the gas phase Pdn clusters adopt compact 3D structures. However, when these Pdn clusters were deposited on the alumina surface, the gas phase geometry significantly distorted in such a way that alternate Pd atoms were placed in up and down fashion. The results reveal that both Pd−Pd and Pd−surface interactions are strong and therefore, a balance of energetics between them decides the lowest energy structure of the composite system. For Pd2 2870



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Adsorption of Small Palladium Clusters on the Î±-Al2O3(0001) Surface

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