Metal-Supported Metal Clusters: A Density Functional Study of Pt

Article pubs.acs.org/JPCC

Metal-Supported Metal Clusters: A Density Functional Study of Pt3 and Pd3 Juan A. Santana and Notker Rösch* Department Chemie & Catalysis Research Center, Technische Universität München, 85747 Garching, Germany S Supporting Information *

ABSTRACT: The geometric, energetic, and electronic properties of metal-supported metal clusters were examined computationally by applying a method based on density functional theory to model systems. To explore lattice strain effects on these systems, Pt3 and Pd3 clusters adsorbed on Au(111) and Cu(111) were studied. The geometric and electronic properties of these small metal-supported clusters were found to differ from the corresponding overlayer system. The d-band centers of the adsorbed clusters on Au(111) were calculated to be very similar to those of the adsorbed clusters on Cu(111), indicating that the support has only a minor effect on the d-band center of the adsorbed clusters. In contrast, the gap between the local d-band centers of Pt and Pd overlayers and the Fermi energy is reduced from −2.41 eV (Pt) and −2.10 eV (Pd) on Cu(111) to −1.41 eV (Pt) and −1.22 eV (Pd) on Au(111). overlayers4,16,20 and strong heterogeneous metal−metal bonds result in a lower activity as the number of atomic layers is reduced. In contrast, the reactivity of Pt and Pd nanoparticles on Au(111) and submonolayers of Pd4,6,7 and Pt4 on Au(111) can not only be ascribed to lattice strain effects. These systems have been shown to exhibit a significantly higher activity than their overlayer counterparts. Other effects such as particle size, density of low-coordinated surface atoms, and a potential direct involvement of the Au substrate in the reactions need to be considered.4,12,16 A first step in a detailed systematic study of these effects on the reactivity of nanoparticles and submonolayers of Pd and Pt on metal substrates is to evaluate lattice strain effects on the surface and electronic properties of these mixed-metal systems. For mono- and multilayer deposition, previous computational studies26 showed large differences between the chemical properties of Pd on Au(111) and Cu(111).26 The local d-band center of a Pd overlayer is more than 1 eV closer to the Fermi energy when supported by Au(111) rather than by Cu(111).26 The reduced energy gap from the d-band center to the Fermi energy correlates with the computational result that hydrogen binds notably stronger to a Pd overlayer that is supported by Au(111) than by Cu(111).16,20,26 As mentioned, these differences in systems with an overlayer are mainly driven by lattice strain and ligand effects. For submonolayer coverage, related computational works are mainly limited to Pd on Au(111).12,13,30,31 These calculations showed that the local dband centers of Au supported Pd clusters resemble those of Pd overlayers on Au(111). However, this need not be the case for

1. INTRODUCTION Supported nano- and subnano-sized metal clusters are of special interest both experimentally1−8 and theoretically1,9−16 because of their application as industrial or model catalysts. Supported catalysts consisting of metal clusters of subnano-size, dispersed on metal oxide, carbon, aluminosilicate, zeolite, or crystalline aluminophosphate are typically used in processes of industrial importance.1,2 In the field of electrocatalysis, metal nanoclusters deposited on different forms of carbon and on well-defined metal surfaces are often used as model systems for fundamental studies.3−8 Such model systems allow one to isolate and explore important effects that control chemical reactivity. These effects include, besides others, particle size,5 density of lowcoordinated surface atoms,4,7,12 and the influence of the substrate material in the case of metal surfaces.4,6 Detailed understanding of the structural and electronic characteristics of such model systems in combination with knowledge of how their chemical reactivity is affected is crucial for improving existing or developing novel electrocatalysts. Pt and Pd nanoparticles, mono- and multilayers deposited on well-defined Au single crystal surfaces, are among the most studied model electrocatalysts by both experimental4,7,17,18 and theoretical techniques.4,11−13,15,16,19,20 The reactivity of Pt and Pd mono- and multilayers on Au is mainly dominated by lattice strain21 and ligand effects.19,22−26 Tensile strain arises from matching the lattice dimensions of the overlayer to those of the Au substrate, while ligand effects derive from the formation of heterogeneous bonds between the overlayer and the substrate. These effects are more pronounced, leading to enhanced activity, when metal overlayers comprise only one or two atomic layers.4,15−17 Related studies addressed Pt27 or Pd4 deposited on Cu(111) and Pt on Ru(0001).28,29 While the interatomic spacing in bulk gold is larger than in palladium and platinum, the reverse relationship holds for copper and ruthenium as the substrate. Lateral compression of Pt or Pd © 2012 American Chemical Society

Received: February 7, 2012 Revised: March 26, 2012 Published: May 1, 2012 10057

dx.doi.org/10.1021/jp301227e | J. Phys. Chem. C 2012, 116, 10057−10063

The Journal of Physical Chemistry C

Article

Table 1. Properties of Optimized Pt3 and Pd3 Clusters in the Gas Phase and Adsorbed on Metal Substratesa ΔEads Pt3 Pd3 Pt1(fcc)/Au(111) Pt1(hcp)/Au(111) Pt3/Au(111), conf. 3c Pt3/Au(111), conf. 3d Pt3/Au(111), conf. 1c Pt3/Pt(111) Pt3/Cu(111) Pd3/Au(111) Pd3/Pd(111) Pd3/Cu(111)

−366 −357 −167 −165 −166 −208 −216 −158 −171 −193

ΔEagg

⟨MC−MC⟩

−237 −125

2.49 2.51

−38 −37 −26 −29 −23 −32 −29

⟨MC−MS⟩

2.62 2.63 2.63 2.66 2.68 2.73 2.71 2.75

2.63 2.63 2.75 2.73 2.75 2.65 2.52 2.73 2.66 2.53

⟨MS1−MS2⟩

μ(MC)

2.96 2.97 2.99 2.99 2.99 2.83 2.57 2.97 2.79 2.57

0.62 0.65 0.00 0.00 0.39 0.38 0.37 0.20 0.01 0.00 0.35 0.00

LDOS(εF)

ε(MC)

ε(MS)

0.5 0.6 2.5 2.5 2.4 2.5 1.5 1.4 2.8 0.7

−1.35b −0.99b −1.11 −1.09 −1.46 −1.44 −1.43 −1.87 −1.51 −1.15 −1.37 −1.29

−3.35 −3.35 −3.32 −3.34 −3.31 −2.31 −2.36 −3.35 −1.77 −2.37

Adsorption energy ΔEads (per atom) of the adsorbate (atom or cluster) on the support, agglomeration energy ΔEagg (per atom) of the clusters M3, in the gas phase from atoms or on a surface from adsorbed atoms, average distance ⟨MC−MC⟩ between the atoms of an adsorbed cluster M3, average nearest-neighbor distance ⟨MC−MS⟩ between atoms of the cluster M3 and the substrate, average nearest-neighbor distance ⟨MS1−MS2⟩ between atoms in the first and second layers of the support, local magnetic moment μ(MC) of the cluster M3 (per atom), local density of state LDOS of cluster M3 at the Fermi energy εF, local d-band center ε(MC) of the cluster M3 relative to εF, d-band center ε(MS) of the substrate relative to εF. Adsorption and agglomeration energies in kJ mol−1, distances in Å, magnetic moments in μB, LDOS in states/atom/eV, one-electron energies in eV. b For the bare clusters, the average energy of the highest occupied and the lowest unoccupied levels are used as reference energy, instead of the Fermi energy εF. cPt3 on Au(111) in conformations 3 and 1, respectively (Figure 1). dPt3 in conformation 3 on a slab of six layers. a

for a dipole correction40 as implemented in VASP. Spinpolarized calculations were carried out to check whether the supported clusters exhibit a magnetic behavior. The surfaces Au(111), Pt(111), Pd(111), and Cu(111) were modeled by slabs of four atomic layers, separated by a vacuum region of 15 Å. A (3 × 3) surface unit cell was employed to model metal-supported Pt3 and Pd3 clusters. During geometry optimization, the two bottom atomic layers of the surface models were kept fixed at the calculated lattice constants, while the remaining atoms were allowed to relax until all residual forces were less than 0.02 eV/Å. The calculated lattice constants (Au, 4.17 Å; Pt, 3.97 Å; Pd, 3.94 Å; Cu, 3.64 Å) agree very well with results of previous GGA calculations (Au, 4.18 Å;13,41 Pt, 3.99 Å;29,42 Pd, 3.96 Å,13,41 3.95 Å;42 Cu: 3.64 Å,41,43 3.63 Å42) as well as with experimental values32 (Au, 4.08 Å; Pt, 3.92 Å; Pd, 3.89 Å; Cu, 3.61 Å). Brillouin zone integrations were carried out with a (5 × 5 × 1) k-point sampling.44 For the analysis of the electronic structure, we used a finer mesh of k-points, 9 × 9 × 1. Pt3 and Pd3 clusters in the gas phase were calculated with a cubic unit cell, with edges of 12 Å. Adsorption energies of an atom or a cluster are defined as reaction energies of the corresponding process, with respect to the ground state of the adsorbate in the gas phase. Thus, adsorption energies ΔEads (per atom) are the more negative, the stronger the corresponding adsorbate−substrate bonds are. In contrast, agglomeration energies ΔEagg are defined as reaction energies of the formation of a cluster, in the gas phase from atoms or on the surface from adsorbed atoms. Hence, agglomeration energies characterize the strength of the internal bonds of a cluster. These energies are measured in kJ mol−1, while we use eV units for describing electronic properties of the systems.

other mixed-metal systems as the present work will demonstrate. In the present work, we employ small model clusters, Pt3 and Pd3, to explore geometric, energetic, and electronic properties of metal-supported Pd and Pt clusters. As mentioned, Pd3 on Au(111) has previously been studied.12,13,30 However, the present work will show that one gains a better understanding of metal-supported metal clusters by extending such studies to other adsorbates and different supporting metals. Therefore, we will address the surfaces Au(111) and Cu(111) as support. Since bulk Au and Cu have significantly different nearestneighbor distances,32 2.88 Å vs 2.55 Å, respectively, these metals serve as model substrate systems for exploring lattice strain and ligand effects at submonolayer coverage of Pt and Pd. Additionally, we also studied clusters adsorbed on their homogeneous surfaces, Pt3 on Pt(111) and Pd3 on Pd(111). Our model calculations are intended as a first step toward studying the catalytic properties of metal supported Pd and Pt planar clusters of increasing diameters. Additionally, the present work aims at elucidating whether electronic properties, particularly the local d-band centers, of supported planar clusters and overlayer systems, are in general similar. The study of substrate effects on the properties of supported clusters may be helpful in the rational design of economic and efficient electrocatalysts.

2. COMPUTATIONAL DETAILS All calculations were carried out within the framework of density functional theory (DFT), employing the Vienna ab initio Simulation Package (VASP).33−35 We used the Perdew, Burke, and Ernzerhof (PBE)36 variant of the generalized gradient approximation (GGA) to represent exchangecorrelation effects. The ionic cores were described by the Projector Augmented-Wave (PAW) method.37,38 The electronic one-particle wave functions were expanded in a planewave basis set up to an energy cutoff of 350 eV. We used the technique of fractional occupation numbers,39 with a level width of 0.05 eV. All total energies were extrapolated to kbT = 0 eV. The interaction between the repeated slabs was modified

3. RESULTS AND DISCUSSION 3.1. Pt3 and Pd3 Clusters in the Gas Phase. Table 1 summarizes the properties of optimized Pt3 and Pd3 clusters in the gas phase and on metal substrates. The calculated geometry of both clusters in the gas phase is very similar, with average 10058



Article

conformations 3 and 4, the center of the cluster is positioned above hollow sites: (3) at a hcp site with the three atoms of the adsorbate at fcc sites and (4) at a fcc site with the three atoms of the adsorbate at hcp sites. The lateral separation between repeated clusters is ∼6 Å, sufficient to avoid direct interaction.12,13,30 All these conformations have similar energetic and geometric properties. In conformations 1 and 3, Pt3 on Au(111) exhibits similar adsorption energies, −166 kJ mol−1/atom and −167 kJ mol−1/atom, respectively (Table 1). These conformations are the most stable ones because the three atoms of the cluster are at fcc sites of the (111) surface. This is the expected behavior from the adsorption properties of a single Pt atom on Au(111); adsorption on a fcc site is 9 kJ mol−1 lower in energy than adsorption on a hcp site. The average Pt−Pt distances in these conformations is also very similar, 2.62 Å (3) and 2.63 Å (1) (Table 1). The average Pt−Au(111) distance is 2.75 Å in either conformation. Also, other properties are very similar, e.g., the local d-band centers of the adsorbed cluster and the d-band centers of the substrate. The other two conformations, 2 and 4, are only slightly higher in energy, by 4−5 kJ mol−1, but again feature very similar geometric and electronic properties; see Table S1 of the Supporting Information. Analogous similarities hold for the various conformations of the system Pd3/Au(111) (Table S1 of Supporting Information). Since all conformations are very similar, further model calculations and analysis were restricted to conformations of type 3. We also checked how the thickness of the slab representing the substrate affects the properties of the adsorbed triangular clusters. Going from a slab with four to a slab with six atomic layers, the adsorption interaction per atom of Pt3 on Au(111) grows only by 2 kJ mol−1. Similarly, the geometry, the local magnetic moment per atom, and the local d-band center of the triangular cluster are very similar for the models of four and six atomic layers. The same holds for the system Pd3/Au(111) (Table S1 of Supporting Information). Therefore, in all further calculations of the present model study, the substrate surfaces were modeled by slabs of four atomic layers. 3.2.1. Energetic and Geometric Properties. The adsorption energy per atom for Pt3 on Pt(111) is −208 kJ mol−1, while it is −171 kJ mol−1 for Pd3 on Pd(111) (Table 1). The adsorption energies per atom for Pt3 on Au(111) and Cu(111) are −167 kJ mol−1 and −216 kJ mol−1, respectively (Table 1). The corresponding values for Pd3 are −158 kJ mol−1 and −193 kJ mol−1. Overall, the clusters Pt3 and Pd3 bind strongest to Cu(111), somewhat weaker to their homogeneous surfaces, Pt(111) or Pd(111), respectively, and finally weakest to Au(111). This strong interaction with Cu favors alloy formation.41,50−52 Nevertheless, small domains of Pd/Cu overlayers53 as well as islands of Pd−Cu alloys54 may be present at low coverage of Pd on a Cu surface. However, the weaker interaction of Pt and Pd with Au(111) leads to the formation of islands and overlayers.4,6,7 The stronger binding of Pt3 and Pd3 to Cu(111), relative to Au, is associated with a notable transfer of electron density from the support to the adsorbate55 as verified by our Badertype charge analysis.56,57 For Pt3, the total transferred electron density from Cu(111), 0.80 e, is larger than from Au(111), 0.16 e, and Pt(111), 0.06 e. Similarly, for Pd3, the transfer of electron density is larger upon adsorption from Cu(111), 0.45 e, than from Au(111), 0.04 e, and Pd(111), 0.02 e. This is the expected behavior when one considers the calculated work functions of these surfaces, approximated as energy difference from the

interatomic distances of 2.49 Å (Pt3) and 2.51 Å (Pd3). The calculated formation energy per atom is −237 kJ mol−1 for Pt3 and −125 kJ mol−1 for Pd3. These values agree quite well with previous computational results: −213 kJ mol−1 to −232 kJ mol−145−48 for Pt3 and −121 kJ mol−149 for Pd3. Both Pt3 and Pd3 were calculated to be in a triplet spin state (S = 1), exhibiting a total magnetic moment of μ = 2.00 μB, in agreement with previously reported results for Pt345−47 and Pd349 in the gas phase. As reference for clusters supported on a metal substrate, we evaluated the average energy of the manifold of d-orbitals of the clusters in the gas phase, relative to the average energy of the highest occupied and lowest unoccupied molecular orbitals, i.e., the center of the HOMO−LUMO gap of the bare clusters. The corresponding values are −1.35 eV for Pt3 and −0.99 eV for Pd3. Similarly, we quantified the influence of the substrate on the magnetic properties of adsorbed clusters. To this end, we approximated the local magnetic moment (LMM) per atom as the difference of spin-up and spin-down charges in atomcentered spheres of standard radii.38 For clusters in the gas phase, these values were calculated at 0.62 μB for Pt3 and 0.65 μB for Pd3. The (small) difference of the total magnetic moments, two unpaired electrons per cluster or 0.67 μB per atom for either system, represents the contribution of the interstitial region between the atomic spheres. As previously pointed out,49 the favorable high-spin state in Pd clusters is due to sd hybridization as a result of cluster formation. The local contribution of d-orbitals in each atom is depleted, leading to an open-shell-like behavior. 3.2. Adsorption of Pt3 and Pd3 on (111) Surfaces. Figure 1 shows the four inequivalent high-symmetry conformations for a triangular cluster on a (111) surface. In Conformations 1 and 2, the center of the triangular cluster is located on top of a surface atom, and the three atoms of the adsorbate are located either at face centered cubic (fcc) (conformation 1) or at hexagonal close packed (hcp) (conformation 2) hollow sites of the (111) surface. In

Figure 1. Stable conformations of M3 clusters on a (111) surface, shown for a (3 × 3) surface unit cell: (1) center of the cluster on top of a surface atom with the three atoms M on fcc hollow sites; (2) center of the cluster on top of a surface atom with the three atoms M on hcp hollow sites; (3) center of the cluster on a hcp hollow site with atom M on fcc site; (4) center of the cluster on a fcc hollow site with atom M on hcp site. 10059



Article

Fermi energy to the electronic potential in the region far from the surface; Cu, 4.74 eV; Au, 5.12 eV; Pd, 5.23 eV; Pt, 5.70 eV. These computational results agree well with measured values of work functions;58 Cu, 4.53−5.10 eV; Au, 5.10−5.50 eV; Pd, 5.22−5.60 eV; Pt, 5.12−5.93 eV. The stronger adsorption energies of Pt3 compared to those of Pd3 also can be rationalized by similar charge transfer arguments. The stronger binding of the clusters to their homogeneous surfaces, compared to adsorption on Au, is expected due to the strong electronic interaction of the clusters with these surfaces, as judged from an analysis of the local density of states; see Figure S1 in Supporting Information for the local density of state (LDOS) of Pt3 on Pt(111), Au(111), and Cu(111). The higher adsorption energy of Pt3 on Pt(111) as compared to Pd3 on Pd(111) can be rationalized by the cohesive energies of the corresponding metals. The experimental values of the bulk materials are −564 kJ mol−1 for Pt59 and −377 kJ mol−1 for Pd.60 The agglomeration energy of M3 clusters, i.e., the formation of adsorbed clusters from isolated adsorbed atoms M at fcc sites of a given metal support (Table S2 of Supporting Information), in general is small, from −38 kJ mol−1 to −23 kJ mol−1 (Table 1). Bonding competition between strong M-support and internal M−M bonds results in a dramatic reduction of the latter interaction energies. These notable agglomeration energies (per atom) of clusters in the gas phase, −237 kJ mol−1 for Pt3 and −125 kJ mol−1 for Pd3, are reduced to less than −40 kJ mol−1. We refrain from interpreting the small differences among the agglomeration energies on different substrate surfaces. As expected from the principle of bond competition, adsorption of the M3 clusters leads to longer M− M distances compared to clusters in the gas phase (Table 1). The average M−M distances increase (on average) by 0.16 Å for Pt3 and 0.22 Å for Pd3. Pd3 expands more than Pt3 because the cohesive energy of Pd3 is weaker than that of Pt3; see Table 1 and the experimental bulk cohesive energies given above. The average interatomic distances ⟨MC−MC⟩ of adsorbed Pt3 are 2.62 Å on Au(111), 2.66 Å on Pt(111), and 2.68 Å on Cu(111). This order correlates with the adsorption energies of the cluster and the average distances ⟨MC−MS⟩ of Pt atoms to their nearest-neighbor of the substrate, 2.75 Å (Au), 2.65 Å (Pt), and 2.52 Å (Cu). The principle of bond competition may also be invoked to rationalize the interatomic distance ⟨MS1− MS2⟩ between the first and second layer of the substrate surface, which is expected to increase due to adsorption. We observe this behavior for Au(111), where ⟨MS1−MS2⟩ increases by 0.05 Å, and Cu(111), where the average distance increase by 0.03 Å (Tables 1 and 2). For Pt(111), the average distance does not change. For Pd3 adsorbed on Au(111), Pd(111), and Cu(111), the average interatomic distances ⟨MC−MC⟩ are 2.73 Å, 2.71 Å, and 2.75 Å, respectively. Here, the value for Pd3/Pd(111) does not follow the order expected from the cluster adsorption energies. The stronger internal bond of Pd3 when adsorbed on Pd(111), −32 kJ mol−1, than on Au(111), −23 kJ mol−1, can help rationalizing the slightly shorter bond of Pd3/Pd(111). In fact, Pt3 on Au(111) and Pt(111) exhibits a similar but inverted correlation. The internal bond of Pt3 on Au(111), −38 kJ mol−1, is stronger than on Pt(111), −26 kJ mol−1, and the average interatomic distance of Pt3 is shorter when adsorbed on Au(111). However, for Pd3, the average distances ⟨MC−MS⟩ to the substrate, 2.73 Å (Au), 2.66 Å (Pd), and 2.53 Å (Cu), follow the same regular trend as for Pt3 on the corresponding

Table 2. Calculated Properties of Pure and Overlayered (111) Metal Surfacesa,b Pd(111) Pt(111) Cu(111) Au(111) Pd/Au(111) Pt/Au(111) Pd/Cu(111) Pt/Cu(111)

⟨MS1−MS1⟩

⟨MS1−MS2⟩

2.79 2.82 2.58 2.95 2.95 2.95 2.58 2.58

2.79 2.83 2.54 2.94 2.83 2.87 2.67 2.68

ε(MS1)

ε(MS)

−1.22 −1.41 −2.10 −2.41

−1.72 −2.25 −2.40 −3.37 −3.23 −3.24 −2.29 −2.28

Average nearest-neighbor distance ⟨MS1−MS1⟩ between atoms in the overlayer (top layer), average nearest-neighbor distance ⟨MS1−MS2⟩ between atoms in the overlayer and atoms in the top-layer of the substrate, local d-band center ε(MS1) of the atoms in the overlayer relative to the Fermi energy εF, d-band center ε(MS) of the atoms in the substrate relative to εF. Distances are in Å, one-electron energies in eV. bCalculated and experimental (in parentheses, ref 32) nearestneighbor distances (in Å) for bulk metals: Pd, 2.79 (2.75); Pt, 2.81 (2.77); Cu, 2.57 (2.55); Au, 2.95 (2.88). a

substrates. These values correlate with the strength of cluster substrate binding. Again, one finds slightly larger (or equal) average nearest-neighbor distances ⟨MS1−MS2⟩ between the first and second layer of the substrate, 0.02 Å (Au), 0.00 Å (Pd), and 0.03 Å (Cu) (Tables 1 and 2), in close analogy to the findings for Pt3, discussed above. 3.2.2. Electronic Properties. Previous calculations showed that adsorbed Pd clusters on Ag(001)61 and Au(111)30 do not exhibit a magnetic polarization. We found similar results for Pd3 on Cu(111) and on Au(111). For Pd3 on Pd(111), the LMM per atom is 0.35 μB (Table 1). Calculations62,63 and experiments64 demonstrated thin Pd films to be magnetic. To examine how the thickness of the slab modeling of the support affects the LMM of Pd3 on Pd(111), we also studied a system with six atomic layers instead of the four used in our standard models. However, the effect on the LMM is rather small; it changes from 0.35 μB (per atom) in the model with four atomic layers to 0.33 μB in the model with six atomic layers. For Pt3 on Au(111), we found the cluster to be magnetic with a LMM of 0.39 μB per atom (0.38 μB in the six-layer model; Table 1). The corresponding values for Pt3 on Pt(111) is 0.20 μB. On Cu(111), Pt3 has a very small magnetic moment, 0.01 μB (Table 1). The magnetic properties of Pt3 and Pd3 upon adsorption can be rationalized by analyzing the LDOS (states/atom/eV) on each Pt or Pd atom at the Fermi energy, εF (Table 1). This analysis usually is customarily carried out on results from analogous non-spin-polarized calculations.61,65 Invoking the Stoner criterion,66 J × LDOS(εF) ≥ 1, for the onset of magnetism and the value J = 0.65 eV for the exchange integral of Pd,67 one estimates the magnetic polarization of Pd to start from LDOS(εF) ≈ 1.5. This value agrees well with the results of the present model calculations on Pd3 and Pt3 (see Figure S2 in Supporting Information). Assuming the same value of J for both Pt and Pd, the Stoner criterion is satisfied for neither Pd3 nor Pt3 on Cu(111) and for Pd3 on Au(111), and these clusters are not magnetic. Pd3 on Pd(111) and Pt3 on Au(111) and Pt(111) exhibit values of LDOS(εF) ≥ 1.5 and thus satisfy the Stoner criterion. As discussed in section 3.1, Pt3 and Pd3 in the gas phase are magnetic (triplet ground state) with LMM values of 0.62 μB (Pt) and 0.65 μB (Pd) (Table 1). In comparison, clusters on a 10060



Article

Pd. Similar results for overlayer systems have previously been reported.68,69 3.3. Comparison with Overlayer Systems. The larger lateral nearest-neighbor distances within Au(111) surfaces, 2.88 Å,32 are known to introduce a stretching strain to the structure of Pt and Pd overlayers on Au surfaces.15,16,20,21,26 However, the small lateral interatomic distances of Cu surfaces, 2.55 Å,32 introduce a compression on the structure of overlayers of Pt and Pd on Cu surfaces.41,50,52 Our calculations for Pt or Pd overlayers on Au(111) and Cu(111) clearly show these large differences in the average interatomic neighbor distances of the Pt and Pd overlayers. In overlayers on Au(111), these distances were calculated to be 0.37 Å longer than on Cu(111); 2.95 Å (Au) and 2.58 Å (Cu) (Table 2). In contrast, the average interatomic distances in the triatomic adsorbed clusters are slightly longer on Cu(111) than on Au(111): by 0.06 Å for Pt3 and by 0.02 Å for Pd3. The interatomic distances within the Pt or Pd overlayers on Au(111) and Cu(111) are determined by the lateral interatomic distances of the substrate surface. This is not the case for small adsorbed metal clusters where the interatomic distances are always smaller than in the corresponding bulk metals. This reduction of nearest-neighbor distances in small clusters is a result of the reduced coordination of these ad-atoms.70−74 The longer interatomic distances of Pt3 and Pd3 on Cu(111), compared to Au(111), show that the strength of the cluster-substrate bonds affects the geometric properties of the adsorbed clusters. Figure 2 shows the energies of the d-band centers of the clusters M3, adsorbed on Au(111) and Cu(111), and those of

metal support feature significantly reduced magnetic moments. The effect is dissimilar for Pt and Pd clusters. The high-spin state of Pt3, in the gas phase, reflects the reduced d-occupation of atomic Pt (4f14 5d9 6s, 3D3), while in Pd3, it results from sd hybridization when the cluster is formed. These differences in electronic structure lead to the observed different LMM values of Pd3 and Pt3 adsorbed on metal substrates. For Pd3, coordination to the metal substrates, broadening the local density of states, suffices to quench in full its LMM value. In contrast, a significant charge transfer is required to completely reduce the LMM value of Pt3 as observed for Pt3/Cu(111). The position of the d-band center, relative to the Fermi energy, of metal surfaces has been shown to correlate with their chemical reactivity.21 Earlier studies13,30 addressed the role of coordination and metal−metal distances on the position of the local d-band center of supported metal clusters. Our results for the local d-band center of Pd3 on Au(111), −1.15 eV (Table 1), agrees adequately with previous results for that system, −1.21 eV30 and −1.31 eV.13 Also, our result for Pd3 on Pd(111), −1.37 eV, reproduces very well the earlier value of −1.39 eV.30 For Pt3, we noted a similar trend of a lower-lying local d-band center for Pt3 on Pt(111) than on Au(111): −1.87 eV on Pt(111) vs −1.46 eV on Au(111) (Table 1). The larger distance of the d-band center of triangular clusters on their homogeneous surfaces is expected due to the stronger coupling with these surfaces;30 see the corresponding values of ΔEads. The local d-band centers of the clusters, on Cu(111) and Au(111), exhibit similar energy separations from the pertinent Fermi energy (Table 1). For Pt3 on Cu(111), the local d-band center is calculated at −1.51 eV, only 0.05 eV lower than for Pt3 on Au(111), −1.46 eV. For Pd3, the difference is slightly larger, −1.29 eV for Pd3/Cu(111) vs −1.15 eV for Pd3/Au(111). The substrates have only a minor effect on the position of the local d-band center of small adsorbed clusters. Different from full metal overlayers (see the following section), these adsorbates do not feel any lateral constraints. Therefore, the electronic properties of adsorbed small clusters are not affected by (strong) tensile or compressive strain. Differences are mostly driven by ligand effects,19,22,23,26,28 which may be interpreted in part as minor constraints. The d-band centers of the various substrate surfaces, free of adsorbates, were calculated at −1.72 eV for Pd(111), −2.25 eV for Pt(111), −2.40 eV for Cu(111), and −3.37 eV for Au(111) (Table 2). These values change only slightly when a Pt3 or Pd3 cluster is adsorbed. For Pt3/Pt(111), the d-band center is shifted downward by 0.06 eV and for Pd3/Pd(111) by 0.05 eV. The effect due to cluster adsorption is also quite small for Au(111) and Cu(111), but the d-band center of the support is shifted upward upon adsorption, by 0.05 eV for Pt3 on Au(111) and 0.09 eV for Pt3 on Cu(111); the corresponding values for adsorption of Pd3 are 0.02 eV on Au(111) and 0.03 eV on Cu(111). Analysis of the layer-by-layer (local) d-band center of Au(111) and Cu(111), with and without adsorbed Pt3 (data no shown), shows that all four layers of the slab model are slightly affected. Similar shifts of the d-band centers of the substrates, downward for adsorption on homogeneous surfaces and upward for adsorption on Au(111) and Cu(111), but of smaller magnitude, were observed for the adsorption of single Pt and Pd atoms; see Table S2 of Supporting Information. For an overlayer of Pt or Pd on Au(111) and Cu(111), the same trend, but of larger magnitude, was observed (Table 2). The dband centers of all substrates studied are shifted upward by ∼0.1 eV when the surfaces are covered by an overlayer of Pt or

Figure 2. Energies of d-band centers of metal clusters M3 adsorbed on various (111) substrate surfaces and the corresponding metal M overlayer, relative to the Fermi energy εF of the system. For M3 clusters in the gas phase, the energies are given relative to the center of the HOMO−LUMO gap. M = Pt (squares, solid line); M = Pd (circles, dashed line).

the corresponding overlayers, all values relative to the corresponding Fermi energies. The average energies of the valence d-manifold of the triatomic clusters in the gas phase, relative to the center of the HOMO−LUMO gap, is also given. That average energy of Pt3, −1.35 eV, is lower than in Pd3, −0.99 eV, due to the lower d-occupation of Pt; the valence configuration of atomic Pt is 4f145d96s 3D3, while that of atomic Pd is d10 1S0.75 When the clusters bind to the substrate, the coordination increases; hence, the d-band centers are shifted downward.22,23 Adsorption on Cu(111) results in a slightly larger shift due to the stronger binding to this surface (Table 1). Yet, differences are rather small: replacing Au(111) by Cu(111) has only a minor effect on the position of the d-band 10061



Article

Pt, and Pd show a particularly strong binding of both Pt3 and Pd3 on the Cu(111) surface. The magnetic moment of Pd3 was found to be totally quenched upon adsorption on Au(111) and Cu(111). However, Pt3 partially retains its magnetic moment upon adsorption on Au(111), while for adsorption on Cu(111), it is almost reduced to zero. Adsorption on Au(111) and Cu(111) affects the interatomic distances of Pt3 and Pd3 in a minor way only. This is at variance with expectations based on results calculated for the corresponding Pt and Pd overlayers where the notably different lateral nearest-neighbor distances of Au(111) and Cu(111) induce very different interatomic distances. These results correlate with the energies of the local d-band centers. For Pt3 and Pd3 on Au(111) and Cu(111), the valence d-electrons are at very similar energies as in the free clusters values, relative to the Fermi energy or the middle of the HOMO−LUMO gap, respectively. In contrast, the local d-band centers of the overlayer systems are significantly shifted for Cu(111) but not for Au(111). We expect the present results to lead to notable size effects in the properties of metal-supported metal clusters and their catalytic activity.

centers of the clusters (Table 1). Moreover, the positions of the local d-band centers of Pt3 and Pd3 clusters, supported by Au or Cu, are quite similar (within 0.3 eV) to those of the corresponding (small) bare clusters. The d-band center of Pd3 on Au(111), −1.15 eV, is very close to the corresponding value of a Pd overlayer on Au(111), −1.22 eV (Table 2), as previously reported.13,30 We also found such similarity for Pt3 adsorbed on Au(111) and the corresponding Pt overlayer on Au(111): −1.46 eV vs −1.41 eV, respectively. In contrast to Au-supported systems, the dband centers of the triatomic clusters adsorbed on Cu(111) are significantly different from those of the corresponding overlayer systems. The d-band center of Pt3 on Cu(111) is 0.90 eV closer to the Fermi energy than the corresponding value of a Pt overlayer (Table 2). Similarly, for Pd3 on Cu(111), the local dband center is 0.81 eV closer to the Fermi energy than that of a Pd overlayer on Cu(111). From the linear relationship between the d-band center and the width of the d-band,22,23 one expects a downward shift of the local d-band center when going from a triatomic cluster to an overlayer. The larger average coordination in an overlayer leads to a wider d-band, hence to a downward shift of the dband center, essentially maintaining the occupation of the dband.22,23 This expected behavior indeed is observed for the M3 clusters and overlayers on Cu(111) but not for analogous systems on Au(111). The lateral constraints imposed by Au(111) to the overlayers result in longer interatomic distances, reducing the interaction within the d-manifolds. In consequence, the widths of the d-manifolds in the adsorbed M3 clusters and the overlayer are similar on Au(111), calculated at 6.84 (6.40) eV for the adsorbed Pt3 (Pd3) clusters and 6.99 (6.61) eV for the Pt (Pd) overlayers. In contrast, the analogous values for clusters and overlayers on Cu(111) are 6.51 (6.20) eV and 8.28 (7.77) eV, respectively. Clearly, the d-band widths of adsorbed M3 and of the corresponding overlayers on Au(111) are comparable, and in turn, the local d-band centers of these systems are very similar (Figure 2). The similarity13,30 between the local d-band centers of adsorbed Pt3 and Pd3 clusters on Au(111) and the corresponding overlayer systems does not seem to hold true in general, as they reflect a compensation of counteracting effects, a strain due to the support vs lateral (intralayer) interaction (higher coordination). On the basis of the well-known correlation between the dband center position of regular surfaces and overlayer systems and catalytic activity of such system,21 the catalytic activity of small metal clusters supported on a different metal substrate should not vary significantly. However, one needs to corroborate for metal-supported metal clusters to what extent the position of the d-band center indeed correlates with the catalytic activity. This correlation may depend on the size and shape of the adsorbed clusters as well as on the location of active sites (terraces, steps, edges, kinks, etc.) These and other questions will be addressed in forthcoming works.

■

ASSOCIATED CONTENT

S Supporting Information *

Table of calculated properties of clusters Pt3 and Pd3 on Au(111), optimized for various conformations; table with properties of Pt1 and Pd1 adsorbed at fcc sites of various metal supports; figure with density of states (DOS), from non-spinpolarized calculations, of clean metal supports M(111) (M = Au, Cu, Pt), Pt3/M(111), and local DOS (×5) of Pt3 in Pt3/ M(111); figure showing the local magnetic moment of M3 clusters adsorbed on Au(111), Cu(111), and M(111) (M = Pt, Pd) as function of the corresponding (local) density of states at the Fermi energy, from non-spin-polarized calculations. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

We thank Dr. Sven Krüger for helpful discussions. J.A.S. gratefully acknowledges support by a research fellowship of the Alexander von Humboldt Foundation. The work was supported by Fonds der Chemischen Industrie (Germany). Computational resources were provided in part by the Jü lich Supercomputing Centre.

4. SUMMARY In the present computational study, we examined geometric, energetic, and electronic properties of small metal clusters, specifically Pd3 and Pt3, supported on a metal substrate. To unravel similarities and differences between supported clusters and the corresponding overlayer systems, we employed Au(111) and Cu(111) as model substrates as they exert different types of lattice strain effects. The calculated adsorption energies of the small clusters on the (111) surfaces of Au, Cu,

(1) Gates, B. C. In Handbook of Heterogeneous Catalysis; Ertl, G., Knözinger, H., Schüth, F., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, Germany, 2008; p 1277. (2) Kulkarni, A.; Lobo-Lapidus, R. J.; Gates, B. C. Chem. Commun. 2010, 46, 5997. (3) Murray, R. W. Chem. Rev. 2008, 108, 2688. (4) Wolfschmidt, H.; Paschos, O.; Stimming, U. In Fuel Cell Science: Theory, Fundamentals, and Biocatalysis; Wieckowski, A., Nørskov, J. K., Eds.; Wiley: Chichester, U.K., 2010; p 1.

10062



Article

(5) Maillard, F.; Pronkin, S.; Savinova, E. R. In Fuel Cell Catalysis: A Surface Science Approach; Koper, M., Wieckowski, A., Eds.; Wiley: Chichester, U..K, 2009; p 507. (6) Kibler, L. A. ChemPhysChem 2006, 7, 985. (7) Steidtner, J.; Hernandez, F.; Baltruschat, H. J. Phys. Chem. C 2007, 111, 12320. (8) Yano, H.; Inukai, J.; Uchida, H.; Watanabe, M.; Babu, P. K.; Kobayashi, T.; Chung, J. H.; Oldfield, E.; Wieckowski, A. Phys. Chem. Chem. Phys. 2006, 8, 4932. (9) Nasluzov, V.; Ivanova-Shor, E.; Shor, A.; Yudanov, I.; Rösch, N. Kinet. Catal. 2010, 51, 832. (10) Petkov, P. S.; Petrova, G. P.; Vayssilov, G. N.; Rösch, N. J. Phys. Chem. C 2010, 114, 8500. (11) Jinnouchi, R.; Toyoda, E.; Hatanaka, T.; Morimoto, Y. J. Phys. Chem. C 2010, 114, 17557. (12) Björketun, M. E.; Karlberg, G. S.; Rossmeisl, J.; Chorkendorff, I.; Wolfschmidt, H.; Stimming, U.; Nørskov, J. K. Phys. Rev. B 2011, 84, 9. (13) Andersin, J.; Honkala, K. Phys. Chem. Chem. Phys. 2011, 13, 1386. (14) Yang, F.; Zhang, Q. F.; Liu, Y. W.; Chen, S. L. J. Phys. Chem. C 2011, 115, 19311. (15) Ferrin, P.; Mavrikakis, M.; Rossmeisl, J.; Nørskov, J. K. In Fuel Cell Science: Theory, Fundamentals, and Biocatalysis; Wieckowski, A., Nørskov, J. K., Eds.; Wiley: Chichester, U.K., 2010; p 486. (16) Groβ, A., Schnur, S. In Catalysis in Electrochemistry: From Fundamental Aspects to Strategies for Fuel Cell Development; Santos, E., Schmickler, W., Eds.; Wiley: Chichester, U.K., 2011; p 165. (17) Kibler, L. A. Electrochim. Acta 2008, 53, 6824. (18) Greeley, J.; Nørskov, J. K.; Kibler, L. A.; El-Aziz, A. M.; Kolb, D. M. ChemPhysChem 2006, 7, 1032. (19) Santos, E.; Quaino, P.; Schmickler, W. Electrochim. Acta 2010, 55, 4346. (20) Santos, E.; Schmickler, W. In Catalysis in Electrochemistry: From Fundamental Aspects to Strategies for Fuel Cell Development; Santos, E., Schmickler, W., Eds.; Wiley: Chichester, U.K., 2011; p 197. (21) Hammer, B.; Nørskov, J. K. In Advances in Catalysis; Academic Press Inc: San Diego, CA, 2000; Vol. 45, p 71. (22) Kitchin, J. R.; Nørskov, J. K.; Barteau, M. A.; Chen, J. G. Phys. Rev. Lett. 2004, 93, 156801. (23) Kitchin, J. R.; Nørskov, J. K.; Barteau, M. A.; Chen, J. G. J. Chem. Phys. 2004, 120, 10240. (24) Pandelov, S.; Stimming, U. Electrochim. Acta 2007, 52, 5548. (25) Behm, R. J. Z. Phys. Chem. 2009, 223, 9. (26) Groβ, A. Top. Catal. 2006, 37, 29. (27) Wolfschmidt, H.; Bussar, R.; Paschos, O.; Brülle, T.; Stimming, U. 21st Meeting of The North American Catalysis Society, 2009. http:// www.nacatsoc.org/21nam/data/papers/Paper1823.pdf (accessed Nov 23, 2011). (28) Hoster, H. E.; Alves, O. B.; Koper, M. T. M. ChemPhysChem 2010, 11, 1518. (29) Lischka, M.; Mosch, C.; Groβ, A. Electrochim. Acta 2007, 52, 2219. (30) Roudgar, A.; Groβ, A. Surf. Sci. 2004, 559, L180. (31) Fratesi, G. J. Phys.: Condens. Matter 2011, 23. (32) Lonsdale, K. International Tables for X-ray Crystallography; Kynoch Press: Birmingham, AL, 1962; Vol. 3. (33) Kresse, G.; Hafner, J. Phys. Rev. B 1993, 47, 558. (34) Kresse, G.; Hafner, J. Phys. Rev. B 1994, 49, 14251. (35) Kresse, G.; Furthmüller, J. Phys. Rev. B 1996, 54, 11169. (36) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (37) Blöchl, P. E. Phys. Rev. B 1994, 50, 17953. (38) Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1758. (39) Methfessel, M.; Paxton, A. T. Phys. Rev. B 1989, 40, 3616. (40) Neugebauer, J.; Scheffler, M. Phys. Rev. B 1992, 46, 16067. (41) Roudgar, A.; Groβ, A. Surf. Sci. 2005, 597, 42. (42) Chen, Z.-X.; Neyman, K. M.; Gordienko, A. B.; Rösch, N. Phys. Rev. B 2003, 68, 075417.

(43) Gajewski, G.; Pao, C.-W. J. Chem. Phys. 2011, 135, 064707. (44) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188. (45) Yang, S. H.; Drabold, D. A.; Adams, J. B.; Ordejón, P.; Glassford, K. J. Phys.: Condens. Matter 1997, 9, L39. (46) Huda, M. N.; Niranjan, M. K.; Sahu, B. R.; Kleinman, L. Phys. Rev. A 2006, 73, 053201. (47) Celik, V.; Unal, H.; Mete, E.; Ellialtioglu, S. Phys. Rev. B 2010, 82, 12. (48) Xiao, L.; Wang, L. J. Phys. Chem. A 2004, 108, 8605. (49) Moseler, M.; Hakkinen, H.; Barnett, R. N.; Landman, U. Phys. Rev. Lett. 2001, 86, 2545. (50) Canzian, A.; Mosca, H. O.; Bozzolo, G. Surf. Sci. 2004, 551, 9. (51) Bozzolo, G.; Garces, J. E.; Noebe, R. D.; Abel, P.; Mosca, H. O. Prog. Surf. Sci. 2003, 73, 79. (52) Demarco, G.; Garces, J. E.; Bozzolo, G. Surf. Sci. 2003, 526, 309. (53) Aaen, A. B.; Laegsgaard, E.; Ruban, A. V.; Stensgaard, I. Surf. Sci. 1998, 408, 43. (54) Howe, C. J.; Cropper, M. D.; Fleming, T. P.; Wardle, R. M.; Bailey, P.; Noakes, T. C. Q. Surf. Sci. 2010, 604, 201. (55) Rodriguez, J. A.; Goodman, D. W. Science 1992, 257, 897. (56) Bader, R. F. W. In The International Series of Monographs on Chemistry; Oxford University Press: Oxford, U.K., 1990; Vol. 22. (57) Henkelman, G.; Arnaldsson, A.; Jonsson, H. Comput. Mater. Sci. 2006, 36, 354. (58) Lide, D. R. CRC Handbook of Chemistry and Physics; 89th ed.; CRC Press: New York, 2008. (59) Kittel, C. Solid State Physics; 5th ed.; John Wiley & Sons: New York, 1976. (60) Wagman, D. D.; Evans, W. H.; Parker, V. B.; Schumm, R. H.; Halow, I.; Bailey, S. M.; Churney, K. L.; Nuttall, R. L. J. Phys. Chem. Ref. Data 1982, 11, 1. (61) Wildberger, K.; Stepanyuk, V. S.; Lang, P.; Zeller, R.; Dederichs, P. H. Phys. Rev. Lett. 1995, 75, 509. (62) Bouarab, S.; Demangeat, C.; Mokrani, A.; Dreysse, H. Phys. Lett. A 1990, 151, 103. (63) Hong, S. C.; Lee, J. I.; Wu, R. Phys. Rev. B 2007, 75, 172402. (64) Huang, X.; Tang, S.; Mu, X.; Dai, Y.; Chen, G.; Zhou, Z.; Ruan, F.; Yang, Z.; Zheng, N. Nat. Nanotechnol. 2011, 6, 28. (65) Blügel, S. Phys. Rev. B 1995, 51, 2025. (66) Stoner, E. C. Proc. R. Soc. London A 1938, 165, 0372. (67) Christensen, N. E.; Gunnarsson, O.; Jepsen, O.; Andersen, O. K. J. Phys 1988, 49, 17. (68) Roudgar, A.; Groβ, A. J. Electroanal. Chem. 2003, 548, 121. (69) Gohda, Y.; Gross, A. J. Electroanal. Chem. 2007, 607, 47. (70) Häberlen, O. D.; Chung, S.-C.; Rösch, N. Int. J. Quantum Chem. 1994, 52, 595. (71) Häberlen, O. D.; Chung, S.-C.; Stener, M.; Rösch, N. J. Chem. Phys. 1997, 106, 5189. (72) Krüger, S.; Vent, S.; Nörtemann, F.; Staufer, M.; Rösch, N. J. Chem. Phys. 2001, 115, 2082. (73) Yudanov, I. V.; Sahnoun, R.; Neyman, K. M.; Rösch, N. J. Chem. Phys. 2002, 117, 9887. (74) Yudanov, I. V.; Metzner, M.; Genest, A.; Rösch, N. J. Phys. Chem. C 2008, 112, 20269. (75) Martin, W. C.; Wiesen, W. L. In Atomic, Molecular, & Optical Physics Handbook; AIP: Woodbury, NY, 1996; p 135.

10063


Metal-Supported Metal Clusters: A Density Functional Study of Pt

Recommend Documents