Article pubs.acs.org/JPCC
Support and Oxidation Effects on Subnanometer Palladium Nanoparticles Christopher J. Heard,† Stefan Vajda,‡,§ and Roy L. Johnston*,† †
School of Chemistry, University of Birmingham, Edgbaston, Birmingham, U.K. B15 2TT Materials Science Division and Nanoscience and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States § Department of Chemical and Environmental Engineering, School of Engineering & Applied Science, Yale University, 9 Hillhouse Avenue, New Haven, Connecticut 06520, United States ‡
S Supporting Information *
ABSTRACT: The effect of cluster size, oxidation state, and the support upon the structures and energetics of subnanometer palladium nanoparticles is investigated within a density functional framework. Gas phase global minima of Pd4 and Pd10 along with their suboxide counterparts are determined using a genetic algorithm and deposited upon MgO (001) and a high-index alumina surface. It is observed that there is an oxidation-dependent transition in the smaller clusters from three-dimensional to two-dimensional structures both in the gas phase and when supported by a surface. MgO strongly promotes a change from tetrahedral- and icosahedral-based structures toward cubic forms, while alumina induces significant distortion of the cluster and the breaking of Pd−Pd bonds. Increased oxygenation contributes cooperatively to these effects, causing disruption of the Pd−Pd bond network, favoring the incorporation of oxygen into the cluster structure, further complicating unambiguous structure prediction.
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has been performed by Xiao and Schneider,21 showing an intermediate binding energy of 1.35 eV to a terminal oxygen site, which is the preferred site for all but Ag atoms. Nasluzov and colleagues investigated the binding to α-alumina of Pd3 and Pt3 clusters in parallel and almost perpendicular modes,22 observing that Pd clusters parallel to the surface are unstable with respect to the metal-on-top23 arrangement. They further observed significant extension of metal−metal bonds within the cluster due to metal−oxygen surface binding, with an associated charge transfer to the clusters of up to 0.5e. In order to investigate the size dependence of cluster structure, Nigam and Majumder carried out a systematic deposition study of Pdn (n = 1−7) on α-alumina (0001);18 the compact structures found to be optimal in the gas phase are distorted to a great degree due to the strong surface effect. They conclude that the delicate balance between Pd−Pd and Pd−surface bonding is crucial to prediction of the optimal geometry.18 The magnesium oxide (001) surface provides a model system for cluster deposition due to its regular rocksalt crystalline form, its strong ionic bonding within a layer, and the resilience to deformation on cluster adsorption. To this end, many studies of cluster structure have been performed for this system, for a range of sizes, in order to isolate ionic bonding effects, which eliminates the surface irregularity introduced by alumi-
INTRODUCTION Supported subnanometer metal clusters are of growing interest in the field of heterogeneous catalysis, owing to recent experiments which suggest high selectivity and activity toward specific reactions of alkanes and alkenes. This interest has been facilitated by the availability of sensitive experimental procedures, which can guarantee size selectivity of produced clusters, and the atomic resolution of analysis techniques used to identify the resultant complexes on substrate deposition and subsequent chemical reaction. Vajda and colleagues have shown that platinum clusters in the 8−10 atom size range have activities up to 2 orders of magnitude greater than previous platinum catalysts for the selective oxidative dehydrogenation of propane.1 More recently, Oliver-Meseguer and co-workers found Au3−10 to be extremely efficient for the ester-assisted hydration of alkynes,2 with a strong dependence of activity on cluster size. Palladium clusters on the nanoscale, particularly those deposited on oxide supports, have been well investigated and characterized, both experimentally3−7 and theoretically,8−13 due to their varied catalytic uses as well as their relative ease of study. Subnanometer palladium clusters have subsequently attracted considerable interest, particularly within the theoretical community,14−20 often with the focus on structures and bond strengths of supported clusters as a function of size and surface site. Comparative work between Pd and other late transition metal atoms (Pt, Ag, Au) upon clean and hydroxylated alumina © 2014 American Chemical Society
Received: November 8, 2013 Revised: January 30, 2014 Published: January 30, 2014 3581
dx.doi.org/10.1021/jp411019t | J. Phys. Chem. C 2014, 118, 3581−3589
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Figure 1. Global minima for Pd4Ox (x = 0, 1, 2, 3, 4) and Pd10 as determined with the BCGA.
na.10−13,24−32 Palladium atoms were considered by Rivanenkov and colleagues,33 whereas Ferrari and colleagues16 investigated Pd and Ag dimers and tetramers, finding a preference for oxygen centers, with the metal-on-top effect suppressed for Pd2. Ferrando and Fortunelli studied a range of tetrameric clusters,27 including Pd4, upon a fixed bilayer of MgO (001) and elucidated the transition pathways to motion upon the surface. Recently, Ismail et al. used the same fixed 5 × 5 bilayer for a study of adsorption properties of mixed AuPd clusters at the nanometre scale.11 Atanasov et al. have developed a new potential to account for the metal-on-top effect and applied it to Pd nanoparticles of up to 80 atoms,13 finding a prevalence of overhanging Pd atoms in compact cluster structures, as found both experimentally34,35 and theoretically36 for Pd on MgO. The overall goal of this wealth of structural work is to benefit future experiments by understanding and predicting the reactive properties of specific cluster systems. Heterogeneous catalysis upon supported subnanoscale clusters appears to be an important future direction. Lee et al. have recently determined the effect of palladium cluster size on catalytic ability for methanol decomposition upon an amorphous alumina support, both theoretically and experimentally,17 finding that clusters of between 8 and 10 atoms have low activity due to poisoning, whereas slightly larger particles of 15−18 atoms show no such poisoning. The same group have subsequently shown Pd6 and Pd17 deposited on ultrananocrystalline diamond to be effective electrodes in the oxygen evolution reaction in the electrochemical water splitting reaction.37 This again shows an atomic scale size dependence, as Pd4 was found to have no such activity, which was attributed to the shape and dimensionality of the clusters. Highly oxidized Pd4 is calculated to be twodimensional and is thus without the Pd−Pd bridging sites which have the correct balance of binding energies to allow the reaction to proceed, in contrast to Pd6 and Pd17. In this work, we consider the binding of subnanometer scale palladium clusters Pd4 and Pd10 to MgO (001) and θ-alumina and determine their preferred binding modes as a function of size and support identity. In addition, we investigate the effect of oxidation on the structures and energetics of these clusters, for catalytically relevant oxidation levels, in order to support experimental work currently ongoing in the field of supported subnanoscale metal cluster catalysis. The first section is comprised of a density functional theory global optimization of the free clusters and their oxide counterparts to determine appropriate structures for deposition. The next section is dedicated to clusters deposited upon MgO (001) and their oxidation, followed by the analogous investigation for aluminasupported clusters. A discussion of the results from the point of view of cluster size, support identity, and oxidation level concludes the study.
erations of 10 individuals are produced and ranked according to a tanh fitness function. Mating is undertaken using the roulette selection criterion and the Deaven−Ho cut and splice method39 while mutation is performed with the single point weighted routine. Electronic structure calculations are performed with the Quantum Espresso plane-wave density functional package (PWscf).40 The Perdew−Berke−Erzenhof exchange correlation functional41 and ultrasoft pseudopotentials of the RRKJ type42,43 are utilized, with ten and six explicitly treated electrons for oxygen and palladium, respectively. The Methfessel−Paxton smearing scheme44 is employed to aid convergence for metallic states. The BFGS algorithm is used for local minimization, with energy and force convergence criteria of 10−4 Ry and 10−3 Ry a0−1, respectively. The MgO surface consists of a 5 × 5 × 3 slab comprising a cubic cell of 150 atoms, based on a bilayer unit cell used in a previous work.11 We extend the number of atomic layers from two to three in order to more accurately represent the bulk surface. Calculations are performed in a periodic framework, such that the surface is represented by an infinite repeat of cells in the plane parallel to the surface. Thirteen vacuum layers are introduced above the surface (in the perpendicular direction) to avoid image−image interactions. The alumina cell is comprised of a seven-layer slab with 18 vacuum layers above the surface. The slab contains 120 atoms in an monoclinic unit cell with a, b, and c vectors of 21.4, 23.2, and 37.8 bohr, respectively, and γ = 76.5°, which is sufficient to avoid cluster− cluster interactions. This cell geometry is taken from a previous work1 and completely locally relaxed before cluster calculations are performed. A series of test calculations showed the MgO substrate to be very resilient to deformation for a wide range of cluster geometries and sizes. Alumina, with a less rigid structure and more unsaturated oxygen bonds, was found to change slightly on cluster adsorption. On relaxation of several atomic layers, these changes were found to occur almost exclusively within the first surface layer, with insignificant changes to atomic positions in the second or third layers. Furthermore, the charge transfer between surface and cluster was observed to be invariant to the number of relaxed surface layers, to within 0.03 electron/atom. Therefore, only the first layer of alumina is allowed to relax in subsequent calculations, and the MgO substrate is constrained to the bulk geometry in order to reduce unnecessary computational cost.
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RESULTS AND DISCUSSION Gas Phase. The global minima for free Pd4Ox, with x = 0, 1, 2, 3, and 4 (up to the stoichiometric PdO composition), and Pd10 are determined using the BCGA and are shown in Figure 1. The bare metal tetramer adopts a tetrahedral geometry, with the planar structure uncompetitive as the next lowest energy isomer at +0.74 eV. C2v (Y-shaped) and linear geometries were found to be significantly higher in energy (>+1.5 eV), in close
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THEORETICAL METHODS The global optimization of cluster structure is achieved using the Birmingham cluster genetic algorithm (BCGA).38 Gen3582
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agreement with Nigam and Majumder,18 and are given no further consideration. For the oxidized clusters, the trend is for the tetrahedron to open out toward the planar shape on increasing oxidation. For Pd4O2, the structure is a defective tetrahedron, with an extended bond between the atoms labeled 1 and 2 in Figure 1. For Pd4O3, this distortion is more extreme as the high density of oxygen atoms upon the frame causes the strained Pd−Pd bond to break. The resultant butterfly skeleton is a transitionary structure between the tetrahedron and the rhombus. By Pd4O4, the full transition to planarity has occurred. Another experimentally relevant cluster size at the subnanometre scale is the Pd10 particle. Currently, the BCGA is limited to around 15 atoms for a monometallic cluster at the level of theory required for this study and around ten atoms for a binary system. Therefore, the global minimum (GM) for only the bare palladium decamer is elucidated. The resultant structure is of the form of an incomplete icosahedron. From observation of both the GM and the library of suboptimal minima produced by the BCGA, it appears there is a competitive energetic balance between steric strain and the maximization of palladium−oxygen bonds. In the unhindered case of Pd4O1, the three-coordinate Pd−O binding mode (denoted by μ-3) is found in preference to the two-coordinate (μ-2) mode. The situation is reversed in general for the more highly oxidized clusters. In these cases, the majority of low-lying isomers adopt predominantly μ-2 binding modes. Pd−O binding is the most important bonding motif, as evidenced by the maximization of Pd−O bonds to the detriment of the Pd−Pd network. This result is supported by experimental data on gas phase dimers, for which the Pd−O bond is approximately 3 times stronger than that of Pd−Pd. Further evidence of this behavior is that bound dimeric oxygen is rarely found to be competitive in our searches. The lowest lying dimer-bound isomer is at +440 meV for Pd4O4 and is not found to be one of the ten lowest energy isomers for any of the less oxidized clusters. Oxidation States. In order to determine whether the cluster is likely to attain the high oxidation states, the relative energy for the reaction in eq 1 can be calculated, which is defined as the difference between the total energy of the global minimum of the Pd4Ox cluster, and the sum of of the energies for the free oxygen molecules and the Pd4 global minimum, as given in eq 2. Pd4 + nO2 → Pd4O2n
(1)
ΔE = E Pd4O2n − E Pd4 − EnO2
(2)
that in a suitably oxygenated environment all of the oxidation states are potentially available. Because barriers to rearrangements between the structures have not been computed, the likelihood of disproportionation cannot be estimated, but the large relative energies suggest that the clusters may be highly oxidized in an oxygen-rich atmosphere. Supported Clusters. To simulate the process of ultrasoft landing upon a substrate in a reduced atmosphere, low-energy gas phase bare metal clusters are deposited upon the surface, and their geometries are locally relaxed. Oxidation is then simulated by addition of oxygen atoms over binding sites on the cluster and the surface and are further relaxed. MgO-Supported Clusters. In the work of Ferrando and Fortunelli,27 five isomeric forms of the tetramer−substrate complex were considered: two tetrahedral structures, differing only by a rotation upon the surface, one planar rhombic structure, and two metal-on-metal structures. The authors note that the tetrahedral complexes are overall the most stable, with the planar geometry the next most favorable. This result, reproduced by our calculations, coupled with the analogous behavior found in the gas phase means we may focus on the tetrahedron and planar structures in the MgO-bound regime. Pd4Ox/MgO(001). In the next step, oxygen atoms are deposited over binding sites upon both the tetrahedral and the rhombic clusters and the resultant minima ranked by total energy. Figure 2 shows the lowest energy geometries for each of Pd4O1, Pd4O2, Pd4O3, and Pd4O4. In the case of Pd4O3, two isomers are very nearly degenerate, and so both are given. The tetrahedral global minimum undergoes a noticeable distortion on the binding of a single oxygen atom, with a Pd− Pd bond along the base of the cluster stretched from 2.61 to 2.90 Å in order to improve epitaxy with the Mg−O surface. Palladium will bind to oxygen in preference to magnesium, and the cluster rearranges slightly to enhance this bonding. The distortion of the palladium frame on oxygen loading is the most striking behavior, with the cluster becoming increasingly cubic in structure by the migration of palladium atoms toward epitaxially favorable positions. By Pd4O2, the basal Pd−Pd bond is broken entirely. The planar cluster is a simpler system, with fewer possible binding modes to oxygen. μ-2 bridging may occur along the plane of the surface or bind atop the palladium. There is an additional μ-4 site above the cluster, which maximizes steric inhibition for additional ligands. Pd4O1 adopts a μ-4 motif, with the oxygen atom located centrally atop a 4-fold Pd4 face, but for subsequent oxygenation levels, this site becomes energetically unavailable, and the oxygen atoms are forced into sites peripheral to the palladium frame, bound both to metal and the surface. The exception to this rule is Pd4O3, for which the cluster is extended along the long axis of the rhombus sufficiently to allow a μ-3 occupation of the top site. It is noted that the next most favorable isomer for this composition follows the trend of occupying the surface + cluster sites, and is energetically competitive, at +40 meV. Pd4O4 shows a rearrangement from rhombic to a squareplanar structure. This configuration allows the palladium atoms to sit over surface oxygen sites, and the oxygen atoms to sit over magnesium sites, without breaking any Pd−Pd bonds. Again, the drive toward epitaxy with the substrate produces a particularly stable structure. It should be recognized that in all cases neither the likelihood of interconversion between particular isomers nor the statistical
Table 1 shows that in each case bound oxygens produce more stable structures than the free case, and so we may predict Table 1. Relative Energies of Pd4Ox Global Minima As Calculated with Eq 2 species
relative energy (ΔE)/eV
Pd4 Pd4O1 Pd4O2 Pd4O3 Pd4O4
0 −1.25 −2.83 −4.99 −6.49 3583
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Figure 2. Putative global minima for Pd4Ox (x = 0, 1, 2, 3, and 4) on a MgO(001) substrate. The two closely competitive isomers for Pd4O3 are labeled d and e. Palladium atoms are in dark blue, and adsorbed oxygen atoms are in red. The surface magnesium atoms are in green, whereas the surface oxygens are maroon, to distinguish them from deposited oxygen atoms.
available for the MgO surface but may play a role for the alumina substrate.)
weights of reaction pathways between them are considered. Therefore, any minima that are found to be energetically similar must be considered as viable alternatives. The relative total energies of the lowest lying isomers for both structural types at all compositions are recorded in Figure 3. The tetrahedron is more stable upon the MgO (001) surface
ΔEPd4Ox = E Pd4Ox @surf − E Pd4Ox − Esurface
(3)
Table S1 (see the Supporting Information) shows the overall deposition energy of each stoichiometry of Pd4Ox is high and largely invariant to additional oxidation, at around −5.35 eV for each composition. Pd10Ox/MgO(001). To investigate the effect of size on the structural behavior of the cluster, the decameric palladium particle is bound to MgO (001). In this case, only the gas phase global minimum structure was deposited upon the center of the substrate. The lower symmetry of the initial cluster−substrate complex means that specific orientations are of less importance than for the tetramers. The cluster is locally relaxed and immediately shows a striking change in structure, from the icosahedral to a cubic form. The remarkable result is that the larger cluster appears more susceptible to the epitaxy effect of the support than the smaller clusters. Palladium atoms rearrange into a compact, two-layer form, with the layer bound directly to the substrate occupying only oxygen-bound sites. Oxidation of Pd10. Oxygen atoms are once again added to binding sites upon the palladium frame; however, the number of possible states grows combinatorially with the number of oxygens, so in order to qualitatively gauge the effects of incremental oxidation, only the suboxides Pd10O1 and Pd10O2, and the experimentally known 1:1 composition, Pd10O10, are considered in this study. The available sites may be separated into three classes: surface + cluster, interlayer, and top sites. Surface + cluster is defined as previously, occupying μ-2 bridging sites between palladium atoms, while also forming a bond to a substrate magnesium atom. Interlayer sites are defined as sites at the height of the top palladium layer which form three bonds to palladium. Top sites are those which bond above the top atoms and may bridge two top sites, or sit atop the 4-fold face, in a similar manner to the Pd4 rhombus. Interlayer oxygens are clearly found to be the most favorable, with each interlayer site producing an isomer lower in energy than any of the top or surface + cluster sites. The top and surface + cluster sites are essentially degenerate, both containing two Pd−O bonds. Pd10O2 isomers were produced by combining the five lowest energy isomers of Pd10O1, which contain all three site types. Ten calculations in total are made, with the order of the
Figure 3. Energetic difference between the best tetrahedral and planar clusters as a function of oxidation level. Positive values denote a preference for the tetrahedron.
in the reduced regime by approximately 70 meV, which represents a 10-fold decease in relative stability when compared with the gas phase clusters; oxidation reduces this difference to within the range (
