Role of Cluster Morphology in the Dynamics and Reactivity of

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Role of Cluster Morphology in the Dynamics and Reactivity of Subnanometer Pt Clusters Supported on Ceria Surfaces Fabio R. Negreiros and Stefano Fabris* CNR-IOM DEMOCRITOS, Istituto Officina dei Materiali, Consiglio Nazionale delle Ricerche and SISSA Scuola Internazionale di Studi Superiori Avanzati, Via Bonomea 265, I-34136 Trieste, Italy S Supporting Information *

ABSTRACT: We study subnanometer (sub-nm) Pt clusters supported by highly reducible oxide surfaces and establish the role of cluster morphology in the thermodynamics and kinetics of surface processes relevant for reactivity, namely cluster mobility, reverse oxygen spillover, and oxygen vacancy formation. The relationships between cluster morphology and reactivity are rarely considered in computational studies because of the large domain and complexity of the potential energy surface, particularly in the presence of strong metal−support interaction. Global optimization algorithms together with Hubbard-corrected density functional theory calculations (DFT+U) are used to identify the stable and metastable morphologies of Pt3−Pt6 clusters supported on pristine and defective CeO2(111) surfaces. Our systematic exploration for these sub-nm Pt particles shows that the charge of the supported cluster, its bonding to the substrate, and the degree of ceria reduction depend on the metal/oxide interface area and on the cluster morphology. Concerning reaction thermodynamics and kinetics, the use of global optimization methods leads to very different results as compared to usual minimization procedures. By allowing for morphology changes during reaction, the energetics of reverse O spillover changes from highly endothermic to exothermic and leads to new minimum-energy reaction and diffusion mechanisms. The diffusion kinetics predicts clusters as small as Pt6 to be resistant to sintering on ceria surfaces. The relevance of these findings for larger metal clusters and for supporting oxide nanoparticles is discussed as well as their connection with the recent literature.

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INTRODUCTION

This interplay between the activity of Pt nanoparticles and the reducibility of the oxide support motivated a large number of experimental and theoretical studies.14−20 They provided evidence of the important role of the nature, size, and morphology of both the metal and oxide nanoparticles, which are particularly relevant for reducing the catalyst cost.17,21 The study of ultrasmall Pt nanoparticles (NPs) supported on ceria surfaces is therefore central in this context. The available information based on spectroscopy, microscopy, and numerical modeling points to the high reactivity of ceria-supported Pt clusters of sizes ≤1−3 nm, often in conjunction with nanostructured ceria particles.22,20 Ultrasmall supported nanoparticles may display rich and dynamical morphological changes, which can impact on cluster reactivity, stability, and mobility and whose effects has so far never been explored. Previous theoretical studies based on the DFT+U method have addressed the stability, charge, and diffusion of isolated Pt atoms and of Pt dimers on pristine ceria surfaces.23−25 The Pt atoms were shown to remain nearly neutral with a very weak charge transfer to the ceria surface, and therefore no Ce+3 centers were created. Diffusion energy barriers of single Pt atom were found to be very low, around 0.1 eV, and sintering

Ceria-based materials are active catalysts for several reactions of industrial interest, including water-gas shift, selective CO oxidation, hydrocarbon reforming, low-temperature hydrogen and methanol oxidation, and oxygen reduction.1−7 They are therefore key technological components in, for instance, exhaust converters and fuel cells. These catalysts are often based on platinum-group metals supported and dispersed on the oxide support. Because of their high cost, there is an ongoing effort for reducing the amount of precious metal without affecting the device efficiency.8 In the specific case of polymer−electrolyte−membrane fuel cell electrodes, national and international targets call for reducing the amount of noble metal.9 In this context, ultralow loading Pt−ceria systems are investigated as potential candidate for meeting these targets.8,10 Although the origins of the enhanced reactivity displayed by Pt−ceria systems are complex and not yet fully disclosed, they are usually related to the high reducibility of the ceria support and to the catalyst activation due to metal−support interaction. The former allows for oxygen buffering during reaction, i.e., the exchange of O atoms between the surface lattice and the gas phase or adsorbed species.11 The latter involves electron transfer at the metal−oxide interface, and the diffusion of O species between the oxide and supported nanoparticle (O spillover).12,13 © 2014 American Chemical Society

Received: June 27, 2014 Revised: August 13, 2014 Published: August 19, 2014 21014

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cluster. A simple shake move was applied at each step of the BH search, which consists in displacing all adsorbed Pt6On atoms (the atoms of the ceria surface are allowed to relax but are not randomly displaced during the BH procedure) simultaneously by 0−1.6 Å in a random direction. In the DFT+U calculations of this first set of BH calculations we used energy cutoffs of 30 and 240 Ry for the wave function and electronic density, respectively, modeled the supporting ceria surface with one O−Ce−O trilayer (the lowermost two atomic layers were constrained during the geometry optimization), and used 12 Å of vacuum between replicated images in the direction perpendicular to the surface. For the second step, the reoptimization was performed using energy cutoffs of 40/320 Ry, three O−Ce−O trilayers (the lowermost O−Ce−O trilayer was constrained during the optimization), and 15 Å of vacuum. The charge analysis was performed by applying to the charge density generated with QE the postprocessing tool40,41 that calculate the Bader and the Voronoi atomic charges. All the values reported in this paper corresponds only to the Bader analysis, but a systematic comparison of both the Bader and Voronoi predictions for each system showed that, although the absolute charges are different, the main qualitative conclusions are unaffected by the specific charge definition adopted. Transition-state searches were performed with the nudged elastic band (NEB) algorithm42 using the Broyden scheme in a two-step approach. First, a NEB with 3−9 intermediate images was performed without climbing image (CI). After convergence, a second NEB with initial and final states close to the previously found transition state was performed using additional 3−5 images with the CI. For the paths not involving subsurface vacancies, only two O−Ce−O trilayers were used in order to speed up the calculations, having checked that for these special cases the energy differences evaluated with two trilayers change by at most 0.1 eV with respect to the calculations employing three trilayers. It is known that there are several metastable positions of the Ce+3 centers with respect to O vacancies or to metal/oxide interfaces.43,44 We considered this issue of electron localization for the Pt6O0V0, Pt6O1V0, and Pt6O0V1 compositions by exploring several possible metastable configurations, whose energy differed by 0.1−0.3 eV. It turns out that the Ce+3 ions always locate at the Pt/ceria interface. We report in the following only the lowest energy of these configurations.

was predicted to occur even at very small temperatures. The structural and electronic properties of larger clusters, such as Pt8, Pt49, and Pt155, supported by ceria surfaces and NPs have also been investigated with DFT+U calculations.8,18,26 However, as the number of atoms increases from dimers to NPs, so does the morphological phase space of the system, and traditional approaches for predicting the minimum-energy morphology of supported clusters (i.e., structural optimization of supported clusters starting from arbitrary or gas-phase initial configuration) may easily remain trapped into metastable highenergy minima. To overcome this limitation, in the last years considerable effort aimed at characterizing and predicting the morphology of surface-supported NPs and the related potential energy surface (PES) that governs the cluster geometry. In this context, basin hopping (BH) and genetic algorithm global optimizations techniques have been applied at the DFT level for nanoclusters of gold supported on MgO,27 nanoclusters of palladium/ palladium oxide supported on MgO,28 and also for nanoalloys of Au24Pd1.29 Highly reducible oxides such as ceria add further complexity to the search since, due to strong metal/support interaction, different isomers might lead to different charge transfer, with important consequences for reactivity studies. In this work we address the interplay between morphology, electronic properties, and stability of a set of small Pt clusters supported by pristine and defective CeO2(111) surfaces. Our exploration of the morphological PES demonstrates the richness of stable and metastable morphologies of the supported clusters and their role in the thermodynamics and kinetics of catalytic processes.

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COMPUTATIONAL METHOD All calculations were performed at the density-functional theory (DFT) level using the plane-wave Quantum Espresso (QE) package.30 The calculations were spin polarized and employed the Perdew−Burke−Ernzerhof (PBE)31 exchange-correlation functional together with ultrasoft pseudopotentials (USPP).32 Structural optimizations and transition state searches were carried out in a spin-unrestricted formalism with the Hubbard U correction as implemented by Cococcioni and de Gironcoli.33 The value of the parameter U was set to 4.5 eV following our previous works21,34−36 and in line with the values used by the current literature.37 The CeO2(111) surface was modeled by using a 3 × 3 cell having thicknesses ranging from one to three O−Ce−O trilayers (see details below) and with 12−15 Å of empty space between replicated cells. The lattice parameter was set to the DFT-PBE optimized one for bulk, which overestimates the experimental one by ≈2%. The Brillouin zone was sampled at the Gamma point only. For each studied composition (i.e., for PtiOkVacj clusters with i varying from 3 to 6 and j and k varying from 0 to 2), the search and determination of the lowest energy clusters’ morphologies were performed with the basin-hopping (BH) global optimization algorithm38,39 with a two-step approach. The potential energy surface was first explored by using a fast and less accurate setup (see below) that allowed for collecting and grouping all distinct structures having energies within 2.0 eV higher than the putative global minimum. The resulting large database was then refined by reoptimizing all the structures with a higher accuracy (see below). In the first step, the BH algorithm was repeated for 200−300 Monte Carlo steps using fictitious temperatures of 1000 and 2500 K and starting from random configurations of the supported Pt

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RESULTS AND DISCUSSION Global Optimization of Pt3−6 Clusters on CeO2. The BH global optimization technique was first applied to determine the morphologies and relative energetics of Pt3− Pt6 clusters supported on the pristine CeO2(111) surface. For each cluster size, the four lowest energy stable isomers resulting from the BH calculations are reported in Figure 1. The relative cluster energetics are referred to the corresponding putative global minimum (GM), whose energy is set to zero. The gap between the lowest energy structure and the first metastable isomer increases almost linearly as a function of the cluster size from 0.19 eV (Pt3) to 0.62 eV (Pt6). The planar motif is the lowest energy only for Pt3, while a 3D morphology is preferred by all the larger clusters, for which the planar structures are higher in energy by more than 0.6 eV. This is a clear effect of the interaction with the support, since the planar morphologies are always preferred in the gas phase for the same cluster sizes (see Supporting Information). Overall, all the clusters are 21015

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Information). We find a clear energy preference for the clusterinduced Ce+3 ions to be close to the Pt cluster, directly below it, or nearby an interfacial Pt atom. Because of the charge transfer from the cluster to the surface, there is a charge depletion at the interfacial Pt atoms with respect to the corresponding unsupported clusters (blue atoms in Figure 2). This overall loss of charge of the Pt atoms in contact with the surface weakens the Pt−Pt bonding, therefore increasing the Pt−Pt distance with respect to the gas phase by ≈4% on average. In addition, the interaction with the support drives an intracluster charge transfer from the interfacial Pt atoms to the undercoordinated Pt atoms farer away from the surface (reddish Pt atoms).45 This charge rearrangement induced by the ionic surface and leading to a local dipole is a common feature for metal clusters supported by oxide surfaces, also called a metal-on-top effect.46 With respect to the lowest energy 3D geometry, one additional Ce+3 center is always present in the case of the planar morphologies, where all Pt atoms are at the interface. This shows that the charge transfer from the Pt cluster to the support depends on both the number of Pt atoms in direct contact with surface oxygen (i.e., interface area) and the number of undercoordinated Pt atoms in the second layer (i.e., cluster morphology). It remains to be determined whether larger Pt NPs can yield larger charge transfer to the ceria support, leading to more than 2−3 Ce3+ ions. Deeper insight into the cluster/oxide bonding is provided by the projected density of states (PDOS) analysis displayed in Figure 3 for two lowest energy Pt4 (a) and Pt6 (b) representative cluster sizes. The formation of a covalent Pt− O bond is evident from the significant contribution of the surface O-2p states (solid red areas) to the discrete Pt-4d energy levels of the cluster (solid black areas). The directional character of these Pt−O bonds is shown by the charge density difference plotted in the inset of Figure 3a. Several of these Pt− O states fall in energy ranges that are away from the valence O2p band, either below it (for energies between −7 and −5 eV) or in the band gap (for energies between −1 and 1 eV). This analysis also shows that there is no interaction between the Pt 4d states and the 4f states of the reduced Ce3+ ions. The PDOS for Pt6, shown in Figure 3b, reveals similar features compared to the Pt4 case, with one additional peaks for the f-orbital of Ce+3 and, most importantly, also a more metallic character near the Fermi energy, a transition that is expected to happen as the cluster size increases.26 Thermodynamics and Mechanisms of Cluster Diffusion. Energy barriers for cluster diffusion were evaluated starting from the putative global minima of the Pt3−6 clusters described above. Three distinct types of diffusion path were explored (Figure 4, right panel):47,48 cluster sliding, involving the translation of two or more cluster atoms parallel to the surface (red lines in Figure 4); cluster rolling, involving rotation of the whole cluster around an axis parallel to the surface (blue lines); and diffusion via cluster deformation and reconstruction (black lines). The activation energies for diffusion calculated for the three mechanisms are reported in the left panel of Figure 4 as a function of cluster size. We show that the databases of stable isomers obtained with the BH approach is critical also for determining the lowest energy paths for cluster diffusion.49 In the resulting minimumenergy paths all the Pt 4−6 clusters diffuse via cluster deformation and reconstruction, taking advantage of the cluster morphological flexibility shown in Figure 1 and visiting

Figure 1. Lowest energy stable isomers found by BH calculations for Pt3, Pt4, Pt5, and Pt6 clusters supported by the pristine CeO2(111) surface. The energies are in eV relative to the most stable motif. Pt atoms are in gray, oxygen from the first (second) layer in red (violet), Ce+4 in yellow, and Ce+3 in orange.

strongly bound to the ceria surface with binding energies ranging from 4.6 eV (for Pt3) to 6.7 eV (for Pt6). Bonding at the metal/oxide contact yields considerable charge transfer and rearrangement, which we quantify with a Bader analysis. Figure 2 reports the differences between the

Figure 2. Bader charge difference between the Ptn/CeO2 system and the noninteracting clean surface and metal cluster. Charge accumulation/depletion is represented by red/blue colors as indicated in the color scale.

Bader charges evaluated for the supported (Ptn/CeO2) and for the noninteracting (isolated Ptn and pristine surface) systems. White color indicates no variation, while red/blue colors represent charge accumulation/depletion, respectively. Each panel displays two replica of the cluster: the one on the right is used to report the cluster’s Bader charges, while the one on the left is used to report the Bader analysis of the surface atoms underlying the cluster. For all cases there is an overall charge transfer from the cluster to the support that reduces the ceria surface and leads to the formation of 2 Ce+3 centers for Pt3−5 and 3 Ce+3 centers for Pt6 (red surface atoms in Figure 2). There are several metastable positions for the localization of the excess electrons on the Ce sites, differing by up to 0.2 eV (see Supporting 21016

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of temperatures and should therefore be resistant to sintering. Indeed, experimental X-ray absorption measurements50 show that Pt clusters of ≈1 nm supported by a closely related substrate, Ce−Zr−Y mixed oxide, are stable up to 800 °C. Vacancy as a Trapping Center. Before addressing the relationship between cluster reactivity and fluxionality, we consider the role of point defects as possible trapping centers for sub-nm Pt clusters. Among the possible ceria point defects, we focus on those with the lowest formation energy, namely surface and subsurface O vacancies, since they have been shown to be the most abundant relative to other types such as Ce vacancies or Schottky defects. In order to predict whether a Pt cluster binds preferentially to an O vacancy, we have performed global BH optimizations for the Pt4 and Pt6 clusters bound to a surface O vacancy. We find that the overall morphology of the resulting lowest energy clusters is qualitatively similar to the one of the corresponding clusters on the pristine surface (Figure 1). Details of the resulting stable morphologies are reported in the Supporting Information. Starting from these structures with an O vacancy on a surface site below the Pt cluster (i.e., at the metal/oxide interface, position 3 in Figure 5), the simulations were repeated for

Figure 3. Projected density of states for supported Pt4 and Pt6. Individual spin up/down contributions are reported above/below the x-axis. The total PDOS is reported by the black thin line. The pink line represents the unoccupied f orbitals of the Ce4+ atoms. The black, red, and blue areas represent the contribution of the Pt and oxygen atoms at the metal/oxide interface and of the Ce3+ ions, respectively.

Figure 5. Adsorption energies (in eV, right panel) of Pt4 and Pt6 clusters on a defective CeO2(111) surface with an O vacancy in three different positions (see sites 1−3 in left panel).

different vacancy positions, namely a subsurface site below the Pt cluster (position 2), and a surface site next-nearest neighbor beside the Pt cluster (position 1). The cluster binding energies to the defective ceria surfaces are reported in Figure 5 and clearly increase as the distance between the cluster and the vacancy increases. All sizes display a clear preferential binding of the cluster to the pristine ceria surface. Formation of O vacancies at the Pt/CeO2 interface is disfavored over formation on the pristine surface. The driving force for displacing the vacancy far from the metal/oxide interface is 1.20 eV for Pt4 and reduces to 0.45 eV for Pt6. We therefore conclude that, from the thermodynamic point of view, O vacancies do not act as trapping centers for Pt clusters of these sizes. The kinetics and mechanisms of the cluster growth as well as of its diffusion may however lead to metastable Pt clusters bound at O vacancies. Given the large activation energy for diffusion, if cluster nucleation were favored at an O vacancy, the cluster could grow over a surface defect (i.e., in a metastable position) without having the chance to diffuse toward a defect-free stoichiometric surface (its global minimum). Another example are O vacancies created at the cluster/oxide interface during reactions involving lattice O (i.e., reverse O spillover or CO oxidation). There will be a driving force for displacing them away from the cluster, as in our Figure 5, but their actual diffusion will be limited by their kinetic barrier for diffusion. Hence, although being formed in a

Figure 4. Activation energy (in eV, left panel) for cluster diffusion through sliding (red lines), rolling (blue), and deformation/ reconstruction (black) mechanisms (see right panel).

structures related to the available metastable geometries. For the Pt3 and Pt6 cases we notice that a diffusion mechanism involving the sliding of a part of the cluster (a dimer or a trimer) is also relevant. Overall, the barrier is strongly dependent on cluster size and increases from ≈1 eV for Pt3− Pt5 to ≈2 eV for Pt6. These results demonstrate the limited mobility of sub-nm Pt clusters on the stoichiometric defect-free CeO2(111) surface. While the barriers for diffusion of Pt adatoms, dimers, trimers, and tetramers are compatible with their mobility at room temperature and above, these calculations predict clusters as small as Pt6 to be very stable on ceria surfaces over a wide range 21017

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structural changes are not considered) to weakly exothermic (when structural changes are included). The binding energy of an O atom to the Pt cluster is therefore comparable to the binding energy of O in the lattice of ceria surface. Moreover, this shows that lattice oxygen transfer to the supported Pt cluster does not necessarily requires nanostructured ceria particles, but the process can be exothermic also for small Pt clusters supported by extended ceria surfaces. The lower value of O vacancy formation induced by nanostructured ceria will instead have an important effect on the reaction thermodynamics, as reported by Vayssilov et al.18 In conclusion, the thermodynamics for reverse O spillover is controlled by O vacancy formation and by the large morphological changes of the cluster induced by the adsorbate. This effect is particularly important for the sub-nm supported clusters considered here, which have access to several metastable isomers. It is likely that, by increasing cluster sizes, the structural changes induced by adsorbates will become less relevant because of the higher stiffness and cohesion of clusters larger than 1−2 nm. Indeed, reverse O spillover was reported to be exothermic by 1.0 eV for a Pt8 cluster, which did not modify substantially its morphology,14 similarly to our value of 1.00 eV obtained when fixing the cluster geometry (Figure 6b). Reverse Oxygen Spillover. Radical morphological changes of supported clusters during chemical reactions are rarely considered in computational studies. Cluster fluxionality during reaction is however particularly relevant for the thermodynamics of O spillover by Pt/CeO2 systems, as we have just shown for the supported Pt6 cluster. The initial and final states for this reaction (Figure 6a−e) identified with global structural optimization approaches indicate that the minimumenergy reaction thermodynamics involves a transition from a 3D to 2D cluster morphology. In order to calculate the combined activation energy for the diffusion of an O atom from the ceria lattice to the Pt cluster and for the related morphology changes, we have considered many different nudged elastic band (NEB) initial paths taking advantage of the large data set of metastable minima identified with the previous analysis. The path with the lowest activation energy resulting from our calculations is displayed in Figure 6 and consists of two fundamental steps: (1) the spillover of a lattice O atom at the ceria/Pt interface involving the formation of an O vacancy (Figure 6a−c) and (2) the adjustment of the cluster morphology driven by the presence of the O adspecies. It turns out that the rate-limiting step is the former, with a calculated activation energy of 1.13 eV (Figure 6b). Hence, while the structural flexibility of supported ultrasmall Pt clusters enters into the O spillover thermodynamics by lowering substantially the energy of the final state, we find that cluster morphology changes have limited effects on the reaction kinetics, since the rate-limiting step does not involve major structural rearrangements of the cluster and is thus mostly related to O vacancy formation. The calculated value of the activation energy for O spillover is quite compatible with that one proposed by the kinetic thermodynamic study of Zhdanov and Kasemo.51 By fitting the parameters of a kinetic model to experimental temperatureprogrammed desorption data, these authors have found that the energy of oxygen atoms adsorbed on Pt is similar to their energies in the surface layer of ceria. Furthermore, the energy barrier for oxygen extraction that gives a good match between the kinetic model and the experimental results was found to be about 1.0 eV, which is also in good agreement with our result.

metastable site, the residence time of O vacancies at the metal/ oxide interface may therefore be long enough to allow their participation into interface reactions. Cluster Fluxionality and Reaction Thermodynamics. Having established that the relevant Pt clusters for reactivity will be bound to a pristine ceria surface, we now focus on the reverse O spillover process, which plays a key role in the catalysis of ceria-based materials. We take the largest cluster in our set as a model system (Pt6) and show the importance of determining minimum-energy cluster morphology in the reaction thermodynamics. The fundamental process in the reverse oxygen spillover is the formation of an O vacancy at the metal/oxide interface, and the migration of the related surface O atom to the supported Pt cluster. Here we study the thermodynamics of this process by comparing two approaches. A traditional one based on a plain energy minimization and structural relaxation procedures, and one in which the conformational phase space of this system is explored with the BH method. We generated 9 initial configurations starting from the Pt6 global minimum described above (Figure 6a). In each of these,

Figure 6. Thermodynamics of reverse O spillover on a Pt6 cluster at a CeO2(111) surface. Initial state (a) and final states resulting from a usual structural relaxation procedure (b) and from a BH optimization (c−f).

one interfacial lattice O atom in contact with the Pt cluster was displaced to the supported cluster. The resulting geometry was then structurally optimized. This is the usual procedure followed by most of the computational studies of supported clusters and, in particular, by all previous works involving ceria as the support. The resulting set of (meta-)stable configurations is reported in the Supporting Information, while only the most stable one is reported in Figure 6b. We find that its energy is 1.00 eV higher than the reference Pt6 GM one. In order to fully account for the possible conformational changes due to reverse O spill over, we have performed a BH optimization of the final state by sampling the position of all Pt atoms in the cluster as well as the lattice-O atom adsorbed to the Pt surface. The four lowest energy isomers resulting from this analysis are shown in Figure 6c−f. All of these have considerably lower energy than the configuration obtained with the plain structural optimization, by up to 1.03 eV. In particular, the global minimum for the final state (Figure 6c) is isoenergetic to the initial state (Figure 6a). Allowing for morphology changes of these sub-nm Pt clusters turns the reaction thermodynamics from highly endothermic (when 21018

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Matteo Farnesi Camellone for useful discussions and for critical reading the manuscript. We acknowledge the FP7-NMP-2012 project chipCAT under Contract No. 310191 and the EU FP7 COST action CM104 for financial support and the ISCRA initiative of CINECA for high performance computing resources.

Figure 7. Lowest energy path found connecting the two minima shown in Figure 6a,b. Spilled oxygen is painted as green.

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CONCLUSIONS In conclusion, by combining DFT+U calculations with global optimization BH algorithms we established the role of morphological changes in the dynamics and reactivity of subnm Pt clusters supported by CeO2(111) surfaces. We first identified the minimum-energy adsorption geometries for the Pt3−Pt6 series as well as several higher energy metastable isomers. The resulting database reflects the richness of available binding morphologies and electronic structures of Pt/CeO2 systems and is used to rationalize the different charge transfer, binding strength, and surface reduction as a function of cluster size and morphology. In all cases, the calculations predict a charge transfer from the interfacial Pt atoms to the ceria surface (leading to 2−3 Ce3+ ions for different sizes/ structures) and to the undercoordinated Pt atoms of the cluster. The same database was also used to guide the search for lowenergy paths for cluster diffusion. Indeed, the lowest energy mechanisms for the diffusion of the Pt3−Pt6 clusters entail their deformation and reconstruction, involving transition-state morphologies closely related to the higher energy metastable structures available to the cluster. The diffusion barrier increases from ≈1 eV for Pt3−Pt5 to ≈2 eV for Pt6, thus suggesting that sub-nm clusters as Pt6 can already be well anchored to the support and stable against sintering. All clusters considered in this study are more strongly bound to the pristine surface than at surface O vacancies of reduced supports. This implies that the formation energy of vacancies close to the Pt/oxide contact is larger than at the pristine surface. The reactivity of these sub-nm Pt clusters appears not primarily related to their effect in assisting O vacancy formation on the ceria substrate. By allowing the morphology of the cluster to change during reverse O buffering, we showed that the reaction thermodynamics can be exothermic also for Pt clusters supported on extended surfaces, similarly to what was predicted for Pt clusters supported by ceria nanoparticles. We show that the kinetics of the process is primarily governed by the O vacancy formation that initially does not drive major structural rearrangements, which instead follow as a consequence of the O adsorption on the Pt cluster.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Lowest energy morphologies of the Pt6 cluster in the gas phase; energy variations with respect to the location of the Ce+3 centers for Pt3−Pt6/CeO2 systems; stable binding geometries of Pt6 on the defective CeO2 surface; set of final states for the O spillover reaction. This material is available free of charge via the Internet at http://pubs.acs.org. 21019

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp506404z | J. Phys. Chem. C 2014, 118, 21014−21020