Co Core–Shell Nanoparticles: Segregation

Article pubs.acs.org/JPCC

Computational Study of Pt/Co Core−Shell Nanoparticles: Segregation, Adsorbates and Catalyst Activity M. Kettner,†,‡ W. B. Schneider,§,⊥ and A. A. Auer*,∥ †

School of Chemistry and Biochemistry, University of Western Australia, 35 Stirling Highway Crawley, Perth Western Australia 6009, Australia ‡ Faculty of Natural Sciences, Chemnitz University of Technology, Reichenhainer Straße 70, D-09126 Chemnitz, Germany § Department of Interface Chemistry and Surface Engineering, Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1 40237 Düsseldorf, Germany ⊥ Center for Electrochemical Sciences, Ruhr-Universität Bochum, Universitätsstrasse 150D-44780 Bochum, Germany ∥ Max-Planck-Institut für Chemische Energiekonversion, Stiftstraße 34-36 45470 Mülheim an der Ruhr, Germany ABSTRACT: A computational study on Pt/Co core−shell nanoparticles, which are promising candidates in the search of more active catalysts for the oxygen reduction reaction in fuel cell technology, is presented. We study the energetics of the segregation process using density functional theory (DFT) and a 37-atom cluster model. The influence of the adsorbates CO, O and O2 on the segregation energy is investigated. Furthermore, Nørskov’s model of the oxygen binding energy is used as an indicator to estimate the activity of the model system and to investigate electronic and geometric effects.

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INTRODUCTION State-of-the-art proton exchange membrane (PEM) fuel cells contain platinum nanoparticles in the cathode material as catalyst for the oxygen reduction reaction (ORR). Availability, costs, and durability of the catalyst material as well as the activity of the ORR are key factors for the economic viability of fuel cell systems.1 In recent years the search for increased activity and reduced catalyst costs has led to intensified research on nanoparticles and especially on core−shell particles (CSP). If the particles are stable under working conditions, the advantage is that small particles offer favorable surface-to-volume ratios and hence increased efficiency. On the other hand, core−shell particles with less noble metals in the core are not only more costeffective but have been reported to exhibit superior activity compared to standard Pt systems. In this respect, CSPs made of PtCu, PtNi, and PtCo have been reported as especially promising candidates as far as catalyst activity is concerned.2−4 CSPs can be synthesized by various means such as chemical leaching and high temperature annealing.3 Another method, as developed by Mayrhofer et al.,5 is the adsorbate-induced surface segregation which can be carried out in the gas phase as well as via electrochemical treatment. In order to perform the gas phase segregation, the cluster is kept at 200 °C under CO atmosphere for 3 h. The electrochemical treatment is carried out on a rotating disk electrode in an alkaline electrolyte saturated with CO. Both procedures take place at much lower temperatures and within a much shorter time than other techniques that yield CSPs.3 Thus, the adsorbate-induced © 2012 American Chemical Society

surface segregation is less affected by the loss of active surface area and material, and the formation of an incomplete shell. Besides extensive experimental work computational investigations have been carried out in order to facilitate understanding of the basic reactions involved in the oxygen reduction reaction on catalytic metal and alloy surfaces and the catalyst stability. Most of this work has been directed at the surface structure of bulk material and the influence of oxygen as an adsorbate.6,7 Several studies exist on segregation of alloying elements in Pt based systems.8−10 Different schemes and models were introduced like the correlation of the adsorption energy of oxygen with the activity of the catalyst.11 For nanoparticles and clusters various models for the catalyst have been proposed.12,13 Sheppard et al. even presented an approach for small metal clusters to tune the oxygen binding energy.14 In their approach of rational compound design, the composition of clusters was determined to obtain maximum activity in an alloy nanoparticle system, although the stability and availability of these systems is unclear. As CSPs seem to be of growing interest in catalyst research, the present investigation is aimed at the understanding of the role cobalt has as alloying metal in Pt nanoparticles. In this paper we present results of a density functional study in which the behavior of cobalt within a platinum cluster system is investigated in order to rationalize the segregation process Received: April 19, 2012 Revised: June 22, 2012 Published: June 22, 2012 15432

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described by Mayrhofer et al.5 The effect carbon monoxide and oxygen might have on this segregation process are considered. Furthermore, the catalytic activity for the ORR is discussed for the model systems using Nørskov’s indicator of oxygen binding energy.

that a cluster containing at least three layers of metal atoms can reproduce the results of infinite slab calculations sufficiently.13 For these calculations, distances between the atoms of the cluster used as a surface model were set to a platinum bulk distance of 2.78 Å, and all except for the seven central surface atoms of the bottom plane were fixed in their position during optimization. All DFT calculations were performed using the TURBOMOLE18,19 program suite which is based on Gaussian basis functions with no periodic boundary conditions assumed. For the analysis of the charge density, the natural bond orbital (NBO) charge analysis has been applied.20 It should be noted that the clusters used are systems with an open shell electron configuration. However, a statistic calculation over several spin states could not be carried out due to restrictions in the program used in this studies. Thus, the best spin configuration is taken as that with the lowest energy. The ground state of the system is determined for every system by optimizing the total spin. For all considered species a geometry optimization is carried out for a set of spins and the spin state with the lowest energy is used to calculate the properties of the species. For the Pt37 cluster, for example, a spin state of S = 5 (BP86/SV(P) level of theory) is found to be the most favorable.

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COMPUTATIONAL DETAILS AND THE MODEL SYSTEM All calculations have been carried out for a hemispherical cubeoctahedral cluster model containing 37 atoms of platinum (and cobalt). This system has been introduced by Eikerling et al.12 and has several advantages. Like rough surfaces it exhibits ⟨111⟩ and ⟨100⟩ facets as well as edges and kinks between them (Figure 1). Its diameter on the bottom plane is about 1.34 nm such that its size is at the lower limit of cluster systems used in experimental studies.

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RESULTS AND DISCUSSION The structure of the Pt37 cluster is displayed in Figure 1 and serves as basis for the comparison with other cluster structures. In order to assess the impact of cobalt as an alloying element on the geometric and electronic properties, platinum atoms were substituted with cobalt atoms (Figure 1). In the following, a certain configuration is denoted with a superscript letter, e.g. PtA36Co represents a derivate of a Pt37 cluster, where the Pt atom at position A is substituted by a Co atom. Pt36Co and Pt34Co3 Systems. In the first part of this investigation only one atom of the Pt37 cluster is substituted. Because of the symmetry of the cluster, only five nonredundant substitutions are reasonable. Positions at the bottom of the cluster were not considered as it is assumed that the cluster is bonded to the support on this side. The set of different structures (Figure 1) consists of one center substitution (A), one in the middle of the Pt(100) facet (B), two at edge positions (C and D) and one at the vertex position (E). In order to compare the thermodynamic stability of the various cluster systems, the energy difference between a

Figure 1. Pt37 cluster with the five substitution positions marked.

Furthermore, analysis of the density of states (i.e., orbital energies) exhibits a metallic electronic structure. All structures have been optimized (except for the ones occurring in the final section) using the resolution of identity algorithm (RI), the B−P86 functional, a SV(P)15 or TZVP16 basis set and effective core potentials (60 electron ECP [6s3p2d]-SV(P) or [6s4p3d1f]-TZVP for Pt, no ECP used for Co).17 The only geometry constraint that has been applied for the cluster model is the planarity of the bottom layer as it is assumed that the cluster rests on a stable substrate surface. In order to simulate adsorption on a ⟨111⟩ bulk surface for the investigation on the ORR activity in the last paragraph of the next section (Figure 7), the bottom plane of the cluster was used as a model for the infinite surface. Previous work shows

Figure 2. Structural (first row) and charge properties (second row, color coding indicated at the bottom) of the Pt36Co clusters. 15433

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Table 1. Selected Properties of the Cluster Systemsa S Eabs [a.u.] ΔESeg [eV] ρcharge,Co [e] d̅Co [Å] NCo

PtA36Co

PtB36Co

PtC36Co

PtD36Co

PtE36Co

4.5 −5686.6003 0.00 0.66 2.76 12

6.5 −5686.5744 0.71 0.96 3.08 8

6.5 −5686.5708 0.80 0.90 3.26 7

6.5 −5686.5641 0.99 0.87 3.25 7

4.5 −5686.5565 1.19 0.78 3.60 5

S, spin state; Eabs, absolute energy; ΔESeg, segregation energy; ρcharge,Co, partial charge on the cobalt atom; d̅Co, average distance of the cobalt atom to its neighbors; NCo, coordination number of the Co atom.

a

Table 2. Selected Properties of the Cluster Systema cluster system

Ptcor 34 Co3

Ptsur 34 Co3

PtA36Co

PtB36Co

S Eabs [a.u.] ΔESeg [eV] ρcharge,Co [e] d̅Co [Å] NCo

7.5 −8213.3758 0.00 0.59 2.73 12

7.5 −8213.3545 0.58 0.63 2.73 (cor), 3.04 (sur) 12 (cor), 8 (sur)

4.5 −5686.6003 0.00 0.66 2.76 12

6.5 −5686.5744 0.71 0.96 3.08 8

S, spin state; Eabs, absolute energy; ΔESeg, segregation energy; ρcharge,Co, partial charge on the cobalt atom; d̅Co, average distance of the cobalt atom to its neighbors; NCo, coordination number of the Co atom.

a

substituted (labeled as Ptsur 34 Co3). The latter structure is chosen as it resembles the equivalent to structure B, the second most stable structure after the core configuration. Furthermore, this structure is the result of the change between a cobalt atom and an adjacent platinum atom. The results of the calculations for these systems are summarized in Table 2. While the change in the geometry of the two Pt34Co3 configurations is negligible, the segregation energy of 0.58 eV clearly shows that the core− shell configuration is more stable. Additionally, the difference is of the same magnitude as for the similar Pt36Co structures. This supports the idea that in the gas phase, the preferred configuration for CoPt particles is a core−shell cluster rather than a structure with cobalt atoms located on the surface of the cluster. Compared to the pure platinum cluster, the core−shell system changes its properties noticeably. The pure Pt37 cluster displays a high electron density in the core, thus rendering the surface partially positive (as seen in Figure 3 last column). However, the partial charge of cobalt is much more positive (by 0.6 e) than that of a platinum atom at this position. Thus, in contrast to the pure platinum cluster with a negatively charged core and a positively charged surface, the charge distribution is the opposite in the core−shell system (see Figure 3, second column). This charge distribution can be found in the Ptsur 34 Co3

structure with the metal atom in the center (structure A) and on the surface (structures B−D), is discussed. This is a common approach to estimate the segregation energy ΔESeg.9 Figure 2 and Table 1 display the converged structures as well as electronic and energetic properties. As the energy values and the positive segregation energies show, the configuration with cobalt located in the center of the cluster (structure A) is the most stable one, followed by the facet position (structure B) and the edge positions (structures C, D). The least favored position of the cobalt atom is the vertex (structure E). This decline in stability corresponds to a decrease of the cobalt coordination number. The energy difference between the first and the second stable configuration is considered large (0.71 eV). Compared to this value, the energy differences between the other configurations is quite small. Thus, it can be concluded that for small PtCo nanoparticles Co atoms are not likely to be found at the surface and will occupy positions with high coordination numbers. Furthermore, these findings suggest how the dissolution processes of Co in PtCo alloy particles might proceed. The results support the idea of a stepwise mechanism, in which probable intermediate structures are first configuration B, which has the minimal energy for a surface Co atom, then configuration C or D and finally configuration E from which a low coordinated Co atom is most likely to dissolve. However, the thermodynamics show that a considerable amount of energy (in order of >0.5 eV) is required to reach the first surface configuration. As a consequence, the process is likely to be dominated by the dynamics of defects on the cluster or the effects of distortions due to adsorbates that also modulate barriers for position exchange. The second part of the investigation involves the substitution of three platinum atoms for cobalt to give Pt34Co3. For this purpose, two possible structures are considered. In the first structure the three inner Pt atoms are substituted which leads to a core−shell system (labeled as Ptcor 34 Co3). For the second structure a surface system is generated, where two coreplatinum atoms and one surface atom at position B are

Figure 3. Structural (first row) and charge properties (second row) of the two Pt34Co3 and the Pt37 clusters. For color coding see Figure 2. 15434

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Table 3. Data Resulting from the Adsorption Calculationsa

cluster too, with the difference being that the cobalt atom in the surface is positively charged. While this effect might be attributed to the small size of our model system, it can be expected that an analogous charge distribution inversion could be observed for smaller cobalt nanoparticles with a one or twolayer platinum skin. Adsorption of CO, O2, and O. In order to investigate the effects adsorbates have on the segregation energy, the adsorption of CO, O2 and O are investigated on the two most stable configurations of the Pt36Co system (PtA36Co and PtB36Co) as well as on the two configurations of the Pt34Co3 sur system (Ptcor 34 Co3 and Pt34 Co3). The adsorption energy of an adsorbate X on a PtCo cluster is compared to the equivalent adsorption energy on a Pt37 cluster, as a reference to give the PtCo relative adsorption energy ΔΔEX = ΔEPt (see Figure 4 X − ΔEX

system

S

Eabs [H]

ΔΔEX [eV]

PtA36Co·CO PtB36Co·CO Ptcor 34 Co3·CO Ptsur 34 Co3·CO Pt37·CO PtA36Co·O2 PtB36Co·O2 Ptcor 34 Co3·O2 Ptsur 34 Co3·O2 Pt37O2 PtA36Co·O PtB36Co·O Ptcor 34 Co3·O Ptsur 34 Co3·O Pt37·O

6.5 6.5 6.5 8.5 5.0 4.5 7.5 6.5 6.5 6.0 5.5 6.5 6.5 7.5 6.0

−5799.9034 −5799.8618 −8326.6851 −8326.6417 −4536.5124 −5836.8692 −5836.8511 −8363.6439 −8363.6309 −4573.4774 −5761.7537 −5761.7385 −8288.5238 −8288.5158 −4498.3638

−0.008 0.419 −0.179 0.427 0.000 −0.030 −0.242 −0.011 −0.234 0.000 0.023 −0.267 0.169 −0.191 0.000

a S, spin state; Eabs, absolute energy; ΔΔEX, relative adsorption energy).

Table 4. Calculated Co Surface Segregation Energies (eV) ΔESeg,Pt for X Adsorbing on Pt36Co and Pt34Co3 system

PtA36Co·X → PtB36Co·X

sur Ptcor 34 Co3X → Pt34 Co3X

X = CO X = O2 X=O no adsorbate X

1.132 0.493 0.414 0.705

1.182 0.354 0.217 0.577

the adsorption energy of CO on PtA36Co is fairly small, for the Ptcor 34 Co3 system it is increased by 0.18 eV. Thus, the adsorption energy of CO on PtCo-alloys will decrease if the surface contains Co atoms. On the other hand, Co atoms in the center of the cluster increase the stability of CO on the surface. This implies that CO preferably binds to platinum at low coverages, and will stabilize a core−shell configuration at high coverages. As with CO, the central atom of the ⟨100⟩ side is chosen for the adsorption of O2 and O. The results from the calculations of the O2 adsorption are listed in Table 3. For all structures O2 prefers to adsorb in a bridged configuration, where each oxygen atom is bound to a platinum atom, resulting in the O−O-bond being aligned nearly parallel to the surface. The adsorption energy of O2 for the clusters containing Co at the surface is increased by 0.2 eV, while the value for ΔΔEO2 of the clusters with a core−shell configuration is almost zero (see Table 3 and Figure 5). Thus, while not inverting the sign of the segregation energy, the adsorption of molecular oxygen lowers the segregation energy for PtA36Co·O2 → PtB36Co·O2 and sur Ptcor 34 Co3·O2 → Pt34 Co3·O2 by 0.2 eV. The results for adsorption of atomic oxygen are listed in Table 3. For all examined structures, O is adsorbed in a μ2 position, bridging the metal atom in the central position and its neighbor. The distance between the surface cobalt atom and B the oxygen atom within Ptsur 34 Co3·O and Pt36Co·O is 1.76 Å, while the equivalent distance between the central platinum atom and the oxygen atom in the compounds Ptcor 34 Co3·O and PtA36Co·O is calculated to be 1.96 and 1.95 Å, respectively. While the adsorption energies on the cobalt atom at the surface (structures PtB36Co and Ptsur 34 Co3) are in the range of the values for O2 (0.27 and 0.19 eV, respectively), this is different for the Ptcor 34 Co3 core structure. In contrast to O2, the relative adsorption energy of O is positive (0.17 eV). Thus, for

Figure 4. Lowering of the Pt34Co3 Pt segregation energy by CO. The PtB36Co·CO → PtA36Co·CO system shows a similar behavior.

for the calculated adsorption energies) ΔΔEX is positive if the adsorption energy of the species X is lower on the alloyed cluster compared to the pure Pt37 cluster and negative if it is larger. The advantage of using the relative adsorption energies is the cancellation of systematic errors in the adsorption energies. The central atom of the ⟨100⟩ facet is the position of the cobalt atom in the most stable surface configuration. For this reason, it is chosen as the adsorption site to study the direct influence of the adsorbates on the segregation process. Here, CO is bound with the carbon atom to the central atom in a perpendicular alignment. An exception is the cluster Ptcor 34 Co3, where CO forms a μ2 type bond between the BPt and the CPt atom instead. The resulting energies and the structural properties are listed in Table 3. As shown in Table 4, the segregation energy for PtA36Co·CO → PtB36Co·CO is 1.13 eV, sur that for Ptcor 34 Co3·CO → Pt34 Co3·CO is 1.18 eV. Compared to the bare cluster, the segregation energies are increased by 0.43 and 0.61 eV respectively, as illustrated in Figure 4. For PtB36Co and Ptsur 34 Co3, the relative adsorption energy ΔΔECO is approximately 0.4 eV (Table 3). While the change of 15435

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the latter is only likely to occur at high coverages as CO favors Pt-adsorption sites. Overall, these results are in qualitative agreement with the with experimental findings of Mayrhofer et al.5 One thing to note in this context is the effect that atomic oxygen has on the geometry of the adsorption site of Pt and PtCo nanoparticles. As can be seen from the geometries displayed in Figure 6 and Figure 7, adsorbed atomic oxygen in a bridging position increases the bond length of the adjacent metal atoms significantly. For O on the Pt37 and Ptcor 34 Co3 system the Pt−Pt distance is increased by 0.4 Å, for O on the Ptsur 34 Co3 system the Pt−Co distance is increased by 0.2 Å. For O on a hcp cite of a Pt ⟨111⟩ surface this effect is much less pronounced,21 indicating that the surface of these small clusters is indeed very ductile due to the large number of atoms with low coordination number. It is especially interesting to compare the segregation behavior found for nanoparticles in this study to results obtained for bulk surfaces. Early studies8 focused on the segregation behavior of impurities in a host material and showed that platinum atoms will segregate in cobalt and that cobalt has no tendency to segregate in platinum. More detailed studies by the Balbuena group reveal an interesting trend while for a Pt3Co-alloy surface Pt tends to segregate, for a Pt skin on a bulk Co structure, Pt antisegregates from the surface until a large fraction of the surface is covered with cobalt.9,10 The authors concluded that while 3d-metals do not tend to segregate in Pt-alloys, the underlying crystal structure is decisive such that strain effects of the host material can induce segregation under certain circumstances. In the case studied here, Co does not segregate under any circumstances investigated, even if oxygen is adsorbed. The effect oxygen has on the segregation of Co in Pt-based nanoparticles is also found for the bulk material - in practically all cases adsorbed oxygen stabalizes Co at the surface. For the nanoparticles, however, this is not enough to compensate the segregation energy of a Pt atom. This effect might originate from the large platinum content of the studied clusters, resembling the behavior of cobalt impurities in platinum. On the other hand, in such small clusters with large surface to volume ratio, regions of extended

Figure 5. Lowering of the Pt34Co3 Pt segregation energy by O2. The PtB36Co·O2 → PtA36Co·O2 system shows a similar behavior. sur Ptcor 34 Co3·O → Pt34 Co3·O the segregation energy is 0.3 eV and hence larger than for O2. Consequently, the influence of atomic oxygen is comparable to that of molecular oxygen in quality - it preferentially binds on surface Co atoms and lowers the energy of the surface configurations compared to the core−shell structures. However, the magnitude of these effects is even larger for atomic oxygen. Hence, the adsorption of atomic oxygen and the surface oxidation of Pt alloy nanoparticles is likely to be a key effect in the dealloying process of these species in oxidizing environments. In summary, adsorption of atomic oxygen reduces the stability of the CSP configuration by at least 0.29 eV, while CO increases the stability of the core−shell configuration by ca. 0.4−0.6 eV, if it is adsorbed over Co (see Figure 6). However,

Figure 6. Comparison of adsorption energies of CO, O2, and O. 15436

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Figure 7. Binding energies of atomic oxygen on the models for Pt nanoparticles, Pt/Co core−shell particles and Pt, Pt/Co, and Au surfaces, calculated by the Nørskov scheme. The red mark indicates the binding energy, for which the highest ORR activity is expected as estimated from Nørskov’s volcano plot.

Therefore, alloying with Co may increase the ORR activity, as the binding energy of oxygen is decreased, although the oxygen bond is slightly weaker than the optimal value suggested by Nørskov et al. In addition, it can also be estimated that the ORR activity on the edge of a CSP would be still lower than on a Pt(111) surface. Although the model system for this study is of very limited size, it can be used to estimate whether the influence of cobalt as an alloying element is due to structural change induced by the alloying atom (geometrical) or due to the atomic substitution itself (electronic). Consequently, the adsorption energy of O on the ⟨111⟩ surface of the Pt37 cluster was compared to the adsorption energy at the ⟨111⟩ surface of the cluster with the same geometry but with the three core atoms replaced by cobalt atoms. Similarly, the adsorption energy of O on the ⟨111⟩ surface of the Pt34Co3 cluster was compared to that on an isogeometrical cluster with a Pt core. Exchanging the core atoms of the Pt37 system yields an oxygen adsorption energy of 1.90 eV which is quite close to the adsorption energy of the relaxed Pt34Co3 system. The exchange of the cobalt atoms in the Pt34Co3 cluster without geometry relaxation lowered the oxygen adsorption energy to 1.51 eV, which is similar to the adsorption energy of the relaxed Pt37 system. This indicates that local structural changes at the adsorption site do not affect the adsorption energy strongly, suggesting that the electronic effects of the alloying element are the more important factor. This is in agreement with former studies of alloys.22 However, due to its limited size, the results obtained for this model system should be regarded with caution. Further investigations are needed to resolve the ongoing debate on whether electronic or geometric effects can be used to improve the ORR activity of platinum in alloy CSP systems.

bulk structures are missing which causes the clusters to be very ductile. So although the electronic structure is clearly metallic, the system does not yet behave bulk-like which may be another reason for the differences in the segregation properties of Co in Pt-based nanoparticles. However, in order to reinforce this conclusion, further systematical investigations on larger clusters are necessary. A Note on the ORR Activity of Pt and PtCo Nanoparticles. Nørskov et al. correlated the ORR activity of various metal surfaces with the binding energy of atomic oxygen, defined by the reaction H2O + * → O* + H2 (* denotes a free adsorption site on the catalyst). While less noble metals like iron bind oxygen strongly, noble metals like gold bind oxygen weakly. The Pt(111) surface has the highest ORR activity, yet from Nørskov’s analysis it can be suggested that the ORR activity of platinum can be improved as it still binds oxygen by ca. 0.3 eV too strongly.11 While this is a very simplified approach due to reducing the catalyzed reaction to the thermodynamics of one intermediate (kinetics, surface mutation or the different adsorption behavior of the other intermediates are neglected), it is still a worthwhile tool for a first insight in the quality of a material as catalyst. This approach is used in the following to estimate the performance of core−shell particles and PtCo alloys as catalyst for the ORR. For this purpose the Ptcor 34 Co3 and the Pt37 cluster (and for comparison, a corresponding Au cluster) have been used to model nanoparticles and CSPs. The Pt surface and the surface of a PtCo alloy has been modeled by the bottom of the Pt37 cluster and the Ptcor 34 Co3 cluster, respectively (see computational details). On top of both clusters the preferred adsorption site is a bridging position, on the edge, between the ⟨111⟩ and the ⟨100⟩ facet. On the bottom, oxygen prefers to adsorb on a fcc site. As presented in Figure 7, the relative adsorption energy of oxygen on the edge of the platinum cluster is 0.54 eV larger compared to the Pt(111) surface. Between the CSP and the model for the PtCo alloy surface, this difference is even larger (0.85 eV). In terms of the Nørskov scheme for the ORR activity it can be concluded that the activity at the edge of the clusters is lower than on the ⟨111⟩ surfaces. Therefore, it is assumed that the ORR preferentially takes place on the facets of nanoparticles rather than on edges and corners. Introducing the alloying element cobalt reduces the binding energy of atomic oxygen on the ⟨111⟩ facet by 0.45 eV. Because of cobalt on the edge of the cluster, the binding energy is decreased by only 0.14 eV. This is still higher than on the Pt(111) surface.

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CONCLUSIONS The stability of PtCo core−shell particles was investigated using a 37 atom cubeoctahedral cluster model. It was found that the segregation energy of core−shell particles, calculated by the energy difference between the core and the most stable shell position range from 0.58 to 0.71 eV. The difference between the various surface positions is less than 0.3 eV and the stability increases with increasing coordination number of the cobalt atom. This implies a dissolution mechanism which includes a gradual decrease in coordination, in which atoms are migrating stepwise from a bulk to a surface to an edge followed by a vertex site from which they are most likely to dissolve. 15437

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(11) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jónsson, H. J. Phys. Chem. B 2004, 108, 17886−17892. (12) Wang, L.; Roudgar, A.; Eikerling, M. J. Phys. Chem. C 2009, 113, 17989−17996. (13) Jacob, T. Fuel Cells 2006, 6, 159−181. (14) Sheppard, D.; Henkelman, G.; von Lilienfeld, O. A. J. Chem. Phys. 2010, 133, 084104. (15) Schäfer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571−2577. (16) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297−3305. (17) Andrae, D.; Häussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta. 1990, 77, 123−141. (18) TURBOMOLE V6.2 2010, a development of University of Karlsruhe and Forschungszentrum Karlsruhe GmbH, 1989−2007, TURBOMOLE GmbH, since 2007; available from http://www. turbomole.com. (19) Ahlrichs, R.; Bär, M.; Häser, M.; Horn, H.; Kölmel, C. Chem. Phys. Lett. 1989, 162, 165−169. (20) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899−926. (21) Gu, Z.; Balbuena, P. B. J. Phys. Chem. C 2007, 111, 9877−9883. (22) Ma, Y.; Balbuena, P. B. Surf. Sci. 2008, 602, 107−113.

Calculations concerning the influence of CO, O and O2 on the segregation energy reveal a preference of CO binding to Pt over Co as well as that binding of CO to Pt increases the segregation energy by 0.60 eV, whereas adsorbing O2 or O decreases this segregation energy by 0.23 and 0.35 eV, respectively. Thus, the much stronger adsorption of CO on Pt atoms explains the fact that CO annealing can effectively be used in the synthesis of core−shell particles, while oxygen species that are present in the ORR have a large influence on dealloying processes in CSP systems. As soon as a less noble atom migrates to the surface of the particle, it is stabilized through preferential binding to atomic or molecular oxygen. Furthermore, the surface of the cluster is found to be rather ductile in comparison to bulk systems which might be attributed to the large surface to volume ratio in these small nanoparticle systems. The scheme of Nørskov was applied to assess the ORR activity of CSP and surface alloys on the basis of cluster models. As the oxygen is bound stronger on the edges of the clusters compared to the surface, it can be assumed that in general, facets should be catalytically more active than edges. It should be noted that this trend would be inverse for materials that bind oxygen weakly. Furthermore, the fact that the oxygen binding energy is decreased on the surface alloy model suggest a higher ORR activity on these systems in comparison to pure Pt. A fist analysis on the small model implies that the influence of Co in these systems is rather electronic than geometrical (strain).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.A.A. and M.K. would like to acknowledge K. Mayrhofer and M. Stratmann (MPIE) for fruitful discussions and generous support as well as the Chemnitzer Hochleistungs-Linux-Cluster (CHiC) team at TU Chemnitz for endorsement of this project. W.B.S. acknowledges funding from the Center for Electrochemical Sciences, Bochum.

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REFERENCES

(1) Vielstich, W.; Lamm, A.; Gasteiger, H. A. Handbook of Fuel Cells: Fundamentals, Technology, Applications; Wiley: New York, 2003. (2) Yang, H.; Vogel, W.; Lamy, C.; Alonso-Vante, N. J. Phys. Chem. B 2004, 108, 11024−11034. (3) Koh, S.; Leisch, J.; Toney, M. F.; Strasser, P. J. Phys. Chem. C 2007, 111, 3744−3752. (4) Mani, P.; Srivastava, R.; Strasser, P. J. Phys. Chem. C 2008, 112, 2770−2778. (5) Mayrhofer, K.; Juhart, V.; Hartl, K.; Hanzlik, M.; Arenz, M. Angew. Chem., Int. Ed. 2009, 48, 3529−3531. (6) Gu, Z.; Balbuena, P. B. J. Phys. Chem. C 2008, 112, 5057−5065. (7) Callejas-Tovar, R.; Balbuena, P. B. Phys. Chem. Chem. Phys. 2011, 13, 20461−20470. (8) Ruban, A. V.; Skriver, H. L.; Norskov, J. K. Phys. Rev. B 1999, 59, 15990−16000. (9) Ma, Y.; Balbuena, P. B. Surf. Sci. 2009, 603, 349−353. (10) Ramirez-Caballero, G. E.; Ma, Y.; Callejas-Tovar, R.; Balbuena, P. B. Phys. Chem. Chem. Phys. 2010, 12, 2209−2218. 15438

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