Fluxionality of Au Clusters at Ceria Surfaces during CO Oxidation

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Fluxionality of Au Clusters at Ceria Surfaces during CO Oxidation: Relationships among Reactivity, Size, Cohesion, and Surface Defects from DFT Simulations Prasenjit Ghosh,*,† Matteo Farnesi Camellone,‡ and Stefano Fabris¶,⊥ †

Department of Chemistry and Physics, Indian Institute of Science Education and Research, Pune-411008, India Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, D-44780 Bochum, Germany ¶ Theory@Elettra Group, CNR-IOM DEMOCRITOS, S.S. 14, km 163.5, I-34149 Trieste, Italy ⊥ SISSA, Via Bonomea 265, I-34136 Trieste, Italy ‡

S Supporting Information *

ABSTRACT: Density functional theory (DFT) calculations are used to identify correlations among reactivity, structural stability, cohesion, size, and morphology of small Au clusters supported on stoichiometric and defective CeO2(111) surfaces. Molecular adsorption significantly affects the cluster morphology and in some cases induces cluster dissociation into smaller particles and deactivation. We present a thermodynamic rationalization of these effects and identify Au3 as the smallest stable nanoparticle that can sustain catalytic cycles for CO oxidation without incurring structural/morphological changes that jeopardize its reactivity. The proposed Mars van Krevelen reaction pathway displays a low activation energy, which we explain in terms of the cluster fluxionality and of labile CO2 intermediates at the Au/ceria interface. These findings shed light on the importance of cluster dynamics during reaction and provide key guidelines for engineering more efficient metal−oxide interfaces in catalysis. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis

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supported gold nanoclusters have been shown to catalyze low-temperature CO oxidation,12,13 hydrogenation,14 the water−gas shift reaction,16 or NOx reduction.15 Several theoretical studies based on DFT have contributed to the characterization of the structural and electronic properties of Au species supported on stoichiometric and reduced ceria surfaces.17−19,21−23 These studies pointed to the importance of cationic Au species for CO oxidation and suggested that O vacancies act as trapping sites for Au adatoms. The interaction between defective ceria and the Au adatom yields negatively charged species that favor nucleation of larger clusters.17,23−25 Concerning CO adsorption and oxidation, these negatively charged Auδ− species at O vacancies prevent adsorption of molecular CO and O2 and are thus inactive.23 Molecular adsorption is energetically favored by larger clusters, which can then promote CO oxidation via different reaction mechanisms, as recently proposed by Henkelman and co-workers for the case of Au13 clusters supported on model ceria surfaces.26,27 In particular, these authors show that CO oxidation pathways by coadsorption of CO and O2 on the Au13 clusters have lower

old nanoparticles dispersed on oxide supports show catalytic properties that contrast with the inactivity of bulk Au and oxide surfaces. This was first recognized in the pioneering work of Haruta et al.1 Since then, there has been a surge in research interests on the activity of supported/ unsupported Au clusters for many important chemical reactions such as CO oxidation,2−4 propylene epoxidation,5 water−gas shift,6,7 and so forth. Among the different reactions catalyzed by Au/oxide systems, the low-temperature CO oxidation is often considered as a model reaction in fundamental studies probing reactivity. Experimental and theoretical studies have shown that the reactivity of oxide-supported Au nanoparticles toward CO oxidation depends on several morphological and electronic factors, which are also sensitively affected by the cluster size. For example, in the case of nonreducible oxide supports, the reactivity of Au clusters on MgO depends on their ability to form structural isomers that are energetically comparable as well as on the oxidation state of the cluster, which is determined by its size and interaction with the substrate.8−10 Reducible oxide supports add a further degree of complexity to gold/oxide catalysts by facilitating charge and oxygen buffering during reaction. Among these, cerium oxide (ceria, CeO2) is one of the most efficient due to the presence of empty and strongly localized f states close to the Fermi level, which form a narrow band and mediate the charge and oxygen exchange, also via facile O vacancy formation.11 Ceria© 2013 American Chemical Society

Received: April 30, 2013 Accepted: June 22, 2013 Published: June 22, 2013 2256

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Figure 1. Lowest-energy configuration and energetics of Aun supported on stoichiometric ceria (a−d) and reduced ceria (e−h). O, Au, and Ce4+/ Ce3+ atoms are denoted in magenta, yellow, and gray/blue, respectively. Main Au−Au bond lengths are reported in Å. Adsorption energy (i) and charge (j) of Au clusters as a function of cluster size on stoichiometric (red circles) and reduced (blue cross) ceria and a comparison of our results with those of Zhang et al.19 (blue star).

sites (see the Supporting Information (SI) for additional details). The reduced Ce3+ ions resulting from the interaction with Au or from surface O vacancies (white squares) are denoted as blue circles in Figure 1a−h. On the stoichiometric CeO2(111) surface, a single Au adatom binds to a bridge site between two surface O atoms. In agreement with previous calculations,17,23−25 the computed BE is −1.18 eV (Figure 1a). The Au2 dimer binds vertically atop a surface O atom with a BE of −1.75 eV/atom (Figure 1b). The Au3 trimer adsorbs in a triangular morphology (BE = −2.24 eV/atom) with one side bridging across two surface O atoms and with the plane perpendicular with respect to the surface (Figure 1c). The binding morphology of the Au4 tetramer is tetrahedral (Figure 1d) with a calculated BE of −2.24 eV/atom. On the reduced CeO2−x(111) surface, all of the Aun (n = 1− 4) metal nanoclusters are predicted to have at least one Au atom adsorbed on the surface O vacancy. The Au1 adatom binds to the vacancy (Figure 1e) with a BE of −2.29 eV and is therefore significantly more stable (>1.1 eV) than that at the stoichiometric surface. Also, the Au2 dimer strongly binds to the reduced support (BE = −2.16 eV), but differently compared to the Au2@CeO2 case, it lies parallel to the surface with one Au atom above the O vacancy (Figure 1f). Au3 displays two lowest-energy metastable structures, one in which the trimer is vertical to the surface (Figure S5(A), SI) and the other in which it lies linearly on the surface with the central Au atom bound to the vacancy (Figure 1g). The former structure is 0.16 eV lower in energy than the latter. The BE of this second structure is −2.13 eV, quite comparable to that one of Au2@ CeO2. A similar BE is predicted for the Au4 cluster, BE = −2.25 eV/atom, which displays tetrahedral morphology, with one vertex at the O vacancy (Figure 1h). Note that the Au3 and Au4 clusters have access to several structural isomers with comparable energy (see some examples in the SI). The calculated energetics of adsorption as a function of the cluster size and substrate stoichiometry is reported in Figure 1i and is compared to the results of Zhang and co-workers19 available for the Aun@CeO2−x case. This plot clearly demonstrates the strong preferential binding of small clusters

activation energies (0.14−0.33 eV) than Mars van Krevelen mechanisms (1.27−2.17 eV). Less information is instead available on the stability and reactivity of small-sized clusters as well as on the dynamics of the Au/ceria interface during reaction. This is expected to play an important role due to the high mobility and fluxionality of Au nanoparticles at temperatures relevant for technological applications. In this computational work, we show that adsorbate-induced changes of the morphology and size of the cluster have an impact on its stability and reactivity. Our systematic investigation of the stability, cohesion, and morphology of the cluster and of CO adsorption demonstrates that certain cluster sizes spontaneously dissociate into smaller units, while in other cases, the nanoparticles remain intact or even reshape. We rationalize this apparent erratic behavior on the basis of three energy factors, namely, the incremental cohesive energy of the cluster, the BE of molecular CO to the cluster, and that to a monomer at an O vacancy. We demonstrate that the smallest active and stable cluster is Au3. It nucleates at an O vacancy and displays the smallest activation energy for CO oxidation reported so far. The structural flexibility of small clusters plays therefore an important role in the reactivity by activating dynamical effects that prevent cluster dissociation while boosting reactivity. Selective Binding of Aun Clusters to Ceria Surfaces. We first address the size dependency of the binding, morphology, and charge state, of Aun clusters at the stoichiometric and reduced ceria(111) surfaces. The ceria(111) surfaces are chosen as the support because these are the most stable low indexed surfaces.28 Additionally, these surfaces serve as model systems on which several experimental and theoretical studies have already been performed.19,20 The results of this systematic study are summarized in Figure 1, which displays the minimumenergy adsorption configurations for the Aun (n = 1−4) clusters, together with the corresponding binding energies (BEs) as a function of cluster size. Figure 1 displays the lowestenergy structures obtained by adsorbing and relaxing a set of low-energy vacuum-optimized configurations at several surface 2257

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Figure 2. Lowest-energy configuration and energetics of CO adsorption on Aun supported on stoichiometric ceria (a−d) and reduced ceria (e−h). The C and the O atoms of the CO molecule are represented by black and red spheres, respectively. Main Au−Au and Au−C bond lengths are reported in Å. ΔE (i), BEiCO (j), and Ecoh (k) as a function of the number of atoms of the Aun cluster supported by stoichiometric (red circles) and reduced (blue stars) ceria. See the text for definitions.

in being more negatively charged than those on the stoichiometric supports. This can be rationalized on the basis of the excess electrons about surface O vacancies. The calculated charges display notable variations as a function of cluster size and do not show a clear correlation with the cluster BE and stability. Selective CO Adsorption on Supported Aun Clusters and Induced Morphology Changes. Building on the results described in the previous paragraphs, here we explore the dependency relationship of CO adsorption on the supported Aun cluster size and on the degree of reduction of the surface. The lowest-energy configurations and the corresponding BE of CO on the Aun clusters (n = 1−4) supported by CeO2 and CeO2−x surfaces are reported in Figure 2. These binding geometries and cluster morphologies result from the structural minimization of a large number of different cluster and adsorbate configurations, as explained in the Computational Details section. A CO molecule strongly binds to a positively charged Auδ+ adatom supported on the stoichiometric CeO2(111) surface (BECO = −2.48 eV; Figure 2a), while it cannot bind to negatively charged Au adspecies incorporated into a surface O vacancy of the reduced surface (BECO > 0; Figure 2e). We have previously shown that the Auδ+ 1 adspecies supported by the stoichiometric surface are metastable in the presence of O vacancies, which readily attract and trap the Au adatoms, 23 reverting their charge to Auδ− 1 species. This different behavior of clusters supported by stoichiometric and reduced ceria surfaces is reversed for the larger clusters (Au2 and Au3), and the variation vanishes already for the tetramer (Figure 2j). The BE of CO on clusters supported by stoichiometric and reduced ceria displays opposite trends as a function of cluster size. On the stoichiometric surface, the large BECO for the Au1 adatom is halved at the dimer (BECO = −1.24) and further reduced for Au3 and Au4, which have similar BECO values, −1.02 and −1.08 eV, respectively. Instead, on the reduced surface, the BECO inverts sign from positive to negative between Au1 and Au2, it further decreases to −1.80 for Au3, and

(Au1 and Au2) to O vacancies of the reduced ceria surface. This preferential binding is quickly lost, and the trend is reversed or leveled for cluster sizes as small as Au3 and Au4. This is because the larger the cluster, the more the structural and electronic effects of the vacancy are distributed among the cluster atoms. In addition, the defective fraction of the metal/oxide interface decreases as the cluster size increases. The latter Au3 and Au4 clusters therefore do not display a clear preference for adsorption at O vacancies. We conclude that single O vacancies acts as trapping sites only for Au adatoms and dimers. Adsorption of Au clusters to stoichiometric and reduced surfaces entails a strong charge transfer at the metal−oxide contact.23 On the one hand, this leads to Auδ+/Auδ− adspecies, and on the other hand, this yields surface reduction/reoxidation through electron localization/delocalization at Ce4+/Ce3+ sites, respectively. The calculations show that for a given size of the Aun cluster, there is a same number of reduced Ce3+ ions on reduced and oxidized ceria surfaces (see the blue spheres in Figure 1), Au2 being the only exception. This has an important technical implication. Conclusions based on differences between the BEs of a cluster at the stoichiometric and reduced surfaces are robust with respect to the parameter U. It is indeed well-established and consistent with the Hubbard approach that the DFT+U energetics weakly depend on the value of U when comparing systems with the same degree of reduction.23,29,44 There is a vigorous debate about the charge of the active species on ceria-supported Au nanoparticles, both in the context of CO oxidation and of the water−gas shift reaction.14,30,31 Although some works have addressed the charge of the cluster for specific cases,19,26 a systematic study of the charge as a function of both the cluster size and support reduction is missing and would be useful in this context. The charge of the Aun nanoclusters on the ceria support obtained by computing the Löwdin charges are +0.10/−0.31/−0.07/+0.10 e (stoichiometric surface) and −0.39/−0.29/−0.57/−0.42 e (reduced surface) for the Au1/Au2/Au3/Au4 clusters, respectively. Overall, the clusters supported by reduced surfaces result 2258

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then it increases to −1.14 eV for Au4. As a result, the selectivity for CO binding determined by the degree of reduction of the surface is lost for cluster sizes of Au4. The values of BECO for Au4, −1.08/−1.14 eV, are comparable to those reported for Au13, −1.28/−1.13 eV.26 Our calculations clearly show that the adsorption of CO affects the morphology, orientation, and even size of the ceriasupported clusters. As an example, adsorption of CO on the Au2@CeO2−x dimer induces its cleavage into two monomers, one bound to the vacancy and the other to the neighboring stoichiometric surface (Figure 2f). Consistently with the preferential CO binding on Au monomers described above, CO is found on the Au adatom at the stoichiometric surface (Figure 2f), yielding the characteristic Au−CO bond length of the monomer (Figure 2a). The splitting of the dimer is reflected in the Au−Au bond length, from 2.59 Å without CO to 3.23 Å after CO adsorption. Quite similarly, adsorption of CO on the Au4@CeO2 cluster breaks it up into a trimer at the vacancy and a monomer at the stoichiometric surface (Figure 2d), with the CO molecule bound again to the monomer. In these cases, the overall BE of CO to the clusters also includes the energy cost to dissociate the cluster. Note that we also identify a metastable binding configuration of CO at Au4 where the cluster does not cleave and whose energy is higher than the that of dissociated case by 0.34 eV (see SI Figure S9(C)). In other cases, the clusters remain intact upon CO adsorption, such as the Au2 and Au3 at the stoichiometric surfaces (Figure 2b and c, respectively). Additionally, we observe an intermediate behavior, namely, a change in the morphology without a full cluster cleavage, as in the case of Au4 at the reduced surfaces, where CO adsorption makes it depart from the tetrahedral geometry toward an almost planar one with elongated Au−Au bond lengths (Figure 2d). Finally, there are cases where different cleaved and intact cluster geometries with CO adsorbed are energetically almost degenerate. One example is the trimer at the reduced surface, which can break up into a dimer and a monomer (with the CO bound to the monomer), or it can rearrange into an almost linear geometry (with CO adsorbed on a bridge position between two Au atoms) (Figure 2g). We present in the following a thermodynamic analysis that rationalizes these irregular changes in cluster size and morphology induced by CO adsorption. Thermodynamic Analysis and Incremental Cohesive Energy. Our results show that whenever the Au cluster is cleaved by CO adsorption, only the atom bound to CO at the metal−oxide interface is detached from the parent cluster. We propose that the driving force responsible for this cluster dissociation is the large CO BE to the Au adatom supported by the stoichiometric surface. This energy (BECO(1) = −2.48 eV) is larger than the BE of CO to any other cluster considered in this work. We propose that the thermodynamics of this process is governed by the balance between three energy terms, (i) the adsorption energy of CO on the intact Aun cluster, BEiCO(n), (ii) the incremental cohesive energy that is required to cleave a Aun cluster into a Aun−1 cluster and one adatom on the stoichiometric surface, Ecoh(n), and (iii) the BE of CO to the latter adatom (BECO(1) = −2.48 eV). We define the following energy difference ΔE = BEiCO(n) − E*(n), where E*(n) = [−Ecoh(n) + BECO(1)]. In these assumptions, CO adsorption would induce cluster dissociation when ΔE is positive. We report these energy terms as a function of cluster size and of the degree of surface reduction in Table 1. The predictions

Table 1. Thermodynamic Analysis of Adsorbate-Induced Cluster Dissociation As a Function of Cluster Sizea n

BEiCO(n)

2 3 4

−1.24 −1.02 −0.74

2 3 4

−0.85 −0.89 −0.91

Ecoh(n) Aun@CeO2 −1.14 −2.04 1.06 Aun@CeO2−x −1.00 −1.80 −1.43

E*(n)

ΔE(n)

−1.34 −0.44 −1.42

0.1 −0.58 0.68

−1.63 −1.59 −1.05

0.63 −0.21 0.14

a

n is the number of atoms, BECO(n) is the BE of CO to the intact cluster, Ecoh is the cluster incremental cohesive energy, and E*(n) = −Ecoh(n) + BECO(1). Positive values of ΔE = BEiCO − E* indicate cluster dissociation. All energies are in eV.

of this model thermodynamics are consistent with the structural modifications observed during the structural relaxation and described above. Two systems display large positive values of ΔE, Au2@CeO2−x (0.68 eV) and Au4@CeO2 (0.63 eV). Indeed, these two clusters dissociated during the structural relaxations in the presence of adsorbed CO. The Au3@CeO2 cluster displays a large negative value of ΔE (−0.58 eV). This is consistent with the structural relaxation of this nanoparticle, which does not dissociate upon CO adsorption. The values of ΔE calculated for all other cases are much smaller ( 2, the trend of Δ correlates with that of E*(n), while BEiCO(n) shows smaller variations. Instead, on the reduced surface (blue circles), ΔE depends on a balance between the two energy contributions. Thus, upon CO adsorption, the cluster morphology changes due to the interplay between the high BECO on Au+1 , the considerably lower BECO on bigger clusters, and the strongly size-dependent incremental cohesive energy of the supported clusters. CO Oxidation on Au3 and Catalyst Regeneration. Recently, Henkelman and co-workers studied the reaction mechanisms for CO oxidation catalyzed by a Au13 cluster on stoichiometric and reduced ceria surfaces.26,27 The CO oxidation pathway may proceed via coadsorbed CO and O2 or via a Mars van Krevelen mechanism. For this cluster size, the latter mechanism was shown to occur in very high energy barriers (>1.3 eV) determined by the desorption of the CO2 products. In a previous work, we have explored the same reaction channel for the case of Au adatoms supported by stoichiometric and reduced CeO2(111) surfaces. The reaction consists of the following steps: CO adsorption on the adatom, CO spillover across the interface, CO oxidation, and CO2 desorption (Figure 3). The spillover is, in this case, the rate-limiting step and has a lower activation energy (0.86 eV) than the Au13 case.23 We found that, although Auδ+ species supported by stoichiometric surfaces can catalyze CO oxidation, the oxygen vacancy that is formed during the reaction attracts and traps the adatom, reversing its charge and deactivating it. The resulting Auδ− species at O vacancies, although very stable, do not allow for the binding of other reactants, either CO or O2, and thus lead 2259

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Figure 3. Calculated reaction pathway and corresponding energetics for oxidation of the first CO molecule by a supported Au3 cluster.

The desorption of CO2 generates an additional O vacancy on the surface, which can potentially lead to the dispersion of the cluster, in analogy to the Au2 case. This vacancy attracts the cluster but does not break it up into an adatom and a dimer, each supported by a vacancy. Although the incremental cohesive energy of this trimer is as low as the Au2 one (0.89 and 0.85 eV, respectively), the flexibility of Au3 allows it to readjust so as to spread across the two vacancies in a structurally stable configuration. In order to provide a perspective on how stable the trimer is on these surfaces, we compare the BEs of Au3 with those of Pt3 on these surfaces. On the stoichiometric and the reduced surfaces, the Pt3 BEs are −3.83 and −3.92 eV, respectively.34 The Pt3 cluster is therefore very strongly bound to the substrate compared to that of the Au3. By comparison with the results reported by Kim and coworkers26 for a Au13 cluster, we can conclude that the effect of a larger cluster size on this reaction pathway is (i) to strongly stabilize the intermediate state (from −0.18 eV for the trimer to −1.65 eV for Au13@CeO2−x), (ii) to significantly reduce the activation energy for CO spillover (from 0.65 to 0.01 eV for the trimer and Au13@CeO2−x, respectively), and (iii) to substantially increase the energy barrier for CO2 desorption, which turns out to be the rate-limiting step for the Au13 cases, with activation energies as large as 1.27/2.17 eV for the stoichiometric/reduced surface. For this particular reaction path, the Au trimer at reduced surfaces is therefore more active than both isolated Au+ adatoms and larger clusters. We now show that, differently from the adatom case, the trimer is not deactivated during reaction and can be regenerated in the presence of molecular O2. This catalyst regeneration is enabled by the high structural flexibility of the Au3 cluster. We find that, similarly to the monomer case, this trimer prevents the binding of O2, but

to catalyst poisoning. These species are however the nucleation sites for the growth of larger clusters similar to those studied in this work. In the following, we aim at identifying the smallest Au cluster that is anchored at an O vacancy that can efficiently catalyze multiple CO oxidations with low activation energies and without being deactivated. As we have shown, CO adsorption, that is, the first stage of the CO oxidation reaction, can break up a Aun cluster into a Aun−1 cluster and a neighboring Auδ+ adatom that binds the CO molecule. In these cases, the reaction is likely to proceed on the detached adatom following the same pathway for Au1 described above (Figure 3, red line) and can in principle lead to the full cluster dispersion and deactivation. In particular, this is the case for CO adsorption on the dimer at an O vacancy, which splits into an inactive Auδ−@vacancy and a CO@Auδ+ adatom (Figure 2f). Hence, also the Au2@CeO2−x cluster is unlikely to be relevant for CO oxidation. Instead, we have seen that the Au3@CeO2−x cluster is very stable and flexible, with several almost isoenergetic structural configurations. The reaction path predicted by the NEB calculations for this Au trimer at an O vacancy is depicted in Figure 3 (Au3, blue line). In the initial state, the CO molecule binds at the end of the linear cluster, which elongates one of the Au−Au bonds from 2.69 to 2.84 Å. The total energy of this configuration differs by less than 0.1 eV from the one displayed in Figure 2g, where CO binds in the middle of the cluster. The activation energy for spillover is 0.65 eV, which is lower than that for the case of an adatom (0.86 eV). The reaction proceeds through an intermediate state where the trimer with CO is bound across the interface, forming a complex with a surface O atom. We note that this state is lower in energy than the initial state by 0.18 eV. The oxidation of CO to CO2 and its desorption from this intermediate state are associated with an activation energy of 0.58 eV. 2260

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Figure 4. Calculated reaction pathway and corresponding energetics for (a) O2 adsorption and catalyst regeneration and (b) oxidation of a second CO molecule by a supported Au3 nanocluster.

clear preferential binding to surface O vacancies of reduced surfaces. Larger clusters Au3−Au4 instead interact with almost equal BEs to stoichiometric and reduced surfaces. Surface defects are therefore trapping sites for Au adatoms but not necessarily for larger clusters. Au adatoms at O vacancies can constitute nucleation sites for the growth of larger clusters, which may remain anchored to the vacancy only in the presence of a sizable activation energy for cluster diffusion but that do not display a clear thermodynamic preference for being located at surface defects. Our calculations demonstrate the structural flexibility of these small Au clusters that can easily adopt different configurations to facilitate CO adsorption. Indeed, the adsorption of molecular CO induces structural and morphological modifications of the supported clusters and, in some case, drives their dissociation and dispersion into smaller particles. This supports the fluxionality of these small clusters in the presence of adsorbates and evidence the importance of cluster dynamics during reaction. A thermodynamic analysis rationalizes this cluster flexibility on the basis of three energy terms, the strong preferential binding of CO to Au monomers at stoichiometric surfaces with respect to larger supported clusters and the incremental cohesive energy of the Au nanoparticles. Our analysis identifies the smallest active and stable cluster capable to catalyze CO oxidation by exploiting the oxygen buffering capacity of ceria. The lower activation energy of the Mars van Krevelen mechanism displayed by this cluster with respect to larger clusters is related to an optimal balance

differently from the adatom, the cluster can now easily shift and rearrange its structure so as to expose one of two vacancies underneath. There is a strong driving force for the binding of molecular O2 to this defect, leading to an O2 adsorbate bridging between the cluster edge and the support (−1.06 eV, Figure 4). This process is accompanied by an energy barrier of 0.52 eV resulting from the displacement and rearrangement of the cluster. Similar configurations for O2 adsorbed at the periphery of Au nanoclusters supported by TiO2 have recently been proposed in refs 32 and 33. This activated molecular oxygen (O−O bond length increases to 1.49 Å) can then take part in the oxidation of a second CO molecule, as discussed below. We report in Figure 4 the energetics for the binding and oxidation of a CO molecule at the catalyst edge close to the activated O2 molecule. The reaction pathway predicted by the NEB calculations consists of the adsorption of molecular CO at the cluster edge involving an activation energy of 0.31 eV. The bond length analysis shows that CO adsorption splits the trimer into a dimer above a vacancy and a monomer, one binding the CO and the other the activated O2 molecules. The reaction proceeds with the CO oxidation and the desorption of the CO2 products, involving an activation energy of 0.27 eV. Note that the calculated activation energies for the second oxidation are lower than the rate-limiting step of the first oxidation. In conclusion, we have presented a systematic study of the cluster structural stability, cohesion, and morphology as a function of particle size and degree of surface reduction, also in the presence of molecular CO. Our study reveals a change of binding regime at very small cluster sizes. Au1 and Au2 display a 2261

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Letter sub X where Esub+X tot , Etot , and Etot are the total energies of the combined system (CO)/Au n /CeO 2 (111), the (Au n )/ CeO2(111) surface in a certain oxidation state, and an isolated adsorbate (CO)/Au atom, respectively and n denotes the number of adsorbate molecules/atoms. All quantities are calculated in the same supercells. In the study of the CO adsorption on the supported Au clusters (as described in detail later), we find that for certain cases, Aun splits into a Aun−1 nanocluster on the stoichiometric/ reduced CeO2(111) surface and a single adatom on the stoichiometric surface. To gain an understanding of this process, we compute the incremental cohesive energy of the supported clusters (Ecoh(n)) as

between the activation energies of CO spillover to the metal− oxide interface and to the facile desorption of the CO2 products. The calculations emphasize the important role of cluster mobility and flexibility about the interfacial O vacancies that assist the CO oxidation. Although these ultrasmall optimal clusters might be unstable at high temperatures and may be elusive to spectroscopy and microscopy characterizations, these results evidence the key role played by the structural dynamics of the Au−oxide interface in the reactivity of these catalysts.

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COMPUTATIONAL DETAILS All of the calculations were performed with the Quantum ESPRESSO package,35 which is an implementation of the DFT36,37 in a plane wave pseudopotential framework. The electron−electron exchange and correlation functional was described with the Perdew−Burke−Ernzerhof (PBE) parametrization of the of the generalized gradient approximation (GGA).38 A Hubbard U term acting on the Ce 4f orbitals was added to the standard density functional for reproducing the electronic structure of the reduced ceria. In the present work, we have used the implementation of Cococcioni and Gironcoli39 and a value of U = 4.5 eV, in line with previous analysis.40−49 Both the oxidized and reduced (111) surfaces were represented with a (3 × 3) nine layer supercell slab. The spin-polarized calculations employed energy cutoffs of 30 and 300 Ry for the wave function and augmentation charge density, respectively. The vacuum between the periodic images along the direction perpendicular to the slab was set to more than 15 Å. The lowest three atomic layers were constrained to bulk equilibrium positions, and the rest were allowed to relax according to the calculated Hellman−Feynman forces until the maximum force was less than 0.02 eV/Å. Integrations over the Brillouin zone used a (2 × 2 × 1) Monkhorst−Pack grid.50 The climbing image nudged elastic band method (CI-NEB)51 was employed to sample the reaction mechanism for CO oxidation. To find the stable Au clusters on the ceria surfaces, we first optimized the Au nanoclusters in the gas phase and then on the oxide surfaces. In the gas phase, the choice of initial configuration is trivial for Au1 and Au2, while larger clusters present a rapidly increasing number of possible isomers. For the Au3 and Au4 cases, we selected the lowest-energy structural isomers reported in the literature.52 Determination of a sufficiently stable structure of the clusters adsorbed on the oxide surface as well as a stable adsorption geometry of CO on the supported Au clusters is very challenging because of the several possible local minima on the potential energy surface. For each cluster size, we have sampled a set of different cluster morphologies, adsorption sites, and orientations (see the SI). The fact that, in several cases, the different initial conditions relaxed to the same energy basin during the structural optimizations gives us confidence that the lowest-energy structures discussed in the text are sufficiently close to the global minimum and statistically representative. The adsorption energies of the Aun clusters on stoichiometric and reduced CeO2(111) surfaces (BE(n)) and of the CO molecule on the Aun/CeO2(111) surfaces (BECO(n)) were calculated according to sub + X sub X (Etot − Etot − nEtot ) n

s/r s s/r s Ecoh(n) = Esurf (n) + Esurf − Esurf (n − 1) − Esurf (1)

Es/r surf(n)

(2)

Essurf

where and represent the total energy of the Aun cluster on the stoichiometric(s)/reduced(r) CeO2(111) surface and the total energy of the clean stoichiometric surface, respectively.

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ASSOCIATED CONTENT

* Supporting Information S

Additional structures of Aun and CO adsorbed on Aun on ceria surfaces. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support by the European Union FP7-NMP-2012 project chipCAT under Contract No. 310191 and COST action CM1104. P.G. would like to thank ICTP for hosting his stay at Trieste under the Regular Associate Scheme during which the author worked on the referee’s comments.

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