Structural Rearrangement of AuâPd Nanoparticles under Reaction

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Structural Rearrangement of Au-Pd Nanoparticles Under Reactions Conditions: An Ab Initio Molecular Dynamics Study Cong-Qiao Xu, Mal-Soon Lee, Yang-Gang Wang, David C. Cantu, Jun Li, Vassiliki-Alexandra Glezakou, and Roger Rousseau ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b07409 • Publication Date (Web): 25 Jan 2017 Downloaded from http://pubs.acs.org on January 27, 2017

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Structural Rearrangement of Au-Pd Nanoparticles Under Reactions Conditions: An Ab Initio Molecular Dynamics Study Cong-Qiao Xu,1,2 Mal-Soon Lee,2* Yang-Gang Wang,2#* David C. Cantu,2 Jun Li,1,3* Vassiliki-Alexandra Glezakou,2 and Roger Rousseau2 1

2

Department of Chemistry, Tsinghua University, Beijing 100084, China Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352, United States 3 William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States

Corresponding Authors: [email protected], [email protected], [email protected]

ABSTRACT The structure, composition, and atomic distribution of nanoalloys under operating conditions are of significant importance for their catalytic activity. In the present work, we use ab initio molecular dynamics simulations to understand the structural behavior of Au-Pd nanoalloys supported on rutile TiO2 under different conditions. We find that the Au-Pd structure is strongly dependent on the redox properties of the support, originating from strong metal-support interactions. Under reducing conditions, Pd atoms are inclined to move toward the metal/oxide interface, as indicated by a significant increase of Pd-Ti bonds. This could be attributed to the charge localization at the interface that leads to Coulomb attractions to positively charged Pd atoms. In contrast, under oxidizing conditions, Pd atoms would rather stay inside or on the exterior of the nanoparticle. Moreover, Pd atoms on the alloy surface can be stabilized by hydrogen adsorption, forming Pd-H bonds, which are stronger than Au-H bonds. Our work offers critical insights into the structure and redox properties of Au-Pd nanoalloy catalysts under working conditions. KEYWORDS: Au-Pd nanoalloy ⋅ TiO2 ⋅ ab initio molecular dynamics ⋅ redox property ⋅ charge transfer 1

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Finite-size nanoparticles have been widely studied in the past decades because of their distinct properties from bulk materials.1-3 Au nanoparticles possess remarkable electronic and optical properties, and are particularly active in various catalytic reactions such as CO oxidation.4-8 Bimetallic alloy clusters have exhibited enhanced activity and selectivity compared with monometallic ones due to their well-known catalytic, optical, and electronic properties.9-15 Among the binary nanoalloys, gold-palladium (Au-Pd) nanoparticles (NPs) have attracted considerable attention because of their superior performance in various catalytic reactions, such as synthesis of H2O2 from H2 and O2;16-22 oxidation of CO, C-H bonds, methane, and alcohols;10, 23-30

vinyl acetate synthesis;31, 32 dehydration of formic acid;33 and N2O decomposition;34 as well

as other functions.35-41 The structure of binary NPs, or clusters, can be quite different from single-component ones. Their atomic properties and functionalities are different on the surface, in the core, or at the interface with a surface.42 Their catalytic activity will depend on the surface structure, composition and distribution of the atomic species.43 However, little is known on how the elements are structured in bimetallic NPs under catalytic operating conditions. Thus, determining the structural properties of a prototypical bimetallic Au-Pd NP under operating conditions is the key to understanding and controlling their catalytic function. Previous studies have indicated that a truncated octahedron (TO) with local fcc packing was the global minimum structure for an ordered Au38 NP,44-49 while a structure having two Au adatoms on the Au4@Au32 core shell was later found to be lower in energy, similar to a structure determined for Au40.50 To study the structure of bimetallic Au-Pd NPs, a 38-atom cluster, Au32Pd6, is chosen in this work as a model NP to represent an Au-rich alloy NP of roughly 1 nm in diameter, yet be tractable enough for large-scale ab initio molecular dynamics (AIMD) simulations. The NP structure depends on its electronic structure and can be affected by various factors.51 Previous works have shown that bimetallic NP restructuring may occur in the presence of NO, CO, or H2, as well as at different temperatures.52-55 Hydrogen chemisorption on Au-Pd clusters affects their structure and stability.56-58 However, to our knowledge, these factors have not been studied systematically. 2


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Support properties also play an important role in NP59-61 and single atom62 properties. For example, TiO2 supports are ubiquitously employed in catalysis and have electronic properties that have been thoroughly studied.63-71 As a reducible metal oxide, the existence of oxygen vacancy defects affects the interactions between the support and clusters.62, 72-77 Bonding patterns, catalytic activities and mechanisms can be quite different when supports are reduced, hydroxylated, or oxidized.61, 78 Herein, TiO2 under reduced and oxidized conditions was chosen to investigate the support effects on the structure dynamics and charge states of supported Au32Pd6 NP. In this work, the effects of support material redox state, hydrogen and oxygen adsorption, and temperature on the structure of Au-Pd NPs are investigated with AIMD simulations. To address this, the following model systems are considered: (1) the gas phase cluster, Au32Pd6; (2) the gas phase cluster in the presence of H2, Au32Pd6-4H; (3) reduced TiO2(110)-supported cluster, Au32Pd6/TiO2-x, with a single surface oxygen vacancy (VO) leading to a 5% coverage similar to the experimentally observed 5-8% oxygen vacancies; (4) Au32Pd6-4H cluster on the reduced TiO2 surface, Au32Pd6-4H/TiO2-x; (5) Au-Pd NP on a stoichiometric TiO2 surface, Au32Pd6/TiO2; (6) Au-Pd NP on an oxidized TiO2 surface with one VO and two extra O atoms (Oad), Au32Pd6/2Oad-TiO2-x; and (7) Au32Pd6-4H cluster on an oxidized TiO2 surface with one VO and two extra O atoms, Au32Pd6-4H/2Oad-TiO2-x. To model support oxidation, one O2 molecule was placed on the TiO2-x surface, which easily dissociated into two O atoms, 2Oad-TiO2-x. The oxygen atoms became strongly bound to Ti atoms with negligible Oad-Oad interactions. Simulations show that Au-Pd nanoalloy structures depend strongly on the reaction conditions. At low temperatures Pd atoms prefer to remain inside the core of the cluster, but they emerge onto the surface at elevated temperatures. Adsorbed H atoms on the alloy can also alter the structures because of bonding competition between Pd-H and Au-H. Also, Pd and Au atoms present different distributions at the interface depending on the redox state of the support.

3


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RESULTS AND DISCUSSION Au32Pd6 structures in the gas phase at 0 K The TO structure of the ordered Au38 can be viewed as an Au6 octahedral core surrounded by an Au32 shell, Au6@Au32, with eight equivalent hexagonal fcc(111)-like faces, as shown in Figure S1A. There are three possible Pd doping sites: a) Pdcore, with the Pd atom in the octahedral core; b) Pdcent, with the Pd located at the centroid of the surface fcc(111) facet; c) Pdhex, where Pd lies at the vertex of the hexagonal faces. Figures S1B-F show the relative energies with respect to the lowest energy structure of several isomers of Au32Pd6 cluster. It shows that the Pd6@Au32 core-shell structure with Oh symmetry is the most stable, in accord with previous work.49, 79-86 In addition, we see that the energy of the cluster increases as Pd atoms are closer to the surface. It is energetically favored for Pd atoms to reside in core sites, potentially to reduce the elastic strain on the cluster due to the smaller size of Pd atoms compared to Au. Among the structures with Pd atoms closer to the surface (Figure S1C-G), Pd atoms are preferentially located at the centroid site of the (111) facet (positions b, b’) rather than hexagonal site at the (100) facet (positions c, c’, c”), in agreement with previous results of Yuan et al.87 The positional preference of Pd atoms, Pdcore > Pdcent > Pdhex, is consistent with the fact that the Pd-Au bond energy is larger than Pd-Pd and Au-Au.84, 88 Indeed, on fcc(111) facet a larger number of Pd-Au bonds are formed with Pdcent, which gives rise to a larger binding energy. The stability trend of centb > centb’ and hexc > hexc’ > hexc” is consistent with the stronger Pd-Au interactions. Temperature and support effects on the Au32Pd6 structure AIMD simulations at 700 K were performed to account for finite temperature fluctuations, as well as to obtain optimized structures under different conditions. The optimized structures are shown in Figure 1 where differences in overall structures, and Au-Pd atom distributions, can be clearly distinguished. Au-Pd nanoparticles display liquid-like properties at 700 K. Their shapes are easily altered when supported on TiO2(110), as shown by their moments of inertia (MOIs) along the x, y, and z-axes that describe their sphericity (Figure S2). Gas phase clusters, with and without H, have 4


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similar MOIs along the three axes, indicating that the cluster is approximately spherical. However, all clusters on a support show elongated shapes along the xy plane, evidenced by a large MOI oriented along the z-axis. The flattened shape of the Au-Pd cluster is due to the interactions between the cluster and support. To quantify the liquid-like character of Au32Pd6 clusters, the root-mean-square (rms) bond length fluctuations (δrms) were calculated using the following equation: 2

1

〈 〉 〈 〉 〈 〉

where rij corresponds to the distance between atoms i and j, 〈⋅⋅⋅〉 denotes a time average, and N is the number of atoms. According to the Lindemann criterion, melting occurs when δrms reaches ~0.1-0.15 for a bulk material, however this value is higher for nanoclusters.89,

90

Au-Pd

nanoalloys are nearly liquid-like, in both spherical gas phase clusters or elongated surface bound ones, as shown in Table S1, where all δrms values are greater than 0.20.

Figure 1. Optimized structures following AIMD simulations for: Au32Pd6-4H, Au32Pd6/TiO2-x, Au32Pd6-4H/TiO2-x, Au32Pd6/TiO2, Au32Pd6/2Oad-TiO2-x, Au32Pd6-4H/2Oad-TiO2-x. Color code: Pd (blue), Au (yellow), Ti (gray), lattice O (red), Oad (green), and H (white). 5


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Surface Pd atoms on Au-Pd nanoparticles are catalytic active sites,55, 91-93 and they appear on surface sites when temperature and support effects are considered. At 700 K, the PdcoreAushell structure is not energetically favored as in 0 K. Au and Pd atomic distributions, with respect to the cluster center of mass (COM) for all cases at 700 K, are illustrated in Figure S3. A new peak in the Pd distribution appears at ~4 Å (Figure S3a) showing a migration of Pd atoms toward the surface, more so for TiO2-supported clusters. Despite the structural rearrangements, pair distribution functions are similar in all cases (see Figure S4) with average coordination numbers (CNs) of ~2 for Pd-Pd, ~5 for Au-Au, and ~8 for Pd-Au. O2 activation preferably occurs on Pd sites over Au sites on Au32Pd6/TiO2-x because O2 binds on Pd atoms. Adsorption of O2 is repulsive on surface Au (Ead = 0.27 eV) but attractive on Pd (Ead = -0.12 eV). After adsorption, Pd-O distances are ~2.10 Å and the O-O bond length increases to 1.27 Å, suggesting O2 activation toward a superoxide. This supports the conclusion that Pd atoms on cluster surfaces can perform dioxygen activation, as likely observed in H2O2 formation.30, 93, 94 The nanoparticle and support interactions can be described in terms of the interatomic Pd-Ti, Au-Ti, Pd-O, and Au-O bonding interactions. These are expressed in the form of radial distribution functions (RDFs), and quantified by the number of bonds for the different support models, i.e. stoichiometric TiO2 or under reduced/oxidized conditions shown in Figure 2a,b and Table 1. The support character has the most significant effect on Pd-Ti bonds: they are present only under reduced conditions. This is evidenced by the distinct peak at ~2.7 Å in Figure 2a for Pd-Ti only present when the surface is reduced. Additionally, a pronounced peak in Figure 2c of Pd-surface distances is observed at ~2.6 Å for the reduced support only, indicating stronger Pd-Ti interactions at the upper surface of the nanoparticle. This observation is extremely relevant for catalysis because Pd atoms at the surface and interface are reaction active sites. Au-Ti bonds are present at ~2.80 Å in all cases. Fewer Au-Ti contacts are present as the oxidizing degree of the support increases compared to those with reduced supports. The opposite is observed for Au-O bonds, while present in all cases at ~2.35 Å, more bonds are present as the degree of oxidation of the support increases, and fewer ones with the reduced supports. No distinct differences for Pd-O 6


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bonds are observed, and no Pd-Oad interactions were observed on the cluster with H on an oxidized surface (Au32Pd6-4H/2O-TiO2-x) due to Pd-H interactions at the surface.

Figure 2. Radial distribution functions (RDFs) of (a) Pd-Ti, Au-Ti, and (b) Pd-O, Au-O relative to the distance between relevant species. (c) Density profiles as a function of the distance between Pd and Au atoms to the upmost Ti layer of TiO2, which is marked as Pd-Sf and Au-Sf.

The charge transfer between the cluster and the support, which will be discussed in more detail in a later section, can easily explain this behavior where the negatively charged Au-Pd cluster present with reduced support prefers to interact with Tiδ+ atoms rather than Oδ- atoms on the surface (Figure 2c). Although strong interactions between Pd atoms and the reduced surface are present, Pd atoms prefer to remain within the cluster than at the interface. This was observed during an AIMD simulation where Pd atoms initially placed at the interface in Au32Pd6-4H/TiO2-x started diffusing into the cluster after only ~1 ps. Table 1. Average Number (N) of Pd-Ti, Au-Ti, Pd-O, Au-O bonds and number of Pd atoms at the interface of the cluster and support calculated by the integral of corresponding RDFs Au32Pd6/TiO2-x Au32Pd6-4H/TiO2-x Au32Pd6/TiO2 Au32Pd6/2Oad-TiO2-x Au32Pd6-4H/2Oad-TiO2-x

NPd-Ti 1.0 1.5 0.2 0.1 0.1

NAu-Ti 7.4 6.6 5.4 5.1 5.2

NPd-O 1.2 1.2 1.3 1.1 1.2

NAu-O 4.7 4.9 5.6 5.4 6.1

NPd-Sf 2.2 3.0 1.4 1.5 1.3 7


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H2 adsorption effects on the Au32Pd6 structure Au-Pd alloys exhibit excellent catalytic hydrogenation activities in CO2 reduction95-97 and alkene hydrogenation,98-103 where intermediate H species are a possible reducing agent. Preferential Pd-H interactions were observed to alter atomic distributions since H surface atoms will rearrange the configuration of the Au-Pd nanoparticle. In the gas phase cluster, Au32Pd6-4H, a Pd atom migrates from the core to the surface to form a Pd-H bond during the AIMD simulation. This is consistent with previous findings53 that show a AucorePdshell structure with H coverage. Contrary to the gas phase structure without H at 0 K, when H is present a structure with Pd at the surface is energetically favored (~0.2 eV) compared to the PdcoreAushell configuration. The restructuring capability of surface H atoms can be attributed to stronger Pd-H interaction compared to Au-H.81 However, during the AIMD simulations, some Pd atoms remained at the core when H is present because some H atoms moved into the nanoparticle to bind with core Pd atoms. Furthermore, the motion of H atoms back to the surface after forming bonds with core Pd atoms was not observed during the simulations, suggesting that a significant energy barrier exists to break stable Pd-H bonds. Favorable Pd-H bond formation drives both Pd and H migration towards the surface, with a marked effect on the distribution of Pd atoms in Au-Pd clusters. A similar phenomenon is observed in Pt-Pd nanoparticles where H atoms migrated to the central core to bind with Pd.104 The formation of H2 molecules from surface H atoms was observed in the AIMD simulations for the cluster in the gas phase and on a reduced support. This is shown by the radial distribution functions of H-Pd, H-Au, and H-H pairs that appear in Figure S5a in the SI. In gas-phase, an H2 molecule recombines on a Pd site within 4 ps of simulation time, and remains as a stable diatomic for the remainder of the simulation. In contrast, for the cluster on a reduced support, two H2 molecules form initially, but they quickly dissociate on Au sites. On a reduced surface, Au-H interactions are more favorable than Pd-H interactions, opposite to what is observed for H atoms on the Au-Pd nanoparticle in the gas phase. This result is consistent with the work by Tierney and co-workers where they found that Pd atoms are not active for H2 dissociation on Au(111).105 After H2 dissociation on the reduced supported cluster, mobile H atoms diffused 8


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toward the TiO2 surface making bridging Au-H-Ti bonds. H2 formation was not observed on the Au-Pd cluster on an oxidized support, which may arise from the difficulty of oxidizing H atoms on a positively charged cluster as discussed in the following section. Electronic properties of Au32Pd6 under reduced and oxidized conditions As alluded to above, the structural changes can be rationalized by the differences in the electronic properties of the Au-Pd clusters as a response to the environment. This becomes evident in the projected density of states (PDOS) of Pd-d, Au-s, and Au-d states, shown in Figure S6 for the gas phase and supported Au-Pd cluster. The gas phase cluster band gap decreases with H adsorption (from 0.8 to 0.4 eV). Supported clusters, in particularly without H atoms, give rise to even smaller band gaps (∼0.2 eV). While the Pd-d states remain close to the Fermi level, the Au-d states show a broad distribution. As a result, for supported clusters, it is the electronic state of the cluster not that of the support, that dominates the highest occupied bands. This conclusion indicates that both the Au and Pd atoms contributing to the redox activity of the nanocluster. Below, we present an in depth analysis of this observation.

Table 2. Bader charge analysis of Au32Pd6, Au32Pd6-4H, Au32Pd6/TiO2-x, Au32Pd6-4H/TiO2-x, Au32Pd6/TiO2, Au32Pd6/2Oad-TiO2-x, and Au32Pd6-4H/2Oad-TiO2-xa [Pdinter] Au32Pd6 Au32Pd6-4H Au32Pd6/TiO2-x Au32Pd6-4H/TiO2-x Au32Pd6/TiO2 Au32Pd6/2Oad-TiO2-x Au32Pd6-4H/2Oad-TiO2-x a

0.02 -0.13 0.07 0.22 0.32

[Pdtop]

[Pd6]

[Au32]

[Au32Pd6]

0.18 0.19 0.50 0.15 0.09

0.28 0.48 0.20 0.06 0.57 0.37 0.41

-0.28 -0.42 -1.17 -0.50 -0.82 0.62 0.32

0.00 0.05 -0.96 -0.45 -0.26 0.99 0.73

[H4]

[2Oad]

-0.05 -0.53

-0.17

-2.05 -2.03

Data listed here are Bader charges (in the unit e-) for Pd atoms at the interface ([Pdinter]), Pd atoms above the

interface ([Pdtop]), all the Pd atoms ([Pd6]), all the Au atoms ([Au32]), Au-Pd bimetallic cluster ([Au32Pd6]), all the H atoms adsorbed at the cluster ([H4]), and two O atoms absorbed at the surface ([2Oad]).

The charge state of the cluster changes when the cluster is supported on TiO2, or when Oad or H atoms are present. Due to electronegativity difference, a charge transfer from Pd to Au atoms 9


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results in a positively charged Pd core in the gas phase Au32Pd6 cluster, as shown in Table 2. When H atoms are adsorbed, they become slightly hydridic, triggering electron transfer from the cluster to the hydrogen atoms, oxidizing the metal cluster. Transfer of electrons also occurs to/from the supporting surface. When the surface is reduced (Au32Pd6/TiO2-x), there is a charge transfer of 0.96 e- from the surface to the cluster as quantified by Bader charge analysis (Table 2). The transfer occurs in the opposite direction when the support is oxidized. In the Au32Pd6/2Oad-TiO2-x system, the net Bader charge of the cluster becomes positive. Like with H adsorption, when supported by an oxidized surface, the Au-Pd nanoparticle loses 0.99 electrons. In all cases and despite the electron transfer, Pd atoms remain positively charged in varying degrees. To further verify the charge transfer, work functions (W) for all cases were obtained by calculating the difference between the Hartree potential (VH) within the vacuum region and the Fermi energy, see Table S2. As noted in Ref. 106, changes in VH can be used to track the net charge flow between subsystems (cluster and support), and is a direct measure of the impact it has on the redox properties of the nanoparticle. While work function values of gas phase clusters are similar with and without hydrogen, ~5.0 eV, those of the reduced, stoichiometric and oxidized TiO2 supports are 5.09, 6.75, and 6.11 eV, respectively. When a cluster is adsorbed onto the reduced surface, work function is nearly the same, W ~ 5.0 eV, leading to a negligible electron transfer from the cluster to the support. In contrast, clusters adsorbed on stoichiometric or oxidized supports show increased work function, W ~ 5.1 to about 5.4 eV, indicating charge transfer from the cluster to the supports. Due to arbitrary nature of all population analysis methods, we also performed charge analysis based on the electrostatic (Hartree) potential, which provides a direct measure of charge transfer. Here, we calculated Hartree potential differences (∆VH) between the total system and that of the sum of a neutral cluster and support, each obtained independently, which are shown in Figure 3. In agreement with the Bader charge analysis, charge flows from the reduced support to the cluster, and from the cluster to the oxidized support. The amount of charge transfer (∆q) is 10


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calculated using a plate-capacitor model: ∆ ∆ ⁄4d where ρ is the density of the cluster at the surface and d the separation distance.106, 107 Results are listed in Table S2 and follow the same general trend as those extracted from Bader analysis, with the difference that the magnitude of ∆q is smaller. When the cluster is on a reduced support (TiO2-x), the cluster gains 0.06 e- from the support. However on the same reduced support, in the presence of H atoms, the cluster only gains 0.01 e-. In summary, the cluster transfers 0.17 e- to the stoichiometric support (Au32Pd6/TiO2), 0.29 e- to the oxidized support (Au32Pd6/2Oad-TiO2-x), and 0.34 e- with adsorbed H atoms and the oxidized support (Au32Pd6-4H/2Oad-TiO2-x). Therefore, the charge transfer potential and the extent of charge transfer depend on the reduced/oxidized condition of both the support and the cluster. Charge transfer assessed via a plate capacitor model are consistent with charge transfer as reported by Bader analysis, though showing a much small magnitude, except for the cluster adsorbed stoichiometric. This discrepancy may be due to the strong bond formation between the support and the cluster as shown in Fig. 2 and Table 1. This results in localized electron density at the interface rather than charge transfer and indicates that the interaction is not as simple as a charge transfer from one side to another side, but partially similar to formation of a localized chemical bond.

Figure 3. The z-projection of Hartree potential difference (∆VH) by subtracting values of the cluster and the support from that of the total system. Here z-axis is defined as being perpendicular to the TiO2 surface. The potentials are shifted so the potentials inside the support surface are ∼0 eV. 11


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To visualize the electronic interactions between the cluster and the support, electron density difference maps were constructed (see Figure 4 and Figure S7). The figures clearly reveal a localization of electrons at the interface, showing the chemical bonding that occurs between the cluster and the support. Interactions between the Au-Pd cluster and the reduced surface are stronger than with the stoichiometric surface. Changes in electron distribution on oxidized supports are largely located on Oad at the interface and on oxygen vacancy sites (Figure S7).

Figure 4. Electron density difference graphs for Au32Pd6/TiO2-x and Au32Pd6/TiO2 from front and top view. The increase of electron density is indicated by green color, while the decrease of electron density by blue color.

Free energetics and stability of supported Au32Pd6 at 700 K The enthalpic and entropic contributions of the free energy were calculated as described in the Computational Details section. The vibrational contributions of cluster and support atoms can be quantified by the vibrational density of states (VDOS), shown in Figure S8. Heavy atoms, such as Au and Pd, show low-frequency vibrations with broad modes in the range of 0–250 cm-1 characteristic of their liquid-like cluster behavior at high temperatures. The vibrational frequencies are distributed below 1000 cm-1 for supported systems, where the low-frequency 12


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regions (0 - 80 cm-1) are dominated by the Au-Pd vibrational modes, and the vibrations between 120 and 1000 cm-1 are dominated by the support. Fa et al.,108 and Sauceda et al.109 also showed that vibrational frequencies of Au clusters are in the 0–200 cm-1 range, while Mitev and Hermansson110 showed that those of bulk TiO2 range between 60 and 1000 cm-1, in agreement with our results. Adsorption free energies of the Au-Pd cluster on the support are shown in Table 3. The most stable system at 700 K occurs when Au-Pd nanoalloy is adsorbed on the oxidized support. The adsorption free energy on the stoichiometric support is the smallest, highlighting the importance of surface defects in stabilizing metal nanoparticle on supports. This can be traced back to the least interaction between them (∆Hads), which is also seen in the electron density difference maps (Fig. 4). A decreased stability (increased free energies) is observed with adsorbed H atoms on the cluster. Adsorption enthalpies of the clusters (∆Hads) are very similar for both reduced and oxidized supports, ranging from -450 to -499 kJ/mol. However, adsorption entropies, particularly the vibrational component, vary significantly, indicative of significant entropic effects on the stability of the system, see Table 3. The large entropic component of the free energy also explains the restructuring behavior of the cluster at high temperatures. The largest entropy change is noted in Au32Pd6-4H/2Oad-TiO2-x due to strong bonding interactions between H and Au/ Pd. Table 3. Adsorption enthalpies (∆Hads, kJ/mol), decomposition of entropies (∆Strans, ∆Srot, ∆Svib, J/mol/K) and total adsorption entropies (∆Sads, J/mol/K), -T∆Sads (kJ/mol/K), and adsorption free energies (∆Gads, kJ/mol) for Au32Pd6/TiO2-x, Au32Pd6-4H/TiO2-x, Au32Pd6/TiO2, Au32Pd6/2Oad-TiO2-x, and Au32Pd6-4H/2Oad-TiO2-x at 700 K

∆Hads ∆Strans ∆Srot ∆Svib ∆Sads -T∆Sads ∆Gads

Au32Pd6/ TiO2-x -499 -184 -201 -17 -402 282 -217

Au32Pd6-4H/ TiO2-x -450 -187 -212 -1 -400 280 -170

Au32Pd6/ TiO2 -233 -184 -201 93 -292 204 -28

Au32Pd6/ 2Oad-TiO2-x -493 -184 -201 129 -256 179 -314

Au32Pd6-4H/ 2Oad-TiO2-x -492 -187 -212 -105 -505 353 -138 13


ACS Nano

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CONCLUSIONS The structure and electronic properties of TiO2 supported Au-Pd nanoalloys are dynamic and change with H adsorption, redox conditions of support, and temperature. At 0 K in the gas phase the Pd6@Au32 core-shell structure is found to be the most stable, with positively charged Pd core atoms. At elevated temperatures entropy drives Pd atoms to migrate to the outer surface of the cluster, more so when adsorbed H atoms are present or the cluster is supported on titania. For supported clusters, we observe charge transfer from the reduced support to the Au-Pd cluster, or from the cluster to the oxidized support. As a result, there are more Pd-Ti and Au-Ti contacts between the negatively charged cluster and the reduced support, whereas there are more Au-O bonds between the positively charged cluster and the oxidized surface. Electron density differences and free energy calculations show that interactions between the Au-Pd cluster and the stoichiometric surface are the weakest due to the relatively small charge transfer between the cluster and the support. On non-stoichiometric surfaces, the Au-Pd binding is stronger on the oxidized surface than on the reduced one. Adsorption of H atoms are adsorbed on the surface of the Au-Pd cluster also reduces the binding of the cluster to the support. These factors influence the redox properties, typified by the work function of the nanocluster, and presumably sintering as well. Understanding the relation between conditions and structure is crucial to catalytic processes because Pd atoms on the surface serve as active sites. Here, we have shown that the location and distribution of Pd is highly dependent on both the adsorbate and redox state of the support.

METHODS Calculations were performed using spin-polarized Kohn-Sham density functional theory (DFT). We used the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE)111 exchange correlation functional as implemented in CP2K package.112 The core electrons were described by Goedecker-Teter-Hutter pseudopotentials113,

114

, the double-ζ

Gaussian basis sets were used for the valence electrons (4s24p64d10 for Pd, 5d106s1 for Au, 14


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ACS Nano

3s23p63d24s2 for Ti, and 2s22p4 for O),115 and an auxiliary plane wave basis with a cutoff116 set to 350 Ry was used for computing the electrostatic terms. All simulations were carried out using

Γ-point sampling. To correct for strong electron correlation, the DFT+U method117 was used with U applied to the Ti 3d electrons within a local spin density approximation to enhance electron localization. The U value of 13.6 eV was assigned to Ti, which was found to adequately reproduce the work function, W = 5.1 eV,118 and location of defect state at ∼0.9 eV below the conduction band.119 The details of generating the U value were discussed in our previous work.107 A 7 × 3 rutile TiO2(110) periodic slab with four O-Ti-O tri-layers was utilized to model the TiO2 (110) surface, where only the bottom Ti layer was fixed to the bulk lattice positions and all other layers were allowed to relax. This large slab model was chosen to make sure the space between the adsorbed cluster and its image is large enough (> 7 Å in xy-plane) to avoid cluster-cluster interactions during the simulation. In addition, a 30 Å vacuum space was used between slabs to ensure minimal interference between slabs in the z direction. The 38-atom truncated octahedron Au32Pd6 cluster was chosen as a model of the Au-Pd nanoalloy because of its typical structure and ratio between the two atoms. Ab initio molecular dynamics (AIMD) simulations in the canonical ensemble (NVT) using a Nose-Hoover thermostat120-121 were performed at 700 K for 40-50 ps with a 1 fs time step. All simulations started with the lowest energy structure of Au32Pd6 to investigate the structural changes of gas phase and supported clusters. Simulations ran for a duration of ~50 ps. At the end of each trajectory, the last configuration was quenched to a temperature of 0 K to obtain final structures under different conditions. For AIMD analysis, only equilibrated data of the last 25-30 ps were considered. Charge analysis was done using the Bader atom-in-molecule approach.122, 123 In addition, we calculated the adsorption free energy as follows: ∆

!"

∆#

!"

$∆%

!"

where Hads and Sads are the adsorption enthalpy and entropy, respectively, and T is the temperature. ∆Hads and ∆Sads were obtained by subtracting the values of the support and the 15


ACS Nano

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nanoalloy from that of the total system, where the deconvoluted adsorption entropy and enthalpy were calculated using: ∆#

∆%

!"

!"

∆%&'

("

∆./01 2 + ∆3&'

+ ∆%'*& + ∆%+,("

+ ∆3'*& + ∆3+,- 4$

with ∆./01 2 being the energy of the optimized structure at 0 K. Vibrational entropy (Svib) was calculated by employing a quasi-harmonic approximation G

%+,- 367 8 9 : ;

Structural Rearrangement of AuâPd Nanoparticles under Reaction

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