Density Functional Study of Trimetallic AuxPdyPtz (x + y + z = 7

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Density Functional Study of Trimetallic AuPdPt (x+y+z=7) Clusters and Their Interactions with the O Molecule 2

Shuang Zhao, Bo Zhao, XinZhe Tian, Yun-Lai Ren, Kaisheng Yao, JianJi Wang, JunNa Liu, and Yunli Ren J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b04411 • Publication Date (Web): 21 Jun 2017 Downloaded from http://pubs.acs.org on June 28, 2017

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The Journal of Physical Chemistry

Density Functional Study of Trimetallic AuxPdyPtz (x+y+z=7) Clusters and Their Interactions with the O2 Molecule Shuang Zhao, Bo Zhao, XinZhe Tian, YunLai Ren, KaiSheng Yao, JianJi Wang,* JunNa Liu, and YunLi Ren School of Chemical Engineering and Pharmaceut, Henan University of Science and Technology, Luoyang, Henan 471003, PR China.

ABSTRACT: Density functional theory calculations were performed to investigate the structural and energetic properties of trimetallic AuxPdyPtz clusters with x+y+z=7. The possible stable geometrical configurations with their electronic states are determined. We analyze the chemical order, binding energies, vertical ionization potential, electron affinity and HOMO-LUMO gaps as a function of the whole concentration range. The affinity of AuxPdyPtz clusters towards one O2 molecule is also evaluated in terms of the changes in geometry, adsorption energy and charge transfer.

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Author to whom correspondence should be addressed. Electronic mail: [email protected]

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1. INTRODUCTION Although its bulk material is chemically inert, nanosized gold cluster is a good catalyst for many reactions. The aberration-corrected scanning transmission electron microscopy demonstrated that the catalytic activity of gold clusters supported on an iron oxide correlates with the presence of very small clusters of ~10 atoms.1 In the last few years, there have been limited reports but rapidly growing interest in Au-based trimetallic clusters and nanoparticles, due to their superior catalytic activity and selectivity than monometallic and bimetallic Au catalysts in oxidation reactions.2-8 The activity of the trimetallic Au-Pt-Ag nanoparticles with an average diameter of 1.5nm is several times higher than that of Au nanoparticles with nearly the same particle size in aerobic glucose oxidation.9 The trimetallic Au-Cu-Sn catalyst towards hydrazine oxidation shows enhanced catalytic activity compared with their binary counterpart Au-Cu.10 The addition of Pt in supported nanosized Au-Pd clusters can significantly enhance the selectivity of benzaldehyde in the oxidation of benzyl alcohol, while still maintaining a high alcohol conversion.11

The structural and electronic properties of metal clusters are believed to be an important link in understanding the fundamental mechanism of catalysis. Since unambiguously determining the structural and electronic properties of a small atomic cluster is an almost impossible task for experiments alone, theoretical calculations based on solutions to Schrödinger equation have been conducted to examine the physical and chemical properties of small clusters. In the last two decades, the 2


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Au-based bimetallic clusters, such as Au-Ag,12-15 Au-Cu,16-18 Au-Pt19-20 and Au-Pd21-22 have been extensively investigated by theoretical calculations. It has been revealed that the geometry, energetics, stability and reactivity of these bimetallic clusters strongly depend on the cluster size, composition and the charge state. For Au-based trimetallic clusters, the synergic interaction between three different metallic elements introduces a mutual influence on neighboring atoms and leads to distinct properties from monometallic or bimetallic clusters. In a recent work, Zhang et al. demonstrated a first-principles investigation of chirality in magnetic AunAlMn (n=1-7) clusters with neutral and negative and positive charge states.23 Chirality turns up with the forms of left-handed and right-handed in stable Au5AlMn, Au6AlMn, and Au7AlMn clusters. Wu et al. reported the geometrical structures of Au-Pd-Pt clusters with 13, 34, 60 and 75 atoms using global optimization algorithm.24 The isosahedron and decahedron were found to be dominant motifs in these clusters. So far, theoretical investigations of Au-based trimetallic clusters are scarce and remain challenging.

The adsorption and dissociation of oxygen molecule on Au-based catalysts is also an intense interest because of their relevance in oxidation catalysis. The negatively charged Au atoms in Au-Pt-Ag particles could activate the molecular oxygen by donating excess charges to the antibonding orbital, and the resulting superoxo- or peroxo-like oxygen promotes glucose oxidation.9 Molecular oxygen usually prefers binding to Cu sites in bimetallic AumCun clusters (m=1, 2, 3; 1≤n≤6) and the dissociation of O2 molecule is observed in AuCu3, AuCu4 and AuCu6 clusters.17 Ajay 3



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et al. reported a B3LYP density functional theory (DFT) analysis of O2 adsorption on 27 bimetallic AunMm (m, n=0-3 and m+n=2 or 3; M=Cu, Ag, Pd, Pt and Na) clusters.25 The alloy trimers containing only one Au atom are most reactive toward O2 while those with two Au atoms are least reactive, which is related to the ensemble effect and coulomb interactions.25 However, theoretical studies have mainly focused on the adsorption behaviors of O2 on pure Au and Au-based bimetallic clusters.17, 25-31 To the best of our knowledge, there has been no theoretical report on the interactions of O2 with Au-based trimetallic clusters. In fact, Au-based trimetallic clusters have been much less explored than pure and binary-alloy Au clusters.

In this letter, we focus on the trimetallic Au-Pd-Pt system and present results of DFT calculations on the geometric and electronic structures of AuxPdyPtz (x+y+z=7) clusters. Ternary diagrams have been introduced to better display the variation of the calculated properties in the whole range of composition. Furthermore, the interactions of molecule oxygen with AuxPdyPtz clusters have also been investigated. The article is organized as follows. Section 2 briefly describes the computational methods used in this work. In Section 3 we present the low-energy structures and discuss the geometries, energetics, stabilities and adsorption behaviors of O2 on trimetallic AuxPdyPtz clusters. Finally, the conclusion is drawn in Section 4.

2. COMPUTATIONAL METHODS The calculations were carried out using GAUSSIAN 09 package.32 The 4


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Stuttgart-Dresden effective core potential (ECP) plus DZ basis set33 augmented with the f polarization functions34 were used for Au (f exponent, 1.05), Pd (f exponent, 1.472) and Pt (f exponent, 0.993) atoms. The Dunning’s correlation-consistent basis set (aug-cc-pVTZ)35 was used on O atoms. To examine the effect of the choice of the exchange-correlation functional, we have calculated the spectroscopic parameters of Au2, Pd2, Pt2 and O2 by using different functionals: PW91PW9136 and TPSSTPSS37 functionals within the generalized gradient approximation (GGA), B3LYP38-39 and B3PW9136,

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methods as hybrid functionals. Table 1 displays the calculated and

experimental values of the bond lengths, vibrational frequencies and binding energies (Eb) for diatomic molecules. In our calculations, the hybrid B3LYP and B3PW91 functional overestimate the bond length and underestimate the binding energy of Au2, Pd2 and Pt2 significantly. On the other hand, PW91PW91 and TPSSTPSS functional have resulted in nearly the same binding energies which are reasonable for Au2 and Pd2. They have also calculated reasonably good bond length and frequency for all the dimers. However, TPSSTPSS has calculated a much better Eb of Pt2 than PW91PW91. Therefore, we have chosen the TPSSTPSS method to study AuxPdyPtz clusters. In order to obtain the lowest-energy structures of pure clusters, apart from starting with configurations as reported previously,20-21, 40-42 few other unbiased initial configurations were also considered. To search the lowest-energy isomers of binary clusters, 424 of initial geometries, including two- and three-dimensional configurations, had been taken into account in our geometry optimizations. For trimetallic AuxPdyPtz clusters, the search for optimal geometries were restricted to the 5



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combination of another element into the most stable bimetallic clusters and the less stable isomers whose energy do not exceed that the most stable bimetallic ones by more than 0.1eV (228 initial configurations were considered). The reason for not employing the global geometry optimization for ternary cluster is two fold: (1) our primary objective is to investigate the influence of two dopants in one Au-based trimetallic cluster on the physical and chemical properties of the cluster; (2) although for x+y+z=7, it is now possible to employ the global optimization but for the whole series of clusters, it is computationally too expensive. Geometry optimization for the minimal-energy configurations was implemented with all degrees of freedom and without any symmetry restriction. Spin-polarized calculations have been done for the first four multiplicities of each initial configuration. Thus, singlet, triplet, quintet and septet states have been worked out for those having even number of electrons, doublet, quartet, sextet, octet states have been worded out for those having odd number of electrons. All calculations were performed with (99,590) pruned grid (ultrafine grid as defined in Gaussian 09). Natural bond orbital (NBO)43 analysis was used to provide the natural

charge

distribution.

Vibrational

frequency

calculations

including

thermochemical analysis were carried out at 298.15K and 1 atmosphere of pressure. These frequency calculations also guarantee the optimized structures locating the minima, not as transition structures.

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3. RESULTS AND DISCUSSION In Fig. 1, we show the most stable structures obtained by TPSSTPSS calculations as a function of the whole concentration range in a ternary diagram (more structures can be seen in the Supporting Information for the conciseness of the text). In the triangle vertices are the pure one-element clusters, triangle edges describe the binary ones, while clusters inside the triangle correspond to the ternary systems. When moving apart from a given vertex, each parallel line contains clusters with one atom less of the constituent element of the pure cluster located at that vertex.44 Such ternary diagrams have been successfully applied in the theoretical investigation of trimetallic FexCoyNiz clusters (x+y+z=5, 6, 7 and 13).44-46

3.1. Pure Au7, Pd7 and Pt7 Clusters The most stable Au7 can be formed by addition of an Au atom on an edge of triangular Au6 with Cs symmetry. Fernandze et al. have obtained planar structures of Aun clusters up to 11 atoms.40 The preference of two-dimensional configurations of small Aun clusters can be attributed to the enhanced s-d electron hybridization caused by strong relativistic effects. Concerning spin multiplicity, unlike Pd or Pt atom, the closed 5d shell and the sharable 6s electron of Au result in low spin multiplicities for Au7 (doublet). The ground state of Pd7 is a distorted pentagonal bipyramid of C2 symmetry and adopt triplet spin multiplicity, in consistent with previous DFT21 and molecular dynamic41 studies. The lowest energy structure of Pt7 is a side capped

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double square with an average bond distance of 2.678Å and has quintet state. Chen et al.20 and Heredia et al.42 have also obtained similar geometry for Pt7.

3.2. Bimetallic Au-Pd, Au-Pt and Pd-Pt Clusters The multiplicities of the most stable Au-Pd clusters are given in Table 2. The clusters enriched in Au atoms maintain the lowest possible spin multiplicity (singlet or doublet) while clusters enriched in Pd atoms have higher spin multiplicities (triplet or quarter). Au6Pd is a planar hexagon with the Pd atoms located at the center site and has D6h symmetry. Starting from Au5Pd2, most stable structures adopt a three-dimensional configuration. It is found that the Au atoms prefer exposed positions which are lower coordinated, while the Pd atoms prefer the inner positions which are higher coordinated. Similar behavior has also been found in AumPdn (m+n=2-6) clusters, where the Pd atoms are preferred to connecting with each other to keep a maximum number of Pd-Pd bonds.22

Similar to Au6Pd, Au6Pt is also a planar hexagon-ring with a centered Pt atom and has doublet electronic state. Higher spin multiplicities are preferred with more than two Pt atoms. The Au-Pt distance in Au6Pt is 2.696Å, slightly shorter than the same D6h structure reported by Yuan et al.20 Except for Au6Pt, other bimetallic Au-Pt clusters favor three dimensional structures. The Au atoms occupy exposed positions while the Pt atoms form a higher number of bonds, which is in agreement with previous studies of AunPt (n=1-12)19 and Aun-xPtx (n=2-14)20 clusters. 8


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For bimetallic Pd-Pt clusters with seven atoms, most of them have never been reported in the literature. The Pd-Pt heptamers present quintet spin states, with the exception of Pd2Pt5, where septet is preferred. The Pt atoms tend to occupy the outer sites which are lower coordinated while Pd atoms take the sites which are higher coordinated. However, our results are different with the previous simulation on (PtPd)n (n=23-28) nanoclusters using Gupta many-body potential combined with genetic-symbiotic algorithm, where the Pt atoms occupy the cluster core and the Pd atoms are situated on the cluster surface.47 This discrepancy may be attributed to that the structural and electronic properties of metal clusters strongly depend on the cluster size and the level of theory used to calculate the potential energy surfaces.

3.3. Trimetallic AuxPdyPtz (x+y+z=7) Clusters As seen from Fig. 1, all the trimetallic AuxPdyPtz clusters are demonstrated to be three-dimensional. The spin multiplicities of the clusters tend to increase as the number of Au atoms decreases. When x=2 and 3, the Au atoms are separated from each other and in capped positions around a compact Pd/Pt core. The general trend confirmed for the low energy isomers of ternary AuxPdyPtz is that the Au atoms favor the exposed positions with the lower coordination number while the Pd atoms favor the center positions with the higher coordination number. The covalent radius of atomic Au, Pd and Pt is 1.34, 1.28 and 1.30Å, respectively.48 The Au atom prefers to

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occupy an exposed position due to its larger covalent radius than Pd and Pt so as to easily reduce geometrical relaxation.

In Fig. 2, we depict the average interatomic distances of the cluster as a function of their composition. Bottom left and right vertices correspond to pure Au7 and Pt7 clusters, respectively, and top vertex indicates species Pd7. As seen from Fig. 2, the shortest interatomic distances are found in the neighborhood of the Pd-Pt bimetallic alloys, and in high Pt concentration region, while the large interatomic distances are located at the Au rich corner. A clear increase in the interatomic distance is observed for the alloys with more than four Au atoms. The largest interatomic distance of AuxPdyPtz clusters occurs at x=5, which is due to that the Au-Au distances get larger when the structures shift from two-dimensional (x=6) to three-dimensional (x=5).

3.4. Binding Energy per Atom The binding energy per atom of AuxPdyPtz cluster with size of x+y+z=7 is calculated as follows: E b(Au x Pd y Ptz ) =

xE Au + yE Pd + zE Pt − E Au x Pd y Pt z The calculated x +y +z

binding energy per atom of the AuxPdyPtz clusters is listed in Table 2. Generally speaking, the binding energy per atom of a given cluster is a measurement of its total thermodynamic stability. Concerning the pure clusters, binding energy is larger for Pt7 (3.11eV) than Au7 (1.79eV) and Pd7 (2.11eV). For bimetallic Au-Pd and Au-Pt clusters, the binding energy increases monotonically with increasing Pd/Pt concentration, and the introduction of Pt atom to the Pd matrix also increases the Eb. 10


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In Fig. 3, we show the binding energies a function of the whole cluster composition. It is notice that the binding energies are smaller for the binary Au-Pd region, that is, when the Pd composition is reduced. The binding energy of AuxPdyPtz clusters is mainly controlled by the number of Pt atoms. Ternary clusters with more Pt concentration exhibit larger Eb and have similar Eb values with the same number of Pt. For example, the binding energies of Au3PdPt3, Au2Pd2Pt3 and AuPd3Pt3 have similar values of 2.46, 2.52 and 2.55eV, respectively. The larger Eb indicates that relatively higher stability is obtained when heterogeneous Au-Pt and Pd-Pt bonds replace Au-Au and Pd-Pd bonds in AuxPdyPtz clusters.

3.5. Chemical Order In order to understand the mutual influence of structure, chemical orders (segregation or mixing) of AuxPdyPtz clusters as a function of the relative composition, were investigated for all configurations. The chemical order parameter, σ, for multi component finite size clusters can be defined with the following characteristics: positive when phase separation (segregation) takes place, negative when mixing is present, i.e. a positive value indicates that the homo-atomic pairs are more favorable while a negative value indicates that the hetero-atomic pairs are more prominent. A value close to zero indicates that transition from segregation to mixing phase is present in the systems. The chemical order parameter (σ) is calculated as:

σ =

N Au − Au + N Pd − Pd + N Pt − Pt − N Au − Pd − N Au − Pt − N Pd − Pt N Au − Au + N Pd − Pd + N Pt − Pt + N Au − Pd + N Au − Pt + N Pd − Pt 11


(1)


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where NA-B stands for the number of nearest neighbors A-B bonds (see the pair distribution columns on Table 2). The order parameter has been proved to be useful in describing the short-range order interactions present in metal clusters and surfaces, and it was introduced by Ducastelle.49

The diagram of the chemical order obtained by Eq (1) is shown in Fig. 4. Pure Au7, Pd7 and Pt7 clusters show a tendency of segregation (σ=1) as expected. Bimetallic Au-Pd and Au-Pt also show segregation at low concentration of Au, while in the dilute limit of Pd and Pt, show zero order parameter, indicating the tendency of mixing accompanied by disorder. The AuPdPt5 cluster also gives a zero order parameter. The transition from segregation to mixing occurs when the composition of the pure metal decreases to a mole fraction of about 0.625. The mixing phase (negative σ) is located mainly in the ternary region, which indicates the tendency for mixing in ternary clusters, and that signifies more stability with more heterogeneous bonds.

3.6. Electronic Properties For the neutral clusters with optimal geometry, the vertical ionization potential (VIP) was calculated as the energy difference between the electronic ground state of the neutral cluster with optimal geometry and the electronic ground state of cation with the geometry of the former. The vertical electron affinity (VEA) is defined as the energy difference between the electronic ground state of the neutral with optimal 12


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geometry and the electronic ground state of anion with the geometry of this neutral. A neutral cluster with higher VIP means that it is more difficult for the cluster to lose electrons. A higher VEA means that more energy is released when an electron is added to a neutral cluster and the production of the corresponding anion is more readily accomplished. The VIP and VEA of the AuxPdyPtz clusters are represented in Fig. 5(a) and Fig. 5(b), respectively.

The values of VIP for pure systems are expected to increase as the number of electrons grows. We notice in Fig. 5(a) that the smallest VIP is located at the Pd corner. Also low values of the VIP are observed midway along the Pd-Pt edge, and in nearby sites. The largest values of the VIP were found to be in the Au rich region. It can be observed that in the Au-Pd edge, the VIP generally increases as the number of Au atoms increases. The calculated VEA is 3.04, 1.92 and 2.71eV for Au7, Pd7 and Pt7, respectively. The clusters with high number of Pd atoms exhibit smaller VEA (see Fig. 5(b)). A clear increase in VEA with increasing Pt composition is observed along the Pd-Pt edge. The larger values of the VEA are located at the Pt corner, and also along the Au-Pd edge in the Au rich region. The values of the VIPs and VEAs, for all possible x, y and z combinations, are given in Table 2.

The energy gaps (Egap) was calculated as the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The HOMO-LUMO energy gaps characterize the ability of an electron to 13



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jump from an occupied orbital to an unoccupied orbital. A large Egap often corresponds to higher chemical stability, that is to say, a large Egap indicates weaker chemical reactivity. The bimetallic Au-Pd clusters containing even number of Pd atoms exhibit larger Egap than their neighbors containing odd number of Pd atoms, while the bimetallic Au-Pt clusters with even number of Au atoms exhibit larger Egap (see Table 2). Fig. 6 illustrates the ternary diagrams of Egap for all the AuxPdyPtz clusters. The larger Egap are located in the ternary region along the midway of Au-Pd edge, which reaffirm that the clusters are stable with respect to alloying. However, the Egaps is not correlated at all with the binding energies (see Fig. 3).

3.7. Interaction of O2 with AuxPdyPtz Clusters The studies of the adsorption behavior of O2 on metal clusters can help in understanding the mechanism of catalytic oxidations. On the basis of the structures of bare clusters, we optimized the structures of the AuxPdyPtz-O2 complex. The structures of the most stable AuxPdyPtz-O2 complexes obtained are displayed in Fig. 7. Their spin multiplicities are 3 or 5 for the cases of even number of electrons and are 2 or 4 for the cases of odd number of electrons (see Table 3). The structures of some clusters are changed substantially upon adsorption of O2. For example, the O2 adsorption drives the transformation of bimetallic Pd5Pt2 framework from the C2 face-capped trigonal bipyramid to a pentagonal bipyramid. Several possible coordination modes for O2 adsorption on metal have been proposed: the Griffiths model50 where two oxygen atoms are bonded to the same metal atom and leads to formation of a O-M-O 14


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triangle, the Pauling model51 where one O atom is connected to one metal atom through a single M-O-O bond, and the Yeager model52 where the two O atoms are attached to two neighboring metal atoms and form a M-M-O-O four-membered ring. Our calculations yielded 1-fold Pauling model for O2 adsorption on Au-rich Au6Pd, Au6Pt and Au5Pd2 clusters. Griffiths model is the most favored for Au5PdPt and AuxPdyPtz clusters with x=4, while other clusters all prefer the 2-fold Yeager model. Concerning the type of atoms to which the O2 is attached, an easily recognized, noticeable feature is that the precedence order follows the trend Pt > Pd >Au. The O2 prefers binding to Pd/Pt site of the Au-Pd/Pt bimetallic clusters, when both Au and Pd/Pt sites co-exists. For Pd-Pt bimetallic clusters, the O2 adsorption predominantly occurs on Pt sites if both Pd and Pt atoms are available. It is also worth noting that the O2 is oriented along the Pt-Pt bond forming the Yeager configuration for the ternary AuxPdyPtz clusters with z≥3.

The calculated bond length and stretch frequency of free O2 is 1.222Å and 1561cm-1, in good agreement with the experimental values of 1.208 Å53 and 1580cm-1.54 Substantial red shifts of O-O frequency and increase of O-O bond can be observed upon O2 adsorption on AuxPdyPtz clusters. Pauling adsorption mode structures show a O-O bond elongation in the range of 1.251-1.269 Å (2.3-3.7% elongation). In Griffith adsorption structures, the O-O distance is extended to a range of 1.296-1.332 Å (5.7-8.3% increase). The O-O distances in Yeager adsorption structures range from 1.33-1.454 Å (8.1-16% elongation), which falls close to the NEXAFS range of 1.37± 15



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0.05Å for the superoxo species,55 indicating that the O2 adsorbed with Yeager configurations is catalytically activated. The O-O distances as a function of cluster composition are illustrated in Fig. 8. It can clearly be seen that the largest elongation of O-O distances are obtained in the ternary region with higher Pt concentration, but not for pure or binary clusters. Specifically, the largest O-O distance is found for Au2Pd3Pt2 (1.454Å) and Au2PdPt4 (1.445Å) clusters. These results suggest that ternary AuxPdyPtz clusters could be better candidates to activate the O2 molecule than pure or binary clusters.

In the molecular adsorption, the O-O bond is lengthened and activated by electron transfer from the metal cluster to the anti-bond orbital of O2. Generally, the greater the charge transfer from the metal to O2 is, the longer is the O-O bond distance and the larger is the red shift of O-O frequency. The ternary diagrams of charge transfer to O2 and the red shift of O-O frequency are displayed in Fig. 9 and Fig. 10, respectively. As expected, the largest charge transfer and red-shifts are located mainly in the ternary region with high number of Pt atoms, consistent with the activation of O-O bond upon adsorption (Fig. 8). Ternary AuxPdyPtz clusters, which donate more electrons to O2 than binary and pure clusters, are more active towards activation of O2 by lengthening the O-O bond. Additionally, we found an approximate linear correlation between the elongation of the O-O bond distance and the NBO charges on adsorbed O2 (R2=0.79, Fig. 11). The red shifts in the O-O stretching frequency upon adsorption also correlate linearly with the elongation of the O-O bond (R2=0.98, Fig. 16


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12) and the charge transfer from metal to adsorbed O2.

The adsorption energy of O2 is defined by the follow equation:

AE = E Au x Pd y Pt z + E O − E Au x Pd y Pt z − O 2

2

Where E Au x Pd y Pt z and E Au x Pd y Pt z − O 2 are the total energies of the bare cluster and the complex cluster, respectively. The more positive the AE is, the stronger the bond is. All adsorption energies were corrected with basis set superposition error (BSSE) estimated by using the counterpoise corrections method.56 The variations of adsorption energies as a function of cluster concentration are presented in a ternary diagram in Fig. 13. The smallest AEs are observed in the Au rich corner, while the largest AEs reside in the neighborhood of the Pt corner, and also along the Pd-Pt edge. From the Au vertex to Pd-Pt edge, a gradual increase in the AEs can be observed as the Au composition decreases. The clusters with a low number of Au atoms and high number of Pd/Pt atoms exhibit stronger adsorption. In fact, we found a decent linear correlation between the O2 AE and the charge transfer from metal cluster to O2 (R2=0.63, Fig. 14) by fitting the entire data set: generally the more the charge transfer to O2, the higher the adsorption energy. The correlation is much better for Griffiths adsorption (individual R2=0.94) than Yeager adsorption (individual R2=0.37, exclude the data point for Au7-O2) and is better in the low AE region than in the high AE region. The lack of very precise correlation maybe attributed to that it is difficult to separate the AE contributions of charge transfer to O2, adsorption configuration,

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coulomb attraction, and extent of overlap of relevant orbitals, since all these factors are interrelated in a complex manner and operate simultaneously.25

4. CONCLUSION We have calculated the stability, geometric and electronic properties of trimetallic AuxPdyPtz (x + y + z = 7) clusters with the density functional method at the TPSSTPSS/SDD level of theory. In AuxPdyPtz clusters a general tendency is that the Au atoms tend to take the exposed positions which are lower coordinated while the Pd atoms prefer the center positions which are higher coordinated. The clusters with high concentration of Pt and low concentration of Au exhibit larger binding energy and smaller interatomic distance. The clusters with high Pd concentration show smaller vertical ionization potential and electron affinity. Analysis based on chemical order parameter indicates that ternary AuxPdyPtz clusters prefer mixing rather than segregation.

Motivated by the experimentally observed higher catalytic reactivity of Au-based trimetallic alloys towards selective oxidation catalysis, we also investigated the interaction of O2 molecule with AuxPdyPtz clusters. The precedence of the atom to which the O2 is attached follows the order Pt > Pd >Au. The O-O axis is oriented along the Pt-Pt bond of AuxPdyPtz when z≥3. The adsorption energy increases with decreasing Au composition and correlates approximately with the electron transfer from metal cluster to O2. The largest elongation of O-O distance is obtained in the 18


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ternary regions with higher number of Pt atoms (especially for Au2Pd2Pt3 and Au2PdPt4 species), indicating trimetallic clusters could be better candidates to weaken and activate the O-O bond, than the corresponding pure or binary clusters.

Undoubtedly, the conclusions above have been extracted from a series of model systems, further theoretical and experimental investigations are required. Nevertheless, some general tendencies can be derived. The doping of a gold cluster with two different atomic impurities can change the geometric and electronic structure of the clusters substantially, thereby modifying and controlling the catalytic activity. It is hoped that the present work of small trimetallic AuxPdyPtz clusters will serve as a starting point for further research of physical and chemical properties of ternary clusters, and tailoring more efficient and active ternary-alloy gold catalysts towards catalytic oxidation reactions.

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Supporting Information Geometries, spin multiplicities, electronic and zero-point energies of AuxPdyPtz clusters

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 21603060), the Program for Science＆Technology Innovation Talents in Universities of Henan Province (Grant No.15HASTIT004), the Henan Joint Funds of the National Natural Science Foundation of China (Grant No.U1504213), and Scientific Research Fund of Henan University of Science and Technology (Grant No.2015QN012).

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Figure Captions:

Figure 1 Optimized geometric structures of AuxPdyPtz (x+y+z=7) clusters as a function of the atomic composition. Yellow balls represent Au atoms, blue for Pd, and white for Pt. Numbers below each model of cluster geometry indicate the number of Au, Pd and Pt atoms in each cluster. Figure 2 Average distances in Å of AuxPdyPtz clusters (x+y+z=7) as a function of the atomic composition. Figure 3 Binding energy per atom in eV of AuxPdyPtz (x+y+z=7) clusters as a function of the atomic composition. Figure 4 Chemical order parameter σ defined by Eq. (1) of AuxPdyPtz (x+y+z=7) clusters as a function of the atomic composition. Figure 5 Vertical ionization potential in eV (a) and vertical electron affinity in eV (b) of AuxPdyPtz (x+y+z=7) clusters as a function of the atomic composition. Figure 6 HOMO-LUMO gap in eV of AuxPdyPtz (x+y+z=7) clusters as a function of the atomic composition. Figure 7 Optimized geometric structures of AuxPdyPtz-O2 (x+y+z=7) clusters as a function of the atomic composition. Yellow balls represent Au atoms, blue for Pd, white for Pt, and red for O. Numbers below each model of cluster geometry indicate the number of Au, Pd and Pt atoms in each cluster. Figure 8 The calculated O-O distance in Å of AuxPdyPtz-O2 (x+y+z=7) clusters as a function of the atomic composition. Figure 9 The charge transfer (in e) from AuxPdyPtz clusters to O2 as a function of the atomic composition. Figure 10 The red-shifts (in cm-1) in O-O stretching frequencies as a function of the atomic composition. Figure 11 Approximation correlation between the increase in the O-O bond length (in Å) and the NBO charge (in e) on adsorbed O2. 30


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Figure 12 Linear correlation between the increase in the O-O bond length (in Å) and the red-shifts in O-O stretching frequency (in cm-1). Figure 13 The O2 adsorption energy in eV of AuxPdyPtz-O2 (x+y+z=7) clusters as a function of the atomic composition. Figure 14 Approximate correlation between the O2 adsorption energy (in eV) and the charge transfer (in e) to the adsorbed O2 molecule.

31



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Page 32 of 49

FIG. 1

070

160

250

340

430

520

610

700

151

052

241

142

331

421

511

601

061

232

322

412

502

043

133

034

223

313

403

124

214

304

025

115

205

32


016

106

007

Page 33 of 49

FIG. 2

0.00

Pd

2.585 2.608

1.00

2.632 2.655 2.679

0.25

2.702

0.75

2.726 2.749

0.50

t

Au -P d

2.773

0.50

-P Pd

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.75

0.25

1.00

Au 0.00

0.00 0.25

0.50

0.75

1.00

Pt

Au-Pt

33



FIG. 3

0.00

Pd

1.785 1.951

1.00

2.118 2.284 2.450

0.25

2.616

0.75

2.782 2.949

Au

0.50

0.75

-Pt

-Pd

3.115

0.50

Pd

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.25

1.00

Au 0.00

0.00 0.25

0.50

0.75

1.00

Pt

Au-Pt

34


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Page 35 of 49

FIG. 4

Pd 0.00

-0.715 -0.501

1.00

-0.286 -0.072 0.142

0.25

0.357

0.75

0.571 0.786

Au -P

d

1.000

0.50

0.50

0.75

t -P Pd

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.25

1.00

Au 0.00

0.00 0.25

0.50

0.75

1.00

Pt

Au-Pt

35



FIG. 5

0.00

(a)

Pd

6.240 6.439

1.00

6.638 6.836 7.035

0.25

7.234

0.75

7.433 7.631

Au

0.50

0.75

-P t

-P

d

7.830

0.50

Pd 0.25

1.00

0.00

Au 0.00

0.25

0.50

0.75

1.00

Pt

Au-Pt

0.00

(b)

Pd

1.915

1.00

2.056 2.196 2.337 2.478

0.25

2.618

0.75

2.759 2.899

0.50

t

Au

-P

d

3.040

0.50

-P Pd

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.75

0.25

1.00

Au 0.00

0.00 0.25

0.50

0.75

1.00

Pt

Au-Pt

36


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FIG. 6

0.00

Pd

0.120 0.395

1.00

0.637 0.864 1.090

0.25

1.316

0.75

1.543 1.769

0.50

0.75

-Pt

Au -P

t

1.995

0.50

Pd

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.25

1.00

Au 0.00

0.00 0.25

0.50

0.75

1.00

Pt

Au-Pt

37



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 49

FIG. 7

070

160

061

250

340

430

520

610

700

421

142

232

322

412

502

052

241

331

511

601

151

043

133

223

313

403

034

124

214

304

025

115

205

38


016

106

007

Page 39 of 49

FIG. 8

Pd

1.250 1.276

0.001.00

1.301 1.327 1.352

0.25

1.377

0.75

1.403 1.429

0.50

0.50

t

Au -P

d

1.454

-P Pd

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.75

1.00 Au 0.00

0.25

0.25

0.50

0.00 1.00

0.75

Pt

Au-Pt

39



FIG. 9

0.00

Pd

-0.526 -0.474

1.00

-0.422 -0.371 -0.319

0.25

-0.267

0.75

-0.216 -0.164

0.50

t

Au

-Pd

-0.101

0.50

-P Pd

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.75

0.25

1.00

Au 0.00

0.00 0.25

0.50

0.75

1.00

Pt

Au-Pt

40


Page 40 of 49

Page 41 of 49

FIG. 10

0.00

212

Pd

300

1.00

388 475 563

0.25

651

0.75

739 826

0.50

t

Au

-P

d

946

0.50

-P Pd

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.75

0.25

1.00

Au

0.00

0.00 0.25

0.50

0.75

1.00

Pt

Au-Pt

41



FIG. 11

0.22

Increase in the O-O Bond Length

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.20 0.18 0.16

R2=0.79

0.14 0.12 0.10 0.08 0.06 0.04

-0.55 -0.50 -0.45 -0.40 -0.35 -0.30 -0.25 -0.20 -0.15 -0.10

NBO charge on Adsorbed O2

42


Page 42 of 49

Page 43 of 49

FIG. 12

1000

Shift in the O-O Stretching Frequency

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


900 800 700 600

R2=0.98

500 400 300 200 0.05

0.10

0.15

0.20

Increase in the O-O Bond Length

43



FIG. 13

0.00

Pd

0.16 0.39

1.00

0.63 0.87 1.11

0.25

1.35

0.75

1.59 1.83

Au

0.50

0.75

- Pt

- Pd

2.06

0.50

Pd

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 49

0.25

1.00

Au

0.00

0.00 0.25

0.50

0.75

1.00

Pt

Au-Pt

44


Page 45 of 49

FIG. 14

2.2

R2=0.63

2.0 1.8

Adsorption Energy of O2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.6 1.4 1.2 1.0 0.8 0.6

Pauling Adsorption

0.4

Griffiths Adsorption Yeager Adsorption

0.2 -0.55

-0.50

-0.45

-0.40

-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

NBO Charge on Adsorbed O2

45



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 49

Table 1. Comparison of the calculated and experimental bond length (R in A), vibrational frequency (Freq in cm-1), and binding energy (Eb in eV) of Au2, Pd2, Pt2 and O2. R

Freq

Eb

Species

Au2

Pt2

Pd2

O2

Au2

Pt2

Pd2

O2

Au2

Pt2

Pd2

B3LYP

2.571

2.365

2.518

1.209

164

228

200

1646

1.88

2.58

0.58

B3PW91

2.546

2.541

2.499

1.202

171

182

206

1686

1.92

2.14

0.60

PW91PW91

2.548

2.363

2.493

1.222

169

225

210

1561

2.21

4.33

1.42

TPSSTPSS

2.539

2.358

2.489

1.224

173

229

213

1556

2.17

3.33

1.45

Expt.

2.47354

2.3357

2.4854 1.20853

158054

2.2954

19154

22358 21059

46


3.1457 1.12±0.2460

Page 47 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Properties of AuxPdyPtz clusters with x+y+z=7. Numbers in the first column indicate the number of Au, Pd and Pt atoms in each cluster. The spin multiplicity (SM), number of nearest neighbor pairs (the order is Au-Au, Pd-Pd, Pt-Pt, Au-Pd, Au-Pt and Pd-Pt, respectively), chemical order (σ), binding energy per atom (Eb in eV), vertical ionization potential (VIP in eV), vertical electron affinity (VEA in eV) and HOMO-LUMO gaps (Egap) for clusters are presented in the table. Alloy

SM

N-N Pair

σ

Eb

VIP

VEA

Egap

700

2

110000

1

1.79

7.03

3.04

1.33

070

3

016000

1

2.11

6.24

1.92

0.56

007

5

001300

1

3.11

6.88

2.71

0.27

610

1

600600

0

1.93

7.83

2.45

1.18

520

2

5 1 0 10 0 0

-0.25

2.00

7.49

2.69

1.95

430

3

230900

-0.29

2.02

7.22

2.61

1.85

340

4

060800

-0.14

2.04

7.11

2.59

1.99

250

3

080700

0.07

2.08

6.70

2.13

0.79

160

4

0 12 0 3 0 0

0.60

2.11

6.66

2.17

1.04

601

1

600060

0

2.08

7.82

2.35

1.21

502

2

501080

-0.14

2.18

7.22

2.47

0.81

403

3

203090

-0.29

2.37

7.16

2.55

1.15

304

4

006060

0

2.56

6.96

2.52

0.77

205

5

009040

0.38

2.76

7.10

2.63

0.98

106

6

0 0 12 0 2 0

0.71

2.95

7.15

2.42

0.87

061

5

0 12 0 0 0 3

0.60

2.26

6.63

2.01

1.02

052

5

090006

0.20

2.43

6.74

2.08

0.89

043

5

052008

-0.07

2.58

6.68

2.12

0.79

034

5

024009

-0.20

2.71

6.70

2.31

0.55

025

7

015008

-0.14

2.85

7.14

2.70

1.65

016

5

009003

0.50

2.98

7.16

2.87

0.19

511

2

500551

-0.38

2.09

7.49

2.68

1.69

421

3

210632

-0.57

2.17

7.14

2.50

1.78

412

3

201362

-0.57

2.28

7.16

2.48

1.57

331

4

030623

-0.57

2.20

7.12

2.54

1.94

322

4

011444

-0.71

2.35

7.13

2.47

1.87

313

4

003263

-0.57

2.46

7.04

2.48

1.43

241

5

060503

-0.14

2.21

7.18

2.64

1.90

232

5

031415

-0.43

2.38

7.20

2.54

1.88

223

5

012236

-0.57

2.52

7.19

2.54

1.81

214

5

005044

-0.23

2.66

7.17

2.49

1.58

151

4

080304

0.07

2.27

6.72

2.13

0.80

142

4

040218

-0.47

2.42

6.75

2.30

0.54

133

6

031118

-0.43

2.55

7.08

2.47

1.52

124

6

013028

-0.43

2.70

7.10

2.56

1.62

115

6

007025

0

2.82

7.05

2.62

1.38

47



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 49

Table 3. Properties of AuxPdyPtz-O2 clusters with x+y+z=7. Numbers in the first column indicate the number of Au, Pd and Pt atoms in each cluster. The spin multiplicity (SM), adsorption energy (AE in eV), O-O distance (in Å), O-O stretching frequency (in cm-1) and the NBO charge on adsorbed O2 (in e) for clusters are presented in the table. Alloy

SM

AE

O-O

O-O

charge on

distance

frequency

O2

700

3

0.55

1.348

1014

-0.52

070

3

1.94

1.354

945

-0.47

007

5

2.06

1.412

746

-0.48

610

3

0.16

1.251

1342

-0.11

520

2

0.67

1.269

1267

-0.12

430

3

0.79

1.296

1192

-0.19

340

4

1.09

1.330

1005

-0.33

250

3

1.68

1.348

945

-0.41

160

4

1.51

1.343

966

-0.40

601

3

0.19

1.252

1336

-0.11

502

2

1.22

1.373

842

-0.35

403

3

1.55

1.332

1097

-0.27

304

4

1.64

1.407

739

-0.44

205

3

1.75

1.425

713

-0.50

106

4

1.74

1.438

685

-0.49

061

3

1.97

1.382

845

-0.50

052

5

1.96

1.383

829

-0.47

043

3

1.85

1.418

705

-0.50

034

3

2.00

1.374

845

-0.45

025

3

2.06

1.416

724

-0.49

016

5

1.98

1.431

683

-0.52

511

2

0.93

1.326

1074

-0.21

421

3

0.85

1.299

1186

-0.20

412

3

1.13

1.325

1107

-0.24

331

2

1.45

1.372

851

-0.43

322

2

1.37

1.397

789

-0.49

313

4

1.42

1.427

695

-0.48

241

3

1.83

1.377

839

-0.45

232

3

1.51

1.454

642

-0.49

223

3

1.69

1.442

671

-0.49

214

3

1.65

1.445

657

-0.51

151

4

1.53

1.368

871

-0.44

142

4

1.81

1.374

858

-0.44

133

4

1.67

1.433

689

-0.48

124

4

1.84

1.434

691

-0.48

115

4

1.86

1.434

691

-0.49

48


Page 49 of 49

TOC Graphic

1.250 1.276 1.301 1.327 1.352 1.377 1.403 1.429 1.454

Pd 0.001.00

0.50

0.50

t

Au -P

0.75

0.75

1.00 Au 0.00

-P

d

0.25

Pd

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.25

0.25

0.50

0.00 1.00

0.75

Pt

Au-Pt

49


Density Functional Study of Trimetallic AuxPdyPtz (x + y + z = 7

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