Pd Bimetallic Nanocatalysts Studied by

Article pubs.acs.org/JPCC

C−Cl Bond Activation on Au/Pd Bimetallic Nanocatalysts Studied by Density Functional Theory and Genetic Algorithm Calculations Bundet Boekfa,†,‡,§ Elke Pahl,∥ Nicola Gaston,⊥ Hidehiro Sakurai,# Jumras Limtrakul,‡ and Masahiro Ehara*,†,§ †

Institute for Molecular Science, Nishigo-naka 38, Myodai-ji, Okazaki 444-8585, Japan Department of Chemistry, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand § Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Kyoto 615-8520, Japan ∥ Centre for Theoretical Chemistry and Physics, Institute of Natural and Mathematical Sciences, Massey University, Auckland, New Zealand ⊥ School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington, New Zealand # Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Osaka 565-0871, Japan ‡

S Supporting Information *

ABSTRACT: The C−Cl bond activation by Au/Pd bimetallic alloy nanocatalysts has been investigated with regard to the oxidative addition of chlorobenzene (PhCl). Fifteen stable structures of the Au10Pd10 nanocluster (NC) obtained by a genetic algorithm were examined by DFT calculations using the M06-L, TPSS, and B3LYP functionals. Triplet states of cagelike C1 and Cs structures are found to be relevant reflecting the quasi-degenerate nature of the Pd moiety, while several other low-lying structures and spin states may also contribute to the oxidative addition. For all examined cluster structures, the oxidative addition step is exothermic, and internal conversion and/or spin crossing are expected to occur as several states are close in energy and geometry. Based on an energetic analysis of a model system consisting of the Au10Pd10 NC surrounded by four poly(n-vinylpyrrolidone) (PVP) molecules, the PVP units activate the system as electron donors and stabilize it. While a neutral NC model overestimates the energy barrier slightly, the opposite holds for an anionic NC model. In the oxidative addition, the interaction between the phenyl group and the Pd atom on the NC surface as well as a dissociation taking place at the Pd site are found to be essential. This indicates the importance of direct coordination effects in the Au/Pd bimetallic NC. NBO analysis shows that a π back-donation of the M(dπ) to σ*(C−Cl) orbital is relevant for the C−Cl bond activation and the interaction energy explains the favorable dissociation at the Pd site compared to the Au site.

1. INTRODUCTION Bimetallic nanoparticle (NP) catalysts have been of special interest in both fundamental chemistry and industrial applications due to their unique properties.1−3 Gold−palladium (Au/Pd) bimetallic NP catalysts, one of the most important bimetallic NP catalysts, have been extensively investigated because of their wide varieties of catalytic activity with respect to various substrates.4−8 For instance, they are applicable in many useful reactions such as the synthesis of hydrogen peroxide,6 the oxidation of benzyl alcohol,9 and in coupling reactions.10,11 As mentioned in an early review,12 the finetuning of the electronic properties in the NP catalysts by controlling the impurity-doping, namely, the bimetallic nature as well as the particle shape and size is crucial for the activity of the bimetallic NP catalysts. In Ullmann coupling reactions a carbon−halogen bond activation is taking place, which provides a useful strategy for organic synthesis.13−15 While the original reaction requires high temperatures, it has been found recently that Au/Pd bimetallic nanoparticles (NPs) supported by poly(n-vinylpyrroline) © 2014 American Chemical Society

(PVP) in liquid media allow the unique C−Cl bond activation in a basic environment under ambient conditions.10 In particular, the Au0.5/Pd0.5:PVP system achieves the homocoupling reaction of general chloroarenes for various substrates with a high yield. This reaction is of interest because it is enabled only by Au/Pd bimetallic alloy nanoclusters (NCs) and not by the pure Au or Pd NCs or a physical mixture of the pure clusters. This means that the electronic structure of the Au/Pd bimetallic NC is crucial for the catalytic activity. It was shown that the cross-coupling reaction of 4-chlorobenzoic acid and phenyl boronic acid is also possible.11 This C−Cl bond activation was previously examined by density-functional theory (DFT) calculations of the oxidative addition step of chlorobenzene (PhCl), which is a key step discriminating the activity of Au/Pd alloy NCs from the pure Au or Pd NCs.10 Received: July 24, 2014 Revised: August 29, 2014 Published: September 3, 2014 22188

dx.doi.org/10.1021/jp5074472 | J. Phys. Chem. C 2014, 118, 22188−22196

The Journal of Physical Chemistry C

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stabilizes the Au or Au/Pd NCs and also activates the NCs by their electron-donating function.27 The XPS measurement of the chemical shift also suggests that a charge reorganization occurs from PVP to the NPs. However, the effects of PVP on the Au/Pd NCs have not yet been fully analyzed theoretically. Also the global minimum structure of the Au10Pd10 NC was reported as a cage-like structure from the GA calculations.24 Thus, a detailed theoretical consideration of the geometric structures and spin states of the Au/Pd NC is also necessary to investigate the mechanism and energetics of the coupling reaction. In this work, we investigate the C−Cl bond activation by the Au/Pd bimetallic alloy NC in the oxidative addition of PhCl. The geometric structure and spin state of the Au/Pd NC itself and the energetics along the oxidative addition step are examined. The global and some possible local minimum structures of the Au10Pd10 NC are initially obtained by a genetic algorithm using the Gupta potential. These NC structures are then optimized by using DFT calculations with M06-L, TPSS, and B3LYP functionals taking into account the relevant structures and spin states. The reaction pathways of the oxidative addition of PhCl are examined in several possible states. The effects of the surrounding PVP are illuminated using the Au10Pd10-PVP model and the model dependence on the energetics is assessed by comparing to neutral and anionic NCs. The C−Cl bond activation by pure Au NCs is also compared to discuss the catalytic activity of the Au/Pd alloy NCs. Our focus is to extract and understand the essential factors of the catalytic activity of Au/Pd bimetallic NCs.

Various ab initio or DFT calculations have been carried out for investigating the geometric structures and catalytic activities of the Au, Pd, and Au/Pd alloy NCs. For the structures and relative energies of the Au NCs, it has been generally recognized that higher-order coupled cluster calculations like CCSD(T) with sufficiently flexible basis sets are necessary for converged results. Various kinds of benchmark calculations have also been performed for the validation of the DFT functionals.16 The CCSD(T) geometry optimization of the Au8 cluster was achieved using the numerical derivative algorithm.17 Recently, Götz et al. investigated the 2D and 3D structures of the Au10 cluster with respect to their relative energies by CCSD(T) calculations.18 It was shown that the pure GGA, TPSS functional performs best, while M06 and M06-L functionals also provide results similar to the CCSD(T) results. For the Au/Pd bimetallic clusters, Zanti and Peeters investigated the stable structures and their spin states for AunPdm (n + m ≤ 14) by B3LYP/Lanl2DZ calculations.19 The high-spin states were found to be stable in the case of bimetallic clusters with the Pd atoms. For the reactions on the Au, Pd, and Au/Pd NCs, various theoretical calculations, mostly DFT calculations, have been carried out. For example, the oxidative addition of iodobenzene and the cross-coupling on Au supported on CeO2 catalysts were studied by DFT/PW91 calculations.20 The oxygen activation on Au supported catalyst was examined with DFT/PW91.21 The M06 functional was previously used for the reaction on Au and Au/Pd NC catalysts.10,16 Recently, complete renormalized coupled-cluster singles and doubles with noniterative triples (CR-CC(2,3)) benchmark calculations were performed for the methanol oxidation on the Au8 NC.16 For the catalytic activity of the Au/Pd bimetallic NCs, the geometric structures of the NC as well as the topology of the Au and Pd atoms or Au−Pd or Pd−Pd bonds on the NC surface are relevant. Experimentally, the structures of Au/Pd bimetallic NPs or relatively large NCs have been directly observed using the TEM technique 22 and Pd core Au shell structures were found for particle sizes of 1−5 nm. Theoretical work on the structures of Au/Pd bimetallic NCs is not so easy because a global optimization of the structure is sometimes difficult, even by DFT calculations and for relatively small NCs. In particular, Zanti et al. showed that the spin states of the Au/ Pd NCs are complicated when Pd atoms are included.19 For the purpose of the global optimization of the Au/Pd bimetallic NCs in various sizes, Pittaway et al. used the Birmingham genetic algorithm (GA) calculations developed by Johnston’s group.23 They used the Gupta many-body empirical potentials with three different parameters to locate the global minimum of the Au/Pd bimetallic cluster with up to 50 atoms.24 The stable structures of the Au/Pd NCs are PdcoreAushell configurations with some Pd atoms on the NC surface. Recently, Bruma et al. used the Gupta potential for 98 atoms of Au/Pd catalysts.25 They also found the Pdcore structures to be stable. However, these GA calculations are usually carried out with simple potentials and without consideration of the spin states. A combination with ab initio or DFT calculations is desirable to investigate the catalytic activity of bimetallic NCs. In order to extract the essential points of the Au/Pd bimetallic NC catalysts, the underlying theoretical model is of special importance. For this purpose, we previously adopted an anionic Au10Pd10 NC for the modeling of the Au0.5Pd0.5:PVP catalysts, which was based on the theoretical model of the Au NC catalysts supported by PVP.10,26 The surrounding PVP

2. METHODOLOGY The geometric structures and spin states of the Au/Pd NC and the oxidative addition of PhCl over the Au/Pd NC catalysts have been investigated by DFT calculations. The selection of the DFT functionals was found to be essential in the calculation of the energetics of the Au/Pd bimetallic NC, in particular for the spin states, and therefore, we examined the small clusters, Au2Pdx (x = 3, 4), using several functionals, namely, B3LYP,28 TPSS,29 ωB97X-D,30 M06, M06-L,31 and PBE32 comparing the results with CISD,33 CCSD,34 and MP235 calculations. In addition, we also examined the adsorption and bond activation of PhCl over Au2Pd3. For the model clusters of the present system, the Au10Pd10 NC was adopted and the low-lying stable structures were examined by means of the Birmingham GA23 followed by DFT calculations with the M06-L, TPSS, and B3LYP functionals. These functionals were selected according to the results for the Au2Pdx (x = 3, 4) clusters. In the GA calculations, the Gupta potentials with three types of parameters, namely DFT-fitted, experimental-fitted, and average parameters as reported previously,24 were used. We also examined other structures like the tetrahedral (Td) one because the Td structure has some characteristics of the partial structures in larger NCs and may therefore be suitable for representing the NC surfaces and to examine the reactivity. All model structures considered in different topologies of the Au and Pd atoms are shown in the Supporting Information (SI). For the oxidative addition step, we examined the reaction pathways starting from some stable Au10Pd10 NCs obtained by the DFT calculations, namely, the C1, Cs, C2, and Td structures in their singlet to quintet spin states. The oxidative addition starts with the adsorption of PhCl (AD). We examined several positions on the NCs as adsorption sites and located the most 22189



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ground state. The bond distances are compared in the Supporting Information. The average Pd−Pd and Pd−Au distances are calculated to be 2.66−2.92 and 2.70−2.76 Å by the various DFT calculations, while the distances are 2.86 and 2.79 Å, for the Au2Pd3 and Au2Pd4, respectively, in the CISD/ LANL2TZ(f) calculations. The oxidative addition of PhCl on Au2Pd3 was also considered. The energies and structures of the adsorption and transition states are given in the Supporting Information. The PhCl is adsorbed via the benzene ring at the Pd atomic site followed by the C−Cl bond activation. Using the M06-L functional, the adsorption energy was calculated to be −23.2 kcal mol−1 and the activation energy was 19.8 kcal mol−1. The energies compare well with the CCSD values of −21.2 and +17.6 kcal mol−1, respectively. For this cluster, the B3LYP gave the lowest adsorption and activation energies compared with other functionals. Based on these results, in particular, in view of the relative energy of the spin states, we have selected the M06-L, TPSS, and B3LYP functionals for the calculations of the structures and bond activation of the larger Au10Pd10 NC. 3.2. Structure and Spin State of Au10Pd10 NCs. For simulating the Au/Pd NC catalysts, the stable structures and spin states of the neutral Au10Pd10 NCs were examined. The validity of the neutral cluster as computational model is discussed later. The approximate stable structures for various Au/Pd topologies were obtained by GA calculations (15 structures), and then the global and some local minima were optimized by the DFT calculations with the M06-L, TPSS, and B3LYP functionals. As discussed later, the three functionals provided almost consistent results in geometric structures and spin states of the Au/Pd NCs, but some differences are observed due to the character of the functionals. Many other local-minimum structures of the Au10Pd10 NCs were also obtained by the GA calculations as shown in the Supporting Information. The most stable structures of the Au10Pd10 NC are cage-like structures with two Pd atoms located in the center of the cluster with different symmetries; namely, the C1, Cs, Cs‑2, C1−2, and C2 structures as shown in Figure 1a−f. The relative energies of the low-lying spin states of these structures are summarized in Table 2. The DFT/M06-L calculation shows that the triplet state of the C1 structure is the most stable, and three other states of C1 and Cs symmetry exist within 2.3 kcal mol−1. These structures

stable dissociative adsorbed states (DA). The transition state (TS) has been confirmed by vibrational analysis as a configuration, which exhibits only one imaginary frequency correlating to the reaction coordinate from AD to DA. The results of vibration analysis for all the transition states are summarized in Tables S2−S4 in the Supporting Information. For simulating the reaction on the Au/Pd NC catalyst stabilized by PVP in liquid media, the effect of PVP is relevant. The PVP acts as an electron donor as well as a stabilizer, which has been supported by theoretical calculations27 and XPS measurements.26 For the computational model of the Au/Pd NC:PVP catalyst, the Au10Pd10 NC surrounded by four PVP molecules represented as Au10Pd10 4PVP was examined and compared with the neutral and anionic Au10Pd10 NCs (without PVP), to find a simpler suitable model for simulating the reaction, in particular, with respect to the activation-energy barrier. All structures were fully relaxed in the geometry-optimization step without any constraint. The Au and Pd atoms were treated by relativistic effective core potentials (RECP) with double-ζ basis set [3s3p3d] (LANL2DZ), while the H, C, O, and Cl atoms were calculated with the 6-31G(d,p) basis set, except for the small Au2Pdx clusters. All the structures along the reaction pathway were examined with three different functionals, namely, the M06-L, TPSS, and B3LYP functionals. NBO analysis36 was performed to examine the bond breaking of PhCl over the Au10Pd10 NC. All calculations were conducted using the Gaussian 09 program.37

3. RESULTS AND DISCUSSION 3.1. Au2Pdx (x = 3, 4) Clusters. The stable structures and the bond activation on small Au/Pd clusters were examined to validate the DFT functionals. The singlet and triplet states of the Au2Pdx (x = 3, 4) clusters were calculated with several functionals and the results were compared with ab initio calculations. The energies of the singlet state relative to the triplet state for Au2Pdx (x = 3, 4) are given in Table 1. In the Table 1. Relative Energies (kcal mol−1) of the Singlet State to the Triplet State of Au2Pdx (x = 3, 4) Clusters by ab Initio and DFT Calculations for Different Functionalsa Au2Pd3

a

Au2Pd4

method

LANL2DZ

LANL2TZ(f)

LANL2DZ

LANL2TZ(f)

CISD CCSD//CISD MP2 B3LYP TPSS ωB97X-D M06 M06-L PBE

26.6 17.3 −1.3 7.2 8.6 6.3 −1.3 4.8 6.8

25.4 23.9 0.5 8.3 8.1 6.4 −1.6 4.6 7.1

15.9 15.6 −7.7 11.2 15.8 5.8 −5.3 16.9 6.1

2.7 1.8 −7.7 11.1 13.8 6.8 −4.3 4.2 7.5

Negative values indicate a singlet ground state.

CISD and CCSD/CISD calculations, the triplet state is more stable than the singlet counterpart. The trends are similar between LANL2DZ and LANL2TZ(f) basis sets. The B3LYP, TPSS, ωB97X-D, M06-L, and PBE functionals gave the triplet ground state, which is more stable than the singlet state by 4.6− 8.3 kcal mol−1 (Au2Pd3) and 4.2−13.8 kcal mol−1 (Au2Pd4). On the other hand, the MP2 and M06 functionals found a singlet

Figure 1. Global- and local-minimum structures of the neutral Au10Pd10 NC in (a) C1, (b) Cs, (c) C2, (d) C1−2, (e) Cs‑2, and (f) Td optimized by the DFT/M06-L calculations. Other local minima are given in the Supporting Information. 22190



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compared to the DFT calculations. In these structures obtained by using the average-parameter potential, the Pd atoms are localized in the center of the NC. The Td structure of the Au10Pd10 NC (Figure 1f) has also been considered because, in the larger clusters, a similar Au−Pd surface structure appears in Au19Pd19 NC.24 Additionally, the comparison with the Au20 NC is of interest. The structure shown in Figure 1f is the most stable structure among the Td clusters, and there are some other Td topoisomers with different Au−Pd positions as shown in the Supporting Information. The pyramidal length is about 8.4 Å. In the model, there are two Au and two Pd atoms at the apex and the facet sites. The apex sites are negatively charged and the Pd atoms at the facet and edge sites are positively charged. The Au20 NC was also examined for comparison. The atomic charge distribution of these NC is compared in the Supporting Information. The adiabatic electron affinity of the bare Au20 NC is calculated as 2.56 eV in good agreement with the experimental value of 2.745 eV.38 3.3. Oxidative Addition of PhCl on Au/Pd NC. The first key step of the coupling reaction, the oxidative addition of PhCl on the Au/Pd NC, is considered. As shown in the previous section, the Au10Pd10 NC exhibits cage-like structures in singlet to quintet spin states, though the stability is dependent on the functionals used for the DFT calculations. Among these structures and spin states, we examined the oxidative addition for the NC structures with relative energy below 3.0 kcal mol−1, namely, the Cs, C1, Cs‑2, and C1−2 structures for each functional; four cases for M06-L (3C1, 1Cs, 3Cs, and 1C1), five cases for TPSS (3C1, 3Cs, 1Cs, 5C1, and 1C1), and six cases for B3LYP (5Cs‑2, 5C1−2, 3C1−2, 3Cs‑2, 3C1, and 5C1), where for simplicity, the structure (X) with spin multiplicity (S) is represented as SX. The relative energies of the adsorption state (AD), transition state (TS), and dissociative adsorption state (DA) are summarized in Table 3 with the activation barrier energy. The C−Cl activation of PhCl on the Au10Pd10 NC was examined at various sites of the NC. Among them, the possible oxidative addition pathways obtained by M06-L are shown in

Table 2. Relative Energy of the Low-Lying Spin States of the Au10Pd10 NC Optimized by DFT Calculations with the M06L, TPSS, and B3LYP Functionalsa structure method

spin state

C1

Cs

C1−2

Cs‑2

C2

M06-L

1 3 5 1 3 5 1 3 5

2.3 0.0 4.7 3.1 0.0 2.9 9.5 3.0 3.9

1.3 2.0 7.4 2.9 1.0 5.0 10.3 7.2 10.5

9.9 9.5 10.0 7.3 6.6 3.6 10.8 2.1 0.0

9.9 9.5 9.9 7.3 6.6 3.6 10.8 2.4 0.0

13.0 11.9 23.2 3.1 10.2 17.8 9.5 14.9 22.2

TPSS

B3LYP

a

Other local-minimum structures are listed in SI. The energy is shown in kcal mol−1 relative to the most stable state obtained by each functional. All structures in each spin state are optimized as local minima.

were obtained from the GA calculation with exp-fit parameters. The C1 structure has two Pd atoms in the core with a distance of 2.57 Å. The largest bond distance of 7.96 Å is found between Au and Pd edge atoms. The charge on the central Pd atoms is +6.5 e and all surface atoms are negatively charged with Au being more charged than Pd, which is relevant for the catalytic activity of the Au/Pd NC (for detailed charges, see the Supporting Information). The calculated adiabatic electron affinity of this NC is 3.25 eV, which is larger than for the Au20 cluster (2.745 eV).38 The singlet state of the Cs structure is located just 1.3 kcal mol−1 above. The size and structure of the Cs NC are similar to those of the C1 NC as compared in Figure 1. In the Cs NC, the center Pd−Pd distance is 2.55 Å and the length of the NC is 8.09 Å. The charge distribution (qPd(center) = +6.5) and the electron affinity (3.25 eV) are nearly the same. The stability of the geometric structures and spin states is dependent on the functionals of the DFT calculations. The 15 structures from singlet to quintet spin states were also examined by the TPSS and B3LYP functionals. As noted above and in the Supporting Information, the M06-L functional suggested that the stable structures are the triplet and singlet states. By using the TPSS functional, the triplet states of the C1 and Cs structures are also stable and their singlet counterparts lie above with a relative energy of ∼3 kcal mol−1. The B3LYP functional favors the higher spin states for this system; the most stable structures are the quintet states of the C1−2 and Cs‑2 structures. Each of these two structures also has the triplet states located about 2 kcal mol−1 above the lowest quintet states. Some of these structures with different spin states are examined for the C−Cl activation in the next section. There are some other cage-like structures, which are located in the low-energy region as shown in the Supporting Information. Fifteen structures were obtained by the GA calculations with three types of potentials. From DFT-fit and exp-fit parameters, C1, Cs, and C2 structures were obtained, and indeed, they were calculated as the low-lying states by the DFT calculations. Some of these structures are regarded as topological isomers differing only by the permutation of Au and Pd atoms. These cage-like structures of the Au10Pd10 NC are more stable than the tetrahedral structure (Td) by about 69.2 kcal mol−1, which was used in the previous work.10 The structures based on the GA using the average parameters are relatively higher in energy with +14.6 to +27.3 kcal mol−1

Table 3. Energetics for the Oxidative Addition of PhCl on the Neutral Au10Pd10 NCa method M06-L

TPSS

B3LYP

structureb 3

C1 1 Cs 3 Cs 1 C1 3 C1 3 Cs 1 Cs 5 C1 1 C1 5 Cs‑2 5 C1−2 3 C1−2 3 Cs‑2 3 C1 5 C1

spin mul.

bare

AD

TS (Ea)

DA

3 1 3 1 3 3 1 5 1 5 5 3 3 3 5

0.0 1.3 1.9 2.3 0.0 1.0 2.9 2.9 3.1 0.0 0.0 2.1 2.4 3.0 3.9

−28.2 −27.8 −29.5 −28.7 −26.2 −23.6 −21.1 −20.5 −28.8 −8.0 −15.1 −12.2 −15.1 −12.2 −10.2

−17.2 (11.0) −14.5 (13.3) −15.4 (14.1) −20.9 (7.8) −21.4 (4.8) −16.3 (7.3) −14.8 (6.3) −14.8 (5.7) −23.9 (4.9) 2.4 (10.4) 2.9 (18.0) 2.5 (14.7) 2.4 (17.5) −2.9 (9.3) 3.4 (13.6)

−41.3 −36.5 −34.7 −37.6 −39.5 −31.9 −32.0 −38.0 −36.7 −25.2 −25.4 −28.4 −31.6 −25.7 −23.6

The energy is shown in kcal mol−1 relative to the most stable NC obtained by each functional and the actual activation energies are in parentheses. bStructure (X) with spin multiplicity (S) is represented as S X. a

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Figure 2 with displaying the structures for the 3C1 case. Focusing on the stable 3C1 case, the PhCl adsorbs on the

are compared in Figure 3. The 3C1 structure was found to be the most stable among these states like in M06-L, however,

Figure 2. Energy diagram for the oxidative addition of PhCl on Au10Pd10 NC calculated by the M06-L functional. Energies are in kcal mol−1; AD, TS, and DA are based on the Au10Pd10 3C1 structure. The structures shown depict only the active Pd region of the whole Au10Pd10 3C1 NC.

Au10Pd10 C1 NC with the phenyl group on the Pd−Pd ridge with a distance of ∼2.3 Å. The calculated adsorption energy is −28.2 kcal mol−1. At the TS, the C−Cl bond activation occurs at the Pd site where the Cl atom is transferred to the Pd−Pd site and the phenyl group is forming a bond with the Pd−Pd site. An imaginary frequency was obtained as 177.8i cm−1 correlating to the reaction coordinate from AD to DA. The C− Pd distance is shortened to 2.00 Å, and the C−Cl bond is elongated to 2.27 Å. The calculated apparent activation energy is −17.2 kcal mol−1 and the actual activation barrier energy is 11.0 kcal mol−1. The oxidative addition is exothermic by −13.1 kcal mol−1. In the DA state, the phenyl group is bridged by two Pd atoms (2.09 and 2.11 Å) and the Cl atom is also bridged by two Pd atoms (2.55 and 2.62 Å). The other three cases (1Cs, 3Cs, and 1C1) show similar trends as found in the 3C1 case. The energy profiles of these cases are compared in Figure 2 and the structures of AD, TS, and DA for these cases are shown in the Supporting Information. The adsorption energies of these cases are in the range of −27.8 to ∼−29.5 kcal mol−1 and the activation-energy barriers ranges between 7.8 and 14.1 kcal mol−1. The structures of these intermediates and transition states are similar to those in the 3 C1 case. The most stable DA structure is the Cs NC in the triplet state. Because all of these intermediate and transition states are energetically very close, these pathways would be possible in the oxidative addition step via thermal distribution and internal conversion/spin crossing. As suggested in the H2 dissociation on pure Au clusters,39 the lowest energy pathway is not necessarily the most favorable pathway in the reactions of cluster catalysis. In this case, the singlet state in C1 structure has the lowest activation energy barrier (7.8 kcal mol−1), and therefore, the spin crossing enhances the activity of the present catalytic system. These low activation energy barriers agree well with the mild conditions and low temperature found in the experiment.10 The low-lying oxidative addition pathways were also considered by the TPSS calculations; namely, the C1 NC in the singlet to quintet states and the Cs NC in the singlet and triplet states. The energy profile is shown in the Supporting Information (Figure S4). The adsorption energy, activation energy, and reaction energy calculated with three functionals

Figure 3. (a) Adsorption energy, (b) activation energy, and (c) reaction energy (kcal mol−1) for the oxidative addition of PhCl on Au10Pd10 NC calculated with M06-L, B3LYP, and TPSS functionals. Values for singlet, triplet, and quintet states of the NCs are shown in green, red, and blue, respectively.

with a lower activation energy barrier of 4.8 kcal mol−1. The 3Cs and 1Cs states are higher in energy by about 3 kcal mol−1 than the corresponding spin states of the C1 structure. Two 3C1 and 1 C1 states are stable at the DA structure and spin crossing may occur among these states. With TPSS, the activation energy barriers tend to be lower (4.8−6.3 kcal mol−1) in all structures and spin states, which was also seen in other cases by GGA functionals. The quintet state of the C1 structure, which was not calculated in the lower state by M06-L, is also stable at the DA structure. Traced by B3LYP, the quintet states appear to be stable in bare NC, while the triplet states become the lowest after the AD step (Figure S5). Namely, the 5Cs‑2 and 5C1−2 structures are the most stable structures of Au10Pd10 NC, however, in the course of the oxidative addition, the 3C1 and 3Cs structures become lowest, similar to the findings with the M06-L and TPSS functionals. In all six cases considered, the adsorption energies calculated were found to be small; two stable AD states are the C1−2 and Cs‑2 structures in the triplet state (Ead = −15.1 kcal mol−1). The activation barrier ranges from 9.3 to 18.0 kcal mol−1 and this step also shows an exothermic C−Cl bond dissociation. The C−Cl bond activation structures in these states are also similar to those obtained by M06-L; namely, the interaction between the phenyl group and the Pd− Pd site (AD), the C−Cl bond activation at the Pd atom (TS), 22192



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which was found to be the most stable structure, was examined in these calculations. Experimentally, PVP is assumed to be relevant to stabilize the Au or Au/Pd alloy NCs in liquid media.40 In Figure 5 the C−Cl bond activation by the C1 NC interacting with four PVP molecules is shown (Au10Pd10 4PVP model). The PVP molecules interact with the NC over their carbonyl group and are coordinated to the Pd atom in a distance range of 2.29−2.46 Å. The averaged adsorption energy of PVP on Au10Pd10 NC (C1) is about −24.3 kcal mol−1 per PVP. The Au10Pd10 NC is charged by −0.73 based on a Mulliken charge analysis. These results confirm that in the Au/ Pd bimetallic NC:PVP system, the PVP works as an electron donor as well as a stabilizer. Let us start our discussion of the oxidative addition step with the Pd atom as the dissociation site. The adsorption energy of PhCl at the Pd site is in the range of −26.7 to −28.2 kcal mol−1 for all three models. The calculated activation energy barrier of the oxidative addition is found in the order of Au10Pd10− < Au10Pd10 4PVP < Au10Pd10 as 7.2, 9.3, and 11.0 kcal mol−1, respectively. These trends correlate with the charge or partial charge of the model NC; the Mulliken charge of the NC in Au10Pd10 4PVP is about −0.73 e. Also in the Au10Pd10 4PVP model, the oxidative addition step is exothermic by 15.0 kcal mol−1 compared to the neutral and anionic NCs with 13.1 and 17.0 kcal mol−1, respectively. This shows that PVP works as an electron donor and activates the NC. Therefore, the neutral NC model overestimates the activation barrier and underestimates exothermicity slightly. The opposite trend is true for the anionic NC model. Thus, we should be aware of and take into account these effects when adopting bare cluster models. The structures of AD, TS, and DA for the Au10Pd10 4PVP model in the oxidative addition are shown in Figure 5. These structures are very similar to those found with the neutral and anionic NCs. In the AD state, the phenyl ring of PhCl interacts with Pd−Pd edge, and in the DA state, the Cl atom and the phenyl ring are bridged by neighboring Pd−Pd edges. Compared to the neutral or anionic NCs, PhCl interacts with Au10Pd10 4PVP more weakly with a lesser adsorption energy. The site dependence of the adsorption and activation barrier energies is also of interest. Two other interaction sites, Pd−Pd and Au sites, were also examined. The AD, TS, and DA structures in these cases are displayed in the Supporting Information. The oxidative addition is calculated to be exothermic in all cases. For the Pd−Pd bridge site, again, the activation energies are in the order of Au10Pd10− < Au10Pd10 4PVP < Au10Pd10 as 5.5, 7.5, and 8.2 kcal mol−1, respectively, though the difference in absolute energies is less. Therefore, the C−Cl activation also occurs at this Pd−Pd site. For the Au site,

and the bridged structure of the dissociated phenyl and Cl species (DA). Summarizing, the calculations with all three considered functionals show that several NP structures with different symmetries and spin states lead to a comparable energetics for the oxidative addition step. We conclude that multiple pathways are accessible and internal conversion and spin crossing might occur. 3.4. Effect of PVP on the Oxidative Addition on Au/Pd NC. It is important to take the effects of PVP into account in the modeling of the Au/Pd NC, in particular, for the further theoretical studies of the reaction mechanism and catalyst design. The most simple model is sought in view of a fast throughput of computations. Thus, the effects of PVP are examined using a model system in which the Au10Pd10 NC is surrounded by four PVP molecules (denoted as Au10Pd10 4PVP). Also the site dependency is taken into account. In Table 4, the results for Au10Pd10 4PVP as well as for the bare Table 4. Relative Energies of the Oxidative Addition of PhCl on the Various Au10Pd10 NC and Au20 NC Modelsa model Au10Pd10 Au10Pd10− Au10Pd10 4PVP Au10Pd10 Au10Pd10− Au10Pd10 4PVP Au10Pd10 Au10Pd10− Au10Pd10 4PVP Au10Pd10 Au10Pd10− Au18Pd2 Au18Pd2− Au20 Au20−

structure

interacting site

AD

TS (Ea)

DA

3

C1 2 C1 3 C1

Pd Pd Pd

−28.2 −27.0 −26.7

−17.2 (11.0) −19.8 (7.2) −17.4 (9.3)

−41.3 −44.0 −41.7

3

C1 C1 3 C1

Pd−Pd Pd−Pd Pd−Pd

−26.5 −24.5 −21.3

−18.3 (8.2) −19.0 (5.5) −13.8 (7.5)

−36.9 −44.0 −34.9

3

C1 C1 3 C1

Au Au Au

−30.4 −28.3 −27.3

−2.8 (27.6) −4.4 (23.9) −2.4 (24.9)

−35.8 −39.3 −36.6

1

Pd edge Pd edge Au Au Au edge Au edge

−30.9 −28.2 −16.4 −12.1 −13.2 −14.7

−17.6 (13.3) −16.4 (11.8) 15.8 (32.3) 10.1 (22.2) 27.0 (40.2) 15.6 (30.3)

−38.0 −41.1 −14.1 −24.4 14.2 −13.7

2

2

Td Td 1 C1 2 C1 1 Td 2 Td 2

Energies are shown in kcal mol−1, and the actual activation energies are in parentheses.

a

neutral and anionic Au10Pd10 NCs are compared for three adsorption sites (Pd, Pd−Pd, and Au) where PhCl dissociates. The interacting sites found for the different NCs are shown in Figure 4. Based on the calculations in section 3.2, the 3C1 NC,

Figure 4. Structure and the interaction site for the oxidative addition of PhCl on (a) Au10Pd10, (b) Au18Pd2, (c) Au10Pd10, and (d) Au20 NCs calculated with M06-L functional. 22193



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Figure 5. Structures of the oxidative addition of PhCl on Au10Pd10 4PVP model in the triplet state optimized by the M06-L calculations: (a) AD, (b) TS, and (c) DA. (Distances are in Å.)

Figure 6. NBO analysis and relevant interaction at the transition state of the oxidative addition of PhCl on Au10Pd10 NC by M06-L calculations (a) Pd−Pd site and (b) Au site.

transition state of the oxidative addition. The relevant orbital interactions are shown in Figure 6 and more detailed data are compiled in the Supporting Information. In both cases, the σ donation from σ(C−Cl) to Pd(pσ) or Au(pσ) and the π backdonation from Pd(dπ) or Au(dπ) to σ*(C−Cl) are energetically relevant except for the π interaction of the benzene ring with the NC. The second order NBO energy is large in σ*(C− Cl) - Pd (dπ) or Au (dπ) and this interaction is the origin of the C−Cl bond scission. Comparing the oxidative addition between Pd and Au sites, the calculated energies are 31.2 and 20.7 kcal mol−1 for the Pd and Au sites, respectively, which qualitatively agrees well with the fact that the Pd site is more favorable than the Au site. This difference originates in the orbital energy difference (εj-εi) reflecting the energy levels of the Pd and Au d orbitals; namely, orbital energy difference is 0.17 and 0.20 au for the Pd−Pd and Au sites, respectively. The coupling term, Fij (Fock matrix elements based on NBO), also contributes to this difference. More details are given in the Supporting Information. We also considered the cases of the Td structure of the Au10Pd10 NC and the Au20 NC. Previously, we demonstrated

although the adsorption energy is relatively large, the activation energy barrier is significantly higher with 27.6 and 23.9 kcal mol−1 for neutral and anionic NCs, respectively. Considering that the C−Cl bond activation occurs at room temperature in experiment, the Pd or Pd−Pd sites should be relevant. The bond activation at the Au site has also been compared for three different NC models; namely, Au20, Au18Pd2, and Au10Pd10. Note that the Au18Pd2 is regarded as a model for a core−shell structure of Au/Pd alloy NC. The respective adsorption energies are calculated to be −13.2, −16.4, and −30.4 kcal mol−1, while the activation energies are 40.2, 32.3, and 27.6 kcal mol−1, respectively. This means that the core Pd atom (Au18Pd2) actually enhances the activity via an indirect effect but is not as strong as the direct effect (dissociation at the Pd or Pd−Pd site). All activation energies are smaller in the case of the anionic NCs. From these results, the direct interaction at the Pd atom is relevant for the C−Cl bond activation in Au/Pd alloy NC, at least for the present PhCl system. To interpret the reactivity at the Pd−Pd and Au sites, a natural bond orbital (NBO) analysis was performed for the 22194



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and Td model of AuPd NC exhibit low-energy activation barriers with less than 20 kcal mol−1 in contrast to the Au NC whose activation barrier is over 30 kcal mol−1. NBO analysis clearly shows that the π back-donation of M(dπ) to σ*(C−Cl) is relevant for the C−Cl bond activation and the interaction energy at the transition state explains the favorable dissociation at the Pd−Pd site compared to the Au site. The effects of the surrounding PVP are considered with the NC 4PVP model. The PVP coordinates with its carbonyl group pointing to the Pd atoms on the AuPd NC and works as an electron donor to activate the Au/Pd NC. The PVP affects the energy level of the Pd d orbitals on the surface and enhances the interaction between the Pd and substrates. This interaction stabilizes the transition states and reduces the activation energy barrier. For the Au/Pd bimetallic NC case, the structures, spin states, and reactivity in the bimetallic NC as well as the effect of the surrounding PVP have been examined in detail. It was demonstrated that the present computational method using GA combined with the DFT calculations is useful for investigating the structures and reactivity on the bimetallic NCs.

the advantage of the Au/Pd NC over the Au NC with the anionic Td model, Au10Pd10− and Au20−, with the facet site adsorption.41 In the present work, we also examined the dissociative adsorption at the Pd edge site in the neutral Au10Pd10 Td NC and the neutral Au20 NC. The AD, TS, and DA structures for these cases are shown in the Supporting Information. The energetics are also summarized in Table 4. Though the Au10Pd10− NC (Td) is activated by the negative charge, the effect is not as significant as in the case of cage-like structures: the differences of the adsorption energy and activation barrier energy are 1.2 and 3.8 kcal mol−1, respectively. Although the Td NC model gives less stable structures than the cage-like model as mentioned in section 3.1, the Td model provides similar energetics of oxidative addition as the cage-like structures. The adsorption of PhCl on the Au20 NC is weak with −13.2 kcal mol−1. The interaction distance, namely, the C−Au distance is large with about 3.2−3.6 Å in the AD state compared to the case of the Au/Pd NC. The calculated activation energy barrier is 40.2 and 30.3 kcal mol−1 for the neutral and anionic NCs, respectively; the effect of the charge is the same in the case of the Au NC. The structures of AD, TS, and DA are almost the same between neutral and anionic NCs.

■

4. CONCLUSION The C−Cl bond activation by the Au/Pd bimetallic NC has been theoretically investigated for the oxidative addition of chlorobenzene, which is a key step of the unique homocoupling reaction.10,42 The Au10Pd10 NC 4PVP as well as bare neutral and anionic Au10Pd10 NCs were examined as the model system. Stable structures and spin states of the NC system and possible reaction pathways were calculated by means of the GA calculations with the Gupta potential followed by the DFT calculations using the GGA and hybrid functionals. The DFT functionals were chosen due to their ability to predict suitable spin states for Au2Pdx (x = 3, 4) clusters. Five stable cage-like structures of Au10Pd10 NCs, namely, the C1, Cs, C1−2, Cs‑2, and C2 structures, were obtained in their singlet to quintet spin states. M06-L and TPSS gave the stable triplet states for the C1 and Cs structures within about 2.0 kcal mol−1, while B3LYP suggested the stable quintet state. These structures are more stable by about 69 kcal mol−1 than the Td structures. Fifteen other topological isomers suggested by the GA calculations were also optimized in various spin states. Several low-lying oxidative addition pathways were then examined for Au10Pd10 NCs. Some of the pathways are thermally accessible even via spin crossing and internal conversion. Therefore, it is to be expected that many lowlying pathways contribute to the reaction in larger NCs. The activation energy barriers are in the range of 7−14 kcal mol−1 for the neutral Au10Pd10 NC model using the M06-L functional, much lower values as found with the TPSS functional. The M06-L, TPSS, and B3LYP functionals all demonstrate that the C−Cl bond activation by Au/Pd NCs is exothermic with a low activation energy. It was found that the adsorption and dissociation of PhCl is preferred at the Pd site with the phenyl group interacting with the Pd atoms, though this depends on the topology of the Au and Pd on the surface of NC. An Au adsorption/dissociation site, on the other hand, gave high activation energies. These results indicate that the direct coordination effect of the Pd atom on the surface of the Au/Pd bimetallic NC is relevant for the C−Cl bond activation of PhCl. Both cage-like structures

ASSOCIATED CONTENT

S Supporting Information *

Fifteen Au10Pd10 stable structures, charge distribution, energy diagram with three functionals, structures of AD, TS, and DA, NBO analysis, structures with PVP and Au2Pdx (x = 3, 4) data. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-564-55-7461. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Prof. P. Schwerdtfeger for valuable discussions. This work was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS), Japan, ACT-C Project by Japan Science and Technology Project (JST), MEXT (Ministry of Education Culture, Sports, Science and Technology, Japan) program “Elements Strategy Initiative to Form Core Research Center” (since 2012). B.B. acknowledges the Thailand Research Fund (TRF) (MRG5480239). E.P. acknowledges support through a MURF Women’s Award Grant. The computations were partially performed in the Research Center for Computational Science, Okazaki, Japan.

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REFERENCES

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