Mechanistic Insight into the Styrene-Selective Oxidation on

Article pubs.acs.org/JPCC

Mechanistic Insight into the Styrene-Selective Oxidation on Subnanometer Gold Clusters (Au16−Au20, Au27, Au28, Au30, and Au32− Au35): A Density Functional Theory Study Sisi Lin and Yong Pei* Department of Chemistry, Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Xiangtan, Hunan Province 411105, China S Supporting Information *

ABSTRACT: We performed a comprehensive study of the reaction mechanism of styrene-selective oxidation to benzaldehye and styrene epoxide on subnanometer gold clusters with the cluster size ranges from around 0.4 to 1.0 nm via the density functional theory (DFT) calculation. The major focuses of the current study are the intrinsic catalytic selectivity and size-dependent activities of gold clusters toward styrene oxidations. The reaction selectivity of styrene oxidation over subnanometer gold clusters, e.g., selective formation to benzaldehyde or styrene epoxide, with the presence of dioxygen as the sole oxidant or the H2/O2 mixture as the reactant is discussed. A new reaction channel leading to the formation of a benzaldehyde product involving the formation of a metastable four-membered ring CCOO* intermediate is proposed, which explains the recent experimental observations of a high yield of benzaldehyde on ∼1.4 nm gold clusters. The effect of the charge state of gold clusters on the reaction selectivity and reaction rate is examined. The results indicated that the reaction selectivity is not affected by the charge state of the cluster by using the Au34− cluster as a benchmark model. However, the reaction rate of styrene oxidation is significantly increased on the anionic gold clusters caused by larger O2 adsorption energies, suggesting higher catalytic activity of anionic clusters. The mechanism of dramatic increase of product selectivity to styrene epoxide using H2 as the additive is explored as well. We find that the major role of the H2 additive is facilitating the dissociation of O2 into an active O atom on subnanometer gold clusters, which leads to high selectivity to the epoxide product. This systematic study enables a quantitative assessment of the size-dependent activity and selectivity of subnanometer gold clusters toward styrene-selective oxidation.

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INTRODUCTION Gold nanoparticles have excited much research interest owing to their unusual and somewhat unexpected catalytic properties. It has been shown that the small gold nanoparticles are active in several catalytic oxidation and hydrogenation reactions, for example, the carbon monoxide oxidation,1,2 selective oxidation of olefin and alcohol,3 synthesis of hydrogen peroxide,4,5 as well as water−gas shift reaction,6 in stark contrast to the properties of their bulk counterpart. Among various gold catalytic reactions, the gold-catalyzed low-temperature CO oxidation received the most intensive research interest. To date, numerous experimental and theoretical studies have been devoted to finding the active site and size-dependent catalytic activity of gold nanoparticles. The reaction mechanism and influence factors of the nanogoldcatalyzed CO oxidation reactions have been studied in great detail.7−17 For the metal-oxide-supported gold nanocluster system, such as the Au/TiO2 system, both experiment and theory have confirmed that the interface between gold and TiO2 is the key active site.18,19 A dual active site involving the perimeter Au atoms and nearby surface Ti atoms was identified. On the other hand, the systematic studies of CO oxidations on © 2014 American Chemical Society

different sized gold clusters also provided insights into the structure and size-dependent catalytic properties of gold clusters toward CO oxidation.20 The triangular Au3 active site and a novel trimolecular CO self-promoting oxidation mechanism have been proposed. In comparison to the well-studied CO oxidation reactions, the mechanisms and selectivity of gold-catalyzed olefin oxidation such as styrene oxidation or propylene oxidation are less understood by both experiments and theory. To date, gold nanoparticles supported on different oxides have been successfully prepared and used as a catalyst for propylene and styrene oxidation. Patil et al. reported21,22 the performance of gold nanoparticles supported on a number of alkaline earth oxides and transitional metal oxides as catalysts for the epoxide of styrene, which found that except for the Au/MnO2 and Au/ U3O8 all the supported gold nanoparticles are active and selective catalysts for the epoxidation of styrene to styrene oxide. In particular, the Au/TiO2 and Au/CuO performed the Received: May 21, 2014 Revised: August 12, 2014 Published: August 14, 2014 20346

dx.doi.org/10.1021/jp505018g | J. Phys. Chem. C 2014, 118, 20346−20356

The Journal of Physical Chemistry C

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toward styrene oxidation.25 They found the key step of styrene oxidation, e.g., the O2 activation, is not the direct dissociation of the O2 molecule. Instead, the O2 is activated via the formation of an oxametallacycle intermediate (OMME) on the gold clusters. After that, the O−O bond scission led to the formation of styrene epoxide. A similar mechanism was also proposed by Gao et al. using the Au43Cu12 as a model catalyst.26 Meanwhile, we note that these theoretical studies did not fully address the catalytic selectivity of gold clusters. As mentioned above, the product population of styrene oxidations on gold is strongly dependent on the size of gold clusters and properties of oxide support. In several gold cluster systems, such as the inert material (BN and SiO2) supported gold clusters, the major partial oxidation product of styrene is benzaldehyde (up to 82%), and the selectivity to styrene epoxide is relatively lower.24 In contrast, when the H2/O2 mixture was used as a reactant, the selectivity to the epoxide product was significantly improved.27−30 To the best of our knowledge, there was no systematical theoretical study about the oxidation selectivity of styrene on gold clusters. The size-dependent catalytic activities of gold clusters and the competitive reaction pathways that lead to different oxidation products still remain to be explored. In this work, we systematically investigate the reaction pathways of styrene oxidation on the gas-phase neutral Aun nanoclusters with n = 16−20, 27−28, and 30−35 using the density functional theory (DFT) calculations. The current theoretical studies aim to provide a systematical investigation on the size-dependent catalytic activity and selectivity of gold nanoclusters toward styrene oxidations. We have first investigated the styrene oxidation processes on different sized gold clusters with their sizes ranging from about 0.4 to 1.0 nm. The catalytic selectivity of gold clusters, e.g., selectively catalyzing styrene into benzaldehyde or styrene epoxide, with the presence of dioxygen as the sole oxidant or the H2/O2 mixture as the reactant is discussed. A new reaction channel involving the formation of a novel four-membered ring CCOO* intermediate is proposed to explain the selective formation of benzaldehyde. The new reaction channel is energetically more favorable than the known pathway that leads to the styrene epoxide, explaining the recent experimental observations of much higher yield of benzaldehyde resulting from the styrene oxidation on ∼1.4 nm gold clusters.24 Furthermore, the mechanism of H2-promoting epoxidation of styrene is elucidated as well. Our results indicate that the coadsorbed H2 molecule can promote the activation of the O2 molecule to hydroperoxo (OOH) and hence the active O atom. The produced active O species on subnanometer gold clusters are responsible for the high catalytic selectivity toward the formation of styrene epoxide.

best among various metal-oxide/nanogold complexes. Haruta et al. showed that the size and shape of gold nanoparticles and the properties of substrate support significantly affect the catalytic performance of gold nanoclusters in propylene oxidations. For example, for Au/TiO2, only hemispherical Au NPs with size of 2.0−5.0 nm were active for propylene epoxide synthesis, whereas spherical Au NPs larger than 5.0 nm in diameter favored the complete combustion of propylene to CO2, and Au clusters smaller than 2.0 nm led to propylene hydrogenation.23 Recently, Turner et al. demonstrated a sharp size threshold in catalytic activity of gold nanoparticles toward styrene oxidation. It was found that the gold nanoparticles with size larger than ∼2 nm are completely inactive to styrene oxidation, while nanoparticles with a mean size of ∼1.5 nm (around 55 atoms) exhibited the highest catalytic activity toward styrene oxidation using the dioxygen as the sole oxidant.24 However, despite considerable efforts that have been devoted to investigating the factors that affect catalytic activities and selectivity of gold nanoparticles toward olefin oxidation, the origin of catalytic activity and selectivity of gold nanoparticles remained unclear. Due to the size dispersion and structural heterogeneity of metal-oxide-supported gold nanoparticles, the remarkable catalytic behavior might in part arise from strong interaction between the gold and the metal-oxide support. As a result, the explanation of the effectiveness of gold nanoparticles is complicated by the presence of support or addition of additives, which led to an incomplete understanding of the catalytic role of tiny gold nanoparticles. Another important issue related to the reaction mechanism of gold-catalyzed olefin oxidation is the product selectivity. In general, the partial oxidation of olefins leads to the formation of epoxides, aldehyde, acetone, etc. Among these oxidation products, the epoxides are versatile intermediates in the organic synthesis of fine chemicals and pharmaceuticals. How to increase the selectivity and efficiency toward the epoxidation of olefins to obtain epoxides is therefore a very important question, which requires an in-depth understanding of the reaction mechanism of catalytic oxidation reactions. In recent years, the DFT calculations have been performed to investigate the reaction mechanism of styrene oxidation on nanogold clusters (Scheme 1). Gao et al. studied the mechanism of styrene epoxidation on Au38 and Au55 clusters stimulated by the experimental discovery of shape size threshold effects of catalytic activity of gold nanoclusters Scheme 1. Styrene-Selective Oxidation on Gold Clusters with the O2 as the Sole Oxidant or the H2/O2 Mixture as Reactants and Populations of Oxidation Products

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COMPUTATION METHOD AND DETAILS In this study, the atomic structures of all subnanometer gold clusters are taken from previously determined global-minimum structures based on a combined photoelectron spectroscopy (PES) measurement and density functional theory (DFT) global minimum search, as shown in Figure 1.31−34 In particular, the pyramid structure of the Au20 cluster has been successfully confirmed by the real-space atomic-resolution images.35 The geometric structures of intermediates and transition states are optimized using either a restricted or unrestricted DFT method associated with the general gradient approximation (GGA) in the form of the Perdew−Burke−Ernzerhof 20347



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Figure 1. Gold cluster models used in calculations. The Au27, Au28, Au30, and Au32−35 have core−shell structures.

(PBE) functional36 implemented in Dmol3 6.1.37,38 The semicore pseudopotential (DSPP) is adopted together with the double numerical (DND) basis set for the geometric optimization and transition state search. The reaction pathway of the styrene oxidation is computed using the combination of a linear synchronous transit (LST)/ quadratic synchronous transit (QST) algorithm with conjugated gradient optimization. In the calculation of O 2 adsorption energy on Au clusters and the first transition state to form the oxametallacycle intermediate, a fixed triplet spin is used for a system with an even number of total valence electrons, while a fixed doublet spin is used for a system with an odd number of total valence electrons. For all other calculations, including transition states, intermediate states, and final (product) states, the spin state of the system is set to be the singlet or doublet for a system with an even or odd number of electrons. The convergence criterion of the geometric optimization is set to be 1.0 × 10−5 Hartree for energy change, 1.0 × 10−3 Hartree/Å for the gradient, and 3.0 × 10−3 Å for the displacement, respectively. The smearing parameter is set to be 0.002 Hartree in geometric optimizations. Note that the spin crossover effect of O2 activation is not considered in this study, but the relative stabilities between the triplet and singlet spin states of the intermediate and transition states are examined. Only the lowest energy states are reported.

Figures 2 and 3 display the most favorable sites of styrene adsorption on various gold clusters. From Table 1, the adsorption energies of styrene range from 0.58 to 0.89 eV. The adsorption configuration of styrene adopts a Au−CC style, in which two carbon atoms in the terminal double CC bond are bonded to a single Au atom via the formation of a σ−π bond. Upon the adsorption to the gold clusters, the CC bond is slightly elongated due to the electron transfer to the antibond π orbital. We find that the adsorption energies of the styrene molecule are not strongly dependent on the size of the gold cluster. Rather, it is more sensitive to the coordination number of surface gold sites. The gold atoms with lower coordination number generally bind stronger with the styrene molecule, as shown in Figures 2 and 3. The most stable adsorption sites of O2 near the preadsorbed styrene molecule are further determined and represented in Figures 2 and 3. It can be found that the adsorption energies of the O2 molecule are much smaller than that of styrene, which are in range of −0.10 to −0.59 eV. Among 12 gold clusters, the Au19 cluster has the largest binding energy to O2. The calculated binding energies of O2 are close to recent theoretical studies on similar gold clusters.20 Apparently, the direct dissociation of O2 on the bare gold cluster is less possible because of weak adsorption and activation of O2. The results of weak activation of molecular O2 on gold clusters are in agreement with previous experimental studies, which show the O2 activation on gold clusters is very sensitive to the size and charge state of gold clusters.39−41 As a result, a Langmuir− Hinshelwood (LH) mechanism was adopted in all mechanism investigations. 2. Reaction Pathways of Styrene Oxidation on the Gold Clusters Using O2 as the Sole Oxidant. Recently, Turner et al. reported that the ∼1.4 nm gold clusters are effective catalysts for the selective oxidation of styrene by dioxygen.24 The results displayed above show 20% conversion of styrene to benzaldehyde and styrene oxide, with the selectivity of 82% and 12%, respectively. The control experiments confirmed that the secondary reaction of conversion of initially formed styrene oxide contributed little to the formation of benzaldehyde, which indicated that the high selectivity of the reaction is largely due to the intrinsic catalytic selectivity of gold nanoparticles. To date, most theoretical studies focused on the epoxidation mechanism of olefin oxidation on gold clusters. The

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RESULTS AND DISCUSSION 1. Adsorption of Styrene and O2 on the Gold Nanoclusters. To illustrate the mechanism of styrene oxidation, we first try to determine the adsorption site of styrene and O2 on the gold cluster. In this part of the investigations, we consider the styrene and O2 molecules to be coadsorbed on the gold catalysts without any supporters and additives. First, we perform an extensive exploration of the adsorption site and adsorption energies of styrene and O2 molecules on various sized gold clusters to determine the most favorable adsorption site. From Figure 1, the gold clusters investigated herein have structures evolved from the cage to pyramid to core−shell structure. Due to the low symmetry of most gold clusters, there are a lot of different adsorption sites on the cluster surface. Our results indicate that the plat and slot surface sites are less favorable for the adsorption of styrene and O2 than the sharp sites, in particular for the styrene adsorption. 20348



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Figure 2. Oxidation reaction pathways of styrene on cage and pyramid gold clusters, Aun (n = 16−20). The black line (Path 1) represents styrene reacting with dioxygen to generate the epoxides. The yellow line (Path 2) indicates the branching pathway leads to a four-membered ring CCOO* intermediate and hence benzaldehyde product.

epoxidation mechanisms of styrene, propylene, as well as ethylene on gold clusters or the gold surface in the presence of O2, a H2/O2 mixture, or atomic O have been studied by DFT calculations.25,26,42,43 However, the intrinsic mechanism of catalytic selectivity of gold clusters, e.g., selectively catalyzing the styrene oxidation into styrene epoxide or benzaldehyde, was not well explored by theory. Torres et al.42 and Xue et al.43 carried out DFT calculations on the selective oxidation reactions of styrene and ethylene on atomic oxygen (O) covered Au(111). The studies both found that the overall reaction energy profile for the molecular mechanism corre-

sponded to the epoxidation process. The calculated energy barriers together with the analysis of reaction rates of different reaction channels indicated that the formation of the epoxide product is favorable over aldehyde, in good agreement with previous experimental observations. To the best of our knowledge, these studies are the only theoretical works that addressed the selectivity of olefin oxidation on gold. On the basis of the stepped adsorption structure of styrene and O2 on gold clusters, we propose two competitive reaction pathways, which lead to different oxidation products such as the styrene epoxide and benzaldehyde, as shown in Scheme 2. 20349



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Figure 3. Oxidation reaction pathways of styrene on core−shell gold clusters, Aun, with n = 27, 28, 30, 32−35. The black line (Path 1) represents styrene reacting with dioxygen to generate the epoxides. The yellow line (Path 2) indicates that the branching pathway leads to a four-membered ring OMME intermediate and hence benzaldehyde product.

oxametallacycle intermediate (OMME). After the formation of the six-membered ring OMME, the scission of the O−O bond leads to a new four-membered ring OMME and an isolated O atom. The four-membered ring OMME includes a −C−C−O−

The reaction Path 1 leading to the formation of styrene epoxide is similar to the mechanism proposed previously.25,26 In the first step of the reaction, the carbon atom in the CC bond can attack the O2 molecule to form a six-membered ring 20350



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Table 1. Absorption Energy and Energy Barrier of Styrene Oxidation on 12 Gold Clusters in Different Reaction Pathsa energy barrier of TS2 AuN

absorption energy of styrene

absorption energy of O2

energy barrier of TS1

Path 1

Au16 Au17 Au18 Au19 Au20 Au27 Au28 Au30 Au32 Au33 Au34 Au35

−0.78 −0.83 −0.89 −0.76 −0.61 −0.82 −0.74 −0.88 −0.58 −0.68 −0.65 −0.65

−0.26 −0.25 −0.17 −0.59 −0.10 −0.24 −0.19 −0.35 −0.05 −0.18 −0.13 −0.22

1.05 0.91 1.08 0.73 0.93 0.93 0.87 0.60 1.06 0.86 0.84 0.69

0.76 1.11 1.23 0.92 1.31 0.83 1.14 1.28 0.90 1.20 0.92 1.20

energy barrier of TS3

Path 2 Path 1 1.12 1.06 0.82 1.35 1.09 0.72 1.22 0.99 0.74 1.07 0.77 1.20

0.57 0.51 0.74 0.46 0.31 0.46 0.28 0.30 0.77 0.67 0.50 0.07

Path 2

k(Path2)/k(Path1) (T = 298 K)

0.15 0.39 0.16 n/a 0.28 0.01 0.08 0.03 0.11 0.18 0.40 0.38

8.4 7.1 8.6 8.4 5.3 72.6 4.4 8.1 5.1 1.6 3.4 1

× 10−7 × 106 × 10−7 × 102 × × × × ×

10−2 104 102 102 102

a The energy is in units of eV. The ratio of the reaction rate between competitive reaction channels in the TS2 step (k(Path2)/k(Path1)) is calculated using the Arrhenius formula with a temperature of 298 K.

Scheme 2. Proposed Catalytic Cycles of Styrene Oxidation on Gold Clusters with O2 as the Sole Oxidanta

a

Path 1 leads to styrene epoxide as the major product, and Path 2 has benzaldehyde as major product.

summarized in Figures 2 and 3 and Table 1. From Figures 2 and 3, the first step of both Path 1 and Path 2 is the formation of a six-membered ring OMME (denoted as OMME(1) in Figures 2 and 3). There are two possible reaction pathways to form the OMME intermediate. That is, either the αC or the βC of adsorbed styrene can attack the O2 molecule to form a C−O linkage. Through comparing the energies of two kinds of attack modes, we find the attack mode including the αC to O2 has relatively lower activation energy, which is about 0.3 eV lower than the βC attack mode. Notably, the O−O bond in the formed oxametallacycle intermediate is significantly elongated in comparison to the gas-phase O2. The measured bond length of O−O in the oxametallacycle intermediate is in the range of 1.41−1.44 Å on various gold clusters. The energy barriers of this step (TS1-Path1) on various sized gold clusters are in the range of 0.65−1.05 eV. We find that the energy barrier of the αC−O linkage formation step during the styrene oxidation is much larger than that in the CO oxidation reaction. The LH mechanism of CO oxidation on gold clusters has a similar reaction step involving the attack of the C atom (in CO) to the O2.20 However, the energy barrier of the C−O bond formation in CO oxidation is generally less than 0.6 eV. The major difference between two kinds of reactions is that the C atom in the adsorbed CO molecule is positively charged, while both the αC and βC in the adsorbed styrene are negatively charged. The

framework linking to a single Au atom. It was proposed that the formation of the OMME intermediate plays a key role in the styrene epoxidation on atomic O covered Au(111).44 The cyclization of OMME results in the epoxide product. Finally, to complete the catalytic cycle, the adsorbed atomic oxygen should be removed. In fact, the atomic O can interact with the styrene molecule directly to form styrene epoxide (in major) or benzaldehyde (in minor), as observed by previous experimental and theoretical studies.25,26,44 Since this reaction step is not the ratio-determining step of overall reactions, we did not further investigate the reaction mechanism and selectivity in this step. Besides the well-studied epoxidation process (Path 1), we propose a branching reaction channel after the formation of the six-membered ring OMME, denoted as Path 2. From Scheme 2, the initial entrance reaction steps of Path 2 are the same as those of Path 1. However, after the formation of the sixmembered ring OMME, we suggest that the Au−βC linkage may break, and the resulted dangling βC terminal can attack the neighboring O atom to form a novel four-membered ring CCOO* intermediate as shown in Scheme 2. The fourmembered ring CCOO* intermediate can further break the O−O and C−C bonds simultaneously to form benzaldehyde and aldehyde to finish the catalytic cycle. The snapshots of reaction intermediate, transition state, and energies of elementary reaction steps in Path 1 and Path 2 are 20351



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βC−Au linkage in the formed OMME intermediate results in a dangling βC atom, which is capable of attacking the O atom that linked to the neighboring gold atom. A novel fourmembered CCOO* intermediate is formed after this step. It is found that the O−O bond length is further elongated to 1.52− 1.58 Å in the four-membered CCOO* intermediate. The fourmembered CCOO* intermediate is unstable and can readily dissociate into the benzaldehyde and aldehyde via the simultaneous breaking of a C−C and an O−O bond. A similar reaction mechanism that leads to selective formation of benzaldehyde was also proposed for styrene oxidation catalyzed by ligand-protected gold clusters.45 The energy barrier of these steps (denoted as TS2-Path2 and TS3-Path2) are given in Table 1. From Table 1, one may find that the simultaneous break of C−C and O−O bonds of the four-membered CCOO* intermediate has low energy barriers (0.01−0.40 eV, TS3Path2) on 12 gold clusters, which indicates that the formation of benzaldehyde from the four-membered CCOO* intermediate is very feasible. Competition of Path 1 and Path 2. It is interesting to correlate currently proposed mechanisms with the observed catalytic selectivity of gold nanoclusters in a styrene oxidation reaction.24 From Figures 2 and 3, the two reaction channels Path 1 and Path 2 are diverse after the formation of a sixmembered ring OMME intermediate. In the Path 1, the OMME intermediate experiences dissociation of a C−O bond and finally leads to the formation of a four-membered oxametallacycle intermediate and hence styrene oxide. In contrast, the six-membered ring OMME intermediate in Path 2 may directly dissociate into benzaldehyde and aldehyde through formation of a novel four-membered ring CCOO* intermediate. The comparison of the energy barriers of two diverse reaction channels, e.g., the dissociation of an O−O bond to a four-membered ring OMME intermediate and the formation of a four-membered ring CCOO* intermediate, may enable us to judge which route is energetically more favorable. From Table 1, the calculated energy barriers clearly show the competition behaviors of two reaction channels. The ratio of the reaction rate between two reaction channels (k(Path2)/ k(Path1)) is calculated via the Arrhenius formula. From Table 1, one may find that besides the Au16, Au19, Au28, and Au35 clusters all other gold clusters possess a much higher reaction rate toward the formation of benzaldehyde, in good agreement with the recent experimental observations.24 Finally, the possible catalytic active sites on different sized gold clusters and the effect of charge state of gold clusters are further discussed. Recently, the triangular Au3 site was suggested as the major active site for the CO oxidation on the neutral gas-phase gold clusters.20 Presently, one may find that the key reaction step involving the formation of a sixmembered ring OMME intermediate in both reaction channels is going through on an edge Au2 site. We have tried to locate an OMME intermediate with the O atom bonded to two different gold atoms like those found in CO oxidations but have been unsuccessful. The most stable structure of the OMME intermediate resulting from the initial attack of αC to O2 only forms a single Au−O linkage on all gold clusters. However, in the reaction Path 1, we find that the break of the O−O bond from the OMME intermediate happens more favorably on the triangular Au3 site. The direct dissociation of the OMME intermediate on a Au2 surface dimer site is energetically higher than that on the Au3 site by around 0.3 eV. The edge Au2 site and the triangular Au3 site on the gold cluster

positive charged C atom in CO facilitates the electrophilic attack to the negatively charged O atoms and hence leads to much lower energy barriers. For the epoxidation reaction of styrene on gold clusters (Path 1), the O−O bond in the six-membered ring OMME intermediate can further break and leads to a new fourmembered ring OMME intermediate and an isolated O atom as displayed in Figures 2 and 3. The OMME intermediate (Au− C−C−O−Au) was proposed as a key intermediate during the styrene oxidation on atomic O-covered Au(111),44 which can dissociate into the epoixde product via cyclization reaction. In the present, the calculated energy barriers of O−O scission in the six-membered ring OMME is in the range of 0.7−1.3 eV on 12 gold clusters (TS2-Path1). The product four-membered ring oxametallacycle intermediate has the βC and O atoms bonded to a single gold atom. In the next reaction step, the βC−Au linkage is broken, and the dangling βC may attack the O atom to form an epoxide product. The energy barriers of this cyclization reaction (TS3-Path1) are in range of 0.07−0.77 eV. Our proposed epoxidation mechanism of styrene is similar to the previous theoretical reports.25,26 From Figures 2 and 3, the overall reaction processes on 12 gold clusters are all exothermic, and the energies of transition states in each reaction step are generally below the zero energy line of initial reactants. The mechanism can well explain the epoxidation of styrene on subnanometer gold clusters in the presence of dioxygen. The further examination of size-dependent catalytic activity of gold clusters indicates that the energy barriers and adsorption energies of reactants are not strongly dependent on the cluster size, even though we also note that the adsorption of styrene becomes weaker with increasing the size of gold clusters. From Table 1, the gold clusters with a core−shell structure (e.g., Au32−Au35) have smaller adsorption energies of styrene in comparison to the other clusters. The decreased adsorption energy of styrene on core−shell gold clusters might be attributed to the increased coordination number of surface gold atoms. From Figure 1, the surface Au atoms are in general penta- or hexa-coordinated on the core−shell clusters Au32− Au35. Moreover, the smooth surface of the core−shell clusters is also unfavorable for the styrene adsorption. However, we note that the energy barriers of the first reaction step toward the formation of the six-membered ring OMME intermediate (TS1-Path1) on core−shell gold clusters (e.g., Au30, Au33− Au35) are lower than the smaller clusters. The formation of a six-membered ring OMME intermediate was considered as a rate-determining step during styrene oxidation.25 The decreased energy barriers of TS1-Path1 on Au30 and Au33−Au35 suggest the core−shell gold clusters have higher conversion efficiency for styrene oxidations. The described Path 1 well explains the formation mechanism of styrene epoxide. Nonetheless, the experimental observations of high yield of benzaldehyde resulting from the styrene oxidation over an ∼1.4 nm gold cluster with the dioxygen as the oxidant cannot be fully understood from Path 1. A possible contribution of the benzaldehyde product might be attributed to the secondary reaction of conversion of formed styrene epoxide. However, such contributions are little as observed by the control experiment.24 The high selectivity of the benzaldehyde product is most likely caused by the intrinsic catalytic selectivity of gold nanoclusters. Path 2 describes a competitive pathway that leads to the formation of benzaldehyde starting from the six-membered ring OMME intermediate. From Figures 2 and 3, the break of the 20352



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both serve as the active site during styrene oxidation. On the other hand, the effect of charge state of the gold cluster on the reaction mechanism is examined. It was known that the anionic gold clusters generally adsorb O2 more strongly than their neutral counterparts.40,41 We have systematically compared the adsorption energy of O2 and styrene on both neutral and anionic clusters (Table S2 in the Supporting Information) and accordingly computed the reaction rate of styrene oxidation on two kinds of clusters based on a microkinetics model. The results indicated that the selectivity of the reaction is not affected by the charge state of the cluster, by using the Au34 cluster as a benchmark model (cf. Table S3, Supporting Information). However, the reaction rate of styrene oxidation is significantly increased on the anionic gold clusters, suggesting higher catalytic activity of anionic clusters. 3. Styrene Oxidation on the Gold Clusters with H2/O2 Mixture. The above discussions of the reaction paths of styrene oxidation with O2 as the sole oxidant well explained the catalytic selectivity of subnanometer gold clusters observed by recent experiments.24 In the following discussions, we investigate a different reaction mechanism for styrene-selective oxidation on subnanometer gold clusters in the presence of a H2 and O2 mixture. A striking difference of product distribution for the propylene oxidation on gold clusters has been previously reported when the H2/O2 mixture is fed as reactants. Haruta et al. showed that the catalytic selectivity to the epoxide product was significantly improved when the H2/O2 mixture was used.27−30 To understand the intrinsic mechanism of change of reaction selectivity caused by the H2 additive, we further investigate the reaction mechanism and selectivity of styrene oxidation on gold clusters with the H2 as an additive. The cooperative coadsorption of hydrogen and oxygen on cationic gold clusters Aux+ (x = 2−7) has been explored experimentally.46 DFT calculations suggested the coadsorbed H2 molecule may promote the O2 activation to hydroperoxo (OOH) with low activation energy barriers. Moreover, it has been established that the Au nanoparticles supported on alumina are capable of generating hydrogen peroxide from H2 and O2 in solution at low temperatures.4 Recent work has also identified the OOH species from the inelastic neutron scattering when the H2 are O2 are reacted over the Au/TiO2 catalyst.47 If the role of gold catalyst is to generate H2O2 from H2 and O2, the remaining steps of the epoxidation mechanism may be described by the initial interactions of O2 an H2 to generate the active O species, as illustrated in Figure 4a. The interactions between the H2 and O2 are therefore likely to be of key importance for determining the reaction process and selectivity. Presently, the optimized coadsorption structure and adsorption energy of H2 or O2 on 12 gold clusters are summarized in Figure S1 (Supporting Information) and Table 2. From Figure S1 (Supporting Information), the adsorption energy of H2 is very slightly affected by the coadsorbed O2 molecule, indicating a weak cooperative effect of coadsorption. Moreover, the adsorption energy of H2 is also not sensitive to the size, site, and structure of gold clusters, as shown in Table 2. The adsorption energy of H2 is in a narrow range of −0.11 to −0.18 eV on different gold clusters. Despite that the adsorption energies of H2 and O2 are both small, the activation of O2 by the coadsorbed H2 molecule is very feasible. From Figure 4a, a H atom transfer process is observed upon the attack of H2 to the neighboring adsorbed O2 molecule. The reaction leads to an OOH species on the gold cluster. The energy barrier of this

Figure 4. (a) Schematic mechanism of H2-promoted epoxidation of styrene on gold. (b) Snapshot of the reactants, intermediate, and transition state of the H2-promoted O2 dissociation reaction on the Au33 cluster. The snapshots of reaction pathways on different sized gold clusters are given in Figure S2 in the Supporting Information.

step (TS1) ranges from 0.02 to 0.49 eV on different gold clusters from Table 2, which is comparable to the coadsorption energy of H2 to O2. The formed OOH species is much more stable in energy than the initial coadsorption structure of H2 and O2, suggesting the H-transfer reaction is thermodynamically favorable. In the formed OOH species, the O−O bond is significantly activated, evidenced by the much elongated bond lengths (cf. Table S1 in the Supporting Information). The OOH species can continually abstract the rest of the H atoms to form a metastable H2OO species (TS2). The energy barriers of this step on 12 gold clusters are generally less than 0.5 eV. In the formed H2OO species, we find the O−O bonds have elongated to 1.50−1.54 Å (Table S1, Supporting Information). The H2OO species is very unstable and can readily dissociate into a water molecule and an isolate O atom via the scission of the O−O bond. The energy barriers of this step (TS3) are typically less than 0.1 eV on various gold clusters. The described reaction mechanism between H2 and O2 on gold clusters is expected to be a key step leading to the high selectivity of the epoxidation process. In comparison to the mechanism that the styrene interacts directly with the O2 molecule (Scheme 2), the presence of H2 significantly facilitates the O2 activation. As a result, when the H2/O2 mixture is fed, the reaction between coadsorbed H2 and O2 is expected to proceed prior to the interaction between styrene and O2. Herein, it is interesting to explore why the adsorbed H2 molecule significantly improves the O−O activation. Table 3 presents the Hirshfeld atomic charges of H(H2−H1*) and 20353



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O(O2−O1*) atoms in the coadsorption structure. It is found that the vast majority of the H atoms are positively charged and the O atoms are negatively charged. When the H2 and O2 approach each other to reach the transition state, a clear charge transfer from the H(2) atom to O(2) atom is found from Table 3. This electron transfer mechanism is similar to the O2 activation by CO, where the positively charged C atom can electrophilically attack the O2 to form an OCOO* intermediate with relatively low energy barrier. For comparison, the charge analysis indicates that the C atoms in the CC bond of styrene are both negatively charged. The high energy barrier during the attack of styrene to O2 (Figures 2 and 3 and Table 1) most likely arises from the negatively charged C atoms. On the basis of the discussions above, the mechanism combined with the high selectivity of the epoxidation reaction of styrene on gold clusters with a H2/O2 mixture can be understood from the formation of active O species via the reaction between adsorbed H2 and O2. It was well-known that the adsorbed O atoms are capable of reacting with styrene to form styrene epoxide with high selectivity on Au(111).44 The mechanism and reaction selectivity has also been well studied by the DFT calculations using the ethylene and styrene as model molecules.42,43 As a result, we do not further study in detail the reaction mechanism between styrene and the adsorbed O atom. The major difference of selectivity of styrene oxidation on gold clusters with the O2 as the sole oxidant or H2/O2 mixture as the reactant can be understood from the different manners of O2 activation. The active O species that were generated from the interactions of H2 and O2 on gold clusters are largely attributed to the high selectivity to the epoxide product.

Table 2. Adsorption Energy and Energy Barriers of O2 Activation by H2 on the 12 Gold Clustersa AuN

adsorption energy of H2/eV

adsorption energy of O2/eV

energy barrier of TS1/eV



Au16 Au17 Au18−1 Au18−2 Au19 Au20 Au27−1 Au27−2 Au27−3 Au28−1 Au28−2 Au28−3 Au30−1 Au30−2 Au32−1 Au32−2 Au32−3 Au33−1 Au33−2 Au33−3 Au34−1 Au34−2 Au34−3 Au35−1 Au35−2 Au35−3

0.00 −0.14 −0.14 −0.13 −0.13 −0.11 −0.13 −0.14 −0.16 −0.12 −0.17 −0.18 −0.17 −0.18 −0.12 −0.14 −0.15 −0.11 −0.15 −0.15 −0.12 −0.13 −0.11 −0.11 −0.14 −0.14

−0.12 −0.08 −0.11 −0.22 −0.38 −0.05 −0.23 −0.28 −0.20 −0.24 −0.28 −0.19 −0.18 −0.21 −0.15 −0.16 −0.12 −0.14 −0.14 −0.21 −0.04 −0.13 −0.08 −0.08 −0.15 −0.19

0.07 0.13 0.08 0.31 0.11 0.49 0.17 0.14 0.24 0.02 0.39 0.28 0.35 0.23 0.36 0.35 0.41 0.36 0.20 0.46 0.47 0.34 0.37 0.09 0.22 0.19

0.46 0.40 0.28 0.46 0.52 0.55 0.24 0.30 0.48 0.06 0.56 0.10 0.24 0.29 0.26 0.45 0.30 0.17 0.16 0.36 0.11 0.17 0.09 0.42 0.20 0.27

0.02 0.09 0.06 0.07 0.04 0.03 0.05 0.05 0.04 0.08 0.11 0.06 0.06 0.04 0.06 0.08 0.11 0.05 0.09 0.11 0.17 0.08 0.11 0.04 0.07 0.04

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a

For core−shell gold clusters, we have investigated the reaction of H2 and O2 over different surface sites, denoted as AuN-1, AuN-2, and AuN3. The definition of TS1, TS2, and TS3 is given in Figure 4 and Figure S1 in the Supporting Information.

CONCLUSIONS In summary, the styrene-selective oxidation on 12 gold clusters (Au16−20, Au27−28, Au30, Au32−35) in size range of 0.4−1.0 nm are investigated by DFT calculations. The intrinsic catalytic selectivity of gold clusters, e.g., selectively catalyzing the styrene into benzaldehyde or styrene epoxide, with the presence of O2 or the H2/O2 mixture is discussed. In the case of the styreneselective oxidation with dioxygen on subnanometer gold clusters, we propose a new reaction channel involving the formation of a novel four-membered ring CCOO* intermediate, which accounts for the selective formation of benzaldehyde. The comparison of energy barriers of different reaction channels indicates that the selective oxidation of styrene to the benzaldehyde is commonly more favorable in energy than to styrene epoxide on subnanometer gold clusters, in good agreement with recent experimental observations.24 The mechanism of high selectivity of styrene epoxidation over gold clusters with the H2/O2 mixture is further explored. The subnanometer gold clusters demonstrate a weak size effect on the O2 activation. All gold clusters currently observed are capable of catalyzing O2 dissociation into active O atoms in the presence of H2 additive. We propose that the activation of O2 by H2 to form an active O species is a key step leading to the high selectivity of epoxidation reaction.

Table 3. Net Charge of H(1)−H(2) and O(1)−O(2) on Different Gold Clustersa numberAu

charge of H(1) (|e|)

charge of H(2) (|e|)

charge of O(1) (|e|)

charge of O(2) (|e|)

Au16 (RE) Au16 (TS1) Au17 (RE) Au17 (TS1) Au19 (RE) Au19 (TS1) Au28 (RE) Au28 (TS1) Au33 (RE) Au33 (TS1) Au34 (RE) Au34 (TS1)

−0.03 0.01

0.03 0.10

−0.11 −0.07

−0.07 −0.07

0.00 0.00

0.04 0.13

−0.06 −0.12

−0.04 −0.14

0.09 0.01

0.04 0.16

−0.21 −0.15

−0.21 −0.16

−0.03 −0.01

0.04 0.15

−0.09 −0.16

−0.05 −0.17

−0.03 0.00

0.03 0.17

−0.08 −0.15

−0.04 −0.16

−0.03 0.01

0.03 0.17

−0.11 −0.15

−0.07 −0.16

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a The RE (H2*+O2*) and TS1 correspond to the structural snapshots in Figure 4 and Figure S1 in the Supporting Information. O(1) and H(1) are defined as the atoms that directly link to the gold cluster. O(2) and H(2) are the atoms invloved in the electrophilic attacking.

ASSOCIATED CONTENT

* Supporting Information S

The bond length of O−O and O−H in the reactions between the adsorbed O2 and H2 and the snapshots of reaction intermediate and transition states are given. The effect of charge state of the gold cluster on the reaction mechanism and 20354



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the details of the microkinetics model are displayed. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NSFC (21103144, 21373176, 21422305) and Hunan Provincial NSFC (12JJ7002, 12JJ1003). REFERENCES

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Mechanistic Insight into the Styrene-Selective Oxidation on

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