Mechanistic Insights into Propene Epoxidation with O2–H2O Mixture

Research Article pubs.acs.org/acscatalysis

Mechanistic Insights into Propene Epoxidation with O2−H2O Mixture on Au7/α-Al2O3: A Hydroproxyl Pathway from ab Initio Molecular Dynamics Simulations Jin-Cheng Liu,† Yan Tang,† Chun-Ran Chang,*,†,‡ Yang-Gang Wang,*,† and Jun Li*,† †

Department of Chemistry and Key Laboratory of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China ‡ Institute of Industrial Catalysis, School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China S Supporting Information *

ABSTRACT: We have studied the mechanism of propene epoxidation with O2−H2O mixture on Au7/α-Al2O3(0001) using global minimum approach, density functional theory (DFT), and ab initio molecular dynamics (AIMD) methods. It is found that water can easily dissociate on coordinatively unsaturated surface Al sites to form a hydroxylated Al2O3 surface. The Au7 cluster on such a surface prefers anchoring at bare Al sites and transfers from upright to flat structures with a decrease of surface hydroxyl groups. The activation of molecular oxygen via a hydroperoxyl (OOH) intermediate (i.e., O2 abstracting a hydrogen atom from coadsorbed water) is identified to be a feasible pathway from both AIMD simulations and static DFT calculations. Additional water promotes the H-transfer process by constructing a hydrogenbonding chain with reactants. The resulting OOH turns out to be a key oxidative species for subsequent propene epoxidation. It can either dissociate into atomically oxygen species to epoxide propene or combine with propene directly, forming an important *C3H6OOH species that could be easily transformed to epoxypropane. This study reveals the important role of water in propene epoxidation and may stimulate further exploration and the usage of O2−H2O mixture as oxidizing agent in other oxidation reactions. KEYWORDS: propene epoxidation, gold cluster, O2 activation, hydroperoxyl, water, AIMD 0.88% and a PO selectivity of 52%.11 Although the current performance is not good enough for an industrial application, it does pave a potential green and cheap alternative to manufacture of PO. Haruta et al. indicated that the O2−H2O process could be further improved by refining the catalyst and controlling the reactivity of oxygen radicals on gold clusters.11 Meanwhile, Molina and co-workers showed that upon replacing H2 by water, the selectivity of PO increased obviously over gold clusters with 6 to 10 atoms soft-landed on Al2O3 substrate.10 Not only in propene epoxidation does water play an important role, but in some other gold-catalyzed reactions, such as CO oxidation, water often presents promotional effects.13−21 However, the role of water in facilitating the reactions and the corresponding active sites remain elusive, requiring further experimental and theoretical studies.17 It is reported that quasi-equilibrated molecular adsorption of O2 and H2O on Au clusters lead to the O2 activation via a hydroperoxyl (denoted as *OOH hereafter, with * representing the adsorbed state) intermediate, which account for the

1. INTRODUCTION Supported gold nanoparticles and nanoclusters have attracted great attention in current catalysis science since Haruta, Hutchings, and others found that nanosized gold were highly reactive for a variety of chemical reactions.1−5 The epoxidation of propene (C3H6) to propene oxide (PO) is an important reaction process in chemical industry because the product PO is a substantial precursor in the production of many commodity chemicals such as polyether polyols and propylene glycol. However, currently PO is usually produced by using chlorine or peroxides as oxidizing agents, which leads to significant amounts of waste byproducts or high costs. In the last decades, tremendous efforts were made to find a new efficient ecofriendly method to replace the old chlorohydrin and peroxide processes.6,7 Haruta and co-workers first showed in 1998 that small gold nanoparticles supported on TiO2 could act as a catalyst for the direct epoxidation of propene by an O2−H2 gas mixture, but the direct use of expensive H2 and the low conversion rate means that significant improvements in this process are still required.8,9 Recently, water is shown to be capable of replacing H2 in propene epoxidation over gold catalysts.10−12 The new oxidant, O2−H2O mixture, is able to achieve a C3H6 conversion of © XXXX American Chemical Society

Received: January 5, 2016 Revised: February 20, 2016

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mixture. Our computational modeling study not only confirms some of the previously proposed mechanisms of waterenhanced propene epoxidation but also provides some novel insights that are essential for understanding gold catalysis. Global minimum searching of the optimal gold cluster structures on the alumina surface demonstrates that Au7 clusters are inclined to transfer from upright to flat structures with more available bare Al sites. AIMD simulations clearly show that *OOH species can be readily formed from coadsorbed *O2 and *H2O at the Au/α-Al2O3(0001) interface, where O2 prefers being adsorbed at the edge of Au clusters while hydrogen can frequently transfer between *OH, *OOH, and *H2O. The reaction of the first propene with dissociated *OOH is divided into three elementary steps with the ratedetermining step (RDS) barrier of 0.82 eV. These findings help to enrich the understanding of gold catalysis on oxide support surfaces.

remarkable reactivity and water effects in gold-catalyzed oxidation reactions.13−16,21−25 Landman et al. first reported the formation of *OOH-like complex from coadsorbed O2 and H2O on gold clusters supported on a defect-free MgO (100) facet by DFT calculations.15 We found in 2011 that the *OOH intermediate could be produced easily by abstracting a hydrogen atom by O2 from coadsorbed water.24 This hydroperoxyl pathway of oxygen activation was later investigated for O2 coadsorbed on gold nanoparticles with alcohols, amines, acids, alkanes, alkenes, silylane, and other hydrogencontaining agents.23,24,26,27 Recently, on the basis of experimental kinetic isotope effect, Saavedra et al. further confirmed the hydroperoxyl pathway and suggested that hydrogen transfer between coadsorbed H2O and O2 at the metal−support interface promotes O2 binding and activation. The resulting Au-OOH species serves as a good oxidizer for CO oxidation.13 Despite previous theoretical efforts, there are still some issues regarding the role of water in computational modeling of the catalytic procedure: (i) the role of the oxide support were either too simplistic or underestimated; (ii) locally optimized surface metal clusters might not lie in the most stable adsorption configuration during theoretical simulations; (iii) static DFT calculations without considering in situ dynamic reaction conditions may lead to subjective judgment. Among the various issues, the following requires special attention in computational modeling and simulation of heterogeneous surface catalysis. First, most nanostructures or bulk surfaces are not naked under catalytic reaction conditions.28 For synthesis, surface ligands are essential and difficult to be removed under usual conditions. For example, the hydroxyl groups are often involved on oxide surfaces while partially oxidized metal atoms might exist on the surface of metal nanoparticles.28,29 Both experiments and theoretical calculations revealed that molecularly adsorbed water on Alterminated surfaces is metastable and can dissociate readily.30−36 Although the hydroxylation state of the support was addressed in a number of theoretical studies,37−45 few computational models have included water or hydroxyl in their supercell slab models when dealing with complex catalytic systems. Many of the previous theoretical calculations regarded water or hydroxyl as a necessary reactant rather than an often existing species in catalytic environment during catalysis processes.10,15,17,45−49 Second, even though the morphology of gold clusters is known to have an important impact on the catalytic behaviors,50 in most cases, only locally optimized surface clusters are chosen as initial subjective configurations, which cannot exclude the possibility that such adsorption structures are merely metastable local-minimums with relatively high energy. Obviously, subsequent mechanistic computations based on unstable adsorption structures might cast shadow on the credibility of the calculated results. For oxide-supported Au catalysts, searching for a stable adsorption structure is viewed as a key to establish a reasonable computational model.50−52 Third, the reaction pathways established by static DFT calculations may involve subjective judgment because the dynamic reaction conditions cannot be fully sampled with preselected models. Using ab initio molecular dynamics (AIMD) simulations to address the surface catalysis issues is desirable to narrow the gap between theoretical models and in situ experimental reaction conditions.53−59 In this paper, we have chosen Au7/α-Al2O3(0001) as a catalyst model, which was reported by Molina experimentally,10 to study the propene epoxidation process with O2−H2O

2. COMPUTATIONAL DETAILS Computational Models. A α-Al2O3(0001)-p(3 × 3) surface slab model was used to model the Al2O3 substrate. The slab consists of three Al−O−Al trilayers (9 atomic layers), where the bottom one Al−O−Al layer and bottom hydroxyl groups were frozen while the remaining layers were allowed to relax. All the supercell slabs were repeated periodically with a 15 Å vacuum layer between the images in the direction of the surface normal. Both the top and bottom slab surfaces were hydroxylated to eliminate the slab dipole (Figure 1). The Au7

Figure 1. Structure of the fully hydroxylated α-Al2O3(0001) (a) side view and (b) top view. The surface layer contains two kinds of hydroxyl groups, OadsH and OsH, where Oads represents oxygen from water and Os represents oxygen from substrate.

cluster that was well characterized in previous studies48,60−63 was chosen to model small subnanometer-sized Au clusters. In this work, we calculated the adsorption energies according to the following equation Eads = E(slab + adsorbate) − E(slab) − E(adsorbate)

where E(slab+adsorbate), E(slab), and E(adsorbate) are the calculated electronic energy of species adsorbed on the surface, the bare surface, and the gas-phase molecule, respectively. Similarly, the dissociative adsorption energy is defined as Edis = E(slab + dissociated adsorbate) − E(slab) − E(adsorbate)

and the reaction energy is defined as ΔE = E(products) − E(reactants). Considering the adsorption−desorption equilibrium, we investigated (a) the fully hydroxylated α-Al2O3(0001) surface with nine H2O molecules dissociatively adsorbed on αAl2O3(0001) in one cell. This full hydroxylation of the alumina surface results in one OadsH on every Al site and one OsH for every three subsurface O atoms (Figure 1), where Oads represents oxygen from water and Os represents oxygen from α-Al2O3 surface; (b) one-water-absent surface with eight H2O molecules dissociatively adsorbed on α-Al2O3(0001) (Figure 2526

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realistic catalytic environment is important to build a reasonable computational model. We first performed AIMD simulations at 700 and 473 K to illustrate how the finite temperature influences the nature of adsorbed water on αAl2O3(0001). Eleven H2O molecules are preoptimized on bare α-Al2O3(0001) surface with each surface Al site occupied by one H2O and the remaining two H2O molecules interacting with other water molecules through hydrogen bonds. All H2O molecules are allowed to bind at the surface during a 20 ps AIMD trajectory at T = 700 K (Figure 2) and T = 473 K (Figure S5).

S3a); and (c) two-water-absent surface with seven H2O molecules dissociatively adsorbed on α-Al2O3(0001) (Figure S3b). All computational models are justified by estimation of free energy at realistic conditions (Figure S1 and S2). DFT Parameters. The Quickstep module in CP2K simulation package64 was used to do all the DFT calculations using periodic boundary conditions and P1 space symmetry. The spin-polarized Perdew−Burke−Ernzerhof (PBE) exchange−correlation functional65 was adopted with mixed Gaussian and plane-wave (GPW)66 basis sets for electronic structure calculations. Molecularly optimized double-ζ valence plus polarization (DZVP) basis sets were used to minimize the basis set superposition errors.67 A 350 Ry energy cutoff was used for the plane wave (PW) calculation of the electrostatic energy terms. Core electrons were modeled by Geodecker− Teter−Hutter (GTH) pseudo potentials68 with 11, 3, 6, 1, and 4 valence electrons for Au, Al, O, H, and C, respectively. The gamma point approximation was used for Brillouin Zone integration. The calculations of the location and energies of the transition states were performed using the dimer method69−72 with convergence criterion for the maximum force of 4.5 × 10−4 atomic units and the maximum geometry change of 3.0 × 10−3 atomic units. To verify transition states corresponding to true saddle points on the potential energy surface, vibrational analysis was performed to test imaginary frequency and vibration mode. Global Minimum Searching of Surface Clusters. TGMin program developed at our group at Tsinghua University was used to search for the global minimum structure of Au7 cluster on different alumina surfaces. The TGMin program, which has been successfully used in searching of the global minimum structures of gas-phase boron clusters73−77 and metal oxides,78 is based on improved basin-hopping (BH) algorithm,51,79 but with a series of chemical constraints implemented so as to reduce the size of the search space. Au7 clusters were searched amounting to more than 500 structures for each ease in gas phase or on hydroxylated Al2O3 surfaces. AIMD Simulations. As gold clusters tend to undergo facile adsorption-induced structural changes,53,54,56,57,80−82 it is essential to explore the geometry and morphology change of gold clusters and nanoparticles during reaction conditions. The Born−Oppenheimer molecular dynamics (BOMD) simulations were carried out in the canonical (NVT) ensemble employing Nosé−Hoover thermostats83,84 with a time step of 1 fs for simulations of Au clusters or 0.5 fs for all the other processes during 15−30 ps of well-equilibrated trajectory. We tested the temperature and potential energy equilibrium for every MD simulation to ensure good MD statistics (Figure S4). During the short time simulation, only low-energy barrier events can be observed and this may preclude the observation of slow processes. To partially overcome this limitation, we have performed simulations at both high temperature (700 K) and practical one (473 K), where the high temperature simulations could rapidly explore a large volume of phase space and the practical ones could represent the real reactivity for the reaction processes. The temperature-controlled strategy was introduced in previous studies.53,54

Figure 2. (a) Eleven molecularly adsorbed H2O before AIMD simulation. (b) Selected snapshots of the AIMD trajectory for 11 H2O on α-Al2O3(0001) after 20 ps at 700 K.

AIMD simulation reveals that hydrogen atoms can spontaneously transfer between molecularly adsorbed H2O and surface oxygen. Meanwhile some H2O molecules desorb from surface Al sites and bind with nearby surface hydroxyl groups through hydrogen bonds. From the snapshots in Figure 2b, after ∼20 ps of simulations, one H2O molecule desorbs from the surface Al site, four H2O molecules dissociate on the surface, four H2O molecules remain molecularly adsorbed on Al site, and the rest of them randomly bind on the surface through hydrogen bonds. This dynamics suggest that water molecules on Al-terminated surfaces are metastable and can dissociate easily to hydroxyls, which is of special importance for the physics and chemistry on the α-Al2O3 surfaces. We have obtained similar results from simulations at 473 K (Figure S5). 3.1.2. Water Dissociation on α-Al2O3(0001). From AIMD simulations, three kinds of H2O dissociation pathways are observed, which are denoted as IS1 → FS1 (i.e., 1−2 dissociation with hydrogen transferred to a neighboring surface oxygen, Figure 3a black curve), IS2 → FS2 (i.e., 1−4 dissociation with hydrogen transferred to a second-neighboring surface oxygen, Figure 3a red curve), and IS3 → FS3 (i.e., H2Oassisted 1−4 dissociation, Figure 3b). After water dissociation, two kinds of hydroxyls appear: OadsH and OsH. The reaction energies of IS1 → FS1 and IS2 → FS2 are calculated to be −0.60 eV and −0.61 eV, corresponding to activation barriers of 0.53 and 0.41 eV, respectively (Figure 3a), which are in good agreement with previous studies.30,33,36 When a second H2O(2) is introduced to the IS2 → FS2 pathway, as depicted in Figure 3b (IS3 → FS3), the two H2O molecules dissociate following an SN2-like mechanism (Figure 3b), i.e., while one hydrogen atom of H2O(1) transfers to H2O(2), another hydrogen atom on H2O(2) is simultaneously abstracted by the surface oxygen. There is virtually no barrier, with the dissociation barrier calculated as only 0.05 eV, which is 0.36 eV lower than that of IS2 → FS2 pathway. Constructing a hydrogen bonding chain with reactants might account for the enhanced role of H2O in many heterogeneous catalytic reactions, which we will discuss further in O2 activation.

3. RESULTS AND DISCUSSION 3.1. Formation of Hydroxylated α-Al2O3(0001) Surface. 3.1.1. AIMD Simulation of Water on α-Al2O3(0001) Surface. Understanding the behavior of Al2O3 surface under 2527

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perform a global-minimum searching of the structures of gasphase Au7 clusters and find out reported structures and several new structures, indicating a good global-minimum searching statistics.60,61,63 All Au7 clusters with relative energy below 0.6 eV are listed in Figure S7. Then, global minimum searching of the structures of Au7 cluster on α-Al2O3(0001) surface are also investigated by TGMin program. Three kinds of substrate surfaces with different hydroxylation degrees are considered, that is, the fully hydroxylated surface, the one-water-absent surface, and the two-water-absent surface (Figures 1 and S3). Figure 4a shows a configurational energy spectrum of Au7 on fully hydroxylated αAl2O3(0001), in which representative configurations are depicted according to their relative energies. The potential energy surface is relative rugged, with hundreds of low-lying structures located within ∼1 eV with respect to the most stable one we searched. All the Au7 clusters present vertical or slightly slant geometry over hydroxyl groups, because Au7 can only weakly bind to the fully hydroxylated surface. Considering the above-mentioned dissociation equilibrium, we also searched the structures of Au7 on partially bare surfaces. It is interesting that Au7 clusters on the one-water-absent surface appears with vertical or flat geometries (Figures 4b), whereas Au7 clusters on the two-water-absent surface only present flat geometries (Figures 4c), which are supported by statistics of low-energy structures (Figure S8) and further AIMD simulations (Figure S9 and S10). Although the α-Al2O3(0001) surface is rugged, the most stable Au7 configurations on the three kinds of surface all present proximate Cs symmetry, which is also the stable symmetry of gas-phase Au7 cluster (Figure S7). These results indicate that the Au7 clusters are inclined to transfer from upright to flat structures with more available bare Al sites, which is in agreement with previous experimental studies that showed the same tendency of structure change with increasing temperature.85 This trend, in fact, originates from the strong gold−support interaction. With the existence of bare Al sites, gold cluster strongly interacts with the support to form a flat structure and as a result creates a clear Au−Al2O3 interface. The interface plays a vital role in the following O2 activation process,

Figure 3. Potential-energy profiles for water dissociation on αAl2O3(0001). (a) 1−2 dissociation (IS1 → TS1 → FS1, red curve), 1− 4 dissociation (IS2 → TS2 → FS2, black curve). (b) H2O-assisted 1−4 dissociation (IS3 → TS3 → FS3).

3.2. Adsorption of Au 7 on α-Al 2O 3 (0001). The interaction between gold cluster/nanoparticle and supported oxide plays a crucial role in the geometric and electronic properties of gold, which strongly affects the catalytic performances. Therefore, it is vital to understand the structural features of gold on the substrate. The program used to search for the stable structure of Au7 on α-Al2O3(0001) is TGMin package developed in our group.73 To test its reliability, we first

Figure 4. Geometric structures and potential energies of Au7 on (a) fully hydroxylated α-Al2O3(0001) surface, (b) one-water-absent α-Al2O3(0001) surface, and (c) two-water-absent α-Al2O3(0001) surface. Representative configurations are depicted according to their relative energies. a1, a2, a3 are selected configurations on fully hydroxylated surface; b1, b2, b3 are selected configurations on one-water-absent surface; c1, c2, c3 are selected configurations on two-water-absent surface. 2528

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ACS Catalysis where the O2 at first adsorbs on the gold cluster and is then protonated by the water at the interface via a hydrogen transfer process. Figure 5 shows the electron density difference between Au7 and the substrate. One can see that there is only little density

Figure 6. Adsorption modes of O2 on gold: (a) μ1-η1 mode, (b) μ2-η2 mode, (c) μ4-η4 mode.

We first studied the adsorption of O2 via μ4-η4 mode, and we found that all the adsorption energies calculated were close to 0 eV and no charge-transfer occurs between Au cluster and O2. Therefore, the μ4-η4 adsorption mode is excluded in this case. From the molecular orbital analysis, we find that the highest occupied molecular orbital (HOMO) of bare Au7 cluster (Figure 7a,c) mainly appears at the edges rather than at the

Figure 5. Electron density difference (δρ) of Au7 supported on (a) fully hydroxylated α-Al2O3(0001) surface, (b, c) one-water-absent surface, and (d) two-water-absent surface (δρ = ρtot − ρAu7 − ρAl2O3). Here a1, b1, b2, c1 correspond to the low-energy structures shown in Figure 4. The green isosurface indicates the increase of electron density, and the blue isosurface indicates the decrease of electron density with an isosurface value of 0.003 electrons/Å3.

Figure 7. Isosurfaces of the highest occupied molecular orbital (HOMO) of b1 (a) and b2 (c), and the most stable adsorption configuration of O2 on b1 (b) and b2 (d).

change between hydroxyls and Au7 cluster (Figure 5a) on the fully hydroxylated surface, whereas a significant density increase occurs between Au7 and surface Al site on partially bare surfaces (Figure 5b−d), which represent stronger chemical bonding. Because of the much higher electronegativity of Au than Al, Au atoms tend to obtain electrons from adjacent Al atoms. Bader charge analysis shows that the charge of Au atoms bound to Al atoms decrease from 0.01 |e| to −0.43 |e|, and the charge on adjacent hydroxyls increases from ∼ −1.96 |e| to ∼ −1.85 |e|, which leads to partial polarization of the adsorbed Au7(OH)n complex as discussed in Supporting Information (Figure S11). Eads(Au7) of a1 in Figure 4 is −1.53 eV, whereas Eads(Au7) on partially bare surfaces of b1 is −3.76 eV and c1 is −5.46 eV, which shows an increasing stability of Au7 with more bare Al sites. From the projected electronic density-of-states (PDOS) analysis about Au7 on one-water-absent surface (Figure S12), the 3s and 3p orbitals of Al have large overlap with the 5d orbitals of Au, suggesting the formation of the Au− Al chemical bond. Subsequent AIMD analysis indicates that the robust Au−Al bonds likely ensure the stability of Au7 on the support even at 700 K. 3.3. Chemical Behavior of O2−H2O on Au7/α-Al2O3 (0001). 3.3.1. O2 Adsorption. Upon the finding of stable and practical Au7/α-Al2O3(0001) structures, we are now in a position to investigate the catalytic activity of such catalysts. Due to the stability of triplet O2, the activation of O2 is often considered to be the RDS in nanogold catalysis.50,86 We first investigated O2 adsorption on b1 and b2, the low-lying structures of Au7 on one-water-absent α-Al 2O 3 (0001). According to previous studies,86−88 we considered three kinds of O2 adsorption modes (Figure 6), that is, μ1-η1 mode with only one oxygen atom bound to one gold atom, μ2-η2 mode with each oxygen atom bound to one gold atom, which is expected to occur on edges of Au clusters or nanoparticules, and μ4-η4 mode with each oxygen bound to two gold atoms, which is expected to happen on (100) facet of gold.

plane of Au7. Previous studies have shown that the key point of O2 adsorption on gold clusters is that the HOMO energy level of the gold clusters should be close to the πg* orbital of O2 and the two orbitals must have a good overlap. 26,50,54,86 Calculations for O2 adsorption on gas-phase gold clusters have verified this point.86 The subsequent calculations of oxygen adsorption on b1 and b2 show that O2 can weakly adsorb on the edge of Au7 clusters with the most stable adsorption energy −0.27 eV on b1 and −0.48 eV on b2. The most stable binding configurations for the two models are displayed in Figure 7b,d, in which one oxygen of O2 approaches to the HOMO of gold cluster via μ1-η1 mode, and another oxygen interacts with the surface hydroxyls through the hydrogen bond, which are supposed to further stabilize the adsorbed O2. After adsorption, the O−O bond length has been elongated to 1.32 Å on b1 and 1.31 Å on b2, nearly 8% longer than that of free O2 (1.21 Å), indicating that molecular O2 is already activated by forming a surface superoxide (Table S2). Further reaction between *O2 and the adjacent *OH has a barrier of 0.92 eV and an endothermic energy of 0.46 eV (Figure 11a, path-i). Therefore, from both thermodynamic and kinetic aspects, this reaction is not easy to occur at relatively low temperature. 3.3.2. AIMD Simulations of O2 Activation. To elucidate how water affects the adsorption of O2, we performed AIMD simulations for O2 adsorption by introducing 20 water molecules into the system at 473 K. The starting configuration is shown in Figure 8a, where O2 adsorbs at the edge site of Au7 cluster. Individual snapshots from this trajectory are displayed in Figure 8a−d. The frequent hydrogen transfer also occurs in this case, similar to the AIMD simulation results of 11 H2O molecules on bare α-Al2O3(0001) (Figures 2 and S5). At ∼8 ps (473 K), we observed that the hydrogen on *H2O is abstracted by *O2, leading to the formation of *OH and *OOH species 2529

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experiments.16,89−92 Based on the evolution of trajectory, the hydrogen peroxide can desorb from the gold cluster after 24 ps. Further investigation of the reaction *H2O + *O2 → *OOH + *OH is performed by analyzing the Owater−H and Ooxygen−H pair. As shown in Figure 9b, separation of the O−H pair can be roughly divided into three zones: (i) 0.8−1.5 Å, hydrogen atom interacts with oxygen atom through chemical bond. A typical trajectory is depicted by the black curve ranging from 7.0−7.2 ps in Figure 9c. The fast vibration has periods around 10 fs (i.e., the frequency is 100 THz); (ii) 1.5−2.5 Å, hydrogen atom interacts with the oxygen atom through the hydrogen bond. A typical trajectory is depicted by the red curve ranging from 7.5− 7.6 ps in Figure 9c. The hydrogen bond does not hold long, and the H atom frequently shuttles between the nearest two O atoms. The vibrational frequency of the original bonded O−H is reduced from 100 to 80 THz (corresponding to a vibration period of ∼12.5 fs) and leads to a larger amplitude, which is beneficial for the protonation reaction; (iii) > 2.5 Å, separation between the O−H pair is too large to cause effective interaction (see the red curve ranging from 7.0−7.2 ps in Figure 9c). Statistical analysis of O−H pair distribution is shown in Figure 9d, during the formation of *OOH (t < 8 ps), there is a small peak at ∼1.5 Å, marking the hydrogen bond distance between *O2 and *H2O. The highest peak at 1 Å represents the O−H chemical bond length. Within 8−14 ps, the distribution of the Owater−H and the Ooxygen−H pair is reversed, which indicates the formation of *OOH and deprotonation of *H2O.

Figure 8. (a−d) Selected snapshots of the AIMD trajectory for *O2 + 20 H2O + Au7/α-Al2O3(0001) at 473 K to show the reaction *H2O + *O2 → *OOH + *OH and *H2O + *OOH → *HOOH + *OH.

(Movie 1). The intermediate *OOH species, albeit a transient product, is considered to be essential for propene epoxidation.11,24 During the AIMD trajectory, we only observed hydrogen transfer from adsorbed H2O or gas-phase H2O but not from adsorbed OH, which is consistent with static DFT calculation results. These AIMD simulation results provide direct evidence for the experimental speculation that *OOH may readily form from coadsorbed O2 and H2O at finite temperature.13−15,24 In addition to *OOH, hydrogen peroxide was also detected during AIMD simulation. At ∼16 ps, *OOH obtains another hydrogen from adsorbed water and forms *HOOH (Figure 8c, Movie 2). *HOOH, *OOH, and *O2 have all been detected in

Figure 9. (a) Sketch of the reaction *H2O + *O2 → *OOH + *OH and O−H pair on Au7/α-Al2O3. (b, c) Owater−H and Ooxygen−H pair trajectory at 473 K within 14 ps simulation. (d) Pair distribution before (t < 8 ps) and after (t > 8 ps) the formation of *OOH. 2530

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depletion of Au7 cluster. Spin density analysis also indicates that electrons on one of the πg* orbital of *O2 has been paired (Figure S13b) with the spin moment about 0.99. Therefore, the calculated bond length, net charge, and spin properties all suggest that the molecularly adsorbed O2 is activated into a superoxo-like species in the presence of coadsorbed water. The subsequent H-transfer from *H2O to *O2 to form *OOH only experiences an activation barrier of 0.34 eV and an exothermic reaction energy of −0.49 eV (Figure 11a, red curve). The barrier is pretty close to those on naked Au10 and Au38,24 but the reaction energy changes from endothermic to exothermic, reflecting the critical influence of support on the elementary steps. When a second H2O molecule is involved in the H-transfer process, the activation barrier and reaction energy is reduced to 0.16 and −0.31 eV, respectively (Figure 11a, black curve), in good agreement with the water-assisted hydrogen transfer on the Au(111) surface.23 The notable reduced barrier, 0.16 eV, could well explain why the hydrogen transfer from two water molecules to *O2 only needs 3 ps during AIMD simulations. The formed *OOH remains a superoxo-like species after hydrogen transfer, as is analyzed in the Supporting Information (Table S2). Recently, Saavedra et al. reached a similar conclusion regarding the activation of O2 via *OOH intermediate on gold/TiO2 by using a Michaelis− Menten (M−M) kinetic model.13 Size effect was investigated by comparing the Au7 cluster with nanoparticles (Figure S14). The possibility of O2 adsorption on the gold particles is low due to its small energy of adsorption, which will hinder the oxidation of O2 to *OOH on nanoparticles.86,97 In summary, O2 prefers being adsorbed on the edge of gold clusters and stabilized by hydroxyl or water on oxide surface. The hydrogenation of *O2 to *OOH turns out to be a feasible pathway for the activation of O2 at gold/oxide interface from both kinetic and thermodynamic aspects.

Another AIMD simulation that starts from a similar stochastic configuration with 20 water molecules and one *O2 is also performed, in which we observed the hydrogen transfer process between *O2 and two H2O molecules. As shown in Figure 10 and Movie 3, while *O2 abstracts a

Figure 10. (a−c) Selected snapshots of the MD trajectory for *O2 + 20 H2O + Au7/α-Al2O3(0001) at 473 K to show the reaction *(H2O)2 + *O2 → *OOH + *OH + H2O.

hydrogen atom from H2O(1), a hydrogen atom of *H2O(2) can simultaneously transfer to OH(1), yielding the final species *OOH, H2O, and *OH. The middle H2O(1) is actually not consumed in the whole reaction but primarily acts as a mediator for transferring hydrogen. Although such a dynamic H-transfer process has been studied in our and other pervious works by using static DFT methods,23,93−96 it is the first time this role of water is observed by AIMD simulations and can be achieved in only ∼3 ps. 3.3.3. Static DFT for O2 Activation. To have a quantitative understanding of the O2 activation, we further performed static DFT calculations in this subsection. The b2 configurations with one or two undissociated water molecule(s) are employed as catalyst models. It is shown that *O2 can be stabilized by *H2O via hydrogen bonds with the adsorption energy of O2 slightly decreasing from −0.48 eV to −0.64 eV, which can be seen in Figure 11. The adsorbed O2 in ii-b has a net Bader charge of −0.47 |e| and a O−O bond length of 1.31 Å (Table S2). The electron density difference in Figure S13a shows that one πg* orbital of *O2 is partially occupied, in synergy with the charge

Figure 11. O2 adsorption and hydrogen transfer process on the catalyst. (a) Energy profiles of the three reactions: (b) path-i: *OH + *O2 → OOH + *O, (c) path-ii: *H2O + *O2 → *OOH + *OH, and (d) path-iii: *(H2O)2 + *O2 → *OOH + *OH + H2O, where the blue balls represent oxygen atoms from dioxygen, and the green balls represent oxygen atoms from water. 2531

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Figure 12. Optimized structures and energy profile for the formation of the first PO by path-iv (black curve) and path-v (red curve). Path-iv: (1) *OOH + CH3CHCH2* → CH3CHCH2OOH*; (2) CH3CHCH2OOH* → CH3CHCH2O* (OMME) + *OH; (3) CH3CHCH2O* (OMME) → CH3CHCH2O* (PO). Path-v: (1) *OOH → *O + *OH; (2) *O + CH3CHCH2* → CH3CHCH2O* (OMME); (3) CH3CHCH2O* (OMME) → CH3CHCH2O* (PO). The blue balls represent oxygen atoms from dioxygen, and the green balls represent oxygen atoms from water as shown in Figure 11.

3.4. Catalytic Process of Propene Epoxidation. In this subsection, we investigate the elementary steps of propene oxidation by *OOH using the static DFT method. 3.4.1. Propene Epoxidation with *OOH. We first investigate the path of propene reacting with *OOH directly (Figure 12, path-iv). As reported by previous experimental and theoretical studies, unsaturated hydrocarbons prefer adsorbing at lowcoordination gold atoms via the CC bond.98−101 Our results show that propene preferentially adsorbs at the edge of gold clusters, because the LUMO and HOMO of gold cluster has most of their distributions at these low-coordinate edge sites on Au7 clusters and stable adsorption configuration require good overlap between LUMO of Au7 cluster and the πu bonding orbital of CC bond. The adsorption of propene on the gold atom that linked with *OOH has an adsorption energy of −0.24 eV, and the adsorption at the adjacent gold site is more stable with a lower adsorption energy of −0.72 eV (Figure S15a, b). Considering the relatively large size of propene, the stable adsorption at the adjacent gold atom will create a steric effect to hinder the adsorption of propene at the site where the OOH adsorbs. After adsorption, CC bond length increases

from 1.32 to 1.39 Å, indicating that the CC bond is weakened. The subsequent CC activation and oxidation can be divided into three steps, as shown in Figure 12, path-iv. First, the *C3H6 combines with *OOH to produce CH3CHCH2OOH* complex, through which the CC bond length increases from 1.39 to 1.51 Å, and the O−O bond length of OOH increases from 1.35 to 1.47 Å (close to the O−O distance in a peroxide species). The barrier and reaction energies are calculated to be 0.89 and 0.31 eV, respectively. The spin moment of *OOH decreases from 0.89 to 0.14, and the net Bader charge decreases from −1.08 to −1.59 |e|, indicating that *OOH is reduced to a peroxide species (OOH−). Second, the O−O single bond breaks, leading to the formation of an oxametallacycle intermediate (OMME), which has a barrier of 0.70 eV and a reaction energy of −0.55 eV. Although the OMME intermediate divides the high-barrier oxidation process into several moderate steps, unfortunately it also leads to the formation of other byproducts such as acrolein, CO2, and propanal.10,24,46 The last step involves breaking Au−C bond to form a three-membered ring with a barrier of 0.68 eV and an exothermic reaction energy of −0.66 eV. At this stage, one PO 2532

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ACS Catalysis is formed with its RDS as *OOH + CH3CHCH2* → CH3CHCH2OOH*. To test the influence of water coverage, we calculated similar propene oxidation pathway on 2.5 monolayer surface, and the results are shown in Figure S16. 3.4.2. Propene Epoxidation with *O and *OH. Previous work by infrared spectroscopic study has shown that *OOH is a precursor for the formation of atomically adsorbed oxygen (*O) and hydroxyl (*OH) on Au/TiO2 system.102 Theoretical studies suggested that *OOH is feasible to dissociate on Pt, Au surfaces or clusters,13,23,94−96 which implies that O* and OH* can act as candidates for propene epoxidation on Au7/αAl2O3(0001). The calculated dissociation barrier of *OOH is 0.82 eV with 0.06 eV endothermic reaction energy (Figure 12, path-v), which is in good agreement with the *OOH dissociation on Au/TiO2 interface and Au(111) surface.13,23 Subsequent adsorption of propene on the same Au atom with preadsorbed *OH and *O is relatively weak with Eads = −0.54 eV, whereas the adsorption energy on adjacent Au atom is −0.72 eV (Figure S15c, d), which is analogous to the case of adsorption with *OOH. Then the *C3H6 combines with *O to produce CH3CHCH2O* (OMME) complex, which has a barrier of 0.78 eV and a reaction energy of 0.03 eV. In both path-iv and path-v, the OMME is a crucial intermediate, which can be produced by dissociation of CH3CHCH2OOH* or combination of *C3H6 and *O. The last step, that is, CH3CHCH2O* (OMME) → CH3CHCH2O* (PO), is basically the same with the step three in path-iv, with a barrier 0.81 eV and an exothermic reaction energy of −0.93 eV. After the three-step reaction processes (path-iv or path-v), the system leaves one extra O on the surface (see the blue ball in Figure 12, iv-6). This O is relatively difficult to be abstracted by another propene to form the second PO with ΔE = 0.20 eV and Ea(RDS) = 1.21 eV, as discussed in the Supporting Information (Figure S17, path-vi), but the *OH on gold cluster is a good candidate for allyl formation, which is postulated to lead to a combustion product, that is, CO2 and acrolein.9,46,103 This reaction involves the abstraction of a hydrogen from the methyl group of propene by *OH (Figure S17, path-vii). The barrier is only 0.37 eV, which is much lower than the barrier for PO formation. Thus, in the case of propene epoxidation, selectivity tends to be poor. To ensure a high selectivity, conversions are usually kept low (