Anatase TiO2-x

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 7

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Theoretical Insights into Propene Epoxidation on Au/Anatase TiO (001) Catalysts: Effect of the Interface and Reaction Atmosphere x

Qianqian Zhang, Xian Zhao, Jing Yang, Mingyue Zheng, and Weiliu Fan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11201 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 26, 2019

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The Journal of Physical Chemistry

Theoretical Insights into Propene Epoxidation on Au7/Anatase TiO2-x(001) Catalysts: Effect of the Interface and Reaction Atmosphere Qianqian Zhang,† Xian Zhao,‡ Jing Yang,§ Mingyue Zheng,‡ Weiliu Fan*,† †School

of Chemistry and Chemical Engineering and ‡State Key Laboratory of Crystal Materials,

Shandong University, Jinan 250100, China §School

of Chemical Engineering, Shijiazhuang University, Shijiazhuang 050035, China

ACS Paragon Plus Environment

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ABSTRACT. Interfacial synergy of metal/oxide catalysts has been extensively studied in heterogeneous catalysis, but elucidating synergy mechanism and controlling interfacial structure remain challenging. Herein, the effect of interface and reaction atmosphere on propene epoxidation on Au7/anatase TiO2-x(001) catalysts is studied using density functional theory calculations. The results indicate that propene epoxidation occurs on topmost Au atoms on perfect Au7/TiO2(001) catalyst. Propene epoxidation with O2 alone has higher barrier and reaction energy, while in the presence of H2, the hydrogenation of O2 to OOH is a feasible pathway for propene epoxidation from both kinetic and thermodynamic viewpoints. On defective Au7/TiO2-x(001)-VO catalyst, oxygen vacancy regulates geometric/electronic structures of interfacial sites, and propene epoxidation instead occurs at the interface of Au7 and TiO2-x(001)VO. Under O2 atmosphere, O atom fills oxygen vacancy and significantly reduces the energy of entire catalytic system; however, this has little effect on kinetics of epoxidation. O2H2 mixture results in the lowest barrier owing to activation of OO bond and interfacial synergy. Our results suggest that moderate and rational regulation of oxygen vacancy on oxide surface can provide highly active sites and better interfacial synergy for propene epoxidation, and highlight the essential role of reaction atmosphere, which can be utilized to design high-efficiency heterogeneous catalysts.


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1. INTRODUCTION The epoxidation of propene to propene oxide (PO) is an important chemical reaction, as PO is a precursor for the production of a variety of commercial chemicals (polyether polyols, propylene glycol, polyurethane foams, etc.).1 Traditionally, chlorohydrin and organic hydroperoxide methods have been used for the industrial production of PO; however, these processes are complex, expensive, and environmentally unfriendly, and generate large amounts of by-products and waste materials.2,3 In 1998, Haruta et al. first proposed that small gold nanoparticles supported on TiO2 could act as catalysts for the direct epoxidation of propene with an O2H2 gas mixture.4 Later, Dow-BASF developed an environmentally friendly direct epoxidation process catalyzed by titanium silicalite-1 using H2O2 as an oxidant.5 Since then, propene epoxidation with O2 or O2H2 mixture has been widely carried out.6-10 However, owing to the diverse reaction systems and complex reaction processes, the reaction mechanism of propene epoxidation is unclear, which greatly limits the design and development of highefficiency green catalysts. It is generally believed that there are three propene epoxidation reaction mechanisms: the molecular oxygen mechanism, the atomic oxygen mechanism, and the OOH-mediated mechanism. Ag- and Cu-based catalysts are believed to follow the molecular oxygen mechanism, which is similar to that of ethylene epoxidation: O2 reacts with co-adsorbed C3H6 to form an oxametallacycle intermediate, which is a critical step for the production of PO.11,12 However, other reports have indicated that O2 is easily dissociated on these catalysts and propene epoxidation occurs via the atomic oxygen mechanism.13,14 When Au-based catalysts are used, H2 or H2O is usually introduced into the reaction gas stream. The O2 can be reduced to H2O2 or OOH species, which can act as the oxidant to attack the C=C bond of C3H6; OOH has been


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suggested to be the main active species.4,15,16 So far, few studies have compared these three mechanisms in the same catalytic system, or the difference between the use of an O2 or O2H2 mixture. More importantly, the limitations of the testing methods and complex reaction processes complicate experimental research into the epoxidation mechanism. Therefore, atomic-level theoretical calculations to explore the reaction mechanism (i.e., the active species, active sites, and reaction path) are essential. Several theoretical studies of propene epoxidation have been reported. For example, Chang et al. reported a comprehensive theoretical investigation of the role of water as a catalytic promoter for propene epoxidation on Au38 and Au10 clusters, and explored the OOH epoxidation process for C3H6.17 Pulido et al. have calculated O2 dissociation and propene epoxidation on the Ag(100) and (111) faces to explore the reaction of O atoms with C3H6.18 Molina et al. investigated propene epoxidation on oxidized silver surfaces, and clarified the effect of the surface oxygen concentration.19 In these studies, one-component metal clusters or surfaces were used as the catalysts. The role of the oxide support was not investigated, and no comparisons of different oxygen species were carried out. However, the majority of experimental catalysts for propene epoxidation are heterogeneous catalysts composed of metal nanoparticles and oxide supports. Studies have shown that the support not only affects the growth, size, and morphology of the metal nanoparticles, but also improves the catalytic activity of the interface sites via the metalsupport interaction,20-22 mainly presented by special configurations and charge transfer between active components.23-25 Additionally, the interface exhibits bifunctional sites during the reaction. Using infrared spectroscopy and density functional theory, Green and coworkers proved that dual TiAu sites at the Au/TiO2 interface promoted CO oxidation.26 Importantly, defects such as oxygen vacancies on the surface of metal oxides can change the distribution, geometry, and


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electronic structure of metal nanoparticles, and thus represent an important aspect in regulating the interfacial structure and metal-support interaction.27-29 In view the importance of interface in catalytic reactions, it is worth studying the role of the interface in propene epoxidation and clarifying its reaction mechanism in heterogeneous catalysis. It has been reported that the planar structure is the most stable structure for small Au clusters in the gas phase,30 and the two-dimensional (2D) Au7 cluster has a high activity for CO oxidation,31 O2 activation32 and propene epoxidation33 in theoretical researches. Meanwhile, experimental results have shown that the planar Au7 cluster on TiO2 catalysts has much higher activity for O2 activation and CO oxidation than others.34,35 Moreover, the stability of Au7 cluster with different configurations in the gas phase and on the TiO2 surface was also investigated in our study (details in Supporting Information). Anatase TiO2(001) surface shows better surface activity because of the higher surface energy (0.90 J/m2), which is almost twice that of the most stable (101) surface (0.44 J/m2).36 Sun et al. indicated that the adsorption of Au on TiO2(001) is much stronger than that on the majority (101) surface, and the valence electrons of Au atoms have been highly delocalized due to the strong metal-support interaction, which may result in the higher catalytic activity.37 Moreover, experimentally, (001)-dominated TiO2 crystals have been widely used in solar-hydrogen production,38 photocatalytic decomposition,39 lithium barriers,40 etc. Therefore, to gain more fundamental insights into propene epoxidation on metal/oxide catalysts, we chose an Au7 cluster supported on perfect and defective anatase TiO2(001) surfaces as the catalyst models for this work. In this paper, we study the mechanism of propene epoxidation on Au7/anatase TiO2-x(001) catalysts under different atmospheres (O2 and O2H2). Our calculations show that propene epoxidation mainly occurs on the topmost Au atoms of Au7/TiO2(001) catalyst, whereas it is


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more likely to take place at the interface after the introduction of an oxygen vacancy due to the metal-support interaction and the dual action of the metal and oxide support on the Au7/TiO2x(001)-VO

catalyst. The OOH formed by O2 and H2 at the interface is more kinetically favorable

than O2 for the epoxidation, because the activated O–O bond of OOH displays superoxo- or peroxo-like characteristics. Our work highlights the importance of the metal-oxide interface as well as the reaction atmosphere in propene epoxidation, and provides a useful approach for improving the catalytic performance of supported-metal catalysts. 2. COMPUTATIONAL DETAILS AND MODELS All density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP) software.41 The projector augmented wave (PAW) method was used to describe the interaction between the core electrons and the valence electrons. The exchange correlation potential was described using the Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation (GGA).42 Because Ti atoms have highly correlated localized 3d electrons, the DFT+U method was employed, and the Hubbard parameter U-J = 3.5 was used for the Ti 3d electrons.43 The electron wave functions were expanded in a plane wave basis set with a kinetic energy cutoff of 400 eV. For structural optimizations, the convergence criterion for the total energy was set to 1×10-5 eV, and that of the force on each atom was set to 0.02 eV/Å. The Monkhorst-Pack scheme kpoint grids were 2×2×1. For electronic structure calculations, the k-point values were set to 15×15×1, similarly to in a previous report.44 Spinpolarized calculations were carried out for all the systems. The transition states were searched using the climbing image nudged elastic band (CI-NEB) method integrated into VASP.45,46 Frequency analysis was carried out to ensure that each transition state had only a single imaginary frequency.


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After optimization, the lattice parameters of the unit cell of anatase TiO2 were a=b=3.856 Å and c=9.501 Å. The p(3×3) supercell was used for the TiO2(001) surface, in which 108 atoms were distributed into six layers. During calculations, the atoms in the four topmost layers were allowed to relax, whereas those of the bottom two layers were frozen. A 15 Å vacuum gap was introduced above the TiO2(001) surface to screen the self-interaction effects of the periodic boundary conditions. The Au7 cluster was optimized using a unit cell of 15 Å×15 Å×15 Å, and only the Γ-point was used. In this paper, the binding energies (Ebind) of the Au7 cluster supported on perfect and defective anatase TiO2(001) surfaces were calculated using the following equation: Ebind = EAu7/TiO2 - EAu7 - ETiO2

(1)

where EAu7/TiO2 is the system energy of the Au7 cluster deposited on TiO2, EAu7 is the energy of the free cluster, and ETiO2is the energy of the bare surface. The stability of each adsorption geometry was evaluated using the adsorption energy (Eads), which is defined as: Eads = Esurf + mol - Esurf - Emol

(2)

where Esurf + mol is the total energy of the Au7/TiO2 system after molecular adsorption, Esurf is the energy of the bare Au7/TiO2, and the Emol is the energy of a gas phase molecule. Hence, a negative value of Eads represents an exothermic process, whereas a positive value represents an endothermic process. The reaction barrier Ea and energy ∆E were calculated as: Ea = ETS - EIS

(3)

∆E = EFS - EIS

(4)


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where EIS, ETS, and EFS represent the energy of the initial state (IS), transition state (TS), and final state (FS), respectively. The formation energy of an oxygen vacancy was calculated as: 1 Ef(VO) = Etot(def) - Etot(no def) + μ(O2) (5) 2 where Etot(def) is the total energy of the defective TiO2 surface, Etot(no def) is the energy of the perfect surface, and μ(O2) is the energy of an O2 molecule. The d band centers (εd) of the Au atoms were calculated as follows: +∞

εd =

∫ -∞ ρd (E)EdE +∞

∫ -∞ ρd(E)dE

(6)

where ρd represents the projected density of states of the d orbitals of the Au atom, and E represents the energy with regard to the Fermi level. The relaxed structures of the perfect and defective anatase TiO2(001) surfaces are illustrated in Figure 1. The outermost atoms of the perfect (001) surface are fivefold coordinated titanium (Ti5c), twofold coordinated oxygen (O2c), and threefold coordinated oxygen (O3c). A high density of unsaturated atoms (Ti5c and O2c), which act as the active sites of the catalyst, contributes to catalytic activity.37 The defective surfaces were built by removing an O2c or O3c atom on the surfaces. Using equation (5) above, we calculated the formation energies of the different oxygen vacancies on TiO2(001) surface. As shown in Figure S3, there are four main types of oxygen vacancies on the TiO2(001) surface because of the different environments around the O atoms. The vacancy created by removing an O2c atom is denoted as VO2c, and that created by removing an O3c atom removed is denoted as VO3c. Based on the formation energies of the different types of oxygen vacancies, we chose VO2c, which had the lowest formation energy, as the type of


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oxygen vacancy on the TiO2(001) surface (denoted as TiO2-x(001)-VO) in this paper; the corresponding configurations are shown in Figure 1.

Figure 1. Side and top views of prefect and defective anatase TiO2(001) surfaces. Red and gray balls represent O and Ti atoms, respectively. The black dotted circles indicate the oxygen vacancies. 3. RESULTS AND DISCUSSION In this paper, the terminal double-bonded C atom was labelled as C1, the center double-bonded C atom as C2, and the C atom of the methyl group as C3. As mentioned above, the oxametallacycle intermediate is a significant precursor for the formation of PO. Due to the different oxygen species and number of Au atoms involved in the reactions, the intermediates were not consistent among the various reaction processes. As depicted in Scheme 1, the oxametallacycle intermediates involving O2 (or OOH) bonded to C1 are referred to as OOMP or OOMMP (where OOMP involves one Au atom and OOMMP involves two Au atoms), and the


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intermediates in which an O atom (or OH) is bonded to C1 are referred to as OMP or OMMP (similarly, M refers to the Au atom).

Scheme 1. Reaction scheme of propene oxidation by molecular oxygen, atomic oxygen and OOH (OH) species. 3.1 Propene epoxidation on Au7 cluster supported on perfect TiO2(001) surface 3.1.1

Propene epoxidation with O2

We first investigated the adsorption of Au7 cluster on perfect TiO2(001) surface; the various possible adsorption configurations are presented in Figure S4. The Au atoms are clearly more likely to combine with O2c than with Ti5c or O3c, indicating that O2c has higher activity than the other atoms. The Au atom can bond with adjacent O and Ti atoms, and breaks the OTi bond on the TiO2(001) surface, enhancing the adsorption stability of Au7 cluster; this result was consistent with the report of Sun et al.37 Moreover, the number of AuO bonds and their lengths also affect the adsorption of Au7 cluster. That is, the greater the number of bonds that are formed and the shorter the bond length is, the more stable the adsorption of Au7 cluster is. Therefore, configuration Au7/TiO2-1 is the most stable structure with the lowest binding energy (-2.97 eV). In this structure, Au-1, Au-2, and Au-3 are bonded to O2c and Ti5c atoms on the surface, the lengths of the AuO bonds are 2.06 Å, 2.09 Å, and 2.15 Å, and there is a weak interaction


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between Au-2 and Ti5c with a bond length of 3.14 Å. Figure 2 and Figure S5 indicate the Bader charges and d-band centers of the Au atoms of the Au7/TiO2(001) catalyst. The Au7 cluster donates electrons (0.325 |e|) to the TiO2(001) surface and becomes positively charged. The Au atoms at the interface (Au-1, Au-2, and Au-3) are in direct contact with the surface O atoms, which have positive charges and lower d-band centers. The center atom Au-4 is also electropositive and has the lowest d-band center. The top Au atoms (Au-5, Au-6, and Au-7) are electronegative and have higher d-band centers due to fact that they are not in contact with the surface O atoms.

Figure 2. Side and top views of the most stable configurations of Au7 cluster on perfect TiO2(001) surface. The Bader charges carried by every Au atom are shown on the right. Red, gray and yellow balls represent O, Ti and Au atoms, respectively. Generally, in O2 adsorption and OO bond activation, the π antibonding (π*) orbitals of the O2 molecule accept electrons; thus, negatively charged sites facilitate the adsorption and activation of O2. Additionally, the d-band states of the transition metal also play an important role in the


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interactions between the metal clusters and the adsorbate.47 When the d-band centers of a transition metal are up-shifted to the Fermi level, the antibonding orbitals will be pushed above the Fermi level. This phenomenon will reduce the Pauli repulsion, increasing the stability of the corresponding adsorption configuration.48 Consequently, we inferred that the topmost Au atoms of the Au7/TiO2-1 system (Au-5, Au-6, and Au-7), with more negative charges and higher dband centers, would promote O2 adsorption. The most stable deposition configuration of the Au7 cluster, Au7/TiO2-1, was used to model O2 adsorption. After evaluating several possible adsorption sites, we chose the six O2 adsorption configurations indicated in Figure 3. The adsorption energies for the formation of OAu bonds with O2 are clearly low for the Au-5, Au-6, and Au-7 atoms. On the Au-4 atom, O2 is physically adsorbed with an adsorption energy of -0.07 eV because of the high-coordination state and electro-positivity of Au-4. In addition, due to the strong repulsion of the surface O atoms, the adsorption of O2 at the interface or on the TiO2(001) surface is difficult: O2 physically adsorbs at the interface (Au-1 atom) with an adsorption energy of -0.07 eV, and the adsorption energy for the bonding of O2 with Ti5c on the TiO2(001) surface is -0.34 eV. The most stable adsorption state of O2 was on the top of Au7 cluster, which were denoted as P-O2-1, with an Eads of -1.07 eV. Consistent with our predictions above, the adsorption of O2 was most likely on the Au atoms with more negative charges and d-band centers closer to the Fermi level. In this state, the OO bond is parallel with Au-6···Au-7; the length of the Au-7O1 bond was 2.14 Å and that of the Au-6O2 bond was 2.12 Å. Moreover, the OO bond was lengthened from 1.23 Å to 1.34 Å, indicating that O2 is pre-activated by adsorption on Au7/TiO2(001) catalyst.


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Figure 3. The O2 adsorption configurations on Au7/TiO2(001) catalyst. Color coding is same with Figure 2, with the dark green balls represent the O atoms of O2. The bond length unit is Å. Before exploring the epoxidation of propene, we calculated the adsorption of C3H6 on the Au7/TiO2(001) catalyst. As depicted in Figure S6, due to the steric hindrance at the interface, C3H6 tends to adsorb on the Au7 cluster, especially the low-coordinated Au atoms on the edge of the Au7 cluster. This is consistent with the results reported for previous experiments and theoretical studies in which alkenes were preferentially adsorbed on the edge metal atoms with low coordination numbers.49,50 Besides, our results indicated that there is a weak interaction


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between propene and the surface Ti5c, which is consistent with the results reported by Driscoll et al. that propene bound to the surface through a π-interaction at TiO2 sites, and the interaction of propene-Au is stronger than propene-TiO2.51 The most stable adsorption state occurs on Au-7 site with an energy of -1.03 eV (P-C3H6-1). A comparison of the adsorption energies of O2 and C3H6 indicates that the Au/TiO2(001) catalyst shows the similar adsorption capacities for O2 and C3H6 and that both adsorption sites involve the Au-7 atom. Consequently, we calculated the epoxidation of propene using two co-adsorption models (P-O2+C3H6-1 is the preferential adsorption configuration for O2, while P-O2+C3H6-2 is the preferential adsorption configuration for C3H6). Interestingly, after the co-adsorption of O2 and C3H6, the conformation of the Au/TiO2(001) catalyst changes slightly. The Au7 cluster is stretched upward, and the Au-1O bond breaks. Propene epoxidation from the initial states in which O2 and C3H6 are co-adsorbed on the top Au atoms occurs via a two-step mechanism (Figure 4): (i) C3H6+O2→OOMMP and (ii) OOMMP→PO+O. In the initial state P-O2+C3H6-1, O2 and C3H6 adsorb on adjacent Au-7 and Au-6 atoms, and the distance between O1 and C1 is 3.44 Å. The co-adsorbed C3H6 and O2 can react to form the OOMMP intermediate via lengthening of the C=C bond from 1.39 Å to 1.53 Å and the OO bond length from 1.32 Å to 1.43 Å. The barrier and reaction energy were calculated to be 1.40 eV and 0.31 eV, respectively. Then, the OO bond of OOMMP breaks to generate PO and leave an O atom bonded to Au-7; this has a lower barrier of 0.62 eV and a reaction energy of -0.36 eV. In this reaction path, the rate-determining step is the generation of OOMMP by the reaction of the adsorbed O2 and C3H6. However, the high barrier of this reaction prevent it from occurring significantly.


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In the case of P-O2+C3H6-2, C3H6 adsorbs on Au-7 and O2 adsorbs on Au-6, and the distance between O1 and C1 is 3.48 Å. The reaction path and the mechanism of propene epoxidation in this state are the same as for P-O2+C3H6-1. The rate-determining step is also the formation of OOMMP, and the barrier and reaction energy were calculated to be 1.38 eV and 0.36 eV, respectively. The barriers for these two reaction processes are similar, indicating that the order in which O2 and C3H6 adsorbing has little effect on the kinetics of propene epoxidation. However, propene epoxidation is slightly exothermic on P-O2+C3H6-1, while it is endothermic on PO2+C3H6-2. Hence, the preferential adsorption of O2 is thermodynamically favorable for propene epoxidation.

Figure 4. Potential energy profiles for propene epoxidation with O2 (a: P-O2+C3H6-1, b: PO2+C3H6-2) on Au7/TiO2(001) catalyst. Color coding is same with Figure 3, with the dark grey and white balls indicate the C and H atoms of C3H6, respectively. The bond length unit is Å.


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Propene epoxidation with OOH

Formation of OOH from O2 and H2. Most of the reported experimental results have indicated that the presence of H2 in the reaction system can greatly increase the selectivity towards PO during the epoxidation of propene with O2.16,52,53 When H2 is present, O2 can react with H2 to produce an OOH intermediate in situ, which can also act as an oxidant to epoxidize propene. In this subsection, we describe a study of the formation of OOH. The initial state of this reaction is based on the most stable adsorption structure (P-O2+H2), in which O2 and H2 are adsorbed on adjacent Au atoms on the top of the Au7 cluster. The reaction coordinate profile in Figure 5a shows that the formation of OOH from O2 and H2 occurs via a two-step mechanism: (i) H2→H+H and (ii) H+O2→OOH. The dissociation of H2 on the Au7 cluster is an exothermic process (ΔE = -0.38 eV) with a low barrier of 0.18 eV (TS-1), indicating that H2 dissociation occurs easily on Au7/TiO2(001) catalyst. Subsequently, an H atom attacks O2 to form OOH with an Ea of 0.57 eV and ΔE of 0.25 eV. The OO bond length of OOH is elongated from 1.36 Å to 1.50 Å, indicating it is activated by the electron transfer from the H atom.


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Figure 5. Potential energy profiles for the formation of OOH from O2 and H2 (a) and propene epoxidation with OOH (b) on Au7/TiO2(001) catalyst. Color coding is same with Figure 4, with purple balls indicate the H atoms of H2. The bond length unit is Å. Propene epoxidation with OOH. Based on the formation of OOH described above, we next discuss the epoxidation of propene with OOH as the oxidant. This epoxidation on the Au7/TiO2(001) catalyst takes place via two steps (Figure 5b): (i) C3H6+OOH→OMP+OH and (ii) OMP→PO. The initial state of the reaction is based on the most stable adsorption structure, in which OOH and C3H6 are adsorbed on adjacent Au atoms (P-OOH+C3H6) and the distance between O1 and C1 is 3.46 Å. Owing to the strong activation of the OO bond (1.50 Å) of OOH, this bond is broken during the bonding of O1 to C1, and O1 is transferred from Au-7 to Au-6, leading to the formation of the OMP intermediate. The presence of H2 not only causes the OO bond to break before OMP formation, but also causes the epoxidation reaction to proceed on a single Au atom, which reduces the barrier (1.19 eV) and reaction energy (-1.03 eV) of OMP


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formation. We also calculated the barriers and reaction energies of the individual dissociation of O2 and OOH on Au7/TiO2(001). As shown in Figure S7a, the barrier for O2 dissociation is 1.87 eV with an exothermic reaction energy of -0.48 eV, indicating that it is difficult to break the OO bond during the formation of OOMMP in the progress of propene epoxidation with O2 because of the higher barrier. In comparison, the barrier and reaction energy of OOH dissociation are 0.98 eV and -2.56 eV, respectively (Figure S7b), which are both lower than the respective values for the dissociation of O2, suggesting that H2 promotes O2 activation from both thermodynamic and kinetic aspects. Moreover, the barrier for OOH dissociation is also lower than the barrier for OMP formation indicating that OO bond will break during the formation process of OMP. Then, O1 bonds to C2 with a bond length of 1.42 Å, leading to the formation of PO; this process has a barrier of 0.72 eV and a reaction energy of -1.41 eV. The remaining OH group combines with Au-7 and a surface Ti5c atom to form a stable AuOHTi structure. The reaction path is consistent with the report of Haruta and co-workers who proposed that hydroperoxo species formation by O2 and H2 can reacts with propene adsorbed on Au to form PO.54 Besides, Bravo-Suárez et al. confirmed that the hydroperoxo species were intermediates in propene epoxidation with H2 and O2 using in situ ultraviolet–visible (UV–vis) spectroscopy.55 Comparing propene epoxidation with O2 and O2H2, although the main reaction sites are Au atoms at the top of the Au7 cluster in both cases, the reaction pathway (i.e., whether or not the OO bond breaks) and reaction intermediate (OOMMP vs OMP) are quite different. When H2 is present, O2 first reacts with a dissociated H atom to produce OOH; the activated O–O bond of OOH intermediate displays superoxo- or peroxo-like characteristics, which is responsible for C3H6 epoxidation.17 Moreover, the Au7/TiO2(001) interface stabilizes the remaining OH group


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(via an AuOHTi structure), which significantly reduces the reaction energy, making propene epoxidation more thermodynamically favorable. 3.2 Propene epoxidation on Au7 cluster supported on defective TiO2-x(001)-VO surface 3.2.1

Propene epoxidation with O2

Oxygen vacancies are the most basic point defects on the surface of oxides, and play an important role in the physical and chemical properties of metal nanoparticles56 and the activation of O2.57 Thus, the presence of oxygen vacancies may affect propene epoxidation on heterogeneous catalysts. First, the adsorption of Au7 cluster on the TiO2-x(001)-VO surface was explored. As is depicted in Figure S8, it is clear that Au atoms have strong interactions with O2c than with Ti5c or O3c. Importantly, Au7 cluster adsorbing on O2c atoms near the oxygen vacancy and adsorbing at the oxygen vacancy site in the vertical manner have the similar adsorption energy, and the stability of these two vertical structures are stronger than that horizontal adsorption configurations above the oxygen vacancy. In addition, it has been confirmed that oxygen vacancy has a significant influence on the adsorption of O2. In order to explore the role of oxygen vacancy in O2 activation and propene epoxidation, we choose the oxygen vacancy exposed configuration Au7/TiO2-x-VO-1 as the model for subsequent calculations. Figure 6 shows the Bader charges of the Au atoms of Au7/TiO2-x-VO-1. By introducing an oxygen vacancy on the TiO2(001) surface, electrons on TiO2-x(001)-VO are transferred to the Au7 cluster, resulting in the Au7 cluster is negatively charged (-0.080 |e|). Although the Au-1 and Au-2 atoms are in direct contact with surface O atoms, their charges are decrease substantially than in Au7/TiO2(001) catalyst, especially that of the Au-2 atom, which decreases from 0.175 |e| to 0.053 |e|. The charge of the middle Au atom Au-3 becomes negative rather than positive, and the other middle Au atom, Au-5, the number of negative charges is nearly doubled. Although the top Au


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atoms (Au-6 and Au-7) are far from the oxygen vacancy, they still become more negatively charged, particularly that of the Au-7 atom, whose charge increases by 0.022 |e|. Figure S9 depicts the d-band centers of the Au atoms of Au7/TiO2-x(001)-VO catalyst; the d-band centers of the negatively charged Au atoms are closer to the Fermi level. As negative charges and higher dband centers are favorable for O2 adsorption, the middle Au atoms have a stronger ability to adsorb O2 than the top Au atoms.

Figure 6. Side and top views of the most stable adsorption configurations of Au7 cluster on TiO2x(001)-VO

surface. The Bader charges carried by every Au atom are shown on the right. Color

coding is same with Figure 2. Black dotted circles indicate the oxygen vacancies. The bond length unit is Å. We then modelled O2 adsorption on the oxygen-vacancy-containing Au7/TiO2-x(001)-VO catalyst. The interface, top Au atoms, and oxygen vacancy were considered as potential adsorption sites, and four stable O2 adsorption configurations were obtained (Figure 7). The Au7 cluster adsorbs in a vertical manner on the TiO2-x(001) surface, allowing O2 to bond with Au-5


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and surface Ti5c to form a stable AuOOTi structure at the interface near the oxygen vacancy (D-O2-1). The adsorption energy of O2 is -1.79 eV, and the OO bond length is 1.42 Å, indicating that the O2 molecule adsorbed at the interface is extremely activated. Major reasons for this phenomenon include the more negative charge of Au-5 and the participation of surface Ti5c. Correspondingly, O2 can also form an AuOOTi structure on the Au-3 site at the interface (D-O2-2), but the OTi bond is very weak (2.51 Å), and the adsorption site is far from the oxygen vacancy, resulting in a higher adsorption energy (-0.94 eV), and the OO bond length is only 1.33 Å. Furthermore, the top Au atoms (Au-6 and Au-7) have more negative charges and can also be used to adsorb O2. In D-O2-3, the OO bond is parallel to Au-6···Au-7 and Au-7O1 (2.16 Å) and Au-6O2 (2.14 Å) bonds are formed, similarly to the O2 adsorption configuration in Au7/TiO2 (001) catalyst. However, the top Au atoms of the Au7 cluster adsorbed on TiO2x(001)-VO surface

are more negatively charged, and can transfer more electrons to the π* orbitals

of O2. As a result, the O2 adsorption energy on the top Au atoms is 0.21 eV lower than that on Au7/TiO2(001) catalyst, and the OO bond is activated with a length of 1.36 Å. Importantly, this adsorption configuration is more stable than the adsorption of O2 on the Au-3 atom (D-O2-2). Previous studies have reported that O2 tends to adsorb on the oxygen vacancy site of oxide surfaces,57,58 and our results confirm this opinion. When O2 adsorbs at the oxygen vacancy site, O2 fills the oxygen vacancy, and O1 bonds with the surface Ti5c with a distance of 1.92 Å, and the OO bond length is stretched from 1.23 Å to 1.46 Å, indicating greater pre-activation. However, the O2 atom is completely bound after filling the oxygen vacancy, which may hinder subsequent epoxidation reaction. In summary, although the oxygen vacancy is the most suitable site for O2 adsorption on Au7/TiO2-x(001)-VO catalyst, the adsorption configurations D-O2-1 and D-O2-3 cannot be ignored due to their lower O2 adsorption energy. Therefore, we next calculated


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the propene epoxidation reaction for these three configurations, and compared the epoxidation activity of the different sites (top Au atoms, interface, and oxygen vacancy) on the Au7/TiO2x(001)-VO

catalyst.

Figure 7. The O2 adsorption configurations on Au7/TiO2-x(001)-VO catalyst. Color coding is same with Figure 3. Black dotted circles indicate the oxygen vacancies. The bond length unit is Å. We first calculated propene epoxidation on the top Au atoms, and compared this reaction with that on Au7/TiO2(001) catalyst. As shown in Figure 8a, the epoxidation process occurs via a twostep mechanism: (i) C3H6+O2→OOMMP and (ii) OOMMP→PO+O. In the initial state, O2 and C3H6 both adsorb on the top Au atoms, and the distance between O1 and C1 is 3.75 Å. The coadsorbed C3H6 and O2 react to form the OOMMP intermediate; the barrier and reaction energy were calculated to be 1.28 eV and 0.15 eV, respectively. We also calculated the separate O2


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dissociation on the top Au atoms (Figure S10a), and the barrier was calculated to be 1.43 eV, indicating that the OO bond is difficult to break prior to the formation of OOMMP. After the formation of OOMMP, the OO bond breaks to form PO and leave an O atom bonded to Au-6 and Au-7 with a barrier of 0.79 eV and a reaction energy of -0.62 eV. Comparing propene epoxidation on top Au atoms of Au7/TiO2(001) and Au7/TiO2-x(001)-VO catalysts, it is clear that the reaction pathways, intermediates, and reaction mechanisms on these two catalysts are similar, suggesting that the epoxidation of C3H6 with O2 on the top Au atoms proceeds according to the above two-step mechanism (OOMMP generation and PO formation) whether or not an oxygen vacancy is present. However, when an oxygen vacancy is introduced onto the TiO2(001) surface, the Au7 cluster becomes negatively rather than positively charged, which increases its adsorption capacity for O2 molecule and reduces the barrier by 0.12 eV compared to that of the Au7/TiO2(001) catalyst. Additionally, the epoxidation process has a lower reaction energy (-0.47 eV vs -0.05 eV) on the Au7/TiO2-x(001)-VO catalyst , making propene epoxidation more thermodynamically favorable. Next, we explored propene epoxidation with O2 at the Au7/TiO2-x(001)-VO interface (Figure 8b). Similarly, the epoxidation process also consists of the following two elementary reactions: (i) C3H6+O2→OOMMP and (ii) OOMMP→PO. Although the adsorption and reaction sites of the reactants are different from D-O2+C3H6-1, this epoxidation process still involves the OOMMP intermediate. This is because the dissociation barrier of O2 (1.27 eV) is slightly higher than the formation barrier of the oxametallacycle intermediate (1.17 eV), and O2 and C3H6 adsorb on two adjacent Au atoms. Then, the OO bond of OOMMP breaks to generate PO, and the O2 atom is transferred to fill the oxygen vacancy; this process has a barrier of 0.96 eV and a reaction energy of -2.52 eV. Although the transfer of the O2 atom to the oxygen vacancy


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involves a longer path, resulting in a higher barrier for this elemental reaction, the filling of the oxygen vacancy greatly reduces the energy of the reaction system, so that the reaction process is more exothermic. The barrier of the overall epoxidation reaction is slightly lower at the interface site than on the top Au atoms (1.17 eV vs 1.28 eV) due to the more negative charges on Au-5 and the bifunctional sites of Au and Ti. On the other hand, the adsorption of O2 at the interface makes it easier for the O2 atom to fill the oxygen vacancy, which reduces the reaction energy (2.15 eV), greatly improving the reactivity of propene epoxidation in terms of thermodynamics.


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Figure 8. Potential energy profiles for the propene epoxidation with O2 (a: D-O2+C3H6-1, b: DO2+C3H6-2, c: D-O2+C3H6-3) on Au7/TiO2-x(001)-VO catalyst. Color coding is same with Figure 4. Black dotted circles indicate the oxygen vacancies. The bond length unit is Å. The epoxidation of propene using the D-O2-4 O2 adsorption model, in which O2 adsorbs at the oxygen vacancy site, was then investigated. As depicted in Figure 8c, this epoxidation process


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consists of the following three elementary reactions: (i) O2→O+O, (ii) C3H6+O→OMP, and (iii) OMP→PO. Differently from the propene epoxidation on D-O2+C3H6-1 and D-O2+C3H6-2, the O2 dissociates into two O atoms before the formation of OMP. The O2 atom fills the oxygen vacancy, and O1 is used in the subsequent propene epoxidation. The barrier and reaction energy of O2 dissociation are 0.84 eV and -1.09 eV, respectively, indicating that O2 is easily dissociated. (The barrier for the dissociation of O2 alone at the oxygen vacancy site is 0.88 eV, as shown in Figure S10c.) Next, O1 reacts with C3H6 to form the OMP intermediate at the interface, and the O1 atom is transferred from Au-2 to Au-5. The barrier and reaction energy were calculated to be 1.89 eV and -0.27 eV, respectively. Due to the high barrier of this elementary reaction, the formation of OMP becomes the rate-determining step of the propene epoxidation, and it is obvious that propene epoxidation with the dissociated O atom as an oxidant is difficult. This is because the Bader charge of the O1 atom was calculated to be -0.787 |e|. The more negative charge on the O1 atom makes it more nucleophilic, which disfavors attacking the C=C bond to generate the OMP intermediate, and it may have a possibility to abstract a hydrogen from the methyl group of propene. This conclusion is supported by the report of Li et al. who proposed that atomically adsorbed oxygen and hydroxyl are good candidates for allyl formation.33 The last step involves breakage of the AuC2 bond and formation of an O1C2 bond to form a threemembered ring, with a barrier of 0.91 eV and an exothermic reaction energy of 0.37 eV. Although propene epoxidation proceeds at the interface for both D-O2+C3H6-2 and DO2+C3H6-3, the barriers, reaction pathways, and mechanisms of the reaction are quite different. In D-O2+C3H6-2, the barrier of OOMMP formation is lower than the O2 dissociation barrier, and OOMMP is generated without OO bond cleavage. In D-O2+C3H6-3, a dissociated O atom reacts with C3H6 to generate the OMP intermediate, because the dissociation of O2 is preferred at


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the oxygen vacancy site. However, the excessively negative charge of the dissociated O atom is unfavorable for attacking the nucleophilic C=C bond, resulting in a higher barrier. Overall, the epoxidation ability of molecular oxygen is better than that of atomic oxygen in terms of both thermodynamics and kinetics. On the other hand, comparing propene epoxidation with O2 at different sites (D-O2+C3H6-1 and D-O2+C3H6-2), the reaction energy is much lower with O2 at the interface due to the filling of the oxygen vacancy, indicating that this route will be thermodynamically preferred. 3.2.2

Propene epoxidation with OOH or OH

Formation of OOH or OH from O2 and H2. As discussed above, when H2 is present, O2 will be reduced to OOH species to participate in propene epoxidation. In this section, we describe the reaction processes of O2 and H2 on the Au7/TiO2-x(001)-VO catalyst with different O2 adsorption configurations (O2 adsorption on the top Au atoms, interface, and oxygen vacancy site) (Figure S11). All reactions occur through two elementary reactions: H2 dissociation and OOH (or OH) formation. In D-O2+H2-1, the formation of OOH from O2 and H2 on top Au atoms is the same as the reaction process on Au7/TiO2(001) (Figure 5a), and the barriers of H2 dissociation and OOH formation are 0.14 eV and 0.54 eV. In D-O2+H2-2, O2 adsorbs at the interface and H2 adsorbs on the adjacent Au-7 atom. A dissociated H atom forms a bond with the O2 atom to generate OOH at the interface, and the OOH interacts with Au-5 and Ti5c to form a stable AuOOHTi structure. The barriers of H2 dissociation and OOH formation are 0.18 eV and 0.63 eV. For DO2+H2-3, O2 adsorbs on the oxygen vacancy site and H2 adsorbs on Au-7 atom. The dissociation of H2 on the Au7 cluster is an exothermic process (ΔE=-0.18 eV) with a barrier of 0.19 eV. Next, when a dissociated H atom reacts with O2, the energy is high enough to break the OO bond without the generation of OOH, because the OO bond is extremely activated (1.46 Å) when O2


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adsorbs at the oxygen vacancy site, and can be broken with very low energy, which is consistent with the work of Liu et al.59 The O2 atom fills the oxygen vacancy, and the OH group formed by O1 and H adsorbs at the interface to form a stable AuOHTi structure. The barrier and reaction energy of this process are 0.71 eV and -3.23 eV, respectively. Propene epoxidation with OOH and OH. We further studied the propene epoxidation process for the three OOH or OH adsorption conditions above, as shown in Figure 9. We first studied the reaction of C3H6 and OOH with the initial state D-OOH+C3H6-1, in which OOH and C3H6 both adsorb on the top Au atoms. First, the distance between O1 and C1 shortens from 3.75 Å to 1.46 Å via the formation of the OOMMP intermediate; the barrier and reaction energy were calculated to be 0.95 eV and 0.63 eV. The OO bond of OOH does not break, but is lengthened from 1.44 Å to 1.48 Å. Then, the OO bond of OOMMP breaks to form PO and leave an OH group bonded to the Au-6 and Au-7 atoms with a barrier of 0.80 eV and a reaction energy of -0.71 eV. The above reaction processes are similar to that of propene epoxidation with O2 on the top Au atoms on the Au7/TiO2-x(001)-VO catalyst. This epoxidation process has a lower reaction energy barrier (0.95 eV vs 1.28 eV) but a higher energy (-0.08 eV vs -0.47 eV), indicating that the OOH path is kinetically preferred. Next, propene epoxidation with OOH adsorbed on the interface was investigated. As depicted in

Figure

9b,

the

epoxidation

reaction

occurs

via

a

two-step

mechanism:

(i)

C3H6+OOH→OMMP+OH and (ii) OMMP→PO. The initial state of the reaction is based on the adsorption structure D-OOH+C3H6-2, in which OOH is adsorbed at the interface and C3H6 is adsorbed on the Au-7 atom. The OO bond breaks during the reaction of C3H6 with OOH to form the intermediate OMMP, since the OO bond of OOH is strongly activated (1.49 Å). The barrier and reaction energy are 0.76 eV and -0.23 eV, respectively, and the barrier is the lowest


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among all the barriers for the generation of oxymetallacycle intermediates in all the calculated systems. Subsequently, OMMP is converted to PO accompanying the distance between O1 and C2 changes from 2.49 Å to 1.42 Å. The barrier and reaction energy are 0.63 eV and -0.17 eV, respectively.

Figure 9. Potential energy profiles for the propene epoxidation with OOH (a: D-OOH+C3H6-1, b: D-OOH+C3H6-2, c: D-OH+C3H6) on Au7/TiO2-x(001)-VO catalyst. Color coding is same with Figure 5. Black dotted circles indicate the oxygen vacancies. The bond length unit is Å.


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Finally, propene epoxidation with OH was investigated in D-OH+C3H6, in which OH adsorbs at the interface to form a stable AuOHTi structure with Au-5 and Ti5c while propene adsorbs on the Au-7 atom (Figure 9c). The epoxidation occurs via a two-step mechanism: (i) C3H6+OH→OMMP+H and (ii) OMMP→PO. C3H6 combines with OH to produce the OMMP intermediate via the breakage of the O1H bond and the formation of an O1C1 bond. The barrier and reaction energies were calculated to be 1.83 eV and 0.82 eV, and the energies are too high to form the OMMP. The main reason for this is the higher electron density on the O atom (1.116 |e|) of the OH group, which is unfavorable for attacking the nucleophilic C=C bond. Finally, OMMP transforms into PO by the reaction of O1 with C2, which is the final step of all propene epoxidation reactions. The barrier is 0.60 eV with an exothermic reaction energy of 0.42 eV. Comparing D-OOH+C3H6-2 and D-OH+C3H6, propene epoxidation occurs at the interface in both cases and involves the same reaction intermediate; however, the barriers and reaction mechanisms are totally different. The adsorption of OOH at the interface, where O–O bond is moderately activated, is more favorable for propene epoxidation due to the dual effect of the metal and the TiO2 support. However, the OH group is unsuitable for attacking the nucleophilic C=C bond because of the high electron density on the O atom. Hence, OOH is a more suitable oxidant for propene epoxidation than OH. Additionally, when OOH acts as the oxidant, the interface site of Au7/TiO2-x(001)-VO catalyst is more reactive than the top Au atoms due to the metal-support interaction and the dual action of the metal and oxide support. 4. CONCLUSION In this study, we conducted a comprehensive DFT study of propene epoxidation with O2 or O2H2 mixture on Au7 cluster supported on perfect and defective anatase TiO2(001) surfaces. On


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the perfect Au7/TiO2(001) catalyst, O2 and C3H6 prefer to adsorb on the top Au atoms due to the excess positive charge of the interfacial Au atoms and the large steric hindrance at the interface, and propene epoxidation occurs on the top electro-negative Au atoms. In the presence of H2, the hydrogenation of O2 to OOH is a feasible pathway for propene epoxidation from both kinetic and thermodynamic viewpoints. The introduction of an oxygen vacancy on the TiO2(001) surface changes the geometric and electronic structures of the interfacial sites on the Au7/TiO2-x(001)-VO catalyst, which not only exhibits a strong capacity for O2 adsorption, but also provides highly active sites and better interfacial synergy for propene epoxidation. Under O2 atmosphere, although O2 dissociation is facilitated by the oxygen vacancy site, the excess negative charge on the dissociated O atom prevents O from attacking the nucleophilic C=C bond, whereas O2 adsorption at the interface near the oxygen vacancy results in higher propene epoxidation activity owing to the dual action of the metal and the oxide support. In the presence of H2, O2 is hydrogenated to OH due to the strong activation of the O–O bond at the oxygen vacancy site, while O2 is reduced to OOH at the interface near the oxygen vacancy. Similarly, the excess negative charge on OH results in a high energy for propene epoxidation, while the formation of OOH at the interface leads to the lowest barrier, due to the moderate activation of the OO bond and the interfacial synergistic effect. Therefore, increasing the adsorption capacity for O2 at the interface near the oxygen vacancy is the key to enhanced propene epoxidation activity. The rational regulation of oxygen vacancies on the oxide surface may be an effective strategy to design high-performance metal/oxide catalysts. The details of the regulation of oxygen vacancies on the oxide surface will be investigated in our future study.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. The models of Au7 cluster (Figure S1 and Figure S2), geometries of TiO2(001) surface with different oxygen vacancies (Figure S3), adsorption configurations of Au7 cluster supported on perfect TiO2(001) surface (Figure S4), project density of states (PDOS) for d-orbit of Au atoms in Au7/TiO2(001) catalyst (Figure S5), C3H6 adsorption configurations on Au7/TiO2(001) catalyst (Figure S6), potential energy profiles for the dissociation of O2 and OOH on Au7/TiO2(001) catalyst (Figure S7), adsorption configurations of Au7 cluster supported on defective TiO2-x(001)-VO surface (Figure S8), project density of states (PDOS) for d-orbit of Au atoms in Au7/TiO2-x(001)-VO catalyst (Figure S9), potential energy profiles for the dissociation of O2 on Au7/TiO2-x(001)-VO catalyst (Figure S10) and potential energy profiles for the formation of OOH or OH from O2 and H2 on Au7/TiO2-x(001)-Vo catalyst (Figure S11) (PDF). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: 86-531-88366330. Fax: 86-531-88364864. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We acknowledge the financial support from Natural Science Foundation of China (grant no. 21771119) and the Taishan Scholar Project of Shandong Province (grant no. ts201712011).


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TOC Graphic


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Figure 1. Side and top views of prefect and defective anatase TiO2(001) surfaces. Red and gray balls represent O and Ti atoms, respectively. The black dotted circles indicate the oxygen vacancies. 80x92mm (300 x 300 DPI)



Scheme 1. Reaction scheme of propene oxidation by molecular oxygen, atomic oxygen and OOH (OH) species. 80x52mm (300 x 300 DPI)


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Figure 2. Side and top views of the most stable configurations of Au7 cluster on perfect TiO2(001) surface. The Bader charges carried by every Au atom are shown on the right. Red, gray and yellow balls represent O, Ti and Au atoms, respectively. 80x85mm (300 x 300 DPI)



Figure 3. The O2 adsorption configurations on Au7/TiO2(001) catalyst. Color coding is same with Figure 2, with the dark green balls represent the O atoms of O2. The bond length unit is Å. 80x147mm (300 x 300 DPI)


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Figure 4. Potential energy profiles for propene epoxidation with O2 (a: P-O2+C3H6-1, b: P-O2+C3H6-2) on Au7/TiO2(001) catalyst. Color coding is same with Figure 3, with the dark grey and white balls indicate the C and H atoms of C3H6, respectively. The bond length unit is Å. 80x93mm (300 x 300 DPI)



Figure 5. Potential energy profiles for the formation of OOH from O2 and H2 (a) and propene epoxidation with OOH (b) on Au7/TiO2(001) catalyst. Color coding is same with Figure 4, with purple balls indicate the H atoms of H2. The bond length unit is Å. 80x92mm (300 x 300 DPI)


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Figure 6. Side and top views of the most stable adsorption configurations of Au7 cluster on TiO2-x(001)-VO surface. The Bader charges carried by every Au atom are shown on the right. Color coding is same with Figure 2. Black dotted circles indicate the oxygen vacancies. The bond length unit is Å. 80x84mm (300 x 300 DPI)



Figure 7. The O2 adsorption configurations on Au7/TiO2-x(001)-VO catalyst. Color coding is same with Figure 3. Black dotted circles indicate the oxygen vacancies. The bond length unit is Å. 80x99mm (300 x 300 DPI)


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Figure 8. Potential energy profiles for the propene epoxidation with O2 (a: D-O2+C3H6-1, b: D-O2+C3H6-2, c: D-O2+C3H6-3) on Au7/TiO2-x(001)-VO catalyst. Color coding is same with Figure 4. Black dotted circles indicate the oxygen vacancies. The bond length unit is Å. 129x169mm (300 x 300 DPI)



Figure 9. Potential energy profiles for the propene epoxidation with OOH (a: D-OOH+C3H6-1, b: DOOH+C3H6-2, c: D-OH+C3H6) on Au7/TiO2-x(001)-VO catalyst. Color coding is same with Figure 5. The bond length unit is Å. 80x137mm (300 x 300 DPI)


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TOC Graphic 80x35mm (300 x 300 DPI)


Anatase TiO2-x

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