Theoretical Study of Propylene Epoxidation over Cu2O (111) Surface

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Theoretical Study of Propylene Epoxidation over CuO(111) Surface: Activity of O , O, and O species 2

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Yangyang Song, and Gui-Chang Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07044 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 21, 2018

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The Journal of Physical Chemistry

Theoretical Study of Propylene Epoxidation over Cu2O(111) Surface: Activity of O2-, O-, and O2- Species

Yang-Yang Song, Gui-Chang Wang* (Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) and the Tianjin key Lab and Molecule-based Material Chemistry, College of Chemistry, Nankai University, Tianjin 300071, P. R. China)

*Corresponding author: Gui-Chang Wang. Telephone: +86-22-23503824 (O)

E-mail: [email protected]

Fax: +86-22-23502458

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Abstracts: Propylene epoxidation catalyzed by cuprous oxide in the presence of oxygen species is important in both technology and scientific fields, and the active species, lattice oxygen (O2-), adsorbed atomic oxygen (O-), or adsorbed molecular oxygen (O2-), plays a significant role in the catalytic reaction. In the present work, the mechanism of propylene epoxidation and dehydrogenation on Cu2O(111) facet with different oxygen species has been studied through density functional theory (DFT) calculations with a Hubbard U correction in detail. The whole reaction processes of propylene oxidation including two different routes: allylic hydrogen stripping (AHS) reactions and propylene epoxidation reactions. Acrolein can be generated by two H-stripping reactions in the AHS path and propylene oxide (PO) is formed through the oxametallacycles propylene (OMP) intermediate. The calculated results show that the adsorbed atomic oxygen (O-) is the most active oxygen species for the selective oxidation of propylene due to the strongest basicity among these three oxygen species, whereas the adsorbed molecular oxygen (O2-) has the highest selectivity for the PO formation among these three different oxygen species because of its relatively low basic properties and the moderate oxidation compared to those of atomic oxygen (too active oxidation) or lattice oxygen (less oxidation due to the close-shelled nature of oxide system). Moreover, the microkinetic simulation was used to confirm the above DFT calculation results. The aim of the present study gives the guide in choosing the efficient active oxygen species for PO formation, which should one with the moderate oxidant.

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1. Introduction The oxidation of alkenes has a series of intermediate products and those can yield to many valuable and widely used chemical materials, which play a crucial role in industrial chemistry.1-2 Among those alkenes oxidation, propylene epoxidation forming propylene oxide (PO) is of significance3-4 and PO can be applied to create polyether polyols, propylene glycol, etc.1, 5 Traditional chlorohydrin method causes serious environmental pollution and organic hydroperoxide method produces large amounts of coproduces.6 Propylene epoxidation catalyzed by metals or metal oxides has been proposed and investigated extensively. Over the past decades, some studies found that the metallic and metallic oxide catalysts (Au,7-8 Ag,9-10 Cu,11-13 Ru-Cu-Na,14 CuAu/SiO2,15 Cu2O,16-17 and CuO18-19) have shown catalytic activity toward propylene epoxidation. Comparing to those of Ag and Au catalysts in IB group, Cu-based catalysts are emerging as important modules in a range of applications20-22 and have been exploited for much cheaper and higher PO selectivity. For examples, the groups of Lopez and Lambert theoretically studied propylene epoxidation on Ag(111) and Cu(111) facets, suggesting that Cu(111) has a higher epoxidation selectivity.23 The experimental results of Lambert et al. found that the PO selectivity can achieve 53% at the propylene conversion of 0.25% when using Cu/SiO2 as the catalyst.11 Wang and coworkers reported that the PO selectivity of 35 % can be obtained over the CuOx-VOx catalyst at the C3H6 conversion of 0.78 %.13 For the active phase of copper in propylene epoxidation, the highly dispersed form of Cu0 was regarded as the active phase by Lambert et al. based on high-resolution electron microscopy and others.11 Chen et al. investigated propylene epoxidation on a model of mixed oxide thin film formed by adding TiOx to a Cu2O surface and found that the Cu+ is the active phase.24 Nevertheless, Li et al. exhibited that both Cu0 and Cu+ on Cu/SiO2 can activate the C=C bond of C3H6 and should be the active phase.12 Zheng’s group demonstrated that the selectivity of PO follows the order of Cu > Cu2O > CuO when using O2 as the oxidant, namely the metallic Cu is favorable for the PO formation instead of the acrolein (the PO selectivity is 54.2 % in the propylene conversion of 2.6 % at 160 ℃), and thus the metallic Cu may be the active phase for PO production.17 It was also found that the metallic Cu can be easily oxidized to Cu2O in propylene oxidation conditions, so Cu0 should be the active phase at the initial stage and the Cu+ maybe the active phase at the steady state.17 Besides, Solomon et al. revealed that Cu2O is an efficient catalyst in propylene selective oxidation and much 3



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stable at the temperature of 900 K in a vacuum.18 Considering the higher stability and better performance for propylene oxidation, cuprous oxide was selected as the catalyst for propylene epoxidation in the present work. The aerobic oxidation of propylene on silver catalysts has been investigated by Pulido’s team25 and found that: (i) the dissociation of O2 can form surface oxygen species to participate propylene selective oxidation, which is the rate determining step in the whole process and (ii) the formation of propylene epoxide or carbonylic compounds from oxametallacycle (OMP) intermediates competes with the allyl route for acrolein producing. Lei et al. have studied propylene oxidation catalyzed by a size-selected cluster of three silver atoms, a large number of possible reaction paths have been introduced extensively.26 Moreover, Song et al. introduced the propylenedioxy (PDO) (the C1 atom and the C2 atom in the C3H6 binding with two neighboring O atoms simultaneously) intermediates mechanism.19 Based on the systematically theoretical discussions,26-33 the proposed mechanism for propylene selective oxidation was shown in scheme 1. Düzenli et al. investigated the partial oxidation of propylene on Cu2O(001) and CuO(001), which shows that the lattice oxygen (Olattice) of CuO or Cu2O phase has low activity for PO formation.34 For the nature of oxygen species during the ethylene/propylene epoxidation processes, it is generally accepted that the electron rich oxygen is usually to be considered as to activate C-H bond breaking and to form CO2 by complete oxidation, whereas the electron deficient oxygen is supposed to active the C=C double bond and prefers the epoxidation path. Palmer et al.35 studied the methane C-H bond activation on La2O3 with different kinds of O species theoretically, in which they found the catalytic activity of C-H bond activation follows the trend of O- > O2- > O2-, namely the adsorbed oxygen atom (O*) with the highest active and Olattice with the least active for the C-H bond activation. Recently Dai et al. investigated the role of O* and absorbed molecular oxygen (O2*) in the propylene epoxidation on IB metals by theoretical calculations, in which they found that the O* favors the AHS reaction and the acrolein is the major product with low selectivity of PO, whereas the PO selectivity is increased greatly when the O2* as the oxidant.36 Inspired by these interesting results, and considering that the Cu2O is a more reasonable catalyst phase compared to that of metallic Cu for PO formation in the presence of oxygen, propylene selective oxidation with the Olattice, O*, and O2* species on Cu2O(111) was illustrated in the present work.

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Scheme 1. Proposed reaction pathways for propylene selective oxidation on Cu2O(111).

2. Calculation methods and surface models 2.1. Methods. All calculations were performed using spin-polarized version of VASP37-38 with the projector augmented wave (PAW)39-40 method. The Perdew-Burke-Ernzerhof (PBE)41 generalized gradient approximation (GGA)42 was chosen for the exchange and correlation functional. For the analysis of electron correlations in transition metal oxides, DFT + U method43-45 was employed with the U-J value of 3.6 eV for the Cu 3d states, which was determined from electrostatically embedded Hartree-Fock calculations46 using the method exploited by Mosey et al.47 The electronic wave functions were expanded in a plane wave basis with the kinetic cutoff energy of 400 eV and the Brillouin zone sampling is carried out using 2 x 2 x 1 Monkhorst Pack k-point mesh48. Self-consistent field computations at the geometry optimizations were repeated until forces acted on the relaxed atoms were below 0.035 eVÅ-1. The climbing image general nudged elastic band (NEB) method49-50 was employed to locate the transition states (TSs) and the frequency analysis was used to confirm the TSs. Zero-point energy (ZPE) correction was added to all activation energies:51

ZPE   1 2hv i , where h denotes the Planck constant,  i denotes the computed real frequencies. i

The rate constant k and pre-exponential factors A0 were calculated based on the TS theory:52 k

 -E   -E  k BT q iTS  exp  ZP   A 0 exp  ZP  , where kB is the Boltzmann constant, IS h q i  k BT   k BT 

EZP

is the

activation barrier with ZPE correction, A0 refers to the pre-exponential factor which is determined by the vibrational partition functions qiIS and q TS (the adsorbed species contain neither translational i

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nor rotational contributions). The adsorption energy ( Eads ), activation energy ( Ea ) and reaction energy ( E ) are calculated by the following formulas: Eads  E A M  E A  E M , Ea  ETS  EIS and

E  EFS  EIS , where EA/M, EM, EA, EIS, ETS and EFS represent the energies of the adsorption system, substrate, adsorbate, initial state (IS), TS and final state (FS), respectively. The van der Waals correction was carried out by considering the weak interaction with the catalyst and using the DFT-PBE-D3 method.53

2.2. Models Symmetric periodic substrates are modeled by the p (2 × 2) unit cell of four layer copper atoms and eight layer oxygen atoms, where the uppermost six layers were relaxed (see Fig. 1). The optimized lattice constant of 4.27 Å is applied in unit cell volume, which is close to the experimental data.54 The type was obtained by cleaving the bulk in (111) directions and a vacuum space of 18 Å was used to prevent spurious interactions between the repeated slabs. On the non-polar surface, there are four chemically different environment atoms, which were denoted as CuCSA, CuCUS, OSUB, and OSUF (see Fig. 1). CuCSA is saturated surface copper atom which binds to two neighboring oxygen atoms, and CuCUS is unsaturated surface copper atom which only binds to one neighboring oxygen atom. OSUB is saturated and fourfold-coordinate subsurface oxygen atom, and OSUF is unsaturated and threefold-coordinate surface oxygen atom. Oxide (O2-, O-, and O2-) species were modeled with lattice oxygen, absorbed atomic oxygen, and absorbed O2 on Cu2O(111). In the propylene molecule, the terminal carbon atom is named as C1, the middle carbon atom is named as C2 and the carbon atom of the methyl is named as C3.

Fig. 1. Side view (a) and top view (b) of Cu2O(111) clean surface and propylene molecule (c).

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3. Results and Discussions 3.1. The analysis of different oxygen role on Cu2O(111) surface Here O2- (superoxo with the bond length of 1.36 Å and the O-O bond energy of 4.10 eV in the gas phase55) can be considered as the O2*, O- is the O*, and O2- is the Olattice. Indeed, the corresponding charge of each oxygen species can be confirmed by its electronic structure. The HOMO distributions for the Olattice, O*, and O2* on Cu2O(111) surface calculated through the magnetization density are shown in Fig. 2. It is well known that the Olattice in stoichiometric Cu2O(111) bulk represents O2-, and can be expressed in Fig. 2a with no single electron in atomic orbitals. Fig. 2b shows a single electron in p atomic orbitals of the O* with the Bader charges of around -0.8 |e|, which means that the O* species can be identified as O-. Fig. 2c demonstrates the magnetization density of the O2* species caused by the fact that the HOMO is the antibonding orbitals (π2p*) of the O2* species, and the calculated Bader charges is around -0.7 |e|, i.e., the O2* species can be taken as superoxide O2-.35, 56

Fig. 2. HOMO distributions of the Olattice, O*, and O2* on Cu2O(111) surface, Isosurface levels were set at 0.01 Å−3. Orbital distribution image was obtained by VESTA visualization software. (Cu atoms are in deep blue only in this figure for viewing clearly.)

3.2. Reaction mechanism of propylene selective oxidation 3.2.1. Reaction mechanism with the lattice oxygen (O2- site) species on Cu2O(111) Propylene selective oxidation with surface O2- species on Cu2O(111) has been divided into two paths: dehydrogenation and epoxidation routes (see Fig. S1). The adsorption properties of reactant, intermediates, and products have been investigated firstly. The optimized adsorption configurations 7



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are displayed in Fig. S2a and the adsorption energies are listed in Table S1 in supporting information. 3.2.1.1. Dehydrogenation mechanism It should be pointed there are three different adsorption configurations for propylene as seen in Fig.S2a, and they related different reaction processes. For example, configuration of C3H6(I) is favor for the AHS route, and C3H6(II), C3H6(III) are favor the epoxidation route. The adsorption energy of propylene in Table S1 corresponding to the most stable adsorption configuration, which was chosen as the energy reference point for both AHS and epoxidation routes in the determining the energy barrier and reaction energy in the following study. The most stable adsorption of propylene for dehydrogenation adopts  type (C3H6(I)), where C1 binds to CuCUS with the distance of 2.06 Å and C2 connects to the same CuCUS with the length of 2.11 Å. The H atom in the methyl group tends to surface OSUF with the distance of 2.69 Å. The adsorption energy of propylene is -1.09 eV. Dehydrogenation reaction mechanism started from absorbed propylene involves hydrogen abstraction from the methyl by OSUF to form allyl and hydroxyl group with a barrier of 1.20 eV, at TS1 the distance of breaking C3-H is 1.50 Å and the length of forming O-H is 1.19 Å. The hydrogen stripping in the methyl is endothermic by 1.05 eV, and the bond distances of C1-C2 and C2-C3 in allyl are quite similar. The formed allyl is favored at η1(C1)-η1(C3) mode with the adsorption strength of -1.56 eV. The following step, C3 in the methylene binds to another nearby surface OSUF directly created C3H5O species. The binding process of C and O requires 0.73 eV and has a barrier of 1.16 eV via TS2, where the length of C3-OSUF is 1.77 Å. The following step, because of the existence of available OSUB active species, hydrogen atom of C3H5O species can be stripped by the OSUB forming acrolein with a barrier of 0.12 eV and the process is exothermic by 0.76 eV. At TS the distance of C3-H is 1.37 Å and the length of O-H is 1.28 Å, respectively. The final product of acrolein prefers  type with the adsorption strenght of -0.92 eV. Then acrolein desorbs from the surface and water can be generated from hydrogen diffusion. The desorption barrier of acrolein should be smaller than its corresponding adsorption barrier (0.92 eV) by considering the entropy change at relatively high reaction temperature or activated by the co-adsorbed species. Lastly water desorbs from the surface ending dehydrogenation reactions. Apparently, the first C-H bond broken in C3H6 is the key step due to its relatively high energy barrier (1.20 eV) in the AHS processes. 3.2.1.2. Epoxidation mechanism via the OMP intermediate 8


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For adsorption configuration of  type of C3H6(II) as seen in Fig. S2, in which the C1 binds to CuCUS with the distance of 2.04 Å and C2 connects to the same CuCUS with the length of 2.17 Å. For C3H6(III), the distance of C1-CuCUS is 2.05 Å and the length of C2-CuCUS is 2.13 Å. The adsorption energy of propylene is -1.00 eV for C3H6(II) and -1.01 eV for C3H6(III). The reaction of propylene epoxidation starts with propylene adsorption close to CuCUS, and the adsorbed propylene acts on neighboring OSUF through carbon atom binding to oxygen atom, which can create two different OMP intermediates. For the adsorption type of C3H6(II), its C1 atom binds to OSUF and C2 connects to CuCUS leading to the formation of OMP1, where the distance of C1-OSUF is 1.51 Å and the length of C2-CuCUS is 1.96 Å. The OMP1 formation process is endothermic by 0.96 eV and has a barrier of 1.22 eV via TS4, where the C1-OSUF distance is 1.90 Å. For the C3H6(III), its C2 atom binds to OSUF and C1 connects to CuCUS generating OMP2 intermediate, where the distance of C2-OSUF is 1.53 Å and the length of C1-CuCUS is 1.95 Å. The generation of OMP2 requires 0.61 eV with a barrier of 0.90 eV, and the length of C2-OSUF is 1.96 Å at TS5. In one route, intermediary OMP1 can be oxidized to epoxide PO, which involves the breaking of the C2-CuCUS bond and the formation of a (-C1OSUFC2-) cycle and needs to overcome the barrier of 1.59 eV. The cyclizing process is endothermic by 0.64 eV, and the distance of C2-OSUF is about 2.18 Å at TS6. Propanal is also formed from OMP1 by H shift from C1 to C2. At TS7 the distance of H-C1 is calculated to be 1.31 Å and the length of H-C2 is decreased to 1.37 Å. The shift of H from C1 to C2 is endothermic by 0.48 eV and the barrier is 0.62 eV higher than that for PO formation. In the other route, OMP2 can ring-close to form the epoxide of PO with a barrier of 1.92 eV. In this process, the bond of CuCUS-C1 is broken and oxygen atom interacting with C2 is directly binding to C1 simultaneously. The ring closure is endothermic by 0.89 eV, and the length of OSUF-C1 is 2.02 Å at TS8. Moreover, OMP2 can also form acetone by H shift from C2 to C1 through TS9, in which the distance of C2-H is 1.34 Å and the length of C1-H is 1.37 Å, respectively. The activation barrier is 2.02 eV and the shift of hydrogen atom is endothermic by 0.33 eV. For the Olattice as the oxidant, the above results indicated AHS process is more favorable than that of PO process because of the energy barrier of key step in AHS (1.20 eV) is smaller than that of PO process via OMP1 type (1.59 eV), so the main product on pure Cu2O(111) should be the acrolein. In the AHS route, hydroxyl groups existed on the surfaces can form the product water through the diffusion of hydrogen. After the product desorbs from the catalyst in the AHS and epoxidation 9



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routes, oxygen-defected surfaces can be created on the surface. Aimed at making the catalyst recovery, molecular oxygen is requisite at the working conditions. Molecular O2 moiety bound in the hole is such that it can form Olattice and O*. This restoration process takes place easily on these two surfaces, and the O* with higher activity becomes the oxidant participating in propylene oxidation.

3.2.2. Reaction mechanism with the adsorbed oxygen atom (O- site) species on Cu2O(111) The selective oxidation of propylene with surface O- species on Cu2O(111) has been studied via dehydrogenation route and epoxidation route. Some key species configurations and reaction routes are summarized in Figs. 3 and S3. The optimized configurations of IS, FSs, and intermediates are shown in Fig. S2b, and the corresponding adsorption energies are displayed in Table S1 in supporting information. For the O* on Cu2O(111), it binds to the surface CuCUS with the CuCUS-O distance of 1.80 Å and the adsorption strength of 1.89 eV with respective to the molecular O2. 3.2.2.1. Dehydrogenation mechanism Two H-stripping reactions from absorbed propylene occur at dehydrogenation mechanism. In this process, the adsorption of propylene adopts  type (C3H6(I) as seen in Table S2b), in which C1 and C2 are equally binding to the same CuCUS with the distance of C1-CuCUS being 2.05 Å and the length of C2-CuCUS being 2.15 Å. The hydrogen atom in the methyl is close to surface O- species with the distance of 2.86 Å. The adsorption energy of C3H6(I) is -1.06 eV. Before the beginning of dehydrogenation, gaseous propylene absorbs at CuCUS nearby surface O- species. The first step involves H abstraction from the methyl by the neighboring O- species forming allyl and hydroxyl via TS10, where the distance of C3-H is 1.32 Å and the length of O-H is 1.34 Å. The formed allyl adopts η1(C1)-η1(C3) model with the adsorption energy of -1.41 eV. In this process, the stripping of hydrogen is exothermic by 0.13 eV and has a barrier of 0.82 eV. The second step, C3 in the methylene binds to nearby OSUF forming C3H5O species with a barrier of 0.22 eV. The formation of C3H5O intermediate is exothermic by 0.58 eV and the distance of C3-OSUF is 2.18 Å at TS11. The third step, the abstraction of hydrogen from C3 by hydroxyl group can produce the final products acrolein and water via TS12, in which the length of C3-H is 1.42 Å and the distance of O-H is 1.39 Å, separately. The second hydrogen stripping requires 0.33 eV and this process has a barrier of 0.74 eV. The formed acrolein is favored at a  type with the adsorption energy of -0.81 eV. In summary, for AHS route, the key step is the first C-H bond activation of C3H6 and with the barrier of 0.82 eV. 10


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Fig. 3. Optimized geometries along the dehydrogenation (a), the epoxidation (b), and the direct epoxidation (c) mechanism of propylene with surface O- species on Cu2O(111). Bond lengths are in Å. Adsorbed oxygen atoms are in Magenta. These notations are used throughout the paper.

3.2.2.2. Epoxidation mechanism a) OMP intermediate mechanism For epoxidation mechanism, two adsorption modes contribute to propylene. The first model, C3H6(II), the adsorption of propylene adopts  type at CuCUS, where the distance of C1-CuCUS is 2.07 11



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Å and the length of C2-CuCUS is 2.13 Å. The adsorption energy is -1.03 eV. The second model, C3H6(III), the adsorption of propylene prefers  type at CuCUS as well, where C1 has a distance of 2.03 Å and C2 has a length of 2.19 Å toward the CuCUS. This type has the adsorption energy of -1.13 eV. Intermediary OMP1 can be generated from the C3H6(II), in which C2 connects to CuSUB with the length of 2.08 Å and C1 binds to absorbed atomic oxygen with a distance of 1.40 Å. The formation of OMP1 requires energy of 0.36 eV with a barrier of 1.71 eV, and the length of C1-O is 2.22 Å at TS13. Intermediary OMP2 can be formed from the C3H6(III), in which C1 links to a CuSUB with 2.02 Å and C2 binds to absorbed atomic oxygen with 1.43 Å. The formation of OMP2 requires 0.69 eV with a barrier of 1.41 eV via TS14, where the distance of C2-O is 2.25 Å. Epoxidation occurring from OMP1 to PO involves the formation of a (-C1OC2-) cycle with a barrier of 0.64 eV. This process of PO formation is exothermic by 0.46 eV and the distance of C2-O is 2.15 Å at TS15. Intermediary OMP1 can be also converted to propanal by hydrogen shift from C1 to C2 and the shift of hydrogen releases 1.58 eV. At TS16 the distance of forming H-C2 is 1.90 Å and the length of breaking H-C1 is 1.17 Å, and the calculated barrier is 0.23 eV. PO can also be generated from OMP2 by Cu-C1 bond breaking and the oxygen atom binding to C1 directly. The formation of PO from OMP2 has a barrier of 0.28 eV and the length of forming O-C1 is decreased to 1.99 Å at TS17, concomitantly exothermic by 0.78 eV. Otherwise, acetone can be created from OMP2 by hydrogen shift from C2 to C1 via TS18, where the length of H-C2 is increased to 1.20 Å and the distance of H-C1 is shorten to 1.71 Å. The barrier of H shift is 0.15 eV higher than PO formation from OMP2 and the process is exothermic by 2.07 eV. For the above OMP type mechanism one can find that the OMP formation step is the rate-limiting step due to the higher energy barrier in compare with the later step for PO formation, and the OMP2 is favorable for PO formation than that of OMP1 when compared the respective energy barrier (171 vs. 1.41 eV). b) PDO intermediate mechanism For the Cu2O(111) system with surface O- species, it should be noted the possible PDO intermediate mechanism inspired by Greeley et al28 and displayed in Fig. S3. PDO species can be generated from intermediary OMPs with the breaking of C-Cu and the forming of C-O. The formation of PDO1 from OMP1 is exothermic by 1.13 eV with the barrier of 0.18 eV through TSa, where the distance of C2-O is 2.22 Å. The generation of PDO2 from OMP2 releases 1.18 eV with the 12


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barrier of 0.20 eV, and the length of C1-O is 2.18 Å at TS. Epoxide PO can be produced from intermediary PDO through ring-closure, which has a barrier of 1.75 eV from PDO1 and 1.64 eV from PDO2. At TS, the distance of C2-O is 2.23 Å and the length of C1-O is 2.16 Å, respectively. Since the formation of PO from PDO intermediate has tremendous barrier in comparison with the OMP pathways and requires rather high temperature, hence this potential pathway is unfavorable in propylene selectivity oxidation. c) Direct epoxidation mechanism Based on above discussions, alternative direct epoxidation mechanism inspired by Özbek et. al 30

has also been referred in the present study, where propylene reacts with absorbed atomic oxygen

forming PO directly. For the direct epoxidation mechanism, it can either be processed through Langmuir-Hinshlwood (L-H) or Eley-Rideal (E-R) types, which are summarized in Fig. 3c. For L-H type, absorbed propylene can be directly binding to O- species with a barrier of 1.61 eV. The process is exothermic by 0.25 eV and the distance of C1-O is 2.10 Å at TS, and then PO desorbs with a barrier of 1.36 eV. Here the desorption barrier was estimated by the formula: Edes  Ea  Eads , where Ea is the energy barrier of adsorption process, and Eads is the adsorption energy of PO. Ea is assumed to be zero in the present work, thus the PO desorption barrier is approximated to its adsorption energy. For E-R type, gaseous propylene acted on absorbed atomic oxygen can create PO directly. The process releases 1.29 eV and has a smaller barrier of 0.62 eV via TSgas, where the distance of C2-O is 1.99 Å. The desorption of PO has a barrier of 1.36 eV, which indicates that PO desorption is rate controlling step in E-R direct mechanism. In fact, if the entropy effect (-TS) was considered, the desorption barrier of PO would be decreased significantly, and thus the desorption step might not be the rate-controlling step at relatively high reaction temperature (~400 K or so). Moreover, as the configuration of IS in E-R type is not stable as that of L-H type (a energy difference equals to the C3H6 adsorption energy in magnitude approximately), so the E-R type would have the similar energy barrier for PO formation as that of L-H type when the same IS was chosen (i.e. 0.62 + 1.13 = 1.75 eV for E-R type). Our calculation indicates that either the L-H or E-R direct mechanism is unfavorable due to its relatively high energy barrier (> 1.6 eV).

3.2.3. Reaction mechanism with the adsorbed molecular oxygen (O2- site) species on Cu2O(111) 13



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The selective oxidation of propylene with surface O2- species on Cu2O(111) has been studied through dehydrogenation path and epoxidation path, and the detailed mechanisms are summarized in Figs. 4 and S4. The adsorption geometries of IS, FSs, and intermediates are very similar with the former circumstances and not be introduced here in detail. The optimized configurations of IS, FSs, and intermediates are exhibited in Fig. S2c, and the corresponding adsorption energies are listed in Table S1 in supporting information. 3.2.3.1. Dehydrogenation mechanism In Fig. S2c, there are three kinds of adsorption configurations for propylene, and the first adsorption model of propylene (C3H6(I)) is favor at dehydrogenation path with the adsorption energy of -1.00 eV. Molecular O2 adsorbs in Cu2O(111) with the bond length of O-O is 1.37 Å, and the adsorption energy is -2.51 eV. At first, the dehydrogenation path involves H stripping from methyl in absorbed propylene by surface O2- species with a barrier of 0.92 eV, forming allyl and OOH species (C3H6* + O2-  C3H5*+OOH*). At TSo-1 the distance of C3-H is 1.45 Å and the length of O-H is 1.18 Å, and the process is endothermic by 0.44 eV. The second step, C3 in the methylene directly binds to nearby OSUF forming C3H5O species through TSo-2, where the length of C3-OSUF is 2.17 Å. This binding process between C and OSUF is exothermic by 0.42 eV and has a barrier of 0.47 eV. The third step, acrolein can be produced with the stripping of hydrogen from C3H5O by surface O2- species, which is endothermic by 0.25 eV. The process has a barrier of 1.33 eV and the distance of C3-H is 1.93 Å at TS. A pair of OH* species can also be formed during the second H-stripping step with surface O2- species, and water can be generated from hydrogen diffusion between the both OH* species. Lastly, the products desorbs from the surface ending the dehydrogenation route. The key step of AHS by O2- is the activation of the second C-H bond with the barrier of 1.33 eV. 3.2.3.2. Epoxidation mechanism Propylene epoxidation mechanism with surface O2- species on Cu2O(111) begins with the interaction between adsorbed propylene and the O2- species, which leads to the formation of a new oxametallacycle species (OOMP)36 through the first O-C bond coupling and the distance of O-O elongates to 1.48 Å from 1.36 Å. The second and third adsorption models of propylene in Fig. S2c prefer epoxidation path, and the elaborate mechanism can be seen in Fig. 4b. The second adsorption model (C3H6(II) the adsorption energy is -0.96 eV) is benefit for the formation of OOMP1, where the distance of C1-O is 1.41 Å and the length of C2-Cu is 2.06 Å. The third model ((C3H6(III) the 14


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adsorption energy is -0.99 eV) tends to the generation of OOMP2, where the distance of C1-Cu is being 2.02 Å and the length of C2-O is being1.45 Å. The formation of OOMP1 is endothermic by 0.95 eV with a barrier of 1.68 eV, and the length of C1-O is 1.84 Å at TS. The generation of OOMP2 requires 1.14 eV and has a barrier of 1.53 eV through Tso-5, where the distance of C2-O is 2.07 Å.

Fig. 4. Optimized geometries along the dehydrogenation (a) and the epoxidation (b) mechanism of propylene with surface O2- species on Cu2O(111). Bond lengths are in Å.

The new OOMP1 intermediate can produce PO involving the formation of a (-C1OC2-) cycle. This prcess is exothermic by 1.32 eV with a barrier of 1.05 eV, and the distance of C2-O is 2.15 Å at TS. Intermediary OOMP1 can also turn into propanal by H shift from C1 to C2 with the calculated 15



Page 16 of 32

barrier of 0.63 eV, and this H shift process releases 2.07 eV. At TS the distance of H-C2 is 1.81 Å and the length of H-C1 is 1.23 Å. PO can still be created from OOMP2 by the breaking of Cu-C1 bond and the binding of O-C1 directly. The formation of PO from OOMP2 is exothermic by 1.51 eV and has a barrier of 0.44 eV, and the length of O-C1 is being 1.94 Å at TS. Acetone can be formed from OOMP2 by hydrogen shift from C2 to C1. The process releases 2.70 eV with a barrier of 0.40 eV, and at TS the length of H-C2 is 1.24 Å and the distance of H-C1 is 1.52 Å, respectively. Furthermore, epoxide PO might be directly generated from strongly absorbed propylene with the O2- (Fig. S4). However, our calculation suggested this process is exothermic by 0.21 eV and has a high barrier of 2.86 eV (and the distance of C1-O is 1.95 Å at TS), so one can ignore such direct epoxidation mechanism when the O2- species as the oxidant.

3.3. Free energy diagrams Free energy diagrams for propylene selective oxidation with surface O2-, O-, and O2- species on Cu2O(111) are plotted in Fig. 5. The free energies are determined by taking into account the entropy contributions. We assume such contributions from the translational entropy, which is calculated as the equation:57-58 S  1.5R ln  2 Mk BT   3R ln h  R ln  k BT P   2.5R , where M, R, kB , h, T, and P are the molecular weight, ideal gas constant, Boltzmann constant, Planck constant, temperature, and pressure, respectively. In Fig. 5 diagrams, the free energies are reported at 433 K and 100 kPa to keep in accordance with the experiment condition17. Under such condition, we estimated that propylene in the gas phase lost 0.73 eV of entropy energy (TS) in the adsorption, and the products (PO, propanal, acetone and acrolein) desorb into the gas phase gaining almost the same entropy energy of 0.75 eV. The values are approximate to the entropic energy calculated by the method of Campbell et al. (0.68 eV)59 For the O2- oxidant in Fig. 5a, because of rather high barriers for the formation of propanal and acetone, the routes of them are neglected. In the epoxidation path, the barrier of PO formation from OMP2 (1.92 eV) is too high to achieve, namely the formation route of PO from OMP2 is hampered. The barrier of OMP1 formation is only 0.02 eV higher than that of allyl (1.22 vs 1.20 eV), however the barrier of PO formation from OMP1 (1.59 eV) is so high that this route is unfavorable. This may 16


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be caused by the reason that the adsorption of OMP intermediates is strong, where unsaturated CuCUS atom becomes saturated for binding to C1 or C2. In the dehydrogenation path, all steps have relative lower barrier than the epoxidation path, and the second H-stripping step has much smaller barrier maybe caused by releasing much energy. Thus, the dehydrogenation route is preferred than the epoxidation route and acrolein is the main product with this species. For the O- oxidant in Fig. 5b, the first H abstraction has the highest barrier of 0.82 eV in the dehydrogenation path, whereas in the epoxidation path, the barriers of OMP formation are higher (1.71 and 1.41 eV) caused by the strong adsorption of OMP, which means that the dehydrogenation path is more favorable. For the O2- oxidant in Fig. 5c, in the epoxidation route, special OOMP1 and OOMP2 intermediates can be formed from absorbed propylene with the barriers of 1.68 eV and 1.53 eV, respectively. In the dehydrogenation route, the formation of acrolein from C3H5O species with the barrier of 1.33 eV is the rate-determining step, which exhibits that propylene selective oxidation with surface O2- species prefers the dehydrogenation route. The difference of barrier energy between the PO formation from OOMP2 and the acrolein formation is very small (i.e. 1.33 eV for AHS and 1.53 eV for PO), indicating that the selectivity of PO may be enhanced with this species. For the three different oxygen species, the formation of acrolein, PO, propanal, and acetone has relatively low barriers with surface O- species compared to surface O2- and O2- species, which indicates that the O- is more active for propylene selective oxidation. However, for the selectivity towards the PO formation, as the energy barrier difference between AHS and epoxidation on O2- is the smallest one 0.20 eV (1.53 eV for epoxiation and 1.33 eV for AHS) as compared to that of 0.59 eV for O- species (i.e. 1.41 eV for epoxidation and 0.82 eV for AHS) and 0.39 eV for O2- (that is 1.59 eV for PO formation and 1.20 eV for acrolein formation).

17



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Fig. 5. Free energy profiles of propylene oxidation at surface O2-(a), O- (b), and O2- (c) sites on Cu2O(111) at 433 K and 100 kPa. The black line shows the formation route of acrolein, the olive line and blue line show the formation routes of PO and propanal via OMP1 (OOMP1), the red line and magenta line show the formation routes of PO and acetone via OMP2 (OOMP2), and the orange line shows the formation of PO from propylene directly.

3.4. Energetic span model analysis for the PO selectivity by O2-, O- and O2The traditional measure of the efficiency of a catalyst is the turnover frequency (TOF), which is expressed as the number of cycles performed per time unit and catalyst concentration. To further understand catalytic activity and selectivity for propylene partial oxidation reactions, we calculate the TOF based on the energetic span model developed by the groups of Kozuch and Shaik.60-61 In catalytic cycle, only one transition state and one intermediate determine the TOF, and they are called the TOF-determining transition state (TDTS) and the TOF-determining intermediate (TDI). The energetic span model offers a quantification of the influence of each intermediate and transition state on the TOF for identifying the TDI and TDTS. Once the TDI and TDTS have been identified, a simple estimation of TOF can be determined by the energetic span model approximately, which gives the model its name: TOF 

With

E





k BT E/RT e h

E(TDTS) E(TDI)

if TDTS appears after TDI

E(TDTS) E(TDI)G

r

if TDTS appears before TDI

E , the energetic span, corresponds to the apparent activation energy of the full cycle. Thus,

knowing the TDTS, the TDI, and when necessary also the reaction energy, this approximation provides all the relevant kinetic information that is required for calculating the TOF of the cycle. We calculated the apparent activation energy and TOFs of acrolein and PO with surface O2-, O-, and O2- species on Cu2O(111), and the result of apparent activation energy was displayed in Fig. 6. The apparent activation energy of acrolein and PO formation with surface O2-, O-, and O2- species are 2.21, 0.99, 1.35 eV and 2.53, 1.62, 1.70 eV, respectively. It can be found that acrolein is easier to synthesize relative to PO, which means that acrolein is the main product with surface O2-, O-, and O2species. The TOFs of acrolein and PO formation with surface O2-, O-, and O2- species are calculated 18


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to be 1.70×10-13 s-1, 26.98 s-1, 1.74×10-3 s-1, and 3.2×10-17 s-1, 1.25×10-6 s-1, 1.47×10-7 s-1, which indicates that surface O- species is the most active. PO selectivity with surface O2- species can be ignored for the higher apparent activation energy, and PO selectivity with surface O- and O2- species can reach 0.00046  and 0.85  , respectively. Thus, the selectivity of PO ( Seli ) with surface O2species is the relatively highest among these three different species (calculated by the formula: Seli 

ri

r

100% , ri represents the reaction rate of product i, namely PO and acrolein).

i

In fact, the experimental results show that the selectivity of PO is 1.8% when using molecular oxygen as the oxygen agent on the pure copper catalyst, close to our present results for the case of O2-.31 Of course, the PO selectivity is low by using pure copper as the catalysts, and this can be promoted significantly by adding K+, or Cs+ metals,31-32 and this will be discussed in our future work.

Fig. 6. The analysis of energetic span model with surface O2-, O-, and O2- species on Cu2O(111).

3.5. Analysis for the difference in the PO formation for different oxygen species Based on above analysis we know that the selectivity of PO with surface O2- species is the highest among these three different species, and the catalytic activity of propylene catalyzed by O- is higher than those of O2- and O2-, so it is important to analyze the physical original in detail. a) Basicity properties of different oxygen species for AHS process For the dehydrogenation mechanism, the ability of stripping hydrogen atom is related to the basicity of oxygen, and the strong basicity means facile to strip hydrogen atom. The proton (acid-like species) was selected as a probe to further detect the basicity of different oxygen species. The binding energy between the H atom and surface O2-, O-, and O2- species are -0.37 eV, -1.60 eV, and 19



Page 20 of 32

-0.80 eV, respectively (Fig. S7). The results indicate that surface O- species is the most active relative to surface O2- and O2- species due to the strongest binding to proton, in agreement well with the first dehydrogenation step in the AHS processes (Fig. S8). The relatively moderate basic properties and oxidation of surface O2- species compared to those of surface O- species (too active oxidation) or surface O2- species (less oxidation) may be lead to the highest selectivity of PO. Secondly, the basicity of lattice and adsorbed oxygen is studied by electronic structure analysis (see PDOS analysis as seen in Fig. S9). As seen from Fig.S9, the population of p-band near the Fermi level is largest for the O*, followed by the O2*, and the smallest one is the lattice oxygen, general agreement with the catalytic activity of oxygen species. b) Quantitative analysis of energetic roles contributions to AHS processes catalyzed by both Oand O2In addition to the basicity properties of oxygen species analyzed above, other key factors determining the AHS process was analyze by energy decomposition scheme. Here the first C-H bond scission step by both O- and O2- was chosen and try to analysis the reason why AHS is favorable by O* instead of O2- species. The energy barrier for an given elemental step can be decomposed into TS three terms,62 E a  E A  E B  Eint , where Ea is the activation energy barrier, EA(EB) is the energy

cost for the activation of reactant A(B) from the IS to the TS without reactant B(A), and

TS is E int

the

interaction energy between A and B at the TS. Here A is O*(or O2-), and B is C3H6*. As seen in Fig. 7, it can be seen the activation of O- (or O2-) is small and almost the same for both O- and O2- to contributes to AHS step barrier, whereas the energy required to active C3H6 is much different for Oand O2*, that is, 0.95 eV for O- species and 1.68 eV for O2-. This can be further confirmed by the TSs type of first C-H bond broken in AHS processes, that is 1.32 Å for O* and 1.48 Å for O2- as seen in Figs. 3 and 4, so larger deformation would be existed in the O2* situation. For the interaction between TS O- and C3H6*, Eint , the associated magnitude is -0.24 eV for O* and -0.78 eV for O2-, which is TS different form the trends of energy barrier of first C-H bond activation, so the Eint is not the major

factor determining the energy barrier of AHS. Considering these three parts of energy barrier, it is believed that the activation of C3H6 ( E C3H6 ) is the major part, and also the interaction between O- and TS C3H6 ( Eint ) contributes to the C-H bond activation barrier to some extent.

20


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Fig. 7. Barrier decomposition analysis of the AHS and the OMP2 epoxidation routes with surface O- species (a) and the AHS route with surface O2- species (b). Blue box represents Ea , Green box represents E O , Red box represents TS TS EC3H6 , and Magenta box represents E int Negative (positive) values of E int indicate attraction (repulsion) between

A and B at the TS.

c) Quantitative analysis of energetic roles contributions to AHS and PO catalyzed by OThe above DFT and energetic span model analysis revealed that the AHS is more favorable than that of PO formation for both O- and O2- situations, so it worthy to explore the possible reasons further. Here the above mentioned energy barrier decomposition scheme was also used, and the chosen step is the first C-H bond activation of C3H6 (i.e., C3H6+O  C3H5+OH) and the first C-O bond formation (i.e., C3H6+OOMP). As seen in Fig.7, for these two routes, the energy cost of O* activation is small for both C-H bond broken and C-O bond formation (ca. 0.1 eV or so) due to less active of strong adsorbed oxygen atom, and major portion of the activation energy barrier was the activation of the C3H6 (i.e., 0.95 eV for C-H bond activation and 1.41 eV for C-O bond formation), but this cannot explain the energy barrier order of C-H bond activation and C-O bond formation (that is 0.82 eV for AHS and 1.41 eV for OMP). On the other hand, the interaction between C3H6 and Ocan be regard as the main reason influenced the activation barriers because it can account for the barrier difference between C-H bond broken and C-O bond formation. At the TSs, the interaction was attraction in the AHS route, whereas the interaction was repulsion in the OMP2 epoxidation route. This may be caused by the fact that the bond distances between C3H6 and O- at the TSs exist a large gap, where the distance of O-C is 2.62 Å in the AHS route and the length of O-C is 2.25 Å in the OMP2 epoxidation route. To further understand the physical original why C-H bond scission is more favorable than 21



Page 22 of 32

that of C-O bond formation one, the electronic analysis of TSs for C3H6 + O → C3H5 + OH and C3H6 + O → OMP2 was performed based on the projected crystal orbital Hamilton population method (pCOHP) developed by Dronskowski et al.63-64 In the pCOHP diagram, the positive and negative values correspond to the bonding and antibonding states, respectively. One can know that the Fermi level lines between bonding and antibonding region in Fig. 8. Moreover, for the C-H bond broken, the integrated antibonding states is 0.72 eV, and the integrated bonding states is 4.98 eV; for the C-O bond formation, the integrated antibonding states is 0.49 eV, and the integrated bonding states is 2.94 eV. Clearly, the population of bonding states at the TSs of C-H bond broken step is much larger than that of C-O bond formation, so more stable of TSs for C-H bond activation can be expected.

Fig. 8. Projected crystal orbital Hamilton population (pCOHP) between C3H6 and Cu2O(111) at TSs: (a) C3H6 + O → C3H5 + OH, (b) C3H6 + O → OMP2.

d) Geometric factor of TSs for the formation of PO To further understand the origin of the barriers variation for PO formation with surface O2- and O- species via OMP1 or OMP2, the calculated geometries of the TSs for the formation of products were analyzed (see Fig. 9). For surface O2- species, the geometry of TSs leading to PO requires substantial elongation of Cu-O (0.48 Å or 0.44 Å) and Cu-C (1.22 Å or 0.77 Å) bonds in OMP1 or OMP2 fragments. The O-atom moves beneath the C3H6 fragment, which is concomitantly shifted 22


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upward of C3H6. In contrast, for surface O- species, the bond distances of Cu-C are slightly changed (0.79 Å or 0.46 Å) at the TSs configuration from the OMP1 or OMP2 fragments, and the bond distances of Cu-O in the OMP1 or OMP2 fragments are almost unchanged for the TSs configuration. The adsorption strength of OMPs is almost same (Table S1), and slight elongation of configuration between OMPs and TSs requires lesser barrier than substantial elongation.65 Thus, it is energetically less demanding to elongate the Cu-O and Cu-C bond with surface O- species than O2- species, and more PO can be produced with surface O- species in propylene selective oxidation.

Fig. 9. Schematic presentations of OMP intermediates with transition states leading to PO with surface O2- (a) (b) and O- (c) (d) species on Cu2O(111).

3.6. Micro-kinetic simulations The detailed microkinetic simulation scheme can be found in our previous work66-67 and others68-69, and is described simple as following: the steady-state surface coverage was calculated by solving the ordinary differential equations describing the coverage of the intermediates. Then the coverage was put into the calculation of the individual elementary reaction rates and the overall rate per surface atom. Finally, the TOF, as the traditional measure of the efficiency of a catalyst, is clear by Arrhenius equation obviously. The barriers and pre-exponential factors with surface O2-, O-, and O2- species were summarized in Tables S2, S3, and S4 and the correlation formulas were shown in supporting information. The TOFs of acrolein and the surface coverage in the AHS route with surface O2-, O-, and O2species on Cu2O(111) facet are plotted against the temperature in Fig. 10. The TOF of PO formation 23



Page 24 of 32

is very small (< 10-6 s-1), so we do not list its simulation results in this work. For surface O2- and O2species, the TOF of acrolein increases and the surface coverage of propylene keep steady with the temperature going up, and the TOFs at different temperature are quite small, which indicates that propylene selective oxidation requires high temperature. For surface O- species, the TOF of acrolein increases at first with the temperature increasing and declines subsequently when the temperature reaches about 433 K, which indicates that the temperature of 433 K is appropriate for propylene selective oxidation. For this species, the surface coverage of C3H6, allyl, and C3H5O species hold smooth, this may be caused by the strong adsorption of them. At the same temperature, the TOF value of acrolein follows O- > O2- > O2-, which means that the active order is O- > O2- > O2- and surface O- species is the most active for propylene selective oxidation.

Fig. 10. Temperature dependence of the total reaction rate and surface coverage with surface O2-, O-, and O2species in the dehydrogenation route on Cu2O(111). 24


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Sensitivity analysis (SA)70-71 is carried out to search for the rate determining step for acrolein forming. SA is performed for selected model response(s) (R) with respect to rate parameters. Typical model responses include reactant conversion, reaction rate, product selectivity, etc. Herein we choose the formation rate of certain product at 433 K as the response and the pre-exponentials (A) as parameters to get the degree control (DRC) for each elementary step defined as equation (6): DRCi , j 

d (ln R j ) d ln( Ai )



Ai dR j

(6)

R j dAi

where R j is denoted as the measured response of certain product j towards the perturbation of Ai i.e. the pre-exponential factor of reaction i . The calculated DRC contribution of the elementary steps in Tables S2, S3, and S4 is shown in Fig. 11.

Fig. 11. The DRC distribution of the elementary steps toward acrolein on Cu2O(111) with surface O2- (a), O- (b), and O2- (c) species at 433 K. Note: the +/- in bracket refers to the positive or negative values of DRC in corresponding to the promotion or inhibition effect on the rate.

For surface O2- species, the rate controlling step for acrolein is the combination of allyl and the Olattice in M3 and has a DRC contribution of 72.58%, caused by the fact that this elementary step has a high barrier and requires much energy. The first H-stripping of propylene forming allyl in M2 also has a significant DRC contribution with the value of 27.42%. Step M3 has positive contribution for the formation of acrolein, however, elementary step M2 has negative DRC contribution. For surface O- species, the first H-stripping in M2 and the second H-stripping in M4 have high DRC 25



Page 26 of 32

contributions being 51.40% and 47.48%, respectively. The formation of C3H5O species in M3 and the desorption of acrolein in M5 have low DRC contributions, which can be neglected in the whole process. Obviously, elementary steps M2 and M4 are proposed to be the rate-determining steps for acrolein forming. Otherwise, the step M2 has positive DRC contribution toward allyl species and promotes the formation of acrolein, which caused by the reason that this process releases the energy of 0.13 eV. Other steps containing M3, M4 and M5 have negative DRC contributions, which show that these steps inhibit the formation of acrolein. For surface O2- species, the DRC values of all the elemental steps for acrolein forming are positive, and the M3 and M4 steps have DRC contributions of 0.11% and 30.84%, respectively. The first H-stripping process is supposed to the rate controlling step with the DRC contribution of 69.05%, which caused by the reason that the process requires the energy of 0.44 eV.

4. Conclusions The present study has computationally discussed the epoxidation and dehydrogenation routes of propylene on Cu2O(111) surface through DFT+U scheme, and the mechanism of propylene selective oxidation with surface O2-, O-, and O2- species has been elucidated. The main conclusion of the present work are: 1) Surface O- species is the most active relative to the O2- and O2- species, and the main product is acrolein; 2) Surface O2- had the highest selectivity of PO formation among these three different oxygen species; 3) Lattice oxygen (O2-) had the smallest activity comparison to those of O2- and O-; 4) The proton probe indicate that the basicity of O- is the more strong than O2- and O2species, indicated its high activity for propylene oxidation. The present results demonstrated that the active oxygen species is the adsorbed molecular oxygen with the modest oxidation capacity, and might be useful for choosing the suitable oxidant or adding promoter like potassium atom to alter the electronic structure of oxygen species for propylene epoxidation reaction.

Supporting Information Reaction mechanism with surface O2- species on Cu2O(111); Optimized adsorption configurations and adsorption energies for various pertinent species with surface O2-, O-, and O2- species on Cu2O(111); Propylenedioxy intermediate mechanism with surface O- species; The direct epoxidation mechanism of propylene with surface O2- species on Cu2O(111); Formation of active oxygen species 26


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on Cu2O(111); The PDOS analysis of CuCUS and CuCSA sites on Cu2O(111) clean surface; The combination between H atom and different oxygen species on Cu2O(111); Electronic structure of different oxygen species on Cu2O(111); Microkinetic simulation; Stability of Cu2O(111) in working conditions.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grants No. 2142100, 91545106, 21773123) the 111 project(B12015) and the Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase).

27



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Theoretical Study of Propylene Epoxidation over Cu2O (111) Surface

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