Dynamic Frustrated Lewis Pairs on Ceria for Direct Non-oxidative

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Dynamic Frustrated Lewis Pairs on Ceria for Direct Non-oxidative Coupling of Methane Zheng-Qing Huang, Tian-Yu Zhang, Chun-Ran Chang, and Jun Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00838 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019

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Dynamic Frustrated Lewis Pairs on Ceria for Direct Non-Oxidative Coupling of Methane Zheng-Qing Huang,† Tianyu Zhang,§ Chun-Ran Chang,*,† and Jun Li‡,# †Shaanxi

Key Laboratory of Energy Chemical Process Intensification, School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China ‡Department of Chemistry and Key Laboratory of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China §Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, IL 62901, USA #Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, China ABSTRACT: While the regulation of surface oxygen vacancy (VO) of ceria is an effective methodology to construct solid frustrated Lewis pairs (FLPs), the intrinsic properties and potential applications of solid FLPs are not well-demonstrated. Herein, we present a theoretical study on the formation rules and dynamic behaviors of FLPs on CeO2(110) and their performance on nonoxidative conversion of methane. The density functional theory studies show that the formation of solid FLPs on CeO2(110) depends dependent on the number of oxygen vacancies (VOs). The FLPs constructed by three or more VOs are stable in thermodynamics, whereas the FLPs containing few VOs (i.e., VO monomer and VO dimer) are less stable but can be dynamically formed via thermal fluctuation and reactant-adsorption, as observed by ab initio molecular dynamics simulations. Hence, the general existence of FLPs on reduced CeO2(110) surface under reaction conditions is revealed. Calculations on methane activation show that FLPs are active for C–H bond breaking with an activation energy as low as 0.63 eV, attributed to the enhanced acidity and basicity of FLPs. Due to the structure of FLPs, the subsequent dissociation of a second methane and the C-C coupling of CH3 to form C2H6 is relatively prevailing at FLP sites, different from the popular gas-phase coupling mechanism. Ethane can be further dehydrogenated at FLP sites to form more valuable ethylene. Based on these findings, our study provides insights into the formation and dynamic behaviors of solid FLPs on CeO2 and more importantly predicts a promising strategy for non-oxidative coupling of methane into more valuable chemicals on abundant oxide-based catalysts. KEYWORDS: dynamic frustrated Lewis pairs, CeO2, methane, C–H activation, oxygen vacancy

1. INTRODUCTION Frustrated Lewis pairs (FLPs), constituted by sterically encumbered Lewis acid and Lewis base combinations, have aroused increasing attention since the pioneering work by Stephan and coworkers in the past decade.1-6 The applications of FLPs have also evolved from the primary stoichiometric reactions to catalytic processes, in particular in the domain of homogeneous catalysis. However, the problematic and costly recycling of homogeneous catalyst hinders the large-scale

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application of soluble FLPs,7 and thus developing heterogeneous FLP catalysts becomes more attractive. Currently, the heterogeneous FLPs can be classified into two categories: semi-solid FLPs8-11 and all-solid FLPs.12-21 The former require appropriate solid substrates and matched molecular components and thus are always difficult to control. The latter constructed by both Lewis acid and base on solid surface are more promising to circumvent the problems of homogenous FLPs. To date, all-solid FLPs are designed on several kinds of materials such as doped graphene,12,13 B/Aldoped phosphorenes,14 Lewis pair-functionalized metal-organic frameworks,15-17 and hydroxylated indium oxide.18-22 Though these FLPs have shown activities for hydrogenation reactions, the preparation in experiment is complex and difficult as two or more sorts of materials are needed to fabricate the fine structures. Therefore, a simpler and more effective way is demanding for constructing all-solid FLPs. Recently, we reported a new strategy to construct all-solid FLPs on metal oxide (CeO2) through the regulation of surface oxygen vacancy (VO),23 which is relatively more effective and simpler than previous methods. As shown in Fig. 1 (right), the FLPs on reduced CeO2(110) are constituted by two adjacent surface Ce cations next to VO as Lewis acid and one neighboring O anion as Lewis base.23,24 The distance between the acid site (coordinatively unsaturated Ce) and the base site (surface O) is ~4 Å, fitting well with the concept of FLPs and much longer than that of Ce–O bond (~2 Å) in classical Lewis pairs (CLPs) on stoichiometric CeO2(110) (Fig. 1, left). The ceria-based solid FLPs exhibit superior activity for H2 activation and hydrogenation of alkenes.23,24 In the asdesigned FLP sites on CeO2(110), the oxygen vacancies (VOs) for constructing stable FLPs are all linearly connected with each other, forming an infinite line of VOs (Fig. 1, right). Though such a structure can form more stable FLPs in thermodynamics, it is difficult to be synthesized in experiments.23 Therefore, exploring the possibility of constructing FLPs through small isolated VO clusters (more general in experiment) is desirable for drawing a complete picture of solid FLP sites on ceria.

Figure 1. Schematic of CLPs on stoichiometric CeO2(110) and FLPs on reduced CeO2(110). The Lewis acidic and basic sites in CLPs and FLPs are labeled by magenta and blue circles, respectively. In addition to attain fundamental understanding of solid FLPs on CeO2 surfaces, it is also necessary to expand the application of solid FLPs in new reactions since the present applications are mainly focused on hydrogen activation and hydrogenation reactions.15-26 Thus, another objective of our study is to extend the application of ceria-based solid FLPs to activation and conversion of methane. For one thing, the activation of sp3 C–H bond of methane is regarded as the “Holy Grail”


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in chemical community,27-32 which requires exceptional highly active site to overcome. For another, the on-going discovery of large-scale reserves of natural gas resources and the advancement in shale gas extraction technology point to the promising future of methane as an alternative for petrochemical industry,33 so that it is desirable to develop novel and effective catalysts for the conversion of methane. Though metal-doped ceria catalysts via forming metal-CH4 σ-complex have been extensively studied in methane activation,34-38 using pure ceria for catalyzing methane conversion is limited.39 Activation of methane using metal oxide catalysts, such as MgO,40,41 Al2O3,42-44 Co3O4,45,46 La2O3,47,48 PdO,49,50 and IrO2,51,52 is well-recognized in methane chemistry. The key aspects of these oxides being capable of activating methane are mainly attributed to the naturally existed Lewis acid -base pairs, where methane adsorbs and dissociates into methyl group on acidic metal cation and atomic hydrogen on basic lattice oxygen of oxide. Because of the acid-base strength of the oxide, the activation/dissociation of methane undergoes a heterolytic manner, resulting in a negatively charged methyl group (CH3δ–) on metal cation and a proton on lattice oxygen. As such, the intrinsic acid-base properties of oxide play an important role in the activation of methane. As reported by Chu et al. that the acid-base properties of La2O3 correlate strongly with its methane activity, the pairs consisting of the metal cation with strong Lewis acidity and the oxygen anion with strong Lewis basicity are the most reactive for C–H bond cleavage.53 Fortunately, the solid FLPs (Ce···O) we constructed on reduced CeO2 surfaces possess enhanced Lewis acidity and basicity compared with classical Ce–O Lewis pairs,24 which together with the elongated distance between acid and base sites may contribute to the potential application of FLPs in the activation of methane. Under the premise of methane activation, conversion of methane to high value-added chemicals is more complex, which can be achieved by indirect routes via syngas54 or direct routes via oxidative coupling of methane (OCM),55-57 selective oxidation of methane (SOM),58-62 or non-oxidative conversion of methane.63-66 Although each of these routes has its pros and cons, the direct, nonoxidative conversion of methane is comparatively more promising due to the advantages of high atomic efficiency, low capital costs and less CO2 emission. In 2014, Bao et al. reported a novel direct, non-oxidative pathway for methane transformation by using singly dispersed iron/silica (Fe©SiO2) catalyst,66 which is regarded as a milestone in methane conversion.67 In this process, methane at a conversion of 48.1% was exclusively converted to ethylene, benzene, naphthalene and hydrogen (the selectivity of methane to hydrocarbons exceeding 99%). Nevertheless, the limitation of this process is the high reaction temperature, mainly resulted from the difficult activation of methane and the subsequent desorption of methyl group to yield higher hydrocarbons via C-C coupling in the gas phase. In particular, the desorption of methyl group into gas-phase has an apparent barrier as high as 4 eV.66 Therefore, seeking catalysts not only having remarkable methane activation ability but also outstanding C-C coupling capability is the key to solve methane conversion issues. As indicated above, the activation of methane on solid FLPs of ceria might be


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improved due to the enhanced acid-base strength and geometric effect. Whether they have superb power to transform of methyl groups to C2 and C2+ hydrocarbons is of great interest. Through the analysis of the FLP structure on CeO2(110) in Fig. 1, solid FLPs naturally process neighboring sites for activating two or more methane molecules, which as a result may provide appropriate active sites for C-C coupling on the surface and circumvent the drawbacks of the gas-phase C-C coupling. In this work, we firstly use combined ab initio molecular dynamics (AIMD) simulations and static density functional theory (DFT) methods to get deeper understanding of the rules of creating FLP sites on CeO2(110) surface, then study their stability and dynamic behaviors under reaction conditions, and finally explore the methane activation and the C-C coupling behaviors of solid FLPs under non-oxidative conditions. The aim of this study is to attain fundamental understanding on solid FLPs and to extend the applications of FLPs to direct, non-oxidative conversion of methane.

2. COMPUTATIONAL DETAILS Electronic Structure Methods. All the electronic structure calculations were performed using the Vienna Ab-initio Simulation Packages (VASP).68-70 The projected-augmented wave (PAW) pseudopotentials were utilized to describe the core electrons.71 A plane-wave kinetic energy cutoff of 400 eV was used to treat the valence electrons. The exchange-correlation potential was treated by the Perdew–Burke–Ernzerhof (PBE) version of the generalized gradient approximation (GGA).72 To treat the on-site Coulomb and exchange interaction of the strongly localized Ce 4f electrons, we used the DFT + U method with an effective U = 4.5 eV.73-75 We also considered the van der Waals dispersion forces between adsorbates and surfaces using the zero damping DFT-D3 method of Grimme.76 The Brillouin zone integration was sampled using the Monkhorst-Pack scheme,77 and the meshes used for CeO2 bulk and CeO2(110) surfaces were 7 × 7 × 7 and 1 × 1 × 1, respectively. All the structures were relaxed until the forces on each ion were less than 0.02 eV/Å, and the convergence criteria for the energy was set as 10–5 eV. The nudged elastic band combined with minimum-mode following dimer method was adopted to determine the transition state structures of elementary steps.78,79 All the transition state structures were identified by vibrational frequency analysis. The reaction energy, ΔE, was calculated by energy difference between the product and the corresponding reactant. The activation energy, Ea, was defined as the energy difference between the transition state and the initial state. Due to the existing problem of multiple local minima with respect to the sites where the excess electrons localize, we tried to set the Ce3+ ions at positions with less coordination number to get the most stable electron localization patterns for our calculated slabs.80-82 All the slab structures corresponding to the data in Fig. 4, Fig. 9, Fig. 12 and Fig. 14 are displayed in the supporting information with Ce3+ ions labeled. The projected crystal orbital Hamilton population (COHP) curves were generated with LOBSTER.83-86 We used the pbeVASPFit2015 basis set for the projection of wave functions.86 The


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charge spilling values are always less than 1%, indicating that LOBSTER is able to transfer more than 99% of the charge density (in the occupied levels) from VASP-generated wave functions into the local orbitals. The atomic charges were computed by Mulliken population analysis using LOBSTER.87,88

Ab initio Molecular Dynamics Simulations. To explore the surface configuration changes of CeO2(110), especially the movement of oxygen atom nearest neighboring to VO, the Born– Oppenheimer molecular dynamics (BOMD) simulations were carried out in the canonical (NVT) ensemble using Nosé–Hoover thermostats. The time step adopted for simulations was 0.5 fs with a time period of 10 ps.89,90 To account for the effect of temperature on surface structures, we performed the simulations at both low temperature (300 K) and high temperature (700 K). All the ab initio molecular dynamics simulations were performed with the Quickstep module in CP2K package91 using periodic boundary conditions and P1 space symmetry. The spin-polarized Perdew– Burke–Ernzerhof exchange-correlation functional was used.72 The mixed Gaussian and plane-wave (GPW) basis sets were adopted for electronic structure calculations.92 Molecularly optimized double-ξ valence plus polarization (DZVP) basis sets were used to minimize the basis set superstition errors.93 A 500 Ry energy cutoff was used for the plane-wave calculations of the electrostatic energy terms. The Geodecker–Teter–Hutter (GTH) pseudo-potentials were used to treat the core electrons.94 We adopted the pseudopotential and basis set for Ce atom modified by Wang et al., which can properly account for localized 4f electronic states.95 The DFT + U method, based on the Mulliken 4f state population analysis, was used to describe the Ce 4f electrons with a U value of 7.0 eV, which is tested and used in previous studies.95,96 The gamma point approximation was adopted for Brillouin zone integration. The convergence criteria for maximum force and maximum geometry change was set as 4.5 × 10–4 atomic unit and 3.0 × 10–3 atomic unit, respectively. The VASP and CP2K calculations produced comparable results.

Surface Models. The optimized lattice parameter for bulk CeO2 was 5.44 Å, which is close to the experimental value of 5.41 Å.97 Our previous study showed that the construction of FLPs by regulating surface VO is possible on CeO2 (110) and (100), but is unsuccessful on CeO2(111).23 Moreover, the surface energies indicate the CeO2(110) is more stable than CeO2(100) and the oxygen vacancy formation energies demonstrate that VO on CeO2(110) is easier to be created than that on CeO2(100).98,99 Therefore, only the CeO2(110) surface was selected and modeled using a periodic five-layer slab as shown in Fig. 2. Supercell p(2 × 5) and p(2 × 3) was adopted to study the formation rules and catalytic performance of FLPs, respectively. The bottom three layers of the slab were fixed at bulk positions, and the top two layers with adsorbates were allowed to relax. Only the top-surface VOs are included hereafter as our calculations in Fig. S1 indicate that the sub-surface VOs are less stable, which is different from the CeO2(111) surface.81,82,100


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Figure 2. (a) Side view of stoichiometric CeO2(110) surface with five atomic layers. (b) Top view of CeO2(110) in the supercells of p(2 × 3) and p(2 × 5).

3. RESULTS AND DISCUSSION 3.1. Dependence of VO Numbers on the Formation of FLPs Solid FLPs on CeO2 surfaces, the pairs of Ce cations and O anions around the oxygen vacancy, need to satisfy two requirements: (i) a longer distance (~4 Å) between the Ce cations and O anions compared with the classical Ce–O bond (~2 Å); (ii) The space between the Ce cations and O anions should not be blocked by surface oxygen atoms and can be accessible for molecules. In our previous studies, Vo monomer and VO dimer in a p(2 × 2) surface are created to construct solid FLPs on CeO2(110).23,24 In the case of VO monomer, the O atom neighboring to VO moves “upwards” out of plane to localize at the bridge site of Ce atoms (B sites in Fig. 3a) and fails to construct FLPs due to the blockage of acidic Ce sites. Those Lewis pairs only satisfying the requirement (i) can be termed as blocked FLPs. In the case of VO dimer, the O atoms neighboring to VOs are able to be kept at the original bulk position in plane (P sites in Fig. 3b) and successfully form FLPs. However, such a structure leads to the VO clusters standing in a line (y direction in Fig. 3c) from a periodic view, which is hardly existed or prepared in experiments. Therefore, it is necessary to unravel the formation rule of FLPs on CeO2 by using more universal small VO clusters rather than VO monomer or VO line. To this end, we performed DFT calculations on CeO2(110) with various VO clusters from VO monomer to VO pentamer in a p(2 × 5) supercell (Fig. 2b).

Figure 3. Optimized structures of reduced CeO2(110) in a p(2 × 2) supercell. (a) CeO2(110) with isolated VO monomer. (b) CeO2(110) with connected VO dimer. (c) Top view of the VO dimer forming a line of VOs in a periodic view. As mentioned above, FLP sites should have O atoms neighboring to VOs being stayed at the P sites rather than B sites. Therefore, to figure out which cases are beneficial for the formation of


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FLPs, we calculated the energy differences between the surface with all the O atoms neighboring to VOs staying at B sites (named as B-site configuration) and the surface with all the O atoms neighboring to VOs staying at P sites (named as P-site configuration) in Fig. 4a. Such an energy difference is denoted as ΔEconfig, where ΔEconfig = E(P-site configuration) – E(B-site configuration). If ΔEconfig < 0 eV, it means that the transformation of surface from B-site configuration to P-site configuration is exothermic and the latter is more stable. As shown in Fig. 4a, when the surface has isolated VO monomer and VO dimer, the B-site configuration is more stable with obviously positive ΔEconfig of 0.31 eV and 0.23 eV, respectively. Whereas on surface with VO tetramer and VO pentamer, the Psite configuration is more stable as indicated by the negative ΔEconfig of –0.12 eV and –0.63 eV. The surface with VO trimer is a transition point where the P-site configuration has nearly the same stability with B-site configuration (ΔEconfig = 0.02 eV). Overall, for surface with small VO clusters such as VO monomer and dimer, CLPs are more stable in thermodynamics, and for surface with larger VO clusters (> VO trimer), FLPs are more stable. It suggests that FLPs are preferentially to be formed on surfaces with high density of VOs or with large VO clusters. Though few studies of VO clusters on reduced CeO2(110), experiments on reduced CeO2(111) have shown that oxygen vacancies are inclined to exist in the form of large VO clusters instead of isolated small ones on deeply reduced surface.101 Further theoretical studies found that high vacancy concentration, increased temperature and surface hydroxyls on CeO2(111) can benefit the clustering of VO clusters.102,103 Based on the above analysis, in the following calculations we only consider the CeO2(110) surfaces with VO monomer, VO dimer and VO trimer as the former two small VO cluster models can represent the surfaces with low density of VOs and the VO trimer model can reflect the surfaces with high density of VOs. The optimized structures of CeO2(110) in p(2 × 3) supercell with VO monomer, VO dimer and VO trimer are shown in Fig. 4b, which is termed as CeO2(110)-VO, CeO2(110)-2VO, and CeO2(110)-3VO surface, respectively.

Figure 4. (a) Dependence of VO numbers on the formation of FLPs. The structures corresponding to the energy differences of transformation (ΔEconfig) from P-site configuration to B-site configuration are also displayed. (b) Optimized side views of CeO2(110)-VO, CeO2(110)-2VO, and CeO2(110)-3VO surfaces. Oxygen atoms neighboring to VOs are colored in magenta.


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3.2. Stability and Dynamic Behaviors of Solid FLPs Stability of FLPs on CeO2(110)-3VO. Though static DFT calculations manifest that solid FLPs constructed on CeO2(110)-3VO surface are stable, the impact of reaction conditions, such as the temperature, on the structure of FLPs is unknown. Therefore, AIMD simulations with a time period of 10 ps were performed on CeO2(110)-3VO surface at low (300 K) and high (700 K) temperatures to recognize the stability of FLPs. To firstly get a general understanding of surface structure change, the root-mean-square deviation (RMSD) of atom positions is calculated as shown in Fig. 5a. At low temperature of 300 K, the surface structure change is small in the majority of time (state I), but at 8.6 ps the diffusion of one surface O atom splits the connected VO trimer into one VO monomer and one VO dimer as shown in state II of Fig. 5a. The VO trimer splitting also occurs on CeO2(110)-3VO surface at 700 K but at an earlier time of 0.5 ps, which indicates that the large VO cluster can be easily divided into smaller ones at high temperatures. The splitting or diffusion of oxygen vacancies can only occur on the top-surface of CeO2(110), different from the case on CeO2(111) which involves the migration of sub-surface VO.100,104,105 To quantitatively describe the time periods when solid FLPs exist on surfaces, we keep track of the movements of three O atoms neighboring to the VOs and analyze the coordinates of the O atoms in the direction perpendicular to surface plane (z) as shown in Fig. 5b and c. A reference coordinate, zr = 8.2 Å, is set to separate the time periods: (i) the period of FLPs being blocked, in which the O atoms with z larger than zr are close to B sites, and (ii) the period of keeping stable FLPs, in which oxygen atoms with z smaller than zr approach to P sites. If one or more O atoms neighboring to VOs is below zr, the structure of FLPs is stable and the corresponding time period is termed as ΔtFLP and listed in Table 1. At low temperature of 300 K (Fig. 5b), the FLPs always exist on the surface in the whole simulation time (ΔtFLP = 10 ps), and O atoms rarely move to the B sites (like state β). At high temperature of 700 K, though the VO trimer splitting occurs at the very beginning, in most of time (ΔtFLP = 9.85 ps) at least one O atom neighboring to the newly formed VO clusters stays at the P sites below zr as shown in Fig. 5c, which reflects the stability of FLPs at high temperature. Meanwhile, the calculated results above also indicate that the dynamic transformation between FLPs and CLPs can happen at high temperatures.


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Figure 5. (a) Root-mean-square deviation of atom positions of CeO2(110)-3VO surface in AIMD simulations at 300 K and 700 K. Time evolution of Cartesian coordinates (z) of selected O atoms (colored in magenta, blue and cyan) neighboring to VOs on CeO2(110)-3VO at both (a) 300 K and (b) 700 K. The dash line at z = 8.2 Å is the reference coordinate. The structures in the upper panels in Fig. 5b and c correspond to the states labeled as green dots in the corresponding time evolution curves. The green circles marked within the structures denote the positions of the intact VOs.

Dynamic Formation of FLPs on CeO2(110)-VO and CeO2(110)-2VO Induced by Thermal Fluctuation. As AIMD simulations on CeO2(110)-3VO indicate that FLPs and CLPs are dynamically transformed into each other with increasing temperature, thus it is necessary to reinvestigate the possibility of the formation of FLPs on CeO2(110)-VO and CeO2(110)-2VO surfaces under the impact of temperature. To this end, AIMD simulations with a time period of 10 ps were also performed on CeO2(110)-VO and CeO2(110)-2VO surfaces. Similar to the simulations on CeO2(110)-3VO, the z-coordinates of O atom(s) neighboring to VO(s) (colored in magenta and/or blue in Fig. 6) are tracked to record the appearance of solid FLPs. On CeO2(110)-VO at 300 K, the tracked O atom only fluctuates slightly around the equilibrium B site position with a small standard deviation (σ) of 0.11 Å as shown in Fig. 6a, indicating that FLPs remains difficult to form on CeO2(110)-VO at low temperature. When the temperature increases to 700 K (Fig. 6b), the fluctuation amplitude of objective O atom becomes larger with a σ of 0.20 Å. The probability of the O atom moving from B site to the P site to form FLPs also increases as manifested by the larger ΔtFLP of 1.36 ps at 700 K than 0.45 ps at 300 K. These results suggest that thermal fluctuation at certain temperatures could enable dynamic formation of FLPs on CeO2(110)-VO surface.


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Figure 6. Time evolution of Cartesian coordinates (z) of selected O atom(s) (colored in magenta and/or blue) neighboring to VO(s) on (a, b) CeO2(110)-VO and (c, d) CeO2(110)-2VO surfaces at 300 K and 700 K. The dash line at z = 8.2 Å is the reference coordinate. The structures in the upper panels correspond to the states labeled as green dots in the time evolution curves. The green circles marked within the structures denote the positions of the intact VOs. On CeO2(110)-2VO at 300 K, the simulations start with two O atoms locating at P sites. The initial states last for a period of nearly 2.5 ps, represented by state α in Fig. 6c. Afterwards two O atoms successively move to B sites with blue one at ~2.5 ps and magenta one at ~3.0 ps. From 3.0 ps to 10 ps, the O atom in blue moves back to P site for several times, such as state γ in Fig. 6c, whereas the O atom in magenta always stays at B sites above the zr coordinate. At high temperature of 700 K, two O atoms move quickly to the more stable B sites as shown in Fig. 6d. Both the two O atoms can move back to P sites below zr coordinate, resulting into a total ΔtFLP of 1.69 ps. Compared with the simulation results on CeO2(110)-VO, the probability of dynamic formation of FLPs on CeO2(110)-2VO are higher as indicated by larger ΔtFLP on CeO2(110)-2VO surface. Overall, the AIMD simulations demonstrate that blocked FLPs on CeO2(110)-VO and CeO2(110)-2VO could be dynamically transformed to FLPs by thermal fluctuation, which also implies the universal existence of FLP sites on reduced CeO2(110) surfaces no matter large VO clusters exist or not. Table 1. Time periods of FLPs appearing on CeO2(110) surfaces (ΔtFLP) detected by AIMD simulations surfaces

T/K

Δta / ps

ΔtFLP / ps

CeO2(110)-VO

300

10

0.45

700

10

1.36

300

10

4.28

700

10

1.69

300

10

10

700

10

9.85

CeO2(110)-2VO CeO2(110)-3VO a The

total simulation time (Δt).


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Dynamic Formation of FLPs on CeO2(110)-VO and CeO2(110)-2VO Induced by Methane Adsorption. In addition to temperature, the reactive species in the atmosphere may also affect the active site by collision or adsorption. To extend the use of FLPs in methane conversion, the adsorption of methane and its impacts on surface configurations are explored on CeO2(110)-VO and CeO2(110)-2VO surfaces as shown in Fig. 7. On CeO2(110)-VO surface, the O atom (colored in magenta) remains staying at the original B site during methane approaching to the surface (Fig. 7a). However, on CeO2(110)-2VO surface methane pushes one O atom (colored in magenta) deviating from the original B site to eliminate the blocking of FLPs during its adsorption (Fig. 7b). The different behaviors of the oxygen atoms on the two surfaces upon methane adsorption originate from the energy changes in oxygen atoms moving from B sites to P sites. Moving one oxygen atom from B site to P site on CeO2(110)-VO surface needs to uptake an energy of 0.37 eV (Fig. 7a), larger than the absolute value of adsorption energy (0.21 eV) of methane on CeO2(110)-VO surface. However, the same process on CeO2(110)-2VO only needs to take in an energy of 0.08 eV (Fig. 7b), which can be compensated by the released energy of methane adsorption (–0.23 eV). Therefore, on CeO2(110)-2VO surface, FLPs are formed upon methane adsorption and then FLPs disappear with methane desorption, indicating the formation of FLPs is dynamic. Moreover, the AIMD simulations of CeO2(110)-2VO surface with five methane molecules at 700 K further demonstrate methane molecules can benefit the formation of FLPs as shown in Fig. S3. In a word, except for the temperature-induced dynamic formation of FLPs, the reactant-adsorption may also lead to the dynamic formation of FLPs.

Figure 7. Impact of methane adsorption on the surface structures of (a) CeO2(110)-VO and (b) CeO2(110)-2VO. The data in eV represents the energy difference for oxygen atom moving to P sites (left) and methane being adsorbed on the surface (right). The O atoms neighboring to VOs are colored in magenta.

3.3. Activation of Methane at Solid FLP Sites The activation and transformation of methane are explored at the Lewis pair sites of four CeO2 surfaces as mentioned above, i.e., CeO2(110), CeO2(110)-VO, CeO2(110)-2VO, and CeO2(110)-3VO surfaces. The Lewis pair sites (the active center) of the four surfaces are denoted as CLP, FLP-VO, FLP-2VO, and FLP-3VO, respectively, as shown in Fig. 8. Although the FLP sites on CeO2(110)VO and CeO2(110)-2VO surfaces are blocked by oxygen atoms, the FLP sites can be dynamically formed under reaction conditions and thus are still denoted as FLP-VO and FLP-2VO.


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Figure 8. Optimized top views of CeO2(110) surfaces with different VOs. (a) Classical Lewis pairs on stoichiometric CeO2(110), CLP sites. (b) Blocked frustrated Lewis pairs on CeO2(110)-VO, FLPVO sites. (c) Blocked frustrated Lewis pairs on CeO2(110)-2VO, FLP-2VO sites. (d) Frustrated Lewis pairs on CeO2(110)-3VO, FLP-3VO sites. The Ce atoms with magenta circle are the acidic sites of Lewis pairs, and the O atoms with blue circle are the basic sites of Lewis pairs.

Methane Dissociation at Solid FLP Sites. On the four Lewis pairs above (including FLPs and CLPs), the activation of methane via cleaving the first C–H bond is probed. As shown in Fig. 9a, methane weakly adsorbs at all the four Lewis pairs with an adsorption energy of –0.21 eV at CLP and FLP-VO sites and –0.23 eV at FLP-2VO sites, and binds more strongly at FLP-3VO sites (–0.32 eV). Though slight differences in adsorption energy, the variations of the distance between methane and CeO2(110) surfaces are significant, which decrease from 3.47 Å at CLP sites to 2.59 Å at FLP-3VO sites as shown in Fig. 9 and Table 2. The enhanced accessibility of methane to the Lewis pairs induced by enlarged space between FLP-acid site and FLP-base site is beneficial for C– H activation, similar to the previous study of hydrogen activation.24 Though the variation of adsorption energy of methane on the four models is no more than 0.11 eV, the difference of energy relevant to C–H cleavage is significant. At the CLP sites on stoichiometric CeO2(110), the methane dissociation has a reaction energy of 0.72 eV and needs to overcome an activation energy of 1.08 eV, obviously lower than the energy barrier (1.44 eV) of abstracting the first hydrogen atom in methane by surface oxygen atom on CeO2(111).39 At the blocked FLP-VO sites, the reaction energy raises to 1.01 eV and the activation energy slightly increases by 0.06 eV. Compared with CLP sites, the blocked FLP-VO sites are less active, which can be attributed to the endothermic process of moving the oxygen atom from B site to P site during methane dissociation. Meanwhile, it is interesting to note that although the FLP-VO is blocked before the reaction, the FLP behavior is presented during CH4 dissociation (see state TS1-2 in Fig. 9c), which can be regarded as a case of reaction-induced dynamic formation of FLPs. At the FLP2VO sites, methane dissociation has a lower activation energy of 0.74 eV and a decreased reaction heat of 0.47 eV compared with CLP and FLP-VO sites. At the FLP-3VO sites, the activation energy and reaction energy further reduce to 0.63 eV and 0.27 eV, respectively. Overall, for the methane activation/dissociation on CeO2(110), the FLP-2VO sites and FLP-3VO sites are more active in both thermodynamics and kinetics than CLP sites and FLP-VO sites. In addition, though even less active for C–H bond cleavage than the CLP sites, blocked FLP-VO sites shares the similar transition state


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structure with both FLP-2VO and FLP-3VO (see states TS1-2, TS1-3 and TS1-4 in Fig. 9), indicating dynamic formation of solid FLPs during methane dissociation.

Figure 9. Methane dissociation on CeO2(110) surfaces. (a) Potential energy diagram of the reactions occurring on CLP sites (black curve) and FLP sites (red, blue and green curves). The zero energy reference corresponds to the sum of energies of CH4(g), and the corresponding clean CeO2(110) surfaces displayed in Fig. 8. (b-e) Geometric structures of the transition states and intermediates of reactions in Fig. 9a. To understand the original chemistry of solid FLPs for methane activation, we first analyzed the geometric structures and atomic charges of initial states (IS) and transition states (TS) of methane dissociation as shown in Table 2. As mentioned above, the distance between methane and surface in methane adsorption, i.e., d(C–surface) of IS in Table 2, at CLP sites is longer than that at FLP sites and decreases along with the increasing numbers of VO. Then in the transition states, at CLPs the methyl group is 2.43 Å above the surface during its approaching to the top site of Ce atom. At the three FLP sites, methyl group is attracted by two Ce atoms, resulting in a shorter distance to the surface (less than 2.00 Å). Interestingly, the changes of d(C–surface) from IS to TS decrease along with increasing numbers of VO, i.e., 1.36 Å for FLP-VO, 1.02 Å for FLP-2VO, and 0.69 Å for FLP3VO, implying that the TS at FLP-3VO sites is relatively closer to the early transient state. For the bond lengths of C–Ha and O–Ha (Table 2), the changes from IS to TS are also getting smaller in the order of CLPs, FLP-VO, FLP-2VO and FLP-3VO. Moreover, the ∠HaCHb and atomic charges in Table 2 also share the same trends of smaller variations from IS to TS along with increasing numbers of VO. Overall, the above results support that transition state in methane dissociation at FLP sites can be regarded as early transition state, especially for the FLP-3VO sites. Our previous study has reported that the acidity of two Ce3+ in FLPs is stronger than that of one Ce4+ in CLPs, and the basicity of O atom is enhanced in FLPs, which cooperatively enable the early transition state in hydrogen dissociation at solid FLP sites.24 Similarly, the early transition state and lower activation energy of C–H bond breaking at FLPs can be also attributed to the enhanced acidity of FLP-acid site and the stronger basicity of FLP-base sites, which cooperatively attract methane closer to surface and reduce the distance between O atom and Ha atom.


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Table 2. Structural Parameters and Atomic Charges (Q) of Initial States (IS) and Transition States (TS) in CH4* → CH3*(Ce) + H*(O) at Lewis Pair Sites on CeO2(110) surfaces sites

states

CLP-110 FLP-VO FLP-2VO FLP-3VO a

distancea / Å

angleb / deg

Qc / e

d(C–surface)

d(C–Ha)

d(Ce–C)

d(O–Ha)

∠HaCHb

∠CeCHa

∠CHaO

CH3

Ha

IS

3.47

1.10

3.67

2.52

111.5

51.8

158.4

–0.25

0.19

TS

2.43

1.60

2.69

1.11

131.0

61.4

168.9

–0.52

0.31

IS

3.13

1.10

4.26, 4.47

2.46

108.3

74.6, 76.9

167.8

–0.26

0.18

TS

1.77

1.52

3.13, 3.19

1.17

141.4

94.0, 91.9

168.8

–0.64

0.28

IS

2.85

1.10

4.10, 3.85

2.29

110.7

69.8, 69.0

175.2

–0.27

0.17

TS

1.83

1.45

3.17, 3.05

1.23

139.2

89.3, 85,8

166.9

–0.59

0.25

IS

2.59

1.11

3.69, 3.65

2.16

113.1

70.6, 70.0

168.9

–0.30

0.18

TS

1.90

1.44

3.10, 3.16

1.24

139.4

86.2, 87.0

166.1

–0.60

0.25

The d(C–surface) represents the distance between C atom of methane and CeO2(110) surfaces. Ha represents the

dissociated hydrogen atom from methane (labeled in Fig. 9). b Hb represents the hydrogen atom in methyl group (labeled in Fig. 9) nearest to the surface. c CH3 represents the methyl group in methane except the Ha atom.

Significant changes of d(C–surface) and d(O–Ha) in methane adsorption (IS in Table 2) from CLPs to FLPs are also observed, and the Mulliken charges of methane in IS manifest that the charge transfer from surface to methane increases along with increasing numbers of VO, which both suggest that methane can be more effectively activated at FLP sites than CLP sites. However, the bond lengths of C–Ha in adsorption states are nearly unchanged from CLP sites to FLP sites. To figure out the changes of C–Ha bond in methane adsorption at Lewis pairs and understand the interactions between C–Ha bond and surface O atom in Lewis pairs, an analysis of the crystal orbital Hamilton population was carried out.83-86 Figure 10 shows the negative projected COHP (pCOHP) curves for the C–H bond of methane adsorbed on the four Lewis pairs and the ones in the vacuum. The positive –pCOHP can be viewed as a bonding interaction, and the negative –pCOHP is regarded as an antibonding interaction. Firstly, the C–H bond of methane relaxed in vacuum has a bonding character in the energy ranging below the Fermi level and possesses an antibonding character above the Fermi level, which results in a stable C–H bond of methane. Then the C–H bond of methane adsorbed on CeO2(110) surface generates almost the same –pCOHP curves with the ones in vacuum. However, the enlarged local –pCOHP curves inserted in each panel reflect that a slightly negative peak under the Fermi level only exists in C–H bond of the adsorbed methane, i.e., from –2 to 0 eV at CLP sites, and from –3 to –1 eV at three FLP sites. As being the occupied C–H antibonding states, these peaks indicate the primary activation of C–H bond.


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Figure 10. Negative pCOHP curves of C–H bond of methane and O···H pairs in methane adsorption at (a) CLP, (b) FLP-VO (c) FLP-2VO and (d) FLP-3VO sites. Blue curves indicate the pCOHP of the C–Ha bond of methane optimized in the middle of the vacuum region, where the interactions between methane and the surfaces are negligible. Red curves and green curves refer to the pCOHP of the C–Ha bond and O···Ha pairs in methane adsorption. The Ha atom (labeled in Fig. 9) is the closest one to the surface. The enlarged local pCOHP of C–Ha bond inserted in each panel corresponds to the regions labeled by magenta dash circles. The Fermi levels of the CeO2 slabs are set to zero in this figure. In the pCOHP profile for the methane in vacuum, one can find that the peak for the lowest C–H antibonding orbital (σ*C–H) appears at around E = 3.6 eV for CLP sites, E = 3.0 eV for FLP-VO sites, E = 2.8 eV for FLP-2VO sites and E = 2.2 eV for FLP-3VO sites. Also, the peaks for the highest C–H bonding orbital (σC–H) can be seen at around E = –5.4 eV for CLPs, E = –6.2 eV for FLP-VO, E = –6.2 eV for FLP-2VO and E = –6.7 eV for FLP-3VO. Since the C–H antibonding orbital is closer to the Fermi level than the bonding one on each surface, the electron transfer between methane and CeO2(110) surface can be expected mainly from the high-lying valence bands to the σ*C–H orbitals.106 In the pCOHP profile for adsorbed methane, the peaks for the highest bonding orbital of O···H pairs localize at the same position with the ones for the σ*C–H below Fermi level, reflecting the electron transfer from surface basic sites (O atoms) to the C–H antibonding orbitals. As previous studies reported that the FLP sites have stronger basicity than CLP sites do24 and a surface with oxygen vacancy is a Lewis base,107 it is reasonable that the FLP sites with more VOs can denote more electrons to methane and active the C–H bond more effectively.


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To further unravel the interactions between methane and FLP sites in the transition states, the electronic structure analysis, including electron density difference and electron localized function (ELP), is also conducted. In Fig. 11a, the electron density difference maps show that electrons accumulate around methyl group, whereas electrons are depleted around Ha atom, in agreement with the trends of atomic charges listed in Table 2. Therefore, methane is dissociated in a heterolytic manner, resulting in a hydrogen atom with more positive charges (Hδ+) and a methyl group with more negative charges (CH3δ–). Moreover, the ELF maps in Fig. 11b show the electrons around CH3 are highly localized. For the regions between methyl group and Ce ion, both the gap of density difference equal to zero (Fig. 11a) and the highly delocalized electrons as demonstrated by ELF (Fig. 11b) suggest that the main interactions between Ce ions and CH4 in TS are Coulombic attraction. Between Ha atom and O atom, there is a significant increase of electron density and the ELF in these regions are around 0.5, indicating the covalent bonding between O atoms and CH4 in TS. Altogether, the electron transfer between CH4 and Lewis basic sites (O atoms) and the local electric field of Lewis pairs both contribute to the activity for methane dissociation at both CLPs and FLPs.

Figure 11. (a) Electron density difference maps and (b) electron localization function maps of the transition states of methane dissociation at CLP, FLP-VO, FLP-2VO, and FLP-3VO sites on CeO2(110) surfaces. The unit for electron density is e bohr–3.

Further Transformation. After methane dissociation, the hydrogen atom (Ha) and methyl group from methane are anchored at surface O site and Ce site, respectively. Then three possible transformation pathways of methyl group including desorption, oxidation, and dehydrogenation may occur. Firstly, the desorption of methyl group from surface is explored, which is an important pathway at high temperatures. As shown in Fig. 12a, the desorption energy of methyl group at CLP sites is only 0.47 eV, mainly owing to the weak bonding between methyl group and one Ce4+ ion. At FLP-VO sites, the desorption energy of methyl group is slightly increased to 0.55 eV and the surface oxygen moves closer to the Ha atom spontaneously after methyl group leaves the surface (see state D2 in Fig. 12c). However, at FLP-2VO and FLP-3VO sites the desorption energies of methyl group further increase to 1.42 eV and 1.81 eV, respectively, indicating the unfavorable


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desorption at FLP sites. Secondly, the adsorbed methyl group may transfer from Ce ions to the nearby O atoms (from state C to state E in Fig. 12), which is termed as oxidation pathway hereafter. The oxidation of methyl group at four sites are all exothermic, and the reaction energy decreases along with the increasing numbers of VO, from –1.78 eV at CLPs to –0.32 eV at FLP-3VO. The activation energy increases from 0.43 eV at CLPs to 1.56 eV at FLP-3VO. Therefore, the more oxygen vacancies the surface has, the more difficult the oxidation will be. Thirdly, the dehydrogenation of CH3* to form CH2* (from state C to state F in Fig. 12) is also a possible pathway. The dehydrogenation reaction produces one CH2* still being adsorbed at Ce sites and one H* binding with another nearby O atom. The reactions at four sites are all unfavorable in thermodynamics with reaction energy ranging from 0.52 eV at FLP-VO to 1.01 eV at FLP-3VO. The activation energy increases along with increasing numbers of VO, from 1.07 eV at CLPs to 1.96 eV at FLP-3VO. Again, the dehydrogenation of adsorbed methyl group becomes more unfavorable with the increasing numbers of oxygen vacancy.

Figure 12. Further transformation of methyl group on CeO2(110) surfaces. (a) Potential energy diagram of the reactions occurring on CLPs (black curve) and FLPs (red, blue and green curves). The zero energy reference corresponds to the sum of energies of CH4(g), and the corresponding clean CeO2(110) surfaces. (b-e) Geometric structures of the transition states and intermediates of reactions in Fig. 12a. As previous studies found that the activation barrier of methane dissociation strongly depends on the formation energy of oxygen vacancy,36,108 we herein further analyzed the relation between the activation barriers of methane dissociation and formation energies of the next oxygen vacancy in CLP, FLP-VO, FLP-2VO and FLP-3VO sites on CeO2(110) surfaces. As shown in Fig. 13, heterolytic dissociation (CH4* → CH3*(Ce) + H*(O), from state B to state C) and homolytic dissociation (CH4*


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→ CH3*(O) + H*(O), from state B to state E) of methane on CeO2(110) possess good linear correlations between activation barriers and the oxygen vacancy formation energies, but the trends are reversed, which can be attributed to the different electron transfers between the surface and methane during reactions. Except for finding a proper descriptor for predicting reaction barrier, the reversed trends in heterolytic and homolytic methane dissociation also show the exceptional catalysis of solid FLPs for methane activation. Details on this part can be found in the supporting information.

Figure 13. Correlations between activation barriers (Ea) of (a) heterolytic and (b) homolytic C–H bond cleavage and the oxygen vacancy formation energies (Δ𝐸VO) in the Lewis pairs. The data in Fig. 13 can be found in Table S1 of the supporting information. Overall, our results show that the FLP-3VO sites are the most active for methane dissociation and also the most stable for stabilizing methyl group as the three pathways of methyl group transformation are most difficult to occur at FLP-3VO sites. The FLP-2VO sites rank second after the FLP-3VO for methane activation and show large differences with FLP-VO sites. The FLP-VO sites as a result of the blockage of Lewis acid sites perform similar reactivity to the CLP sites in methane activation and further transformation. Based on the calculations above, it is interesting to find that the FLP-2VO and FLP-3VO sites are promising for methane activation as the cleavage barrier of C–H bond of methane is as low as 0.74 eV and 0.63 eV, respectively. In particular, the increasingly unfavorable transformation of methyl group on FLP-2VO and FLP-3VO sites paves a potential avenue for the direct coupling of methyl group to produce C2 products.

3.4. Non-oxidative Coupling of Methane to Ethane and Ethylene As indicated above, further transformation of individual methyl group at FLP-2VO and FLP-3VO sites is largely impeded due to the high barriers, which creates the possibility for on-surface conversion of methyl groups directly to C2 products. Such an assumption is based on the two prerequisites: (i) FLP-2VO and FLP-3VO sites have neighboring FLP sites for attracting another methane molecule, and (ii) the activation/dissociation of one more methane is expected to be easier than further transformation of methyl group. Therefore, the direct coupling of two methyl groups to


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produce C2 hydrocarbons is explored in this section. As shown in Fig. 14, the adsorption of a second methane is slightly affected by the product of the first methane dissociation with adsorption energies of –0.25 eV at FLP-2VO and –0.33 eV at FLP3VO. The dissociation of the second methane becomes a little difficult with an endothermic reaction heat of 0.67 eV and an activation energy of 0.90 eV at FLP-2VO sites. Similarly, the dissociation step at FLP-3VO sites has an endothermic heat of 0.69 eV and an activation energy of 0.73 eV, higher than the corresponding energies of the first methane dissociation. Nevertheless, the activation energies of the second methane dissociation are still lower than those of further transformations of methyl group. In the subsequent C-C coupling of two methyl groups to produce ethane at FLP-3VO sites, the reaction experiences a downhill energy release of 2.09 eV and a reaction barrier of 1.12 eV, which is only 0.06 eV higher than that of oxidation of the methyl group. At FLP-3VO sites, the reaction is also highly exothermic by 1.66 eV and has an activation barrier of 1.16 eV, which is much lower than that of the oxidation of the methyl group (1.56 eV). The desorption of ethane is facile at both FLP-2VO and FLP-3VO sites with desorption energies of 0.37 eV and 0.33 eV, respectively. Therefore, the coupling of two methyl groups to form ethane at two FLPs on CeO2(110) surfaces is plausible because the C-C coupling are more exothermic and reaction barriers are similar at FLP-2VO sites and much lower at FLP-3VO sites compared with the dominant pathway of methyl group transformation. It is worth noting that our proposed C-C coupling at FLPs is different from the mechanism occurred between two methyl radicals in the gas phase,109,110 which can be attributed to the different properties of metal oxides and diverse ability to stabilize the methyl group on surface.

Figure 14. Non-oxidative coupling of methane to ethane on CeO2(110) surfaces. (a) Potential energy diagram of the reactions at FLP-2VO sites (blue curve) and FLP-3VO sites (green curve). The zero energy reference corresponds for the sum of energies of CH4(g), and the corresponding clean CeO2(110) surfaces. (b-c) Geometric structures of transition states in dissociation of the second


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methane, formation of ethane and hydrogen. The structures of other transition states and intermediates can be seen in Fig. 9 and Figs. S9-S12. After the desorption of ethane, two hydrogen atoms bound to surface oxygen atoms need to be removed to close the catalytic cycle. Herein, a four-step mechanism of hydrogen formation is proposed in Fig. 14, including: (1) transformation of proton to hydride, from state J to state K; (2) hydride transfer, from state K to state L; (3) hydrogen formation from proton and hydride, from state L to state M; (4) desorption of hydrogen, from state M to state N. Our calculated results in Fig. 14 show that the transformation of proton to hydride is endothermic with the reaction energies of 1.14 eV at FLP-2VO and 1.19 eV at FLP-3VO. Moreover, the activation energies are very high with values of 2.24 eV at FLP-2VO and 2.21 eV at FLP-3VO, corresponding to the transition states of TS6-3 in Fig. 14b and TS6-4 in Fig. 14c. In the second step of hydride transfer, the hydride moves to the Lewis acidic sites of the same FLPs where the proton binds with the FLP-base site (O atom). The reactions are almost neutral (0.01 eV) at FLP-2VO and exothermic (–0.39 eV) at FLP-3VO. The activation energy is 1.02 eV at FLP-2VO and decreases to 0.65 eV at FLP-3VO. In the third step of hydrogen formation from proton and hydride, the reaction has an endothermic reaction energy of – 0.58 eV and an activation energy of 0.59 eV at FLP-2VO. In the desorption of hydrogen, the desorption energy at FLP-2VO sites is only 0.14 eV. At FLP-3VO sites, hydride and proton directly combine to generate gas-phase hydrogen with a desorption energy of 1.01 eV as the hydrogen is unable to be adsorbed as molecular state at FLP-3VO sites. Throughout all the steps in hydrogen formation (from state J to state N in Fig. 14), the rate-determining step is the transformation of proton to hydride with high activation energy around 2.20 eV, which is mainly owing to the breaking of the strong O–H bond. Therefore, high temperature is needed to remove residual hydrogen and regenerate the FLP sites, similar to the OCM and methane dehydroaromatization (MDA) catalyzed by oxides being always operated at high temperatures.30,55 The calculated rate constant of hydrogen transformation of proton to hydride is as high as 104 s–1 at 1200 K (Table S2), which indicates the feasibility of this route at high temperatures. In addition, the activation energy of rate-determining step at FLP sites is still much lower than that at CLP sites (3.26 eV) on CeO2(110) reported from previous study,24 suggesting the advantages of FLPs in the conversion of methane. Importantly, such transformation of methane to ethane is performed under non-oxidative conditions, which could effectively avoid the overoxidation of methane to CO2 that OCM and SOM suffer. As ethylene is more valuable than ethane in industry, we thus further studied the dehydrogenation of ethane to ethylene at FLP-2VO and FLP-3VO as shown in Fig. S13. The calculated results show the rate-determining step is the second C–H bond cleavage with a reaction barrier of 1.29 eV at FLP-2VO and 1.46 eV at FLP-3VO, a little higher than that of C-C coupling of two adsorbed methyl groups. However, as dehydrogenation of ethane has circumvented the difficult step of the


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transformation of proton to hydride in hydrogen formation, the regeneration of the FLP sites is easier than that in transformation of methane to ethane.

4. CONCLUSIONS In this study, we investigated the formation rules and dynamic behaviors of solid FLPs on CeO2(110) surfaces using combined static DFT and AIMD calculations. Then, the activation and non-oxidative coupling of methane to ethane and ethylene were explored at solid FLPs for the first time. The main conclusions are drawn as follows. (1) The formation of stable FLPs on CeO2(110) is dependent on the number of oxygen vacancies. The FLPs constructed by three or more oxygen vacancies are naturally stable in thermodynamics. The FLPs containing less oxygen vacancies (VO monomer and dimer) are less stable under vacuum, but they can be dynamically formed at reaction conditions such as thermal fluctuation at high temperatures and reactant-adsorption. (2) The dissociative activation of methane can be readily achieved at solid FLPs with the activation energy being as low as 0.74 eV on FLP-2VO and 0.63 eV on FLP-3VO. The superb activity of FLPs for methane activation is attributed to the stronger interaction with methane via the local electric field of Lewis pairs and the increased electron transfer from FLP-base sites to the antibonding orbital of C–H bond of methane. (3) The non-oxidative coupling of methane into ethane and ethylene is revealed to be feasible at CeO2(110)-based FLPs. The critical C-C coupling of methyl groups at FLP sites has a reaction barrier of ~1.1 eV, much lower than that of previously reported methyl group desorption and gasphase C-C coupling. Although the formation of hydrogen to regenerate FLPs seems to be difficult, it can be overcome at high temperatures. On the whole, our results provide insights into the formation of solid FLPs on CeO2 surfaces and methane activation at FLP sites. More importantly, this study uncovers a possible strategy for non-oxidative coupling of methane into valuable hydrocarbons on FLPs-contained oxide catalyst and may provide guidance for experimental design of FLP catalysts.

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ASSOCIATED CONTENT

Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Supplementary surface structures involved in the main text, correlation between the activation barrier of methane dissociation and the formation energy of oxygen vacancy, and dehydrogenation of ethane to ethylene at solid FLPs (PDF)



AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] ORCID


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Zheng-Qing Huang: 0000-0003-3766-5068 Chun-Ran Chang: 0000-0003-1084-8307 Jun Li: 0000-0002-8456-3980 Notes The authors declare no competing financial interest.



ACKNOWLEDGMENTS

We thank Prof. Yongquan Qu and Dr. Sai Zhang for helpful discussion. This work is supported by the National Natural Science Foundation of China (21603170, 91645203 and 21590792), the China Postdoctoral Science Foundation (2018T111034) and the Fundamental Research Funds for the Central Universities (xtr0218016 and cxtd2017004). C. R. C. also acknowledges the support from the K. C. Wong Education Foundation, the Young Talent fund of University Association for Science and Technology in Shaanxi, and the Open Project of Shaanxi Key Laboratory of Catalysis. The calculations were performed by using the HPC Platform at Xi’an Jiaotong University and National Supercomputing Center in Guangzhou.

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ACS Catalysis

TOC Graphic


Dynamic Frustrated Lewis Pairs on Ceria for Direct Non-oxidative

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