Origin of Efficient Catalytic Combustion of Methane over Co3O4(110

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Origin to Efficient Catalytic Combustion of Methane over CoO(110): Active Low-Coordination Lattice Oxygen and Cooperation of Multi Active Sites Wende Hu, Jinggang Lan, Yun Guo, Xiao-Ming Cao, and Peijun Hu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01080 • Publication Date (Web): 12 Jul 2016 Downloaded from http://pubs.acs.org on July 16, 2016

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Origin to Efficient Catalytic Combustion of Methane over Co3O4(110): Active Low-Coordination Lattice Oxygen and Cooperation of Multi Active Sites Wende Hu†, Jinggang Lan†, Yun Guo†, Xiao-Ming Cao*,†, and P. Hu*,†‡

† Key Laboratory for Advanced Materials, Center for Computational Chemistry and Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China ‡ School of Chemistry and Chemical Engineering, The Queen’s University of Belfast, Belfast, BT9 5AG, U. K.

E-mail: [email protected] and [email protected]

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ABSTRACT A complete catalytic cycle for methane combustion on the Co3O4(110) surface was investigated and compared with that on the Co3O4(100) surface based on the first principle calculations. It is found that the two-fold coordinated lattice oxygen (O2c) would be of vital importance for methane combustion over Co3O4 surfaces, especially for the first two C-H bonds activation and the C-O bond coupling. It could explain the reason why the Co3O4(110) surface significantly outperforms the Co3O4(100) surface without exposed O2c for methane combustion. More importantly, it is found that the cooperation of homogeneous multi-sites for multi elementary steps would be indispensable. It not only facilities the hydrogen transfer between different sites for the swift formation of H2O to effectively avoid the passivation of the active low-coordinated O2c site but also stabilize surface intermediates during the methane oxidation, optimizing the reaction channel. The understanding of this cooperation of multi active sites might not only be beneficial to develop the improved catalysts for methane combustion but also shed light on one advantage of heterogeneous catalysts with multi-sites compared to single-site catalysts for the catalytic activity.

KEYWORDS Spinel Cobalt Oxides; DFT; Methane Combustion; C-H Bond Activation; Multi Active Sites

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1. INTRODUCTION

The catalytic combustion of methane (CH4)1-3 which is the major constituent of natural gas is an alternative technology to the conventional thermal combustion, which can not only promote the energy transforming efficiency but also effectively lower the combustion temperature to reduce the discharge of noxious NOX, CO and hydrocarbons. This process is of great significance for the energy production such as heater and power generation and for the abatement of the direct emission of methane with the potent greenhouse effect.4 In recent years, the lower level of methane emission is required to meet the demands of much stricter environmental standards such as Euro-VI standard for natural gas vehicles. As a result, it becomes increasingly important to design the catalysts of methane combustion for the higher energy transforming efficiency and lower emissions of noxious and greenhouse gas from the exhaust.

In spite of extensive studies, a catalyst with an excellent ignition activity and high thermal stability is still desired for the methane combustion. Noble metals especially supported palladium catalysts are found to have the best activities for CH4 combustion and the formed palladium oxides during the reaction processes are generally regarded as the main active phase.3 Nevertheless the lower activities were frequently achieved below 400 °C and the decomposition of palladium oxides at the temperature above about 650 °C could make the catalysts deactivation.5-11 Recently, Cargnello et al. reported that the Pd@CeO2 nanoparticles homogeneously dispersed on a modified hydrophobic alumina could catalyze the complete oxidation of methane below 400 °C and remain the thermal

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stability up to 850 °C.6 Despite the great breakthrough, it is worth noting that the high price of noble metal and its scarcity may restrain its wide application in the long term.

In the recent years, a few cobalt-based spinel oxides as a kind of earth-abundant materials which are possible alternatives to expensive noble metal exhibit high activities for the catalytic combustion of methane.12-14 This is possibly attributed to the structure and electronic properties of spinel-type cobalt oxides with variable valence states of transition metal cations in either tetrahedron or octahedron interstices, and multifold surface lattice oxygen anions with diversified Lewis basicity as well as the lower bonding energy of Co-O bond.15 These properties are frequently regarded as the main reasons why Co3O4 nanomaterials are well applied in CO oxidation, NOX reduction, Li batteries, gas sensor, and anode material for electrochemical water splitting.16-22 In particular, with the development of facet- and morphology-controlled synthesis technique, the facet-controlled spinel Co3O4 nanomaterial with the exposed surface of the specific coordination combinations between surface lattice oxygen anions and metal cations could be prepared, leading to the more versatile redox properties compared to the conventional materials. Co3O4 nanorods predominantly exposing their (110) facets were firstly reported to exhibit a much higher activity for CO oxidation than that of conventional nanoparticles mainly exposing (001) and (111) facets.12, 16-17 It was also found that mesoporous Co3O4 and Au/Co3O4 catalysts partly exposing (110) surfaces show a good catalytic activity in the oxidation of ethylene.23 Hu et al.12 showed that the Co3O4 nanomaterials exposing the (110) plane could effectively catalyze the complete oxidation of methane below 400 °C, which are much more active than those only exposing the (100) or (111) surface. In addition, the respective

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results from Gao’s13 and Tao’s14 research groups suggested that (110) surface of cobalt-based spinel oxides is likely to be the highly active surface for the C-H activation of methane.

Compared to the intensive understanding about the palladium-catalyzing methane combustion based on the extensive mechanistic studies,24-28 the mechanistic studies are still limited on the complete catalytic cycle of the methane combustion on Co3O4. The theoretical investigations of Co3O4 materials mainly concentrated on its bulk and surface structures29-31 as well as magnetic and electronic properties32-33. Recently, these studies inspired the mechanistic studies of CO oxidation34-37, oxygen evolution reaction37-38 on the surfaces of spinel-type cobalt oxides to understand the structure-activity relationships. Especially for CO oxidation which is generally regarded as a model reaction, its mechanistic studies have provided some fundamental insights into the geometric and electronic properties of Co3O4 surfaces as well as their catalytic performances for the oxidation processes.39-43

As compared to CO oxidation process, the complete combustion of methane is a far more complex process in which CO oxidation is only one of probably steps. The complete oxidation of CH4 must go through at least four C-H bond breaking steps and two C-O bond coupling steps to form CO2. The first C-H bond activation of CH4 is generally regarded as a difficult step due to the high C-H bonding energy of 435 kJ/mol and the high symmetry of CH4, which is frequently accepted as the key factor governing the activity44 of methane combustion. However, it is worth noting that the produced hydrogen after the C-H bond breaking could hydroxylate Co3O4 surface. Wang et al.36 and Xu et al.37 investigated the effect of hydroxylated surface due to adsorbed surface water, respectively.

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They both found that the energy barrier of C-O bond coupling between adsorbed CO and formed lattice hydroxyl is significantly higher than that between CO and lattice oxygen over Co3O4 surfaces. This is responsible for the negative effect of water on the catalytic activities of CO oxidation over spinel cobalt oxides, which was observed in the experimental work.45-48 Analogically, the hydroxylation of Co3O4 surface is also likely to aggravate the difficulty of the subsequent C-O bonding coupling for methane combustion over Co3O4 surfaces, possibly making it even more formidable than the first C-H bond activation. The removal of these produced surface hydrogen may recover the active lattice oxygen to avoid the drastic C-O coupling process but it will involve the hydrogen transfer and the desorption process of the surface containing-hydrogen species as well. Accordingly, aiming to understand the catalytic performance of Co3O4 on methane combustion, it is essential to investigate the whole catalytic cycle of methane combustion including not only the complex reaction channels of the methane oxidation over Co3O4 surfaces but also the regeneration of active lattice oxygen. Yet the detailed theoretical investigations on the complete catalytic cycle of methane combustion over Co3O4 surfaces are still scarce. Hence, in this paper we are dedicated to utilizing density functional theory calculations to explore the complete catalytic cycle of CH4 combustion on the active Co3O4(110) surface. It is expected that our results can provide the overall landscape of the whole process of methane combustion on spinel-type cobalt oxides surfaces at the molecule level and be contributed to the comprehensive understanding of the key factors governing the activity of spinel-type cobalt oxides catalysts.

2. COMPUTATIONAL DETAILS

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All the spin polarized calculations were performed using Perdew−Burke−Ernzerh (PBE) generalized gradient approximation (GGA) functional49 within the framework of density functional theory implemented in VASP code50-51. The projector-augmented-wave (PAW) pseudopotentials52 with the cores of 1s-2p, 1s, and 1s for Co, O, and C, respectively were utilized to describe the valence-core interactions and a plane-wave kinetic energy cut-off of 500 eV was employed. Regarding the strong correlation between the electrons in partially occupied 3d orbitals of Co, the on-site Coulomb repulsion correction term of U within Hubbard scheme (PBE+U) was applied in the 3d electrons of Co. The value of effective Ueff, namely U-J, was set to 2.0 eV, which has been demonstrated to well simulate the properties of Co3O4 such as the band gap, lattice constant, and magnetic moment in previous computational work.14, 35, 39, 53 In addition, we further checked the reliability of the value of Ueff = 2 eV for the surface reaction as well. We calculated the energy barrier of the dehydrogenation of CH4 by O2c to form CH3* over Co3O4(110) utilizing different U values for Co 3d electrons varying from 2.0 eV to 4.0 eV. From Ueff of 2.0 eV to 4.0 eV, the calculated energy barrier only increases from 0.84 eV to 0.86 eV. Hence, Ueff = 2.0 eV could accurately simulate the methane combustion process over Co3O4 surfaces. Due to the overestimation of O-O bonding energy for PBE functional, the energy of gas-phase O2 is corrected by the energies of two atomic oxygen from DFT and the experimental bonding energy of 5.16 eV (E(O2) = 2×EDFT(O) – Eexp(bond)).

A cubic conventional unit cell containing 32 O anions and 24 Co cations as shown in Figure 1A was selected to model the antiferromagnetic bulk Co3O4 with normal spinel structure. Its lattice constant of 8.124 Å was optimized via the energy volume fit according to the Murnaghan equation of state (EOS)

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experimental value of 8.065 Å 12. The magnetic moments of Co cations have been further checked. It has been found that the only the Co2+ ions at tetrahedral sites have the magnetic moments of 2.53 µB whereas the Co3+ ions at octahedral sites are nonmagnetic at bulk Co3O4. The stoichiometric Co3O4(110) surface was modeled by the p(2×1), 11.489 Å × 8.124 Å slabs separated by a ~15 Å vacuum in Z direction. Each periodic slab contains eight layers of Co-O plane. A 2 × 3 × 1 k-point mesh was used for these surface slabs. For Co3O4(110), it has two different terminations, usually denoted as the A and B terminations: the (110)-A termination exposes two types of cations (Co2+ and Co3+) and one type of anions (three-fold coordinated O3c), whereas the (110)-B termination has only one type of cations (Co3+) and two types of anions (two-fold coordinated O2c and three-fold coordinated O3c). Xu et al.29 and Chen et al.32 have reported the curves of Gibbs surface energies of these two terminations against oxygen chemical potential. The terminal B surface has a lower Gibbs surface energy in oxygen-rich conditions. Considering the oxygen chemical potential under the common working conditions ( PO2 ⁄Po = 0.2; T = 500 K~700 K ), the (110)-B surface was energetically more stable than the (110)-A surface. Hereby the (110)-B terminal was chosen to model the catalyst surface as displayed in Figure 1. At Co3O4(110)-B terminal, due to the broken symmetry compared to bulk and the reduced coordination number, the exposed Co3+ ions have the magnetic moments of 2.44 µB, which are similar to those of the Co2+ ions in bulk. This result is also similar to the magnetic ground-state configurations suggested by Chen et al.32. All the geometry structures of adsorption intermediates were optimized using a force-based conjugate gradient algorithm until the forces on all the relaxed atoms were below 0.05 eV/Å. During the geometry optimization, the bottom four Co-O layers were fixed while the top four Co-O layers and adsorbates

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could be relaxed. The transition states (TSs) were located with a constrained minimization technique with the same force convergence criterion.54-57 Each TS was further verified as the first-order saddle with only one imaginary vibrational frequency and the corresponding vibrational mode along the reaction coordination based on the numerical vibrational frequency analysis. In addition, the adsorption energy of surface species is defined as:

Ead(X) = E(sur) + E(X) - E(X/sur)

where E(sur), E(X) and E(X/sur) are the energies of the catalyst surface, X in the gas phase and X adsorbed on the catalyst surface, respectively. The more positive Ead(X) is, the more strongly the species X binds with the surface.

3. RESULTS AND DISCUSSIONS

3.1 Complete Oxidation of Methane The complete catalytic combustion of CH4 undergoes the complex reaction network to realize both the complete oxidation of CH4 to CO2 and the regeneration of the active sites at Co3O4 surface. We started with the investigation on the complete oxidation of CH4 to CO2. 3.1.1 The First C-H Bond Activation The first C-H bond activation always initiates the whole reaction process. On the one hand, due to the high C-H bonding energy of 435 kJ/mol, the C-H bond breaking is likely to be a tremendous job on the specific surface. Thus the catalytic activity would be low at the desired lower reaction temperature. On the other hand, due to the high symmetry of CH4, once the first C-H bond is able to

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be activated, it is challenging to avoid the deep dehydrogenation of CH4 to form CH* or C* with the high valence state since this could lead to the high bonding orders between these intermediates and the surface sites. As a result, the subsequent formation of C-O bond may be tough, possibly resulting in the low catalytic activity as well. These are the reasons why the first C-H bond activation is generally thought of as a crucial step for the methane combustion.

On Co3O4(110)-B, only the sites of Co3+ cations, O2c and O3c anions are exposed while all the Co2+ sites are located at the subsurface or bulk. This geometry character of the surface makes only Co3+ cations and the surface lattice O2- anions available to catalyze the C-H activation. Hereinafter Co3O4(110)-B is referred to Co3O4(110).

To identify the reaction site and the C-H dissociation mode, we firstly investigated the energy barriers and reaction energies of this step at these possible reaction sites. The calculated geometry structures of the located transition states and the corresponding dissociated adsorption final states (FSs) on the Co3O4(110) surface are displayed in Figure 2. Hereinafter the transition states (TSs) or intermediate states (MSs) are numbered according to their positions displayed in figures along top view. For example, TS2B represents the TS displayed in Figure 2B and MS2C represents the MS displayed in Figure 2C. On Co3O4(110), the C-H bond breaking could occur via the heterolytic dissociation. The CH3 and H fragments stay at Co3+ and lattice O2- at the TSs and FSs, respectively. Since both surface O2c and O3c anions are likely to participate in the C-H activation, we further compare their energy barriers and reaction energies. The calculation results show that the energy barrier of 0.97 eV at Co3+-O3c site is 0.13 eV higher than that at Co3+-O2c site (0.84 eV). This implies

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the stronger interaction between H and lattice O2c, namely the stronger basicity of O2c. The activated C-H bond of TS2E is elongated by 0.283 Å at Co3+-O2c site compared to gaseous CH4 while it is elongated by 0.311 Å at Co3+-O3c site (TS2B). In addition, the left three C-H bonds of CH3 at TS are also slightly elongated by 0.008 Å at Co3+-O2c site while by 0.006 Å at Co3+-O3c site. This indicates that the interaction between C and Co3+ could slightly facilitate the activation of C-H bond. Furthermore, despite both exothermic process, the first C-H bond dissociation at Co3+-O2c site (∆H = -0.33 eV) releases more energy than that at Co3+-O3c site (∆H = -0.11 eV). Considering the possible adsorbed oxygen species on Co3O4 surfaces58-59, we also investigated the possibility of the first C-H bond activation by surface oxygen species. However, due to the drastic energy barrier of adsorbed O2* (each adsorbate X bonded with Co3+ will be denoted as X* in the whole paper) dissociation to form O* at Co3+(1.83 eV), O* is difficult to be formed on the clean Co3O4(110) surface. Furthermore, we found that the energy barrier of the oxidative dehydrogenation of CH4 to CH3* by O2* is as high as 1.53 eV. Accordingly, the first C-H bond of CH4 would mainly dissociate at Co3+-O2c site. Since there are four Co3+-O2c site pairs on the Co3O4(110) p(2×1) surface, in order to assist the description hereinafter, we denoted the four Co3+ cations with CoI, CoII, CoIII, CoIV and the four O2c cations with OI, OII, OIII, OIV at Co3O4(110) p(2×1), respectively, as shown in Figure 1C. Hereinafter this Co3+-O2c site for initial CH4 dissociation is denoted as CoI-OI as shown in Figure 3.

3.1.2 The First C-O Coupling After the first C-H bond dissociation, the two-fold coordinated OI site with the high activity turns to OIH. The CH3* at CoI could further react via one of four possible reaction pathways: (I) the C atom coupling with the O3c to form CH3O3c; (II) the C atom coupling with OIH to form CH3OIH; 11


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(III) the further C-H activation of CH3* to CH2* by O3c; (IV) the further C-H activation of CH3* to CH2* by OIH. We investigated the energy barriers and the reaction energies of these possible elementary steps, respectively. The calculated energy barriers of steps I, II, III, and IV are 1.32 eV, 2.39 eV, 1.45 eV and 1.54 eV, respectively. The corresponding reaction energies are -0.61 eV, -0.49 eV, 0.58 eV and 0.51 eV, respectively. The geometry structures of the corresponding TSs and FSs are shown in Figure S1.

Different from the relatively rapid process on metal surfaces60, it is clear from the results above that the C-H bond breaking from CH3* to form CH2* on Co3O4(110) is endothermic and the energy barrier is remarkably high. It could be attributed to the significantly weakened Co-O bonding due to the formation of the double bond between CH2 and single Co3+ to realize the bivalence of CH2 while the formation of two single bonds between CH2 and two metal atoms at the metal surfaces avoids the expense on the significantly weakened bonding of the substrate. This implies the low possibility of the direct deep dehydrogenation from CH4 to CH or C on Co3O4(110).

For two possible C-O coupling processes above, in spite of both exothermic processes, the high energy barriers mean the rather drastic processes for these two steps as well. In particular, when relatively active OI turns to OIH, the energy barrier of CH3* reacting with OIH is as high as 2.39 eV, which is even much higher than CH3* coupling with O3c. This is ascribed to the significant damage of Co2+-OI bonds for activating OIH at TS.60 Regarding the higher activity of two-fold coordinated O2c compared to O3c as found above, it is expected that the lower C-O coupling energy barrier might be achieved between CH3 and clean O2c. Yet the removal of the H from OIH is requested for the

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regeneration of two-fold coordinated OI. Importantly, we found that the H atom can easily transfer between two O2c sites with a slight energy barrier of 0.11 eV. The details of H migration over the surface will be discussed in the section of 3.2 on water formation. Consequently, the deactivated OIH site can readily transfer H to the adjacent OIV with a slight enthalpy change of -0.06 eV and the OI in the neighbor of CH3* is regenerated (MS3A). Similar to the first C-H activation, the two-fold coordinated OI does exhibit greater activity than O3c. As illustrated in Figures 3A-3C, the reaction between CH3* at CoI and OI requires an evidently lower barrier of 0.90 eV, which is 0.42 and 1.49 eV lower than those of CH3*+O3c and CH3*+OIH, respectively. Moreover, this step is able to release 1.00 eV heat and the CH3* transfers from CoI to OI after the C-O bond formation. Despite the higher energy barrier of 0.90 eV compared to 0.84 eV of the first C-H activation, the apparent barrier of C-O coupling with respect to gaseous CH4 of 0.51 eV is still lower than the first C-H bond activation of 0.84 eV due to the exothermic process of the first C-H bond activation step. Hence the first C-H activation is still the slower elementary step up to now. Nonetheless, if OI could not be reactivated, the C-O coupling had to occur between CH3* and O3c with an apparent energy barrier of 0.91 eV, which could make the first C-O coupling step become more crucial step for methane oxidation. This sheds light on the important role of active O2c in the methane combustion.

3.1.3 The Second C-H Bond Activation Due to the swift diffusion of H between two-fold coordinated oxygen anions as stated above, the H atom of OIVH from the OI is also able to readily transfer to another O2c, OIII (the neighbor O2c of OIV if extending the periodic boundary of Figure 3D). Once the H migrates to OIII, the reactivated OIV can capture one hydrogen from CH3OI to form CH2OI and the C-OI bond would be converted to 13


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C=OI double bond to meet the valence of C. This step only needs to overcome an energy barrier of 0.74 eV. The relatively lower energy barrier compared to the first C-H bond activation is attributable to the weakened C-H bond due to the connected more electronegative oxygen. It is thermodynamically favorable as well, slightly releasing the heat of 0.06 eV. The optimized geometry structures of IS, TS and FS in this step are shown in Figure 3D, 3E and 3F, respectively. The distance of C-H bond at TS3E is 1.332 Å, which is shorter than that of TS2C for the first C-H activation. For O3c, due to the too long distance, it is not possible to seize the hydrogen from CH3O2c.

In addition, due to the geometric accessibility and the adequate chemisorption energy of the O2 at CoI (the chemisorption energy increases to 1.94 eV while the chemisorption energy of O2 on pristine Co3O4(110) is 1.27 eV) after the formation of CH3OI, we also check the likelihood that the CH3OI is dehydrogenated by the O2* adsorbing at CoI. The IS and TS of this step are shown in Figure S2. However, the energy barrier of this step is as high as 1.31 eV. This suggests that the CH3O directly activated by adsorbed O2 is still difficult. Therefore, it is manifested that CH3OI could be dehydrogenated by the active lattice OIV. OIV is a stronger nucleophilic site compared to adsorbed O2*.

Aim to further clarify the important role of O2c in the CH3O oxidative dehydrogenation, we also investigated the CH3O3c oxidative dehydrogenation. Provided that the C-O coupling had to occur between CH3 and O3c at the last elementary step, the energy barrier of oxidative dehydrogenation of formed CH3O3c to CH2O3c by O3c would soar to 1.53 eV. Furthermore, this step needs to absorb the heat of 0.84 eV. The geometry structures of CH3O3c, CH2O3c and the dehydrogenation TS are shown

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in Figure S3. Except for the lower reactivity of O3c compared to O2c, this high energy barrier and significantly endothermic process mainly stem from the low stability of CH2O3c. In order to meet the valence of C, the C=O double bond would be formed between C and O3c once CH2O3c is formed. However, O3c is originally bound to three Co cations. Due to the limited bonding coordination number of O, the formed C=O double bond must significantly weaken the bonding of three Co-O bonds while it will only influence two Co-O bonds for O2c. The lower stability of CH2O3c makes this step thermodynamically unfavorable. This also leads to the lower desorption energy of CH2O3c of 0.84 eV compared to its formation. From the above results, we could find that the low-coordinated lattice oxygen anion is of vital importance for the oxidative dehydrogenation process.

3.1.4 The Second C-O Coupling Although the formation of CH2OI is much easier than CH2O3c, to meet the demand of the tetravalence of C at CH2OI, the formed C=O double bond would weaken the Co-O bonding after all. As a consequence, the distances of CoI-OI and Co2+-OI (Co2+ is located at the tetrahedron interstice at the sub-surface) are stretched by 0.095 and 0.286 Å compared to the pristine surface, respectively. Intuitively, as a close shell molecule, the HCHOI is therefore likely to desorb from the Co3O4(110) surface. However, the desorption of HCHOI needs to consume the energy of 1.12 eV. Interestingly, we found that CH2OI could barrierlessly react with the other adjacent O2c of OII to form OICH2OII. Thus the second C-O bond coupling readily occurs. Moreover, this process energetically declines by 1.24 eV. As displayed in Figure 4A, the C is located in the center of OICH2OII with a tetrahedron structure. Therefore, the C atom switches from the sp2 hybridization in CH2OI to sp3 hybridization of OICH2OII. Importantly, the average bond length of C-OI and C-OII are 1.418 Å, which is close to the 15


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standard length of single C-O bond of 1.420 Å61. The single bond instead of double bond between C and OI could avoid the severe damage of Co-OI bond and the formed C-OII bond could saturate the low-coordinated OII. Hence, CH2OI tends to effortlessly form OICH2OII rather than its desorption.

3.1.5 The Third C-H Bond Activation Due to the open shell character of gaseous OCH2O, the formed OICH2OII is very difficult to desorb. Its desorption energy will be as high as 2.74 eV. As Figure 4A displayed, the hydrogen atoms of OICH2OII are too far to be accessible to lattice oxygen ions for the possible H abstraction unless some nucleophiles adsorb at the Co3+ cations of CoII or CoIII site. Since the bonding of C-OI weakens the Co-OI bond, the gaseous O2 can strongly adsorbed at the partially reduced Co3+ site with the adsorption energy of 2.02 eV. Moreover, as shown in Figure 4B, at this adsorption configuration, one oxygen atom of adsorbed O2 is close to one of the hydrogen atom of the OICH2OII. The H-O distance is 2.157 Å. Hence the adsorbed O2* is likely to take one hydrogen of OICH2OII by nucleophilic attack. The calculated results do manifest this assumption. The calculated energy barrier of OICH2OII oxidative dehydrogenation by O2* (TS4C) is 0.74 eV, which is even lower than the energy barrier of the first C-H bond breaking. This step is exothermic as well, releasing the energy of 0.25 eV. These show that the third C-H activation step could occur via the oxidative dehydrogenation of OICH2OII by O2* to form OICHOII. Interestingly, the intermediate of OCHO was also found by in situ IR spectra, which was thought of as a possible important surface intermediate.14 In addition, compared with the result of the previous OICH3 oxidative dehydrogenation by O2* as discussed above, the energy barrier of analogous oxidative dehydrogenation of OICH2OII is evidently lowered. This could be attributed to the decreasing C-H bond with the increasing number of highly 16


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electronegative oxygen atoms bound with C. Consequently, the less active O2* compared to lattice O2c anion is able to abstract the hydrogen of OICH2OII. The corresponding structures of TS and FS of this step are displayed in Figures 4C and 4D. The activated C-H bond at TS4C is 1.389 Å, which is elongated by 0.285 Å compared to OICH2OII. Thus the further oxidative dehydrogenation of OICH2OII forms OICHOII and O2* is reduced to OOH* at the same time. When the hydrogen is taken away from OICH2OII, the carbon at formed OICHOII tends to change back to sp2 hybridization geometry to meet its tetravalence. As a result, the geometry structure of OICHOII switches to the trigonal planar structure as displayed in Figure 4D. This also implies the increasing bond order between C and OI or OII. Moreover, the average length of two C-O bonds is shortened to 1.286 Å from 1.420 Å of OICH2OII, implying that the C-O2c bonds are considerably strengthened. In addition, the average bond length of four Co-O bonds involving both OI and OII is elongated to 2.152 Å from 1.988 Å when OICH2OII exists. This indicates that the Co-O bonding interactions for both OI and OII are weakened. Interestingly, as shown in Figure 4E, OII of OICH2OII could leave its lattice position, rise along [110] direction, and approach to CoII until the Co2+-OII is broken and the OII adsorbs at CoII to form OICH2O* (MS4E). Likewise, OI is also likely to migrate from the lattice position to CoI to form OIICH2O* (MS4F). It is exothermic for OII migration, releasing 0.24 eV whereas it requires the energy of 0.25 eV for OI migration. Since the energy barriers for these two possible pathways are both facile, only 0.02 eV and 0.30 eV, respectively, these two lattice oxygen migration steps could easily occur. This structure configuration switch from OICHOII to OICHO* or OIICHO* might be attributed to the reduced Co-O bonding energy when OICHOII is formed from the oxidative dehydrogenation of OICH2OII. As a result, the relatively lower energy loss due to the Co2+-O bond

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breaking could possibly be compensated by enhancing the Co3+-O bonding and C-O bonding, meeting the valence of O as well. Yet due to the competition adsorption between OOH* and OIICH2O* at CoI, the bonding promotion of CoI-OI will be limited. Consequently, OIICH2O* is thermodynamically less favorable than OICH2O*. However, once OI leaves its lattice position, as shown in Figure 4G, the adsorbed OOH* could be readily dissociated to fill the oxygen vacancy of OI and generate OH* at CoI as well. This process can release a great amount of energy of 2.08 eV and the energy barrier of 0.10 eV is trivial. The corresponding final state of the OOH* dissociation is shown in Figures 4H. Since the increased energy of 0.24 eV originating from the transfer of OI is slight and the subsequent rapid OOH dissociation can make the whole process thermodynamically extremely favorable, the OICHOII tends to be converted to the OIICHO* at CoI and OOH* will be subsequently dissociated to form OH* at CoI.

3.1.6 The Fourth C-H Bond Activation For the last C-H bond activation, there are three kinds of possible oxygen species to abstract the hydrogen from OCHOII as displayed in Figure 5C: (I) the adsorbed O2* at CoII; (II) the adsorbed OH* at CoI from OOH* dissociation; (III) the lattice OI. The corresponding reaction pathways are compared as follows:

For the pathway I, similar to the third C-H activation step, the formed OIICHO* decreases the bonding of CoII-OII compared to clean surface, leading to the adequate O2 adsorption strength with the chemisorption energy of 1.81 eV at CoII (MS5A). The transfer of hydrogen atom from OIICHO* at CoI to O2* at CoII needs to overcome an energy barrier of 1.14 eV (TS5B). Since the competitive

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adsorption with OH* at CoI further weakens the originally weak adsorption of CO2, the formed CO2 will immediately desorb, which only needs to conquer the desorption energy of 0.20 eV, and an OII vacancy is simultaneously produced (MS5C).

For the pathway II, the direct oxidative dehydrogenation of OIICHO* by OH* at CoI cannot occur due to the too long distance. However, it is found that the O terminal of OIICHO* at CoI can be transferred to CoIII as shown in Figure 5D. For this shift process, the calculated energy barrier is only 0.25 eV and the energy goes downhill by 0.64 eV, indicating that this process is easy to occur. Then the hydrogen of OIICHO* is transferred to the OH* at CoI, and the H2O* at CoI and CO2* are produced. For this dehydrogenation step, the energy barrier is 0.94 eV, which is lower than the reaction pathway I. The corresponding TS and FS structures of this step are shown in Figures 5E and 5F. This step energetically climbs uphill by 0.65 eV. Although it is endothermic, the formed CO2 can readily desorb away from the surface, releasing the energy of 0.45 eV. Regarding the large entropy of CO2 which will additionally lower the energy by 1.57 eV (T = 600K), this process is thermodynamically feasible. Therefore, compared with the pathway I, the pathway II with the lower energy barrier is more favorable. Interestingly, regardless of pathway I or II, both oxygen atoms of CO2 originate from the lattice O2c of OI and OII.

We also investigated the OIICHO* at CoI (MS5C) is directly dehydrogenated to OI (pathway III-1) and is dehydrogenated to OI after the removal of OH* at CoI via H2O desorption (pathway III-2). The energy barriers of these two oxidative dehydrogenation pathways are 1.10 eV and 1.24 eV, respectively. The structures of the dehydrogenation step in these two pathways are shown in Figures

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5G-5I and Figures 5J-5L, respectively. Hence, comparing all these four pathways, due to the lowest energy barrier, the fourth C-H activation is most likely to occur via pathway II.

3.2 Regeneration of Active Surface Site Despite identifying the complete oxidation of CH4 to desorbed CO2, four hydrogen atoms from CH4 are still left at the surface. As discussed above, the two-fold coordinated lattice oxygen anion is the key nucleophilic site for the oxidative dehydrogenation, especially for the first two C-H bonds activation. However, it would be poisoned when it is converted to lattice OH. Although the hydrogen transfer between O2c is likely to temporarily recover the reactivity of O2c close to the reaction intermediate, the continuously accumulated H atoms will block these sites eventually once they cannot leave the surface. Hence, the desorption of hydrogen-containing species for the removal of these hydrogen atoms are of great importance to the whole catalytic cycle. Following the previous mentioned optimal mechanism of methane oxidation, after CO2 desorption, two hydrogen atoms are located at two O2c, respectively, and the other two are in the form of H2O* at CoI as Figure 5F shown. The desorption energy of H2O* is 0.84 eV. Regarding the entropy of gaseous H2O (T∆S = 1.33 eV, T = 600K), H2O can readily leave the surface. Hence the key question is how to remove the left hydrogen located at O2c. Since the desorption of possible close-shell HCHO species containing H is more difficult compared to its deep oxidation as discussed above and HCOOH cannot be formed in the optimal pathway of methane oxidation, these left H atoms are pretty likely to leave the surface in the form of H2O as well. Yet on Co3O4(110), the surface H2O includes three possible states: H2O*, H2O2c, and H2O3c. Their formations depend on the hydrogen transfer between different oxygen species. Hence we furthermore investigated the hydrogen transfer among different oxygen species 20


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and identified the formation pathways of H2O*, H2O2c and H2O3c, and their desorption processes, respectively.

Due to the remained hydrogen atoms stay at O2c and the regeneration of O2c is of vital importance for both the first C-O coupling step and the second C-H bond activation step, we firstly explored the hydrogen transfer between two O2c. The transfer of the hydrogen atom between two clean O2c is very facile where the energy barrier is only 0.36 eV. Therefore, the hydrogen transfer between two O2c could temporarily recover the O2c close to the reaction intermediate, assuring the formation of CH3O2c as discussed above. However, when the hydrogen at O2c transfers to the adjacent HO2c instead of O2c, namely two HO2c form one H2O2c and rebirth one O2c, it becomes endothermic, absorbing the energy of 0.79 eV. What’s more, the formed H2O2c is very difficult to desorb from Co3O4(110). The desorption energy is as high as 2.07 eV. This is in agreement with the previous study on water poison effect for CO oxidation on Co3O4 surface53. This indicates that although the single pathway of hydrogen transfer between O2c sites is capable of regenerating the active O2c site temporarily, it cannot eventually avoid the passivation of active O2c sites with the accumulation of H from CH4 due to the drastic desorption of H2O2c. Hence, it is necessary to test the other possible pathways for the water formation.

As we have found that the produced H2O* at Co3+ site is not so difficult to desorb, we then investigated the likelihood that the accumulated hydrogen transfers from O2c to the possible surface oxygen-containing species adsorbing at Co3+ site such as O* and OH* towards the production of H2O*. Since there is an oxygen vacancy at OII after the CO2 desorption, gaseous O2 can adsorb at CoII and

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immediately be dissociated. Then one oxygen fills the oxygen vacancy of OII and the other becomes O*, as shown in Figure S9. The calculated energy barrier and enthalpy variation of this O2 activation step are 0.49 eV and -0.47 eV, respectively, indicating that the gaseous O2 can easily activated by oxygen vacancy on Co3O4(110). Then the stepwise hydrogen transfer from O2c to O* is possible to form OH* and H2O*. The calculated energy barrier and enthalpy variation are 0.25 eV and -0.40 eV for the hydrogen transfer from O2c to O*. The exothermic process and the slight energy barrier make this process favorable. Then for the hydrogen transfer from O2c to OH*, the energy barrier is even lower, only 0.11 eV and it is also exothermic, releasing the energy of 0.20 eV. As stated above that the desorption energy of H2O* is not difficult, the facile stepwise hydrogen transfer from O2c to O* for the H2O formation enables the regeneration of active O2c. Interestingly, the O* and OH* can also play a role of a bridge in the hydrogen transfer between O2c and O3c. As shown in Figure 6, for the hydrogen transfer from O* to O3c, the energy barrier and the enthalpy variation are 0.50 eV and 0.37 eV, respectively, while they are 0.18 eV and 0.10 eV for the hydrogen transfer from OH* to O3c, respectively. Despite these endothermic elementary steps, the whole hydrogen transfer processes for O2c to O3c are exothermic. Considering these facile hydrogen transfer energy barriers, the hydrogen is able to migrate to O3c and it is possible to further form H2O3c. Therefore, we further investigated the hydrogen transfer between O3c for the H2O3c formation and the subsequent H2O3c desorption as well. For the O3c, the hydrogen transfer barrier is 0.92 eV between two adjacent clean O3c, implying that it is not easy. When the hydrogen shuttle between two adjacent HO3c for the formation of H2O3c and the regeneration of O3c, the energy barrier is even higher than 1.32 eV and the enthalpy variation is 0.64 eV. These indicate that H2O3c is not easy to be formed. Furthermore, albeit lower than the

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H2O2c, the desorption energy of H2O3c is still as high as 1.85 eV. Thus although O* and OH* are possible to work as the bridges to facilitate the formation of HO3c, the gaseous water could not be formed via the desorption of H2O3c, either. Nonetheless, it provides more sites and channels to store surface H*, which is able to facilitate the regeneration of O2c temporarily.

To sum up, the accumulated H from CH4 oxidative dehydrogenation at the surface would leave the surface via H2O* for the regeneration of active O2c site. Since the O* comes from the dissociative adsorption of O2 at the oxygen vacancy of O2c, the oxygen of the formed water in the gas phase could possibly originate from the O2 instead of lattice oxygen.

3.3 The Complete Reaction Pathway of Methane Combustion We have investigated the mechanisms of methane oxidation to CO2, the hydrogen transfer and water desorption for the regeneration of active O2c sites in the previous sections 3.1 and 3.2, respectively. We further take these two parts into account altogether for the comprehensive understanding of the key step for the whole catalytic cycle of methane combustion over Co3O4(110). The optimal reaction pathway for the formation of gaseous CO2 and H2O are summarized in Scheme 1 and the calculated energy profile of the optimal reaction pathway of the methane catalytic combustion on Co3O4(110) is displayed in Figure 7.

As illustrated in Figure 7, the whole energy profile of CH4 combustion is energetically downhill following the optimal reaction pathway. The maximum energy barrier is 0.94 eV for the fourth C-H activation step, the dehydrogenation of OIICHO* to gaseous CO2. Moreover, OIICHO* is energetically the most stable intermediate for the whole methane oxidation process, it is therefore

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expected the relatively higher coverage of OIICHO*. This explains why the vibrational peak of OCHO could be observed in the IR spectra14.

However, taking the entropy into account, the first C-H activation barrier for the dissociative adsorption of gaseous CH4 to CH3* and HO2c is higher regarding the great translational and rotational entropy of gaseous molecule compared to the surface species (T∆S = 1.34 eV, T = 600K). This entropic effect will be more prominent with the increase of the temperature, which makes the first C-H activation step more importantly. What’s more, TS2E, the TS of the first C-H activation step is the least stable TS in the whole energy profile, further indicating that it is the rate-determining TS of methane combustion.

Following the optimal reaction pathway, the apparent energy barrier of the first C-O coupling between CH3* and active O2c with respect to gaseous CH4 is slightly lower than that of the first C-H activation and the energy barrier of this elementary step is only second to the fourth C-H bond activation. It is worth noting that once the active O2c is covered with H, the energy barrier of the first C-O coupling via the other pathways would roar. This would lead to the result that this step becomes the rate-determining step for the whole CH4 combustion. As a result, the Co3+ sites could possibly be covered with a mass of CH3*. Hence, this result also highlights the importance of the rapid H transfer and the moderate formation of H2O* which could easily desorb for the regeneration of active O2c.

In addition, the rapid hydrogen transfer relies on the multi neighboring active O2c sites as well. Except for the hydrogen transfer, both the second C-O coupling step and the fourth C-H bond activation involve the carbon-containing species migration between different O2c and Co3+ sites. If

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taking the connected Co3+-O2c as a group site (For example, group site I means CoI-OI, group site II means CoII-OII, and etc.), we can find that the highly efficient methane combustion extremely relies on the co-work of four active site groups. The first C-H activation and CH3-O coupling reacts at group site I, the CH3O oxidative dehydrogenation occurs at group sites I & IV, the OCH2-O coupling and the OCH2O oxidative dehydrogenation react at group site I & II, and the OCHO oxidative dehydrogenation needs the co-work of group sites I & II & III. As stated before, without the synergetic effect between these sites, the energy barriers of the corresponding steps will dramatically increase. These could be attributed to three reasons: (I) each group site only has one active O2c site and the proper distance of Co3O4(110) makes the active O2c at the neighboring group site could be the other reactive site; (II) The intermediates located at different group sites could avoid the direct bonding competition; (III) The less space limitation due to the compositions of several group sites rather than the single group site makes the intermediate possibly reach more stable hybridization state like the second C-O coupling step for the formation of O2cCH2O2c. Hence these results shed light on the significance of O2c site and its regeneration for the whole methane combustion process.

3.4 Origin to the Different Catalytic Activities between Co3O4(110) and Co3O4(100) As Hu et al suggested that the catalytic activity order for methane combustion of these crystal planes follows: (110) ≫ (100)12, we further investigated the key steps of methane combustion over Co3O4(100) to understand the key factors of the surface structure governing the catalytic activity. For Co3O4(100), at the reaction condition, the Co3O4(100)-B terminal which only exposes Co3+ cations and three-fold coordinated oxygen anions O3c is thermodynamically more stable than Co3O4(100)-A. Hence, Co3O4(100) surface denotes Co3O4(100)-B hereinafter. Intriguingly, the calculated results 25


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show that there are no remarkable differences for the first C-H activation and the first C-O coupling elementary steps between over (110) and (100) surfaces. The first C-H bond activation of CH4 also undergoes oxidative dehydrogenation to form CH3* at Co3+, but H can only stay at O3c to form HO3c, as illustrated in Figures 8A-8C. The energy barrier is 0.77 eV on Co3O4(100), which is similar to that on Co3O4(110), even slightly lower. This might be attributed to the relatively larger structure relaxation for this O3c site at TS.53 Yet the enthalpy variation is -0.13 eV, releasing less heat than that on Co3O4(110). This suggests that the difference of the first C-H bond activation could not directly determine the different catalytic activities of CH4 combustion on different catalysis surfaces. We also investigated the possibility of the first C-H bond activation by surface oxygen species. Similar to Co3O4(110), the formation of O* from O2* is difficult with the high energy barrier of 1.29 eV on the clean Co3O4(100) surface and the energy barrier of the first CH4 dehydrogenation by O2* is also as high as 1.45 eV. Hence CH4 is easier to be activated by O3c at the Co3O4(100) surface. For the subsequent first C-O coupling, the formed CH3* coupling with the clean O3c to form CH3O3c on Co3O4(100) (the structures of TS and FS are shown in Figure 8D & 8E, respectively). This step is a bit more difficult than that on Co3O4(110). The energy barrier increases to 0.98 eV, and the enthalpy variation is -0.64 eV. However, the apparent TS of the first C-O coupling with respect to gaseous CH4 is energetically less stable than that of the first C-H bond activation by 0.08 eV. This indicates that the first C-H activation is not the most difficult step on Co3O4(100) any longer. Furthermore, when investigating the second C-H bond activation on Co3O4(100), namely the dehydrogenation of CH3O3c, the catalytic cycle encountered the fatal trouble. As compared to 0.74 eV on Co3O4(110), the energy barrier of this step on Co3O4(100) surges to 1.72 eV, and this step gets endothermic,

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absorbing the extra heat of 0.50 eV. The corresponding TS and FS structures of this step are shown in Figures 8F and 8G. This rises from the higher coordination number of O3c. As discussed in section 3.1.3, in order to meet the tetravalcence of C, C=O double bond must be formed for CH2O3c. This will significantly weaken the surface lattice Co-O bond. The higher coordination number the lattice oxygen ion possesses, the more Co-O bonds will be weakened. Hence, the absence of low-coordinated O2c at Co3O4(100) prevents CH3O from the further dehydrogenation. As a consequence, the catalytic activity of methane combustion on Co3O4(100) is evidently lower than that on Co3O4(110)12. This again sheds light on the key role of active O2c.

4. CONCULSIONS

In this study, we investigated the whole catalytic cycle of CH4 combustion on Co3O4(110) at the molecular level and also compared the corresponding key steps on Co3O4(100) utilizing DFT calculations. The main conclusions could be drawn as follows: 1.

The optimal reaction pathway of methane oxidation process over Co3O4(110) would undergo CH4 CH3* CH3O2c CH2O2c O2cCH2O2c O2cCHO2c O2cCHO* CO2.

2.

The active low-coordinated O2c would be of vital importance for methane combustion over Co3O4 surfaces, especially for the first two C-H bonds activation and the C-O bond coupling. The presence of active low-coordinated O2c is the origin to the higher catalytic activity over the Co3O4(110) surface compared to the Co3O4(100) surface without exposed O2c.

3.

Another key factor for high activity over Co3O4(110) is that H is able to readily transfer among different surface oxygen species to form adsorbed H2O* for the rapid regeneration of active O2c.

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The possibility of the swift H transfer among neighboring O2c sites enables the regeneration of the active O2c temporarily while the adsorbed oxygen species springing from gaseous O2 is able to readily accept the H from O2c to form adsorbed H2O* which is easier to desorb from the surface unlike H2O2c or H2O3c, enabling the active O2c sites to be eventually recovered. 4.

The multi-sites are crucial for the whole methane combustion. The co-work of different sites could guarantee the facile progress of the whole catalytic cycle, which maximally prevents from the passivation of lattice oxygen and competitive adsorption between different intermediates, as well as facilitates the formation of more stable surface intermediates.

5.

Regarding the entropic effect, the first C-H bond activation step would be the rate-determining step for the whole catalytic cycle on the Co3O4(110) surface following the optimal reaction pathway. Nevertheless, its importance will edge lower with the decrease of the temperature due to the lower entropy of gaseous methane. Moreover, the result that the first C-H bond activation step works as the rate-determining step is under the preconditions of the presence of active O2c which could also be regenerated rapidly. Otherwise, the first C-O bond formation and the second C-H bond breaking would become more crucial steps like that on the Co3O4(100) surface. These interesting insights into the high activity of methane combustion on Co3O4(110) not only

would be beneficial to develop the improved catalysts for methane combustion but also shed light on the general importance of heterogeneous catalysts, especially for transition metal oxides. The presence of the efficient surface species migration among multi-sites could avoid the rapid passivation of single-site such as active lattice oxygen anion and also provide more possibilities for

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the surface intermediates to reach more stable structures or distributions to optimize the reaction pathway while these would be difficult for single-site catalysts.

SUPPORTING INFORMATION

The structures of branch reactions including CH3* coupling with O3c and OIH, the further dehydrogenation of CH3* to CH2* by OIH and O3c, the oxidative dehydrogenation of CH3O3c by O2* and O3c, the vibrational direction of H-Olat bond stretching, and the formation of H2O2c, H2O3c and H2O*; Table including the reaction energy and energy barrier for each elementary step we investigated. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGEMENTS

This project was supported financially by NSFC (21333003, 21303051), Shanghai Natural Science Foundation (13ZR1453000), and the Fundamental Research Funds for the Central Universities. The authors also thank the support from Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) for computing time.

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Figure 1. (A) bulk Co3O4, (B) side view and (C) top view of Co3O4(110)-B surface. Co3+, Co2+, O, H and C atoms are shown in dark blue, light blue, red, white and grey, respectively. In addition, in the top view of structures, the atoms at the top and sub layers are illustrated in ball-and-stick and stick model, respectively and the rest are in line model. The exposed four Co3+ cations and the four O2c cations at the surface were labeled as CoI, CoII, CoIII, CoIV, OI, OII, OIII, and OIV, respectively. These illustrations are utilized throughout this paper.

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Figure 2. The transition state (A: side view, B: top view) and the final state (C: top view) of the first C-H activation at Co3+-O3c site; and the transition state (D: side view, E: top view) and the final state (F: top view) of the first C-H activation at Co3+-O2c site. The energy barrier, the enthalpy variation, and the imaginary vibrational frequency (unit: cm-1) of TS along the reaction coordination are shown below the corresponding TS and MS structures for each elementary step.

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Figure 3. The optimized structures from CH3-CoI to CH2OI: (I) CH3* coupling with OI (A, B and C are the initial, transition and final states, respectively); (II) the dehydrogenation of CH3OI dehydrogenate by OIV (D, E and F are the initial, transition and final states, respectively).

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Figure 4. A: OICH2OII; B: the bidentate adsorption of O2 at CoI; C-D: TS and FS of the third C-H activation for the formation of OICHOII; E-F: the structures of the OII and OI rising up; G: the OOH* filling into OI vacancy; H: the OOH* dissociate into OH* and OI.

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ACS Catalysis

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Figure 5. Four pathways of the 4th C-H activation: A-C: the IS, TS and FS of pathway I: the dehydrogenation of CoI-OCHOII by the adsorbed O2*; D-F: the IS, TS and FS of pathway II: the dehydrogenation of CoIII-OCHOII by OH* at CoII; G-I: the IS, TS and FS of pathway III-1: the dehydrogenation of CoI-OCHOII by OI; J-L: the IS, TS and FS of Pathway III-2: the dehydrogenation of CoI-OCHOII by OI after the H2O formation and desorption.

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Figure 6. Schematic diagram of H-transfer among O*, O2c, O3c, and OH* sites including the energy barrier and the enthalpy variation for each step.

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Figure 7. The energy profile of the minimum energy path. TS2E, TS3B, TS3E, TS4C, and TS5E are the TSs for the elementary reactions of the 1st C-H bond activation, the 1st C-O coupling, 2nd C-H bond activation, 3rd C-H bond activation and 4th C-H activation of which the reaction formulae are listed, respectively. The reconstruction step contains the steps undergoing the TSs and MSs which are displayed from Figure 4D to 5G.

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tr a

tra ns fer

H ns r

H-

fe rd

3

H Con ati tiv ac

.

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Scheme 1. Schematic diagram of the whole catalytic cycle of methane combustion over Co3O4(110) following the optimal reaction mechanism.

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Figure 8. The first three elementary steps of CH4 catalytic combustion on Co3O4(100). A: the top view of Co3O4(100); B-C: the TS and FS of the first C-H bond activation on Co3+-O3c, D-E: the TS and FS of the CH3* coupling with O3c; F-G: the TS and FS of the CH3O3c dehydrogenation to O3c.

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Table of Contents Table of Contents 44x36mm (600 x 600 DPI)


Origin of Efficient Catalytic Combustion of Methane over Co3O4(110

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