Mechanistic Insight into the C2 Hydrocarbons Formation from Syngas

Article pubs.acs.org/JPCC

Mechanistic Insight into the C2 Hydrocarbons Formation from Syngas on fcc-Co(111) Surface: A DFT Study Congbiao Chen, Qiang Wang, Guiru Wang, Bo Hou,* Litao Jia, and Debao Li State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, The Chinese Academy of Sciences, Taiyuan 030001 People’s Republic of China S Supporting Information *

ABSTRACT: A comprehensive density functional theory (DFT) calculation of C2 hydrocarbons formation in Fischer− Tropsch synthesis (FTS) on the close-packed fcc-Co(111) surface has been carried out. The activation barriers and reaction energies for CO dissociation, CHx hydrogenation, CHx + CHy coupling and C(HO) insertion into CHx, CHxCHy−O bond scission, and successive hydrogenation reactions involved in C2 hydrocarbons formation have been examined, and the following conclusions could be concluded: (i) CH is the dominant monomer, which is formed via CO + H → CHO → CH + O; (ii) CHO insertion is more plausible for C−C chain formation compared with CO insertion and CHx−CHy coupling. The rate-determining steps for C2 hydrocarbons are CO + H → CHO and CHCH + H → CH2CH. Meanwhile, CH3 hydrogenation to form CH4 is more facile than C2 hydrocarbons, which will lead to the low productivity and selectivity to C2 hydrocarbons. (iii) Stepped-Co(111) surface has been modeled to clarify the role of defects during C2 hydrocarbons formation, and the calculation results indicate that CHO and CH2CH formation could be facilitated and CH4 formation could be suppressed, suggesting that the step sites could effectively promote the catalytic activity and selectivity for C2 hydrocarbons formation.

1. INTRODUCTION As there is a limited supply of readily available petroleum, people are stimulated to search for alternative energy sources. Because of wide varieties of sources, syngas has become increasingly popular as a valuable alternative feedstock for the synthesis of fuels and chemicals. The Fischer−Tropsch (FT) synthesis plays an important role in long-chain hydrocarbons production from syngas,1 which has received considerable attention both in academics and industry.2−5 The catalyst, which contains reactive metal such as Fe, Co, and Ru as the active component, exhibits FT activity;6,7 however, only Fe and Co-based catalysts have been applied in industrial fuel synthesis.8 In particular, Co has attracted widespread attention because of its high activity and selectivity for producing long-chain hydrocarbons.9−13 There has no consensus on the underlying the mechanism of FTS since its discovery in the 1920s.14,15 Several mechanisms for chain growth have been proposed in experimental16−18 and theoretical3,4,19,20 works for FT synthesis: (a) carbide mechanism,21 (b) CO insertion mechanism,22,23 and (c) hydroxycarbene mechanism.24 Among these mechanisms that described the reaction step for chain growth, carbide mechanism versus CO insertion mechanism are at the focus of this debate. In the carbide mechanism, CO dissociates into C and O, and C adatoms undergo sequential hydrogenation to CHx species,25 which act as the chain initiators;16−18 CH26 as well as CH216,17 species are the most debated monomer intermediates, which are the responsible C−C chain formation for the carbide mechanism. In the CO insertion mechanism, the CO first undergoes © XXXX American Chemical Society

hydrogenation to form CHxO, then C−O bond scission to form an alkyl chain initiator, followed by insertion of CO, which acts as the chain propagator.27,28 Within the hydrogen-assisted scheme of FT synthesis, CHO,29,30 CHOH,31,32 and CH2O33,34 all have been suggested as possible key intermediates on Ru and Co surfaces according to the results of the DFT calculations. For C−C chain formation in carbide mechanism and CO insertion mechanism, various mechanisms have been proposed. Originally, surface CH2 coupling with surface CH323,35 is responsible for C−C chain formation, which has been proposed by Fischer and Tropsch. In addition, Biloen and Sachtler,36 as well as Maitlis et. al,37 described the formation of hydrocarbons by the insertion of the CH2 into the alkyl−metal bond. In recent years, CO insertion into CH2 was found to be responsible for C−C chain formation on the flat Co(0001) surface, which is proposed by Saeys and co-workers.38,39 Further, a theoretical study by the Zhao group40 pointed out that CHO insertion exhibits a superior and/or competitive activity to CO insertion for C−C chain formation on the Co(0001) surface. Meanwhile, the competing reaction of C−C chain formation is complete hydrogenation of the CHx to methane, which is the least desirable product in FT synthesis.41−43 It is well-known that Co can exist in two crystallographic structures, namely, the hexagonal close packed (hcp) phase and the face-centered cubic (fcc) phase, and both phases are observed Received: October 2, 2015 Revised: April 5, 2016

A

DOI: 10.1021/acs.jpcc.5b09634 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

surface calculations was sampled with 5 × 5 × 1 Monkhorst−Pack mesh,54 spin polarized calculations were included to account for the magnetic nature of Co, and the geometry optimization would be converged when the energy difference are smaller than 5 × 10−6 eV, and the forces are less than 0.01 eV/Å. To investigate C2 hydrocarbon formation in FTS process, the transition states were determined by the climbing-image nudged elastic band method (CI-NEB),55,56 and the optimization was carried by dimer method.57,58 The transition state structures would be converged when the forces for all the atoms are less than 0.05 eV/Å. The located TS for the elementary reaction were also confirmed by frequency analysis.

in FT synthesis.44 Although the hcp structure is preferred for the bulk Co, the crystallites with particles below 100 nm are more stable as fcc phase.45,46 The formation of C2 hydrocarbons on the surfaces of fcc-Co is therefore of interest to us. What is the favorable pathway for CO dissociation, direct dissociation or H-assisted dissociation? Which is the most favorable monomer, C, CH, CH2, or CH3? Which way is more plausible for C−C chain formation, CHx−CHy coupling or C(H)O insertion into CHx followed by C−O bond scission? What is the selectivity of CH4? To the best of our knowledge, the C2 hydrocarbons formation on the fcc-Co catalyst has not been systematically tackled with DFT calculations. Therefore, it is of interest to investigate the reaction pathways on the flat Co(111) surface in order to determine the active sites C2 hydrocarbons formation. To resolve the issues above, the investigation of the FTS mechanism has been divided into four parts, the adsorption of surface intermediates, CO activation, CHx formation, and C−C chain growth. The oxygenate production can be neglected because the amount is negligible under typical FTS reaction conditions.47 In the Results and Discussion, the formation of C2H4 and C2H6 will be clearly described and compared with previous investigations; also, some conclusions will be drawn. We expect that this study will offer a better understanding of the reaction mechanism during FTS process and provide some clues for the design of high-performance catalysts.

3. RESULTS AND DISCUSSION 3.1. Adsorption of Reactants and Possible Intermediates. The adsorption energy Eads is defined as follows Eads = Emol + Eslab − Emol/slab

Table 1. Adsorption Energy (kJ·mol−1) and Key Geometrical Parameters (Å) of All Possible Species Involved in FTS Reaction from Syngas on Co(111) Surface species CO C O H

2. MODELS AND COMPUTIONAL METHODS 2.1. Surface Model. In this study, the three-layer Co(111) surface model was chosen and the bottom layer was fixed at the bulk positions, whereas the atoms in the top two layers and the adsorbates were allowed to relax, which is shown in Figure 1.

CHO CH2O

85.2

hcp via C and O

C2H4

The optimized bulk lattice constant (3.53 Å) is in agreement with the experimental value of 3.55 Å.48 The repeating slab is separated by a vacuum spacing of 10 Å to reduce interactions. The adsorption energies change was within 5 kJ·mol−1 with respect to vacuum spacing. Increasing the slab thickness to five layers decreased the adsorption energy of CO less than 4.5 kJ·mol−1, which agrees well with the previous study.49 2.2. Calculations Methods. In this work, the plane-wave based density functional theory (DFT) calculations are performed with Perdew−Burke−Ernzerhof50 that implemented in the Vienna Ab initio Simulation Package (VASP).51,52 The interactions between ion cores and valence electrons were calculated with the projector augmented wave (PAW) method.53 A plane wave cutoff of 400 eV was used in the calculation, and the Brillouin zone of the

279.0 419.4 361.1 623.8 385.3 201.4 2.2 288.9 153.4 34.5 257.1 76.6

hcp via O hcp via C hcp via O hcp via C hcp via C hcp via C upon the surface hcp via C fcc via C and O top via O hcp via α-Ca and fcc via β-C fcc via α-C and top via β-C bri upon the surface hcp via α-C and fcc via β-C bri via α-C and top via β-C bri via α-C and O bri via α-C and O, fcc via β-C hcp via α-C and O, bri via β-C hcp via α-C and β-C

C2H6 CHCO

14.1 329.2

CH2CO

103.4

CH3CO CHCHO

196.3 362.7

CH2CHO

225.7

CH2CH

270.1

CH3CH CH3CH2 CHC

354.2 145.9 532.3

CH2C

402.7

hcp via α-C fcc via α-C hcp via α-C and fcc via β-C hcp via α-C and β-C

CH3C

552.8

hcp via α-C

a

B

configurations hcp via C hcp hcp fcc hcp hcp via C and O

CH3O COH OH CH CH2 CH3 CH4 CHOH CH2OH CH3OH C2H2

Figure 1. Co(111) surface morphology and its adsorption sites: top, bri, hcp, and fcc refer to top, bridge, fcc, and hcp sites. The balls in blue, green, and pink represents the cobalt atoms in the first, second, and the third layer, respectively.

Eads 174.1 680.4 585.9 268.5 266.1 216.7

key parameter Co−C: 1.970, 1.975, 1.976 Co−C: 1.775, 1.777, 1.777 Co−O: 1.865, 1.865, 1.867 Co−H: 1.748, 1.749, 1.749 Co−H: 1.751, 1.751, 1.751 Co−C: 1.860, 2.044; Co−O: 2.029 2.068 Co−C: 2.059, 2.073; Co−O: 2.018, 2.033 Co−O: 2.002, 2.002, 2.002 Co−C: 1.893, 1.893, 1.862 Co−O: 2.001, 2.001, 2.002 Co−C: 1.862, 1.863, 1.863 Co−C: 1.944, 1.947, 1.965 Co−C: 2.152, 2.153, 2.158 Co−H: 3.092 1.951, 1.969, 1.972 Co−C: 2.052, 2.168; Co−O: 2.165 Co−O: 2.206 Co−α-C: 1.992, 2.024, 2.028; Co−β-C: 1.996, 2.029, 2.029 Co−α-C: 2.000; Co−β-C: 2.085, 2.248, 2.248 Co−α-C: 4.176; Co−β-C: 4.128 Co−α-C: 2.045, 2.046; Co−β-C: 2.054, 2.056; Co−O: 2.137 Co−α-C: 1.923, 2.089; Co−β-C: 2.086; Co−O: Co−α-C: 2.227; Co−O: 2.228 Co−α-C: 2.069; Co−β-C: 1.945, 1.976, 1.984; Co−O: 1.968 Co−α-C: 2.018; Co−β-C: 2.096, 2.248; Co−O: 2.026, 2.054 Co−α-C: 1.933, 2.002, 2.013; Co−β-C: 2.098 Co−α-C: 1.961, 1.964, 1.976 Co−α-C: 2.126, 2.195, 2.210 Co−α-C: 2.063, 2.083, 2.084; Co−β-C: 1.841, 1.942, 1.945 Co−α-C: 1.883, 1.884, 1.917; Co−β-C: 2.192 Co−α-C: 1.880, 1.880, 1.881

α-C denotes the C atom linked with functional groups. DOI: 10.1021/acs.jpcc.5b09634 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 2. Species involved in C2 hydrocarbons formation during FT reaction from syngas on Co(111) surface, gray, red, white, and blue spheres represent C, O, H, and Co atom, respectively.

46.6 and 101.2 kJ·mol−1 lower than that for CO direct dissociation. In addition, both reactions are endothermic; the reaction energies are 88.1 and 114.4 kJ·mol−1, respectively. It clearly shows that CO activation via COH and CHO intermediates is more plausible than direct dissociation into C + O because the higher activation barrier is required for the direct C−O bond scission. Meanwhile, CO hydrogenation to CHO is more favorable than COH, thus H-assisted CO dissociation by CHO intermediate is a dominant CO activation pathway on the Co(111) surface, which is in accordance with the previous studies on fcc-Co.52,59 3.3. The CHx (x = 1−3) Formation. As the dominant product for CO activation, CHO dissociation, and hydrogenation are further investigated, the pathway for CHx (x = 1−3) formation, as well as the geometries of the IS, TS, and FS are shown in Figure 4. The C−O bond scission of CHO into CH and O (R4, TS4) is highly exothermic by 63.1 kJ·mol−1, and the activation barrier is 62.9 kJ·mol−1. In TS4, C−O bond of CHO is broken, CH adsorbs at the bridge site and O adsorbs at the hcp site with the C−O bond length of 1.801 Å. The H adatom moves to O atom of CHO to form CHOH (R5, TS5, Ea = 104.2 kJ·mol−1), followed by dissociation into CH and OH (R6, TS6, Ea = 71.3 kJ·mol−1). Alternatively, the addition of H atom will approach the C atom of CHO to form CH2O intermediate

Here Emol/slab is the total energy of the substrate slab with adsorbed structure; Eslab and Emol represents the energy of clean metal surface and adsorbate in the gas phase, respectively. With this definition, the positive value Eads denotes the stable adsorbate structure. In this section, the adsorption energy and the corresponding key geometric parameters of all possible reactants, intermediates, and products involved in the possible pathways of C2 hydrocarbons formation from syngas on the Co(111) surface are shown in Table 1. The most stable adsorption configurations of these species on Co(111) surface obtained from our DFT calculations are shown in Figure 2. 3.2. The Dissociation and Hydrogenation of CO. For the CO activation, the direct dissociation and hydrogenation of CO have been taken into consideration, and the pathway for CO activation as well as the geometries of the initial states (IS), transition states (TS), and final states (FS) are shown in Figure 3. As can be seen in Figure 3, the direct dissociation of CO into C and O (R1, TS1) proceeds with an activation barrier of 231.4 kJ·mol−1, and it is endothermic (ΔH = 89.2 kJ·mol−1). For CO hydrogenation, there are two alternative products, one is COH and the other is CHO. CO hydrogenation to COH (R2, TS2) and CHO (R3, TS3) requires overcoming the activation barriers of 184.8 and 130.2 kJ·mol−1, which are C


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Figure 3. Potential energy diagram for CO dissociation and hydrogenation in together with the structures of initial states, transition states, and final states. Bond lengths are in angstroms. See Figure 2 for color coding.

(R7, TS7), which is slightly endothermic (ΔH = 13.7 kJ·mol−1) with the activation barrier of 51.9 kJ·mol−1. Therefore, CH2O production via CHO hydrogenation is kinetically favorable than CHOH formation and direct dissociation into CH+O. Once CH2O is formed, CH2O hydrogenation and dissociation will be further discussed. The C−O bond breaking of CH2O (R8, TS8) is exothermic (ΔH = −54.8 kJ·mol−1) with a modestly barrier of 65.3 kJ·mol−1. CH2O is hydrogenated to form CH2OH (R9, TS9) with an activation barrier and reaction energy of 88.9 and 31.8 kJ·mol−1. Subsequently, CH2OH dissociates into CH2 and OH (R10, TS10), which need to overcome an activation barrier of 54.6 kJ·mol−1. In addition, CH2O can form CH3O (R11, TS11) via adding a H atom to C of CH2O, and the reaction is exothermic (ΔH = −48.3 kJ·mol−1) with the barrier of 46.6 kJ·mol−1. Again, CH3O formation is more feasible than either CH2OH formation or direct dissociation into CH2 and O. Hence, CH3O dissociation and hydrogenation will be further taken into consideration. Finally, upon the C−O bond cleavage of the CH3O intermediate where CH3 is formed (R12, TS12), the reaction is exothermic (ΔH = −25.8 kJ·mol−1), which corresponds to an activation barrier of 142.9 kJ·mol−1. When the H atom approaches the O atom of CH3O to form CH3OH (R13, TS13), the reaction is endothermic (ΔH = 63.9 kJ·mol−1) with the relatively high barrier of 148.6 kJ·mol−1. As shown in Figure 4, summarizing our calculated activation barriers, it is concluded that CH, CH2, and CH3 are mainly

formed via direct dissociation of CHxO (x = 1, 2, 3) (R4, R8, R12), the corresponding effective barriers for CH, CH2, and CH3 formation are 62.9, 79.0, and 108.3 kJ·mol−1, respectively. It clearly can be seen that CH is the dominant monomer that is responsible for C−C growth. In addition, CO + 3H → CHO + 2H → CH2O + H → CH3O (R3, R7, R11) is the plausible pathway for CO hydrogenation, and CH3OH is formed via CH3O + H → CH3OH (R13), the effective barrier for CH3OH formation is 114.0 kJ·mol−1, which is unfavorable compared with CHx (x = 1−3) formation. 3.4. CH4 Formation. As shown in Figure 4, CH is initially formed via H assisted CO dissociation then it can undergo sequential hydrogenation to form CH4. The activation barrier and reaction energy of hydrogenation of adsorbed CH to CH4 have been listed in Table 2. The pathway for CH4 formation together with the geometries of IS, TS, and FS are shown in Figure 5. As CH is the dominant monomer responsible for C−C chain formation, CHx (x = 1−3), CH dissociation (R14, TS14), and hydrogenation (R15, TS15) are first considered. Both reactions are endothermic by 27.6 and 36.0 kJ·mol−1 correspond to the activation barrier of 101.8 and 55.9 kJ·mol−1, respectively. Since the activation barrier of CH hydrogenation (55.9 kJ·mol−1) is much lower than the dissociation barrier (101.8 kJ·mol−1), CH hydrogenation instead of dissociation should be favored. Subsequently, CH2 undergoes to successive hydrogenation D


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Figure 4. Potential energy diagram for CHx (x = 1−3) and CH3OH formation together with the structures of initial states, transition states, and final states. Bond lengths are in angstroms. See Figure 2 for color coding.

to form CH4. CH2 + H → CH3 (R16, TS16, Ea = 54.2 kJ·mol−1, ΔH = −11.5 kJ·mol−1) and CH3 + H → CH4 (R17, TS17, Ea = 91.7 kJ·mol−1, ΔH = 4.7 kJ·mol−1). The results indicate that CH2 and CH3 can be easily formed via the hydrogenation of CH and CH2. Further hydrogenation to CH4 is more difficult because of high barrier, which suggests that the last hydrogenation step (CH3 + H → CH4) is the rate-determining step for CH4 formation. In summary, CH4 is formed via CH + 3H → CH2 + 2H → CH3 + H → CH4, the effective barrier for CH4 formation is

116.2 kJ·mol−1. Besides, CH comes from direct CHO dissociation, and CH2 as well as CH3 come from CH hydrogenation. As a result, C−C chain formation starting with CHx (x = 1−3) will be taken into consideration in the following section. 3.5. C−C Chain Formation via C(H)O Insertion into CHx (x = 1−3). 3.5.1. C(H)O Insertion into CHx (x = 1−3). In order to study the feasibility of the C(H)O−insertion mechanism, five different reactions (R18−R22) between CHx (x = 1−3) and C(H)O were considered for C−C chain formation. E


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66.9, and 132.6 kJ·mol−1, they are all endothermic by 58.9, 60.8, and 46.1 kJ·mol−1, respectively. Comparing these three reactions, CO insertion into CH2 is kinetically favorable than CO insertion into CH and CH3. An alternative carbonyl-containing species for insertion is CHO. For CHO insertion into CH (R21, TS21), the reaction is endothermic (ΔH = 19.3 kJ·mol−1), and the activation barrier is 39.4 kJ·mol−1. Another product CH2CHO formation via CHO insertion (R22, TS22) is slightly endothermic by 0.1 kJ·mol−1, and the activation barrier is rather low (Ea = 1.7 kJ·mol−1). Therefore, CHO insertion into CH2 is more preferable in kinetics and thermodynamics. The results demonstrate that CHO insertion into CHx (x = 1, 2) is more preferable than CO insertion, and CHO insertion into CH2 is the most favorable among all the C(H)O insertion into CHx (x = 1−3). 3.5.2. CHxC(H)O Hydrogenation. After CO/CHO insertion, the resulting CHxC(H)O intermediates can be hydrogenated to form a CHxCHyO species. For each CHxCO intermediate, the hydrogenation of either α-C or β-C atom has been considered. Five reactions (R23−R27) involved are displayed in Table 2, and the corresponding geometries of IS, TS, and FS are shown in Figure 7. Starting with CHCO, there are two possible hydrogenation products; one is CH2CO (R23, TS23), which is produced via adding an H atom to β-C, and it is endothermic (ΔH = 32.9 kJ·mol−1) with an activation barrier of 61.2 kJ·mol−1. The other hydrogenation product is CHCHO (R24, TS24), which is formed via adding an H atom to α-C. This elementary reaction has to overcome a relatively high activation barrier of 95.7 kJ·mol−1, and it is an endothermic reaction (ΔH = 68.3 kJ·mol−1). Starting with CH2CO, there are two possible hydrogenation products, CH2CHO (R25, TS25) and CH3CO (R26, TS26). The activation barriers for CH2CHO and CH3CO formation are 31.4 and 33.2 kJ·mol−1, and the corresponding reaction energies are −37.3 and −33.2 kJ·mol−1. In addition, CH2CHO formation via CHCHO hydrogenation (R27, TS27) is exothermic by 18.9 kJ·mol−1 with the activation barrier of 88.3 kJ·mol−1. 3.5.3. CHxCHy-O Bond Scission. The C−O bond of the CHxCHyO surface intermediate must first cleave to terminate the propagation process. Five such elementary reaction steps (R28 ∼ R32) involved were considered. The reactions involved are shown in Table 2, and the corresponding geometries of IS, TS, and FS are given in Figure 8. The activation barriers for the C−O bond cleavage of CHxCO (x = 1−3) to form CHxC (x = 1−3) and O (R28, R29, R30) are 138.8, 83.6, and 63.0 kJ·mol−1, and these reaction energies are exothermic by 22.0, 48.8, and 65.4 kJ·mol−1, respectively. In addition, the C−O bond scission of CHCHO and CH2CHO were investigated. C−O bond scission of CHCHO (R31, TS31) is a highly exothermic process (ΔH = −127.4 kJ·mol−1), and the barrier is rather low (Ea = 13.0 kJ·mol−1). The C−O bond scission of CH2CHO (R32, TS32) requires a barrier of 109.4 kJ·mol−1, and it is a slightly exothermic process (ΔH = −17.3 kJ·mol−1). 3.5.4. CHxCHy Hydrogenation. After C−C chain formation, higher hydrocarbons may desorb from the surface as olefins and paraffins. Such chain termination steps for CHxCHy species have been investigated in this section. There are a lot of hydrogenation routes, which contain a large number of elementary reactions from CHC to C2H4 and C2H6. The possible reactions involved are shown in Table 2, and the geometries of IS, TS, and FS are shown in Figure 9. From CHC, two possible hydrogenation routes need to be considered. The addition of the H atom will approach the α-C of CHC to produce C2H2 (R33, TS33); this elementary reaction

Table 2. Activation Barrier and Reaction Energy of Elementary Reactions Involved in C2 Hydrocarbons Formation from Syngas on Co(111) Surface (R1) (R2) (R3) (R4) (R5) (R6) (R7) (R8) (R9) (R10) (R11) (R12) (R13) (R14) (R15) (R16) (R17) (R18) (R19) (R20) (R21) (R22) (R23) (R24) (R25) (R26) (R27) (R28) (R29) (R30) (R31) (R32) (R33) (R34) (R35) (R36) (R37) (R38) (R39) (R40) (R41) (R42) (R43) (R44) (R45) (R46) (R47) (R48) (R49) (R50) (R51)

reaction

Ea/kJ·mol−1

ΔH/kJ·mol−1

CO → C + O CO + H → COH CO + H → CHO CHO → CH + O CHO + H → CHOH CHOH → CH + OH CHO + H → CH2O CH2O → CH2 + O CH2O + H → CH2OH CH2OH → CH2 + OH CH2O + H → CH3O CH3O → CH3 + O CH3O + H → CH3OH CH → C + H CH + H → CH2 CH2 + H → CH3 CH3 + H → CH4 CH + CO → CHCO CH2 + CO → CH2CO CH3 + CO → CH3CO CH + CHO → CHCHO CH2 + CHO → CH2CHO CHCO + H → CH2CO CHCO + H → CHCHO CH2CO + H → CH2CHO CH2CO + H → CH3CO CHCHO + H → CH2CHO CHCO → CHC + O CH2CO → CH2C + O CH3CO → CH3C + O CHCHO → C2H2 + O CH2CHO → CH2CH + O CHC + H → CHCH CHC + H → CH2C CH2C + H → CH2CH CH2C + H → CH3C CH3C + H → CH3CH CHCH + H → CH2CH CH2CH + H → C2H4 CH2CH + H → CH3CH CH3CH + H → CH3CH2 CH3CH2 + H → C2H6 CH + CH → C2H2 CH2 + CH → CH2CH CH2 + CH2 → C2H4 CH3 + CH → CH3CH CH3 + CH2 → CH3CH2 CH3 + CH3 → C2H6 O + H → OH OH + H → H2O OH + OH → H2O + O

231.4 184.8 130.2 62.9 104.2 71.3 51.9 65.3 88.9 54.6 46.6 142.9 148.6 101.8 55.9 54.2 91.7 99.0 66.9 132.6 39.4 1.7 61.2 95.7 31.4 33.2 88.3 138.8 83.6 63.0 13.0 109.4 64.2 43.0 0 22.3 68.0 107.4 31.5 39.1 44.2 61.4 62.0 60.8 40.1 98.4 83.5 201.2 47.4 150.7 48.3

89.2 88.1 114.1 −63.1 25.1 −73.5 13.7 −54.8 31.8 −77.5 −48.3 −25.8 63.9 27.6 36.0 −11.5 −4.7 58.9 60.8 46.1 19.3 0.1 32.9 68.3 −37.3 −33.2 −18.9 −22.0 −48.8 −65.4 −127.4 −17.3 −26.5 −15.7 −0.1 −46.0 58.7 68.2 −11.6 −4.2 1.7 −32.4 −53.4 −15.7 −53.0 −2.6 −23.7 −31.8 16.6 60.0 4.7

The activation barrier and reaction energy of C(H)O insertion into CHx (x = 1−3) are shown in Table 2, and the geometries of IS, TS, and FS are given in Figure 6. For CO insertion into CH (R18, TS18), CH2 (R19, TS19), and CH3 (R20, TS20), the initial states for CO and CHx (x = 1−3) are similar, both CO and CHx (x = 1−3) adsorb at the hcp site. The activation barriers for these reactions are 99.0, F


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Figure 5. Potential energy diagram for CH4 formation in together with the structures of initial states, transition states, and final states. Bond lengths are in angstroms. See Figure 2 for color coding.

is exothermic by 26.5 kJ·mol−1 with an activation barrier of 64.2 kJ·mol−1. The other is forming CH2C via adding an H atom to β-C (R34, TS34); this elementary reaction is exothermic by 15.7 kJ·mol−1 with an activation barrier of 43.0 kJ·mol−1. From CH2C, two possible hydrogenation routes should also be considered. CH2CH formation (R35) via adding an H atom to α-C is a spontaneous reaction, and the corresponding reaction energy is −0.1 kJ·mol−1. The other is forming CH3C (R36, TS35) by adding an H atom to β-C, this elementary reaction is exothermic by 46.0 kJ·mol−1 with an activation barrier of 22.3 kJ·mol−1. CH3CH is the sole hydrogenation product for CH3C (R37, TS36), it proceeds with an activation barrier of 68.0 kJ·mol−1, and the corresponding reaction energy is 58.7 kJ·mol−1. Starting with CHCH, a possible hydrogenation product CH2CH (R38, TS37) is formed with a barrier of 107.4 kJ·mol−1 and an endothermicity of 68.2 kJ·mol−1. Then, CH2CH is further hydrogenated, and the possible products are C2H4 (R39, TS38) and CH3CH (R40, TS39), respectively. For these two products formation, we obtain the barriers of 31.5 and 39.1 kJ·mol−1, and the reaction energies are −11.6 and −4.2 kJ·mol−1, respectively.

Finally, C2H6 is formed by CH3CH successive hydrogenation at the α-C site. For CH3CH hydrogenation to form CH3CH2 (R41, TS40), it is slightly endothermic by 1.7 kJ·mol−1 and requires a barrier of 44.2 kJ·mol−1. The further hydrogenation of CH3CH2 to C2H6 (R42, TS41) has a barrier of 61.4 kJ·mol−1 and it is modestly exothermic (ΔH = −32.4 kJ·mol−1). 3.6. C−C Chain Formation via CHx (x = 1−3) Coupling with CHy (y = 1−3). CHx−CHy coupling reaction is also the alternative pathway for C−C chain formation; six reactions (R43−R48) between CHx (x = 1−3) and CHy (y = 1−3) species have been considered, which are shown in Table 2. The corresponding geometries of initial, transition, and final states are shown in Figure 10. Starting with CH, CH coupling with CH (R43, TS42) was considered. The activation barrier for CH−CH coupling is 62.0 kJ·mol−1, and it is exothermic by 53.4 kJ·mol−1. Starting with CH2, CH2 coupling with CH and CH2 have been taken into consideration. The activation barriers for CH2 coupling with CH (R44, TS43) and CH2 (R45, TS44) are 60.8 and 40.1 kJ·mol−1, and the corresponding reaction energies are −15.7 and −53.0 kJ·mol−1, respectively. Starting with CH3, CH3 coupling G


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Figure 6. Structures of initial states, transition states, and final states involved in C(H)O insertion into CHx (x = 1−3). Bond lengths are in angstroms. See Figure 2 for color coding.

Figure 7. Structures of initial states, transition states, and final states involved in CHxCHyO hydrogenation. Bond lengths are in angstroms. See Figure 2 for color coding.

taken into consideration. The corresponding geometries of IS, TS, and FS are shown in Figure 11. Starting with O, O is first hydrogenated via TS48 to OH (R49, TS48), and it is endothermic by 16.6 kJ·mol−1 with a moderate barrier of 47.4 kJ·mol−1. Then, the water could be formed via two possible pathways: one is via further hydrogenation of OH to H2O and the other is interacting with another OH to form H2O and O. For OH hydrogenation via TS49 to form H2O (R50, TS49), it proceeds with a rather high barrier of 150.7 kJ·mol−1, and it is endothermic with the reaction energy of 60.0 kJ·mol−1. Two OH interact to form H2O (R51, TS50) and O via TS50, it requires a relative lower barrier of 48.3 kJ·mol−1, and it is a slightly endothermic process (ΔH = 4.7 kJ·mol−1). Comparatively, the

with CH (R46, TS 45), CH2 (R47, TS46), and CH3 (R48, TS47) to form CH3CH, CH3CH2 and C2H6 have been taken into account. The activation barriers for CH3CH, CH3CH2, and C2H6 formation are 98.4, 83.5, and 201.2 kJ·mol−1, and they are all exothermic by 2.6, 23.7, and 31.8 kJ·mol−1, respectively. It can be found that CH2 coupling with CH2 is the most feasible among all the CHx−CHy coupling reactions. However, CH3 coupling with CH3 is the most unfavorable, which agrees well with early extended Hückel method results proposed by Zheng et al.60 3.7. H2O Formation. In the FTS reaction, the surface oxygen is an important intermediate, because it could result in poisoning of the catalyst, meanwhile the O removal via hydrogenation (H2O formation) is an effective pathway and therefore it was H


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Figure 8. Structures of initial states, transition states, and final states involved in CHxCHy−O scission reaction. Bond lengths are in angstroms. See Figure 2 for color coding.

Figure 9. Structures of initial states, transition states, and final states involved in CHxCHy hydrogenation reaction. Bond lengths are in angstroms. See Figure 2 for color coding. I


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Figure 10. Structures of partial initial states, transition states, and final states involved in CHx (x = 1−3) coupling with CHy (y = 1−3). Bond lengths are in angstroms. See Figure 2 for color coding.

Figure 11. Structures of initial states, transition states, and final states involved in H2O formation. Bond lengths are in angstroms. See Figure 2 for color coding.

formed via CH + CHO → CHCHO → C2H2 + O, then, C2H2 + 2H → CH2CH + H → C2H4; C2H6 is formed via CH + CHO → CHCHO → C2H2 + O, then, C2H2+4H → CH2CH+3H → CH3CH+2H → CH3CH2+H → C2H6, the effective barrier for C2H4 and C2H6 formation are the same (39.4 kJ·mol−1). In CHx (x = 1−3) coupling with CHy pathways, five pathways for C2H6 formation and three pathways for C2H4 formation have been taken into consideration, which are displayed in Table S3 and Table S4 in Supporting Information. Among these pathways, CH coupling with CH and further hydrogenation is the most plausible pathway for C2H6 and C2H4 formation, the corresponding potential energy diagram is shown in Figure 13. As shown in Figure 13, in C−C chain formation via CH coupling with CH,

pathway via two OH to form H2O and O is more favorable both kinetically and thermodynamically. 3.8. The Comparison between C(H)O−CHx Insertion and CHx−CHy Coupling. On the basis of the investigation above, 17 possible pathways for C2H6 formation and 11 possible pathways for C2H4 formation have been taken into account in the C(HO) insertion into CHx (x = 1−3) pathway, which are displayed in Table S1 and Table S2 in Supporting Information. Among these pathways, CHO insertion into CH, followed by C−O bond scission and hydrogenation to form C2 hydrocarbons is the most favorable pathway to generate C2H6 and C2H4, the corresponding potential energy diagram is shown in Figure 12. In C−C chain formation via CHO insertion into CH, C2H4 is J


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Figure 12. Potential energy diagram for the most favorable pathway of C2H4 and C2H6 formation in CHO insertion into CHx (x = 1−3) pathway.

Figure 13. Potential energy diagram for the most favorable pathway of C2H4 and C2H6 formation in CHx (x = 1−3) coupling with CHy (y = 1−3) pathway.

C2H4 is formed via CH + CH → C2H2, then, C2H2 + 2H → CH2CH + H → C2H4; C2H6 is formed via CH + CH → C2H2, then, C2H2 + 4H → CH2CH + 3H → CH3CH + 2H → CH3CH2 + H → C2H6. The effective barrier for C2H4 and C2H6 formation are 62.0 and 73.7 kJ·mol−1, respectively. On the basis of the results above, it can be found that C2H4 and C2H6 formation via CHO insertion into CH is more favorable than CH coupling with CH pathway. 3.9. Brief Summary of C2 Hydrocarbons Formation on Co(111) Surface. As presented in Figure 14, 48 elementary reactions explicitly illustrate the initiation, growth and termination steps involved in the C2 hydrocarbons formation. Meanwhile, the optimal reaction pathway for C2 hydrocarbons formation is obtained, as shown in Figure 15. As depicted in Figure 15, it can be found that C2H4 and C2H6 formation from syngas on the flat Co(111) surface primarily proceeds via the pathway of CO + H → CHO → CH + O to produce CH; subsequently, CHO insertion into CH can form CHCHO then, followed by C−O scission of CHCHO, further hydrogenation to form C2H4 and C2H6. Among these steps, CHO formation via CO hydrogenation (Ea = 130.2 kJ·mol−1, ΔH = 114.1 kJ·mol−1) is the rate-determining step for the whole

conversion. Second, CHCH hydrogenation to form CH2CH is also a rate-determining step for C2 hydrocarbons formation (Ea = 107.4 kJ·mol−1, ΔH = 68.2 kJ·mol−1). Besides, CH4 is formed via the process of CH + 3H → CH2 + 2H → CH3 + H → CH4, and the last hydrogenation step CH3 + H → CH4 is the rate-determining step for CH4 formation (Ea = 91.7 kJ·mol−1, ΔH = −4.7 kJ·mol−1). According to the results obtained, it can be found that the optimal pathway of C2H4 and C2H6 formation from syngas proceeds via CHO, CHCHO, CHCH, and CH2CH intermediates. Meanwhile, CO hydrogenation to form CHO is the ratedetermining step for the whole reaction which leads to the C2 hydrocarbons formation. Meanwhile, CH2CH formation via CHCH hydrogenation and side product (CH4) formation are the competitive reactions which affect the selectivity for C2 hydrocarbons formation; in other words, more CH2CH formation and/or less CH4 formation would lead to more C2 hydrocarbons. Hence, on the basis of our calculations on fcc-Co catalyst we can predict that to achieve high productivity and selectivity for C2H4 and C2H6 formation, CHO formation, and CH2CH formation should be facilitated, and/or CH4 formation should be minimized. K


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Figure 14. Possible elementary reactions for C2 hydrocarbons formation involved in the FT synthesis on the Co(111) surface.

Figure 15. Optimal pathway for C2 hydrocarbons formation on the Co(111) surface.

three rows from the top layer of a p(4 × 6) Co(111) slab, which is shown in Figure S1 of Supporting Information. The stable configurations of all possible reactants, intermediates, and products involved in CO hydrogenation, CHCH hydrogenation, and CH4 formation, as well as the corresponding elementary reactions on stepped-Co(111) surface have been presented in Figure S2 and Table S6 in Supporting Information. 3.10.1. The Comparison of Key Steps for C2 Hydrocarbons Formation between Flat and Stepped-Co(111) Surface. As shown in Table 3, the activation barriers of CHO (117.8 kJ·mol−1) and CH2CH (95.7 kJ·mol−1) formation on the stepped-Co(111) surface are decreased by 12.4 and 11.7 kJ·mol−1 compared with those on the flat-Co(111) surface (130.2 and 107.4 kJ·mol−1); On the other hand, the activation barrier of CH4 formation increased

3.10. The Role of Defects in C 2 Hydrocarbons Formation. It has been reported that the cobalt surface can restructure and form defects under the syngas (CO + H2) atmosphere, such as steps.61 Meanwhile, Step sites exhibit a better catalytic activity to CO activation on the metal catalysts, which has been attracted considerable attention in experimental and theoretical studies.31,62−66 Liu and Hu64 showed that surface defects such as steps and kinks can largely facilitate bond breaking with DFT calculations. Gong et al.65 demonstrated that most of the surface intermediates favored to adsorb at the step sites and the steps could play an essential role on the behavior of CO dissociation and water formation in FTS process. Thus, to clarify the role of defect role in C2 hydrocarbons formation, a (4 × 6)-stepped-Co(111) surface was modeled by removing L


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The Journal of Physical Chemistry C Table 3. Activation Barrier (kJ·mol−1) and Reaction Energy of Rate-Determining Steps for C2 Hydrocarbons Formation on Flat and Stepped-Co(111) Surface flat surface

activity for CHO, CH2CH, and CH4 formation with the increase of temperature. Meanwhile, at the same temperature, CHO and CH2CH formation rates on the stepped-Co(111) surface are faster than those on the flat-Co(111) surface, while the CH4 formation rate on the stepped-Co(111) surface is slowed down dramatically, which suggests that the step sites can facilitate the formation of CHO and CH2CH, as well as suppress the formation of CH4 at the same temperature. The results of rate constant are in line with our kinetic results.

stepped surface

reaction

Ea

ΔH

Ea

ΔH

CO + H → CHO CHCH + H → CH2CH CH3 + H → CH4

130.2 107.4 91.7

114.1 68.2 −4.7

117.8 95.7 97.6

77.1 32.5 29.7

by 5.9 kJ·mol−1 on the stepped-Co(111) surface in comparison with that on the flat-Co(111) surface (97.6 vs 91.7), demonstrating that the step sites facilitate the formation of CHO and CH2CH, as well as suppress the formation of CH4, thus the productivity and selectivity of C2 hydrocarbons could be promoted. 3.10.2. The Effect of Temperature on the C2 Hydrocarbons Formation. In this section, the rate constant of the rate-determining step for C2 hydrocarbons formation on the flat and steppedCo(111) surface were determined with the harmonic transition state theory (TST),67−69 aiming at probing into the role of temperature during the FTS process. k=

4. GENERAL DISCUSSION A full comparison between the proposed mechanism and those suggested earlier would require a further discussion. The first point concerns the CO activation. On the flatCo(111) surface, the hydrogenation may be a preferred path for CO activation, which is consistent with the experimental work70−72 and theoretical investigations.44,59 Yang group72 has used experimental and DFT methods to investigate the kinetic isotopic effects in clarifying the reaction mechanism and confirmed CO activation via H-assisted by combining the integrated transient and steady-state kinetic modeling. In addition, it was reported that the rate of dissociating CO increases on exposure to hydrogen when the cobalt particle size is at the range of 4−15 nm.71 The first-principles kinetic studies are employed to identify the structure-sensitivity of the CO dissociation in FTS by Liu and co-workers.44 Co (11−21), Co (10−11), Co(10−12), and Co (11−20) were chosen to represent hcp Co, whereas Co (100), Co (311), and Co (110) represent fcc Co on the basis of Wulff construction; the results indicated that hcp Co is more active than fcc Co, and the direct dissociation is more favorable on hcp Co whereas the hydrogen-assisted pathway is more preferable for fcc Co. Interestingly, in the experimental and theoretical work of Wintterlin73 and Weststrate,74,75 they both insist that CO dissociation on the defect sites (e.g.monatomic step edges) of Co(0001) surface proceeds readily. In order for carbon build-up to proceed, the active sites have to be regenerated, namely, carbon and oxygen have to be removed.75 O removal via CO2 formation is difficult because of high barrier needs to be overcome, and a significant concentration of COad is needed. Indeed, the CO2 selectivity in Co-catalyzed FTS is low thus oxygen is removed primarily as water. In our study, on flatCo(111) surface, 231.4 and 130.2 kJ·mol−1 are required for CO direct dissociation and CHO formation via CO hydrogenation, demonstrating that CO activation dominantly via CHO intermediate rather than direct dissociation. Namely, O is not mainly formed via direct CO dissociation. But, there is no doubt that O is the important intermediate during the FTS process, and it is mainly produced by C−O bond scission of CHxO and CHxCHyO intermediates in our work, and O removal proceeds via hydrogenation leading to H2O formation. The second point concerns the CHx species inserted. On the flat Co(111) surface, CH is considered as the most favorable monomer for C−C chain formation, which is consistent with the proposition suggested by Weststrate74 and finding on the Co(0001)76 surface. CH2 and CH3 formation via CH and CH2 hydrogenation are facile, while C formation via CH dissociation is difficult, namely, CH2 and CH3 are also possible for C−C chain formation. In addition, in the experimental work of Brady and Pettit16,17 CH2N2 decomposed into CH2 species and N2 on a range of transition metals without H2 and the main product was C2H4, indicating that the CH2 + CH2 coupling is indeed feasible on transition metals. Meanwhile, Maitlis et al. suggested that CH2 is inserting C1 species,77 while van Santen

⎛ E ⎞ kBT qTS exp⎜ − a ⎟ h qR ⎝ kBT ⎠

where kB is the Boltzmann constant; h is the Planck constant; T is the absolute temperature; ZPVE-corrected is also considered; qTS and qR are the vibrational partition functions for the TS and reactant of elementary reaction, respectively. For partition function, q, the vibrational degrees of freedom are only considered in the surface reaction, and they are calculated in the harmonic model. 1 q= hvi ⎤ Vibrations ⎡ ∏i = 1 ⎢⎣1 − exp − kBT ⎥⎦

( )

where vi is the vibrational frequency. It has been reported that the experimental temperature of FTS on Co catalysts is at the range of 200−240 °C, because higher temperature will lead to methane.8,14 As a result, the rate constants for these key steps have been calculated under 473 K, 493 and 513 K, respectively. The results are listed in Table 4. Table 4. Rate Constant k for the Rate-Determining Steps of C2H4 and C2H6 Formation on the Flat and Stepped-Co(111) Surface at the Different Temperatures rate constant k/s−1 elementary reaction flat-Co(111)

steppedCo(111)

473 K CO + H → CHO CHCH + H → CH2CH CH3 + H → CH4 CO + H → CHO CHCH + H → CH2CH CH3 + H → CH4

493 K

513 K

2.26 × 10−02 9.75 × 10−02 3.75 × 10−01 1.11 × 1001 3.69 × 1001 1.12 × 1002 2.07 × 1003 5.95 × 1003 5.51 × 10−01 2.08 × 1000 3.46 × 1002 1.02 × 1003

1.58 × 1004 7.11 × 1000 2.78 × 1003

4.67 × 1002

3.83 × 1003

1.40 × 1003

It is worthy to note that the rate constant of CHO, CH2CH, and CH4 formation increases with the increasing of temperature on both flat and stepped-Co(111) surfaces, indicating that both flat and stepped-Co(111) surfaces would exhibit a better catalytic M


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Second, for C−C chain formation, CHO insertion into CH is the most feasible among all the CHx−CHy (x = 1−3, y = 1−3) coupling and C(H)O insertion into CHx (x = 1−3) pathways. C2H4 is mainly formed via CHO + CH → CHCHO → CHCH + O, then, CHCH + 2H → CH2CH + H → C2H4; C2H6 is mainly formed via CHO + CH → CHCHO → CHCH + O, then, CHCH + 4H → CH2CH + 3H → CH3CH + 2H → CH3CH2 + H → C2H6. Third, in the optimal pathway of C2 hydrocarbons formation, CO hydrogenation to CHO is the rate-determining step for the whole reaction, and CHCH hydrogenation to form CH2CH is the rate-determining step for C2 hydrocarbons formation; in addition, the undesirable product CH4 formation via CH3 hydrogenation is the side reaction that would affect the selectivity to C2 hydrocarbons, which means that the productivity of C2 hydrocarbons would be improved if CHO and CH2CH formation could be promoted, and/or CH4 formation could be suppressed. Fourth, the presence of step sites, CHO and CH2CH formation are facilitated and CH4 formation is suppressed. Therefore, it can be concluded that defects (step site) could improve the catalytic activity and selectivity for C2 hydrocarbons formation.

et al. proposed that both CH and CH2 are supposed to be the inserting species.78 These studies suggest CH2 is also responsible for C−C chain initiation, and it agrees well with the results of our study. The third point concerns the C−C chain formation. On the flat Co(111) surface, 17 possible pathways for C2H6 formation and 11 possible pathways for C2H4 formation in the C(H)O insertion mechanism CHx (x = 1−3), as well as five possible pathways for C2H6 formation and three possible pathways for C2H4 formation in the CHx−CHy (x = 1−3, y = 1−3) coupling mechanism have been taken into account. The calculated results indicate that C2H6 and C2H4 formation via CHO insertion into CH, followed by C−O scission and hydrogenation has the lowest effective barrier (39.4 kJ·mol−1) is the most feasible among all of C(H)O insertion into CHx and CHx−CHy coupling reactions, namely, chain initiation proceeds via CHO insertion into CH. Meanwhile, the rate-determining step for this pathway is C2H2 + H → CH2CH, therefore, CH2CH is considered as the important intermediate for C2 hydrocarbons formation in our work. In the previous study of Zhao et al.,40 CHO insertion into CHx (x = 1−3) exhibits superior or comparative activity to CO insertion, which is consistent with our results. In the work of Weststrate,74 CH was proposed as the dominant C1 species, and the chain initiation undergoes via coupling with two CH species, namely, C2H2 is the primary product of coupling (−30 °C, 57 kJ·mol−1). As the high adsorption energy of C2H2 (∼230 kJ·mol−1), the desorption of C2H2 is rather difficult, meanwhile, acetylenic compounds are very reactive, therefore, after coupling a net addition of one H is required to produce CH3C, the most stable candidates. Therefore, the same route (CH + CH + 4H → C2H2 + 4H → CH3C + 3H → CH3CH + 2H → CH3CH2 + H → C2H6) on the Co(111) surface is compared with the optimal route (CHO insertion route) for C2 hydrocarbon formation, the effective barrier for CH3C route is 136.7 kJ·mol−1, while the effective barrier for CHO route is 39.3 kJ·mol−1, indicating that CHO route is more favorable pathway for C2 hydrocarbons formation on the flat-Co(111) surface. In addition, monatomic step is the most abundant “defect”,74 therefore, the stepped-Co(111) has been modeled to investigate the role of defects for the key-steps during the C2 hydrocarbons formation. In comparison with those on the flat-Co(111) surface, CHO and CH2CH formation are facilitated, while CH4 formation is minimized on the stepped-Co(111) surface, indicating that the step sites could increase the productivity and selectivity to C2 hydrocarbons. The better understanding of the role of defects (step sites) on the catalytic performance at the atomic level can potentially provide some clues for designing the more excellent Co-based catalysts for hydrocarbons formation. Except for defects, other structure modified Co-based will be investigated in our following work, aiming at improving the activity and selectivity to C2 hydrocarbons formation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09634. The description of all possible pathways for C2H6 and C2H4 formation via C(H)O insertion into CHx (x = 1−3) and CHx (x = 1−3) coupling with CHy (y = 1−3) on the flat-Co(111) surface, the model of stepped Co(111) surface, and the corresponding stable configurations of possible intermediates as well as the description of key steps for C2 hydrocarbons formation on the steppedCo(111) surface have been presented in detail. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-351-4121793. Fax: +86-351-4041153. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the National Natural Science Foundation of China (Nos. 21273265, 21203232, 21503252, and 21303241).The authors are grateful to Lvliang’s cloud computing center for high-performance computing for CPU.

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REFERENCES

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5. CONCLUSION In this work, the activation barrier and reaction energy of possible elementary reactions for C2 hydrocarbons formation have been systematically investigated, and the following conclusions are drawn. First, on the flat-Co(111) surface the formation of CHxO as intermediates is more preferred both kinetically and thermodynamically to direct CO dissociation. Meanwhile, CH, which comes from CHO direct dissociation (CHO → CH+O), is the dominant monomer among all the CHx (x = 1−3). N


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