Mechanisms of H- and OH-assisted CO activation as well as C-C coupling on the flat Co(0001) surface Revisited

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1 Electronic Supplementary Material (ESI) for Catalysis Science & Technology. This journal is The Royal Society of Chemistry 2016 Mechanisms of H- and OH-assisted CO activation as well as C-C coupling on the flat Co(0001) surface Revisited Shaoli Liu, a,b,c Yong-Wang Li, a,b* Jianguo Wang a and Haijun Jiao a,d* a) State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, , China; b) National Energy Center for Coal to Liquids, Synfuels China Co., Ltd, Huairou District, Beijing, , China; c) University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing, , PR China; d) Leibniz-Institut für Katalyse e.v. an der Universität Rostock, Albert-Einstein Strasse 29a, Rostock, Germany. ywl@sxicc.ac.cn; haijun.jiao@catalysis.de Table of content Micro-kinetics Page S3 Table S1. Most stable adsorption site, adsorption energy (E ads, ev), and relative bond distances (d Co-X, Å) of the surface species Page S6 Table S2. The adsorption energy (E ads, ev) and the relative bond distances (d Co-X, Å) of the surface species, as well as the reaction energy (E r, ev) and the barrier energy (E a, ev) in the diffusion reaction of the surface species Page S7 Table S3. Computed adsorption energies (E ads, ev), and the relevant bond distances (d, Å) of the IS, TS and FS for the reactions via H-assisted pathway Page S8 Table S4. Computed adsorption energies (E ads, ev), and the relevant bond distances (d, Å) of the IS, TS and FS for the reactions via OH-assisted pathway Page S9 Table S5. Energy barrier E a (ev) and reaction energy E r (ev) as well as the rate constant k (483K) of CO hydrogenation in the H- assisted pathway on the Co(0001) surface Page S10 Table S6: Barriers E a (ev) and reaction energies E r (ev) as well as the rate constant k (483K) of CO activation via OH-assisted pathway Page S11 S1

2 List of Figures Figure S1. Top (a) and side (b) reviews as well as possible adsorption sites of the hcp Co(0001) surface Page S12 Figure S2. Top and side views of the relative surface species on the Co(0001) surface Page S13 Figure S3. The geometries of the initial state and the final state as well as the transition state of the surface species in the diffusion reaction Page S14 Figure S4. The geometries of the initial state and the final state as well as the transition state for the reactions starting from CO+H in the H-assisted pathway Page S15 Figure S5. The geometries of the initial state and the final state as well as the transition state for the reactions starting from CH 2 O in the H-assisted pathway Page S16 Figure S6. The geometries of the initial state and the final state as well as the transition state in the reaction of the coupling of CH x (x=1-3) with H or CO Page S17 Figure S7. The geometries of the initial state and the final state as well as the transition state for the reactions starting from CO+OH in the OH-assisted pathway Page S18 Figure S8. The geometries of the initial state and the final state as well as the transition state for the reactions starting from COH in the OH-assisted pathway Page S19 S2

3 Micro-kinetics (a) To distinguish the selectivity of CH 3 O hydrogenation and dissociation, we computed the rate constant (k) on the basis of the transition state theory. 1,2 The rate constant of each reaction is calculated according to equation (1), where k B is the Boltzmann constant, T denotes the reaction temperature, h is the Planck constant, E a stands for the activation barrier of each reaction, and q IS,vib and q TS,vib are the harmonic vibrational partition functions for the initial state and the transition state, respectively; i.e., q vib is calculated on the basis of equation (2), where i is the vibrational frequency of each vibrational mode of the adsorbed intermediate derived from our DFT calculations. The computed energy barriers and reaction energy as well as rate constants are listed in the Supporting Information. k kbt h q q TS, vib IS, vib e Ea kbt (1) q vib i 1 hv i 1 exp( ) k T B (2) (b) The rate equation of the related competitive reactions In order to investigate the competition between hydrogenation and dissociation, we consider the following rate equations of the elementary steps involved on the potential energy surface. CO + S = C + O Eq. 1 CO + H = CHO + S Eq. 2 CO + H = COH + S Eq. 3 CHO + S = CH + O Eq. 4 CHO + H = CH 2 O + S Eq. 5 CHO + H = CHOH + S Eq. 6 CH 2 O + S = CH 2 + O Eq. 7 CH 2 O + H = CH 3 O + S Eq. 8 CH 2 O + H = CH 2 OH + S Eq. 9 CH 3 O + S = CH 3 + O Eq. 10 CH 3 O + H = CH 3 OH + S Eq. 11 The rate equation of each reaction can be expressed as equations in the following: r 1 k 1 [ (3) r 2 2 [ ][ (4) r 3 3 [ ][ (5) r 4 4 [ (6) r 5 5 [ ][ (7) r 6 6 [ ][ (8) r k [ S][ CH O] (9) r k [ H ][ CH O] r k [ H ][ CH O] r k [ S][ CH O] r k [ H ][ CH O] (10) (11) (12) (13) Under hydrogen rich conditions, the surface active sites can be occupied by hydrogen atoms; and these surface hydrogen atoms S3

4 can be used for the hydrogenation reactions; on the other hand, there will be no free sites available to accept the O atom from the dissociation reactions. The availability of surface hydrogen atoms will accelerate all hydrogenation steps and at the same time suppress the dissociation steps. (c) The rate equation of CH 3 OH and CH 4 formation In the process of the formation of CH 4 and CH 3 OH on the Co(0001) surface, CO CHO CH 2 O CH 3 O is an optimal pathway. At first, H 2 molecule adsorbs dissociative on the surface (Eq. 12); and CO molecule adsorbs on the surface (Eq. 13); and the subsequent hydrogenation of CO to form CH 3 O (Eq. 2, 5 and 8), which is further hydrogenated to CH 3 OH (Eq. 11); finally the surface CH 3 OH desorbs (Eq. 14) from the surface. H 2(g) + 2S = 2H Eq. 12 CO (g) + S = CO s Eq. 13 CO + H = CHO + S Eq. 2 CHO + H = CH 2 O + S Eq. 5 CH 2 O + H = CH 3 O + S Eq. 8 CH 3 O + S = CH 3 + O Eq. 10 CH 3 O + H = CH 3 OH + S Eq. 11 CH 3 OH = CH 3 OH (g) + S Eq. 14 CH 3 + H = CH 4 + S Eq. 15 CH 4 = CH 4(g) + S Eq. 16 O + H = OH + S Eq. 17 OH + H = H 2 O + S Eq. 18 H 2 O = H 2 O (g) + S Eq. 19 For the CH 4 formation, starting from CH 3 O, CH 3 is formed via the dissociation reaction (Eq. 10), then through hydrogenation reaction to obtain CH 4 (Eq. 15), followed by desorption of CH 4 from the surface (Eq. 16). Finally the surface O is hydrogenated by H into OH (Eq. 17) and H 2 O (Eq. 18), followed by H 2 O desorption (Eq. 19) and the regeneration of the free surface sites. On the basis of the potential energy surface and the computed energy barriers of these reactions, we found that CH 3 O hydrogenation to CH 3 OH and dissociation to CH 3 + O are competitive reactions, and these two reaction have highest barrier (1.55 and 1.56 ev, respectively), and should be the rate-determining steps for the formation of CH 3 OH and CH 4, respectively. Therefore, the rate of the formation of CH 3 OH and CH 4 can be defined by reactions 11 and 10. Thus, the rate equation of the formation of CH 3 OH and CH 4 can be expressed as equation (14) and (15): z / L r[ CH OH ] k ( )[ H ][ CH O] r[ CH ] k [ S][ CH O] (14) (15) Where k 11 is the rate constant of CH 3 O hydrogenation (Eq. 11), (z/l)[h] represents the probability of finding adjacent [CH 3 O] surface species to [H] species, L represents the total surface number of active sites S for H 2 adsorption, and [CH 3 O] and [H] denote surface concentrations of CH 3 O and dissociated hydrogen on the surface, respectively. Since all the other steps are assumed to be in quasi-equilibrium, we can obtain the concentrations of the surface species as follows: [H]: [ H ] K [ S][ H ] (16) 1 2 g [CO]: [ CO] K [ CO ][ S] (17) 2 g S4

5 k [CHO]: k 2 [CO][H] k 5 [CHO][H] = 0, thus [ CHO] 2 [ CO] (18) k 5 k [CH 2 O]: k 5 [CHO][H] k 8 [CH 2 O][H] = 0, thus [ CH O] 5 [ CO] (19) 2 k 8 [CH 3 O]: k 8 [CH 2 O][H] k 11 [CH 3 O][H] k 10 [CH 3 O][S] = 0, thus [ CH O] k [ CO][ H ] 2 (20) 3 k [ H ] k [ S] Considering site balance for S, [L] = [S] + [H] + [CO] + [CHO] + [CH 2 O] + [CH 3 O] + [CH 3 OH] Where K 1 is the equilibrium constant of H 2 adsorption and desorption, K 2 is the equilibrium constant of CO adsorption and desorption, [S] denotes surface concentrations of free site, k n is the rate constant of the corresponding reaction. Substitute the equations (16) - (20) to (14) and (15) we can get the rate equation of CH 3 OH and CH 4 formation: k K [ CO ][ S] K [ S][ H ] 2 2 g 1 2 g r[ CH OH ] k ( )[ H ] z / L k K S H k S g 10 [ ][ ] [ ] (21) k [ CO][ H ] 2 r[ CH 4 ] k10[ S] k [ H ] k [ S] (22) S5

6 Table S1. Most stable adsorption site, adsorption energy (E ads, ev), and relative bond distances (d Co-X, Å) of the surface species; the adsorption energies in the parentheses are without ZPE corrections (* own results in present work) Species E ads (ev) d Co-C (Å) d Co-O (Å) CH -5.65* (-5.79) a ; b ; ; ; a ; b CH * (-3.37) a ; b ; ; ; CH * (-1.49) a ; b ; ; ; CH * (-0.04) CHO -1.53* (-1.61) 1.873; 1.876; ; a ; b 1.962; 1.965; ; 2.209; ; ; COH -3.54* (-3.64) 1.861; 1.911; CHOH -2.32* (-2.32) 1.978; 1.986; CH 2 O -0.19* (-0.23) ; 2.073; CH3O -2.18* (-2.32) 2.028; 2.033; CH 3 OH 0.00* (-0.02) S6

7 Table S2. The adsorption energy (E ads, ev) and the relative bond distances (d Co-X, Å) of the surface species, as well as the reaction energy (E r, ev) and the barrier energy (E a, ev) in the diffusion reaction of the surface species; the adsorption energies in the parentheses are without ZPE corrections Species E ads E a E r d Co-C d Co-O CH-hcp (-5.83) 1.873; 1.876; TS (-5.54) ; CH-fcc (-5.64) 1.877; 1.888; CH 2 -hcp (-3.40) 1.962; 1.965; TS (-3.09) ; CH 2 -fc (-3.37) 1.971; 1.977; CH 3 -hcp (-1.50) 2.183; 2.209; TS (-1.31) ; CH 3 -fcc (-1.50) 2.179; 2.196; CHO (-1.61) 1.912; ; TS (-1.46) ; CHO (-1.61) 1.912; ; COH (-3.64) 1.803; 1.907; TS (-3.45) ; COH (-3.46) 1.857; 1.911; CH 2 O (-0.23) ; 2.073; TS 0.13 (0.14) CH 2 O (-0.23) ; 2.073; CH 3 O (-2.32) 2.028; 2.033; TS (-2.11) ; CH 3 O (-2.24) 2.028; 2.035; S7

8 Table S3. Computed adsorption energies (E ads, ev), and the relevant bond distances (d, Å) of the IS, TS and FS for the reactions via H-assisted pathway; the adsorption energies in the parentheses are without ZPE corrections E ads d Co-C d Co-O d Co-H d C-O d C-H d O-H CO+H-t (-1.60) ; 1.751; TS (1.15) 1.794; 1.841; ; ; 1.689; C+O+H 0.02 (-0.21) 1.758; 1.787; ; 1.890; ; 1.734; CO+H-t (-1.56) ; 1.770; TS (0.53) 1.772; 2.023; COH (-0.66) 1.861; 1.911; TS (-0.16) CHO (-0.44) 1.880; ; CHO+H-t (-0.75) 1.881; ; ; 1.756; TS (0.27) 1.829; ; 1.983; ; CH+O+H (-1.01) 1.847; 1.860; ; 1.870; ; 1.743; CHO+H-t (-0.58) 1.867; ; ; 1.763; ; TS (0.45) 1.948; 2.039; ; CHOH 0.12 (-0.45) 1.978; 1.986; TS (-0.34) 1.887; ; CH 2 O (-0.65) ; 2.073; ; CH 2 O+H-t (-0.92) ; ; 1.763; ; TS (-0.03) 1.932; ; 1.964; ; 1.746; ; CH 2 +O+H (-1.17) 1.931; 1.980; ; 1.893; ; 1.753; ; CH 2 O+H-t (-0.89) ; 2.106; ; 1.747; ; TS (0.17) ; ; CH 2 OH 0.17 (-0.70) 2.104; ; CH 2 O+H-t (-0.93) ; ; 1.761; ; TS (-0.51) ; 2.067; ; CH 3 O (-1.64) 2.028; 2.033; ; 1.100; CH 3 O+H-t (-1.93) 2.033; 2.034; ; 1.763; ; 1.100; TS (-0.37) 1.910; 1.919; ; 1.773; ; 1.090; CH 3 +O+H (-1.85) 2.153; ; 1.878; ; 1.741; ; 1.106; CH 3 O+H-t (-1.95) 2.026; 2.026; ; ; ; ;1.100 TS (-0.40) ; 1.103; CH 3 OH (-1.57) ;1.102; TS (-0.82) 1.754; ; 1.911; ; 1.099; CH 4 +O (-2.52) ; 1.878; ; 1.098; 1.098; S8

9 Table S4. Computed adsorption energies (E ads, ev), and the relevant bond distances (d, Å) of the IS, TS and FS for the reactions via OH-assisted pathway; the adsorption energies in the parentheses are without ZPE corrections E ads d Co-C d Co-O d Co-H d C-O d O-H CO+H+OH (-1.52) ; 2.049; ; 1.776; TS (-0.59) 1.847; 1.955; ; 1.951; ; 1.754; COH+O+H (-0.76) 1.840; 1.906; ; 1.905; ; 1.755; TS (0.04) ; 1.964; ; 1.756; CHO+O+H (-0.37) 1.959; 2.146; ; 1.892; ; 1.748; TS (-0.24) COOH (-0.69) ; TS (-0.23) 1.868; 1.917; ; 1.893; HCOH+O (-0.39) 1.943; 1.991; ; 1.885; TS (0.39) ; CO 2 +H (-0.60) 2.122; ; ; 1.764; ; HCOH+OH+H (-0.68) 1.978; 1.986; TS (0.13) 1.858; 1.974; ; CH+OH+OH+H (-1.25) 1.836; 1.872; ; 2.059; TS (0.33) 2.066; 2.198; ; 1.930; CH 2 OH+O (-0.56) 2.245; 2.273; ; 1.972; TS (-0.60) 1.987; 2.024; HCO+H 2 O (-0.86) 1.873; ; 2.208; ; TS (-0.11) 1.939; 2.121; CH 2 OH(+OH) (-0.67) 2.165; S9

10 Table S5. Energy barrier E a (ev) and reaction energy E r (ev) as well as the rate constant k (483K) of CO hydrogenation in the H- assisted pathway on the Co(0001) surface E a E r A k CO+H TS1 C+O+H CO+H TS2 COH CO+H TS3 CHO CHO+H TS4 CH+O+H CHO+H TS5 CHOH CHO+H TS6 CH 2 O CH 2 O+H TS7 CH 2 +O+H CH 2 O+H TS8 CH 2 OH CH 2 O+H TS9 CH 3 O CH 3 O+H TS10 CH 3 +O+H CH 3 O+H TS11 CH 3 OH CH 3 +O+H TS12 CH 4 +O CH+H+CO TS CH 2 +CO CH+H+CO TS CHCO+H CH 2 +H+CO TS CH 3 +CO CH 2 +H+CO TS CH 2 CO H CH 3 +H+CO TS CH 4 +CO CH 3 +H+CO TS CH 3 CO+ H S10

11 Table S6: Barriers E a (ev) and reaction energies E r (ev) as well as the rate constant k (483K) of CO activation via OH-assisted pathway E a E r A k CO+H+OH TS13 COH+O+H CO+H+OH TS14 CHO+O+H CO+OH TS15 COOH COH+O+H TS16 HCOH+O COOH TS17 CO 2 +H HCOH TS18 CH+OH HCOH+OH+H TS19 CH 2 OH+O+H HCOH+OH+H TS20 HCO+H 2 O+H HCOH+OH +H TS21 CH 2 OH+OH S11

12 Figure S1. Top (a) and side (b) reviews as well as possible adsorption sites of the hcp Co(0001) surface: top site (t), three-fold face-centered cubic site (fcc), three-fold hexagonal close packed site (hcp) and bridge site (b) S12

13 Figure S2. Top and side views of the relative surface species on the Co(0001) surface S13

14 Figure S3. The geometries of the initial state and the final state as well as the transition state of the surface species in the diffusion reaction S14

15 Figure S4. The geometries of the initial state and the final state as well as the transition state for the reactions starting from CO+H in the H-assisted pathway S15

16 Figure S5. The geometries of the initial state and the final state as well as the transition state for the reactions starting from CH 2 O in the H-assisted pathway S16

17 Figure S6. The geometries of the initial state and the final state as well as the transition state in the reaction of the coupling of CH x (x=1-3) with H or CO S17

18 Figure S7. The geometries of the initial state and the final state as well as the transition state for the reactions starting from CO+OH in the OH-assisted pathway S18

19 Figure S8. The geometries of the initial state and the final state as well as the transition state for the reactions starting from COH in the OH-assisted pathway S19

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