Crystallographic Dependence of CO Activation on Cobalt Catalysts: HCP versus FCC

Transcription

1 Crystallographic Dependence of CO Activation on Cobalt Catalysts: HCP versus FCC Jin-Xun Liu, Hai-Yan Su, Da-Peng Sun, Bing-Yan Zhang, and Wei-Xue Li* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academic of Sciences, Dalian, , China Supporting Information S1

2 Calculation details: All spin-polarized DFT calculations were performed by using projector augmented wave (PAW) 1 potentials and the Perdew Burke Ernzerhof(PBE) functional 2 implemented in the Vienna ab initio simulation package (VASP) 3, 4. The influence of the exchange-correlation function on the proper description of CO adsorption (energetics and site) could be found in a recent paper (and reference therein) 5. The cutoff energy was set at 400eV. The p(1 1) slab models used here have a thickness of at least 15.5Å and are separated by a vacuum of 15 Å for the accurate surface energies calculations. Three equivalent (111) layers in each of the two sides of the slab were fully relaxed for the surface energy calculations. Monkhorst-Pack k-points sampling 6 of were used for the HCP Co (0001) surface, scaled for the other surface energies calculations. The p(2 2) slab models were used for the calculations involving CO dissociation on the surfaces exposed in the morphologies of HCP Co and FCC Co. All the surfaces were simulated by four equivalent (111) layers (except FCC Co(100) surface with five layers) slab. Neighboring slabs were separated by a vacuum of 15 Å to avoid the interactions between them. The density of k-points were kept at ~ 0.04 Å -1. All the adsorbates and the topmost two equivalents (111) layers were relaxed. The improved force reversed method 7 was used to determine the transition states (TS) and the force tolerance of 0.03 ev/å was used. And some of the TS are verified by climbing-image nudged elastic band (CI-NEB) methods 8, 9. The located TS for CO dissociation were also confirmed by frequency analysis. The surface energy is determined by E sur. = (E slab N E bulk )/2 A, where E slab and E bulk are the total energy of the slab and one bulk Co atom, respectively. N is the number of Co atoms in the slab and A is the surface area. The reaction energies can be calculated as the difference between the total energies of the products and the reactants. We chose the separate most stable adsorbed fragments on the surface as the initial and final states. The determined equilibrium lattice constants based on density functional theory calculations for bulk HCP Co and FCC Co are a=b=2.494 Å, c=4.031å and a=b=c=3.520 Å, respectively. S2

3 Table S1. Calculated surface energies (E sur, in mev/å 2 ) for the low index surfaces of HCP Co and FCC Co crystallines. Most HCP Co surfaces have two possible terminations (labeled as A and B, respectively), and the terminations with lower surface energy (bold font) are used for Wulff construction and activity study. HCP Co FCC Co (hkil) E sur (hkil) E sur (hkl) E sur (hkl) E sur (0001) 131 (11-20) 155 (111) 127 (320) 160 (10-10)A 140 (10-10)B 178 (221) 145 (210) 162 (10-11)A 149 (10-11)B 178 (110) 151 (310) 165 (20-21)A 149 (20-21)B 166 (211) 151 (21-30)A 154 (21-30)B 169 (100) 154 (10-12)A 156 (10-12)B 159 (311) 156 (11-21) 163 (321) 156 S3

4 Table S2. Adsorption energy (in ev) and favorable adsorption sites of the various adsorbates (CO, C and O) on HCP and FCC Co. The adsorption energies are calculated with respect to the corresponding molecules or free radicals in the gas phase. HCP Co E CO site E C site E O site (10-11) fold fold fold (10-10) fold Long bridge fold (0001) Top Hcp Hcp (10-12) fold fold fold (11-20) Short bridge Hollow Hollow (11-21) fold fold fold FCC Co E CO site E C site E O site (111) Hcp Hcp Hcp (100) fold fold fold (311) Edge fcc fold Edge hcp (110) Short bridge Long bridge fold S4

5 Table S3. Optimized distance (d C-O, in Å) and the only imagination frequency (ω, in cm -1 ) between the C and O atoms at the transition states for the CO direct dissociation on HCP Co and FCC Co surfaces. HCP Co d C-O ω FCC Co d C-O ω (10-11) (111) (10-10) (100) (0001) (311) (10-12) (110) (11-20) (11-21) S5

6 Micro-kinetic model for CO direct dissociation process We assumed that CO molecules directly dissociate on cobalt surfaces obeying Langmuir-Hinshelwood (L-H) mechanism. In other words, CO molecules first chemisorb on cobalt surfaces, and then adsorbed CO molecules will be dissociated to C and O atoms: k1 k- 1 CO + * CO * k2 CO * C * +O * (1) (2) Micro-kinetic model was used to describe the reaction mechanism. Assuming CO adsorption process is very fast compared to dissociation and that the CO adsorption and desorption are in equilibrium. Elementary step (2) is often considered to be irreversible. The equilibrium constant is always the ratio of the forward and backward rate constants. Therefore, the equilibrium constant K for step (1) can be given by K = k k 1 θ CO θ P -1 * CO K 0 P 0 Where θ and θ CO * stand for the coverages of CO molecules and free sites on the cobalt surface, respectively. k 1 and k -1 are the forward and backward rate constants of step (1). P CO and P 0 are the partial pressure of CO gas and standard pressure, respectively. At the same time, the coverages of CO molecule and free active sites obey the sum rule: θ + θ = 1. CO * So we can obtain that θ KP CO CO 1 KPCO and θ * 1 1 KP CO Meanwhile K can be calculated as follows: K 0 0 -ΔG / (kt) = e 0 ΔG = E (T, P ) - E (T, P ) - μ (T, P ) CO/slab 0 slab 0 CO 0 = E ( 0 K, P ) - E ( 0 K, P ) - (E ( 0 K, P ) + Δμ (T, P ) ) CO/slab 0 slab 0 CO 0 CO 0 = E - Δμ (T, P ) ads. CO 0 S6

7 where ΔG is the change of the standard Gibbs free energy for CO adsorption. Eads. is the adsorption energy of the CO molecule. E (0 K, P ) and E (0 K, P ) CO/slab 0 slab 0 given by DFT calculations are the total energies of CO adsorption on the surface system and the clean surface, respectively. While μ (T, P ) stands for the standard CO 0 chemical potentials of CO in the gas phase. In order to obtain Δμ (T, P ), entropy CO 0 and enthalpy changes were taken from thermochemical tables 10 for CO molecules and Δμ (T, P ) = -TS(T, P ) + (H(T, P ) - H( 0 K, P )). Here, T = 500 K and P = CO Pa corresponding to low coverage of CO that were used in our study. In addition, Δμ (T, P ) = ev was applied in our paper. CO 0 Finally, the initial rate for adsorbed CO dissociation reaction (2) can be written as r = k θ θ k KP 2 CO 2 CO * 2 (1 + KP ) CO - a. Where k = A e E / kt, and A is the prefactor and E 2 a. denotes the CO activation barrier. In order to estimate the reaction rate, we denoted the value of prefactor A to be s -1. Assuming all the active sites can be used for active CO molecules, CO conversion rate of the specified surface can be given by C = r * N act, where r and N act are the CO dissociation rate for each active site and the number of the active sites correspondingly. N act can be calculated as the total surface area of the specified surface divided by the surface area per site. S7

8 Table S4. Calculated kinetic parameters involved in CO direct dissociation process. K and k (s -1 ) are the equilibrium constant and CO direct dissociation rate constant, respectively. θ CO and θ * are the coverages of CO and free sites of the surfaces. r (molecules s -1 site -1 ) and C (molecule s -1 ) stand for the reaction rate and conversion rate for CO direct dissociation process at T=500 K and P CO = Pa. The total surface area of HCP Co or FCC Co is assumed to 1 m 2 for convenience. In the main context, the reaction rate given is normalized by the reaction rate of HCP(0001) for comparison. HCP Co surface K k θ CO θ * θ CO *θ * r C (10-11) (10-10) (0001) (10-12) (11-20) (11-21) FCC Co (111) (100) (311) (110) S8

9 Table S5. Calculated adsorption energies of H, HCO and CH (in ev) with respect to gas phase H 2 and the gaseous radicals HCO and CO at favorable adsorption sites of the adsorbates on HCP and FCC Co surfaces. Surface H site HCO site CH site HCP(11-21) fold site C-4-site-O-bridge fold site HCP(10-11) fold site C-bridge-O-bridge fold site HCP(10-12) fold site C-bridge-O-bridge fold site FCC(311) fold site C-3-site-O-bridge fold site FCC(110) fold site C-bridge-O-bridge Long bridge FCC(100) fold site C-bridge-O-bridge fold site S9

10 Table S6. Calculated energies and geometric information for hydrogen assisted CO dissociation on each surface of HCP Co and FCC Co crystallines. E 1 (E 2 ). and ΔH 1 (ΔH 2 ) (in ev) are assigned to elementary reaction barriers and reaction energies, respectively. d 1 (d 2 ) (in Å) is the distance between the C and H atom (or O atom) at the transition state. E total is the total activation energy for the CO*+H* CH*+O* process, see Figure S4 and S5 below. Surface CO*+H* HCO* HCO* CH*+O* CO*+H* CH*+O* E 1 ΔH 1 d 1 E 2 ΔH 2 d 2 E total HCP(11-21) HCP(10-11) HCP(10-12) FCC(311) FCC(110) FCC(100) S10

11 Figure S1. Structural information for the initial state (CO favorable adsorption state), the transition state, and the C and O atoms isolated adsorption geometries on HCP Co surfaces (blue: Co atom; red: O atom; grey: C atom). S11

12 Figure S2. Structural information on the initial state (CO favorable adsorption state), the transition state, and the C and O atoms isolated adsorption geometries on FCC Co surfaces (blue: Co atom; red: O atom; grey: C atom). S12

13 Figure S3. Structural information on the adsorption of the intermediates (H, HCO and CH) and the transition state involved in the H-assisted CO activation process on several HCP and FCC Co surfaces (blue: Co atom; red: O atom; grey: C atom; white: H atom). S13

14 Figure S4. The potential energy surface diagram (in ev) for the direct CO dissociation (CO*+H C*+O*+H*) (dash line) and the H-assisted CO dissociation (CO*+H* CHO* CH*+O*) (solid line) CO on HCP Co (11-21) (a), (10-11) (b) and (10-12) (c) facets. The corresponding overall activation barriers with respect to co-adsorbed CO* and H* (horizontal dot line) separating a sufficient large distance are indicated. The zero energy reference is CO+1/2H 2 in the gas phase. S14

15 Figure S5. The potential energy surface diagram (in ev) for the direct CO dissociation (CO*+H C*+O*+H*) (dash line) and the H-assisted CO dissociation (CO*+H* CHO* CH*+O*) (solid line) CO on FCC Co (110) (a), (100) (b) and (311) (c) facets. The corresponding overall activation barriers with respect to co-adsorbed CO* and H* (horizontal dot line) separating a sufficient large distance are indicated. The zero energy reference is CO+1/2H 2 in the gas phase. S15

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