TY - JOUR
T1 - Catalytic Conversion of CO and H2into Hydrocarbons on the Cobalt Co(111) Surface
T2 - Implications for the Fischer-Tropsch Process
AU - Santos-Carballal, David
AU - Cadi-Essadek, Abdelaziz
AU - de Leeuw, Nora H.
N1 - Funding Information:
We acknowledge the Engineering and Physical Sciences Research Council (EPSRC grant EP/K009567) for funding. Via our membership of the UK’s HEC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202/1 and EP/R029431/1), this work used the ARCHER UK National Supercomputing Service ( http://www.archer.ac.uk ). This work was performed using the computational facilities of the Advanced Research Computing @ Cardiff (ARCCA) Division, Cardiff University. We also acknowledge the use of HPC Wales, Supercomputing Wales, and associated support services in the completion of this work. All data are provided in full in the Results and Discussion section of this paper.
Publisher Copyright:
© 2021 The Authors. Published by American Chemical Society
PY - 2021/6/10
Y1 - 2021/6/10
N2 - The Fischer-Tropsch (FT) process consists of the reaction of a synthesis gas (syngas) mixture containing carbon monoxide (CO) and hydrogen (H2), which are polymerized into liquid hydrocarbon chains, often using a cobalt catalyst, although the mechanistic pathway is not yet fully understood. Here, we have employed unrestricted density functional theory calculations with a Hubbard Hamiltonian and long-range dispersion corrections [DFT+U−D3−(BJ)] to investigate the reaction of syngas and the selectivity toward the hydrocarbons formed on the cobalt Co(111) surface. The single CO and dissociated H2molecules prefer to adsorb at two different types of trigonal surface sites, and we discuss how the interatomic distances, fundamental vibrational modes, charge transfers, surface-free energies, and work functions are modified by the adsorbates. The coadsorption of the syngas molecules in close proximity provides enough energy for the system to cross the saddle points on the minimum energy pathway (MEP), leading to the catalytic hydrogenolysis of the C-O bond. The adsorbed CO, alongside the intermediates CH and OH, are further stabilized when the ratio of equilibrium coverage (C) isCH/CCO,CH,OH> 6:1 under the temperature conditions required for the FT process. We propose several mechanistic pathways to account for the formation of ethane (C2H6), as a model for long-chain hydrocarbons, as well as methane (CH4) which is an undesirable product. The MEPs for these processes show that the coupling of the C-C bond followed by hydrogenation is the most favorable process, which takes precedence over the production of CH4. The termination reaction suggests that water (H2O) remains weakly physisorbed to the surface, allowing the reutilization of its catalytic site. The simulated fundamental vibrational frequencies and scanning tunneling microscopy images of the surface-bound intermediates are in agreement with the available experimental data. Our findings are important in the interpretation of the elementary steps of the FT process on the Co(111) surface.
AB - The Fischer-Tropsch (FT) process consists of the reaction of a synthesis gas (syngas) mixture containing carbon monoxide (CO) and hydrogen (H2), which are polymerized into liquid hydrocarbon chains, often using a cobalt catalyst, although the mechanistic pathway is not yet fully understood. Here, we have employed unrestricted density functional theory calculations with a Hubbard Hamiltonian and long-range dispersion corrections [DFT+U−D3−(BJ)] to investigate the reaction of syngas and the selectivity toward the hydrocarbons formed on the cobalt Co(111) surface. The single CO and dissociated H2molecules prefer to adsorb at two different types of trigonal surface sites, and we discuss how the interatomic distances, fundamental vibrational modes, charge transfers, surface-free energies, and work functions are modified by the adsorbates. The coadsorption of the syngas molecules in close proximity provides enough energy for the system to cross the saddle points on the minimum energy pathway (MEP), leading to the catalytic hydrogenolysis of the C-O bond. The adsorbed CO, alongside the intermediates CH and OH, are further stabilized when the ratio of equilibrium coverage (C) isCH/CCO,CH,OH> 6:1 under the temperature conditions required for the FT process. We propose several mechanistic pathways to account for the formation of ethane (C2H6), as a model for long-chain hydrocarbons, as well as methane (CH4) which is an undesirable product. The MEPs for these processes show that the coupling of the C-C bond followed by hydrogenation is the most favorable process, which takes precedence over the production of CH4. The termination reaction suggests that water (H2O) remains weakly physisorbed to the surface, allowing the reutilization of its catalytic site. The simulated fundamental vibrational frequencies and scanning tunneling microscopy images of the surface-bound intermediates are in agreement with the available experimental data. Our findings are important in the interpretation of the elementary steps of the FT process on the Co(111) surface.
UR - http://www.scopus.com/inward/record.url?scp=85108446836&partnerID=8YFLogxK
U2 - 10.1021/acs.jpcc.1c00254
DO - 10.1021/acs.jpcc.1c00254
M3 - Article
AN - SCOPUS:85108446836
SN - 1932-7447
VL - 125
SP - 11891
EP - 11903
JO - Journal of Physical Chemistry C
JF - Journal of Physical Chemistry C
IS - 22
ER -