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Catalytic Conversion of CO and H2 into Hydrocarbons on the Cobalt Co(111) Surface: Implications for the Fischer–Tropsch Process

The Fischer-Tropsch (FT) process consists of the reaction of a synthesis gas (syngas) mixture containing carbon monoxide (CO) and hydrogen (H₂), which are polymerised 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 H₂ molecules prefer to adsorb in the two different types of trigonal surface sites, and we discuss how the interatomic distances, fundamental vibrational modes, charge transfers, surface free energies and work function are modified by the adsorbates. The co-adsorption 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 stabilised when the ratio of equilibrium coverages (C) is CH: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 are the most favourable processes, which take precedence over the production of CH₄. The termination reaction suggests that water (H₂O) remains weakly physisorbed to the surface, allowing the re-utilisation of its catalytic site. The simulated fundamental vibrational frequencies and scanning tunnelling 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.