Atomistic simulation techniques have been employed to investigate the effect of molecular adsorption of water on the low-index surfaces of calcite, aragonite, and vaterite. Calculated surface and hydration energies agree with experiment and previous calculations where available. Known experimental surface features are reproduced, i.e., 1 × 1 symmetry and structural features of the calcite {101̄4} surface and bulk termination of the {101̄1} and {112̄0} surfaces. Surface carbonate groups tend to rotate to lie flat in the surface. The morphologies of the hydrated crystals agree with experimentally found morphologies. The bulk lattice energies of the polymorphs reflect their thermodynamic stability.

The Fischer−Tropsch (FT) process consists of the reaction of a synthesis gas (syngas) mixture containing carbon monoxide (CO) and hydrogen (H 2 ), 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 H 2 molecules 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) is C H /C CO,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 (C 2 H 6 ), as a model for long-chain hydrocarbons, as well as methane (CH 4 ) 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 CH 4 . The termination reaction suggests that water (H 2 O) 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.

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