Adsorption of methane on carbon models of coal surface studied by the density functional theory including dispersion correction (DFT-D3)

The density functional theory including dispersion correction (DFT-D3) has been used to investigate the adsorption of methane on carbon models of coal surface, including C6H8, pyrene, and coronene. For the small model complex C6H8–CH4, the interaction energies obtained using four kinds of the functionals, BLYP-D3, TPSS-D3, BP86-D3, and PBE-D3, were benchmarked against the best available result that was provided by the complete basis set (CBS) limit of CCSD(T) method, and the BLYP-D3 functional with the best performance was selected to treat the remained larger systems. Several adsorption positions and orientations of CH4 on the hydrocarbon clusters were systematically considered. Our results indicated that the interaction energy in the complex increases as the size of the complex increases and the up configuration of CH4 (with the hydrogen tripod directed to the surface) adsorbed on pyrene and coronene is greatly preferred when compared with both the down and bidentate configurations. The center adsorption site above the six-membered ring is preferred by the methane molecule adsorbed on coronene. It was shown that in coronene–methane model the interaction energies of −3.17 to −3.32 kcal/mol and the molecular distances of 3.36–3.39 Å obtained from the BLYP-D3 calculations are close to the values available in experiment for the binding of methane on graphite and can provide the accurate prediction to those in the coal surface–methane complex.