CO2-adsorption promoted CH4-desorption onto low-rank coal vitrinite by density functional theory including dispersion correction (DFT-D3)

The density functional theory including dispersion correction method was utilized to investigate the promotion effects of CO2-adsorption on the CH4-desorption onto the coal vitrinite. The adsorption energy (ΔE) of vitrinite-CO2 is significantly lower than those of vitrinite-CH4 independent of adsorption locations, indicating that the vitrinite has a higher adsorption affinity for the CO2 than for CH4. When CH4 absorbs onto the vitrinite-CO2, the ΔE reductions (the difference of this co-adsorption state to the single adsorption state) of CH4 onto vitrinite-CO2 are all greater than 0, indicating that the CH4 is less stable onto the vitrinite-CO2 surface than single CH4 onto the pure vitrinite. However, at the same locations, the ΔE reductions of CO2 onto the vitrinite-CH4 are less than 0, indicates that the stability of CO2 on vitrinite-CH4 is higher than single CO2 absorbing on pure vitrinite surface. When CO2 and CH4 simultaneously interact with the surface, CO2 compels CH4 to adsorb onto the less stable site, resulting in the significant ΔE decrease of CH4 onto the vitrinite surface with pre-adsorbed CO2. Our results show that the adsorption of CO2 can promote the CH4 desorption while CH4 can't replace CO2 on the vitrinite surface.