First-principles calculations of methane adsorption at different coverage on the kaolinite (001) surface

Abstract

Understanding gas adsorption capacity on shale sediments is great significance for the evaluation of shale gas reservoir potential and the exploration of potential gas shale. Clay minerals have an important impact on the adsorption of methane according to previous research. Density-functional theory was employed to explore the process of methane molecules adsorption on the kaolinite (001) surface. The coverage dependence of the adsorption position and energetics was systematically studied for a wide range of coverage Θ [from 1/16 to 1 monolayers (ML)]. It is found that the two-fold bridge site is the most stable, followed by the hollow and top sites. The adsorption energy of CH4 decreased with increasing coverage, thus indicating the lower stability of surface adsorption due to the repulsion of neighboring CH4 molecules. The coverage has obvious effects on the methane adsorption process. Other properties of the CH4/kaolinite (001) system, including the lattice relaxation and changes of electronic density of states, were also studied and discussed in detail.