Insight into the adsorption mechanisms of CH4, CO2, and H2O molecules on illite (001) surfaces: a first-principles study

Abstract Understanding the sorption states of shale gas on shale has an important significance for the exploration and development of shale gas. In the present study, the adsorption mechanisms of CH4, CO2, and H2O molecules on the undoped and Al or Al&Mg doped illite (001) surfaces have been investigated using density functional theory. The nine different adsorption sites (H1-H3, T1-T3, and B1-B3) on the surfaces were considered. The calculations show that for all the three molecules, the Al&Mg-doped illite (001) surface has the strongest adsorption ability while the undoped one is the weakest. Besides, the adsorption ability on each of the three surfaces increases in the order of CH4, CO2 and H2O. The H2O molecule having the strongest adsorption on the three surfaces is ascribed to the formation of hydrogen bonds between the adsorbate and adsorbent. The adsorption of the CH4 and CO2 molecules on each of the three surfaces is characterized as a physical adsorption, but that of the H2O molecule is not. In-depth PDOS (partial density of states) and Hirshfeld charge analyses suggest that there exists a small amount of electron transfer from the adsorbent to adsorbate after their adsorptions on the three surfaces. The Al&Mg-doped illite surface having the strongest adsorption ability for each of the three molecules should be attributed to its surface O atoms having more negative charges.