Molecular level investigation of methane and carbon dioxide adsorption on SiO2 surface

Adsorption in porous media has a direct impact on the reserves of fluid in shale gas reservoirs, but they are rarely studied in combination with molecular simulation and quantum mechanisms. The adsorption behaviors between methane, carbon dioxide and quartz are investigated with Grand Canonical Monte Carlo, Molecular Dynamics and Density Functional Theory in this work. Adsorption isotherm, density distribution, adsorption energy and Density of States (DOS) are calculated and analyzed. These results are as below. The adsorption isotherms of methane, carbon dioxide and their mixture (mole ratio 1:1) can be described as Langmuir type. The adsorption amounts increase with the rising of pressure, and the adsorption capacity of carbon dioxide is stronger than methane. The position near the wall is occupied by methane and carbon dioxide, and density distribution shows that the thickness of adsorption layer is 5 angstrom. The distribution of adsorption energy states clearly physical adsorption. DOS of methane systems have a little bit difference, and a relatively large difference of peak value is in carbon dioxide systems. DOS of adsorbents (maximum and minimum adsorption energy) are basically coincided with each other, while DOS of methane and carbon dioxide shift with different degrees to the lower energy region.