High-Pressure CO<sub>2</sub> and CH<sub>4</sub> Transport\nin Smooth Crystalline Silica Mesopores: A Molecular Dynamics Study

In this study, we use molecular dynamics (MD) simulation\nto study\npressure-driven CO<sub>2</sub> and CH<sub>4</sub> flows and their\nslippage behaviors in β-cristobalite mesopores. The result illustrates\nthat both CO<sub>2</sub> and CH<sub>4</sub> have an apparent adsorption\nlayer on pore surface. However, significant differences in gas slippage\nare observed: CH<sub>4</sub> flow shows considerable slippage, while\nit is negligible for CO<sub>2</sub> flow. This disparity is attributed\nto the collective effect of gas molecular configurations and surface\nstructure. The linear molecular structure of CO<sub>2</sub> allows\nit to align perpendicular to the surface, even penetrating into the\nsurface. Notably, the perpendicular orientation of CO<sub>2</sub> molecules\nis energetically favored near the center of the equilateral triangle\nformed by adjacent oxygen atoms on β-cristobalite surface. Conversely,\nthe symmetric molecular structure of CH<sub>4</sub>, coupled with\nits larger size, prevents its penetration into pore surfaces. Therefore,\ndespite smooth crystalline surfaces, CO<sub>2</sub> topological accessible\nplane is much more curved than that of CH<sub>4</sub>. Consequently,\nCO<sub>2</sub> displays hesitating motions undergoing rotational movements,\nwhich significantly hinders its slippage. This study highlights the\ncollective influences of gas molecular characteristics and surface\nstructure on gas slippage, affording important insights into gas\nsequestration and the development of functional materials for gas\nseparation.