Adsorption and Diffusion of Gas Molecules on Graphite Surface

The adsorption and diffusion characteristics of gas molecules on graphite surface are investigated by molecular dynamics simulations. The results show that the adsorption intensity of gas molecules on graphite surface is related to the atomic interactions between gas molecules and carbon atoms in graphite; different gas molecules present distinct adsorption intensities on graphite surface. The adsorption characteristics can be described by the Langmuir isothermal adsorption model, where the molecular number density in the adsorption layer increases with increasing gas pressure. From the decreasing diffusion coefficients with increasing gas pressure and the normal probability distribution of molecular moving distance in a certain time period, it is concluded that the gas diffusion on graphite surface is mainly controlled by the molecular collisions and approaches the bulk diffusion phenomenon. Accordingly, the molecular number density in the adsorption layer has a great effect on the surface diffusion coefficients, resulting in the obviously lower diffusion coefficients of CO₂ and H₂S molecules with stronger adsorption intensities than those of CH₄ and N₂ molecules with weaker adsorption intensities.