Effect of carbon dioxide and nitrogen on the diffusivity of methane confined in nano-porous carbon aerogel

The microscopic diffusivity of methane (CH₄) confined in nano-porous carbon aerogel was investigated as a function of added carbon dioxide (CO₂) and nitrogen (N₂) pressure using quasi-elastic neutron scattering (QENS). In the range of the external pressure of 1-2.5 MPa, the self-diffusivity of methane was found to increase with CO₂ pressure and remain practically unchanged in the N₂ environment. Increasing mobility of methane with CO₂ pressure suggests that the adsorbed CH₄ molecules become gradually replaced by CO₂ on the surface of carbon aerogel pores, whereas the presence of N₂ does not induce the replacement. The molecular mobility of the methane, with or without added carbon dioxide and nitrogen, is described by the unrestricted diffusion model, which is characteristic of methane compressed in small pores. On the other hand, both nitrogen and carbon dioxide molecules in carbon aerogel, when studied alone, with no methane present, follow a jump diffusion process, characteristic of the molecular mobility in the densified adsorbed layers on the surface of the aerogel pores.