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.

(Left) Small Angle Neutron Scattering profile of carbon aerogel and the data fit to the Beauacage unified model to obtain pore size. (Right) Diffusion of methane under carbon dioxide and nitrogen pressure, diffusivity of methane increases with carbon dioxide pressure but remain unchanged with nitrogen pressure.Figure optionsDownload as PowerPoint slideHighlights
► We studied CH4 diffusion in carbon aerogel pores as a function of CO2 and N2 pressure.
► CH4 diffusion increases with CO2 pressure.
► CH4 diffusion remain same with N2 pressure.
► CH4 is displacing from the pore walls due to CO2.