Slow diffusion of methane in ultra-micropores of silicon carbide-derived carbon

We investigate macroscopic uptake kinetics of CH4 in silicon carbide-derived carbon (SiC-DC). Ultra-microprosity in SiC-DC is found based on CO2 adsorption at 273 K, but which has poor accessibility to Ar at 87 K. The adsorption kinetics of CH4 is found to follow a bidisperse pore structure model, considering relatively rapid particle scale diffusion in large micropores, and a much slower local grain (or microparticle) scale diffusion in ultra-micropores. The grain scale activation energies are comparable with values for carbon molecular sieves, and consistent with values expected for the size range of the ultra-micropores, while the activation energies for transport in the larger particle scale micropores are comparable to those for conventional activated carbons. The particle scale diffusivities compare well with the results of equilibrium molecular dynamics simulations using a hybrid reverse Monte Carlo simulation constructed model of SiC-DC, with similar activation energy. On the other hand microscopic quasi-elastic neutron scattering measurements are found to probe only short-range barriers with lower activation energy. It is anticipated that ultra-micropores will not make a significant contribution to the transport in any membrane or adsorption-based process based on SiC-DC, due to the extremely slow transport in these ultra-micropores and their small pore volume.