Barriers to diffusion of CO2 in microporous carbon derived from silicon carbide

We investigate the adsorption equilibrium and uptake kinetics of CO2 in nanoporous silicon carbide-derived carbon (SiC-DC) experimentally, as well as by simulation through grand canonical Monte Carlo (GCMC) and equilibrium molecular dynamics (EMD) simulations using a hybrid reverse Monte Carlo (HRMC) simulation-based atomistic structural model of the carbon. The kinetics is best explained when a grain surface barrier resistance mechanism is incorporated in a bidisperse pore structure model, considering particle scale diffusion in large micropores and a local grain scale diffusion in ultra-micropores. Good agreement is found between simulated and experimental isotherms; however, experimental particle scale diffusivities are almost two orders of magnitude smaller than those obtained from EMD, suggesting the presence of long range barriers for CO2 not captured by the HRMC model structure. These barriers also lead to about two orders of magnitude reduction in the particle-scale diffusivity of CO2 compared to CH4. On the other hand activation barriers for grain scale diffusion of CO2 are comparable to those for CH4 in the SiC-DC, and in carbon molecular sieves, consistent with the ultra-microporous nature of the grains. The activation barrier for interfacial mass transfer coefficient at the grain surface is also consistent with values for carbon molecular sieves.