Abstract
Pore size is an essential factor in controlling gas sorption in porous separation media. The overlap of the potential energy surface (PES) of CO2 interacting with a cylindrical pore wall can be used to tune gas sorption inside a porous material, but how such overlap can benefit post-combustion CO2 capture has not been fully addressed from a computational perspective. Here we use van der Waals density functional (vdW-DF) theory to assess the overlap of PES of CO2 inside cylindrical pores as represented by carbon nanotubes (CNTs) of different diameters. Then we employ grand-canonical Monte Carlo simulations to obtain the adsorption capacity and selectivity of a CO2/N2 mixture with a CO2 partial pressure of 0.15 bar at room temperature. We find that the maximum PES overlap and maximum amount of CO2 adsorbed are both achieved at a CNT diameter or cylindrical pore size of 7.8 Å, which corresponds to an accessible pore size of 4.4 Å. A further investigation of N2 adsorption corroborates the idea of PES overlap. GCMC simulations reveal that a maximum CO2/N2 selectivity of ∼33 is reached at a CNT diameter of 7.05 Å for the gas mixture. This work suggests that a cylindrical pore size between 7 and 8 Å would be most beneficial for post-combustion CO2 capture from overlap of PES.
