Monte Carlo simulation of O2 and N2 adsorption in nanoporous carbon (C168 Schwarzite)

The adsorption Of O-2 and N-2 in nanoporous carbon (NPC) has been investigated by the grand canonical Monte Carlo simulation. The gases are represented as diatomic molecules, and the NPC is represented as a rigid C-168 Schwarzite structure. Interactions between gas-gas and gas-carbon atoms in the NPC are modeled with additive pairwise site-site Lennard-Jones potentials with parameters reported by others for O-2 and N-2 adsorption on graphite. At low loading, the energetically favored N-2 is slightly more adsorbed. At high loading, the size of the gas molecule dominates, and O-2 is preferentially adsorbed. Two steps with an inflection along the adsorption isotherm are observed. A two-site Langmuir model is used to fit the adsorption isotherms and agrees well with the simulation results. Slight orientational preference of the adsorbed gas molecules in the direction of the intersectional channel is observed, but it tends to disappear with increasing external gas pressure. The density distribution of the center-of-mass of the adsorbed gas molecules within the NPC changes from continuous to discontinuous with increasing external gas pressure, first seen in the small pores and then in the large pores at higher pressure. This results from the differences in available spaces and loading capacities of the two types of pores. The adsorption isotherms, Henry's constants, and isosteric heats of adsorption for each gas are calculated at different temperatures, and the temperature dependence of the isosteric heat of adsorption is found to be small.