Molecular Level Models for CO2 Sorption in Nanopores

Adsorption of carbon dioxide in slit-shaped carbon micropores at 273 K has been studied by means of the grand canonical Monte Carlo (GCMC) simulations and the nonlocal density functional theory (NLDFT). Three molecular models of CO2 have been used. Long-run GCMC simulations were performed with the three-center model of Harris and Yung (J. Phys. Chem. 1995, 99, 12021). For NLDFT calculations, we developed an effective Lennard-Jones (LJ) model. GCMC simulations of the effective LJ model of CO2 have been performed for comparison. For each model used, parameters of intermolecular potentials have been determined and validated against two-phase bulk equilibrium data and experimental adsorption isotherms on graphite at 273 and 195 K. In the range of pore widths from 3 to 15 Å, the NLDFT isotherms of CO2 adsorption are overall in a satisfactory agreement with the GCMC isotherms generated using the three-center model. Some deviations have been observed between 6.5 and 8.5 Å, where the adsorbate undergoes a transition from a single-layer to a two-layer structure. The models developed are recommended for studying carbon dioxide adsorption in microporous adsorbents and also for calculating pore size distributions in carbonaceous materials and soil particles. The NLDFT model has the advantage of being much less computationally demanding, whereas the three-center GCMC model serves as a benchmark for quantitative estimates and can be used for studying CO2 sorption at ambient conditions close to the critical temperature.