The Structure of Adsorbed CO2 in Slitlike Micropores at Low and High Temperature and the Resulting Micropore Size Distribution Based on GCMC Simulations

Abstract

The Monte Carlo method is used in its grand ensemble variant in combination with CO2 experimental isotherm data at low (195.5 K) and high temperatures (at 298 and 308 K, i.e., slightly below and above the CO2 critical temperature, respectively) to characterize microporous carbons and obtain the corresponding pore size distribution (PSD). Specifically, the CO2 density inside a single, slit-shaped, graphitic pore of given width is found on the basis of grand canonical Monte Carlo (GCMC) simulations for a predefined temperature and different relative pressures. The simulation results provide useful insights concerning the densification process in the micropores and the structure of the CO2 molecules packing in the individual pores as the temperature or pressure changes from 195.5 K to ambient or from very low to 70 bar, respectively. Effects of temperature, pore size, quadrupole interactions, and molecule elongation on the local density profile within the pore are examined and discussed. In an additional step, we determine the optimal PSD for which the best match is obtained between computed and measured CO2 isotherms. Comparisons are made between the PSDs found for the same carbon sample at low and high temperatures and conclusions are drawn concerning the applicability of the method and the reliability of the resulting micropore size distributions.