Isotherm and heat of adsorption in porous solids with defective pores—adsorption of argon and nitrogen at 77 K in Saran activated carbon

Abstract

The isotherm and isosteric heat of a porous solid are studied in terms of the local isotherms and isosteric heats of individual pores with defective walls, rather than graphitic walls as commonly assumed in the literature. We point out the incorrect formulas that have been used in the literature, and present a correct formula to calculate the isosteric heat for a porous solid. The correct formula is illustrated with a direct Monte Carlo (MC) simulation of systems of two pores of different sizes, and finally we apply our theory to experimental data of argon and nitrogen adsorption at 77 K on S600H and S84 Saran charcoals to derive their pore size distributions (PSD). We show that the PSD derived from the fitting either the isotherm only or the heat curve only may not be reliable. It is necessary to utilize both the isotherm and heat curves in the derivation of a more reliable PSD. We also show that it is essential to use defected walls of carbon pores to model adsorption in pores as the model using graphitic walls can not describe isotherm and heat of adsorption adequately.