A thermodynamic analysis of gas adsorption on microporous materials: Evaluation of energy heterogeneity

Abstract

This paper presents a thermodynamic isotherm derived from solution thermodynamics principles to describe gas adsorption on microporous materials. This isotherm relies on a potential relationship between the integral free energy of adsorption relative to saturation, Ψ / R T , expressed by the Kiselev equation, and the variable Z = 1 / − Ln ( Π ) , being Π the relative pressure. A mathematical analysis reveals that the adsorption energy heterogeneity in the micropores is collected in a characteristic parameter of the isotherm, m, that can be related to the α parameter of the Dubinin–Astakhov isotherm in a simple way ( m = α + 1 ) . The isotherm also predicts a plateau in Ψ / R T at extremely low pressures ( Π < 10 −7 ). Neimark's thermodynamic equation accounting for gas adsorption on mesoporous solids is found to be a particular case of the isotherm presented in this study. The Langmuir isotherm only shows consistency with the thermodynamic isotherm for a reduced combination of values of the relevant parameters, not usually found in common adsorbents. The suitability of the thermodynamic isotherm is experimentally assessed by testing a collection of microporous materials, including activated carbons, carbon nanotubes, and zeolites.