The role of adsorbed phase volume on the thermodynamics of supercritical methane adsorption on microporous carbon

Abstract

Experimental determination of the isosteric heat of adsorption at the fluidsolid interface is an important undertaking in the chemical sciences since this fundamental thermodynamic quantity is closely related to the binding energy of the adsorbate on the adsorbent surface. The usual methods employed to calculate the isosteric heat from measured gas adsorption equilibria, however, are unsuited to the treatment of adsorption under a high-pressure adsorptive fluid (where the difference in molar volume between the two phases becomes small and depends significantly on that of the adsorbed phase). Herein we employ a methodological approach to the thermodynamic analysis of adsorption up to high pressures in the supercritical regime, with a specific focus on methane adsorption on microporous carbonaceous materials at T/T c between 1.25-2.75 and P/P c up to 2. The aim is to achieve a meritorious description of the thermodynamics of the adsorbed phase with as few independent parameters as possible. We compare several simple approaches to estimating the molar volume of the adsorbed phase, and demonstrate that among the several well-known sources of error involved in the isosteric approach, that attributed to molar volume estimations is not itself prohibitive to achieving meritorious results. We contrast the isosteric approach with that of the so-called 'isoexcess' methodology, and thereby shed new insights into the key role of the finite adsorbed phase volume in assessments of adsorption thermodynamics.