On an isotherm thermodynamically consistent in Henry’s region for describing gas adsorption in microporous materials

The Dubinin–Astakhov and Dubinin–Radushkevich isotherms, originally formulated from the classical volume filling theory of micropores, constitute the most accepted models for describing gas adsorption in microporous materials. The most important weakness of these equations relies on the fact that they do not reduce to Henry’s law at low pressures, not providing therefore a proper characterization of adsorbents in the early stage of adsorption. In this paper, we propose a way out of this inherent problem using the thermodynamic isotherm developed in a previous study [J. Llorens, M. Pera-Titus, J. Colloid Interface Sci. 331 (2009) 302]. This isotherm allows the generation of a series of equations that make available a comprehensive description of gas adsorption for the whole set of relative pressures (including Henry’s region), also providing explicit information about energy heterogeneity of the adsorbent through the two characteristic m   parameters of the thermodynamic isotherm (i.e., m1m1 and m2m2). The obtained isotherm converges into the Dubinin–Astakhov isotherm for relative pressures higher than 0.1, the characteristic αα parameter of this isotherm being expressed as α=m2-1α=m2-1 and the affinity coefficient (β)(β) as a sole function of m2m2. An expression differing from the Dubinin–Astakhov isotherm has been obtained for describing Henry’s region, providing relevant information about confinement effects when applied to zeolites.

Overcoming the intrinsic limitations of the Dubinin–Astakhov isotherm in Henry’s region at low pressures using a new thermodynamic isotherm formulation.Figure optionsDownload high-quality image (118 K)Download as PowerPoint slide