A Continuous Binding Site Affinity Distribution Function from the Freundlich Isotherm for the Supercritical Adsorption of Hydrogen on Activated Carbon

A continuous binding site affinity distribution model based on the Freundlich isotherm is proposed to explain the supercritical adsorption of gases on heterogeneous surfaces. The proposed model is successfully applied to determine the binding site affinity distribution of five different pitch-based activated carbons for hydrogen molecules at 77 K (up to 100 kPa) and 298 K (up to 10 MPa). The theoretical limitations and advantages of the proposed model over the commonly used discrete binding model are discussed. According to the proposed model, the affinity distribution curves of the activated carbons are found to be exponentially distributed for hydrogen molecules at the experimental conditions used. Pitch-based activated carbons are used to demonstrate the applicability of the proposed model within the theoretical limitations; this approach is generally applicable to other heterogeneous adsorbents for any target molecule.