Temperature-dependent adsorption of hydrogen, deuterium, and neon on porous Vycor glass

Adsorption isotherms of ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, and Ne have been measured in the temperature range from 15 K to the corresponding critical points in samples of porous Vycor glass. From the Brunauer-Emmett-Teller theory the surface layer coverages are determined. These are found to be temperature dependent. A model-independent approach allows us to fit the data for coverages ranging from submonolayer to thin film, below capillary condensation, for each adsorbate at all temperatures with a temperature-independent curve. This characteristic curve represents the distribution of adsorption sites versus the adsorption potential. In the intermediate coverage range, the isotherms exhibit the modified Frenkel-Halsey-Hill (FHH) behavior. The adsorption saturates for low-adsorption potentials. The characteristic curve is a useful universal curve since it is roughly the same for the three species investigated. We examine the relative strengths of the surface potentials and densities of the two isotopic modifications of hydrogen and of the more classical Ne adsorbed on porous Vycor glass. The characteristic adsorption curve is compared with results from two models for the adsorbate: Dubinin’s isotherm for microporous solids and its extension to rough surfaces which places importance on the porosity of the surface, and Halsey’s model, which is an extension of the FHH isotherm that takes into account the long-range variations of substrate adsorption potential.