Theoretical submonolayer adsorption isotherms for hydrogen on a heterogeneous surface

Abstract

The results of theoretical calculations of physical adsorption isotherms for hydrogen on a heterogeneous surface over a pressure range 10 exp–16<P (Torr)<104 and at temperatures of 3.0, 4.2, 6.0, 10, 15, 20, 25, 30, and 40 K are presented. The values of the relative coverage, Θ, span the range 10−11 up to unity. The form of the isotherm used was the Dubinin–Radushkevich equation: ln Θ=−B[RT ln(P/P0)]2 with B−1/2=528 cal/mol, R=1.9865 cal/mol deg, and P0=vapor pressure of hydrogen at the chosen temperature. In regions where this isotherm is not applicable the results have been rounded off to give the correct limiting values, i.e., Θ=kP (Henry’s law with k constant) for parts of the isotherms at T=30 and 40 K, and P=P0 for Θ≫1. Although the calculations predict only relative coverage, they provide a useful way of evaluating the surface roughness factor for hydrogen of an experimental surface. For a porous adsorbent the theory indicates how to convert the experimental data to relative coverage. Once this is done, experimental results can be compared directly with these calculations.