Kinetics of Isothermal Gas Adsorption on Heterogeneous Solid Surfaces: Equations Based on Generalization of the Statistical Rate Theory of Interfacial Transport

Two generalizations of the statistical rate theory (SRT) approach have been developed for the kinetics of adsorption/desorption on/from heterogeneous surfaces and then compared to each other. The first of them is an improved version of the description based on the condensation approximation (CA) used by us. The other one is an exact generalization for the Langmuir model of adsorption. The latter generalization does not lead to simple analytical expressions for the rate of adsorption/desorption as the CA approach does for typical adsorption energy distributions. The comparison of these two approaches suggests, however, that at small and very high surface coverages the CA approach may not be sufficiently accurate. Very intriguing results are obtained when the developed SRT kinetic equations are applied to describe the kinetics of sorption in carbon molecular sieves. It appears that the fit of experimental data is then equally good as that obtained by assuming that the rate of sorption is controlled by surface diffusion in pores. Also, it is shown that the square-root dependence on time of adsorption at small initial coverages cannot be treated as a definite proof for that sorption proceeds via surface diffusion, as is commonly assumed.