Physical Adsorption on Non-Uniform Surfaces

Abstract

Analysis of adsorption on a uniform surface using the quasi‐chemical theory of interaction shows that the hypotheses of the BET theory lead to substantially no adsorption beyond the first layer if E₂ = E_L, and stepwise isotherms if E₁>E₂>E₃>···>E_L. For the monolayer adsorption of gases on silver, platinum, and steel, the heterogeneous nature of the adsorbing surface is clearly indicated. Cooperative adsorption on the non‐uniform surface is treated, and the cooperative analog of the Freundlich equation derived. The typical multi‐layer isotherm is shown to be composed of three regions: non‐cooperative adsorption on a strongly heterogeneous surface; cooperative adsorption on a still heterogeneous surface; and cooperative multi‐layer adsorption induced by small van der Waals perturbations some distance from the surface. The isotherm p/p₀=exp{—a/θ^r} is derived and shown to be a good representation of adsorption data that conform to the BET Type I, II, or III shapes.