Determination of adsorption energies and kinetic parameters by isosteric methods

Abstract

If adsorption is reversible and sufficiently fast, isotherms or isobars can be measured in adsorption–desorption equilibrium at low pressures on single crystal surfaces. The isosteric heat of adsorption can easily be derived from them using the Clausius–Clapeyron equation. Reaction orders and frequency factors can, in principle, be deduced from a fit of the isobars (or isotherms) using the kinetic equations for adsorption and desorption. Immobile and mobile precursor kinetics can be included in the analysis but the fit fails when structural phase transitions in the substrate or the adlayer cause the kinetics to become complex. We review the methods, strong points and limitations of isobar (isotherm) measurements and of their kinetic fits by discussing the adsorption of water, ethylbenzene and styrene on FeO(111), Fe₃O₄(111) and Pt(111) and the adsorption of ammonia on germanium surfaces. Where the kinetic fit was successful, mobile precursor kinetics is quite common and frequency factors for desorption deviate considerably from the often assumed value of 10¹³ s⁻¹.