Chapter 4 - The Interpretation of Empirical Isotherms in Terms of Surface Heterogeneity

Abstract

This chapter discusses the interpretation of empirical isotherms in terms of surface heterogeneity and two of the simpler methods of relating adsorption isotherms to the site energy distribution. In surface heterogeneity of adsorption isotherms, the theoretically correct true kernel (or the local isotherm) is replaced by the step function or the condensation isotherm. This method is called the condensation approximation (CA). In the so-called asymptotically correct condensation approximation (ACCA) method, the local isotherm is taken to be a combination of a linear and a condensation isotherm. In that case, in effect, as the pressure increases, the surface patches are filled gradually in the sequence of decreasing adsorption energies. In the absence of electronic degeneracy, the molecular partition function of the adsorbed molecule q a is the product of three harmonic oscillator partition functions. Numerous calculations have shown that the energy distribution functions evaluated from experimental data at various temperatures are really quite similar. The empirical equations of heterogeneous surface are indeed simplified forms of some other “correct” isotherm equations.