On the gamma-type heterogeneity of adsorbing surface

Abstract

The gamma-type distributions in adsorption energies of sites, subject to the Langmuir probability of occupancy, are analyzed with the aim to develop criteria for the reliability of estimates of the energy distribution from experimental adsorption data. For broad distributions this model leads to preferential occupation of sites with highest adsorption energies, which causes the breakdown of the virial expansion of adsorption isotherms. The resulting isotherms can be discriminated from simple Langmuir isotherms only for not too narrow distributions and the critical criterion is given. Finally, it is pointed out that experimental data do not allow to discern between a continuous and discrete distribution. All the conclusions can be generalized to other types of distribution in adsorption energies, including composite distributions.