Stepwise adsorption at the same site. A thermodynamic treatment

Abstract

Either elementary statistical thermodynamics or the use of chemical equilibria permits the treatment of the adsorption of L ligands on S centres with more than one coordination vacancy. The case of two coordinative unsaturations, i.e. the formation of surface complexes up to SL₂, is treated in an exact, analytical way. The properties computed are: (i) the partition of L ligands into S, SL and SL₂ species, (ii) the overall adsorption isotherm and (iii) the molar and differential energies of adsorption. The case of an unstable SL complex is also considered. With regard to the case of three coordinative unsaturations, i.e. the formation of surface complexes up to SL₃, the problem can only be solved numerically, although some overall features can be calculated analytically. The case of unstable complexes, either SL or SL₂, is also treated. In the absence of interactions within the same site, the problem is analytically soluble: the adsorption isotherm is Langmuirian, and the partition of the adsorbate into different SL_i surface complexes is governed by a statistical binomial distribution.