TEMKIN-TYPE MODEL FOR THE DESCRIPTION OF INDUCED HETEROGENEITY - CO ADSORPTION ON GROUP-4 TRANSITION-METAL DIOXIDES

CO sigma-coordinated at room temperature on the virtually homogeneous surfaces of some titania and zirconia preparations (showing only one kind of coordinatively unsaturated Me4+) reveals the occurrence of inductive effects among adspecies, as witnessed by the red shift of the stretching frequency and the decrease of the heat of adsorption with increasing coverage. A generalized version of the Temkin equation represents satisfactorily the adsorption isotherm and allowed the calculation of the repulsion energy among adparticles. With structurally heterogeneous surfaces (other preparations of titania, zirconia and hafnia) showing two or three types of coordinatively unsaturated Me4+ cations, revealed by a multiplicity of CO bands, the same model satisfactorily applies to the individual isotherms and also provides evidence that the distribution of the sites is not patchwise. Standard adsorption entropies DELTA(a)S-degrees for both one-species and multispecies systems show a linear dependence upon standard enthalpy DELTA(a)H-degrees, constituting a compensation effect in the free enthalpy term.