MECHANISMS OF DEPOSITION OF SPECIES CONTAINING CATALYTICALLY ACTIVE IONS ON THE OXIDIC SUPPORT ELECTROLYTIC SOLUTION INTERFACES - A UNIFIEDAPPROACH BASED ON THE 2-PK ONE-SITE AND TRIPLE-LAYER MODELS/

A general methodology for investigating the mechanisms of deposition o f ionic species containing catalytically active elements on the oxidic support/electrolytic solution interfaces has been developed. The meth odology is based on the ''two-pK/one-site'' and ''triple-layer'' model s (for the charging mechanism of the surface of the support, and for t he electrical double layer developed between the surface of the suppor t, and the impregnating solution, respectively) and exploits the exper imental data of the adsorption isotherms at various pH of the impregna tion suspension. Starting from a quite general, ''postulated'', deposi tion mechanism, comprising all equilibria that could possibly take pla ce at the ''support/impregnation solution'' interface, the application of this methodology leads to the ''proposed'' deposition mechanism, n amely, the set of equilibria that actually take place, the kind of dep osited species formed, their relative concentrations, and their depend ence from the impregnating parameters (pH, concentration of the precur sor, ionic strength, and temperature). The derived equations, describi ng the deposition of a given ionic species on the support surface, sho w that the concentration of the deposited species depends on various f actors among which the most important are the charge of the ionic spec ies, its concentration in the bulk impregnating solution, the surface concentration of the receptor sites on the support surface, the pH, an d the Galvani and Volta potentials developed, respectively, on the sur face of the solid particles and at the inner Helmholtz plane. As a tes t for the validity of the proposed deposition model, the theoretical v alues of some ''support/electrolyte interface'' parameters (calculated on the basis of the deposition mechanism revealed by applying this me thodology) are compared with the corresponding experimental values det ermined by potentiometric titrations and microelectrophoresis.