Platinum modified by electrochemical deposition of adatoms

Platinized platinum or platinum/graphite catalysts can be modified 'in situ' by submonolayer deposition of different additives (sulphur, copper, lead, tin, iron). An electrochemical method is used based on the underpotential deposition of metals on another more noble metal substrate, by control of its potential at values higher than the Nernst potential of bulk deposition of this additive. Linear potential sweep cyclic voltammetry is used for 'in situ' characterization of modified platinum catalysts, allowing evaluation of the quantity and the degree of coverage of deposited adatoms. Moreover, their adsorption stoichiometry and their oxidation state can be determined. The study of the adsorption, on modified platinum, of olefinic acids (maleic acids), used as probe molecules, allowed evaluation of the 'ligand effects' induced by these additives. Sulphur, copper, lead and tin adatoms behave as 'poisons' of the hydrogenation properties of platinum. The deactivation curves obtained (a = f(theta(poison))) and the initial toxicities of these poisons can be explained by the occurrence of 'ensemble effects' (in the case of copper and sulphur), or by the high surface diffusion of adatoms having low melting points (lead, tin). A restructuring effect of iron, deposited on Pt/graphite has been remarked and can account for the obtained promotion of the hydrogenation catalytic activity of platinum.