PLATINUM CATALYSTS MODIFIED BY TIN

Platinum catalysts were modified by tin deposition and characterized by electrochemical and spectroscopic methods. They were tested by their adsorption properties and by their activities for the hydrogenation of unsaturated C = C compounds. Tin strengthens bonds between adsorbed hydrogen and platinum and changes the distribution between strongly and weakly adsorbed hydrogen. The effect of tin on the catalytic properties of platinum depends on the previous "history" of the platinum catalysts. On pure platinum the free energy of adsorption of maleic acid decreases (in absolute value), while On "regenerated" platinum (after tin deposition) this parameter increases. In the same way, in the first case, a high initial toxicity of tin is obtained, followed by a plateau for medium tin coverage (0.05 < THETA(Sn) < 0.30-0.50). In the case of a "regenerated" platinum catalyst after elimination of tin by successive oxidation cycles, a progressive and irreversible deactivation as a function of the number of experiments occurs. This result is related to tin diffusion in the bulk of the platinum. Such a complex effect of tin on the platinum catalysts can be explained by the formation of tin species in different oxidation states, which can act as electron donors or acceptors.