PD-PT AL2O3 CATALYSTS - PREPARATION BY REDOX REACTIONS, CHARACTERIZATION AND CATALYTIC ACTIVITY/

Pd-Pt/Al2O3 catalysts are prepared by controlled surface redox reactio ns, the refilling or the direct redox reaction. By the refilling metho d, the reduction of platinum salt is made by hydrogen preadsorbed on t he palladium parent catalyst. By the direct redox reaction, the metall ic palladium of the parent catalyst acts directly as reducer. The char acterization of bimetallic catalysts by different techniques (metal ac cessibility, IR, EXAFS) points out a platinum selective deposit over p alladium according to the preparation method under consideration. By u sing preadsorbed hydrogen, platinum is deposited on low coordination s ites of palladium, whereas by direct redox reaction, the deposit occur s preferentially on the palladium faces. The knowledge of the bimetall ic species structure allows to explain the catalytic results for the O -18/O-16 isotopic exchange.