PREPARATION AND CHARACTERIZATION OF PD-PT AL2O3 CATALYSTS/

Bimetallic Pd-Pt/Al2O3 catalysts were prepared by controlled surface r eactions. The reduction of PtCl62- can be made by using as reductant h ydrogen preadsorbed on a parent monometallic Pd/Al2O3, catalyst (refil ling), or metallic palladium itself (direct redox reaction). Two precu rsors (Pd(NH3)(4)Cl-2 and Pd(Acac)(2)) were used to prepare the (1 wt % Pd) parent catalyst. The platinum loading is within the range 0.13-0 .14 wt %. The characterization of the catalysts was made by hydrogen c hemisorption, infrared spectroscopy of adsorbed CO (as probe molecule) and E.X.A.F.S. The two preparation methods lead to different kind of platinum deposit, whatever the palladium parent catalyst. By using pre adsorbed hydrogen, platinum can be deposited in decoration on low coor dination sites (corners, edges,...) of palladium particles. By direct redox reaction, the deposit of platinum atoms occurs preferentially on the (111) faces of palladium particles. However some sintering can oc cur, especially during hydrogen treatment in aqueous medium, That sint ering is enhanced by chloride ions.