Catalytic activity and EXAFS characterisation of three way automotive Pt-Rh/Al2O3-CeO2 catalysts from different preparations

Three way automotive Pt-Rh catalysts were prepared either by coimpregnation of two noble metals (CI catalysts) or by an original method of successive impregnations of Pt and Rh, a reduction step following the platinum impregnation (SI catalysts). According to the preparation procedure it was shown that, especially after oxidative treatment, rhodium can bring about opposite effects on the activity for propane oxidation of platinum atoms deposited on alumina-ceria. The coimpregnation preparation induces an inhibition of platinum activity by the formation of alloy particles with rhodium surface enrichment. On the other hand, SI procedure leads to a selective deposition of rhodium on reduced atoms of cerium avoiding the formation of Pt-Rh alloy. By this way the platinum Pt-0 particles remain active for propane oxidation and are stabilised at high temperature. Such simple description of these two kinds of catalysts, based on catalytic results and some previous physical analysis, must be adjusted by taking into account results of EXAFS characterisation. With this complementary data it is then possible to give a better view of the bimetallic entities in the catalysts, especially concerning the distribution of the two metals in such entities. (C) 1998 Elsevier Science B.V. All rights reserved.