PERFORMANCE AND STRUCTURE OF PT-RH 3-WAY CATALYSTS - MECHANISM FOR PTRH SYNERGISM/

The catalyst performance and structure of Pt and Rh catalyst systems a re studied for fresh catalysts and samples aged to mimic in-use cataly st performance. It was observed that both fresh Pt and Rh catalysts ar e highly active for three-way conversions of HC/CO/NO. However, after exposure to exhaust at elevated temperature, Pt and Rh catalysts deter iorated severely, and only the Pt-Rh catalyst remains highly active fo r the three-way conversions. In order to understand this observed syne rgism between Pt and Ph in the Pt-Ph catalyst, AEM, XPS, and XRD were used to characterize catalyst structural change and establish a correl ation between catalyst performance and structure. Pt catalysts deactiv ate at high temperature under reducing conditions and more severely un der oxidizing conditions. Ph catalysts deactivate under oxidizing cond itions by forming Ph-aluminate species. The regeneration of these Rh-a luminate species to highly active Ph metal particles under reducing co nditions take place at elevated temperature, e.g., 1173 K, which is si gnificantly higher than normal operating temperature (673 K). For the Pt-Rh catalyst, Pt and Ph go through deactivation cycles the same as t hat for the individual Pt and Rh catalysts in the simulated aging proc ess. However, Ph in the aged Pt-Ph catalyst can regenerate in exhaust at temperatures as low as 560 K, which is significantly lower than the normal operating temperature. This is attributed to H-2/CO spillover on Pt particles contained in the Pt-Ph catalyst which facilitate the r egeneration of the inert Ph-aluminate species to active Rh metal parti cles. (C) 1998 Academic Press.