EFFECT OF HIGH-TEMPERATURE LEAN AGING ON THE PERFORMANCE OF PT,RH/CEO2 AND RARE-EARTH ALKALINE-EARTH DOPED PT,RH/CEO2 CATALYSTS

The effect of high temperature lean aging on the performance and physico-chemical properties of a series of Pt,Rh/CeO2 catalysts doped with rare earths and Ba has been studied. Catalyst performance was evaluated in a full complement exhaust gas mixture containing H2, CO, C3H6, C3H8, NO, CO2, H2O, N2 and SO2. It was found that doping the CeO2 lattice with rare earths such as La, Y and Nd leads to severe loss of activity after aging. The undoped and Ba doped catalysts showed high activity and extensive activation after exposure to rich exhaust gas mixtures. Characterization of the aged catalysts using TPR correlated the loss of activity with decoupling of the low temperature synergistic reduction of the noble metal (NM) and surface Ce. Characterization of the catalysts after exposure to rich exhaust gas mixtures further shows that catalyst activation and subsequent high activity was associated with reduction of both the NM and surface Ce. In situ XPS analysis showed that the rare earth led to migration or burial of the NM in the CeO2 support with accompanying retardation of both surface Ce and NM reduction. The results are consistent with a previously proposed hypothesis that the active state of the catalyst is reduced surface Ce in contact with reduced NM.