Role of the state of the metal component on the light-off performance of Pd-based three-way catalysts

摘要

The behavior of Pd-based systems supported on alumina and ceria-zirconia and the complex ceria-zirconia on alumina was investigated during light-off-tests using a stoichiometric C_3H_6+CO+NO+O_2 mixtur.In situ XANES/DRIFTS studies of the Pd chemical state at the surface and bulk of the materials were conducted to determine the noble metal response to the gas atmosphere during light off.The initial,oxidized PdO-like phase was converted to Pd(0) in all cases except for the ceria-zirconia-supported catalyst.Contact between Pd and bulk ceria-zirconiaparticles appeared to stabilized Pd clusters in the 373-673K region with an average Pd(I)-like oxidation state.The presence of hydrocarbon in the gas phase was essential in order to stabilize these Pd(I)-like species.The role of such a phase in providing a source of active oxygen for hydrocarbon oxidation is shown here to strongly decrease light off temperature and this,together with its stability over a wide range of temperatures,may offer a contribution to reducing hydrocarbon emissions during cold start.