Redox behavior of palladium at start-up in the Perovskite-type LaFePdOx automotive catalysts showing a self-regenerative function

Abstract

In order to elucidate the superior start-up activity of LaFePdOx catalysts in practical automotive emission control, the redox property of Pd species in a Perovskite-type LaFe0.95Pd0.05O3 catalyst was studied at temperatures ranging from 100 to 400°C using X-ray spectroscopic techniques. In a reductive atmosphere, and even at temperatures as low as 100°C, Pd0 species is partially segregated out onto the catalyst surface from the B-site of the Perovskite-type matrix of LaFe0.95Pd0.05O3. Passing through successive oxidizing atmospheres, the segregated Pd0 species is re-oxidized into Pd2+ at 200–300°C. The formation of a solid solution between the re-oxidized Pd species and the Perovskite-type matrix begins to be seen at around 400°C and accelerates at higher temperatures. Thus a quasi-reversible redox reaction between the surface Pd0 and the cationic Pd in the LaFe0.95Pd0.05O3 matrix takes place. The start-up activity of LaFePdxOx catalysts can be attributed to Pd0 that segregates under the reductive atmosphere which is a natural part of the redox fluctuation in automotive exhaust gases at 100–200°C.