Self-regeneration of palladium-perovskite catalysts in modern automobiles

It has been found using X-ray diffraction and X-ray absorption fine structure (XAFS) techniques that perovskite-based automotive catalysts, LaFe0.57Co0.38Pd0.05O3 and LaFe0.95Pd0.05O3, retain the high metal dispersion due to the structural responses to the inherent redox fluctuation in exhaust-gas composition with the state-of-the-art gasoline engines. Palladium occupied the B-site (6-fold coordination) of the perovskite structure in the oxidative atmosphere, and segregated out to form small metallic particles in the reductive atmosphere. The local structure around palladium could be changed in a completely reversible manner. The catalysts retained a predominantly perovskite structure throughout a redox (reduction-oxidation) cycle. The agglomeration and growth of the metal particles is suppressed as a result of the palladium movement between inside and outside the perovskite lattice. Moreover, iron plays an important role both to stabilize the perovskite structure and to suppress the growth of palladium particles, although it hardly contributes to the catalytic activity.