Oxygen Exchange between Palladium and Oxide Supports in Combustion Catalysts

Temperature programmed isotopic exchange (TPIE) of oxygen and pulsed methane combustion experiments were used to investigate the contribution of oxygen from the support in the methane combustion reaction over alumina- and zirconia-supported palladium. TPIE experiments show that PdO is more active than reduced palladium for oxygen exchange between the catalyst and gas phase. Experiments performed on initially reduced catalysts produced a partially oxidized palladium phase containing both oxygen isotopes, suggesting that oxygen exchange between solid phases proceeds simultaneously with Pd oxidation. Reactivity tests performed with labeled reaction mixture pulses showed an important contribution of the support in the overall oxygen balance. Adsorbed oxygen is much more rapidly exchanged on zirconia-supported Pd than on alumina-supported Pd, and the isotopic exchange between the gas phase and the catalyst is strongly accelerated in the presence of methane. This is assigned to an increase in the concentration of oxygen vacancies in the support via oxidation of reaction intermediates on the support.