Contributions of lattice oxygen in methane combustion over PdO-based catalysts

Zirconia-supported (PdO)-O-16 and bulk (PdO)-O-16 were used as methane combustion catalysts for a reaction mixture containing 1% methane and 4% O-18(2) in helium. The methane oxidation reaction was performed in pulsed experiments and the distribution of oxygen isotopes among the reaction products was monitored. The O-18 content of the catalyst following labeled reaction mixture pulses was determined by catalyst reduction with either diluted hydrogen or diluted methane pulses. The first reaction pulse at each temperature resulted in both CO2 and water containing primarily O-16. AS the O-18(2) uptake from the gas phase increased, however, the O-18 distribution in water and carbon dioxide changed differently. The isotopic composition of water molecules reflected the oxygen isotopic distribution in the bulk of the catalyst particles, as determined by reduction titration experiments carried out after the reaction sequences. The larger concentration of O-18 in the carbon dioxide is explained by the differences in residence time and mobility of the products water and CO2 on the catalyst. The hydrogen/water samples the bulk, while the CO2 reflects the isotopic composition of the surface. The behavior of the zirconia-supported catalyst was similar to that of the bulk PdO at the lowest temperature; however, as the temperature was increased above 600 K, oxygen exchange with the support became important. The catalyst behavior is explained by the presence of a single oxygen species at the catalyst surface: a bridge-bound oxygen to two palladium atoms. The surface is involved in the methane reaction mechanism by successive reduction/reoxidation cycles. Reoxidation uses both bulk and gas phase oxygen, and also oxygen from the support in the case of the zirconia-supported catalyst. Under these conditions the gas phase oxygen exchange with the catalyst is limited by the methane oxidation surface reaction. (C) 2001 Academic Press.