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 comb ustion catalysts for a reaction mixture containing 1% methane and 4% O-18(2 ) in helium. The methane oxidation reaction was performed in pulsed experim ents and the distribution of oxygen isotopes among the reaction products wa s monitored. The O-18 content of the catalyst following labeled reaction mi xture pulses was determined by catalyst reduction with either diluted hydro gen 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) u ptake 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 cat alyst particles, as determined by reduction titration experiments carried o ut after the reaction sequences. The larger concentration of O-18 in the ca rbon 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. T he behavior of the zirconia-supported catalyst was similar to that of the b ulk PdO at the lowest temperature; however, as the temperature was increase d above 600 K, oxygen exchange with the support became important. The catal yst behavior is explained by the presence of a single oxygen species at the catalyst surface: a bridge-bound oxygen to two palladium atoms. The surfac e is involved in the methane reaction mechanism by successive reduction/reo xidation cycles. Reoxidation uses both bulk and gas phase oxygen, and also oxygen from the support in the case of the zirconia-supported catalyst. Und er these conditions the gas phase oxygen exchange with the catalyst is limi ted by the methane oxidation surface reaction. (C) 2001 Academic Press.