Final Report on Pd Catalysts , March , 2001

Zirconia-supported Pd160 and bulk Pd160 were used as methane combustion catalysts for a reaction mixture containing 1% methane and 4% l8Oz in helium. The methane oxidation reaction was performed in pulsed experiments and the distribution of oxygen isotopes among the reaction products was monitored. The “0 content of the catalyst following labeled reaction mixture pulses was determined by catalyst reduction with either diluted hydrogen or diluted methane pulses. At low reaction temperature both COz and water contained primarily I6O. As temperature increased, however, the l80 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 post reaction reduction experiments. The larger concentration of l 8 0 in the carbon dioxide is explained by the differences in residence time and mobility of the products water and COz on the catalyst. The hydrogedwater samples the bulk, while the C02 reflects the surface composition. The behavior of the zirconia supported catalyst was similar to the bulk PdO at the lowest temperature; however, as the temperature was increased above 600K, oxygen exchange with the support became important. catalyst surface: a bridge-bound oxygen to two palladium atoms. The surface is involved in the methane reaction mechanism by successive reductiodreoxidation cycles. Reoxidation uses both bulk and gas phase oxygen, and also oxygen from the support in the case of zirconia supported catalyst. Under these conditions the gas phase oxygen exchange with the catalyst is limited by the methane oxidation surface reaction. The catalyst behavior is explained by the presence of a single oxygen species at the