Isotopic Studies of Methane Oxidation Pathways on PdO Catalysts

Mechanistic details of CH4 oxidation were examined on PdO/ZrO2 catalysts using isotopic tracer methods and measurements of kinetic isotope effects. Normal kinetic isotope effects were observed using CH4/O2 and CD4/O2 reactant mixtures. The (kH/kD) ratio was between 2.6 and 2.5, and it decreased slightly as the reaction temperature increased from 527 to 586 K. These kinetic isotope effects reflect a combination of kinetic and thermodynamic effects, and the measured values are consistent with rate-determining C–H bond activation steps on surfaces predominantly covered with OH groups. Isotopic equilibration rates for CH4/CD4/O2 mixtures were much lower than methane combustion rates, suggesting that C–H bond activation steps are irreversible on PdO at 473–600 K. Reactions of CH4/18O2 mixtures on Pd16O–Zr16O2 led to the initial formation of C16O2, followed by a gradual increase in the concentration of other CO2 isotopomers as lattice 16O atoms are replaced by 18O from 18O2. The involvement of lattice oxygens in C–H bond activation steps is consistent with a Mars–van Krevelen redox mechanism. Reactions of CH4/16O2/18O2 mixtures lead to all CO2 isotopomers without the concurrent formation of 16O18O. Thus, dissociative oxygen chemisorption is also irreversible during methane combustion. Oxygen atoms in C16O2 exchange with Pd18O–Zr18O2 catalysts at temperatures lower than those required for methane combustion, suggesting that CO2 desorption is quasi-equilibrated. These mechanistic conclusions are consistent with the measured dependence of CH4 oxidation rates on O2, CH4, H2O, and CO2 concentrations. The resemblance between the reaction kinetics on PdO/ZrO2 and on other supported PdO catalysts suggests that the mechanistic conclusions reached in this study are generally valid for methane combustion catalysts based on PdO.