Isotopic studies of methane oxidation pathways on PdO catalysts
Mechanistic details of CH{sub 4} oxidation were examined on PdO/ZrO{sub 2} catalysts using isotopic tracer methods and measurements of kinetic isotope effects. Normal kinetic isotope effects were observed using CH{sub 4}/O{sub 2} and CD{sub 4}/O{sub 2} reactant mixtures. The (k{sub H}/k{sub D}) ratio was between 2.6 and 2.5, and it decreased slightly as the reaction temperature increased from 537 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 CH{sub 4}/CD{sub 4}/O{sub 2} mixtures were much lower than methane combustion rates, suggesting that C-H bond activation steps are irreversible on PdO at 473--6,090 K. Reactions of CH{sub 4}/{sup 18}O{sub 2} mixtures on Pd{sup 16}O-Zr{sup 16}O{sub 2} led to the initial formation of C{sup 16}O{sub 2}, followed by a gradual increase in the concentration of other CO{sub 2} isotopomers as lattice {sup 16}O atoms are replaced by {sup 18}O from {sup 18}O{sub 2}. The involvement of lattice oxygens in C-H bond activation steps is consistent with a Mars-van Krevelen redox mechanism. Reactions of CH{sub 4}/{sup 16}O{sub 2}/{sup 18}O{sub 2} mixtures lead to all CO{sub 2} isotopomers without the concurrent formation of {sup 16}O{sup 18}O. Thus, dissociative oxygen chemisorption is also irreversible during methane combustion. Oxygen atoms in C{sup 16}O{sub 2} exchange with Pd{sup 18}O-Zr{sup 18}O{sub 2} catalysts at temperatures lower than those required for methane combustion, suggesting that CO{sub 2} desorption is quasi-equilibrated. These mechanistic conclusions are consistent with the measured dependence of CH{sub 4} oxidation rates on O{sub 2}, CH{sub 4}, H{sub 2}O, and CO{sub 2} concentrations. The resemblance between the reaction kinetics on PdO/ZrO{sub 2} 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.
- Research Organization:
- Univ. of California, Berkeley, CA (US); Lawrence Berkeley National Lab., CA (US)
- OSTI ID:
- 20003855
- Journal Information:
- Journal of Catalysis, Vol. 188, Issue 1; Other Information: PBD: 15 Nov 1999; ISSN 0021-9517
- Country of Publication:
- United States
- Language:
- English
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