Role of Lattice Oxygen in the Combustion of Methane over PdO/ZrO2: Combined Pulse TG/DTA and MS Study with 18O-Labeled Catalyst

Abstract

The contribution of a redox mechanism involving lattice oxygen in the catalytic combustion of methane over PdO/ZrO₂ catalysts, prepared from amorphous Pd?Zr alloys, has been studied by means of gas pulse methods, including a novel technique “pulse thermal analysis”, and using labeled catalysts containing Pd¹⁸O. Special emphasis was devoted to the influence of the isotope exchange (scrambling) of reactants and products, especially O₂ and CO₂, with the catalyst on the quantity of ¹⁸O-containing reaction products. Substantial amounts of H₂¹⁸O and C¹⁸O¹⁶O were detected during pulses of a reactant mixture consisting of methane and ¹⁶O₂ in a ratio 1:4 at 300 and 500 °C. The effect of the oxygen exchange of molecular oxygen with the solid phase proved to be negligible due to its low extent. At 300 °C, at least 20% of the CO₂ formed originated from the redox mechanism involving lattice oxygen. At 500 °C, oxygen exchange of CO₂ with the catalyst became predominant and precluded determining reliably the proportion of CO₂ formed by the redox process. The results indicate that a substantial part of methane is oxidized via a redox process. Consequently, this reaction has to be taken into account when interpreting the catalytic behavior of palladium-based catalysts and explaining the structure?activity relations previously observed.