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
H2
18O 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.
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