ROLE OF LATTICE OXYGEN IN THE COMBUSTION OF METHANE OVER PDO ZRO2 - COMBINED PULSE TG/DTA AND MS STUDY WITH O-18-LABELED CATALYST/

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 method s, including a novel technique ''pulse thermal analysis'', and using l abeled catalysts containing (PdO)-O-18. Special emphasis was devoted t o the influence of the isotope exchange (scrambling) of reactants and products, especially O-2 and CO2, with the catalyst on the quantity of O-18-containing reaction products. Substantial amounts of (H2O)-O-18 and (COO)-O-18-O-16 were detected during pulses of a reactant mixture consisting of methane and O-16(2) in a ratio 1:4 at 300 and 500 degree s C. The effect of the oxygen exchange of molecular oxygen with the so lid phase proved to be negligible due to its low extent. At 300 degree s C, at least 20% of the CO2 formed originated from the redox mechanis m involving lattice oxygen. At 500 degrees C, oxygen exchange of CO2 w ith the catalyst became predominant and precluded determining reliably the proportion of CO2 formed by the redox process. The results indica te that a substantial part of methane is oxidized via a redox process. Consequently, this reaction has to be taken into account when interpr eting the catalytic behavior of palladium-based catalysts and explaini ng the structure-activity relations previously observed.