INVESTIGATION OF THE ACTIVE STATE OF SUPPORTED PALLADIUM CATALYSTS IN THE COMBUSTION OF METHANE

Possible relationships between the structure and activity of supported palladium catalysts for the oxidation of methane have been explored. Samples have been exposed to a wide range of oxidation and reduction pretreatments and the activity for methane oxidation has been determined as a function of time-on-stream, both for pre-oxidised and for pre-reduced samples. Parallel experiments on the rate and extent of oxidation at 300 degrees C have been performed, The results show that the rate of oxidation of pre-reduced palladium samples is very sensitive to the previous thermal history and samples retain a 'memory' of their treatment. In all cases a monolayer of adsorbed oxygen is taken up very rapidly but further oxidation is slower, although the samples approach the bulk oxide state within a few minutes. There are no obvious correlations between the catalytic activity for methane oxidation and the previous treatment of the samples, suggesting a sensitive dependence of activity on the morphology of the samples. It is clear, however, that metallic palladium is not active for methane oxidation. Similarly, chemisorbed oxygen is not as active as oxidised palladium. Moreover, it is found that there is no loss of activity when the palladium is fully oxidised, suggesting that previously reported changes in activity may reflect long-term morphological effects rather than simple over-oxidation of palladium particles.