Catalytic oxidation of methane over palladium supported on alumina : Effect of aging under reactants

Abstract

A Pd/Al 2O 3 catalyst was aged at 600°C under a methane, oxygen and nitrogen mixture with an oxygen to methane ratio of 4. The aged catalyst was more active than the fresh one, especially at low temperature (below 400°C). The temperature of half conversion was decreased by 70°C. Aging led to a decrease of the metal dispersion: the palladium particle size increased from 7-16 nm. The turnover number (activity per surface palladium atom) was strongly enhanced (factor 20 at 400°C) on the aged sample. In that sense, catalytic combustion of methane must be considered as a structure-sensitive reaction. The reactivity of adsorbed oxygen towards hydrogen increased with the metal particle size; this parallels the increase in the rate of methane oxidation with particle size. Temperature-programmed oxidation measurements suggested that palladium was in the form of bulk palladium oxide for reaction temperatures in excess of 400°C. The formation of bulk PdO appears to be also metal particle size dependent. An instability of the conversion as a function of time at a given temperature was observed. The instability was associated with the slow formation of bulk PdO.