Oxidation of methane over supported precious metal catalysts

Abstract

Studies of the kinetics of methane oxidation over palladium and platinum catalysts supported on a number of metal oxides were made using a pulse-flow microreactor technique. The reaction was investigated at temperatures in the range 500–800 K using reactant mixtures with CH4:O2 ratios varying from 1:10 to 10:1. The supports themselves, with the exception of tin(IV) oxide, did not appreciably catalyze the oxidation reaction. Nevertheless, the activity of both palladium and platinum catalysts was enhanced by the use of metal oxides of large surface area, such as γ-aluminum(III) oxide. The support also influenced the ability of palladium to adsorb oxygen and there was a correlation between the adsorption capacity of the supported precious metal and its catalytic activity for methane oxidation. The presence of adsorbed oxygen was confirmed using X-ray photoelectron spectroscopy but oxygen adsorbed at high temperatures proved detrimental towards methane oxidation. Prolonged exposure of the precious metals to oxygen at high temperatures caused structural changes in palladium, although this did not occur with platinum. The different catalytic behaviour of palladium and platinum was attributed to the differing abilities of these precious metals to adsorb oxygen prior to reaction. Measurement by transmission electron microscopy of the diameters of palladium particles dispersed on a γ-aluminum(III) oxide support showed that the catalytic activity for methane oxidation was independent of particle size.