Abstract

A variety of transition metal oxides, consisting of a transition metal (manganese, copper, cerium or cobalt) oxide and zirconia, were investigated for the combustion of methane and propane and attempts were made to associate surface, redox and structural properties with catalytic activity. In particular, the mangancse-zirconia system was studied in detail. The effect of high temperature (800°C) aging on the surface areas, structure, reducibility and activity of the calcined catalysts was determined. The preparation method used had a significant effect on the structures o f all the oxides studied. Ceria-zirconia mixed oxides prepared by a modified sol-gel method had higher surface areas and higher activity than the corresponding samples prepared by the coprecipitation method. Manganese-zirconia, cobalt-zirconia and copper-zirconia mixed oxides were all prepared by the co-precipitation method and the final properties o f the oxides were very sensitive to changes in preparation parameters within this method. Variations investigated included stirring time of precursor solution prior to addition o f base, precursor concentrations, pH or volume of base added and addition o f base to precursor solution or vica versa. Of all the mixed oxides studied, the manganese-zirconia system, prepared by addition o f ammonium hydroxide, show the best potential as possible alternatives to noble metal catalysts. Manganese oxide and manganese-zirconia mixed oxides were more active for methane combustion than a 0.5 wt% Pt/A^Os. On aging, the mixed oxides gained activity in propane oxidation, while the temperatures at which 50% conversion o f methane occurred remained the same, despite a significant decrease in surface. An increase in reducibility, as indicated by a decrease in the temperature corresponding to maximum reduction, occurred on aging for the mixed oxides. Under the same conditions, only a small decrease in activity o f manganese oxide for both propane and methane oxidation occurred on aging, suggesting that the preparation method, as well as the presence o f zirconium, may play an important role in the increased activity and reducibility on aging.