METHANE OXIDATION OVER ALUMINA-SUPPORTED NOBLE-METAL CATALYSTS WITH AND WITHOUT CERIUM ADDITIVES

Laboratory reactor experiments have been conducted to evaluate alumina-supported noble metal catalysts, both in the presence and absence of cerium additives, for their effectiveness in the catalytic oxidation of methane under conditions likely to be encountered in natural-gas vehicle exhaust. Under oxidizing conditions, all of the catalysts promoted the complete oxidation of methane to CO2 and H2O, with the methane oxidation activity ranking given by Pd > Rh > Pt in the absence of Ce and by Rh > Pd ≈ Pt in the presence of Ce. Under reducing conditions, methane oxidation produced substantial amounts of CO and H2 as the principal partial oxidation products. In the absence of Ce, the methane oxidation activity decreases in the order Pd > Rh > Pt, with the tendency to form CO decreasing in the order Rh > Pd > Pt. The activity ranking for methane conversion in reducing feedstreams was not affected by the presence of Ce; however, the addition of Ce to the Pt/Al2O3 and Pd/Al2O3 catalysts almost completely suppressed the formation of the partial oxidation product CO. At a fixed temperature of ≈550°C, the methane conversion over each of the noble metal catalysts goes through a maximum as the feedstream concentration of O2 is varied. The data suggest that O2 inhibits the CH4 oxidation under oxidizing conditions by excluding the more weakly adsorbed species, CH4, from the active sites. Also, methane oxidation experiments in the presence of CO in the feed showed that the methane conversion characteristics of the noble metal catalysts are little affected by the CO. © 1991.