Patricia J. Mitchell

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.

Exhaust-catalyst development for methanol-fueled vehicles: 2. Synergism between palladium and silver in methanol and carbon monoxide oxidation over an alumina-supported palladium-silver catalyst

Methanol and carbon monoxide oxidation were examined over 0.01 Pd, 5% Ag, and 0.01% Pd/5% Ag catalysts—all supported on γ-alumina. The bimetallic catalyst showed greater CO and CH3OH oxidation activity than either of the single-component catalysts; moreover, the Pd and Ag interacted synergistically in the bimetallic catalyst to produce greater CO and CH3OH oxidation rates and lower yields of methanol partial oxidation products than expected from a mixture of the single-component catalysts. Temperature-programmed oxidation experiments and reactivity experiments involving changes in O2 partial pressure both provided evidence that the PdAg synergism results from Pd promoting the rate of O2 adsorption and reaction with CO and CH3OH on Ag. The data also indicate that virtually all of the Pd in the bimetallic catalyst is present in Pd-Ag crystallites.

Effects of rhodium addition on methane oxidation behavior of alumina-supported noble metal catalysts

Abstract Laboratory methane oxidation experiments were conducted using a series of catalyst samples prepared by impregnating Rh-free noble metal catalysts (i.e., Pt/Pd, Pt or Pd) with various amounts of rhodium (0.0035, 0.005, and 0.014 wt.-% Rh). The addition of these small amounts of rhodium did not significantly change the temperature required for the onset of the methane oxidation. However, variable-composition experiments conducted at a temperature characteristic of warmed-up catalytic converters (ca. 550°C) reveal that the rhodium addition tends to shift the optimum feedstream stoichiometry (at which a maximum methane conversion occurs) toward more reducing conditions. These Rh-induced effects on methane conversion behavior appear to be independent of how the rhodium is incorporated in the catalyst.