Catalytic Oxidation of Methane over ZrO2-Supported Pd Catalysts

Abstract

The catalytic oxidation of methane has been examined in an integral reactor over Pd/ZrO2catalysts in this study in order to determine how various preparation pretreatment and reaction variables influence activity. The conversion of methane versus temperature data indicate that mild oxidative and reductive treatments enhance the activity of a 5 wt% Pd/ZrO2catalyst while a higher-temperature reductive pretreatment produces a less efficient catalyst. Increasing the Pd loading from 0.1 to 10 wt% improves catalytic performance while higher loadings yield negligible improvement. Decay studies were performed on a 5 wt% Pd/ZrO2catalyst and compared to those of an optimized Pd/Al2O3catalyst. Under the conditions used in this study, the activity of the Pd/ZrO2catalyst remains fairly constant over a 50-hr period while the Pd/Al2O3catalyst initially exhibits an increase in activity but then a decrease after approximately 16 hr. At 250°C the Pd/ZrO2converts 56% of the methane to CO2and H2O after approximately 45 hr while the Pd/Al2O3catalytically oxidizes only 32% of the methane under the same conditions. An optimized Pd/ZrO2catalyst achieves a methane conversion of 100% below 300°C, which is 40°C lower than that obtained using the optimized Pd/Al2O3catalyst.The 5 wt% Pd/ZrO2CH4-oxidation catalyst also was characterized using X-ray photoelectron spectroscopy before and during heating in vacuum at 180°C and after treatment in a 2 : 1 mixture of O2and CH4at 180°C and 100 Torr for 45 min. The near-surface region of the as-entered catalyst consists mostly of ZrO2and PdO along with some Pd metal. Some of the PdO is reduced to Pd metal at 180°C, which is near the onset temperature for methane oxidation, and the Pd signal is diminished probably due to agglomeration of the Pd. Exposure of the catalyst to a 100 Torr mixture of 2 : 1 O2and CH4at 180°C for 45 min results in the formation of adsorbed CHx, CO and CO=3and/or HCO−3species. These data suggest that the CH4hydrogen bonds are first broken to form an adsorbed carbon species, which reacts with surface oxygen to form an adsorbed CO. This CO then reacts to form a surface carbonate or bicarbonate species which decomposes to form CO2.