Methane Oxidation over Palladium on Zirconia Prepared from Amorphous Pd1Zr3 Alloy

Abstract

Catalysts for methane oxidation have been prepared by oxidation of amorphous Pd1Zr3 metal alloy under different conditions. A sequence of chemical and morphological changes occurring during the alloy oxidation process leads to highly active microporous palladium/zirconia catalysts suitable for the deep oxidation of methane. Depending on the activation temperature, distinct PdO species interacting with zirconia exist in these catalysts, as revealed by oxygen desorption and thermoanalytical measurements. Smaller PdO species strongly interacting with zirconia were found to decompose at a markedly higher temperature than larger PdO species, which only interact weakly with the support. Kinetic studies were carried out in a fixed-bed microreactor at temperatures of 400-800 K and atmospheric pressure using a reactant mixture with a ratio CH4 : O2 = 1 : 4. Turnover frequencies, based on the exposed palladium surface atoms determined by CO chemisorption, were more than two times higher than those of palladium on zirconia catalysts prepared by impregnation or by coprecipitation. Methane oxidation measurements carried out over the conventionally prepared palladium/zirconia catalysts and a corresponding catalyst derived from Pd1Zr3 showed CO and H2 selectivities higher than 90% under reducing conditions, i.e., with a reactant ratio CH4 : O2 = 8 : 1 at 870 K.