Mechanistic Study of the Selective Catalytic Reduction of Nitric Oxide with Methane over Yttrium Oxide

Abstract

The catalytic activity of nanocrystalline Group IIIB metal oxides for the reduction of nitric oxide with methane was shown to be comparable to that of Co-ZSM-5. The mechanism of selective catalytic reduction of nitric oxide with methane in excess oxygen was examined over nanocrystalline yttrium oxide. A series of heterogeneous and homogeneous reaction steps was proposed to account for the observed trends in catalytic properties. Methyl radicals generated at the catalyst surface desorb into the gas phase, where they react with nitric oxide to form nitrosomethane. Nitrosomethane then decomposes in a series of homogeneous and heterogeneous reactions to produce nitrogen and nitrous oxide. Evidence for gas-phase reaction of methyl radicals with nitric oxide was found in the adsorption studies of nitric oxide on yttrium oxide, the presence of ethane and ethene in the reactor effluent, catalytic studies involving nitrosomethane and nitromethane, as well as the successful prediction of methane selectivities based on a homogeneous reaction mechanism for methyl radical consumption. The proposed pathway for nitrogen production was supported by the observation of hydrogen cyanide under certain operating conditions, as well as adsorbed NCO species detected by infrared spectroscopy.