Mechanistic study of the selective catalytic reduction of nitric oxide with methane over yttrium oxide

A heterogeneous/homogeneous reaction mechanism was proposed to explain the high catalytic activities observed for the SCR of NO_x with CH₄ over yttrium oxide. CH₃ radicals generated at the catalyst surface desorbed into the gas phase, where they reacted with NO to form nitrosomethane. Nitrosomethane then decomposed in a series of homogeneous and heterogeneous reactions to produce N₂ and N₂O. Predictions of CH₄ selectivity based on a homogeneous reaction model for the consumption of CH₃ radicals via reactions with O₂, NO, and NO₂ agreed well with experimental data. Evidence for gas-phase reaction of CH₃ radicals with NO was found in the adsorption studies of NO on yttrium oxide, the presence of ethane and ethylene in the reactor effluent, and catalytic investigations involving nitrosomethane and nitromethane. The proposed pathway for N₂ production was supported by the observation of hydrogen cyanide and adsorbed NCO species. Both the homogeneous reaction model and an analysis of catalytic reactions of possible intermediates suggested that nitrosomethane is the key reaction intermediate in the SCR of NO_x with CH₄ over yttrium oxide.