Infrared spectroscopy studies of the mechanism of the selective reduction of NOx over Cu-ZSM-5 catalysts

The adsorption and subsequent reaction of nitric oxide, propene, molecular oxygen and combinations of these gases has been studied on samples of Cu-ZSM-5 catalysts of high activity in the selective reduction of NO by hydrocarbons, using infrared spectroscopy. When only propene is adsorbed, it first forms an adsorbed allyl and then undergoes stepwise oxidation to adsorbed acrolein, carboxylic acid species and carbon oxides. Since no gas phase oxygen is present, these oxidations involve extra-lattice oxygen from the zeolite. These steps highlight the importance of small, oxygen containing copper clusters in the reaction. Exposure of a fresh catalyst to NO and propene results in the formation of an organic nitro compound, as well as the species observed for propene alone. No co-ordinated NO or dimeric species, of the sort that are important in the absence of hydrocarbon, are observed. Continued exposure of the nitro compound to propene results in the formation of nitrile species, which we have reported previously. Almost identical results are observed for the full gas mixture of propene, NO and oxygen, in helium. To probe the possible role of nitro species in the reaction pathway, the interaction of nitromethane with the catalyst has been studied. Nitromethane decomposes selectively, liberating nitrogen in the presence of oxygen over both Cu-ZSM-5 and H-ZSM-5 catalysts. We propose that organic nitro compounds are important intermediates in the reaction, and speculate on a decomposition pathway to nitrogen, based on known organic chemistry, which passes through azoxy or dinitroso species. Nitriles are proposed to form under reducing conditions, but these may also decompose on exposure to oxygen, liberating nitrogen.