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, molec ular oxygen and combinations of these gases has been studied on sample s 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 stepwis e oxidation to adsorbed acrolein, carboxylic acid species and carbon o xides. Since no gas phase oxygen is present, these oxidations involve extra-lattice oxygen from the zeolite. These steps highlight the impor tance of small, oxygen containing copper clusters in the reaction. Exp osure of a fresh catalyst to NO and propene results in the formation o f an organic nitro compound, as well as the species observed for prope ne alone. No co-ordinated NO or dimeric species, of the sort that are important in the absence of hydrocarbon, are observed. Continued expos ure of the nitro compound to propene results in the formation of nitri le species, which we have reported previously. Almost identical result s 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 pa thway, the interaction of nitromethane with the catalyst has been stud ied. Nitromethane decomposes selectively, liberating nitrogen in the p resence of oxygen over both Cu-ZSM-5 and H-ZSM-5 catalysts. We propose that organic nitro compounds are important intermediates in the react ion, and speculate on a decomposition pathway to nitrogen, based on kn own organic chemistry, which passes through azoxy or dinitroso species . Nitriles are proposed to form under reducing conditions, but these m ay also decompose on exposure to oxygen, liberating nitrogen.