REDUCTION OF NO OVER IMPREGNATED CU/ZSM-5 IN THE PRESENCE OF O-2

Cu/ZSM-5 catalysts prepared by impregnation of Cu acetate are active for NO reduction. In mixtures containing both NO and O2, reductants such as CO or CH4 preferentially react with O2, but propane reacts preferentially with NO. Small amounts of O2 actually increase the reduction of NO by C3H8. The N2 yield reaches a maximum near an O2/C3H8 ratio = 5, i.e. the stoichiometry of total C3H8 oxidation. At much higher than stoichiometric O2 contents for total C3H8 oxidation the N2 yield with C3H8 (54%) is still substantially higher than, for instance, with CO of the same or larger concentration. The hypothesis that an intermediate of C3H8 oxidation is responsible for the enhanced NO reduction is discussed. Temperature-programmed reduction shows that after reaction with NO + C3H8 and sub-stoichiometric amounts of O2 Cu in the catalyst is mainly metallic, but CuO particles appear to be formed in the presence of an excess of O2.