INFLUENCE OF NITROGEN-DIOXIDE ON THE SELECTIVE REDUCTION OF NOX WITH A CATALYST OF COPPER AND NICKEL OXIDES

The selective reduction of NO(x) with ammonia on alumina-supported copper catalysts is shown to be effective when O2 or NO2 are present in the feed. Under steady state conditions, the presence of NO2 in the feed stream increases the overall rate of reduction of NO(x) and simultaneously reduces its dependence on the oxygen concentration. A maximum in activity is found for a molar inlet ratio NO2/NO almost-equal-to 1. It has also been observed that the stoichiometry of the process is a function of the reaction temperature, with a secondary NH3 oxidation reaction appearing at temperatures above 500 K. As revealed by X-ray photoelectron spectroscopy, the copper remains mainly as Cu+ species at steady state conditions. Such species are reoxidized by NO2 more easily than by 02 or NO. An analysis of the transitional states when the feed composition is changed and infrared spectra of the adsorbed species allows us to propose a mechanism which explains the observed results.