NO REDUCTION OVER ALUMINA-SUPPORTED CU AND CU-CR STUDIED WITH THE STEP-RESPONSE METHOD

The NO reduction by CO over alumina-supported Zu and Cu-Cr has been In vestigated by means of transient experiments using step changes in the feed concentration, First, the isothermal reduction by CO and oxidati on by NO of the catalysts was studied, Nest, the influence of oxygen o n the NO conversion was determined. NO is adsorbed on the oxidized cat alysts as nitrite/nitrate complexes and as Cu2+-NO. The nitrite/nitrat e complexes decompose only in a reducing atmosphere, whereas NO readil y desorbs from Cu2+ in, He, CO is retained on the catalysts in a CO/NO /He atmosphere. It was found that the conversion of NO is not only lim ited by O-2 in the gas phase, but that NO also competes with subsurfac e oxygen to oxidize the surface of the catalysts, Ln other words, NO i s only converted when the catalysts are in a reduced slate, Since the activity of the Cu-Cr catalyst in NO reduction was found to he higher than that of the Cu catalyst, it is concluded that er helps to keep th e Cu in a more reduced state. N2O may be formed under dynamic conditio ns even when it is not observed in the steady state, It was nicely dem onstrated that the step-response experiments allow the calculation of the concentrations of the different surface species. These concentrati ons strongly depend on the reaction conditions. (C) 1997 Academic Pres s.