Surface and bulk properties of Cu–ZSM-5 and Cu/Al 2O 3 solids during redox treatments. Correlation with the selective reduction of nitric oxide by hydrocarbons

Abstract

This paper deals with the redox properties of Cu ions implanted in ZSM-5 and supported on Al 2O 3, catalysts active in the selective reduction of NO by hydrocarbons such as propane. Data on the reducibility of the Cu systems in various atmospheres (vacuum, CO, H 2, O 2) and on their DeNO x activity are presented. The methods used to obtain informations on the surface and bulk transformations (and their link with catalytic behaviour) are complementary: UV–visible diffuse reflectance spectroscopy being useful to detect the presence of Cu 2+ and Cu 0, while Cu + is detected indirectly by the analysis of the IR spectrum of CO bound selectively to this cation. The main contributions to the previous knowledge are the following: it is possible to distinguish CO bound to isolated and non-isolated Cu + ions; the isolated Cu 2+ ions are reducible under vacuum without participation of organic impurities; the more active solids for the NO reduction into N 2 are characterized by the presence of isolated Cu n+ ions beside the additional influence of the zeolitic framework; after the formation of Cu + ions the redox cycles are reversible but, after the formation of Cu 0, the reversibility or irreversibility of the redox cycles and the restoration of the SCR activity are function of the copper content; the activity decreases after agglomeration into bulk oxides; there is no formation of bulk CuO during the reaction and, with reducing and moderate oxidizing mixtures, part of the copper remains as cuprous ions.