Surface and bulk properties of Cu–ZSM-5 and Cu/Al2O3 solids during redox treatments. Correlation with the selective reduction of nitric oxide by hydrocarbons

This paper deals with the redox properties of Cu ions implanted in ZSM-5 and supported on Al₂O₃, 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, H₂, O₂) 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 Cu²⁺ and Cu⁰, 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 Cu²⁺ ions are reducible under vacuum without participation of organic impurities; the more active solids for the NO reduction into N₂ are characterized by the presence of isolated Cuⁿ⁺ 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 Cu⁰, 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.