STRONG INTERACTION OF NANOSTRUCTURED CuO-LIKE SPECIES WITH ZnO IN CuO-ZnO CATALYSTS AS EVIDENCED BY A TEMPERATURE-PROGRAMMED REDUCTION (TPR) STUDY

Catalysts based on CuO-ZnO mixed oxides are of great scientific and industrial interest due to their activity for the low temperature-low pressure synthesis of methanol and the water-gas shift reaction [1]. The catalytic activity remarkably increases if a higher homogeneity and interdispersion of the oxides is achieved, this influencing the interaction of CuO with ZnO. Moreover, also the reactivity of metal copper in the reduced catalysts can be influenced by ZnO. It is not easy to show that all the above effects actually occur. We showed that Temperature-Programmed Reduction (TPR) is a powerful tool for a fine tuning of the nature and extent of the interaction in CuO-ZnO catalysts either in the oxidized and in the reduced state. A set of CuO-ZnO catalysts at different copper content was prepared by thermal decomposition at 623 K in air of hydroxycarbonates precursors obtained by coprecipitation at constant pH. This preparation method led to catalysts formed by well dispersed CuO-ZnO mixed oxides of relatively high surface areas and of crystallite sizes below 100 Å. The TPR study in H2 has evidenced that there is a significant interaction of CuO with ZnO. Such an interaction remarkably increases on decreasing the copper content. Indeed in the catalyst with the lowest copper loading (Cu:Zn=5:95, as atomic ratio) a fraction of very small and likely nanostructured CuO-like particles are so strongly interacting with ZnO that they are reduced at a temperature lower than 50 K with respect to the CuO species in the other catalysts. Also copper metal in the reduced catalysts can be partially reoxidized by CO2 to form nanostructured CuO-like species that are extremely reactive, even much more than those of the fresh catalysts. [1] G. C. Chinchen, P.J. Denny, J.R. Jennings, M.S. Spencer and K. C. Waugh, Appl. Catal., 36, 1 ( 1988).