Nanostructured Cu species in low loading copper-based methanol synthesis catalysts

CuO-ZnO based catalysts are of great scientific and industrial interest due to their activity for the low temperature-low pressure synthesis of methanol. Before use, these catalysts are typically activated in a H2 diluted stream. During this treatment, copper oxide species are reduced to Cu metal. The extent of reduction and, in particular, the temperature at which this process occurs are strongly influenced by the interaction between the CuO and ZnO particles. Moreover, also the reactivity of metal copper in the reduced catalysts can be influenced by ZnO. All these effects are remarkably affected by homogeneity and interdispersion of the oxides particles. Indeed, the catalytic activity remarkably increases if a higher homogeneity and interdispersion of the oxides is achieved in the catalyst. A TPR (Temperature-Programmed Reduction) investigation revealed 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 of relatively high surface area and crystallite size below 100 Å was obtained by thermal decomposition, at 623 K in air, of hydroxycarbonates precursors prepared by coprecipitation at constant pH, this method leading to well dispersed CuO-ZnO mixed oxides. The TPR study in H2 has evidenced that there is a strong interaction of CuO with ZnO. Such an interaction remarkably increases on decreasing the copper content. Indeed in the catalyst at lower copper loading (Cu:Zn=10:90 and 5:95 as atomic ratio), a fraction of very small, and then also very likely to be 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. A further relevant finding is that copper metal in the reduced catalysts can be partially reoxidized by CO2 at 573 K for 0.33 h to form very tiny and even more nanostructured CuO-like species that are extremely reactive, much more than those of the fresh catalysts.