Mechanochemical reactions in Cu/ZnO catalysts induced by mechanical milling

Preparation of Cu/ZnO catalysts is discussed in which mechanochemical reactions are involved. These reactions include oxidation of copper and copper oxides during milling in the presence of oxygen, reduction under vacuum, and formation of carbonates in CO2. Oxidation and reduction are promoted by the presence of ZnO. Formation Of Cu2O-like intermediates is suggested in oxidic mixtures, which are not observed when milling occurs without ZnO. This effect may be partly due to defect formation. The various resulting copper species reduce at different temperatures in H-2 atmosphere and this gives an indication for the intimacy of the Cu-Zn interaction. When milling is carried out in the presence Of CO2, mixed carbonates are formed. Intimate mixtures of Cu and Zn are thus obtained, exhibiting high surface areas after reduction that can be explained by evolved porosity. A linear relation between Cu-0 surface area and activity for methanol synthesis at 2 bar is observed. Irreversible deactivation is explained by over-reduction and sintering. Both Cu-0 specific surfaces and BET surfaces (determined after reduction in hydrogen) are substantially increased for all milled Cu/ZnO samples, but especially for samples that undergo mechanochemical reactions. This method thus allows investigation of mixed oxide catalyst precursors and provides an interesting alternative for the preparation of promoted heterogeneous catalysts.