The role of copper oxidation state in Cu/ZnO/Al2O3 catalysts in CO2 hydrogenation and methanol productivity

CO2 hydrogenation was carried out over a series of Cu/ZnO/Al2O3 catalysts, which were prepared by different synthesis methods (co-precipitation, ultrasound-assisted, sol-gel and solid-state). The applicative physicochemical properties of the materials and their catalytic performance were investigated. Subsequently, the preparation methodology influence on the average Cu particle size, the interactions of metals, the exposed copper phases surface area and the ratio of Cu0/Cu+, as well as its specific effect on reactions, were assessed in detail. The ultrasonic synthetic route provided increased basic active sites' number and significant methanol selectivity, compared to the conventional precipitation procedure, while it also improved the dispersion of separate Cu metallic particles, which ultimately changed the intrinsic reactive activity of CuO–ZnO. The trend of apparent CH3OH productivity rate was consistent with the Cu+/Cu0 ratio or content as well. The presence of Cu+ species at high concentration levels is thus crucial for high methanol selectivity and an extremely low selectivity towards CO at the conventional industrial operating conditions analysed. The dependence of the selective methanol production on the fraction of determined alkaline moieties was found analogous to all catalysts studied. Cu+/Cu0 oxidation state role and distribution may, therefore, aid in CO2 conversion catalyst design and optimisation.