Cu-ZnO catalysts for CO2 hydrogenation to methanol: Morphology change induced by ZnO lixiviation and its impact on the active phase formation

Abstract

Synthesis of Cu-ZnO catalysts for the direct hydrogenation of CO2 to methanol requires catalyst precursors able to display a strong Cu-Zn interaction. Copper and zinc mixed hydroxynitrates are good candidates to fulfill this specification. The synthesis of such a phase was achieved by a wet impregnation of copper nitrate on ZnO. Solvent evaporation and catalyst drying at moderate temperatures allowed retaining the metals hydroxynitrate structure, leading to a memory effect for the reduced catalysts. Depending on the Cu content in the reaction mixture, a ZnO lixiviation gradually occurs, what induces an important change in the morphology of the reduced catalyst. Catalyst activity is proportional to the amount of Zn migrated in the active phase, the latter consisting most probably of an oxygen deficient mixed oxide comprising both Cu and Zn. The most active catalyst is obtained when ZnO is fully lixiviated during the catalyst precursor synthesis, giving a core-shell structure after reduction.