Influence of ZrO₂ Structure and Copper Electronic State on Activity of Cu/ZrO₂ Catalysts in Methanol Synthesis from CO₂

Cu/ZrO₂ catalysts obtained by impregnation of ZrO₂ and complexation with citric acid were studied for CO₂ hydrogenation to methanol. The catalyst structure, texture, and active copper surface were determined using XRD, BET, and reactive adsorption of N₂O, respectively. The XPS and Auger spectroscopies were used to determine the surface structure and copper electronic state. FT-IR pyridine adsorption was studied to determine acidity of the catalysts. The results of quantum-chemical calculations concerning the formation of oxygen vacancies in monoclinic and tetragonal ZrO₂ have been also presented. It was found that selection of the appropriate conditions of the catalyst preparation influences the degree of copper dispersion, its electronic state, and contents of the zirconia polymorphic phases (tetragonal and monoclinic). The presence of oxygen vacancies stabilizes both the thermodynamically unstable t-ZrO₂ phase and Cu¹⁺ cations, which are present in the vicinity of oxygen vacancies. Complexes formed preferentially on tetragonal ZrO₂ built from Cu cations and oxygen vacancies are the acid centers active in methanol synthesis reaction; therefore the catalytic activity toward methanol increases with increasing t-ZrO₂ content. The implications of the obtained results for the mechanism of the catalytic hydrogenation of CO₂ are discussed