Influence of Polymorphic ZrO₂Phases and the Silver Electronic State on the Activity of Ag/ZrO₂Catalysts in the Hydrogenation of CO₂to Methanol

Abstract

Ag/ZrO2 and Ag/ZrO2/ZnO catalysts obtained by the coprecipitation method were studied in the CO2 hydrogenation to methanol. The catalyst structure was determined using X-ray diffraction (XRD) and Thermo-Programmed Reduction (TPR). The X-ray Photoelectron Spectroscopy (XPS) and Auger spectroscopies were used to determine the silver electronic state. It was found that selection of the appropriate conditions of the catalyst preparation influences silver dispersion degree, 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 the Ag+ cations, which are present in the vicinity of oxygen vacancies. The catalytic activity to methanol increases with increasing t-ZrO2 content but RWGS reaction is accelerated by dispersed metallic silver. The implications of the obtained results for the mechanism of the catalytic hydrogenation of CO2 are discussed.