First principles investigations of structural and electronic properties of Ga-doped ZnZrOx solid solutions for catalytic reduction of CO2

Abstract

ZnZrOx and gallium-doped ZnZrOx solid solutions exhibit excellent catalytic properties for the hydronation of CO2 to methanol. However, the details of the structures and catalytic properties are not fully understood. Herein, density functional theory has been used to explore the structural, electronic and catalytic properties of ZnZrOx and gallium-doped ZnZrOx catalysts. We used zinc-doped Zr sites of tetragonal ZrO2 with different Zn concentrations as a model to simulate the ZnZrOx solid solution. For the Ga-doped ZnZrOx catalysts, Zr atoms were substituted by Ga atoms. The doping of Zn atoms in ZrO2 reduce the coordination numbers of Zr atoms. Heavily Zn-doped ZrO2 systems do not maintain the tetragonal structure and, hence, deviate from the perfect tetragonal phase. At ∼29.16% Zn concentration, the bandgap of ZnZrOx has been reduced from 4.45 to 2.45 eV. The insertion of Ga in ZnZrOx solid solutions shifts the conduction band edges back, increasing the bandgap of the solid solutions. The Ga doping improves the CO2 adsorption ability of ZnZrOx solid solutions and efficiently dissociates the H2 molecules. On the surface of Ga-Zn doped ZrO2(101), the hydrogenation of CO2 to methanol via the formate pathway is more favorable than the CO pathway.