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

ZnZrO_x and gallium-doped ZnZrO_x 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 ZnZrO_x and gallium-doped ZnZrO_x catalysts. We used zinc-doped Zr sites of tetragonal ZrO₂ with different Zn concentrations as a model to simulate the ZnZrO_x solid solution. For the Ga-doped ZnZrO_x 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 ZnZrO_x has been reduced from 4.45 to 2.45 eV. The insertion of Ga in ZnZrO_x solid solutions shifts the conduction band edges back, increasing the bandgap of the solid solutions. The Ga doping improves the CO₂ adsorption ability of ZnZrO_x 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.