The Role of Oxygen Vacancy on the Carbon Dioxide Photoreduction Using Monoclinic and Tetragonal-phase Zirconium Oxide

Abstract

Various catalysts are used in processes for the conversion CO₂. Photocatalysts with higher concentrations of surface oxygen vacancies have been reported to exhibit excellent CO₂ photoreduction activity. This study systematically investigates monoclinic ZrO₂(001) and tetragonal ZrO₂(101) surfaces using experimental and theoretical approaches. Results of photocatalytic CO₂ conversion experiments under ultraviolet visible light irradiation showed formation rates of ¹³CO on the surfaces of monoclinic phase-major ZrO₂(001) and tetragonal-phase ZrO₂(101) of 0.65 and 0.041 µmol h⁻¹ g_cat⁻¹, respectively. Dissociation of CO₂ was observed at the oxygen vacancy sites on both surfaces. Results of our calculations indicate that the second lowest unoccupied molecular orbital (LUMO+1) of CO₂ adsorbed on the site of the oxygen vacancy is located within the conduction band of monoclinic ZrO₂(001) and facilitates the photodissociation of CO₂. Furthermore, oxygen vacancy sites were re-formed by H₂O desorption through the addition of hydrogen to monoclinic ZrO₂(001). On the contrary, the desorption of H₂O at the oxygen vacancy sites are less likely to occur in tetragonal ZrO₂(101), and this reduces the amounts of CO₂-to-CO conversion products.