Highly efficient photocatalytic conversion of CO2 into solid CO using H2O as a reductant over Ag-modified ZnGa2O4

Abstract

Highly crystalline spinel phase ZnGa₂O₄ modified with a Ag cocatalyst exhibited high activity and selectivity toward CO evolution in the photocatalytic conversion of CO₂ using H₂O as a reductant under UV light irradiation. The stoichiometric evolution of CO, H₂, and O₂ clearly indicated that H₂O worked as an electron donor for the photoreduction of CO₂. Highly crystalline ZnGa₂O₄ was synthesized by a solid-state reaction method at a calcination temperature as low as 973 K. Upon optimizing the fabrication conditions, such as calcination temperature and duration, the photocatalytic activity of ZnGa₂O₄ was maximized because of the optimum balance between crystallinity and surface area in the catalyst material. Furthermore, the formation of metallic Ag particles with different sizes and dispersions on the surface of ZnGa₂O₄ influenced the evolution of CO. When Ag nanoparticles were loaded onto the ZnGa₂O₄ calcined at 1123 K for 40 h using a chemical reduction method, the highest formation rates of CO, H₂, and O₂ (155, 8.5, and 74.3 μmol h⁻¹, respectively) were obtained, and the selectivity toward CO evolution reached 95.0%. An isotope-labeling experiment using ¹³CO₂ confirmed that the origin of the evolved CO was not from organic contamination but from the CO₂ gas introduced during the reaction process.