Visible-light induced oxo-bridged ZrIV−O−CeIII redox centre in tetragonal ZrO2–CeO2 solid solution for degradation of organic pollutants

Abstract

Tetragonal ZrO₂–CeO₂ solid solutions with composition Zr_1−xCe_xO₂ (x = 0.1, 0.2, and 0.3) were synthesized in a citrate complexation route and characterized by XRD, XPS, UV-vis diffuse reflectance and ESR measurements. The formation of the homogeneous solid solution Zr_1−xCe_xO₂ constructed the oxo-bridged bimetallic Zr^IV−O−Ce^III linkage between two neighboring flattened tetrahedrons of the structural framework. As compared to their parent oxides, the ZrO₂–CeO₂ solid solutions exhibited optical absorption extending to longer wavelengths in the visible region. The red shift in the absorption spectrum was demonstrated to be partially due to a metal-to-metal charge transfer (MMCT) transition of the oxo-bridged Zr^IV−O−Ce^III linkage. The visible-light induced MMCT transition of Zr^IV−O−Ce^III → Zr^III−O−Ce^IV resulted in the generation of the additional Ce(IV) and superoxide anion radical formed by the interaction of Zr(III) with adsorbed O₂. Catalytic activity evaluation revealed that the photoexcitation of the MMCT over the solid solution can initiate the degradation of RhB and 2,4-DCP upon visible-light irradiation, whereby Zr(III) and Ce(IV) act as a site-specific reducing and oxidizing center, respectively. The structure of the solid solution Zr_1−xCe_xO₂ and the oxidation states of Zr and Ce species are also discussed in detail.