Fe, N Dual-Doped Cerium Oxide Photocatalyst Manifesting the Simultaneous Degradation of Dye–Heavy Metal Mixed Pollutants under Sunlight

This study explored the effect of dual doping with iron (Fe) and nitrogen (N) into cerium oxide/ceria (CeO₂) at different concentrations, specifically 5, 10, and 15 mol %, using a sol–gel method. Structural, chemical environment, and oxidation state analyses revealed that dual doping effectively influenced the crystal structure, switchable oxidation states of cerium ions (Ce³⁺ ↔ Ce⁴⁺), and dopant-mediated compensation of the charge imbalance created by defect structures. In line with these observations, changes such as enhanced optical absorbance in extended wavelengths and decreased charge recombination were confirmed through optical analyses. As a result, the dual-doped ceria demonstrated enhanced individual and simultaneous degradation and reduction of rhodamine B and chromium VI heavy metal ions under sunlight irradiation. Over 2.5 h, the bare system degraded around 50% of the pollutants, while the optimized dual-doped ceria (10 mol %) degraded nearly 100% of the pollutants under both sole and mixed conditions. This observed enhancement was attributed to the shifting of the conduction and valence bands of ceria to more negative and positive potentials, respectively, positioning these potentials more suitably for effective simultaneous photoredox reactions. Furthermore, the cyclic efficiency of the doped system remained consistent across all 5 cycles, demonstrating stability confirmed by post-characterization, making it promising for scale-up applications.