Ti₄O₇/g-C₃N₄ Visible Light Photocatalytic Performance on Hypophosphite Oxidation: Effect of Annealing Temperature

The oxidation of hypophosphite to phosphate is the key to recover the phosphorus resource from the hypophosphite wastewater. In the present work, Ti₄O₇/g-C₃N₄ composites were synthesized at two different temperatures (100 and 160°C) and their performance on photocatalytic oxidation of hypophosphite under visible light irradiation and the corresponding mechanism were evaluated. A hydrolysis method using g-C₃N₄ and Ti₄O₇ was applied to synthesize the Ti₄O₇/g-C₃N₄ composites with their hybrid structure and morphology confirmed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectra (XPS). The annealing temperature significantly affected the photocatalytic performance of Ti₄O₇/g-C₃N₄ that the 160-Ti₄O₇/g-C₃N₄ composite (fabricated at 160°C) showed the highest oxidation efficiency of hypophosphite of 81% and the highest photocatalytic oxidation rate of 0.467 h^-1 comparing with the 100-Ti₄O₇/g-C₃N₄ composite (fabricated at 100°C) and pure g-C₃N₄. The enhanced photocatalytic performance of 160-Ti₄O₇/g-C₃N₄ could be ascribed to the effective charge separation and enhanced photoabsorption efficiency. Additionally, electron spin resonance (ESR) results showed that hydroxyl radicals and superoxide anion radicals were mainly responsible to the oxidation of hypophosphite with superoxide anion radicals accounting for a more significant contribution. Moreover, Ti₄O₇/g-C₃N₄ photocatalysts showed the remarkable stability in the repetitive experiments.