Photocatalytic reduction of aqueous chromium(vi) by RuO₂/g-C₃N₄ composite under visible light irradiation

Graphitic carbon nitride (g-C₃N₄) has been extensively investigated as a novel nonmetallic visible-light response photocatalyst. However, its uses in photocatalytic reductions were limited because of the sluggish oxygen evolution reaction (OER) and the resulting self-decomposition. In this paper, a ruthenium dioxide loaded g-C₃N₄ composite (RuO₂/g-C₃N₄) was prepared by forced oxidative hydrolysis of ruthenium(iii) chloride on the surface of g-C₃N₄ that was obtained by direct condensation polymerization of melamine. Photocatalytic reduction of aqueous Cr(vi) by it under illumination from a 400-410 nm light-emitting diode was examined. It was shown that the Cr(vi) reduction rate was much higher in RuO₂/g-C₃N₄ than in pure g-C₃N₄. Without any sacrificial electron donor and at initial solution pH 2.3, Cr(vi) removal (200 mL and 0.5 mM) was 34% and 76.4% with 0.1 g pure g-C₃N₄ and 0.1 RuO₂ (1.0 wt%)/g-C₃N₄, respectively. The optimum initial solution pH was 2.4. Methanol accelerates while acetone suppresses the Cr(vi) reduction significantly. Ferric ions catalyze the reduction, especially in the later stage. UV-Vis diffusion reflectance spectroscopy and theoretical analysis showed that RuO₂ not only boosts the charge separation but also protects the g-C₃N₄ from decomposition by its extraordinary catalytic action for OER. The used RuO₂/g-C₃N₄ was separated from the solution by microfiltration, with little leaching and residue remaining in the filtrate. The reclaimed RuO₂/g-C₃N₄ was recycled for 5 cycles and no obvious decrease in catalytic activity was observed, indicating its superior potential in industrial applications.