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.