Effect of Ruthenium Modification of g-C3N4 in the Visible-Light-Driven Photocatalytic Reduction of Cr(VI)

Graphitic carbon nitride (g-C3N4) is a promising heterogeneous photocatalyst in the visible range. It can be used, among others, for reductive conversion of the toxic hexavalent chromium occurring in various wastewaters. Its photocatalytic efficiency, however, has to be improved, which can be realized by modification with different dopants or co-catalysts forming heterojunctions. In our work, ruthenium-modified g-C3N4 has been prepared by ultrasonic impregnation of the pristine g-C3N4, which was synthesized from thiourea. The morphology, microstructure, and optical properties of the photocatalysts were characterized by XRD, SEM, FT-IR, TEM, XPS, and DRS. Their compositions were analyzed by EDS and XPS measurements, indicating 0.5% and 1.4% Ru, due to the different penetrating depths. XPS study showed mainly +2 for the oxidation state of Ru. DRS analysis indicated a slight change in both the CB (from -1.14 to -1.22 eV) and the VB (from 1.49 to 1.56 eV) energies of Ru/g-C3N4, compared to those of g-C3N4. The photocatalytic Cr(VI) reduction efficacy increased from 50.1 to 96.8%. Low pH (=2) was preferred for the photocatalytic Cr(VI) reduction due to the favorable surface charge and E(Cr(VI)/Cr(III)) redox potential. Ru modification proved to be promising for improving the photocatalytic performance of g-C3N4.