Visible light driven photocatalysis in chromate(VI)/TiO2 systems—Improving stability of the photocatalyst

Titanium dioxide photosensitization to visible light can be observed in the presence of chromate(VI) ions adsorbed at the surface of TiO2. The resulting materials do not show, however, sufficient stability and photostability—chromium(VI) species desorb from the surface and can be reduced to chromium(III). The stability of such systems has been improved by TiO2 impregnation with fluorochromate(VI), K[CrO3F], instead of K2Cr2O7, yielding yellow F-CrVI@TiO2 material. Fluorides playing the role of anchors enable stronger adsorption of chromates at titanium dioxide. Also composites of water insoluble chromates(VI) with TiO2 release significantly lower amounts of CrVI ions. Such materials have been synthesized according to two main strategies—the first one involves precipitation of water insoluble chromates in the presence of TiO2 particles (MCrO4@TiO2) while the second strategy comprises of TiO2 synthesis in the presence of insoluble chromate particles (TiO2@MCrO4). Visible light induced photocatalytic activity of synthesized materials was studied in the process of 4-chlorophenol (4-CP) oxidation. A considerable photocatalytic activity was observed in the case of F-CrVI@TiO2 and TiO2@MCrO4 type materials (Mn+: Ba2+, Co2+, Mn2+, Cr3+). Among the composites the highest photoreactivity was shown by TiO2@CoCrO4 (1.7% CrVI, mol/mol, calcined at 450 °C) and TiO2@Cr2(CrO4)3 (5% CrVI, mol/mol, calcined at 450 °C).