Revisiting the mechanism of hexavalent chromium ion reduction: The parallel photodecomposition and photocatalytic reduction of chromate(VI) ester

• Published in: Applied catalysis. B, Environmental Environmental, 2017-08, Vol.210, p.444-453
• Main Authors: Xu, Fei, Webster, Richard D., Chen, Jinfan, Tan, Timothy T.Y., Sit, Patrick H.-L., Teoh, Wey Yang
• Format: Article
• Language: English
• Subjects: Aluminum oxide; Catalysts; Chromate; Chromates; Chromium; Conduction; Conduction bands; Decomposition reactions; Density functional theory; Electron paramagnetic resonance; Electron transfer; Hexavalent chromium; Injection; Interfacial charge transfer; Light scattering; Low concentrations; Methanol; Photocatalysis; Photocatalysts; Photodecomposition; Reduction; Semiconductors; Spectroscopy; Titanium dioxide; Trivalent chromium; Tungsten oxides
• ISSN: 0926-3373; 1873-3883
• DOI: 10.1016/j.apcatb.2017.04.024

[Display omitted] •The often neglected photodecomposition of chromate(VI) ester reaction runs in parallel and non-independently to the photocatalytic reduction of Cr(VI).•Addition of low concentration of photocatalysts (TiO2 and WO3) suppresses the homogeneous photodecomposition of Cr(VI).•Suppression in homogeneous photodecomposition is due to the electron injection from Cr(VI) ester to the photocatalyst conduction band.•Increasing the photocatalysts concentration raises the rate of heterogeneous Cr(VI) reduction and eventually overcomes the suppression effect. The formation and homogeneous photodecomposition of chromate(VI) ester are often overlooked during the studies of photocatalytic reduction of hexavalent chromium, Cr(VI), in the presence of sacrificial electron donors such as methanol. When photoexcited (λ≤525nm), the chromate(VI) ester, formed by the spontaneous reaction of aqueous Cr(VI) with methanol, undergoes homogeneous stepwise decomposition to trivalent chromium, Cr(III). The addition of semiconductor photocatalysts, i.e., TiO2 and WO3, at relatively low concentrations (