Electrochemically assisted photocatalytic oxidation of nitrite over Cr-doped TiO2 under visible light

To investigate the photocatalytic oxidation (PCO) of nitrite ions under visible light, Cr-doped TiO2 supported on a titanium plate was used as a photoelectrode operated under anodic bias potential. The results showed that applying bias potential (E-appl) played an important role in the PCO of NO2. Without bias potential the PCO reaction could not be initiated due to the rapid recombination rate of the photogenerated carriers, but when the potential was larger than 1.2 V (vs. saturated calomel electrode, SCE) the removal efficiency of NO2- increased up to 90% within 3 h for an initial concentration of 8 mg L-1. Zero-order kinetics were observed at E-appl = 0.4 V (vs. SCE), while pseudo first order kinetics were found at E-appl >= 0.8 V (vs. SCE). The effects of the initial concentration of NO2-, initial solution pH, the gas atmosphere, and cathodic reaction on the PCO of NO2- were studied as well. Furthermore, the PCO mechanism of NO2 was investigated by using tert-butyl alcohol or benzoic acid as a diagnostic probe. It showed that the PCO of NO2- under visible light most probably proceeded indirectly via OH radicals, not directly via the valence-band holes.