Does hydrogen peroxide really accelerate TiO2 UV-C photocatalyzed decolouration of azo-dyes such as Reactive Orange 16?

• UV-C TiO2 photocatalysis forms OH and decolours Reactive Orange 16 (RO16) azo dye.
• UV-C photolyses H2O2 to form hydroxyl radicals which accelerate RO16 decolouration.
• H2O2 addition to TiO2/dye suspensions increases decolouration caused by UV-C TiO2.
• But, H2O2 by itself is more effective than either TiO2 or H2O2/TiO2.
• Because, TiO2 absorbs UV but forms fewer hydroxyls radicals than H2O2.

A comparison of UV-C initiated decolourations of the anionic azo-dye Reactive Orange 16 by H2O2, by anatase TiO2 (PC500), and by H2O2/TiO2 is reported. The decolouration rates induced by H2O2, by TiO2 and by H2O2/TiO2, were compared in order to determine whether H2O2/TiO2 was more beneficial than either H2O2 or TiO2 alone. UV-C photolyses H2O2 to form hydroxyl radicals. UV-C can also initiate hydroxyl radical formation at the surface of TiO2, and in this study the photocatalytic conditions were chosen to enhance hydroxyl radical formation. In both cases the hydroxyl radicals lead to decolouration of the azo-dye Reactive Orange 16. Decolouration by irradiation of H2O2/TiO2 was ∼5 times faster than by the TiO2 photocatalyzed reaction but slower than that caused by irradiation of H2O2 alone. Thus, the relative order of decolouration rates is: UV-C/TiO2 < UV-C/TiO2/H2O2 < UV-C/H2O2. Estimates of the absorption of UV by both H2O2 and H2O2/Reactive Orange 16 solutions are used to show that this order is a consequence of the photonic efficiency of H2O2 photolysis being ten to a hundred times larger than the photonic efficiency of hydroxyl radical generation at the surface of TiO2.

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