Abstract
UV-photoexcitation of TiO2 in contact with aqueous solutions of azo dyes does not imply only its photocatalytic degradation, but the reaction fate of the dye depends on the experimental conditions. In fact, we demonstrate that the presence of sodium formate is the switch from a degradative pathway of the dye to its transformation into useful products. Laser flash photolysis experiments show that charge separation is extremely long lived in nanostructured TiO2 thin films, making them suitable to drive both oxidation and reduction reactions. ESR spin trapping and photoluminescence experiments demonstrate that formate anions are very efficient in intercepting holes, thereby inhibiting OH radicals formation. Under these conditions, electrons promoted in the conduction band of TiO2 and protons deriving from the oxidation of formate on photogenerated holes lead to the reductive cleavage of N=N bonds with formation and accumulation of reduced intermediates. Negative ion ESI–MS findings provide clear support to point out this new mechanism. This study provides a facile solution for realizing together wastewater purification and photocatalytic conversion of a waste (discharged dye) into useful products (such as sulfanilic acid used again for synthesis of new azo dyes). Moreover, the use of TiO2 deposited on an FTO (Fluorine Tin Oxide) glass circumvents all the difficulties related to the use of slurries. The obtained photocatalyst is easy to handle and to recover and shows an excellent stability allowing complete recyclability.
Highlights
Very large amounts of dyes are annually produced and used in different industries, including textile, leather and paper industries [1]
Electrons promoted in the conduction band of TiO2 and protons deriving from the oxidation of formate on photogenerated holes lead to the reductive cleavage of N=N bonds with formation and accumulation of reduced intermediates
The use of TiO2 deposited on an fluorine tin oxide (FTO) (Fluorine Tin Oxide) glass circumvents all the difficulties related to the use of slurries
Summary
Very large amounts of dyes are annually produced and used in different industries, including textile, leather and paper industries [1]. Hydroxyl radical formation and their non-selective reaction with dyes have been supposed as the main degradation pathway. We report that two alternative photocatalytic processes based on TiO2 can be operative in aqueous solutions containing an azo dye. It has already been reported that illuminated TiO2 is able to reduce nitroaromatic molecules to the corresponding anilines [25,26,27,28,29,30], as far as we know, the one presented here is the first example concerning the possibility of reaching two simultaneous goals by photocatalysis, namely the removal of a waste from water by converting it into useful building blocks. We circumvented all the difficulties related to the use of slurries of TiO2, since we deposited the semiconductor oxide onto a TCO-glass substrate [31,32], obtaining a photocatalyst that is easy to handle and to recover, which shows excellent stability, allowing complete recyclability and operation at ambient temperature and atmospheric pressure
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