Photocatalytic and electrochemically assisted photocatalytic oxidation of formic acid on TiO2 films under UVA and UVB irradiation

Abstract

Titanium dioxide (TiO2) photocatalysis is a possible alternative/complementary technology for water purification. Attempts to increase the overall efficiency of the process include using higher energy UV to gain better quantum efficiency and electrochemically assisting the process by the application of an external electrical potential. In this work, nanocrystalline TiO2 films, prepared on borosilicate glass and indium-doped tin oxide (ITO) borosilicate glass, were used to investigate the photocatalytic and electrochemically assisted photocatalytic oxidation of formic acid under UVA and UVB irradiation. The experiments were carried out in a stirred tank reactor with high mass transfer characteristics. The rate of formic acid oxidation under UVB irradiation was 30% greater as compared to UVA irradiation. A maximum Φapp of 9% was obtained under UVA irradiation in 100% O2 under open circuit or +1.0 V (SCE) applied potential. A maximum Φapp of 20.3% was obtained under UVB irradiation with 100% O2 using TiO2 on borosilicate glass. Φapp was 19% for +1.0 V, 100% O2, using TiO2 on ITO borosilicate glass under UVB irradiation. The increase in oxidation rates and Φapp with UVB irradiation are due to the higher extinction coefficient of TiO2 at shorter wavelengths and/or the promotion of conduction band electrons to higher more stable states, thus reducing the rate of recombination of charge carriers. The use of a UVB source as compared to a UVA source results in a significant increase in the rate of oxidation and increased apparent quantum yields, however, a cost analysis of the process would be required to determine the economic viability of employing UVB sources. Electrochemically assisted photocatalysis may prove beneficial in large-scale reactors where mass transfer limitations exist.