The photoelectrocatalytic oxidation of aqueous nitrophenol using a novel reactor

This paper reports the oxidation of aqueous 4-nitrophenol solutions in a photo-electrochemical bubble column reactor (BCR) in which mass transfer has been shown not to be rate limiting. The work represents the first steps in the scale-up of active photoanodes and efficient reactors for the disinfection and detoxification of water. The preparation, optimization and application of two types of electrode are described and the results are compared with those for a TiO₂ electrode supplied by Ineos Chlor. Photocurrents measured in tap water and in aqueous methanol were used for the initial characterization of the electrodes. The methanol was employed for diagnostic purposes only, as discussed below; methanol can react either by direct hole transfer or by hydroxyl radical recombination, but the balance of these reactions depends upon the nature of the electrode surface. The most active thermal electrodes were fabricated by heating titanium metal in air at 750 °C for 10 min, whilst the most active sol–gel electrodes were heated at 600 °C for 10 min. Three of the central achievements of the work were to: (1) show that it is possible to design and fabricate photoelectrochemical reactors capable of effecting the mineralization of strongly absorbing organics; (2) confirm that the photocatalytic decomposition of 4-NP in reactors with a 4 dm³ capacity can be increased by the application of a small positive potential and (3) that the application of such a potential significantly enhances the mineralization of 4-NP. For the mineralization of 0.25 mM nitrophenol solutions the reactivity sequence is: Photoelectrocatalytic > Photocatalytic > Photochemical > Electrochemical. However, even at 3 V applied potential, charge recombination is not eliminated. The order of electrode activity was: Ineos > Sol Gel > Thermal. Differences between the activities of different electrodes were attributed to changes in the structure and morphology of the TiO₂. It is noteworthy that although, for nitrophenol oxidation, the thermal electrodes were the least active, for photoelectrocatalytic disinfection in the same type of reactor, thermal electrodes were the most active.