Photoelectrocatalytic destruction of organics using TiO2 as photoanode with simultaneous production of H2O2 at the cathode

Research on the photoelectrocatalytic (PEC) oxidation of organic contaminants, the role of cathodes during photocatalysis has usually been disregarded. This paper reports a study of the PEC decomposition of aniline and salicylic acid with simultaneous production of hydrogen peroxide in a divided reactor using TiO2 as a photoanode. Two types of TiO2 electrode were used. Thermal oxidation electrodes (TO–TiO2) were made by oxidation of titanium metal sheet; sol–gel electrodes (SG–TiO2) were made by coating and then heating a layer of titania gel on titanium sheet. Saturated photocurrent was used to carry out an initial characterization and optimization of both electrode types. The best TO–TiO2 electrodes were prepared by heating titanium at 600–700 °C in air. For the SG–TiO2 electrodes, optimum performance was obtained by heating at 500 °C. These electrodes were then used to photodegrade aniline and salicylate. The SG–TiO2 electrodes turned out to be superior to the TO–TiO2 electrodes in terms of PEC rate under the same conditions but the difference in rate between the two electrodes was comparable under a high enough bias potential. The most important factors affecting the production of H2O2 in the cathode compartment are presented. The current efficiencies for the accumulation of H2O2 were remarkably affected by the cathode used, pH value, current density, and metal cations such as copper and iron ions. An expected H2O2 concentration could be obtained by controlling either the magnitude of the photocurrent or illumination time. The maximum current efficiency for the cathodic reduction of oxygen to H2O2 was as high as 90.1% when graphite was used as the cathode. Compared to the SG–TiO2 electrode, the TO–TiO2 electrode had a higher light to electricity conversion efficiency, thus it turned out to be more suitable for the production of H2O2. Furthermore, the role of in-situ reduced oxygen species in the PEC decomposition of aniline was evaluated.