The pH-dependence of thermally-coated Ti/TiO2 electrodes in the absence and presence of nitrobenzene

Cyclic voltammetric measurements on thermally coated Ti/TiO₂ electrodes in 0.05–3.0 m H₂SO₄, Na₂SO₄ and NaOH media were carried out to evaluate the cause of pH sensitivity of these electrodes. The electrode activity decreased in the order H₂SO₄ > NaOH > Na₂SO₄. Ionic strength had a significant influence on the electrode activity. In the redox catalysis of nitrobenzene reduction the pH dependence increased significantly. The electrode activity again decreased in the same order. In acid media the TiO₂ surface is protonated. In alkaline media OH- ions are specifically adsorbed on the TiO₂ surface. The concentration of these charged species, as well as the ease of counter ion transport through the charged layers during surface redox reactions in the oxide layer, is identified as the cause of the pH sensitivity of the electrodes. In acid media proton transport in the film occurs through a ‘surface Grothus’-like mechanism. In alkaline medium proton abstraction by OH^- ions from the trapped H₂O molecules may facilitate proton transport.