Mechanistic investigation of the conductive ceramic Ebonex® as an anode material

Ebonex® is a conductive and corrosion-resistant ceramic with the approximate composition Ti4O7. Anodic and/or cathodic polarization of a pair of Ebonex electrodes changed their surface composition, as shown by the development of a potential difference between them. In consequence, the activity of an Ebonex anode with respect to oxidation of an organic substrate depends on its past history. The anodic oxidation of p-nitrosodimethylaniline, which has been used as a model compound for the detection and quantitation of hydroxyl radicals, was studied in order to determine whether hydroxyl radicals are produced upon anodic polarization of Ebonex. The results were ambiguous, because direct oxidation of this substrate and oxidation of water to hydroxyl radicals occur at similar potentials. p-Benzoquinone (BQ) was found to be a more satisfactory mechanistic probe because it is resistant to direct oxidation. The rates of both disappearance and overall mineralization of BQ at Ebonex were intermediate between the corresponding rates at boron-doped diamond (BDD) and Ti/IrO2–Ta2O5 anodes, which promote one-electron and two-electron oxidations respectively. However, it is not yet clear whether mineralization is initiated by hydroxyl radicals formed in lower yield than at ‘active’ materials such as BDD, or whether oxidation involves less reactive intermediates such as HO2 radicals.