Benzene Oxidation on Boron-Doped Diamond Electrode: Electrochemical-Impedance Study of Adsorption Effects

Benzene oxidation at a boron-doped diamond anode in 0.5？M K2SO4 aqueous solution is studied by cyclic voltammetry and electrochemical impedance spectroscopy. It is shown by measurements of differential capacitance and anodic current that in the ideal-polarizability potential region benzene either is not adsorbed at the diamond electrode or the benzene adsorption does not affect its capacitance. At more positive potentials, the adsorption of some intermediate of the benzene oxidation occurs at the electrode. The intermediate partially blocks the electrode surface and lowers the anodic current. The very fact of the electrode surface blocking is reflected in the complex-plane presentation of the impedance-potential plots. 1. Introduction Boron-doped diamond (BDD) proved being a corrosion-stable electrode material, particularly suitable for deep anodic oxidation [1]. Indeed, the diamond electrode makes it possible reaching high anodic potentials at which hydroxyl radicals (OH？) are formed at the anode surface (the oxygen evolution overpotential for diamond is sufficiently large, so this electrochemical reaction occurs with high current efficiency). The radicals oxidize organic and inorganic solutes (carboxylic acids, alcohols, phenols, aromatics) in the course of homogeneous chemical reaction. For this process, a kinetic model was suggested [2], according to which one of the two oxidation mechanisms is realized. In the potential region, where water is electrochemically stable, direct electron transfer occurs, whereas at high anodic potentials, indirect oxidation involving the above-mentioned hydroxyl radicals as mediator takes place (with concurrent oxygen evolution). The anodic oxidation at boron-doped diamond is an effective method of the nature and waste water purification from organic and inorganic pollutants [3, 4]. Benzene and its derivatives are typical water pollutants. The benzene oxidation at boron-doped diamond in 0.5？M H2SO4 solution was studied in [5]. It was shown by using high-performance liquid chromatography that a mixture of the benzene oxidation intermediates (hydroquinone, resorcinol, p-benzoquinone, catechol, and phenol) was formed in solution at the anode potential of 2.5？V (versus Ag/AgCl-electrode). The benzene complete incineration yielding CO2 occurs at potentials more positive than 2.5？V. In our preceding paper [6], the benzene oxidation at boron-doped diamond anode was studied in 1？M HCl solution by electrochemical impedance spectroscopy, with special emphasis on the revealing of the role of adsorption in the process. In this