Reactivity of electrogenerated free hydroxyl radicals and activation of dioxygen on boron-doped diamond electrodes

Synthetic boron-doped diamond (BDD) thin film is an electrode material with high chemical and dimensional stability, low background current and a very wide potential window of water stability. Upon anodic polarization, BDD generates hydroxyl radicals that mediate the oxidation processes in the vicinity of the electrode surface. These hydroxyl radials are assumed to be free, i.e., not adsorbed on the electrode surface. Hydroxyl radicals are formed on BDD during water discharge, which is the rate determining step of oxygen evolution reaction. Oxygen evolution on BDD occurs at a high overpotential (over 1 V) with respect to the thermodynamic potential for O2 formation (E0OER = 1.23 V vs. SHE), but very close to the thermodynamic potential of HO• formation (E0HO• = 2.38 V vs. SHE). In the presence of organics, the onset potential of HO• formation changes, which affects the current–potential curves. As a consequence, oxidation of most of organic compounds on BDD results in a shift of the current–potential curves toward lower potentials. An exception is acetic acid, which adsorbs on the electrode surface causing auto-inhibition of its oxidation (oxidation is shifted toward higher potentials). In general, however, oxidation of organics on BDD is a fast reaction, controlled by the mass transport of organics to the anode surface. In the presence of an organic compound, hydroxyl radicals initiate chain reactions resulting in involvement (activation) of molecular oxygen dissolved in aerated aqueous solution. Direct evidence for this process was found during oxidation of acetic acid solution saturated with isotopically labelled 18O2. As a result, C18O2 and C16O18O were evolved proving that molecular oxygen participates in the mineralization of acetic acid. This electrochemically induced activation process is probably initiated by organic free radicals (R•) formed during reaction between organic compounds and hydroxyl radicals. Organic free radicals react further with molecular oxygen to form peroxy radical (RO2•). Organic peroxy radicals are very reactive and can initiate subsequent chain reactions leading, via several possible intermediates, to complete oxidation of organic compound. In fact, similar mechanism has been reported in radiolysis (upon X-ray or γ-ray) of aqueous solutions of acetic acid. This non-faradaic enhancement of the electrooxidation processes opens the possibilities for designing a less energy consuming degradation of organic pollutants on BDD anodes.