Electrochemical oxidation of benzoic acid and its aromatic intermediates on boron doped diamond electrodes

Electrochemical oxidation using Boron-doped diamond (BDD) electrodes is a promising purification technique to remove organic pollutants, such as benzoic acid, from wastewaters. In this study the oxidation of benzoic acid has been investigated on rotating disk electrodes and in flow cells with well-defined mass transfer. The oxidation of benzoic acid on BDD proceeds via the formation of 4-hydroxybenzoic acid, which reacts in consecutive steps towards hydroquinone and benzoquinone. The ring opening in the oxidation of benzoquinone results in the formation of aliphatic carboxylic acids. It was shown that the intermediates 4-hydroxybenzoic acid and hydroquinone can be oxidized prior to water oxidation, which suggests the reaction occurs via a DET mechanism. Benzoquinone is not oxidized before water oxidation and can presumably solely be oxidized by OH radicals. This means that in order to oxidize benzoic acid completely to CO2 the formation of OH radicals is needed. Finally, the effects of mass transfer were analyzed. It was shown that the conversion rates of benzoic acid, benzoquinone and 4-hydroxybenzoic acid were below the limiting mass transfer rates, implying that these reactions are primarily limited by kinetics. [Display omitted] •Oxidation of benzoic acids to CO2 occurs via a number of aromatic intermediates.•4-hydroxybenzoic acid and hydroquinone can be oxidized via direct electron transfer.•Benzoquinone can only be oxidized by OH or other radicals.•Prior to water oxidation the reactions are predominantly limited by kinetics.