Mechanistic aspects of phenol electrochemical degradation by oxidation on a Ta/PbO2 anode

The electrochemical oxidation of phenol in an aqueous solution is a complex transformation involving several transfer steps of oxygen atoms and electrons. Transfer of the oxygen atom occurs through the intermediary of hydroxyl radicals adsorbed on the active sites of the anode. Galvanostatic electrolyses of phenol (10.5 to 105 mmol dm(-3)) in aqueous solution at pH 2 on a Ta/PbO2 anode were followed by high-pressure liquid chromatography and by analysis of the total organic carbon. Hydroquinone, catechol, 1,4-benzoquinone (1,4-BQ), maleic and fumaric acids, and carbon dioxide are the main products. The nonidentified products consist mainly of polymers. Study of the influence of temperature shows that the rate consumption of phenol initially at 21 mmol dm(-3) is mass transport limited. CO2 is immediately formed following the 1,4-BQ-maleic acid pathway involving 20 faradays and forming 4 mol of CO2 and/or the 1,4-BQ-intermediary in C2 pathway at 16 faradays with formation of 2 mol of CO2. The faradaic yield values show that a phenol molecule adsorbed on a catalytic site undergoes a succession of oxidation steps involving, on average, five electrons without desorption of the intermediate products. This number of electrons varies according to the operating conditions (temperature, anodic current density, initial phenol concentration, hydrodynamic conditions, etc.). The mean faradaic yield decreases during electrolysis; it can reach 70% at the beginning of electrolysis of a 21 mmol dm(-3) phenol solution for an anodic current density of 100 mA cm(-2). The phenol conversion into insoluble polymers increases as a function of its initial concentration and the anodic current density but it does not exceed 10%.