MECHANISTIC ASPECTS OF PHENOL ELECTROCHEMICAL DEGRADATION BY OXIDATION ON A TA PBO2 ANODE/

The electrochemical oxidation of phenol in an aqueous solution is a co mplex transformation involving several transfer steps of oxygen atoms and electrons. Transfer of the oxygen atom occurs through the intermed iary of hydroxyl radicals adsorbed on the active sites of the anode. G alvanostatic electrolyses of phenol (10.5 to 105 mmol dm(-3)) in aqueo us solution at pH 2 on a Ta/PbO2 anode were followed by high-pressure liquid chromatography and by analysis of the total organic carbon. Hyd roquinone, catechol, 1,4-benzoquinone (1,4-BQ), maleic and fumaric aci ds, and carbon dioxide are the main products. The nonidentified produc ts consist mainly of polymers. Study of the influence of temperature s hows that the rate consumption of phenol initially at 21 mmol dm(-3) i s 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 forma tion of 2 mol of CO2. The faradaic yield values show that a phenol mol ecule 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 t he operating conditions (temperature, anodic current density, initial phenol concentration, hydrodynamic conditions, etc.). The mean faradai c yield decreases during electrolysis; it can reach 70% at the beginni ng of electrolysis of a 21 mmol dm(-3) phenol solution for an anodic c urrent 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%.