Role of anode material on the electrochemical oxidation of methyl orange

The anodic oxidation of methyl orange (MO, 5-(4-nitrophenylazo)salicylic acid) has been studied by cyclic voltammetry and bulk electrolysis, using a range of electrode materials such as Ti–Ru–Sn ternary oxide, lead dioxide and boron-doped diamond (BDD), glassy carbon (GC) and gold anodes. The results of voltammetries show that with all the electrode materials, in the potential region before oxygen evolution, the oxidation of MO involves simple electrode transfer that produces a polymeric film that deactivates the electrode surface, as confirmed by Fourier Transform Infrared Reflection-Absorption Spectroscopy (FTIRRAS) analysis. A very different behaviour was observed among the electrodes in the region of water decomposition. While BDD and PbO2 regained their initial activity by simple polarisation at 2.3 V vs. saturated calomel electrode (SCE) due to the production of high amount of hydroxyl radicals that destroy the polymeric film, TiRuSnO2, GC and gold cannot be completely reactivated, because they have a low overpotential for oxygen evolution, and this secondary reaction is favoured over polymer mineralization. The results of bulk electrolysis showed that after 3 h of polarisation at 10 mA cm−2, complete colour and chemical oxygen demand (COD) removal were obtained only with BDD anode. Using PbO2 MO was oxidised but a residual COD remains in the solution, while TiRuSnO2 permitted only a partial oxidation of MO.