A STUDY OF ELECTRODE PASSIVATION DURING AQUEOUS PHENOL ELECTROLYSIS

The process of electrode passivation during phenol electrolysis at a p latinum electrode was studied in a sulfuric acid electrolyte (pH0-1). Passive film growth and the effects of concentration and potential wer e investigated using chronoamperometry, x-ray photoelectron spectrosco py, and gel permeation chromatography. The main products of the phenol oxidation are oligomers/polymers with weight-averaged molecular weigh ts typically around 1000 g/mol after a 30 ms anodic pulse. X-ray photo electron spectroscopy shows that the passivating polymer film is oxidi zed incompletely with many hydroxyl groups present. Increased potentia l increased the polymerization rate, but above 1.0 V vs. SCE film deco mposition reactions also occurred. Increased phenol concentration incr eased the charge required to initiate passivation. Potential steps to the open-circuit potential or to more cathodic values can interfere wi th the passivation process. Chronoamperometric results show that the c urrent decay at the passivated electrode is roughly inversely proporti onal to time and that the currents for a fixed amount of polymerizatio n reaction follow a Tafel relationship. This type of decay is not due to a limitation caused by reactant diffusion through, nor IR drop acro ss, a growing film but is more characteristic of electron tunneling th rough a growing insulating barrier layer. The model proposed for the o bserved behavior involves the formation of a region of high molecular weight, oxidized material at the electrode surface which blocks furthe r reaction at the electrode. The rate-determining step at the passivat ed electrode is therefore electron tunneling through this unreactive m aterial.