A STUDY OF THE OXIDATION OF PHENOL AT PLATINUM AND PREOXIDIZED PLATINUM SURFACES

The oxidation of phenol at the platinum electrode was studied in aqueo us acidic solutions. The effects of electrode surface oxide on the oxi dation reactions of phenol and on electrode passivation by reaction pr oducts were investigated using cyclic voltammetry and chronoamperometr y. X-ray photoelectron spectrometry was used to detect changes in the nature of the passive film. Phenol reacted at both the inner and outer Helmholtz layers at platinum metal electrodes. Phenol in the inner He lmholtz layer is adsorbed irreversibly and is conductive. Its oxidatio n involves ring cleavage with a z(eff) greater than 18 eq/mol. The out er Helmholtz layer reactions are characterized by rapid simple oxidati ons involving minimal rearrangement of the reactant molecule. This imp lies that once stable oxidized products such as benzoquinone and polym ers with quinone or ether structures are formed they must move from th e outer to the inner Helmholtz layer to be oxidized further by ring-cl eavage reactions. We postulate that the bulk of the initial current fl ow during phenol oxidation is due to the simple fast outer Helmholtz r eactions. This initial current continues until the buildup of unreacti ve products blocks further outer Helmholtz reactions and the slower in ner layer reactions predominate. This electrode behavior changed if th e electrode was preoxidized producing a platinum oxide coating. The in ner layer reactions were greatly reduced at a platinum oxide coated el ectrode resulting in lower passivated electrode current flow. The onse t of passivation however was delayed at the oxide coated electrode. Th is is attributed to a weaker adsorption of reaction products at the el ectrode surface requiring additional reaction to produce a stable pass ive film. The final resulting passive films at platinum and platinum o xide electrodes were chemically similar based on x-ray photoelectron s pectrometric analysis but differed in thickness indicating that the el ectrode passivation is not due simply to the thickness of the passive film.