Selective inhibition of electrode reactions by organic compounds. I. The inhibition of Br₂ and I₂ evolution on platinum by phenol

The oxidation of Br^- and I^- on Pt electrodes in HClO₄ was found to be selectively inhibited by phenol. The rate of oxygen evolution is essentially unaltered under the same experimental conditions. The species causing inhibition is not phenol but a radical (most likely O·) formed from it in an anodic charge transfer adsorption process. The initial potential for inhibition was related to the concentration of phenol in solution and to the pH. The adsorption of the radical is controlled by the combined effects of charge transfer and competition with adsorbed water molecules. Very good agreement with the theory of electrosorption of organic species based on a competition-with-water model is obtained. The number of water molecules replaced by each phenol radical is five, indicating that the radical is adsorbed through the oxygen atom, with the aromatic ring at an angle to the surface. Very high fractional surface coverages were found (values of 1-σ being in the range 10^-2-10^-4) in the potential region where inhibition takes place.