Potential oscillations and S-shaped polarization curve in the continuous electro-oxidation of CO on platinum single-crystal electrodes

The occurrence of an S-shaped polarization curve in a simple model for the continuous electrochemical oxidation of CO on a platinum electrode is discu ssed. In the model, the S-shaped polarization curve is caused by the compet itive Langmuir-Hinshelwood mechanism between surface-bonded CO and OH. The reaction is studied experimentally on single-crystal platinum rotating disk electrodes in perchloric and sulfuric acid solution, and it is shown that the voltammetry is in good agreement with the model predictions. When studi ed under current-controlled conditions, a fast galvanodynamic scan indeed s uggests the existence of the S-shaped polarization curve. At lower scan rat es, however, irregularities and small-amplitude irregular fluctuations or o scillations in potential are observed. Very regular potential oscillations under current-controlled conditions are observed only on Pt(111) in sulfuri c acid. The possible origin of these irregularities and oscillations is dis cussed in relation to the existing theories of electrochemical instabilitie s.