The electrooxidation of CO on oxide-free smooth Pt in 1 M NaOH has been studied by electrochemical techniques and by UV-Visible Potential-Modulated Reflectance Spectroscopy (PMRS). The influence of the potential at which CO was first admitted to the electrolyte, E(adm), on the electrocatalytic activity of Pt for CO oxidation depended on the electrochemical technique used, viz. stationary or potential sweep, since the poisoning CO species formed at high E(adm) was desorbed after cathodic polarization of the electrode for a few minutes. Voltammograms in CO-saturated 1 M NaOH showed three anodic peaks, the most negative of which corresponded to the oxidation of dissolved CO, and the other two to oxidation of surface species. Apparently, the simultaneous presence of three peaks in the voltammogram under these conditions has not been reported before. A third surface peak was detected from voltammograms for CO-free solution; it did not appear in CO-saturated solution because it was hidden under the peak due to dissolved CO. Although all three surface species blocked the adsorption of hydrogen, only the CO species which is oxidized at the most positive voltammetric peak acted as a poison for CO electrooxidation, in the same way as in the acidic medium. The PMR spectrum of chemisorbed CO shows a maximum at 280 nm, which has been assigned to bridge-bonded CO, although perhaps mixed with linear CO.
