The effect of chloride on the electrooxidation of adsorbed CO on polycrystalline platinum electrodes

We have used the inhibition of the electrooxidation of adsorbed CO at low overpotentials by chloride ions in the solution to elucidate, in an indirect way, the nature of this process. After a characterisation of the polycrystalline platinum surface by means of cyclic voltammetry, we show that the amount of CO adsorbed on the surface at saturation is independent of the chloride concentration in the solution, ruling out completely the possibility of a mixed CO-chloride adlayer. Our data suggest that the process occurring in the pre-peak in CO-stripping voltammograms corresponds to the oxidation through a Langmuir–Hinshelwood mechanism of a fraction of a compact CO adlayer. The resulting less compact CO adlayer is somewhat more strongly adsorbed because of the lower repulsive interactions between neighbouring CO molecules and is, therefore, more oxidation resistant, its oxidation giving rise to the main CO peak. Chloride adsorption blocks the adsorption sites left free by the first CO molecules oxidised, inhibiting OH adsorption and, therefore, the oxidation of the CO adlayer, which can proceed at higher potentials only.