Effect of chloride ions on the electrooxidation at low potentials of dissolved carbon monoxide on platinum

Abstract

We have studied by cyclic voltammetry and Fourier-transform infrared spectroscopy (FTIRS) the influence of chloride ions on the adsorption and electrooxidation of CO on polycrystalline Pt in 0.5 M H 2SO 4 and in 0.1 M NaOH, using a low CO dosing potential, 0.07 and 0.18 V versus RHE, respectively. As is well known, under these circumstances a subsequent stripping CV of Pt in CO-free solution shows a pre-peak and a main peak of chemisorbed CO, usually attributed to weakly and strongly adsorbed CO, respectively. It is also well known that in CO-saturated solutions, for low CO dosing potentials dissolved CO electrooxidation on Pt occurs already at 0.5 V, well below the usually reported potential of 0.85 V observed when CO is dosed at open circuit. It has been found here that in acidic, but not in alkaline medium, chloride ions inhibit the adsorption on Pt of the weakly chemisorbed CO far more strongly than that of the strongly chemisorbed CO, and also inhibit the electrooxidation of dissolved CO on Pt at low potentials, in such a way that there exists a linear relationship between the peak current density of dissolved CO electrooxidation and the charge of the pre-peak in stripping CVs of chemisorbed CO. This correlation gives further confirmation to the hypothesis that electrooxidation of dissolved CO on Pt at low potentials occurs only on the Pt sites liberated from CO in the pre-peak. Inhibition by chloride ions is due to their adsorption on Pt, since the peak current density of dissolved CO electrooxidation decreases linearly with the logarithm of the chloride concentration, and therefore with the adsorption of chloride ions, which increases linearly with the logarithm of the chloride concentration for [Cl −]>10 −5 M.