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

We have studied by cyclic voltammetry and Fourier-transform infrared spectr oscopy (FTIRS) the influence of chloride ions on the adsorption and electro oxidation of CO on polycrystalline Pt in 0.5 M H2SO4 and in 0.1 M NaOH, usi ng 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, usual ly attributed to weakly and strongly adsorbed CO, respectively. It is also well known that in CO-saturated solutions, for low CO dosing potentials dis solved CO electrooxidation on Pt occurs already at 0.5 V, well below the us ually reported potential of 0.85 V observed when CO is dosed at open circui t. It has been found here that in acidic, but not in alkaline medium, chlor ide 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 el ectrooxidation of dissolved CO on Pt at low potentials, in such a way that there exists a linear relationship between the peak current density of diss olved CO electrooxidation and the charge of the pre-peak in stripping CVs o f chemisorbed CO. This correlation gives further confirmation to the hypoth esis that electro oxidation of dissolved CO on Pt at low potentials occurs only on the Pt sites liberated from CO in the pre-peak. Inhibition by chlor ide ions is due to their adsorption on Pt, since the peak current density o f dissolved CO electrooxidation decreases linearly with the logarithm of th e chloride concentration, and therefore with the adsorption of chloride ion s, which increases linearly with the logarithm of the chloride concentratio n for [Cl-1] > 10(-5) M. (C) 2001 Elsevier Science B.V. All rights reserved .