Chloride and iodide electrosorption on Ag(1 1 1)

Chloride and iodide adsorption on Ag(1 1 1) was investigated from mixed c M KX + (0.02 − c) M KPF6, with X = Cl, I, using the potential-step chronocoulometric technique. Measurements were carried out at potentials negative of –0.5 V/SCE, where STM does not provide images of ordered adlayers of these halides. Their adsorption behavior was compared with that of bromide ion on Ag(1 1 1), reported in a previous work. The plot of the applied potential E, decreased by the potential difference ϕd across the diffuse layer, shows a maximum when plotted against the specifically adsorbed charge density σi of the halides, at constant charge density σM on the metal. This behavior is explained by an appreciable contribution from reorientation and displacement of adsorbed water molecules by the adsorbing halides. The minimum of the E − ϕd vs. σi plot is much shallower for iodide than for chloride and bromide. The σi vs. log c plots at σM = 0 are linear, thus satisfying the Temkin isotherm. The resulting interaction factor is appreciably less repulsive for iodide than for chloride and bromide. These results indicate that partial charge transfer from iodide to Ag(1 1 1) is appreciable even at negative potentials at which adsorption is random, as distinct from chloride and bromide.