Adsorption Competition Effetcs in the Electrocatalytic Reduction of Organic Halides on Silver

A systematic study of (silver inorganic and/or organic halide) interphases is described, based on cyclovoltammetric and impedance experiments carried out on polycrystalline electrodes of high reproducibility (controlled by comparison with monocrystalline ones), in water and in acetonitrile. The adsorption competition is brought into focus (i) between the three inorganic halides Cl-, Br-, and I-, with the possible interference of ions from the supporting electrolyte, and (ii) between organic and inorganic halides, especially iodides. Actually adsorption competition proves to be a determining factor in the electrocatalytic mechanism (a remarkable threshold effect being observed for the reduction potentials of organic bromides in correspondence with the limiting negative potential for iodide anion adsorption). Vice versa, the variation of the reduction potentials of a given reacting molecule with the background electrolyte provides significant clues on the nature of the specifically adsorbed species.