The underpotential deposition of cadmium on Pt(1 1 1): effect of the anions and CO displacement experiments

The underpotential deposition (upd) of cadmium on Pt(1 1 1) electrodes in acidic solutions has been investigated. The formation of two upd layers prior to the onset of bulk deposition has been shown. Voltammetric and charge displacement experiments (as CO can displace Cd adatoms) allow an estimation of the Cd coverage corresponding to the upd layers. The first layer is relatively open (θcd≈0.31), probably because of important Cd–Cd repulsion, whereas the second is more compact. Charge displacement experiments also provide an estimate for the potential of zero charge of the Cd/Pt(1 1 1) electrode, which, as expected, lies between those of Pt(1 1 1) and Cd(0 0 0 1) electrodes. Finally, the effect of chloride and bromide on the upd process has been studied. Both anions lead to an extraordinary sharpening of the voltammetric peaks, which results from the existence of strong attractive Cd–halide interactions at the electrode surface. In the case of bromide the main upd peak splits depending on cadmium and bromide analytical concentrations. This unusual behavior is thought to be caused by the separate adsorption of Cd2+ and CdBr+ species. The adsorption of CdBr+ would occur at more positive potentials probably induced by the presence on the electrode surface of preadsorbed bromide. The limitations of coulometry (either from voltammetry or from charge displacement experiments) for providing values of upd adatom coverage are discussed.