Voltammetric and rotating ring-disk studies of underpotential deposition of Ag and Cu on polycrystalline Au electrodes in aqueous H2SO4

The underpotential deposition (UPD) of Cu and Ag from sulphuric acid solutions was studied on polycrystalline Au surfaces by cyclic voltammetry at stationary and rotating ring-disk electrodes. Since the voltammetric profiles also include the surface oxide/reduction contributions, the charges were also calculated by means of the collection technique of the rotating ring-disk system. The charge values obtained from cyclic voltammetry at stationary and rotating ring-disk electrodes were 405 and 386 mu C cm(-2), respect ively, for the oxidation of a complete monolayer of UPD Cu. The proximity of these values suggests epitaxial deposition in a two-electron reduction process. On the other hand, the charge values found for UPD Ag were 108 and 115 mu C cm(-2), which corresponds to the amount required for the oxidation of only half a monolayer of Ag. Since the atomic radii of the three atoms involved (Cu, Ag and Au) are very similar, the differences in the deposition models were associated with the co-adsorption of anions (sulphate or hydroxide ion) which hinders the formation of a full monolayer of Ag on polycrystalline Au surface. (C) 1998 Elsevier Science Ltd. All rights reserved.