X-ray and electrochemical studies of Cu upd on single crystal electrodes in the presence of bromide:: comparison between Au(111) and Pt(111) electrodes

Results from electrochemical and in-situ surface X-ray scattering studies of Cu underpotential deposition (upd) on Au(111) and Pt(111) surfaces in the presence of bromide anions are compared. On Au(111) two different ordered structures have been found by GIXD. At the initial stages of Cu deposition, an enhancement of bromide adsorption is observed resulting in the formation of a bromide incommensurate hexagonal structure, which compresses when the potential is scanned in the negative direction. At +0.32 V, there is a phase transition giving rise to the formation of a commensurate (4 x 4) bromide structure. This phase transition coincides with the appearance of a very sharp peak in the voltammetric profile. In this structure, copper adatoms are probably sandwiched between the electrode surface and the bromide adlayer. Unlike Cu upd on Pt(111) electrodes, no ordered CuBr adlayer is observed. The differences in behavior of Cu upd on Au(111) and Pt(111) electrodes is ascribed not to energetic considerations, but rather to geometric constraints imposed by the lattice structure of the metal relative to the deposited adlayer. The stability (to rinsing) and kinetics of the adlayers formed on Pt(111) electrodes in the presence of bromide have also been examined. (C) 1999 Elsevier Science S.A. All rights reserved.