Underpotential deposition of mercury on Au(111): Electrochemical studies and comparison with structural investigations

Electrochemical studies of the UPD of mercury on Au(111) electrodes have been carried out in sulfuric acid and in perchloric acid alone as well as in the presence of sulfate and chloride anions and in acetate/acetic acid buffer with emphasis on the processes taking place in the presence of (b)sulfate anions. These voltammetric studies have been compared to recent in-situ AFM, STM, and surface X-ray scattering studies in an effort to correlate voltammetric features with surface structures and their transitions. The different processes taking place during mercury UPD on A(111) in sulfuric acid can be summarized as follows: (ii) At potentials higher than the first mercury UPD peak, there exists an ordered (b)sulfate adlayer. The onset of mercury deposition triggers an order/disorder transition that gives rise to the first set of UPD peaks; C-1/A(1). (ii) Mercury deposition is governed mainly by mercury-gold interactions, As monolayer deposition reaches completion a disorder/order transition takes place giving rise to the peaks C-2/A(2). The adlayer is likely composed by Hg2SO4 with the coadsorption of additional H3O+ cations, as an analysis of the coulometric charges appears to indicate. (iii) At +0.816 V there appears a small voltammetric feature (peak C-3) which corresponds to the disappearance of the ordered coadsorbed structure, (iv) Reduction of Hg2+ to Hg-2(2+) in solution occurs at +0.54 V. (v) The final process is the completion of the mercury monolayer (peak C-5) followed by the formation of a mercury-gold amalgam at +0.50 V that leads to the roughening of the electrode surface. These interpretations are consistent with both voltammetric and recent in-situ surface X-ray results as well as with previous electrochemical and STM investigations and provide a very detailed microscopic picture of the processes taking place.