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 ha ve been carried out in sulfuric acid and in perchloric acid alone as w ell as in the presence of sulfate and chloride anions and in acetate/a cetic acid buffer with emphasis on the processes taking place in the p resence of (b)sulfate anions. These voltammetric studies have been com pared to recent in-situ AFM, STM, and surface X-ray scattering studies in an effort to correlate voltammetric features with surface structur es 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 trigge rs an order/disorder transition that gives rise to the first set of UP D peaks; C-1/A(1). (ii) Mercury deposition is governed mainly by mercu ry-gold interactions, As monolayer deposition reaches completion a dis order/order transition takes place giving rise to the peaks C-2/A(2). The adlayer is likely composed by Hg2SO4 with the coadsorption of addi tional H3O+ cations, as an analysis of the coulometric charges appears to indicate. (iii) At +0.816 V there appears a small voltammetric fea ture (peak C-3) which corresponds to the disappearance of the ordered coadsorbed structure, (iv) Reduction of Hg2+ to Hg-2(2+) in solution o ccurs at +0.54 V. (v) The final process is the completion of the mercu ry monolayer (peak C-5) followed by the formation of a mercury-gold am algam at +0.50 V that leads to the roughening of the electrode surface . These interpretations are consistent with both voltammetric and rece nt in-situ surface X-ray results as well as with previous electrochemi cal and STM investigations and provide a very detailed microscopic pic ture of the processes taking place.