ANION EFFECTS ON THE KINETICS OF MERCURY UNDERPOTENTIAL DEPOSITION ONAU(111) ELECTRODES

The kinetics and mechanism of mercury underpotential deposition (UPD) on Au(111) electrodes have been investigated in the presence and absen ce of strongly interacting anions including bi-sulfate, chloride, and acetate. In the absence of strongly interacting anions, i.e. in perchl oric acid, the mercury UPD process is largely controlled by mercury-go ld surface interactions. The presence of sulfuric acid in the supporti ng electrolyte alters the kinetics of the initial and final stages of mercury deposition/dissolution. The presence of two well-ordered struc tures at potentials below (a mercurous sulfate root 3x root 19 structu re) and above (a root 3x root 7 bi-sulfate structure) mercury depositi on leads to the appearance of two pairs of sharp spikes in the cyclic voltammogram. Analysis of the current transients obtained for depositi on and dissolution processes reveals that three different processes ar e taking place during the adsorption/desorption of the mercury bisulfa te layer: adsorption/desorption processes governed by Langmuir kinetic s, a nucleation and growth process linked to an order/disorder transit ion to form the mercury bi-sulfate adlayer, and an order/disorder tran sition related to the formation/disruption of the root 3x root 7 bi-su lfate layer. In chloride medium, the voltammetric profile is very simi lar to that obtained in sulfuric acid solution, with the presence of t wo sharp spikes. However, no nucleation and growth kinetics mechanism was found linked to the process of formation/disruption of the mercury chloride adlayer. The transients show a clear deviation from the idea l Langmuir behavior, probably associated with the presence of attracti ve interactions in the mercury chloride adlayer. The kinetics of mercu ry UPD in acetate media are significantly slower than in the previous media, as revealed by voltammetric and chronoamperometric measurements . The slow kinetics appear to be related to the formation of Hg2+-acet ic acid complexes in solution. Although ordered structures are formed at potentials below the main UPD peak, no nucleation and growth mechan ism was observed.