Anion Effects on the Kinetics of Mercury Underpotential Deposition on Au(111) Electrodes

Abstract

The kinetics and mechanism of mercury underpotential deposition (UPD) on Au(111) electrodes have been investigated in the presence and absence of strongly interacting anions including bi-sulfate, chloride, and acetate. In the absence of strongly interacting anions, i.e. in perchloric acid, the mercury UPD process is largely controlled by mercury−gold surface interactions. The presence of sulfuric acid in the supporting electrolyte alters the kinetics of the initial and final stages of mercury deposition/dissolution. The presence of two well-ordered structures at potentials below (a mercurous sulfate √3×√19 structure) and above (a √3×√7 bi-sulfate structure) mercury deposition leads to the appearance of two pairs of sharp spikes in the cyclic voltammogram. Analysis of the current transients obtained for deposition and dissolution processes reveals that three different processes are taking place during the adsorption/desorption of the mercury bi-sulfate layer: adsorption/desorption processes governed by Langmuir kinetics, a nucleation and growth process linked to an order/disorder transition to form the mercury bi-sulfate adlayer, and an order/disorder transition related to the formation/disruption of the √3×√7 bi-sulfate layer. In chloride medium, the voltammetric profile is very similar to that obtained in sulfuric acid solution, with the presence of two 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 ideal Langmuir behavior, probably associated with the presence of attractive interactions in the mercury chloride adlayer. The kinetics of mercury 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+−acetic acid complexes in solution. Although ordered structures are formed at potentials below the main UPD peak, no nucleation and growth mechanism was observed.