Expulsion of Borate Ions from the Silver/Solution Interfacial Region during Underpotential Deposition Discharge of Pb(II) in Borate Buffers

The electrochemical quartz crystal microbalance, EQCM, was used to study underpotential deposition (UPD) discharge of Pb(II) at gold and silver electrodes in borate buffers in the pH range 8.0-10.5. The EQCM technique provides a wealth of information concerning the details of the UPD reduction mechanisms. The mechanism of UPD of lead in borate buffers is different at gold and silver. At gold, the source of UPD of lead is a lead species present in the bulk of solution, and the mass change coincides with the charge-transfer reaction. No evidence for a significant change in the population of any other species in the electrode/solution interfacial region is observed. Contrary to this behavior at gold, the UPD mechanism at silver was markedly different. The mechanism at silver initially involves the adsorption of a lead/borate anionic complex from solution. The Pb(II) in this adsorbed complex is then reduced at more negative potentials to underpotentially deposited Pb(0).This surface species reduction step occurs concomitantly with significant expulsion of borate ions from the electrode surface. The stoichiometry of the adsorbed complex has been reported. At still more negative potentials, reduction of dissolved lead borate species occurs to complete the underpotentially deposited lead layer.