Radiotracer study of thallium underpotential deposition on a polycrystalline gold electrode in alkaline solutions

Surface concentration of thallium atoms at the polycrystalline gold electrode in 0.1 M KOH supporting electrolyte containing 1 x 10(-5) M and 1 x 10(-4) M Tl+ ions was measured by the radioactive labeling (Tl-204) method. Kinetics of thallium electrodeposition, anodic dissolution, and underpotential deposition (upd) as well as the equilibrium surface concentration of thallium atoms in the potential region of thallium upd were determined from the radiotracer data. The potential dependence of the equilibrium surface concentration obtained from the radiometric method was compared with that calculated from the cyclic voltammetry. At potentials close to the Nernst potential for the Tl/Tl(I) couple, the surface concentration of thallium atoms is close to the close-packed thallium monolayer capacity. In the potential region from the Nernst potential to the main upd peak, the thallium surface concentration decreases monotonically with the electrode potential. However, at more positive potentials, the surface concentration does not drop to zero, but goes through a minimum. Around this minimum, the surface concentration is apparently determined both by the upd of thallium atoms and electrostatic adsorption of thallium cations induced by the specific adsorption of hydroxyl anions. (C) 2000 Elsevier Science B.V. All rights reserved.