COMBINED ULTRAHIGH VACUUM ELECTROCHEMISTRY STUDY OF THE ADSORPTION OFLEAD ON CLEAN AND SULFUR-MODIFIED NICKEL SURFACES IN AQUEOUS ENVIRONMENTS/

The adsorption of lead on clean and sulfur-modified nickel surfaces wa s studied as a function of immersion time. The effect of preadsorbed s ulfur and lead adsorption on nickel oxidation was also examined by ope n-circuit potential(OCP) measurements. Sulfur adsorbed on nickel inhib its the formation of the inner NiO layer, which results in an enhanced nickel oxidation/dissolution in aqueous solutions. The adsorption of lead is enhanced by as much as 1200% when the nickel electrode is firs t modified by a monolayer of adsorbed sulfur. The chemical shift in Pb (4f) binding energies indicates that lead (Pb2+) ions interact directl y with the preadsorbed sulfur on nickel. Lead adsorbed on sulfur-modif ied nickel electrode is also found to retard the oxidation/dissolution of nickel, which is enhanced by sulfur. On sulfur-modified nickel sur faces, 70% of the lead adsorption occurs during the initial 30 s of im mersion. The rate of lead adsorption approaches zero after an immersio n time of 10 min. On the basis of the observations of this study, a mo del for the sulfur-induced lead adsorption has been formulated.