Adsorption of Anions on Ultra-Thin Metal Deposits on Single Crystal Electrodes.Part 1. Voltammetric and Radiochemical Study of Bisulphate Adsorption on Pt(111) Electrodes Containing Cadmium Adatoms

摘要

The formation of submonolayer coverages of cadmium on Pt(111) and polycrystallinePt electrodes has been characterized using cyclic voltammetry, and adsorption of bisulphate on the Cd-covered surfaces has been measured by radioactive labelling. Cyclic voltammograms of platinum (111) and polycrystalline platinum electrodes in Cd 2+ Containing solutions have been compared. The voltammograms differ with respect to current-potential peak symmetries which shows that the deposition reversibility depends on the surface crystallographic orientation of platinum. Assuming that a total discharge of Cd2+ takes place, the charge measured in the potential range studied would indicate that no more than half a monolayer of cadmium is formed on Pt(111). On the contrary, if the discharge is incomplete and a full monolayer of cadmium is formed, the apparent cadmium electrosorption valency is 0.8, However, the radioactive labelling data indicate that some well-resolved cadmium adatom structure are inactive while some are very active toward anion adsorption. We conclude that the inactive Cd deposits are oxidized and are present on the surface in either the oxide or the hydroxide form. Therefore, at this stage of research, the coulometric determination of electrosorption valencies is merely formal, Once a part of the oxidized cadmium deposits is transformed to the metal phase, bisulphate adsorption begins and conforms to the well-known surface behaviour categorized as enhanced anion adsorption on underpotentially deposited ultra-thin metal films on foreign metal substrates.