Upd mechanisms of copper and thallium on a Pt(111) electrode studied by in-situ IRAS and EC-STM

Underpotential deposition (upd) of copper and thallium on a Pt(lll) electrode in a sulfuric acid solution were studied by time-resolved in-situ infrared spectroscopy and electrochemical scanning tunneling microscopy. Bisulfate anions adsorbed on a Pt(111) electrode were replaced by copper or thallium ions upon upd. The nu(S-O) stretching frequency of the bisulfate anion adsorbed on a Pt(111) electrode showed drastic changes with these replacements. During copper underpotential deposition (0.5 to 0.8 V vs. SHE), sulfate anions were coadsorbed with copper ions on a Pt(111) electrode in the form of root 3 x root 7 structure. Even at a potential more negative than 0.5 V, the image of root 3 x root 7 structure was observed steadily froth fn-situ STM. The spot in the STM image was ascribed to sulfate anions adsorbed on upd copper on Pt(111). In contrast to the copper upd, sulfate anions coadsorbed with upd thallium on a Pt(111) electrode were likely to desorb in a negative sweep. Various kinds of upd process were classified systematically in terms of interactions exerted for an electrolyte anion, a upd atom and an electrode surface. The roles of an electrolyte anion and an electrode potential are especially important for upd mechanisms, and are discussed in detail.