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) el ectrode in a sulfuric acid solution were studied by time-resolved in-s itu infrared spectroscopy and electrochemical scanning tunneling micro scopy. 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 depositi on (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 ro ot 7 structure was observed steadily froth fn-situ STM. The spot in th e STM image was ascribed to sulfate anions adsorbed on upd copper on P t(111). In contrast to the copper upd, sulfate anions coadsorbed with upd thallium on a Pt(111) electrode were likely to desorb in a negativ e sweep. Various kinds of upd process were classified systematically i n terms of interactions exerted for an electrolyte anion, a upd atom a nd an electrode surface. The roles of an electrolyte anion and an elec trode potential are especially important for upd mechanisms, and are d iscussed in detail.