Bi adsorption on Pt(111) in perchloric acid solution: A rotating ring-diskelectrode and XPS study

The surface electrochemistry of reversibly and irreversibly adsorbed bismut h (Bi-ad) was studied on Pt(111) electrodes in 0.1 M HClO4. The valence sta te of irreversibly adsorbed Bi (Bi-ir) was determined by ex situ X-ray phot oelectron spectroscopy (XPS). It was shown that Bi-ir does not change its v alence state during potential cycling and is adsorbed in its metallic (i.e. , zero-valent) state. Underpotential deposition (UPD) of Bi (Bi-upd) onto P t(111) was studied using the rotating ring-disk electrode (RRDE) technique for measurements of the Bi3+ ion specific flux. The total amount of Bi depo sited at underpotential (Theta(Bi,upd)) was determined by integration of th e ion specific flux, and found to be approximate to 0.16 ML (1 ML=1 Bi/1 Pt ). The UPD Bi is assumed to deposit at bare Pt sites not occupied by the ir reversibly adsorbed Bi. The difference between Theta(Bi,upd) and the maximu m coverage of a close-packed monolayer of fully discharged Bi adatoms (Thet a(Biad)=0.56 ML) is taken to equal the coverage by Bi-ir, or approximate to 1/3 ML. The charge under the reversible peak in the Pt(111)/Bi-ir voltamme try (Q=160 mu C cm(-2)) for Theta(Bi,ir)similar to 1/3 ML is ascribed to en hanced adsorption of OH on Pt sites adjacent to Bi due to a change of the l ocal potential of zero charge (p.z.c.) induced by Bi. Contrasting kinetic e ffects of Bi-ir were observed on the hydrogen and carbon monoxide oxidation reactions. These effects are discussed in terms of the known role of OHad in these reactions on the Pt(111) surface.