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

The surface electrochemistry of reversibly and irreversibly adsorbed bismuth (Bi-ad) was studied on Pt(111) electrodes in 0.1 M HClO4. The valence state of irreversibly adsorbed Bi (Bi-ir) was determined by ex situ X-ray photoelectron spectroscopy (XPS). It was shown that Bi-ir does not change its valence state during potential cycling and is adsorbed in its metallic (i.e., zero-valent) state. Underpotential deposition (UPD) of Bi (Bi-upd) onto Pt(111) was studied using the rotating ring-disk electrode (RRDE) technique for measurements of the Bi3+ ion specific flux. The total amount of Bi deposited at underpotential (Theta(Bi,upd)) was determined by integration of the 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 irreversibly adsorbed Bi. The difference between Theta(Bi,upd) and the maximum coverage of a close-packed monolayer of fully discharged Bi adatoms (Theta(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 voltammetry (Q=160 mu C cm(-2)) for Theta(Bi,ir)similar to 1/3 ML is ascribed to enhanced adsorption of OH on Pt sites adjacent to Bi due to a change of the local potential of zero charge (p.z.c.) induced by Bi. Contrasting kinetic effects 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.