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

Abstract

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 (Θ_Bi,upd) was determined by integration of the ion specific flux, and found to be ≈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 Θ_Bi,upd and the maximum coverage of a close-packed monolayer of fully discharged Bi adatoms (Θ_Biad=0.56 ML) is taken to equal the coverage by Bi_ir, or ≈1/3 ML. The charge under the reversible peak in the Pt(111)/Bi_ir voltammetry (Q=160 μC cm⁻²) for Θ_Bi,ir∽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 OH_ad in these reactions on the Pt(111) surface.