Underpotential deposition of lead onto Pt(100) in acid solutions: Adsorption isotherms and interface structures

Abstract

The underpotential deposition (UPD) of Pb onto Pt(100) in pure perchloric acid and in solution containing Br^- was examined by classical rotating ring disk electrode (RRD_Pt(100)E) electrochemical methods in combination with in-situ surface X-ray scattering measurements. The total amount of Pb deposited underpotentially in 0.1 M HClO₄ is ≈0.62±5% ML (1 ML equals 1 Pb atom per Pt surface atom), nearly equivalent to a close-packed monolayer of fully discharged Pb adatoms, e.g. 0.63 ML. In pure perchloric acid, deposition of Pb adatoms is accompanied by complete displacement of OH_ad from the Pt(100) surface, and in the potential range 0.15 V<E<0.35 V we found an ordered surface structure with a c(2×2) unit cell. In perchloric acid containing bromide, the maximum Pb_upd surface coverage is ≈0.5 ML; the deposition of 0.5 ML of Pb adatoms is accompanied by desorption of about 0.22 ML of the Br_ad adsorbed on the Pt surface at 0.6 V (ca. 0.42 ML). Thus, Pb UPD on Pt(100) from solution containing Br^- is a competitive process, with the total amount of Pb deposited at underpotential being less than the amount needed to form a close-packed monolayer. As a consequence, the formation of an ordered c(2×2) structure is observed only at E<0.15 V.