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 (RRDPt(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 OHad 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 Pbupd 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 Brad 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.