Underpotential deposition of lead on Pt(111) in the presence of bromide: RRDPt(111)E and X-ray scattering studies1

The underpotential deposition (upd) of Pb onto Pt(111) in solution containing Br^? was examined by classical RRD_Pt(111)E electrochemical methods in combination with in situ surface X-ray scattering measurements. The results show that Pb upd onto Pt(111) in the presence of bromide is a competitive process with the total amount of Pb deposited at underpotential being only ca. 0.51±5% ML, less than the amount needed to form a close-packed monolayer, 0.63 ML (1 ML equals 1 Pb atom per Pt surface atom). Deposition of Pb adatoms is accompanied by desorption of about one-half of the bromide adsorbed on the Pt surface at higher potentials, leaving co-adsorbed bromide both in and on-top-of the adlayer. In the potential region where the ratio of co-adsorbed species is close to one (E≈0 V) there is the slow formation (over a period of ca. 2 h) of an ordered surface alloy, which has a p(2×2) unit cell. No other ordered structures of Pb are observed in the underpotential region. © 1997 Elsevier Science S.A.