UNDERPOTENTIAL DEPOSITION OF LEAD ONTO PT(100) IN ACID-SOLUTIONS - ADSORPTION-ISOTHERMS AND INTERFACE STRUCTURES

The underpotential deposition (UPD) of Pb onto Pt(100) in pure perchlo ric acid and in solution containing Br- was examined by classical rota ting ring disk electrode (RRDPt(100)E) electrochemical methods in comb ination with in-situ surface X-ray scattering measurements. The total amount of Pb deposited underpotentially in 0.1 M HClO4 is approximate to 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 ac companied by complete displacement of OHad from the Pt(100) surface, a nd in the potential range 0.15 V < E < 0.35 V we found an ordered surf ace structure with a c(2 x 2) unit cell. In perchloric acid containing bromide, the maximum Pb-upd surface coverage is approximate to 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 c ompetitive process, with the total amount of Pb deposited at underpote ntial being less than the amount needed to form a close-packed monolay er. As a consequence, the formation of an ordered c(2 x 2) structure i s observed only at E < 0.15 V.