UNDERPOTENTIAL DEPOSITION OF LEAD ON PT(111) IN PERCHLORIC-ACID SOLUTION - RRDPT(111)E MEASUREMENTS

Underpotential deposition (UPD) of lead (Pb-upd) on Pt(111) in perchlo ric acid solution was studied using the rotation ring disk, electrode (RRDPt(111)E) method for measurements of the Pb2+ ion-specific flux. T he total amount of Pb deposited underpotentially is 0.62 +/- 5% monola yer (1 monolayer = 1 Pb per Pt) equivalent to a close-packed monolayer of fully discharged Pb adatoms. The electrosorption valence, gamma(Pb ), of Pb-upd is approximate to gamma(Pb) = 2, implying that two electr ons per Pb-upd adatom are exchanged through the interface. In this wor k we showed that the formation of this close-packed monolayer of Pb-up d occurs through four distinctive voltammetric features: two major sha rp peaks at approximate to 0.5 V (vs saturated calomel electrode) and 0.3 V and smaller reversible peaks at 0 V and -0.15 V. We concluded th at the adsorption of OHad is shifted negatively by Pb-upd adatoms, wit h the shift attributed to induced adsorption of OHad onto Pt atoms nei ghboring the Pb-upd adatoms due to lowering of the local point of zero charge (pcz) by the Pb-upd. Consequently, the pH dependence of the Pb UPD peak at 0.3 V appears entirely through a pH-dependent adsorption of OHad onto the Pt(111) surface modified by Pb-upd adatoms. The pH in dependence of the peak at 0.5 V is consistent with the Pb2+ ion-flux m easurements that the main process associated with this pseudocapacitan ce is Pb UPD. The smaller peaks observed at lower potentials are proba bly associated with a final deposition of Pb-upd adatoms along step ed ges and compacting of adatoms on the (111)-terraces to form an ordered (3 x root 3) close-packed structure.