Underpotential Deposition of Lead on Pt(111) in Perchloric Acid Solution: RRD_Pt(111)E Measurements

Underpotential deposition (UPD) of lead (Pb upd ) on Pt(111) in perchloric acid solution was studied using the rotation ring disk electrode (RRD Pt(111) E) method for measurements of the Pb 2+ ion-specific flux. The total amount of Pb deposited underpotentially is 0.62 ± 5% monolayer (1 monolayer ≡ 1 Pb per Pt) equivalent to a close-packed monolayer of fully discharged Pb adatoms. The electrosorption valence, γ Pb, of Pb upd is ≈ γ Pb = 2, implying that two electrons per Pb upd adatom are exchanged through the interface. In this work we showed that the formation of this close-packed monolayer of Pb upd occurs through four distinctive voltammetric features: two major sharp peaks at ≈0.5 V ( vs saturated calomel electrode) and 0.3 V and smaller reversible peaks at 0 V and −0.15 V. We concluded that the adsorption of OH ad is shifted negatively by Pb upd adatoms, with the shift attributed to induced adsorption of OH ad onto Pt atoms neighboring 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 OH ad onto the Pt(111) surface modified by Pb upd adatoms. The pH independence of the peak at 0.5 V is consistent with the Pb 2+ ion-flux measurements that the main process associated with this pseudocapacitance is Pb UPD. The smaller peaks observed at lower potentials are probably associated with a final deposition of Pb upd adatoms along step edges and compacting of adatoms on the (111)-terraces to form an ordered (3×√3) close-packed structure.