VOLTAMMETRIC STUDY OF UNDERPOTENTIAL DEPOSITION (UPD) OF ZN2- EFFECT OF ADSORBED ANION( IONS ON PT(111) )

The underpotential deposition (upd) of Zn2+ ions on Pt(111) was examin ed in 0.1 M KH2PO4 of pH 4.4 with and without halide ions (Cl-, Br- an d I-) by cyclic voltammetry. Cyclic voltammograms of Zn upd were compa red with those in the absence of Zn2+ ions. The charge transfer to Zn2 + ions was initiated near the desorption potential of adsorbed anions, often with the phase transition on Pt(111). Zn upd potentials negativ ely shifted by anion adsorption according to the order of the adsorpti on strength of H2PO4- < Cl- < Br- < I-. The presence of Br- and I- ion s narrowed the peak width of Zn deposition voltammograms in comparison with the situation with and without Cl- ions. These results suggest t hat Zn upd is correlated closely with the behaviour of pre-adsorbed an ions of the so-called anomalous wave on Pt(111). The anion phase trans ition is discussed to be correlated closely with the Zn upd potential. The number of electrons transferred to one Zn2+ ion, gamma, was estim ated as about two by correlating with the number of electrons for the anion discharge with and without Br- ions. The coverage of upd Zn was estimated as roughly 1/3 down to 0.05 V vs. RHE with gamma = 2.