CORRELATION OF THE UNDERPOTENTIAL DEPOSITION (UPD) OF ZINC IONS ON PT(111), PT(100), AND PT(110) WITH ANION-SPECIFIC ADSORPTION

We report two roles of the adsorbed anions in the underpotential depos ition (upd) of zinc ions giving cooperative and competitive interactio n with the upd metals under different circumstances. The effect of spe cifically adsorbed anions on Zn upd was investigated systematically at Pt(111), Pt(100), and Pt(110) in solutions of pH 1-4.6 by cyclic volt ammetry, where the anions were (bi)sulfate, phosphate, chloride, bromi de, and iodide anions. Zn upd hardly occurred on Pt(111) in acidic sol utions of pH 1. However, the growth of sharp Zn upd voltammetric waves was observed on Pt(111) in phosphate solutions with pH increase in th e pH range of 2-4.6. In sulfate and perchlorate solutions, such behavi or was not observed on Pt(111) with pH increase. On the other hand, at Pt(110), the Zn upd was clearly observed in phosphate solution of pH 1. In 0.1 M KH2PO4 (pH 4.4) with 10(-3) M halides, the onset potential of Zn upd on Pt(111) shifted negatively according to the order of the adsorption strength of Cl- < Br- < I-. The negative shift of 0.02 V b y Cl- adsorption on Pt(111) was smaller than that of 0.10 V on Pt(100) . These results are discussed and correlated to the strength of anion adsorption at the onset potentials of Zn upd, in terms of cooperative and competitive interaction of the adsorbed anion with the upd Zn; ads orbed phosphate anions on Pt facilitate the Zn upd kinetically by the desorption action, but halides tightly adsorbed at the Pt surface obst ruct the initiation of Zn upd. (C) 1998 Elsevier Science S.A. All righ ts reserved.