KINETIC-STUDY OF UNDERPOTENTIAL DEPOSITION OF ZINC IONS ON PD(111) INACIDIC PHOSPHATE SOLUTION

The formation and desorption kinetics of Zn2+ ion underpotential depos ition (upd) on Pt(111) were investigated with respect to the effects o f specifically adsorbed anions in the solution by potential sweeping a nd potential step methods. Results by both methods are consistent with each other; in phosphate solution, Zn2+ upd formation took place acco rding to the Langmuir adsorption kinetics, while on the addition of Br - in the solution the kinetic mechanism of Zn2+ upd formation was chan ged to that of a two-dimensional phase transformation, although the de sorption processes of Zn2+ upd always seemed to take place according t o the nucleation-growth mechanism, being indifferent to the presence o f Br-. This indicates that the Zn underpotentially deposited on Pt(111 ) becomes somewhat ordered with the increase of its coverage, and deso rbs according to a nucleation-growth mechanism. In the absence of Br- in the phosphate solution, Zn upd formation obeys the Langmuir adsorpt ion mechanism, which corresponds to the mechanism of phosphate desorpt ion that takes place at the potential where Zn2+ upd occurs. Random Zn 2+ upd formation seems to be induced by the random desorption of the a dsorbed phosphate. On the addition of Br- ions, the Zn2+ ion upd poten tial shifts 0.12V to a more negative potential. The desorption of spec ifically adsorbed anions is suggested to trigger upd of Zn. These fact s are discussed with regard to the adsorbed anions that are likely to play the role of a 'footprint' for upd; empty sites available to upd o f Zn2+ ions are the sites which were just occupied by specifically ads orbed anions. This means that the desorption of the anions guides the upd of Zn2+ as a 'footprint'. (C) 1997 Elsevier Science S.A.