Underpotential deposition of zinc ions and specific adsorption of hydroxyl species at Pt(111) in alkaline solutions

Abstract

Underpotential deposition (upd) of zinc ions is observed on Pt(111) in alkaline solution as a sharp cyclic voltammogram (CV) peak, in contrast to upd on polycrystalline Pt, for which several broad upd peaks are observed. The CV characteristics on Pt(111) were studied from changes in the sweep rates, zincate ion concentrations ZnO 2 2−, and pH. The Zn upd formation/desorption mechanism is analyzed and discussed in terms of a nucleation-growth mechanism. Changes in the upd peak potential with changes in the concentration of ZnO 2 2− and OH − were 60 mV/log[ZnO 2 2−] and −180 mV/pH. The former value gives an apparent electron transfer number n a=1. The OH − adsorption/desorption peak on Pt(111) shifted with −60 mV/pH and CV peak width 90 mV, independent of sweep rate. These results show that the reaction OH −=OH ads+e − takes place according to the Langmuir adsorption mechanism. The different pH dependences for Zn upd and OH − adsorption/desorption processes suggest that the electrode reaction in alkaline solution, ZnO 2 2−+2H 2O+2e −=Zn+4OH −, is involved in upd on Pt(111). The value n a=1 for the ZnO 2 2− upd process can be discussed in terms of two-electron transfer, such that Zn 2+ is reduced to a Zn 0 state at Pt(111) with a true electron transfer number of n=2, and the induced OH − adsorption takes place as OH ads onto upd Zn at Pt(111) by an oxidative process. The overall upd process can be expressed as ZnO 2 2−+2 H 2 O+ e ⇌ Pt(111) OH|Zn|Pt+3 OH −, which is in good accord with the observed pH dependence of −180 mV/pH. Consequently, the apparent electron transfer number n a becomes 1. From the charge densities of the upd layer and the OH layer, the coverage ratio of OH ads/Zn upd is equal to 0.59/0.45 with respect to surface Pt atoms.