THE EFFECTS OF ORGANIC ADSORBATES ON THE UNDERPOTENTIAL AND BULK DEPOSITION OF SILVER ON POLYCRYSTALLINE PLATINUM-ELECTRODES

The effects of various heterocyclic coadsorbates (pyridine, pyrazine, 2,2'-bipyridyl, 4,4'-bipyridyl, 4-phenylpyridine, 4-mercaptopyridine, and 2-mercaptopyridine) on the electrodeposition of silver onto polycrystalline platinum electrodes in 0.10M H2SO4 have been studied using cyclic voltammetry. Adsorbates bonding through a nitrogen heteroatom significantly hinder both the silver underpotential (UPD) and bulk deposition processes giving rise to a diminution in the underpotential shift in the former and an overpotential in the latter. The diminution of the underpotential shift suggests a strong interaction of the adsorbate adlayer with the platinum substrate. In addition, the existence of a significant overpotential necessary for initiation of bulk silver deposition suggests a significant interaction between the UPD silver monolayer and the adsorbate adlayer. A Pt/Ag/adsorbate intermediary structure is proposed based on this voltammetric evidence. The sulfur-containing adsorbates completely inhibit silver UPD as well as the formation of a PtO film on the polycrystalline electrode. The only observable processes were bulk deposition (at overpotentials of the order of 125 mV) and subsequent bulk stripping of the deposited silver. The effects of having a polycrystalline surface have been assessed by comparing these results to those from studies of silver deposition onto Pt(111) surfaces in the presence of the same coadsorbed species.