Underpotential electrodeposition of Ag on iodine-covered Pt single-crystal electrodes

The voltammograms for underpotential deposition of Ag were measured on a stepped surfaces of single-crystal Pt in the series n(111)×(100), n(111)×(110) and n(100)×(111) for various n. These surfaces were precovered by monolayers of iodine by immersion into iodide solutions. Silver deposits preferentially on terraces rather than steps, with the ordered structure on the (111) terrace more stable than the (100) terrace structure. Introduction of steps introduces disorder, until, for terraces with fewer than five or six atoms wide, the voltammetry peak for the terrace deposition is no longer resolved. There are no distinct voltammetry peaks associated with adsorption at step sites. A temperature-dependent study was made of the voltammetry of silver deposition on Pt(111)(√7×√7)R19.1°–I, prepared by exposing Pt(111) to iodine vapor. Thermodynamic analysis of this data shows that the reaction has ΔrG°=44.0±0.3 kJ mol−1 and ΔrS°=30±6 J mol−1 K−1 at 25°C, where the standard state for the adsorbed phases is taken as the full-coverage ordered phase.