Underpotential deposition of silver on polycrystalline platinum studied by cyclic voltammetry and rotating ring-disc techniques

Abstract

The underpotential deposition of Ag (UPD Ag) on polycrystalline Pt surfaces has been studied combining cyclic voltammetry and rotating ring-disc measurements. A new voltammetric methodology is proposed for the proper calculation of the anodic charge density in UPD Ag desorption measurements. It involves an evaluation of the surface oxidation contribution by measurement of the charge related to PtO reduction in the following scan of the voltammetric experiment. This methodology allows one to obtain the individual contributions to the total anodic charge density. Collecting experiments carried out with the rotating ring-disc system fully confirmed the validity of the procedure. In this way, a complete monolayer of Ag deposited on polycrystalline Pt from a 0.5 M H₂SO₄–5.0 × 10⁻⁶M AgNO₃ solution showed an experimental maximum coverage of 90%. The remaining surface was covered by residual H atoms. This phenomenon was evident for either pure UPD Ag or for depositions involving also bulk Ag. Therefore, apart from the misfit between ad-atoms and substrate, UPD Ag occurs by a straightforward mechanism involving one-electron transfer and resulting in a complete monolayer of Ag_ads. The reappearance of adsorbed hydrogen was observed for depositions beyond two monolayers. This was associated with a surface rearrangement followed by three-dimensional nucleation of Ag that generates free-sites for H-adsorption. No evidence of Ag insertion in the Pt phase was detected in this work.