UNDERPOTENTIAL DEPOSITION OF SILVER ON POLYCRYSTALLINE PLATINUM STUDIED BY CYCLIC VOLTAMMETRY AND ROTATING RING-DISC TECHNIQUES

The underpotential deposition of Ag (UPD Ag) on polycrystalline Pt sur faces 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 m easurements. It involves an evaluation of the surface oxidation contri bution by measurement of the charge related to PtO reduction in the fo llowing scan of the voltammetric experiment. This methodology allows o ne to obtain the individual contributions to the total anodic charge d ensity. 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 x 10(-6) M AgNO3 solution showed an experimental maximum cov erage of 90%. The remaining surface was covered by residual H atoms. T his phenomenon was evident for either pure UPD Ag or for depositions i nvolving also bulk Ag. Therefore, apart from the misfit between ad-ato ms and substrate, UPD Ag occurs by a straightforward mechanism involvi ng one-electron transfer and resulting in a complete monolayer of Ag-a ds. The reappearance of adsorbed hydrogen was observed for depositions beyond two monolayers. This was associated with a surface rearrangeme nt followed by three-dimensional nucleation of Ag that generates free- sites for H-adsorption. No evidence of Ag insertion in the Pt phase wa s detected in this work.