Isotherm and Electrosorption Valency for Underpotential Deposits of Silver on Gold

The isotherm and electrosorption valency for underpotential deposits of silver on gold were determined in 2.00 10-5 M Ag(I) –0.2 M H2S04 solution using the rotating ring-disk electrode (RRDE). Both disk electrode currents and ring electrode currents were required to obtain the electrosorption valency and ring electrode shielding or collection currents were used to determine the surface coverage. Underpotential deposition (UPD) occurs at a convective diffusion controlled rate during deposition of the first monolayer and at a considerably reduced rate during subsequent atom layer deposition. Equilibrium silver coverages well in excess of three atom layers are obtained at potentials near the Nernst potential. Proof of equilibrium was established by experiments starting with bare gold or thick silver deposits. As shown by RRDE shielding and collection experiments, all silver adsorbed in the underpotential region could be removed. The electrosorption valency varies from 0.66 at low coverages (0.1 to 0.4 monolayer), has a minimum of 0.63 at coverages between 0.4 to 0.7 monolayer, and then approaches one at higher coverages. The transient surface coverage dependence of the electrosorption valency is independent of UPD potential for potentials 5 mV or more anodic than the Nernst potential.