Underpotential-overpotential transition of silver overlayer on platinum Part 1. Formation of a Pt + Ag surface alloy

Abstract

Abstract The underpotential deposition (UPD) of silver on a polycrystalline platinum electrode was studied. The amount of Ag deposited in the UPD region, positive to the reversible potential, corresponds to about 2.5 monolayers. Two types of binding of Ag to the surface can be distinguished based on the anodic stripping behaviour. A first layer is deposited on top of the Pt atoms. The amount of Ag in this layer slightly exceeds that needed to form a monolayer, probably due to the strain caused by a slight misfit between the atomic radii of Pt and Ag. The Ag atoms in this layer undergo place exchange, forming a Pt + Ag surface alloy. This process occurs at room temperature on a time-scale of minutes, reaching the steady state in about an hour. This leads to a new (modified) electrode surface, consisting of a mixture of Pt and Ag atoms, upon which further deposition of Ag in the underpotential region occurs. The amount of Ag in the second layer depends on the deposition potential and can reach a charge corresponding to 1.5–2.0 atomic layers, when deposition occurs at a small underpotential of a few millivolts, indicating that some degree of clustering has already occurred in this region. The two layers differ not only in their binding energy, as evidenced by the difference in the peak potentials for anodic stripping. They also play a different role in the bulk deposition of Ag. Thus the first layer forming the surface alloy is totally unaffected by the deposition of further Ag on top of it. The second, more loosely bound, layer is the precursor for bulk deposition. This layer is not involved in the formation of the surface alloy.