Underpotential deposition on alloys

Results from underdeposition experiments on alloy surfaces offer the exciting possibility of providing information on site-specific chemical interactions between an adsorbate and one of the constituents in the alloy. Eventually, this information may be useful in developing a quantitative understanding of how site-specific chemical interactions influence such phenomena as electrocatalysis or dealloying. Herein, we report the first such set of results. We examined the underpotential deposition of copper on polycrystalline AupAg(1-p) alloys. Copper is known to underpotentially deposit on elemental Au electrodes, but not on Ag electrodes. We found that the charge associated with the adsorption process decreased with increasing Ag atomic fraction in the alloy. No measurable amount of copper adsorption was detected for p < 0.28. The peak potential of the copper stripping waves shifted toward the Cu2+/Cu metal/metal ion equilibrium electrode potential with increasing Ag atomic fraction in the alloy. A percolation cluster model was used to explain the underpotential deposition of copper in the system investigated. According to the model each Ag atom blocks three adsorption sites in the Au-rich composition range. In Ag-rich alloys an adsorption site must be formed by at least three Au atoms. A theoretical dependence for the copper coverage as a function of alloy composition was derived and compared to the experimental results. (C) 2001 The Electrochemical Society.