Underpotential deposition on alloys

Results from underdeposition experiments on alloy surfaces offer the exciti ng possibility of providing information on site-specific chemical interacti ons between an adsorbate and one of the constituents in the alloy. Eventual ly, this information may be useful in developing a quantitative understandi ng of how site-specific chemical interactions influence such phenomena as e lectrocatalysis or dealloying. Herein, we report the first such set of resu lts. We examined the underpotential deposition of copper on polycrystalline AupAg(1-p) alloys. Copper is known to underpotentially deposit on elementa l Au electrodes, but not on Ag electrodes. We found that the charge associa ted with the adsorption process decreased with increasing Ag atomic fractio n 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 t he Cu2+/Cu metal/metal ion equilibrium electrode potential with increasing Ag atomic fraction in the alloy. A percolation cluster model was used to ex plain the underpotential deposition of copper in the system investigated. A ccording 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 covera ge as a function of alloy composition was derived and compared to the exper imental results. (C) 2001 The Electrochemical Society.