Anion effects in the UPD of copper on Pd/Pt(111) bimetallic electrodes

The underpotential deposition (UPD) of copper has been used to characterize the structure of palladium films supported on Pt(111). Depending upon the nature of the anions in solution (HSO4-, Cl-, Br-), a variety of adsorption behaviors are revealed. In pure sulfuric acid, palladium islands give rise to copper stripping peaks some 100-150 mV negative of the copper desorption states associated with Pt(111). As already deduced from hydrogen electrosorption measurements, palladium islands form at coverages of palladium < 1 monolayer (mL), and long-range (111) order is preserved within regions of the surface free of palladium, even at coverages close to the completion of the first monolayer. The palladium films themselves are stable up to 0.6 V (Cu2+/Cu), but desorption is facilitated at 0.8 V (Cu2+/Cu), leaving behind a somewhat disordered Pt/Pd phase. In contrast, copper UPD in the presence of strongly adsorbing anions (Cl-, Br-) leads to a much narrower range of stability [0.3 V (Cu2+/Cu)] for the palladium film although, from inspection of copper UPD data, island growth of palladium for theta(Pd) < 1 mL is again implied with significant preservation of long-range Pt(111) order. It is reported that desorption of multilayer palladium films may be catalyzed by Cl- and Br- anions, resulting ultimately in a highly ordered palladium monolayer. Further oxidative desorption of the ordered palladium monolayer produces new copper adsorption states associated with place exchange between the halide and the palladium monolayer and also copper UPD onto the resulting Cu-Pd surface alloy. Repetitive potential cycles between 0.1 and 0.8 V (Cu2+/Cu) transform all palladium islands into a Cu-Pd surface alloy. Further oxidative desorption beyond this stage generates a disordered copper-rich Cu-Pd alloy phase and palladium-free regions of Pt(111). After complete desorption of palladium, the UPD voltammogram of Pt(111)-copper is fully restored with no evidence of surface perturbation having occurred. The relevance of these findings in the context of halide-catalyzed dissolution of palladium surfaces is discussed.