INFLUENCE OF COMPETING ADSORBATES ON THE UNDERPOTENTIAL DEPOSITION OF COPPER ON PT(111)

We present a survey of the voltammetry for the underpotential deposition (UPD) of copper from aqueous sulfuric acid solution on a flame annealed Pt(111) electrode pretreated with or in the presence of aqueous solutions of a variety of adsorbates: because of its sensitivity to the presence of competing adsorbates, variations in the Cu UPD voltammetric features are used to infer substrate/adsorbate interactions. The adsorbates studied included both ionic and molecular species and were: sulfate, cyanide, thiocyanate and cis-1,2-dicyanoethylene-1,2-dithiolate anions; dimethylformamide (DMF), acetonitrile, dimethylsulfoxide (DMSO), pyridine (from 0.1 M H2SO4 and PDW (pyrolytically distilled water)); formic acid, acetic acid, hydroquinone (HQ); pyrocatechol, 2,2'-bipyridyl (0.1 M H2SO4 and PDW), 4,7-dihydroxy-1,10-phenanthroline, and 1,3-propanedithiol. Anionic adsorbates caused a diminution in the underpotential shift (relative to that in sulfuric acid alone), dependent on the strength of adsorption of the anion. Neutral adsorbates gave a variety of responses in terms of underpotential shifts and generally produced changes in the voltammetric peak shapes. Sulfur containing ligands gave rise to the most dramatic effects, generally causing a large reduction in the underpotential shift and in some cases (e.g. propanedithiol) complete inhibition of the UPD process. DMSO, acetonitrile, and DMF did not appreciably affect the potential of UPD but caused substantial kinetic effects in terms of variations in peak heights and widths. For adsorbates that could undergo protonation (e.g. pyridine, bipyridine and dihydroxy phenanthroline) the observed behavior was strongly dependent on pH, reflecting the fact that protonation of the nitrogen can lead to profound alteration in the mode of binding. Deposition in the presence of formic or acetic acid suggests the presence of discrete coexisting domains.