Characterization of single-crystal electrode surfaces as a function of potential and pH by Auger spectroscopy and LEED: Pt (111) in aqueous CaCl 2 and HCl solutions

Abstract

Experiments were performed in which a well-characterized Pt (111) surface was immersed into aqueous CaCl 2/HCl solutions at controlled potential, after which the surface was removed from solution, evacuated, and characterized by LEED, Auger spectroscopy and related techniques. Potential-dependent and pH-dependent adsorption of halogen was observed, predominantly as Cl atoms. Stable chemisorption of Cl occurred only when the pH was less than about 4 with the electrode potential more positive than about 0.5 V (vs. Ag/AgCl reference). An order adsorbed layer, Pt (111)(3×3)−Cl, containing θ Cl = 0.25, was formed at potentials at which Cl was the predominant adsorbed species. Chemisorbed oxides or hydroxides were formed at the more positive potentials. Retention of Ca 2+ by the surface was potential-dependent, with a minimum at about 0.3 V, and never exceeded θ Ca = 0.08 (Ca 2+ ions per surface atom). Water was retained by the surface following evacuation, to the extent of 10–20 water molecules per Ca 2+ ion, depending upon the pH.