ADSORPTION OF HALIDE ANIONS AT THE PT(111)-SOLUTION INTERFACE STUDIEDBY IN-SITU SURFACE X-RAY-SCATTERING

In this paper we present x-ray scattering results of iodide, bromide, and chloride adsorption onto the Pt(lll) surface in solution. Iodide f orms two commensurate adlayer structures, a (root 7x root 7)R19.1 degr ees phase and a hexagonal (3x3) phase, which coexist on the Pt surface . Formation of the (3x3) phase appears to be kinetically limited, wher eas the root 7 phase shows a hysteretic effect as a function of the el ectrode potential, associated with an order-disorder transition. Bromi de forms a series of high-order commensurate structures on Pt(lll) tha t are poorly ordered unless the size of the unit cell is small. No ord ered structures for chloride adsorption are observed and specular x-ra y reflectivity results suggest that the chloride coverage at low poten tial is too small to form a close-packed monolayer on the surface. The differences between the structures formed by the adsorbed anions, and the differences between results for Pt(111) and Au(111), are discusse d in terms of the strength of the metal-halide interaction.