ELECTRODE-SURFACE COORDINATION CHEMISTRY - LIGAND SUBSTITUTION AND COMPETITIVE COORDINATION OF HALIDES AT WELL-DEFINED PD(100) AND PD(111) SINGLE-CRYSTALS

Ligand (adsorbate) substitution (displacement) and competitive-coordin ation (chemisorption) reactions between chloride, bromide and iodide a nions have been studied at well-defined Pd(100) and Pd(111) single-cry stal electrode surfaces in aqueous solutions. Experiments involved: (i ) pretreatment of the Pd(hkl) surfaces with a full monolayer of one ha lide followed by exposure to a dilute aqueous solution of another hali de, and (ii) exposure of a clean Pd(hkl) electrode to a solution that contained a binary or ternary mixture of the halides. The resulting mo nolayers were then characterized by low-energy electron diffraction, A uger electron spectroscopy and temperature-programmed desorption. The results were as follows: (i) the subject halides were oxidatively chem isorbed to produce well-defined halogen adlattices. (ii) In the absenc e of the heavier halides, chloride ions were adsorbed to form Pd(100)- (2 x 2)-Cl and Pd(111)-(root 3 x root 3)R30 degrees-Cl. (iii) These we re displaced, spontaneously, irreversibly and quantitatively, by bromi de ions to yield Pd(100)-(2 x 2)-Br and Pd(111)-(root 3 x root 3)R30 d egrees-Br, respectively. (iv) The latter, in turn, were spontaneously, irreversibly and quantitatively displaced by iodide to produce Pd(100 )-c(2 x 2)-I and Pd(111)-(root 3 x root 3)R30 degrees-I. (v) Only Pd(1 00)-(2 x 2)-Br and Pd(111)-(root 3 x root 3)R30 degrees-Br were produc ed when the electrodes were exposed to a solution that contained a mix ture of Cl- and Br-. (vi) Only Pd(100)-c(2 x 2)-I and Pd(111)-(root 3 x root 3)R30 degrees-I were produced when the electrodes were immersed in a solution that consisted of all three halides. These results, con sistent with the thermal desorption data, demonstrate that the interac tion of the subject halides with Pd electrode surfaces closely follows the homogeneous coordination chemistry of halo-Pd complexes: the stre ngth of chemisorption or surface coordination decreases in the order I - > Br- > Cl-.