Ligand (adsorbate) substitution (displacement) and competitive-coordination (chemisorption) reactions between chloride, bromide and iodide anions have been studied at well-defined Pd(100) and Pd(111) single-crystal electrode surfaces in aqueous solutions. Experiments involved: (i) pretreatment of the Pd(hkl) surfaces with a full monolayer of one halide followed by exposure to a dilute aqueous solution of another halide, and (ii) exposure of a clean Pd(hkl) electrode to a solution that contained a binary or ternary mixture of the halides. The resulting monolayers were then characterized by low-energy electron diffraction, Auger electron spectroscopy and temperature-programmed desorption. The results were as follows: (i) the subject halides were oxidatively chemisorbed to produce well-defined halogen adlattices. (ii) In the absence 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 were displaced, spontaneously, irreversibly and quantitatively, by bromide ions to yield Pd(100)-(2 x 2)-Br and Pd(111)-(root 3 x root 3)R30 degrees-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(100)-(2 x 2)-Br and Pd(111)-(root 3 x root 3)R30 degrees-Br were produced when the electrodes were exposed to a solution that contained a mixture 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, consistent with the thermal desorption data, demonstrate that the interaction of the subject halides with Pd electrode surfaces closely follows the homogeneous coordination chemistry of halo-Pd complexes: the strength of chemisorption or surface coordination decreases in the order I- > Br- > Cl-.
