Bromide adlayer adsorption

In a preliminary report of the results for bromide adsorption onto Pt(lll), it was shown that an incommensurate (3 X 3) hexagonal bromide adlayer is present on the Pt(lll) surface in the potential range 0.05-0.7 V [60]. From fits to the scattering profile at the lowest-order diffraction peak, the peak position... 

The isotherm for bromide adsorption (Fig. 14c) shows that increasing the potential in the double-layer region causes a continuous increase in the bromide coverage. The data shown in Fig. 15 and in other experiments, however, are not consistent with the notion that the additionally adsorbed bromide leads to a continuous compression of the incommensurate bromide adlayer, as this would cause the sharp peak at/ = 0.67 to shift gradually to higher wave vector. An alternative explanation of our results is that bromide forms a series of high-order commensurate (HOC) structures on the Pt(lll) surface, that is, at all potentials the structure corresponds to a close-packed monolayer but that the periodicity depends critically on the ratio of the bromide and Pt lattice parameters. At 0.2 V, the unit cell corresponds to a (3 x 3) structure with a basis of four Br atoms. At 0.6 V, the unit cell is a (7 x 7) structure containing 25 Br atoms, that is, with a 7 5 ratio of the Br and Pt lattice parameters. 

Magnussen et al. studied the adsorption of bromide adlayers on Au lll electrodes in 0.1 mol dm HCIO4 + 0.001 to 0.1 mol dm NaBr. In this case, Br ions were selected as a model system, in contrast to the more usually studied I , as Br ions are less strongly adsorbed but are still classed as specifically adsorbed ions [19]. At potentials more positive than a critical potential, which would depend on the NaBr concentration, it was found that Br ions would form a hexagonal adlayer rotated relative to the 3 direction of the Au l 11 substrate, with the extent of the rotation and adlayer density being dependent on the potential and NaBr concentration. The Br -Br spacing within the layer was found to vary continuously, from 4.24 A at the critical potential to 4.03 A at a potential 0.3 V more positive. 

Bromide adsorption on Au(lll) has also been studied, applying in situ surface X-ray scattering (SXS) and STM [56]. The potential-dependent adlayer density agreed well with the earlier pubKshed bromide surface excess densities, obtained in electrochemical measurements. At very positive potentials, a bromide-induced step-flow etching of Au occurred. 

A physical model and a theory have been proposed [72], which might be helpful in comparative studies on electrocompres-sive behavior of electrodeposited chloride, bromide, and iodide monolayers on the Au(lll) electrode. The theoretical results were in good agreement with the experimental data, which evidence that the adatom-adatom interactions (especially repulsive ones) and electrosorption valency of halide anions determine the compressibility within halide adlayers. Also, Lipkowski et al. have discussed various aspects of adsorption of halide anions on Au(lll) in a review paper [36]. From this paper, we have taken quantitative data concerning adsorption of halide anions on Au(lll) (cf Fig. 3). 

The adsorption of anions such as iodide, bromide, cyanide, and sulfate/bisulfate on electrode surfaces is currently one of the most important subjects in electrochemistry. It is well known that various electrochemical surface processes, such as the underpotential deposition of hydrogen and metal ions, are strongly affected by coadsorbed anions [1, 2]. Particularly, structures of the iodine adlayers on Pt,... 

---