Adsorbed halides

Conversely, an atom in Fig. 6.23 with an affinity level that initially is empty becomes partly occupied upon adsorption. Hence, charge is transferred from the metal to the atom. This sets up a dipole that increases the surface contribution to the work function. This is the case for adsorbed halides, which will be negatively charged at the surface. We will later see that such dipole fields can explain promotion and inhibition effects caused by various adsorbates in catalysis. 

It was surprising that SERS spectra of adsorbed water were observed in the presence of halide ions but not weakly adsorbing anions, like F or CIOJ. Hence, these spectra could be ascribed to water molecules coadsorbed with specifically adsorbed halide ions. ... 

The following subheadings in Section IV relate to the adsorbed species themselves rather than, as in the rest of Parts I and II, to the adsorbed molecules from which the spectra are derived. The structure of the section is different here because the surface hydrocarbon groups of interest are often derived from a number of different adsorbed halides or nitrogen-containing adsorbates. 

Bridge mediation mechanisms in heterogeneous outer sphere electrochemical reactions has also been theoretically treated using the pull—push and push-pull mechanistic concepts [84]. Schmidt [85] has considered theoretically homogeneous inner sphere bridge electron transfer reactions without atom or ion transfer. Bridge mediation in electron transfer reactions may also involve simultaneous atom or ion transfer. Heyrovsky [86] invoked mediation of electron transfer by formation of bridges to explain the enhancement of the rate of electroreduction of indium (III) ions in the presence of specifically adsorbed halide ions on mercury. 

In this equation, M represents a surface metal cation and the sum of m and n fulfills the coordination of M. Such a change in surface conditions is reflected in the pH dependence of the flat-band potential of ZnO electrodes (19J. Judging from the point of zero charge (pzc) of ZnO, which is at about pH=8.7 (20), the surface of the electrode must be charged positively in the soltuion chosen in the experiments. Then, there may arise specific adsorption of halide ions onto the cationic sites, and a mechanism is postulated that the observed pH effects of (X ) is due to contribution from the specifically adsorbed halide ions. Measurements of the flat-band potential of ZnO electrodes as a function of the concentration of iodide ions, however, gave no indication of the specific adsorption. Then, this model is ruled out. 

Mechanistically, the transformation of the N-NDR into an HN-NDR can be explained by the fact that adsorbed halides inhibit the dissociative adsorption of H2O2. The decrease in the reaction current due to the loss of PtOH or the formation of upd-H upon a negative voltage shift is overcompensated by the increase in current density due to the desorption of halide ions. Sustained periodic oscillations appear under potentiostatic as well as galvanostatic conditions in the presence of halides [57] (Fig. 23). The oscillations that are associated with the NDR in the upd-H region were termed oscillations D, those connected to the autocatalytic adsorption of H2O2 oscillations C. 

Mussini, P.R., Ardizzone, S., Cappelletti, G., Longhi, M., Rondinini, S. and Doubova, L.M. (2003) Surface screening effects by specifically adsorbed halide anions in the electrocatalytic reduction of a model organic halide at mono- and polycrystalline silver in acetonitrile. J. Electroanal. Chem. 552, 213-221. 

Tribasic calcium phosphate, Ca3P208, may be made by washing precipitated calcium phosphate with ammonia, which dissolves any excess of phosphoric acid above that required to form this compound. It is a white earthy powder which retains water tenaciously, and also adsorbs halides, bicarbonates and hydroxides.1 It has also been prepared from CaH4(P04)a by dissolving this in a large excess of water... 

Here the oxidation involves an adsorbed halide Iron-surface metal atom complex. In both cases oxidation Is to ferrous ion In solution. The final concentration of FeOff and Fe" is strongly pH dependent. The two Ions will only be stable in acid or oxygen-free systems. In the presence of oxidants the Fe" will be oxidized to various insoluble ferric compounds such as FeaOi,. 

Alternatively, SFIRS can be done in the attenuated total reflection mode [341]. A gold film is evaporated onto a silicon hemicylinder. Vibrational modes of adsorbed halide ions have been detected [344],... 

In the first step of the reaction (Initiation) an electron is transferred from the metal to the adsorbed halide molecule thus generating a phenyl radical and a bromine anion ... 

Figure 9.31 Image of surface cluster. Two TEG molecules form complex with Cu" ion that is attached to copper surface via specifically adsorbed halide X .

K. Ogura and M.D. Salazar-Villalpando, Electrochemical reduction via adsorbed Halide anions, OM 63, 2011, 35-38. 

The addition of a supporting electrolyte, such as a potassium halide salt, has the added effect in SERS studies of introducing a species which can adsorb on the metal surface and give SERS bands. For example, at Ag electrodes, in addition to molecular SERS bands, one finds bands for adsorbed halides. 

The SERS of methyl viologen cations, and MV, and a wide variety of cationic metal complexes have also been observed (Table VIII). With the MV radical cation, both a SERS and a RR process can be observed from the surface Raman spectrum. " The MV and MV cations seem to interact with the Ag surface via adsorbed halides. On adding an electron, the SERS carbon-carbon stretching frequency shifts from 1292 cm for MV to 1352 cm" for MV for the carbons bridging the pyridine rings in the molecule. This upward frequency shift is indicative of increased electron density in the bridging carbon-carbon bond on electron transfer. 

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