Halides, adsorption

Franke C, Piazza G and Kolb D M 1989 The influence of halide adsorption on the electronic surface states of silver electrodes Electrochim. Acta 34 67-73... 

Three possible effects of halide adsorption are envisaged ... 

Oxidative UPD involves the oxidation of species to form an atomic layer where the precursor contains the element in a negative oxidation state. A classic example is the formation of oxide layers on Pt and Au, where water is oxidized to form atomic layers of oxygen. Halide adsorption can be thought of similarly, where a species such as I oxidatively adsorbs on a metal surface as the halide atom. In that case, a bulk film is not formed at more positive potentials, but the diatomic is generated and diffuses into solution. With respect to compound formation, oxidative UPD from a sulfide solution is a good example ... 

The SHG surface spectroscopy is another method useful in the studies on halide adsorption. ApplicabiKty of the SHG spectroscopy method to Au surface properties has been discovered by Pettinger, Kolb, and coworkers [74, 75]. It has also been shown that reconstruction of Au(lll) and Au(lOO) electrodes remarkably affects SHG anisotropy [74]. For this reason, an interference second-harmonic generation anisotropy (ISHGA)... 

When Nakato et al. incorporated adsorption and desorption of halides into their mathematical description, the new type of oscillations could be reproduced in simulations [57], Furthermore, the calculations revealed that oscillations D required, besides the poisoning of the electrode by halide adsorption, the transient current due to the upd-H formation. Hence, the oscillations necessitate two N-NDR hiding factors the adsorption of a poison and an additional current (stemming from a transient process and not a independent current carrier as in Strasser s type HN-NDR subcategory 3). This led the authors to introduce a new HN-NDR subcategory 4. 

Strong adsorption of Cl, Br, or I occurred when Ag(lll) was immersed in aqueous KC1, KBr, or KI solutions, respectively, at open circuit and throughout most of the useful range of electrode potentials. As for Pt surfaces, halide adsorption on Ag(lll) was a redox process. 

Since the partial derivative (<3Ejfdas)Cs for halide adsorption on the... 

Potential dependent halide adsorption, including order-disorder phenomena, is well known [134, 246, 247], and it is reasonable to expect that the breakdown of the PEG-C1 blocking layer might also be potential dependent. In a similar fashion, the composition and structure of thiols and disulfides adsorbed on gold from simple electrolytes have been shown to be potential dependent [282]. In the present example, it is also possible that the approach of SPS to the electrode surfaces is screened by complexation with the potential dependent concentration of Cu+ that is generated at the electrode. Importantly, the equilibrium Cu+ concentration in the additive-free system (i.e. Equations 2.1 and 2.2) is of the order of 400 (tmol L 1 and Cu+ is known to form complexes with all the additives under consideration [239, 279, 280, 283-285]. Furthermore, the equilibrium Cu+ concentration decreases exponentially with potential, that is, 60 mV per decade of concentration [283-285]. Thus, the increasing rate of SPS adsorption with... 

Preparation As compared to single-crystal Ag surfaces, the preparation of pc-Ag electrode may seem to be a relatively simple task. However, a pc-Ag surface, which ensures reproducibility and stability, also requires a special procedure. Ardizzone et al. [2] have described a method for the preparation of highly controlled pc-Ag electrode surface (characterized by electrochemical techniques and scanning electron microscopy (SEM)). Such electrodes, oriented toward electrocatalytic properties, were successfully tested in halide adsorption experiments, using parallelly, single-crystal and conventional pc-Ag rods as references. 

Hx F/RT, respectively, with tim and Hx the number of electrons transferred during metal and halide adsorption, respectively. The respective (potential-independent) rate constants are denoted by km and kx ... 

Figure 17.1.11 Reflectance changes caused by halide adsorption on gold in 0.2 M HCIO4. [From T. Takamura, K. Takamura, and E. Yeager, Symp. Faraday Soc., 4, 91 (1970), with permission.]...

It is important at this point to recall the difference between atom and ion adsorption. Figure 12 shows that chlorine and iodine adsorption become stronger towards negative fields, whereas it is known that halide adsorption on metal electrodes becomes more favorable towards more positive electrode potentials, and hence fields. This apparent contradiction is readily resolved by noting the different reference states involved (i. e., atom and ion), leading to different E, -F slopes as expressed in Eq. (11). 

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