Electrosorption valencies

There is a formal similarity between adsorption and reactions such as metal deposition which gives rise to the concept of electrosorption valence. Consider the deposition of a metal ion of charge number on an electrode of the same material. If the electrode potential 4> is kept constant, the current density j is ... 

The definition of the electrosorption valence involves the total surface excess, not only the amount that is specifically adsorbed. It is common to correct the surface excess F, for any amount that may be in the diffuse double layer in order to obtain the amount that is specifically adsorbed. This can be done by calculating the excess in the... 

Usually the electrosorption valence is denoted by 7, which we use for the surface tension. The symbol l was used earlier by Lorenz and Salie [2]. 

The interpretation of the electrosorption valence is difficult. The following, somewhat naive argument shows that it involves both the distribution of the potential and the amount of charge transferred during the adsorption process. Suppose that an ion Sz is adsorbed and takes up A electrons in the process. A need not be an integer since there can be partial charge transfer (cf. Chapter 4). We can then write the adsorption reaction formally as ... 

Table 18.1 Electrosorption valences of a few simple ions at the pzc and at low coverage.

The electrosorption valence can be related to the dipole moment of an adsorbed species introduced in Chapter 4. For this purpose consider an electrode surface that is initially at the pzc and free of adsorbate. When a small excess charge density o is placed on the metal, its potential changes by an amount A given by ... 

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). 

Cd deposited underpotentially on Au(lll) reaches a limiting coverage of 0.66 monolayer. Electrosorption valency corresponding to this coverage is equal to 0.5, which indicates that Cd adatoms are not fully discharged at the surface [427]. 

Thallium UPD on Au(lll) has been studied, applying potential-step chronocoulometry and quartz crystal microbalance [482]. The UPD surface coverage increased with the increasing cathodic potential. At low coverage, the sublayer was not completely discharged, as it appeared from electrosorption valency. 

Innocenti et al. have studied the kinetics [101] of two-dimensional phase transitions of sulfide and halide ions, as well as electrosorption valency [102] of these ions adsorbed on Ag(lll). The electrode potential was stepped up from the value negative enough to exclude anionic adsorption to the potential range providing stability of either the first or the second, more compressed, ordered overlayer of the anions. The kinetic behavior was interpreted in terms of a model that accounts for diffusion-controlled random adsorption of the anions, followed by the progressive polynucleation and growth. 

Another parameter, the electrosorption valence. Y introduced by Vetter and Schulze (1973), is also widely used in electrochemistry. It is a macroscopic measure defined as y——(1/ (dq /dT). For most conditions, X = —y. 

B. E. Conway and H. Angerstein-Kozlowska, Interaction Effects in Electrodeposited Monolayers and the Role of the Electrosorption Valency Factor, J- Electroanal. Chem. 113 63 (1980). 

Partial charge transfer during adsorption is difficult to evaluate because separation of the charge transferred to the electrode and that part of the charge transferred across the double layer to give specifically adsorbed ions cannot be done through measurements of the total charge in the external circuit. Vetter and Schultze [102] defined the electrosorption valence as... 

The electrosorption valency usually increases as the underpotential decreases to approach the ionic charge (total discharge of the cation) close to the Nernst potential, for instance in the case of lead and thallium upd on silver [114]. However, the co-adsorption of anions may contribute to the observed apparent electrosorption valence, as rotating ring disc electrode (RDE) experiments have shown [113]. 

Fig. 12.20. Electrosorption of Br and I" on polycrystalline gold. Plot of charge passed vs. coverage determined by EQCM. Slope is the electrosorption valency (from Ref. 60 with permission).

Foresti, M.L., Innocenti, M., Forni, F. and Guidelli, R. (1998) Electrosorption valency and partial charge transfer in halide and sulfide adsorption on Ag(lll). Langmuir 14, 7008-7016. 

In this review, we will consider the adsorption of a single species coadsorption phenomena will not be considered, since it is generally impossible to divide the flow of charge among several species. We will present the thermodynamics on which the concept of the electrosorption valency is based, discuss methods by which it can be measured, and explain its relation to the dipole moment and to partial charge transfer. The latter can be explained within an extension of the Anderson-Newns model for adsorption, which is useful for a semi-quantitative treatment of electrochemical adsorption. Our review of concepts and methods will be concluded by a survey of experimental data on thiol monolayers, which nowadays are adsorbates of particular interest. 

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