Electrosorption valency, and

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. 

Substituting B from Eq. (74) into Eq. (73), the following relationship between the electrosorption valency and the dipole moment is obtained ... 

IV. ELECTROSORPTION VALENCY AND PARTIAL CHARGE TRANSFER COEFFICIENT IN SELF-ASSEMBLED THIOL MONOLAYERS... 

Chapter 3, by Rolando Guidelli, deals with another aspect of major fundamental interest, the process of electrosorption at electrodes, a topic central to electrochemical surface science Electrosorption Valency and Partial Charge Transfer. Thermodynamic examination of electrochemical adsorption of anions and atomic species, e.g. as in underpotential deposition of H and metal adatoms at noble metals, enables details of the state of polarity of electrosorbed species at metal interfaces to be deduced. The bases and results of studies in this field are treated in depth in this chapter and important relations to surface -potential changes at metals, studied in the gas-phase under high-vacuum conditions, will be recognized. Results obtained in this field of research have significant relevance to behavior of species involved in electrocatalysis, e.g. in fuel-cells, as treated in chapter 4, and in electrodeposition of metals. 

The structural studies with singleelectrosorption valences and partial charge of metal adatoms at large adatom coverages. Close to the reversible deposition potential, the adatoms are neutral species otherwise, the electrocompression would not be observed. In low-coverage phases, electrosorption valences below Unity may be expected, with the charge on the adatoms usually compensated by the coadsorbed anions. 

J. W. Schultze and F. D. Koppitz [1976a] Bond Formation in Electrosorbates. I. Correlation Between the Electrosorption Valency and Pauling s Electronegativity for Aqueous Solutions, Electrochim. Acta 21, 327-336. 

In the electrosorption of ions, the charge of the ions can be neutralized by the electons in the metal electrode substrate. If this happens, then the normally repulsive effective interaction between equally charged ions can become attractive because of the formation of a metallic bond. The charge is known as the electrosorption valency, and has been studied extensively by Schultze and coworkers [118] and will be discussed in the next section. From the structure of Eq.(1.158) and Eq.(1.159) it is clear that no first order phase transition will occur if the adsorbed ions keep their charge and their repulsive interaction as the potential changes. The conclusion is that the addions, in this case Cu, attract each other in the adsorbed layer, and therefore are chemically different in the adlayer than in the bulk solution. 

J. W. Schultze, D. Rolle, The electrosorption valency and charge distribution in the double layer. The influence of surface structure on the adsorption of aromatic molecules, J. Electroanal. Chem., 2003,552, pp. 163-169. 

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 ... 

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). 

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).

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. 

Another useful thermodynamic relationship that allows the potential dependence of l to be determined from the differential capacity Co, of the interphase at constant rs is readily obtained from the very definition of l. Choosing Ez as the reference potential and denoting by lz the electrosorption valency at Ez, the l value at any other applied potential E is given by ... 

Lorenz refers to f as the macroscopic pet coefficient and regards it as experimentally accessible. C M can be measured by the extrapolated value of the differential capacity C at infinite frequency, when the surface concentration rs is frozen, i.e., when it cannot keep up with the ac signal. Since, however, the partial charge associated with chemisorption does follow the ac signal, this is only true if A can be regarded as potential independent. It can be readily seen that l in Eq. (31) is the opposite of the electrosorption valency l. In fact, replacing from Eq. (9) into Eq. (2) yields ... 

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