Partial charge and electrosorption valency

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. 

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. 

The electrosorption valency expresses the charge flow during the electrosorption process at a constant potential. It takes values ranging between zero and the ionic charge z. Values very close to z show that the adsorbed species are almost completely discharged, while values of i lower thanz indicate adsorption with partial charge transfer. The adsorbed species are recognized as ad-atoms when... 

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 ... 

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... 

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. 

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 ... 

In contrast to this thermodynamic interpretation of /, an attempt was made by Vetter and Schultze [3.226, 3.227] to obtain information on the location and the partial charge of (IP) in the compact double layer from the electrosorption valency. 

A similar criticism refers to the concept of electrosorption valency introduced by Vetter and Schultze starting from the partial charge concept [34, 35]. 

The concept of pseudocapacitance had arisen early in electrochemistry, being associated with potential-dependence of chemisorption of H atoms, e.g. at Pt (Conway and Gileadi [1962]), and of anions such as the halide ions which are adsorbed at a variety of electrode metals, with partial charge transfer (electrosorption valency), inclnding Hg, (Schnltze and Koppitz [1976]). 

In some electrode reactions it may be appropriate to consider that partial charge transfer occurs. In the case of the underpotential deposition of metals, the electrode process leads to the formation of a monolayer or sub-monolayer of metal atoms which interact with the substrate. In the one extreme, the metal-metal bond may have considerable ionic character, so that the reaction approaches ion adsorption, whereas at the other limit an essentially electroneutral surface atom is formed. These complex electrode processes have been discussed extensively in recent years, and the concept of the electrosorption valency [30] has emerged as a way of describing the nature of the under-potential deposit. 

The same approach may also apply to the adsorption of redox particles other than the adsorption of proton-hydrogen atom on metal electrodes. To understand electrosorption phenomena, various concepts have been proposed such as the charge transfer coefficient and the adsorption valence [Vetter-Schultze, 1972]. The concept of the redox electron level in adsorbed particles introduced in this textbook is usefiil in dealing with the adsorption of partially ionized particles at electrodes. 

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