Adsorption electrical potential-dependent

Hence, in Eq. (36), which sign, positive or negative, should be chosen depends on the adsorption state of ionic species in the Helmholtz layer if any kind of specific adsorption is neglected or such adsorption is not so intense, the positive sign can be adopted because there is no inversion of the signs of the electric potentials, as depicted in Fig. 23. This means that the sign of the potential difference in the Helmholtz layer is the same as that of the potential difference in the diffuse layer, i.e.,... 

It is noted that the molecular interaction parameter described by Eq. 52 of the improved model is a function of the surfactant concentration. Surprisingly, the dependence is rather significant (Eig. 9) and has been neglected in the conventional theories that use as a fitting parameter independent of the surfactant concentration. Obviously, the resultant force acting in the inner Helmholtz plane of the double layer is attractive and strongly influences the adsorption of the surfactants and binding of the counterions. Note that surface potential f s is the contribution due to the adsorption only, while the experimentally measured surface potential also includes the surface potential of the solvent (water). The effect of the electrical potential of the solvent on adsorption is included in the adsorption constants Ki and K2. 

The third exponential term in eqn. (187) is identical to the exponential term in the Butler—Volmer equation, eqn. (80), in the absence of specific adsorption. The first two exponential factors in eqn. (187) corresponding to the variation in the electrical part of the free energy of adsorption of R and O with and without specific adsorption A(AGr) and A(AG0), respectively. The explicit form of as, the activity of the adsorption site, and potential dependence of as, A(AGr) and A(AG0) is necessary for a complete description of the electrode kinetics. 

One effect of special interest to electrochemists is the potential-dependent chemisorption of ions and molecules on electrode surfaces. A particularly well-studied example is the adsorption of CO on platinum single-crystal electrodes. In collaboration with the Weaver group at Purdue, we have recently undertaken detailed DFT calculations of the potential-dependent chemisorption of CO on platinum-group (111) surfaces [47,54,55], modeled as clusters, for comparison with the extensive vibrational characterization of these systems as carried out by the Purdue group [56,57]. The electrode potential in these studies is modeled as a variable external electric held applied across the cluster, an approach many others have taken in the past. 

The electrode surface participates actively in an electrochemical reaction sequence by providing adsorption sites for at least one reactant and for the reaction intermediates. Thus, the reaction rate and selectivity depend strongly upon the surface properties and its mode of interaction with reactive species and electrolytes. The existence, however, of the structured double layer interface and of the electric field under which electrosorption takes place distinguishes the latter from gas phase adsorption. Electrolyte ions, solvent molecules, and impurities may adsorb and compete with reactants for surface sites or they may poison the surface or contribute to surface changes under reaction. Despite the wealth of experimental information on the potential dependence of surface coverage and on the nature of some adsorbing species, a fundamental understanding of electrosorption mechanisms is still incomplete. 

The adsorption of either ions or neutral molecules on the electrode surface depends on qn, i.e., on the apphed electric potential. Correspondingly, the electric field at the electrochemical interface is an additional free-energy contribution that either favors or restricts the adsorption of species on the electrode from the ionic conducting phase. A variety of adsorption isotherms has been proposed to account for the behavior of different electrochemical systems. Among them are the Langmuir, Frumkin, and Temkin isotherms [2]. Frumkin and Temkin isotherms, at variance with the Langmuir one, include effects such as adsorbate—adsorbate or adsorbate—surface interactions. 

Specific adsorption of anions can give rise to unconventional temperature dependence of Tafel slopes on account of the temperature dependence of the ion adsorption and consequent changes of the structure and electric potential profile across the double layer where the transition state is established. 

Infrared methods have been used to study adsorbed species (reactants, intermediates, products), to examine species produced in the thin layer of solution between electrode and window, and to probe the electrical double layer. These approaches have been especially useful with species that have a high infrared absorption coefficients, like CO and CN . In favorable cases, information about the orientation of an adsorbed molecule and the potential dependence of adsorption can be obtained. For example, the SNIFTIRS spectrum obtained with a 0.5 mM aqueous solution of p-difluorobenzene in 0.1 M HCIO4 at a Pt electrode is shown in Figure 17.2.6 (62). The spectrum results from both dissolved (positive AR/R-values) and surface adsorbed (negative AR/R-values) p-difluorobenzene. 

PDEIS is a new technique based on fast measurements of the interfacial impedance with the virtual instruments [3] that benefits from the efficient synchronization of direct hardware control and data processing in the real-time data acquisition and control [4], The built-in EEC fitting engine of the virtual spectrometer divided the total electrochemical response into its constituents those result from different processes. Thus, just in the electrochemical experiment, we come from the mountains of raw data to the characteristics of the constituent processes - the potential dependencies of the electric double layer capacitance, charge transfer resistance, impedance of diffusion, adsorption, etc. The power of this approach results from different frequency and potential dependencies of the constituent responses. Because of the uniqueness of each UPD system and complex electrochemical response dependence on the frequency and electrode potential, the transition from the PDEIS spectrum (Nyquist or Bode plot expanded to the 3D plot... 

After the brief introduction to the modem methods of ab initio quantum chemistry, we will discuss specific applications. First of all, we will discuss some general aspects of the adsorption of atoms and molecules on electrochemical surfaces, including a discussion of the two different types of geometrical models that may be used to study surfaces, i. e. clusters and slabs, and how to model the effect of the electrode potential in an ab initio calculation. As a first application, the adsorption of halogens and halides on metal surfaces, a problem very central to interfacial electrochemistry, will be dealt with, followed by a section on the ab initio quantum chemical description of the adsorption of a paradigmatic probe molecule in both interfacial electrochemistry and surface science, namely carbon monoxide. Next we will discuss in detail an issue uniquely specific to electrochemistry, namely the effect of the electric field, i. e. the variable electrode potential, on the adsorption energy and vibrational properties of chemisorbed atoms and molecules. The potential-dependent adsorption of carbon monoxide will be discussed in a separate section, as this is a much studied system both in experimental electrochemistry and ab initio quantum electrochemistry. The interaction of water and water dissociation products with metal surfaces will be the next topic of interest. Finally, as a last... 

The definition of Gibbs elasticity given by Eq. (19) corresponds to an instantaneous (Aft t ) dilatation of the adsorption layer (that contributes to o ) without affecting the diffuse layer and o. The dependence of o on Ty for nonionic surfactants is the same as the dependence of o on Ty for ionic surfactants, cf Eqs (7) and (19). Equations (8) and (20) then show that the expressions for Eq in Table 2 are valid for both nonionic and ionic siufactants. The effect of the surface electric potential on the Gibbs elasticity Eq of an ionic adsorption monolayer is implicit, through the equilibrium siufactant adsorption T y which depends on the electric properties of the interface. To illustrate this let us consider the case of Langmuir adsorption isotherm for an ionic surfactant (17) ... 

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