Adsorption on electrodes

While methods employing radiaoactive tracer techniques have become a classical tool for the study of adsorption on electrodes, optical methods for the study of electrodes and processes occurring on them at an atomic or molecular level have undergone enormously rapid progress, which is characteristic for the contemporary development of electrochemistry. 

Electrode reactions are inner-sphere reactions because they involve adsorption on electrode surfaces. The electrode can act as an electron source (cathode) or an electron sink (anode). A complete electrochemical cell consists of two electrode reactions. Reactants are oxidized at the anode and reduced at the cathode. Each individual reaction is called a half cell reaction. The driving force for electron transfer across an electrochemical cell is the Gibbs free energy difference between the two half cell reactions. The Gibbs free energy difference is defined below in terms of electrode potential,... 

One general method may always be used to reduce the effect of impurity adsorption on electrodes, and that is to work only for shorl times. Impurities take substantial times to adsorb. If the time in which the measurement is made is short enough, the adsorption aspect of impurity interference with electrode kinetic measurements can be reduced. Many of the techniques for doing this are described in Chapter 8 (transients). However, this approach does not eliminate the difficulty that at low current densities impurities in the solution may compete with electrons from the electrode. Further, although transient measurements may greatly reduce the adsorption of impurities during the measurement, it is difficult to arrange techniques so that the electrode is in contact with the solution for seconds only. 

The simplest effect of pure electrostatic ionic adsorption on electrode reaction rates of ions is the Frumkin double layer effect already been discussed in Sect. 3.5. 

Fig. 2 illustrates the significance of T. The validity of certain equations that we shall develop in Sect. 2.2 requires that Tq and Tr remain constant. Certain experimental methods, both thermodynamic and kinetic, can be used to investigate the extent of adsorption on electrodes, but these are beyond the scope of the present discussion. 

Abnormal Infrared Effects in CO Adsorption on Electrodes of Nanometer-Scale Thin Eilm of Platinum... 

ABNORMAL INFRARED EFFECTS IN CO ADSORPTION ON ELECTRODES OF NANOMETER-SCALE THIN FILM OF PLATINUM... 

Figure 14.2.1 Examples of specific adsorption on electrodes, (a) Adsorption of (1) a disulfide or protein on Hg (2) an olefin on Pt (3) an organized Langmuir-Blodgett film on Au. (b) Adsorption of a metal ion or complex through an anionic ligand bridge. [From A. J. Bard, Integrated Chenucal Systems, Wiley, New York, 1994, with permission.]...

The macroscopic description of the adsorption on electrodes is characterised by the development of models based on classical thermodynamics and the electrostatic theory. Within the frames of these theories we can distinguish two approaches. The first approach, originated from Frumkin s work on the parallel condensers (PC) model,attempts to determine the dependence of upon the applied potential E based on the Gibbs adsorption equation. From the relationship = g( ), the surface tension y and the differential capacity C can be obtained as a function of E by simple mathematical transformations and they can be further compared with experimental data. The second approach denoted as STE (simple thermodynamic-electrostatic approach) has been developed in our laboratory, and it is based on the determination of analytical expressions for the chemical potentials of the constituents of the adsorbed layer. If these expressions are known, the equilibrium properties of the adsorbed layer are derived from the equilibrium equations among the chemical potentials. Note that the relationship = g( ), between and , is also needed for this approach to express the equilibrium properties in terms of either or E. Flere, this relationship is determined by means of the Gauss theorem of electrostatics. 

An approach similar to PC has been proposed by Mohihier et al. (MNM theory) Their basic idea was to treat the adsorbed layer as a two-component non-electrolyte solution called the surface solu-The field effect as well as any correlation to molecular or structural properties of the surface solution are missing from the original MNM theory. At this stage this theory differs a little from a curve fitting procedure. In subsequent papers the introduction of the field effect has been attempted following the TPC approach.Thus the MNM theory and its extensions do not offer a real alternative approach to the theoretical description of adsorption on electrodes. 

In any case there is still a long way until the development of a truly satisfactory molecular theory capable of predicting a priori and quantitatively the adsorption features of any solute. Until that occurs, the two trends mentioned above, together with computer simulations, will co-exist for different scopes The first trend for analyzing experimental data, and for applications to complicated adsorption phenomena as well as to interfacial phenomena affected by adsorption. The second trend along with computer simulations for a better understanding of the molecular nature of the adsorption on electrodes. 

Infrared spectroscopy is frequently applied to investigate CO adsorption on electrodes, because CO is important as an intermediate and surface poison in many electrocatalytic reactions and the C-O stretching vibrational modes of the adlayer are sensitive to the chemical environment at the metal/solution interface. Infrared spectra of CO adsorbed on low-index surface planes of Pt single-crystal electrodes have become a benchmark for use in understanding the behavior of CO on other surfaces. Related approaches have been extended to bulk single-crystal metal electrodes that include Pd [66, 67], Ir [68-71], Rh [13, 70], Ru [72-74], Ni [75, 76] and Au [77]. 

In addition to electrocatalysis and double-layer phenomena, SEIRAS has proved particularly useful in the study of organic adsorption on electrodes [99, 146-149]. This is an area where SEIRAS can have an especially important impact, as traditional in-situ infrared methods are often limited by the low absorption cross-sections of organic adsorbates. Along these same lines, SEIRAS is... 

The properties of surfactant molecules properties are (i) their ability to form different aggregate structures (micelles) above die critical micellar concentration (CMC), (ii) their ability to solubilize water-insoluble organic molecules (M) by hydrophobic-hydrophobic interactions, and (iii) their adsorption on electrodes changes the solution-metal interface, which alters redox reactions and produces template effects on the electrode surface (79) (Schem 2). SDS can be used to electropolymerize various thiophene derivatives such as EDOT, BT and MOT in aqueous solution. 

Before presenting the experimental data for adsorption of organic compounds, it will be instructive to examine the fundamental differences between electrosorption, i.e., adsorption on electrodes and adsorption from the gas phase. 

The major difference between gas phase adsorption and electrosorption is that in the former case adsorption occurs on a bare surface, while in the latter case the metal substrate is solvated, i.e., covered with an adsorbed layer of solvent molecules. Thus it is evident that adsorption on electrodes is a replacement reaction. The observed standard free energy, enthalpy and entropy of adsorption can therefore only be related to the type of interaction between the adsorbate and the electrode if the corresponding thermodynamic quantities for the solvent are known and the number of solvent molecules replaced by each adsorbed organic molecule can be estimated. 

The standard free energy of adsorption is thus the difference between the standard free energies of adsorption for the organic and for n water molecules. The same statement may be applied to the standard enthalpy and entropy of adsorption on electrodes. 

The book gives clear introductions to the theories of electron transfer and of diffusion in its early chapters. These are developed to interpret voltammetric experiments at macroelectrodes before considering microelectrode behaviour. A subsequent chapter introduces convection and describes hydrodynamic electrodes. Later chapters describe the voltammetric measurement of homogeneous kinetics, the study of adsorption on electrodes and the use of voltammetry for electroanalysis. 

From the initiative of the director of Polarographic Institute, professor A. A. Vlcek (1927-1999) (Fig. 3.1.9), a regular international electrochemical forum called Heyrovsky Discussion was fotmded in 1967, to which foreign scientists both from East and West were being invited. Every year a different topic was selected the first discussion in 1967 was about adsorption on electrodes and its effect on electrode processes, the next one in 1968 was about adsorption and processes on catalytic electrodes, and in 1969 it was about the mechanism of redox reaction proper (cf. Sect. 3.2). 

The electrode reactions considered so far in this chapter did not involve adsorbed reactants or products. However, it is unrealistic to expect that most actual processes are so simple. Therefore let us deal now with the effect of various types of adsorption on electrode processes. First let us classify adsorption phenomena. 

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