Electrosorption

Although there is an immense amount of literature concerned with adsorption phenomena at electrodes, few practical applications of electrodes as sorption surfaces are reported for the recovery of chemicals, as occurs in the classical adsorption process. Electrosorption, however, could be an alternative means of separation of small quantities of organics and other species from effluent streams. High surface-area adsorbent electrodes would clearly be required. The technique has been demonstrated in the adsorption of P-naphthol onto a packed bed of glassy carbon spheres [95], and cyclic electrosorption was experimentally demonstrated. 

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A question of practical interest is the amount of electrolyte adsorbed into nanostructures and how this depends on various surface and solution parameters. The equilibrium concentration of ions inside porous structures will affect the applications, such as ion exchange resins and membranes, containment of nuclear wastes [67], and battery materials [68]. Experimental studies of electrosorption studies on a single planar electrode were reported [69]. Studies on porous structures are difficult, since most structures are ill defined with a wide distribution of pore sizes and surface charges. Only rough estimates of the average number of fixed charges and pore sizes were reported [70-73]. Molecular simulations of nonelectrolyte adsorption into nanopores were widely reported [58]. The confinement effect can lead to abnormalities of lowered critical points and compressed two-phase envelope [74]. 

In Section 24.3, use of electrosorption for effluent purification was mentioned. The same principle of an electrochemically controlled hemosorption (sorptive blood purification) is used in modem toxicology to extract toxins from blood. By appropriate potential control of the carbon sorbent, particular toxins can be removed selectively without traumatizing the blood, that is, without removing essential blood components such as the thrombocytes. 

Claviher J, Orts JM, Gomez R, Pehn JM, Aldaz A. 1996. Comparison of electrosorption at activated polycrystaUine and Pt(531) kinked platinum electrodes. Surface voltammetry and charge displacement on potentiostatic CO adsorption. J Electroanal Chem 404 281-289. 

Beden B, Lamy C, Bewick A, Kunimatsu K. 1981. Electrosorption of methanol on a platinum electrode. IR spectroscopic evidence for adsorbed carbonyl species. J Electroanal Chem 121 343-347. 

Beden B, Bewick A, Lamy C. 1983. A study by electrochemically modulated infrared reflectance spectroscopy of the electrosorption of formic acid at a platinum electrode. J Electroanal Chem 148 147-160. 

Brankovic SR, Wang JX, Zhu Y, Sabatini R, McBreen J, Adzic RR. 2002a. Electrosorption and catalytic properties of bare and Pt modified single crystal and nanostnictured Ru surfaces. J Electroanal Chem 524/525 231-241. 

Wang WB, Zei MS, Ertl G. 2001. Electrosorption and electrooxidation of CO on Ru(OOOl). Phys Chem Chem Phys 3 3307. 

Papoutsis A, Leger JM, Lamy C. 1987. New results for the electrosorption of methanol on polycrystalline platinum in acid medium obtained by programmed potential voltammetry. J Electroanal Chem 234 315-327. 

For example, the investigations of the current-generating mechanism for the polyaniline (PANI) electrode have shown that at least within the main range of potential AEn the "capacitor" model of ion electrosorption/ desorption in well conducting emeraldine salt phase is more preferable. Nevertheless, the possibilities of redox processes at the limits and beyond this range of potentials AEn should be taken into account. At the same time, these processes can lead to the fast formation of thin insulation passive layers of new poorly conducting phases (leucoemeraldine salt, leucoemeraldine base, etc.) near the current collector (Figure 7). The formation of such phases even in small amounts rapidly inhibits and discontinues the electrochemical process. 

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 ... 

Electrochemical Surface Characterization of Platinum Electrodes Using Elementary Electrosorption Processes at Basal and Stepped Surfaces... 

The same type of voltammogram has been obtained with a Pt (111) electrode after its ordered surface was subjected to argon ion bombardment, introducing structural defects like randomly distributed steps (14). The similar effects of oxygen electrosorption and ion bombardment show clearly that the former perturbs the surface order,... 

The processes classified in the third group are of primary importance in elucidating the significance of electric variables in electrosorption and in the double layer structure at solid electrodes. These processes encompass interactions of ionic components of supporting electrolytes with electrode surfaces and adsorption of some organic molecules such as saturated carboxylic acids and their derivatives (except for formic acid). The species that are concerned here are weakly adsorbed on platinum and rhodium electrodes and their heat of adsorption is well below 20 kcal/mole (25). Due to the reversibility and significant mobility of such weakly adsorbed ions or molecules, the application of the i n situ methods for the surface concentration measurements is more appropriate than that of the vacuum... 

Hydrogen Electrosorption and Oxidation of Formic Acid on Platinum Single-Crystal Stepped Surfaces... 

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