Standard potential surface reactions

A comprehensive list of standard potentials is found in Ref. 7. Table 2-3 gives a few values for redox reactions. Since most metal ions react with OH ions to form solid corrosion products giving protective surface films, it is appropriate to represent the corrosion behavior of metals in aqueous solutions in terms of pH and Ufj. Figure 2-2 shows a Pourbaix diagram for the system Fe/HjO. The boundary lines correspond to the equilibria ... 

It was concluded from this and related works that suppression of the photodissolution of n-CdX anodes in aqueous systems by ions results primarily from specific adsorption of X at the electrode surface and concomitant shielding of the lattice ions from the solvent molecules, rather than from rapid annihilation of photogenerated holes. The prominent role of adsorbed species could be illustrated, by invoking thermodynamics, in the dramatic shift in CdX dissolution potentials for electrolytes containing sulfide ions. The standard potentials of the relevant reactions for CdS and CdSe, as well as of the sulfide oxidation, are compared as follows (vs. SCE) [68] ... 

This analogy to a surface redox mediated process is significant. In a way very similar to the reaction sequence (1.14), the standard potential of the redox surface system Pt(H20)/Pt-0Hads (0.80 V with respect to RHE) determines the active (reduced) site population at any cathode potential E, and consequently is the critical parameter in determining the ignition potential for the ORR process. 

When the B/C conversion is fast, C is produced close to the electrode surface and is likely to diffuse back and be oxidized there. The situation is similar to the ECE case in the ECE-DISP problem discussed in Section 2.2.5. In the ECE case, the cyclic voltammetric responses depend essentially on the dimensionless rate constant, 2 = (7ZT/F)(k/v), of the B/C reaction in the framework of two subcases according to the order in which the two standard potentials, Z yBand c, lie (note that in the D/C couple, D is the oxidized form). Typical cyclic voltammograms are shown in Figure 2.25a and b for the two subcases. 

Electrochemistry is in many aspects directly comparable to the concepts known in heterogeneous catalysis. In electrochemistry, the main driving force for the electrochemical reaction is the difference between the electrode potential and the standard potential (E — E°), also called the overpotential. Large overpotentials, however, reduce the efficiency of the electrochemical process. Electrode optimization, therefore, aims to maximize the rate constant k, which is determined by the catalytic properties of the electrode surface, to maximize the surface area A, and, by minimization of transport losses, to result in maximum concentration of the reactants. 

In general, when a metal is immersed in a solution of (i.e., contairung) its own ions, some surface atoms in the metal lattice do become hydrated and dissolve into the solution. At the same time, ions from the solution are deposited on the electrode. The rate of these two opposing processes is controlled by the potential differences at the metal-solution interface. The specific potentials at which these two reaction rates are equal, called standard potentials, are usually given in the literature for solutions at 25°C (room temperature) and at an activity value of unity. 

A bare surface of silicon can only exist in fluoride containing solutions. In reality, in these media, the electrode is considered to be passive due to the coverage by Si— terminal bonds. Nevertheless, the interface Si/HF electrolyte constitutes a basic example for the study of electrochemical processes at the Si electrode. In this system, the silicon must be considered both as a charge carrier reservoir in cathodic reactions, and as an electrochemical reactant under anodic polarization. Moreover, one must keep in mind that, according to the standard potential of the element, both anodic and cathodic charge transfers are involved simultaneously (corrosion process) in a wide range of potentials. 

More than at mercury, it makes a difference whether the electrode is inert or not. In the first case, the electrode reaction is of the type Fe3+/ Fe2+ etc. and the modelling of processes is the same as with mercury. However, if the electrode reaction is of the type Zn2+/Zn, e.g. at a gold electrode, at least the electrode surface will be modified by the deposited zinc, Frequently, it is observed that the first monolayer of the foreign metal is deposited at a potential substantially positive to its standard potential. This phenomenon is named underpotential deposition and bears some resemblance to an electrode reaction that involves adsorption of the reacting species (see Sect. 6). 

FIGURE 12.11 Although aluminum has a negative standard potential, signifying that it can be oxidized by hydrogen ions, nitric acid stops reacting with it as soon as an impenetrable layer of aluminum oxide has formed on its surface. This resistance to further reaction is termed passivation of the metal. 

As for the reoxidation of reduced heteropoly compounds in the solid state, few reliable studies have been reported. It was reported that the reoxidizability increases with an increase in standard electrode potentials of countercations (108). In the case of reoxidation by O2 of le -reduced CsxHj - PMo 12O40, the rates divided by the surface area show a monotonic variation (Fig. 53e) as in Figs. 53c and d, indicating a surface reaction. A similar variation was observed for the Na and K salts. The presence of water vapor sometimes accelerates the migration of oxide ion, probably in the form of OH- or H20, and makes surface-type reactions more like bulk type II reactions (266). 

It is natural that the possibility of ionic reaction can be reliably predicted from the standard potential series only when the activities of all components taking part in the reaction equal unity. At other activities the mutual relations of substances in the potential series can be changed. There is a second limitation, namely, no retardation of the reactions by various foreign phenomena (e. g. by overvoltage or mechanical pasivity of the surface, due to the existence of oxide films). 

The standard potential series can be used as only a rough guide with respect to the ability of a metal to resist corrosion. In most of the corrosion reactions, the potential values shown in the table are not applicable because of the presence of a film on the metal surface, and the change in potential because the activity of metal ions is less than unity. 

The measurement of ket for single electron-transfer reactions is of particular fundamental interest since it provides direct information on the energetics of the elementary electron-transfer step (Sect. 3.1). As for solution reactants, standard rate constants, k t, can be defined as those measured at the standard potential, E, for the adsorbed redox couple. The free energy of activation, AG, at E°a is equal to the intrinsic barrier, AG t, since no correction for work terms is required [contrast eqn. (7) for solution reactants] [3]. Similarly, activation parameters for surface-attached reactants are related directly to the enthalpic and entropic barriers for the elementary electron-transfer step [3],... 

In the field of chemical reactions, separating the system into fast and slow degrees of freedom is a standard method. It is based on the gap in characteristic time scales for motion of electrons and nuclei. This is the Bom-Oppenheimer approximation, and it is used for constructing potential surfaces for the motion of the nuclei. 

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