Oxide ion transfer

A second example is reaction (27a) which has recently been examined in detail by Feltham and Kriege (90a). The reaction is presumed to occur via an intramolecular oxide ion transfer through structure (17e). 

We may call these ion transfer processes the basic M2+ and O2 transfer processes, in contrast to the acidic M2+ and O2 transfer processes shown in Reactions 22.36 and 22.37. The basic oxide ion transfer, for instance, is the transfer of O2- from the metal oxide into the O2 -acceptor level of H20 molecules, while the acidic oxide ion transfer is the one into the 02 -acceptor level of H+ ions in acid solution. The basic oxide ion transfer in basic solution and the acidic oxide ion transfer in acidic solution obviously differ from each other, and so do their reaction kinetics. 

In comparison with the ionic solid corrosion discussed earlier, it is also worth noting that nickel ion transfer from the film into the solution is assumed to control the transpassive nickel dissolution whose rate increases with increasing interfacial potential. We may also see that the rate-determining process changes from the metal ion transfer to the oxide ion transfer near the oxygen evolution potential, beyond which the dissolution rate of the transpassive oxide film decreases with increasing interfacial potential, AH. 

Lux described reactions in oxide melts in terms of oxide ion transfer reactions (oxidotropism)2i. 22 i3ase is defined as an oxide ion donor and an acid as an oxide ion acceptor ... 

A moderate concentration of CaO promotes a better reaction rate due to enhanced oxide ion transfer across the electrolyte [6]. The CaO content in the electrolyte is a key parameter to the process. The proper CaO concentration suitable for reduction has been rarely investigated, despite the fact that calcium oxide addition to molten calcium chloride yields positive results. Namely, in the purely calciothermic reduction of Ti02 without an external electrochemical potential [14], 0.5 mol% CaO in the CaCl2 melt resulted in a better deoxidation. 

The protective quality of the passive film is detennined by the ion transfer tlirough the film as well as the stability of the film with respect to dissolution. The dissolution of passive oxide films can occur either chemically or electrochemically. The latter case takes place if an oxidized or reduced component of the passive film is more soluble in the electrolyte than the original component. An example of this is the oxidative dissolution of CrjO ... 

Oxidation is a process in which one or more electrons are transferred from the chemical being oxidized to the chemical initiating the transfer. The main purpose of treating wastes by oxidation is detoxification. Oxidation can also aid in the precipitation of ions in cases of oxidized ions that have a solubility lower than that of the original ions. 

The whole sequence of reactions represents a tour de force in the elegant manipulation of extremely reactive compounds. F3CIO2 is a violent oxidizing reagent but forms stable adducts by fluoride ion transfer to Lewis acids such as BF3, AsF5 and PtFe. The structures of F3CIO2 and [F2C102] have C2v symmetry as expected (Fig. 17.26e and i). 

The classical syntheses of phenanthrene and fluorenone fit well into the electron transfer scheme discussed in Section 8.6 and in this chapter. The aryl radical is formed by electron transfer from a Cu1 ion, iodide ion, pyridine, hypophosphorous acid, or by electrochemical transfer. The aryl radical attacks the neighboring phenyl ring, and the oxidized electron transfer reagent (e. g., Cu11) reduces the hexadienyl radical to the arenium ion, which is finally deprotonated by the solvent (Scheme 10-76). 

We recognize redox reactions by noting whether electrons have migrated from one species to another. The loss or gain of electrons is easy to identify for monatomic ions, because we can monitor the charges of the species. Thus, when Br ions are converted into bromine atoms (which go on to form Br2 molecules), we know that each Br ion must have lost an electron and hence that it has been oxidized. When 02 forms oxide ions, 02-, we know that each oxygen atom must have gained two electrons and therefore that it has been reduced. The difficulty arises when the transfer of electrons is accompanied by the transfer of atoms. For example, is chlorine gas, Cl2, oxidized or reduced when it is converted into hypochlorite ions, CIO" ... 

It appears that Cluster C catalyzes the chemistry of CO oxidation and transfers electrons to Cluster B, which donates electrons to external acceptors such as ferredoxin. Since a crystal structure of this protein does not exist, the proposed structure of Cluster C is based on spectroscopic measurements. In some cases, the EPR spectrum of a metal center is diagnostic of the type of center. However, the EPR spectra of Cluster C are unusual. The paramagnetic states of Cluster C (Credi and Cred2) have g-values that are atypical of standard [4Fe-4S] clusters (Table III) and are similar to those in a variety of structurally unrelated systems including a t-oxo bridged ion dimer), a [Fe4S4] ... 

In the case of Tl(III) the overall rate coefficient has been resolved into a product kK for the two steps The large positive AS is due almost entirely to the initial association, which was also studied spectroscopically. An alternative rate determining step in the Pd(II) oxidation is hydride ion transfer to Pd(II) . 

Thus an acid-base reaction involves the transfer of an oxide ion (compared with the transfer of a proton in the Bronsted theory) and the theory is particularly applicable in considering acid-base relationships in oxide, silicate and aluminosilicate glasses. However, we shall find that it is subsumed within the Lewis definition. 

The field of electrochemical ion transfer reactions (EITRs) is relatively recent compared with that of electron transfer reactions, and the application of molecular dynamics simulations to study this phenomenon dates from the 1990s. The simulations may shed light on various aspects of the EITR. One of the key questions on this problem is if EITR can be interpreted in the same grounds as those employed to understand electron transfer reactions (ETRs). Eor example, let us consider the electrochemical oxidation reaction of iodine ... 

MEMED has also been used to investigate the nature of coupled ion-transfer processes involved in spontaneous electron transfer at ITIES [80]. In this application, a key strength of MEMED is that all of the reactants and products involved in the reaction can be measured, as shown in Figs. 19 and 20. The redox reaction studied involved the oxidation of either ferrocene (Fc) or decamethylferrocene (DMFc) in a DCE phase (denoted by Fcdce) by either IrCle or Fe(CN)g in the aqueous phase (denoted by Ox ) ... 

Solid-oxide electrolytes are natural choices for oxygen transport since they transfer oxide ions directly ... 

At the O/S interface, for each molecule of alumina formed inside the oxide layer, i.e., three O2- ions transferred across the O/S interface, six hydrogen ions are formed. Thus, the acidity at the interface tends to rise to an extent which depends on the rate removal of these ions by some mechanism. In view of Eqs. (13) to (15), this should lead to oxide dissolution and a further decrease... 

Figure 6 indicates a change in the charge transfer mechanism at a pH between 9 and 9.5, corresponding to the pH of zero charge of aluminum oxide.33,34 Experimental results on the slopes enabled speculation on the values of transfer coefficients and reaction orders. From that, Valand and Heusler concluded that the most probable mechanism of oxygen ion transfer [reaction (21)] is... 

It is obvious that such an ion transfer must be preceded by some association of the aluminum ion from the oxide lattice with OH- ion (directly from the solution or adsorbed at the interface) [Eq. (22) or (23)] or by protonation with H+ ions from the solution (Eq. (25)]. Valand and Heusler maintain the first case to be operative. This conclusion must, however, be taken as tentative, and further arguments of an experimental nature are warranted. 

Depending on the fabrication techniques and deposition parameters, the pH sensitive slope of IrOx electrodes varies from near-Nemstian (about 59 mV/pH) to super-Nemstian (about 70mV/pH or higher). Since the compounds in the oxide layers are possibly mixed in stoichiometry and oxidation states, most reported iridium oxide reactions use x, y in the chemical formulas, such as lr203 xH20 and IrOx(OH)y. Such mixed oxidation states in IrOx compounds may induce more H+ ion transfer per electron, which has been attributed to causing super-Nerstian pH responses [41],... 

Because of its high negative charge density, the oxide ion is a very strong Bronsted base. Therefore, when an ionic oxide is placed in water, there is proton transfer to produce hydroxide ions. 

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