Decomposition anodic dissolution reactions

Three anodic partial reactions are considered active dissolution of two metals M and M with different kinetics in the absence of their ions in bulk solution and decomposition of water with the evolution of oxygen. The kinetics of the latter process is so slow on most corroding metals that only at very negative potentials can oxygen present in the solution be electroreduced and this eventually becomes limited by mass transport due to the limited solubility of oxygen in water. At even more negative potentials, hydrogen evolution takes place on the electrode surface. The cathodic reduction of some metal ions present on the electrode surface as a consequence of corrosion is also considered in Fig. 13(b). 

Several conclusions may be drawn from the results discussed in this section. Firstly, it appears that in almost all cases studied, the stabilization reaction involves decomposition intermediates instead of free holes. We will not comment on this point here (for a discussion, see ref. [52]). Similarly, we will not enlarge on the observation that in certain cases, Xj and in other cases X2 intermediates are involved, as these problems are beyond the scope of the present paper, which essentially pertains to anodic dissolution and etching. As far as this subject is concerned, two important points emerge, i. e., the fact that, due to the interconnection between stabilization and dissolution, the latter reaction tends to dominate at sufficiently high current densities, and the fact that, depending on the semiconductor and on the circumstances, dissolution either occurs by the DH or by the DX mechanism. In what follows, independent information on the latter point will be gathered, and the factors which determine the dissolution mechanism will be investigated. 

In early attempts to oxidize hydrocarbons electrochemically, organic solvents and corrosion-resistant electrodes (PbO, C, Pt) were used to overcome low reactant solubility and anode dissolution at extreme potentials, -I-1.8 V and up to 4.5 V (326, 327). The primary anodic reaction was usually oxygen evolution or solvent decomposition. The electrode material, nonetheless, affected the product even at the small attainable yields. Thus, toluene oxidized to traces of aldehydes on PbO2 (333), while on Pt it yielded up to 19% benzaldehyde (326). The catalytic efifect of the anode, however, on rate and selectivity was not realized. 

Hoar Ci J found that in the corrosion inhibition of iron in hydrochloric acid by g-naphthoquinoline, the corrosion potential increases monotonically with increasing inhibitor concentration, while in the case of o-tolylthiourea one observes first a decrease of the corrosion potential followed by an increase at higher inhibitor concentrations. A similar predominant inhibition of the cathodic partial reaction at small inhibitor concentrations is exhibited also by phenylthiourea according to Kaesche. Furthermore, in the series of the thiourea derivatives one often finds corrosion acceleration at small concentrations, as for instance in the case of phenylthiourea at concentrations of 10- moles per liter. This appears to be due to a small cathodic decomposition of thiourea and its derivatives in the course of which hydrogen sulfide is formed. As is well known, hydrogen sulfide tends to accelerate corrosion, in particular the anodic partial reaction of dissolution of iron, which has been demonstrated independently by other authors (17). 

The synthesis of anhydrous metal halides is often a complex process. In thermal dehydration reactions complete removal of water is often complicated by decomposition of the compound. The reactions of metals with gaseous hydrogen halides or with halogens usually takes place at high temperatures and requires apparatus that may not be avaUable routinely. For example, CrBra can be obtained by the action of Br2 on chromium metal at a temperature of 750 °C.53 An alternative method has been proposed for the preparation of this compound that involves the anodic dissolution of chromium in the presence of bromine with a platinum cadiode immersed in benzene connected by a salt bridge to a chromium anode immersed in MeOH/Br2. The electrolytic cell is represented by Eq. 7.7 ... 

In principle, another anodic reaction can take place instead of semiconductor decomposition (dissolution), for example, oxidation of dissolved substance or oxygen evolution from water. Apparently, in the latter case, the illumination of semiconductor leads to photoelectrolysis of water with the formation of hydrogen and oxygen, that is, conversion of the energy of light into chemical energy of the photoelectrolysis products. 

The sustained dissolution or the surface transformation into passivating films of semicOTiductors in contact with electrolytes is limiting the lifetime of energy-converting devices and considerable efforts have been made to overcome this deficiency [65-69]. Rather early, criteria have been developed and recently been addressed again that allow to determine whether a semiconductor is thermodynamically stable [70, 71]. The method relates the position of the quasi-Fermi levels at the surface (see Eqs. 17 and 18) with those of the anodic or cathodic decomposition energies. The calculation of the decomposition levels is based on the respective corrosion reaction and a few... 

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