Cyclic oxides

Fig. 15. Compaiison of MoSi and othei intermetaUics and supeiaUoys (a), isothermal and (b), cyclic oxidation (1200°C) behavior (80).

Cyclic Oxidation In many industrial applications it is particularly important for the component to be resistant to thermal shock for example, resistance-heating wires or blading for gas turbines. Chromia, and especially alumina, scales that form on nickel-base alloys are prone to spalling when thermally cycled as a result of the stress build-up arising from the mismatch in the thermal expansion coefficients of the oxide and the alloy as well as that derived from the growth process. A very useful compilation of data on the cyclic oxidation of about 40 superalloys in the temperature range 1 000-1 I50°C has been made by Barrett et... 

Finally in this section on deracemization via cyclic oxidation/reduction methods, there has been some limited work carried out on the deracemization of secondary alcohols. Soda et al. [22] employed lactate oxidase in combination with sodium borohydride to deracemize D/i-lactate (18) via the intermediate pyruvate (19) (Figure 5.12). 

Although the evidence for a cyclic oxide S5O is relatively weak, its existence is likely in the light of the properties of the homologous oxides SsO, S7O, and SsO which show analogous spectra and reactions (see below). 

The oxygen atoms of the homo cyclic oxides occupy axial positions in solid S7O and SsO. In fact, recent ab initio MO calculations at the G3X(MP2) level of theory have demonstrated that these isomers are, by 7 and 9 kj mol respectively, more stable than the alternative structures with the oxygen atoms in equatorial positions [89]. In the case of SsO, however, the energy difference is only 1 kJ mol and the conformation with the oxygen atom in the equatorial position is more stable. This result agrees with the observation that SsO exists as two isomeric forms in the solid state depending on the crystallization conditions (see above). 

SCHEME 1 Schematic illustration of the biological process of 02 dismutation into 02 and H202 catalyzed by Cu, Zn-SOD via a cyclic oxidation-reduction electron transfer mechanism. (Reprinted from [98], with permission from Elsevier.)... 

Stepped surfaces withstand cyclic oxidation-reduction treatments (146) like [111] and some other low-index planes. Steps have either [311] or [110] structures. They are claimed to be the only places where orbital hybridization does not take place (136). No wonder that such platinum (138) and iridium (147) surfaces have enhanced activity in Cg dehydrocyclization of n-heptane. 

The enzyme 3a-hydroxysteroid dehydrogenase plays a key role in this transport across the hepatocyte. A particularly elegant experiment demonstrated the role of the 3a-hydroxysteroid dehydrogenase, by using [ H] at the 3p hydrogen to show cyclical oxidation-reduction of the 3a-hydroxyl with no accumulation of 3-keto bile acids. Confirmation was obtained by use of indo-methacin, an inhibitor of 3a-hydroxysteroid dehydrogenase, which decreased... 

Scheme 27. Enantioselective ring opening of cyclic oxides G and H by TMSN3 catalyzed by chiral Cr(III) Schiff base complexes within the cavities of zeolites Y 53, EMT 54 and into the interlamellar region of K-10 montmorillonite 55.

Without it the alloys show significantly less resistance to cyclic oxidation which would suggest that, as with yttrium in Fecralloy, the lanthanum is acting to provide a firmly bound oxide barrier. 

To prepare metal hexacyanoferrate films, very frequently the following procedure was followed first a film of the respective metal, for example, cadmium [79], copper [80], silver [81], or nickel [82, 83] was elec-trochemically plated on the surface of a platinum electrode, and that was followed by chemical oxidation of the metal film in a solution of K3[Fe(CN)6], leading to the formation of the metal hexacyanoferrates. The same method has been used to produce films of nickel hexacyanoruthen-ate and hexacyanomanganate using the appropriate anions [83]. It is also possible to perform the oxidation of the deposited metals in solutions containing hexacyano-ferrate(II) by cyclic oxidation/reduction of the latter. In a similar way, films of copper heptacyanonitrosylferrate have been deposited [84]. 

Morton, M., and A. Rembaiim Polamcrization of cyclic oxides initiated by electron transfer. J. Polymer Sci. 32, 530 (1958). 

Figure 9. An idealized model for the cyclic oxidation-reduction process using thiol-functionalized surfactant, incorporated into DODAC vesicles together with Rh-coated colloidal CdS, for sustained hydrogen generation under visible light irradiation in one part of the cycle.

---