Redox properties of oxides

It is well known that redox properties of oxides can be modified by the formation of mixed oxides (12). Effect of mixed oxide formation on AG° of M-0 dissociation is illustrated in Figure 1. The free energy changes of the following reactions, AGA° and AGp°, are calculated from thermodynamic data (13, 14). 

Redox properties of oxide surfaces and their relationships to catalytic activity. 

In most cases, the selective oxidatitMi of hydrocarbons by oxides occurs through the so-called Mars-van Krevelen mechanism [68], in which the organic molecule reacts with the lattice oxygen, and then the oxygen vacancy is replenished by gaseous oxygen. As a result, the redox properties of oxides influence their ability to complete such a catalytic cycle. We have demonstrated that the activation of an alkane C-H bond usually undergoes H-abstraction mechanism on a transition metal oxide [69] ... 

Catlow, C. R. A. The effects of dopants on the redox properties of oxide fuel. Proc. IAEA Specialists Meeting on Internal I el Rod Chemistry, Erlangen, FRG, 1979 IAEA Report IWG FPT/3, p. 127-130 (1979)... 

Temperature-prograimned reduction, oxidation and desorption (TPR, TPO, TPD), belong probably to the most widely used in situ techiuques for the characterization of oxidation catalysts and are discussed in more detail in Section 19.4. While TPD (with ammonia as the probe molecule) is frequently used to examine surface acid sites, TPR and TPO (with H2 or O2, respectively) provide information on the redox properties of oxide catalysts being crucial for their performance in catalytic oxidation reactions. Important information on reaction mechanisms can be obtained when the catalysts are heated in the presence of reactants combined with mass spectrometric product analysis. This is called temperature-programmed reaction spectroscopy (TPRS). As far as reaction mechanisms and kinetics are concerned, transient techniques which reflect the response of the catalytic system to a sudden change of reactant are inevitable tools. Two such techiuques, namely the temporal analysis of products (TAP) reactor and steady-state isotopic transient kinetic analysis (SSITKA) will be described in more detail in Section 19.5. 

A closer analysis of die equilibrium products of the 1 1 mixture of methane and steam shows the presence of hydrocarbons as minor constituents. Experimental results for die coupling reaction show that the yield of hydrocarbons is dependent on the redox properties of the oxide catalyst, and the oxygen potential of the gas phase, as well as die temperamre and total pressure. In any substantial oxygen mole fraction in the gas, the predominant reaction is the formation of CO and the coupling reaction is a minor one. 

The second step involves the transfer of electrons from the reduced [FMNHg] to a series of Fe-S proteins, including both 2Fe-2S and 4Fe-4S clusters (see Figures 20.8 and 20.16). The unique redox properties of the flavin group of FMN are probably important here. NADH is a two-electron donor, whereas the Fe-S proteins are one-electron transfer agents. The flavin of FMN has three redox states—the oxidized, semiquinone, and reduced states. It can act as either a one-electron or a two-electron transfer agent and may serve as a critical link between NADH and the Fe-S proteins. 

The redox properties of quinones are crucial to the functioning of living cells, where compounds called ubiquinones act as biochemical oxidizing agents to mediate the electron-transfer processes involved in energy production. Ubiquinones, also called coenzymes Q, are components of the cells of all aerobic organisms, from the simplest bacterium to humans. They are so named because of their ubiquitous occurrence in nature. 

The second direction in which redox properties of sulfones and sulfoxides could manifest themselves in photochemistry is redox photosensitization108,110-114. In such a photosensitization the photosensitizer is transformed by light into a short-lived oxidant or reductant able to react with the substrate to be activated. Tazuke and Kitamura115 have discussed the parameters to play with when one... 

One-electron reduction or oxidation of organic compounds provides a useful method for the generation of anion radicals or cation radicals, respectively. These methods are used as key processes in radical reactions. Redox properties of transition metals can be utilized for the efficient one-electron reduction or oxidation (Scheme 1). In particular, the redox function of early transition metals including titanium, vanadium, and manganese has been of synthetic potential from this point of view [1-8]. The synthetic limitation exists in the use of a stoichiometric or excess amount of metallic reductants or oxidants to complete the reaction. Generally, the construction of a catalytic redox cycle for one-electron reduction is difficult to achieve. A catalytic system should be constructed to avoid the use of such amounts of expensive and/or toxic metallic reagents. 

The consequences of polychlorination of porphyrins on redox properties of complexes has been investigated.1404 The highly chlorinated porphyrin 3-octachloro-/ /c.vo-tetrakis(3,5-dichloro-2,6-dimethoxyphenyl)porphyrin exhibits a substantial anodic shift for reduction of over 0.5 V and a smaller shift for oxidation versus the unchlorinated precursor. Contrastingly, small potential shifts for the octabromo-substituted 5,10,15,20-tetraphenylporphyrinate arise from the dominance of macrocycle ruffling over electronic effects. In the polychloro complex, distortion does not compensate fully for electron-withdrawing effects of the Cl substituents. 

Also, the structure of the Ni11 complex (379) with the related ligand dimethyltetrathiomalonate has been determined.997 The geometry at the nickel center is approximately square planar with an inversion center. (379) shows reversible redox properties 998 Oxidation with I2 affords 1,3-bis (methylthio)-1,2-dithiolium triiodide. 

Tricyclic dithiine derivatives of tetrathiafulvalene (TTF) 32 have been prepared for their increased electropolymerization potential <2000CC1005>. The effect of different tricyclic heterocycles upon the redox properties of TTF analogues was explored, and showed that the furan derivative 33 had an unusual nonplanar conformation that allowed for the attainment of higher oxidation states at relatively low oxidation potentials <2004JMC2822>. 

Figure 8.7 Redox properties of complexes in different oxidation states.

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