Hydrogen redox properties

The pros and cons of oxidative dehydrogenation for alkene synthesis using doped cerianites as solid oxygen carriers are studied. The hydrogen oxidation properties of a set of ten doped cerianite catalysts (Ce0.9X0.1Oy, where X = Bi, In, La, Mo, Pb, Sn, V, W, Y, and Zr) are examined under cyclic redox conditions. X-ray diffraction, X-ray photoelectron spectroscopy, adsorption measurements, and temperature programmed reduction are used to try and clarify structure-activity relationships and the different dopant effects. 

NMR and UV-visible techniques have been used in the characterization of intermediates in the [Fe (edta)]" -promoted decomposition of hydrogen peroxide7 Fe complexes of edta, nta, and dtpa react with FISOs by an inner-sphere one-electron transfer mechanism with transient production of S04, in contrast to Cu, which reacts by an outer-sphere mechanism to give S04 and hydroxy radicalsFe -edta redox properties are relevant to Fe /Cu /H202 systems. ... 

High-potential iron proteins, 45 313-314, 344 cluster stability, 45 324-332 function, 45 315-316 residues, 45 322-344 structure and, 45 317-322 redox properties, 45 333-344 solvent accessibility, 45 330, 332-333 source and function, 45 314-316 structure, 45 316-322 hydrogen bonding and, 45 321-322 intermolecular aggregation, 45 322 primary, 45 317-318 secondary and tertiary, 45 318-321... 

V) are more biologically relevant because of the solvent and associated hydrogen-bonding properties. However, specific interactions from the protein cannot be fully reproduced. Indeed, the redox potential of [Fe(SCH2CH20H)4] f in water is close to the lower limit of FeS4 sites in rubredoxin proteins (—0.1 to +0.1 V versus standard hydrogen electrode (SHE)). 

A solvent, in addition to permitting the ionic charges to separate and the electrolyte solution to conduct an electrical current, also solvates the discrete ions, by ion-dipole or ion-induced dipole interactions and by more direct interactions, such as hydrogen bonding to anions or electron-pair donation to cations. Lewis acidity and basicity of the solvents affect the latter. The redox properties of the ions at an electrode depend on their being solvated, and the solvation effects electrode potentials or polarographic half-wave potentials. 

The herbicidal activity of the bipyridyliums depends on their redox properties. Their abilities as one-electron acceptors of the right redox potential (-350 mV for diquat and -450 mV for paraquat) allow them to siphon electrons out of the photosynthetic electron-transport system, competing with the natural acceptors. The radical anion produced is then reoxidized by oxygen, generating the real toxicant, hydrogen peroxide, which damages plant cells. Structure-activity relationships in this series have been reviewed (60MI10701). 

One other important criterion for successful water cleavage that must be considered is the solution pH. Although the potential difference between the two half reactions for water decomposition is fixed at 1.23 V and is independent of pH, the half-cell reactions are dependent upon pH (Figure 4). Thus, by altering the pH of a solution it is sometimes possible to alter the half-cell potentials to be compatible with the redox properties of a photosensitizing catalyst. The oxidant must have a redox potential above the oxygen line, whilst the reductant must have a redox potential below the hydrogen line. The effect of pH is illustrated in subsequent sections of this chapter. 

Much of the work in this area has centred around efforts to optimize the photochemical and redox properties of the Ru11 complexes which are related to water cleavage reactions, e.g. lifetime of excited state, absorption maxima, etc. A detailed account of these properties is found in Chapter 8.3 and hence it is only intended here to present the results of these studies on hydrogen producing systems. 

The chemical properties of solvents have obviously a strong bearing on their applicability for various purposes. The solvents should selectively dissolve the desired solutes and not some others, they should be inactive in the chemical reactions undergone by the solutes, but solvate, again selectively, reactants, transition states, intermediates, and products. These aspects of the behaviour can be achieved by the proper blend of the chemical properties of structuredness, polarity, electron-pair and hydrogen bond donation and acceptance ability, softness, acidity and basicity, hydrophilicity or hydrophobicity, and redox properties, among others. Such chemical characteristics can often be derived from physical properties, but in other cases must be obtained from chemical interactions, for instance by the use of chemical probes ( indicators ). 

A model of a flavin-based redox enzyme was prepared.[15] Redox enzymes are often flavoproteins containing flavin cofactors flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN). They mediate one- or two-electron redox processes at potentials which vary in a range of more than 500 mV. The redox properties of the flavin part must be therefore tuned by the apoenzyme to ensure the specific function of the enzyme. Influence by hydrogen bonding, aromatic stacking, dipole interactions and steric effects have been so far observed in biological systems, but coordination to metal site has never been found before. Nevertheless, the importance of such interactions for functions and structure of other biological molecules make this a conceivable scenario. 

The metal ion is a typical catalyst for a reaction of Class B, but because H+ and OH- ions are formed in the process, the reaction may be expected to be pH dependent as well. The catalyst does not necessarily have to be a metal ion. Every substrate that can change oxidation state at the right potential and is capable of reacting with the hydrogen peroxide molecule in its two valency states can be considered a catalyst for this reaction. This means that the best parameter to correlate this reaction with would be the redox potential of the catalyst, which unfortunately is very difficult to measure on a solid material. The best thing to do is to use a catalyst which can be dissolved in a liquid medium of some kind, and to study the redox properties in the dissolved state. Measurements of this kind will be discussed in the section on oxidation and dehydrogenation. 

This is probably also a reaction of Class B, which however may be expected not to be pH dependent. Therefore, taking into account the scanty evidence available, we may propose that the catalytic activity for this reaction is also dependent on the redox properties of the catalyst, but that the direction of the reaction, either into ammonia and nitrogen or into nitrogen hydrogen respectively, may be connected with acid/base properties. 

The mechanism of melatonin s interaction with reactive species probably involves donation of an electron to form the melatoninyl cation radical or through a radical addition at the site C3. Other possibilities include hydrogen donation from the nitrogen atom or substitution at position C2, C4, and C7 and nitrosation [169]. The mechanisms by which melatonin protects against LP most likely involve direct or indirect antioxidant and free-radical scavenging activities of this indoleamine [169,171]. 2-Phenyl indole derivatives have redox properties because of the presence of an electron-rich aromatic ring system that allows the indoleamine to easily function as an electron donor. For these derivatives, the possible antioxidant mechanism might be most probably toward carbon-centered radicals described by Antosiewicz et al. [172]. 

The reductive/oxidative properties of transitional metal elements in these zeolite catalysts were also examined by TPR and TPO, and it is shown that metallic species in certain cation locations may migrate under calcination, reduction, and reaction conditions [7], The different treatment, e g, coking or even the oxidative regeneration, will produce metallic species of varied oxidation states with different distributions in the molecular sieve structures as exemplified by the above XPS data. The redox properties of these metallic cations exhibit the influence of hydrogen and/or coke molecules, and it is further postulated that the electron transfer with oxygen species are considered responsible for their catalyzed performance in the TPO regeneration processes, as shown in Figure 2. 

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