Reduction intermediates, redox potentials

The increase in activity as the M(lll)/(11) redox potential of the catalysts is more positive is in contrast to what was previously found in volcano correlations where a maximum activity is observed for intermediate redox potentials not only for the reduction of O2 but for other reactions such as the oxidation of thiols or of hydrazine [99]. When studying a series of unsubstituted and substituted Mn phthalocyanines. 

Nitric oxide and NjO are direct intermediates in the denitrification pathway, the reduction of NO3 to Nj. Reduction to Nj is often incomplete, so that both NjO and Nj are equally important end products of denitrification, the ratio of NjO/Nj production being determined by soil physical properties. For example, NjO is the main end-product in acid soils, whereas low redox potentials and high organic matter content favour the further reduction to Nitric... 

Most quinone reductions go through an intermediate radical or semiquinone stage, usually revealed by a one-electron step in the redox potential.100 The radical formed by the reduction of compound VI is especially stable, probably because of the additional involvement of the benzoyl group.101 The ordinary semiquinones are more stable in basic solution since some of the resonance structures of the neutral radical involve separation of charges. 

The delocalised radical formed by protonation of the radical-anion is more easily reduced than the starting arene. For some polycyclic aromatic hydrocarbons, the redox potential for this radical species can be determined using a cyclic voltammetry technique [10]. Reduction in dimethylformamide is carried out to the potential for formation of the dianion. The dianion undergoes rapid monoprotonation and on the reverse sweep at a fast scan rate, oxidation of the monoanion to the radical can be observed. The radical intermediate from pyrene has E° = -1.15 V vs. see in dimethylformamide compared to E° = -2.13 V vs. see for pyrene,... 

Phenoxyl radicals are oxidizing radicals (for a compilation of redox potentials see Wardman 1989). Thus, in their reactions with 02 (E7 = -0.3 V) there is ample driving force for a reduction by ET [cf. reaction (16)], and this has been thought for a long time to be the only (Hunter et al. 1989) or at least a major process, depending on the reduction potential of the (substituted) phenoxyl radical (Jonsson et al. 1993). Yet in the tyrosine system, despite of the high reduction potential of tyrosine phenoxyl radical (E7 = 0.64 V), the by far dominating process is addition, and the intermediate adduct is locked in by a Mannich reaction [reactions (14) and (15) Jin et al. 1993],... 

The values for the redox potential for the couple M3 + /M2+ have been estimated57 using a simple ionic model and available thermodynamic data. The results (Table 2) correlate closely with the ionization potentials for the M2+ ions, and are in good agreement with both chemical observations and other estimates obtained by spectroscopic correlations. Irreversible oxidation of terbium(m) to terbium(iv) in aqueous K2C03-K0H solutions has been observed electrochemically 58 the discovery of an intermediate of mixed oxidation state explains partly the reduction behaviour of terbium(iv) deposits. Praseodymium(iv) and terbium(iv) have also been detected in nitrate solutions. 

Unfortunately, the redox potential of the Pt4 + /3+ couple is not known in literature. Although some stable Ptm compounds have been isolated and characterized (37), the oxidation state III is reached usually only in unstable intermediates of photoaquation reactions (38-40) and on titania surfaces as detected by time resolved diffuse reflectance spectroscopy (41). To estimate the potential of the reductive surface center one has to recall that the injection of an electron into the conduction band of titania (TH) occurs at pH = 7, as confirmed by photocurrent measurements. Therefore, the redox potential of the surface Pt4 + /3+ couple should be equal or more negative than —0.28 V, i.e., the flatband potential of 4.0% H2[PtClal/ TH at pH = 7. From these results a potential energy diagram can be constructed as summarized in Scheme 2 for 4.0% H2[PtCl6]/TH at pH = 7. It includes the experimentally obtained positions of valence and conduction band edges, estimated redox potentials of the excited state of the surface platinum complex and other relevant potentials taken from literature. An important remark which should be made here is concerned with the error of the estimated potentials. Usually they are measured in simplified systems - for instance in the absence of titania - while adsorption at the surface, presence of various redox couples and other parameters can influence their values. Therefore the presented data may be connected with a rather large error. 

Figure 3.7. The energy profile of a nitrogenase reaction. Eo is the standard redox potential of the reactants, intermediates and products of the reaction Fd = ferredoxin FeP = Fe protein FeMo = FeMo protein. The arrow indicates the increase of the reduction potential upon ATP hydrolysis (Likhtenshtein 1988a). Reproduced with permission.

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