Four-electron reduction pathway

An increase in the fraction of the four-electron reduction pathway at more reducing potentials (Fig. 18.10a, b) may be rationalized within at least two mechanisms. The first is based on the kinetic competition between the release of H2O2 from the ferric-hydroperoxo intermediate [Reaction (18.16) in Fig. 18.11] and its (reversible) reduction to a ferrous-hydroperoxo species, which undergoes rapid 0-0 bond heterolysis (18.13b). Because H2O2 and particularly HO2 are more basic ligands... 

In contrast to Co-porphyrin complexes, the direct four-electron reduction of 02 has been only very rarely claimed to be catalyzed by a cobalt phthalocyanine 404 407 In particular cofadal binuclear Co-Pc complexes immobilized on pyrolytic graphite catalyze only the two-electron electroreduction of 02 to H202.408,409 However, recent work has established that an electropolymerized Co-Pc derivative provides a stable four-electron reduction pathway over a wide pH range 410... 

Clearly a plot of JV/D//R vs. o> X 2 will allow us to make some deductions. If, for example, kx = 0 i.e. there is no direct four-electron reduction pathway for 02 at the electrode, and we obtain from equation 6 ... 

Several characterized NRPSs utilize alternative methods for chain termination. In some synthetases, the TE domain of the final module is replaced by an NAD(P)H-dependent reductase domain. Reduction of a peptidyl-S-PCP substrate through a two-electron reaction leads to the formation of a transient aldehyde, which is subsequently converted into a cyclic imine or hemiaminal through intramolecular cyclization. This two-electron reaction is utilized in the biosynthesis of nostocyclopeptides, the saframycins, ° and anthramycin. Alternatively, a four-electron reduction to the primary alcohol is observed in the biosynthesis of mycobacterial peptidolipids, linear gramicidin," " the myxalamides, lyngbyatoxin, " and myxochelin A 75,76 alternative four-electron reduction pathway involving aldehyde formation, transamination, and reduction to a primary amine occurs in the biosynthesis of myxochelin B. ... 

Figure 6.15. Simplified schematic of the most important reaction pathways of the oxygen reduction reaction. The four-electron pathway results in the formation of water. The two-electron pathway forms hydrogen peroxide. Adsorption of molecular oxygen can form atomic oxygen (dissociative pathway) or form a superoxide species (associative pathway). The formation of Pt—OH and Pt— from water molecules represents the backward reactions of the later portion of the four-electron reduction pathway.

Table3.1 Standard reduction potentials for reactions of relevance to the two- and four-electron reduction pathways of dioxygen in acid and alkaline aqueous solutions.

On Ft electrode surface the adsorption and reduction of O2 via a four-electron reduction pathway follows the steps (Eq. (9.13a)-(9.13d)) ... 

The key step in the reduction of oxygen at a catalytic surfece is the breaking of the 0—0 bond that requires four coupled proton and electron transfers, opening up the possibility of many side reactions and products (see Figure 2.4) [6]. The complexity of the ORR and its numerous potential side products means that it is still relatively poorly understood, although the consensus is that it proceeds either via a direct four-electron reduction pathway or via a peroxide intermediate in a 2 + 2 serial four-electron pathway [16-18]. 

Figure 7.104 shows the plot of the value of J vs. S obtained for different potentials on the bare iron region from the data plotted in Fig. 7.103. A straight line obtained with an intercept much greater than 1 indicates that Oz reduction on reduced iron proceeds by the direct four-electron reaction pathway. Formation of HjOj as an intermediate in the consecutive reaction pathway is less than 1 % of the total reduction current Conversely, in the potential region corresponding to passive iron, the slope, S, of the /disk//ring plot is zero, and the intercept J = (l/N) indicates that 02 reduction on passive Fe is a two-electron process in which is the product, and not an intermediate, of the reaction.

On an actual Ti02 microparticle, depending upon wnether there is a depositee catalyst, there can also be varying contributions of these latter three reactions. As discussed later, some photocatalytic reaction pathways can involve 02", and therefore it appears to be a desirable product as such, metallic catalysts may in fact be counterproductive in this respect, f they promote the two or four- electron reduction reactions. 

In the discussion of the biochemistry of copper in Section 62.1.8 it was noted that three types of copper exist in copper enzymes. These are type 1 ( blue copper centres) type 2 ( normal copper centres) and type 3 (which occur as coupled pairs). All three classes are present in the blue copper oxidases laccase, ascorbate oxidase and ceruloplasmin. Laccase contains four copper ions per molecule, and the other two contain eight copper ions per molecule. In all cases oxidation of substrate is linked to the four-electron reduction of dioxygen to water. Unlike cytochrome oxidase, these are water-soluble enzymes, and so are convenient systems for studying the problems of multielectron redox reactions. The type 3 pair of copper centres constitutes the 02-reducing sites in these enzymes, and provides a two-electron pathway to peroxide, bypassing the formation of superoxide. Laccase also contains one type 1 and one type 2 centre. While ascorbate oxidase contains eight copper ions per molecule, so far ESR and analysis data have led to the identification of type 1 (two), type 2 (two) and type 3 (four) copper centres. 

The OER is a multi-electron reaction which may include a number of elementary steps and involve different reaction intermediates. There are several pathways for 02 electroreduction (1) a direct four-electron reduction to HzO (in low pH media) or to OH- (in high pH media) (2) a two-electron pathway involving reduction... 

Nowadays, it has been demonstrated that the reaction is indeed structure sensitive with a multielectron transfer process that involves several steps and the possible existence of several adsorption intermediates [93-96]. The main advantage that we have with the new procedures with respect to cleanliness is that we have well-ordered surfaces to study a complex mechanism such as the oxygen electroreduction reaction [96-99]. In aqueous solutions, the four-electron oxygen reduction appears to occur by two overall pathways a direct four-electron reduction and a peroxide pathway. The latter pathway involves hydrogen peroxide as an intermediate and can undergo either further reduction or decomposition in acid solutions to yield water as the final product. This type of generic model of a reaction has been extensively studied since the early 1960s by different authors [100-108]. 

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