Four-electron mechanism

Diphenylhydrazine was not detected as an intermediate. The authors postulate a four-electron mechanism involving the intermediate formation of 1,2-diphenylhydrazine. 

Electrocatalytic Reduction of Dioxygen The electrocatalytic reduction of oxygen is another multi-electron transfer reaction (four electrons are involved) with several steps and intermediate species [16]. A four-electron mechanism, leading to water, is in competition with a two-electron mechanism, giving hydrogen peroxide. The four-electron mechanism on a Pt electrode can be written as follows ... 

This conclusion appears to agree fully with the concept of the aforementioned thermodynamically favorable four-electron mechanism of N2 reduction (Likhtenshtein and Shilov, 1970) and with the evolution of free hydrazine at the acid or base treatment of nitrogenase during turn-over (Lowe et al., 1993), (5) Histidine is the only aniino acid side chain capable of donating protons in neutral pH. The Fe atoms are not saturated. Though aforementioned theoretical calculations are based on simplified truncated models of FeMoco and use an approximate computational approach, they allow the... 

In Photsystem II, the water oxidation with evolution of dioxygen occurs under the action of a relatively mild oxidant the cation of chlorophyll which is the product of one-electron oxidation with redox potential E0 = 1.1 eV (Anderson, 2001). The potentials of the oxidation of water by one-, two- and four electron mechanisms are equal to 2.7 V (hydroxyl radical), 1.36 V (hydrogen proxide), and 0.81 (dioxygen). Enclosed in... 

If catalysts are not good, the hydrogen peroxide is generated at some stages of electrode process. Hydrogen peroxide is very strong oxidizer and destroys the construction of the fuel cell. Therefore, the catalyst must provide four-electron mechanism of reaction. Such catalysts are showed in the Tables 3, 4. 

In the second four-electron mechanism, electrochemical reaction of the Si-Si backbonds with holes is assumed to be fast in comparison with electrochemical ligand exchange or chemical reaction with HF or H2O. For the case shown in Fig. 32,... 

It is also very important to monitor the effects of the upper potential limit since the potential at which oxygen reduction begins implies hydroxide or oxide co-formation on platinum. There are many studies of the reaction on the three low-index platinum surfaces [95,98]. The catalytic activity of these surfaces decreases in the order of Pt(l 10) > Pt(l 11) > Pt(100) in perchloric acid solution [96], while the order is Pt(110) > Pt(100) > Pt(l 11) in sulfuric acid solution [93]. In the case of Pt(lll), the formation of a two-dimensional ordered ad-layer of specifically adsorbed (bi) sulfate anions is the main reason for the inhibition of oxygen reduction. Moreover, the direct four-electron mechanism was found for the three surfaces in acidic media, while the reaction mechanism varied to a two-electron reduction on the Pt(lll) and Pt(100) due to the shielding of the hydrogen adatoms. 

FIG. 8 Proposed four-electron mechanism of water photo-oxidation. 

In Nature (on membranes of chloroplasts) water is oxidized by the four-electron mechanism and the life-time of chlorophyll can be as long as one day. In a model octane/water system, oxygen evolution occurs also during several hours. This is indirect evidence in favor of a many-electron reaction. Therefore we shall consider a four-electron mechanism of water oxidation sensitized by chlorophyll adsorbed at the oil/water interface proposed by Volkov [81]. It was shown above that the interface is the most likely site for the water photooxidation reaction. Thus it is assumed that water can be oxidized by a reaction complex adsorbed at the interface that consists of a hydrated oligometer of chlorophyll, a hydrophilic electron acceptor and hydrophobic proton acceptor [81,82,86]. The water in the reaction complex is linked coordinatively with the magnesium of one of the chlorophyll molecules, by hydrogen bonds with the carbonyl group of another chlorophyll molecule, and with the phenol anion also... 

An important requirement that must be met by a catalyst for the oxygen electrode is that of accelerating the reduction reaction that follows the four-electron mechanism ... 

The oxygen reduction reaction (ORR) is the primary electrochemical reaction occurring at the cathode of a PEMFC, and is central to this promising technology for efficient and clean energy generation. The ORR is a multi-electron reaction that follows the direct four-electron mechanism on platinum-based electro-catalysts. It appears to occur in two pathways in acid electrolytes (Adzic and Lima, 2009) ... 

The reduction of molecular nitrogen to ammonium and water oxidation to molecular oxygen causes six- and four-electron transfer to occur eventually in these reactions, respectively. Such processes obviously cannot occur in a single step. Analysis of toe thermodynamics of plausible intermediates rules out one-and two-electron transfers for both reactions and only four-electron mechanisms are energetically allowed.. Evidently, toe direct transport of four electrons fix>m (or to) a mononuclear or even binuclear transition metal complex appears to be ruled out Practically toe only possible variant of toe four-electron mechanism is toe conversion in toe coordination here of a transition metal polynuclear complex. 

The better way for realizing such a mechanism is involving polynuclear complexes of transition metals. This conclusion was confirmed by studies of isolated nitrogenase and model systems (Fig. 18.7). This reaction is proceeded by consecutive four long-distance electron transfer along the path FeP P-cluster FeMOCo). The decisive step of reduction of N2 occurs in polynuclear FeMoCo most probably by four-electron mechanism. 

Liu et al. (2003) studied the oxidation of BH2 on nickel electrocatalyst and reported that the reaction proceeds by the four-electron rather than eight-electron pathways. Normally, Na" " or K" cations in the solution do not influence the four-electron reaction pathways. The four-electron reaction pathways and hydrolysis reaction leads to a decrease in efficiency of oxidation. A suitable catalyst should be identified such that the eight electron mechanism of electro-oxidation of NaBH4 is followed. Otherwise, hydrogen gas generated from four-electron mechanism of electro-oxidation should be utilized to maintain higher efficiency. 

Lund [28] proposed an empirical rule which, nevertheless, has only been verified for a limited number of examples, namely that compounds with the C = N - Y grouping are reduced under polarographic conditions with consumption of two electrons to form a saturated compound if Y is a carbon atom if Y is nitrogen or oxygen, four electrons are consumed in the reaction, the N - Y bond splits, and the double bond is then saturated. However, as shown by later investigations, the Lund rule is not always valid, and even related compounds can be reduced by both a two-electron and a four-electron mechanism depending on the pH of the medium or on the nature of substituents (amidines for example). It would be better to restrict the use of this relationship to the reduction of azomethines in acidic solutions. 

Souchay and Graizon [151] considered that two electrons were involved in the reduction of benzophenone semicarbazone. Lund [28] demonstrated a four-electron mechanism for the electrode reaction with initial cleavage of the N —N bond in benzaldehyde semicarbazone, as with benzophenone semicarbazone in mineral acid solution. In the reduction of benzophenone semicarbazone in a solution buffered at pH 4.0 1-benzhydrylsemicarbazide, i. e., the product from the addition of two electrons, was isolated with 30% yield. The semicarbazones of cinnamaldehyde and benzalacetone can be reduced both in the protonated and in the unprotonated form four electrons are required in alkaline solution, but it has not been possible to determine... 

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