Multielectron reactions

The potential level of the 02 evolving site of the photosynthesis (see Fig. 1) ranging around 0.82 V shows that a four-electron process occurs in it. The water oxidation site of the photosynthesis contains more than four Mn ions interacting with each other, thus leading to the four-electron reaction of water to give 02, Such a multielectron reaction leads to the generation of H2 from proton reduction as described later in chapter 4 on water photolysis. 

Second, in order to involve the oxidant and reductant in the multielectron reactions of water oxidation and reduction, one has to introduce appropriate catalysts into the aqueous phases separated by the membrane. 

The ORR is a multielectron reaction that may include a number of elementary steps involving different reaction intermediates. The detailed mechanism is still not known, since neither ex situ nor in situ techniques are capable of identifying all reaction intermediates formed under genuine reaction conditions exist. Of the various possibilities [13], it has been proposed that on metal surfaces the most plausible reaction pathway for the ORR in both alkaline and acidic electrolytes can be described by the so-called serial reaction pathway (Scheme 3.1) where after the transfer of two electrons and (simultaneous) fast protonation of superoxo/peroxo radicals (not included in the reaction scheme), O is reduced to H O (with rate constant k. ... 

Note that Eq. (n) applies only to one-electron reactions. Whereas the theoretical treatment can be extended formally to multielectron processes, such reactions commonly occur in microscopically separate, one-electron steps. If the first step is rate controlling, n can be set at unity in Eq. (b) in 12.3.7.1, regardless of the number of electrons transferred in the overall reaction. A general difficulty for such multistep processes is that AG, cannot be extracted from rate measurements unless the standard potential for the redox couple comprising the elementary reaction is known. For multielectron reactions, only the formal potential for the overall reaction normally can be obtained. Similar remarks apply to other multistep electrode reactions, such as those involving phase transfer (e.g., metal deposition or gas evolution). 

MG Goldfeld, LA Blumenfeld, LG Dmitrovsky and VD Mikoyan (1980) Plastoquinone function in photosystem 2. Mol Biol 14 804-813 AG Volkov (1986) Molecular mechanism of the photooxidation of water during photosynthesis Cluster catalysis of synchronous multielectron reactions. Mol Biol 20 728-736... 

It is also possible to separate multielectron reactions into slightly more complicated separate one-electron steps. The common gas evolving, electro-catalytic, reactions are of this type. However, this is only useful if the intermediate one-products are stable and show a definite presence in the electrochemical kinetics. As an example, consider the consequence of splitting eqn. (38) into its component one-electron steps... 

The term synchronous multielectron reaction does not mean that all n electrons started synchronously, since this is impossible according to quantum mechanics. Instead, each electron is transferred from donor to acceptors individually. However, the time required for intermediate formation is much less than the time of the reorganization of the medium, so that intermediates as individual chemical compounds do not exist. 

I. INTRODUCTION KINETIC ASPECTS OF SYNCHRONOUS MULTIELECTRON REACTIONS... 

Vectorial charge transfer and a molecular recognition at the interface between two dielectric media are important stages in many bioelectrochemical processes such as those mediated by energy-transducing membranes [1-4], Many biochemical redox reactions take place at aqueous medium/membrane interfaces and some of them are multielectron processes. About 90% of the oxygen consumed on Earth is reduced in a four-electron reaction catalyzed by cytochrome c oxidase. Multielectron reactions take place in photosynthesis, which is the most important process on earth [5-10]. Life on Earth began as photosynthesis. 

For the multielectron reaction 2 1 1, according to Eq. (1), E for 2-electron reactions between O2, a, and Cub strongly depends on the geometry and distances in a catalytic site. Only 2 1 1 mechanism of oxygen reduction by cytochrome oxidase can be realized in vivo in both hot and cold conformations. 

The redox map of photosynthesis in green plants can be described in terms of the well-known Z-scheme proposed by Hill and Bendal [29]. The main advantage of the currently accepted Z-scheme depicted in Fig. 4 lies in the specific mechanism of charge transfer at the stage of water oxidation, which is a multielectron reaction mechanism involving no unknown intermediates [6,12]. 

Oxygen reduction is a multielectron reaction that involves several elementary... 

In spite of the considerable effort expended in trying to unravel the fundamental aspects of the O2 electroreduction reaction, many details about the mechanism are not fully understood. The electrochemical reduction of oxygen is a multielectron reaction that occurs via two main pathways one involving the transfer of two electrons to give peroxide, and the so-called direct four-electron pathway to give water. The latter involves the rupture of the 0-0 bond. The nature of the electrode strongly influences the preferred pathway. Most electrode materials catalyze the reaction via two electrons to give peroxide Peroxide pathway in acid... 

CO2 reduction reactions are shown in Fig. 19 classified by the number of electrons involved in the reaction. In Fig. 19, proton reduction is also shown as a reference. When the number of electrons involved in the reactions shown in Fig. 19 increases, the redox potential for the reaction tends to shift positively, suggesting that CO2 reduction becomes thermodynamically easier with an increase in the munber of electrons. By contrast, the reduction becomes difficult with an increase in the number of electrons due to a mechanistic difficulty in pooling the electrons for a multielectron reaction. In most CO2 reductions catalyzed by molecules, the reaction proceeds with two electron reductions producing CO or HCOOH. CO2 reduction with four, six, or eight electrons is usually a difficult process. 

As explained earlier, and due to the application of Equation 8.4, the capacitance of the AC/AC capacitor is limited by the relatively low capacitance of the negative EDL electrode. For this reason, an AC/AC capacitor has been developed by using 1 moll" KI as anolyte and 1 moll" VOSO4 catholyte separated by a Nafion membrane [111]. High capacitance values of 1200 and 670 Fg" were measured at 5 mV s for the positive and negative electrodes, respectively. The high values at the negative electrode were explained by the multielectron reactions (8.18)-(8.22) ... 

Electrochemical reduction of O2 is a multielectron reaction that has two main possible pathways one involving the transfer of two electrons (2e ) to produce H2O2 and the other, a direct four-electron (4e ) pathway to produce water. In alkaline media, the 2e pathway can be written [27] as ... 

The electrochemical reduction of molecular oxygen on different electrode materials has been studied extensively during the last 20 years principally in connection with electrochemical energetics. Oxygen electroreduction is a multielectron reaction and may include a number of elementary steps forming various parallel-consecutive combinations. 

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