Multi-electron reaction

Synthetic polymers stabilize metal colloids as important catalysts for multi-electron reactions. Polynuclear metal complexes are also efficient catalysts for multielectron processes allowing water photolysis. 

Electrocatalysis is, in the majority of cases, due to the chemical catalysis of the chemical steps in an electrochemical multi-electron reaction composed of a sequence of charge transfers and chemical reactions. Two factors determine the effective catalytic activity of a technical electrocatalysts its chemical nature, which decisively determines its absorptive and fundamental catalytic properties and its morphology, which determines mainly its utilization. A third issue of practical importance is long-term stability, for which catalytic properties and utilization must occasionally be sacrificed. 

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... 

HOR and the ORR involve two and four electrons, respectively. Since the Butler-Volmer equation is important for expressing the relationship between the current density of an electrochemical reaction and the overpotential, the rate-determining step (RDS) of a multi-electron reaction can be simplified as a pseudo-elementary reaction involving multiple electrons. The Butler-Volmer equation for this reaction is usually written as follows ... 

The principles of redox catalysis applied successfully here to photodecomposition of water, can also be profitably employed to carry out efficiently other multi-electron reactions. Of special interest in the area of solar energy conversion are processes such as reduction of C02, N2 and NAD+ and oxidation of halide ions (Cl-, Br- or I-). With the latter, there exists the interesting possibility of complete decomposition of hydrogen halides with the reverse reaction used in a fuel cell to generate power. 

Numerous reviews of the oxygen reduction reaction and its mechanisms have been published. The most recent ones include Adzic [23], Gatteerell and MacDougall [24], and Ross Jr. [25]. The ORR is a multi-electron reaction, which may include a number of elementary steps and involve different reaction intermediates and pathways. Figure 5.1 illustrates the possible reaction pathways. 

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 regioselectivity of palladium-catalyzed additions of organoboronic acids to unsymmetrical alkynes is strongly dependent on steric and electronic effects (Scheme 12).62 Multi-component reaction has been reported for the synthesis of tetrasubstituted alkenes.58 The aryl group from an aryl iodide is generally added to the less hindered... 

CV is extensively used for the study of multi-electron transfer reactions, adsorbed species on the electrode surface, coupled chemical reactions, catalysis, etc. Figure 18b.9 shows some of the examples. 

Fig. 18b.9. Example cychc voltammograms due to (a) multi-electron transfer redox reaction two-step reduction of methyl viologen MV2++e = MV++e = MV. (b) ferrocene confined as covalently attached surface-modified electroactive species—peaks show no diffusion tail, (c) follow-up chemical reaction A and C are electroactive, C is produced from B through irreversible chemical conversion of B, and (d) electrocatalysis of hydrogen peroxide decomposition by phosphomolybdic acid adsorbed on a graphite electrode.

Because the charge separation is a one-electron process but the watersplitting reactions are multi-electron processes (although they have been written above as one-electron processes for simplicity), suitable catalysts are needed to accelerate these multi-electron processes so they can be brought about during the lifetime of the photoinduced species. 

Another advantage of SWV over CV can be seen when dealing with a separate multi-electron transfer reaction. The CV current wave of each or each group of electrons always contains the contribution from the previous electron transfer, particularly the diffusion-controlled current. Separating currents from different electron transfers can be tedious, if not impossible. It can be even worse when we have to take into account the capacitive charging current. Since both capacitive and diffusion-controlled currents are absent or at least minimized on the 7net vs E curve of an SW voltammogram, current waves from each electron transfer are much better resolved and more accurate information can be obtained. 

As shown by Re. 1—2, methanol oxidation to carbon dioxide is a six-electron reaction. This reaction, however, does not proceed by a simple single step. On the contrary, it is considered as a complex multi-step reaction involving several intermediates and by-product which may be different depending on the catalysts, media, temperature and other conditions. Although the details are yet to be found, it is widely agreed that some intermediates or by-products accumulate on the surface and poison the catalyst to decrease its activity. 

Indirect electrochemical reactions usually involve a multi-electron-transfer system that consists of a set of electron transmission units (Fig. 4). Although the overall feature of an electron-transfer process in indirect electrosynthetic reactions is understandable, each step of the electron transmission has not yet been elucidated [10]. 

Spectrophotometry has been a popular means of monitoring redox reactions, with increasing use being made of flow, pulse radiolytic and laser photolytic techniques. The majority of redox reactions, even those with involved stoichiometry, have seeond-order characteristics. There is also an important group of reactions in which first-order intramolecular electron transfer is involved. Less straightforward kinetics may arise with redox reactions that involve metal complex or radical intermediates, or multi-electron transfer, as in the reduction of Cr(VI) to Cr(III). Reactants with different equivalences as in the noncomplementary reaction... 

Oxidation of Alcohols in a Direct Alcohol Fuel Cell The electrocatalytic oxidation of an alcohol (methanol, ethanol, etc.) in a direct alcohol fuel cell (DAFC) will avoid the presence of a heavy and bulky reformer, which is particularly convenient for applications to transportation and portable electronics. However, the reaction mechanism of alcohol oxidation is much more complicated, involving multi-electron transfer with many steps and reaction intermediates. As an example, the complete oxidation of methanol to carbon dioxide ... 

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 ... 

As described in Section 3 of Chapter 2, multi-electron processes are important for designing conversion systems. Noble metals are most potent catalysts to realize a multi-electron catalytic reaction. It is well known that the activity of a metal catalyst increases remarkably in a colloidal dispersion. Synthetic polymers have often been used to stabilize the colloids. Colloidal platinum supported on synthetic polymers is attracting notice in the field of photochemical solar energy conversion, because it reduces protons by MV to evolve H2 gas.la)... 

The polyneclear and mixed valent structure of the PB allow multi-electron processes of Eqs. (22) and (23) as well as both the reduction and oxidation catalyses. The reactions are schematically shown in Fig. 18. 

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