Oxygen four-electron transfer

The WOC is oxidized stepwise by a nearby tyrosine residue (Tyrz), which is itself oxidized by the chlorophyll cation radical P680+ (formed by light-induced charge separation). The electrons are eventually used by PSII for the reduction of plastoqui-none. After the WOC has lost four electrons, the accumulated oxidizing power drives the formation of molecular oxygen from two substrate water molecules, and the catalytic system is reset. The sequence of the four electron-transfer steps is summarized in the Kok cycle [32] of Figure 4.5.3, where the most probable spectroscopically derived oxidation states of the Mn ions [33] are shown for each of the five redox state intermediates S (n - 0-4). 

Photosystem 11 (PS II) within higher plants represents a solar-energy-driven process that removes hydrogen atoms from water to form molecular oxygen (O2, dioxygen) through an overall four-electron transfer reaction (equation 20), ... 

Recently, it was reported that loading small amount of platinum onto tungsten(VI) oxide enhances the visible-light photocatalytic activity significantly and this is caused by the catalytic action of platinum to induce multiple-electron transfer to oxygen 44). Reactions of two and four-electron transfer processes are as follows (potential in parentheses is standard electrode potential versus standard hydrogen electrode at pH 0). 

Anderson and co-workers used similar extensive ab initio DFT-GGA electronic structure calculations to study an equally important electrochemical reaction, namely oxygen reduction. The oxygen reduction was supposed to take place in the following four electron transfer steps ... 

More recently, flavin-dependent nicotinamide oxidases, such as YcnD from Bacillus subtilis [754] or an enzyme from Lactobacillus sanfranciscensis [755] were employed for the oxidation of nicotinamide cofactors at the expense of molecular oxygen producing H2O2 or (more advantageous) H2O via a two- or four-electron transfer reaction, respectively [756-758]. Hydrogen peroxide can be destroyed by addition of catalase and in general, both NADH and NADPH are accepted about equally well. 

Unfortunately, Ir is one of the rarest and most expensive metals which would render its use very costly. Nonetheless, the complex should serve as a suitable model, such that theoretical and experimental knowledge gained from its study may serve to tailor the synthesis of improved catalysts. Table 7.3 lists some metallomacrocyclic complexes which accomplish the reduction of oxygen via the four-electron transfer pathway in acidic electrolytes. 

Some studies have shown that certain modification procedures can be used to transform two-electron reduction metalloN4-macrocyclic complexes into hybrid materials with the capability to reduce oxygen to water, either via the direct four-electron transfer pathway or in the series two-electron transfer pathway. Carbon nanomaterials, carbon nanotubes in particular [58-65], have been reported to significantly increase the catalytic oxygen reduction current, with a substantial reduction of the overpotential for ORR reported in some cases, as shown by the examples in Table 7.4. 

Oxygen can be reduced to H2O2 by benzo-, naphtho-, and anthraquinone-modified electrodes [74] and by poly(violo-gen)-modified electrodes [75]. A Prussian blue-modified electrode catalyses electroreduction of O2 to H2O by the four-electron transfer reaction [76]. 

The electrochemical reduction of molecular oxygen through a four electron transfer at the cathode of a fuel cell. The CF2SO3H group is the protogenic group on ionomers and membranes utilized in catalyst layer and electrolyte in a fuel cell. 

The usage of the cathode and anode reactants can be calculated based on Faraday s law. For each mole of oxygen, four electrons are transferred therefore,... 

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