Oxygen reduction site

COX is an extensive membrane-bound ensemble in which cytochrome a3 and Cub cooperate to form an oxygen reduction site and reduced cytochrome c containing iron(II) is oxidized through proximity to Cua- As a consequence of this reaction four protons are pumped across the membrane to set up a potential that helps to power ATP synthase. In plants, and some bacteria, the latter enzyme is serviced by another complex, membrane-bound protein ensemble, photosystem II. 

Chromates are excellent inhibitors of oxygen reduction in near neutral and alkaline solutions. In these environments, they can stifle corrosion by suppressing this cathodic partial reaction. The inhibition mechanism appears to involve reduction of Cr(VI) to Cr(III) at a metal surface and formation of Cr(III)-0-substrate metal bonds (38). This surface complex is likely to be substitutionally inert and a good blocker of oxygen reduction sites, as suggested by the exceedingly small water exchange rate constant for the first coordination sphere of Cr3+ (39). 

Horita, T., Yamaji, K., Sakai, N., Yokokawa, H., Kawada, T., Kato, T. Oxygen reduction sites and diffusion paths at Lao <>Sro iMnO j x/yttria-stabilizcd zirconia interface for different cathodic overvoltages by secondary-ion mass spectrometry. Solid State Ionics 2000,127, 55-65. 

Statistical-mechanical calculations have suggested that the hydrophobic cavity may contain 3-4 water molecules, at least transiently, and these may be crucial not only for proton transfer as such, but also in the mechanism of coupling between electron and proton transfer, as outlined below. Finally, it should be emphasised that the hydrophobic cavity is next to the binuclear oxygen reduction site, and that it is likely that the product of the redox reaction, water, is initially ejected into the hydrophobic cavity during the... 

CH4 -I- 2Q2- C2H4 -I- 2H2O -I- 4e" through membrane to oxygen reduction site... 

Verkhovsky, M.I., J.E. Morhan, and M. Wikstrom (1995). Control of electron delivery to the oxygen reduction site of cytochrome c oxidase A role for protons. Biochemistry 34,7483-7491. 

Imai H, Matsumoto M, Miyazaki T, Fujieda S, Ishihara A, Tamura M, Ota K (2010) Structural defects working as active oxygen-reduction sites in partially oxidized Ta-carbonitride core-shell particles probed by using surface-sensitive conversion-electron-yield x-ray absorption spectroscopy. Appl Phys Lett 96 191905... 

Various anticorrosion mechanisms have been described, the most common being ennobhng, dislocation of the oxygen reduction site, and the release of dopant anions. 

F re 6.21 Schematic representation depicting the dislocation of oxygen reduction site. 

Nitric acid treatment lowered the methane uptake by about ten percent. This could be due to oxygen occupying sites within pores, but may be the result of weaker interaction between methane and an oxide surface as is observed for silica. Reduction of these treated carbons with hydrogen restored their original methane uptake. Clearly for methane storage, there is no advantage in modifying the carbon surface by nitric acid treatment. 

It follows from equation 1.45 that the corrosion rate of a metal can be evaluated from the rate of the cathodic process, since the two are faradai-cally equivalent thus either the rate of hydrogen evolution or of oxygen reduction may be used to determine the corrosion rate, providing no other cathodic process occurs. If the anodic and cathodic sites are physically separable the rate of transfer of charge (the current) from one to the other can also be used, as, for example, in evaluating the effects produced by coupling two dissimilar metals. There are a number of examples quoted in the literature where this has been achieved, and reference should be made to the early work of Evans who determined the current and the rate of anodic dissolution in a number of systems in which the anodes and cathodes were physically separable. 

In the Lai.,CsxMn03 catalyst, the T decreases with an increase of x value and shows an almost constant value upon substitution of x>0.3. It is thought that the oxygen vacancy sites of perovskite oxide increase with an increase of amount of Cs and the oxidation activity also increases. This result is also verified by the TPR result of these catalysts(Fig. 3). As shown in Fig. 3, the reduction peak appears at low temperature with an increase of x value and no change is shown at more than x=0.3. It can thus be concluded that the catalytic performance of these oxides increases as the amount of Cs in the crystal lattice increases. However, the substitution of Cs to more than x=0.3 leads to excess Cs, which is present on the surface of mixed oxides might have no effect on the catalytic activity... 

Stamenkovic V, Fowler B, Mun BS, Wang G, Ross PN, Lucas CA, Markovic NM. 2007a. Improved oxygen reduction activity on Pt3Ni(lll) via increased surface site availabihty. Science 315 493-497. 

It also consists with the spinel hypothesis for the structure of active catalytic sites the spinel compound NiCo204 is well known for its catalytic activity in the reaction of oxygen reduction [21],... 

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