Oxygen reduction reaction electron transfer number

Is this an oxidation-reduction reaction Historically, it surely is, for the term oxidation originally referred specifically to reactions with oxygen. Yet our electron-transfer view of oxidation-reduction reactions provides no help in deciding so. Where in reaction (76) is there any evidence of electrons being gained or lost In such a doubtful case, our oxidation number scheme provides an answer. Applying the same assumptions used in treating the HSOf-HSOi"... 

As in the case of H2 oxidation, the oxygen reduction reactions (ORR) on the cathode are also assumed to take place in a multi-step manner. The adsorption of O2 on the cathode surface is followed by the dissociation into two O atoms, and the surface diffusion to the three-phase boundary region. The O atoms take part in a number of electron transfer steps, reducing O to 0 . The rate limiting process, however, has not yet been identified conclusively. The overall oxygen reduction reaction and the incorporation of the ions into the electrolyte can be written in Kroger-Vink notation as... 

A widespread interest for the electrochemical oxygen reduction reaction (ORR) has two aspects. The reaction attracts considerable attention from fundamental point of view, as well as it is the most important reaction for application in electrochemical energy conversion devices. It has been in the focus of theoretical considerations as four-electron reaction, very sensitive to the electrode surface structural and electronic properties. It may include a number of elementary reactions, involving electron transfer steps and chemical steps that can form various parallel-consecutive pathways [1-3]. 

In summary, theoretical predicted activation energies are in tine with experimental numbers. Activation energy studies suggest that with Pt as the catalyst, proton transfer precedes O2 dissociation and is involved in the rate determining step of the oxygen reduction reaction. Without catalysts, the third-eleetron transfer step has the largest activation energy, followed by the lirst-eleetron transfer step. An efficient ORR catalyst should activate the first- and third-electron transfer steps. 

Electron-transfer chains in plants differ in several striking aspects from their mammalian counterparts. Plant mitochondria are well known to contain alternative oxidase that couples oxidation of hydroquinones (e.g., ubiquinol) directly to reduction of oxygen. Semiquinones (anion-radicals) and superoxide ions are formed in such reactions. The alternative oxidase thus provides a bypass to the conventional cytochrome electron-transfer pathway and allows plants to respire in the presence of compounds such as cyanides and carbon monoxide. There are a number of studies on this problem (e.g., see Affourtit et al. 2000, references therein). 

This chapter mainly focuses on the reactivity of 02 and its partially reduced forms. Over the past 5 years, oxygen isotope fractionation has been applied to a number of mechanistic problems. The experimental and computational methods developed to examine the relevant oxidation/reduction reactions are initially discussed. The use of oxygen equilibrium isotope effects as structural probes of transition metal 02 adducts will then be presented followed by a discussion of density function theory (DFT) calculations, which have been vital to their interpretation. Following this, studies of kinetic isotope effects upon defined outer-sphere and inner-sphere reactions will be described in the context of an electron transfer theory framework. The final sections will concentrate on implications for the reaction mechanisms of metalloenzymes that react with 02, 02 -, and H202 in order to illustrate the generality of the competitive isotope fractionation method. 

Whereas redox reactions on metal centres usually only involve electron transfers, many oxidation/reduction reactions in intermediary metabolism, as in the case above, involve not only electron transfer, but hydrogen transfer as well — hence the frequently used denomination dehydrogenase . Note that most of these dehydrogenase reactions are reversible. Redox reactions in biosynthetic pathways usually use NADPH as their source of electrons. In addition to NAD and NADP+, which intervene in redox reactions involving oxygen functions, other cofactors like riboflavin (in the form of flavin mononucleotide, FMN, and flavin adenine dinucleotide, FAD) (Figure 5.3) participate in the conversion of [—CH2—CH2— to —CH=CH—], as well as in electron transfer chains. In addition, a number of other redox factors are found, e.g., lipoate in a-ketoacid dehydrogenases, and ubiquinone and its derivatives, in electron transfer chains. 

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