Oxygen outer-sphere reduction

Anderson AB, Alhu TV. 1999. Ah initio determination of reversible potentials and activation energies for outer-sphere oxygen reduction to water and the reverse oxidation reaction. JAm Chem Soc 121 11855-11863. 

In this notation, anodic current is positive, while cathodic current is negative. As the later section on oxygen reduction will show, the Tafel slope can change with overpotential. This is because the Butler-Volmer law only applies to outer-sphere reactions. Although it can describe electrode reactions, the equation does not account for repulsive interactions of the adsorbates or changes in the reaction mechanism as potential is changed. 

Reactivity of Fe(III)(hydr)oxide as measured by the reductive dissolution with ascorbate. "Fe(OH)3" is prepared from Fe(II) (10 4 M) and HCO3 (3 10 4 M) by oxygenation (po2 = 0.2 atm) in presence of a buffer imidazd pH = 6.7 (Fig. a) and in presence of TRIS and imidazol pH = 7.7 (Fig. b). After the formation of Fe(III)(hydr)oxide the solution is deaerated by N2, and ascorbate (4.8 10 2 M) is added. The reactivity of "Fe(OH)3 differs markedly depending on its preparation. In presence of imidazole (Fig. a) the hydrous oxide has properties similar to lepidocrocite (i.e., upon filtration of the suspension the solid phase is identified as lepidocrocite). In presence of TRIS, outer-sphere surface complexes with the native mononuclear Fe(OH)3 are probably formed which retard the polymerization to polynuclear "Fe(OH)3" (von Gunten and Schneider, 1991). 

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. 

In the context of the present discussion, it is worth noting that virtually all the experimental systems that exhibit such "anomalous temperature-dependent transfer coefficients are multistep inner-sphere processes, such as proton and oxygen reduction in aqueous media [84]. It is therefore extremely difficult to extract the theoretically relevant "true transfer coefficient for the electron-transfer step, ocet [eqn. (6)], from the observed value [eqn. (2)] besides a knowledge of the reaction mechanism, this requires information on the potential-dependent work terms for the precursor and successor state [eqn. (7b)]. Therefore the observed behavior may be accountable partly in terms of work terms that have large potential-dependent entropic components. Examinations of temperature-dependent transfer coefficients for one-electron outer-sphere reactions are unfortunately quite limited. However, most systems examined (transition-metal redox couples [2c], some post-transition metal reductions [85], and nitrobenzene reduction in non-aqueous media [86]) yield essentially temperature-independent transfer coefficients, and hence potential-independent AS orr values, within the uncertainty of the double-layer corrections. 

The cathodic reduction of oxygen on catalytically active substrates is very inhibited by deposited hydrated oxide layers. Such is the case of Fe00H-xH20 (sandwich oxide electrode) on gold at pH 7.2 [94] and Ni(OH)2 deposited on platinum [447]. This inhibition is probably related to the electronic properties of the surface layer, since simple outer sphere couples are highly inhibited also (Sect. 5.3). 

The -CN radicals appear to dimerize rapidly to produce dicyanogen, NCCN. ) The reduction rate of [TPPFe (CN)2] depends on a variety of factors. Light and increased cyanide concentrations accelerate the reduction, while small amounts of water inhibit the reaction. Reoxidation of the dicyano Fe complex by molecular oxygen leads directly to the formation of the low-spin complex, as opposed to IJ.-OXO dimer formation, because of the presence of excess strong-held ligand (i.e. the reaction with O2 is probably outer-sphere). Autoreduction of a series of dicyanoiron(III) porphyrins studied by White-Dixon showed that... 

Anderson and Albu investigated an outer-sphere oxygen reduction with ab initio calculations, whereby the electrode was assumed to not directly interact with the intermediate species. They proposed four one-electron transfer steps involving proton transfer from the electrolyte as ... 

Oxidases couple the oxidation of an organic substrate to the two- or four-electron reduction of O2, producing H2O2 or two molecules of H2O, respectively. Oxygen atoms from dioxygen are not incorporated into the product, unlike reactions catalyzed by oxygenases. Oxidase reactions may proceed via iimer-sphere or outer-sphere mechanisms. 

The thermodynamic and kinetic data are summarized in Table 4. The equilibrium constants as listed in Table 4 are calculated from log K = A °/0.059 and A °= — (°M) + (°0i) and involve the standard reduction potentials of the melal couple and of oxygen at [02] = 1 M. This standard state is more suitable for kinetic calculations than the more widely used convention p0l = 1 atm. The calculation of thermodynamic equilibrium constants for the surface complexes if more difficult (Sposito, 1983) and requires further work. Figure 8 displays the resulting LFER. A theoretical line of slope one (its the data over a broad range of 13 log k units. This analysis supports an outer-sphere mechanism for the... 

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