Oxygen evolution reaction, mechanistic studies

Surendranath Y, Kanan MW, Nocera DG (2010) Mechanistic studies of the oxygen evolution reaction by a cobalt-phosphate catalyst at neutral pH. J Am Chem Soc 132 16501-16509 McAlpin JG, Surendranath Y, Dinca M, Stich TA, Stoian SA, Casey WH, Nocera DG, Britt RD (2010) EPR evidence for Co(IV) species produced during waten oxidation at neutral pH. J Am Chem Soc 132 6882-6883... 

Surendranath Y, Kanan MW, Nocera DG (2010) Mechanistic studies of the oxygen evolution reaction by a cobalt-phosphate catalyst at neutral pH. J Am Chem Soc 132 16501-16509... 

Anthracene (404) and its derivatives are reported to yield 9,10-sndoperoxides (405). No mechanistic studies were made with respect to the influence of substituents and their positions on the reactivity of the anthracenes toward oxygen, except those discussed earlier. However, substituent effects have been observed with regard to the thermal stability of the anthracene endoperoxide and its thermal transformation reactions, which either lead to quinone formation or to evolution of oxygen and reformation of the hydrocarbon (Table XIII). 

The marked irreversibility of the oxygen evolution and reduction reactions in aqueous solutions has imposed severe limitations on the mechanistic information which can be obtained for both reactions. In general, at the current densities normally employed for kinetic studies, the current-potential data are insensitive to the back reaction, which normally occurs early on in the multi-step reaction sequence. Further, the reduction and oxidation processes are usually studied only at widely separated potentials. Thus, the surface conditions, whether in the case of metals or bulk oxides, probably differ sufficiently such that the reduction and oxidation pathways may not be complementary. The situation is complicated further by the large number of possible pathways for both reactions. 

Because of the irreversible and not well-understood change of the electrocatalyst surface above 1.0 V, early mechanistic studies were conducted under ill-defined conditions. Thus, while anodic evolution of Oj takes place always in the presence of oxygen-covered electrodes, the cathodic reaction proceeds on either oxygen-covered or oxygen free surfaces with different mechanisms (77,158). The electrochemical oxide path, proposed for oxide-covered platinum metals in alcaline electrolytes (759,160), has been criticized by Breiter (7), in view of the inhibition of oxygen reduction by the oxygen layers. Present evidence points to the peroxide-radical mechanism (77,... 

The kinetics of product evolution in a typical reaction of adamantane hydroxylation showed an initial induction period followed by a fast, apparently zero-order phase with the maximum rate and highest efficiencies (Fig. 2). Deviation from linear behavior took place only after 90% oxygen donor and 80% of the substrate had been consumed. When Ru (TPFPP)(0)2, prepared by reaction of Ru"(TPFPP)(CO) with 3-chloroperbenzoic acid was used as the catalyst, no induction time was detected and zero-order kinetics were observed as well. The well defined and characteristic UV-vls spectra of metalloporphyrins provide an invaluable tool for the mechanistic studies. Thus, monitoring the state of the metalloporphyrin catalysts during the course of both model reactions by UV-vis spectroscopy revealed that the initial form of the catalyst remained the predominant one throughout the oxidation, i.e. in the Ru°(TPFPP)(CO) catalyzed reaction c.a. 80% of the porphyrin catalyst existed as Ru"(TPFPP)(CO) and in Ru (TPFPP)(0)2 catalyzed reaction more than 90% of... 

Many reactions of industrial importance are electrocatalytic, i.e., they involve the specific adsorption of intermediates, for example hydrogen, chlorine, and oxygen evolution, oxygen reduction, and methanol or ethanol oxidation in fuel cells. Many different electrochemical techniques were used to study these reactions, and EIS is one of them, providing interesting kinetic and surface information. Certain model reactions will be presented in what follows with a detailed method of relating impedance parameters with mechanistic and kinetic equations. 

The oxidation of alkanes by r-butyl hydroperoxide (TBHP) has been catalysed by titanium alkoxides, producing the corresponding alcohols and ketones. A radical mechanism is proposed in which r-butoxyl radical formed from TBHP and titanium alkoxide initiates the reaction. The evolution of oxygen (from the decomposition of peroxide) and the abstraction of hydrogen from alkane to form alkyl radical occur competitively. A method for the determination of both the primary and secondary KIEs at a reactive centre based on starting-material reactivities allows the determination of the separate KIEs in reactions for which neither product analysis nor absolute rate measurements are applicable. It has been applied to the FeCls-catalysed oxidation of ethylbenzene with TBHP, which exhibits both a primary KIE and a substantial secondary KIE the findings are in accordance with previous mechanistic studies of this reaction. The oxidation of two l-arylazo-2-hydroxynaphthalene-6-sulfonate dyes by peroxy-acids and TBHP catalysed by iron(III) 5,10,15,20-tetra(2,6-dichloro-2-sulfonatophenyl)porphyrin [Fe(ni)P] is a two-step process. In single turnover reactions, dye and Fe(in)P compete for the initially formed OFe(IV)P+ in a fast reaction and OFe(IV)P is produced the peroxy acid dye stoichiometry is 1 1. This is followed by a slow phase with 2 1 peroxy acid dye stoichiometry [equivalent to a... 

Elucidating all the fascinating details of this reaction will require further mechanistic, structural, and model studies. Finally, the discovery of self-processing redox enzymes (see Section 6 see Metal-mediated Protein Modification) may be relevant to understanding aspects of the evolution of enzymes. Metal-ion mediated redox chemistry with oxygen can modify several amino acids, especially tyrosine, tryptophan, cysteine, and histidine. This may have provided a path to generate new redox cofactors prior to the advent of the complex biosynthetic pathways. 

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