Oxygen electroreduction reaction mechanism

Nowadays, it has been demonstrated that the reaction is indeed structure sensitive with a multielectron transfer process that involves several steps and the possible existence of several adsorption intermediates [93-96]. The main advantage that we have with the new procedures with respect to cleanliness is that we have well-ordered surfaces to study a complex mechanism such as the oxygen electroreduction reaction [96-99]. In aqueous solutions, the four-electron oxygen reduction appears to occur by two overall pathways a direct four-electron reduction and a peroxide pathway. The latter pathway involves hydrogen peroxide as an intermediate and can undergo either further reduction or decomposition in acid solutions to yield water as the final product. This type of generic model of a reaction has been extensively studied since the early 1960s by different authors [100-108]. 

Zinola, C. F. Triaca, W. E. Arvia, A. J. Kinetics and mechanism of the oxygen electroreduction reaction on faceted platinum-electrodes in trifluoromethane-sulfonic acid solutions. J. Appl. Electrochem. 1995, 25, 740-754. 

These mechanisms could explain the fact that, on such surfaces, the kinetics of the oxygen electroreduction reaction into hydrogen peroxide is higher in an alkaline medium than in an acid medium. Indeed, if the S-O2 species is likely to be stable in an alkaline medium, the free energy for the formation of this species is certainly higher in an acid medium. The formation of such an entity must involve the simultaneous transfer of an electron and a proton, which affects the kinetics of the process. 

The change in transition states with electrode potential is observed, for instance, in the complex 4-electron reaction of oxygen electroreduction on several electrode materials, the mechanism of which may also change with pH. At low negative overpotentials, the rds is reaction (124) with reaction (123) in pre-equilibrium. 

The Most Likely Intermediate Species Involved in the Oxygen Electroreduction Mechanism on Pt(111) and Pt( 100) Clusters The adsorption configuration study for the oxygen adsorbates on platinum, described above, provides the possibility of exploring further mechanistic aspects of the oxygen electroreduction, particularly in relation to the possible species involved in the reaction that account for the different behavior of Pt(l 11) and Pt(100) electrode surfaces. 

The mechanism of the reactions in such a system is not clear at present the authors, however, presume that the oxidation of the P450 substrate occurs via intermediates (probably of a peroxide nature) arising during oxygen electroreduction. 

In spite of the considerable effort expended in trying to unravel the fundamental aspects of the O2 electroreduction reaction, many details about the mechanism are not fully understood. The electrochemical reduction of oxygen is a multielectron reaction that occurs via two main pathways one involving the transfer of two electrons to give peroxide, and the so-called direct four-electron pathway to give water. The latter involves the rupture of the 0-0 bond. The nature of the electrode strongly influences the preferred pathway. Most electrode materials catalyze the reaction via two electrons to give peroxide Peroxide pathway in acid... 

Platinum/ platinum family metals/ and silver are classified in the second group. The oxygen electroreduction on these metals occurs both directly to water and via intermediate formation of hydrogen peroxide. The hydrogen peroxide formed in the parallel reaction is decomposed by chemical or electrochemical mechanisms. The relation between the rates of these processes depends in a complicated way on the nature of the metal, the potential, the coverage of the electrode by the chemisorbed species, and their adsorption character. The polarization curves are characterized by a single wave with a limiting current close to the diffusion current for the four-electron process. 

This type of reaction is often called the branched mechanism and corresponds to many real systems such as, for example, the electroreduction of oxygen. At platinum electrodes, a mechanism that explains the experimental data is17... 

Solvent effects on the redox potentials and electroreduction mechanisms of (TPP)CrCl have been discussed by Bas-solo and Hoffman [151, 152], as well as by Bottomley and Kadish [153]. The former research group demonstrated that (TPP)CrCl can readily coordinate with Lewis bases containing oxygen, sulfur or nitrogen donor atoms to form six-coordinate Cr(III) species of the type (TPP)CrCl(L), while the latter group indicated that the Cr(III)/Gr(II) process was accompanied by chemical reactions coupled with reversible electron transfers. The current-voltage curves led the authors to propose the electron-transfer mechanism shown in Sch. 1. 

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