Peroxide pathway, oxygen reduction

Figure 5.2. Pathways of oxygen reduction. The sequential reduction of molecular oxygen, ultimately to water, is shown. Abbreviations 02 % superoxide free radical H202, hydrogen peroxide OH, hydroxide ion -OH, hydroxyl free radical e, electron H+, hydrogen ion. Singlet states of oxygen are also shown as i +02and Ag02.

Figure 6.15. Simplified schematic of the most important reaction pathways of the oxygen reduction reaction. The four-electron pathway results in the formation of water. The two-electron pathway forms hydrogen peroxide. Adsorption of molecular oxygen can form atomic oxygen (dissociative pathway) or form a superoxide species (associative pathway). The formation of Pt—OH and Pt— from water molecules represents the backward reactions of the later portion of the four-electron reduction pathway.

This second pathway, which is indirect oxygen reduction pathway, is commonly designated as the series pathway. The total number of exchanged electrons (nO involved for catalysts which performs the ORR following the series pathway can vary from two to four depending on the amount of water peroxide present in the reaction products (Eq. 45) ... 

In the case of iron phthalocyanine catalyst dispersed on carbon, infrared spectroscopy measurements enabled to confirm the series mechanistic pathway for the ORR in the high overpotential domain (below 0.7 V vs RHE). Two intermediates species, 02 and H02 and two products, water (majority product) and hydrogen peroxide could be identified. However, since only few studies are led on the oxygen reduction reaction by infrared spectroscopy, a confident attribntion of the different vibrational transitions observed to a particnlar intermediate remains difficult. 

A second problem arises for the protection to the reactive intermediates of the oxygen reduction pathway. A perfect protection should be achieved if oxidations with 02 in biological systems would be activated by biocatalysts which completely avoid any formation of free intermediate species. Though many enzymes catalyzing 02-reduction, like cytochromeoxidase (see 6.4.), seem to act in this manner, a complete suppression of free superoxide radical, Oj, and peroxide formation could not be attained. Thus, nature has evolved special enzyme systems acting as scavengers which catalyze rapid disproportion. There exist two types of enzymes superoxide-dismutases 183,184) an(j catalases185,186 Their catalytic function is described by ... 

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]. 

The reduction of oxygen is itself a complex electrochemical process. As described in Section 9.2, O2 reduction is considered to proceed along two parallel pathways, the direct four-electron pathway and the peroxide pathway (Eq. 9-9). Both pathways... 

The oxygen reduction reaction can proceed by two pathways in aqueous electrolytes. The first one, the so-called direct pathway, involves releasing four electrons per oxygen molecule to yield HgO. The indirect pathway involves releasing two electrons to yield hydrogen peroxide (HgOg) that in successive steps can produce water. Two further pathways, which are combinations of the above, can be envisaged. The series pathway implies sequential two or... 

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... 

Figure 1 shows electrochemical oxygen reduction mechanism in alkaline system. Oxygen reduction in alkaline system is cmisidered to proceed by two overall pathways. One is the direct 4-electron pathway (O2 + 4e + 2H2O = 40H ), and the other one is the peroxide pathway (O2 + 2e + H2O = H02 + OH ), which produces hydrogen peroxide as an intermediate product. The H02 is further reduced to OH by either electrochemical reaction or catalytical decomposition reaction. The reactions are dependent on the kind of electrocatalysts [4]. 

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