Oxygen reduction reaction peroxide formation

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.

Depending on the amount and the distribution of selenium, up to 5% hydrogen peroxide formation has been observed in the oxygen reduction reaction. As hydrogen peroxide leads to a rapid degradation of the membrane and the gas diffusion media and enhances the corrosion of the carbon catalyst support and finally to a loss of performance due to this side reaction, the formation of hydrogen peroxide has to be minimized. Further details on the effect of particle size, particle distribution and the effect of the carbon support on the hydrogen peroxide production has been mentioned above. 

It should also be noted that the platinum surface is very sensitive to the presence of species in solution and to electrode pre-treatments (anodization, pre-reduction). Damjanovic etal reported a very strong dependence of the reaction pathway on the purity of the solution. They concluded that the oxygen reduction reaction occurred without hydrogen peroxide intermediate formation on a pre-reduced platinum electrode, and therefore that the production of hydrogen peroxide was effective only on sites affected by the presence of adsorbed impurities. 

O Grady et al. [227] studied the reduction and evolution of oxygen on RuO on titanium and reported that the cathodic reaction involves the formation of peroxide in solution. From y 1/2 vs. rotating disk electrodes, the authors concluded that the reaction 02/H202 was in equilibrium at the surface [227], Miles et al. [428] also studied the oxygen electrode reactions on several metal oxides (Ir, Ru, Pd, and Rd)... 

Figure 2 Peroxidation of oolyunsaturated fatty acids exemplified with a -linolenic acid (=0 -lin). In this scheme radicals are produced by the presence of protoporphyrin IX. In the light, either singlet oxygen may be generated or a superoxide anion provided a suitable redox reaction occurs in case an appropriate reductant is available (part A II, reactions 1,2). Singlet oxygen may lead to formation of a peroxo compounds (of --linolenic acid, part B 1/1) the protoporphyrin IX-radical may form a hydroxy radical HO via superoxide, see part 1/2 (41,42). Both, peroxo linolenic acid and the hydroxy radical will initiate a radical chain reaction (part B, II, III) with linolenic acid leading to the OJ-lin radical possibly in two ways as indicated by the arrows numbered (1) and (2). This radical supports the chain reaction.

The cyclic oxidation and reduction of the iron and copper in the oxygen reduction center of cytochrome c oxidase, together with the uptake of four protons from the matrix space, are coupled to the transfer of the four electrons to oxygen and the formation of water. Proposed Intermediates in oxygen reduction Include the peroxide anion Oz ) and probably the hydroxyl radical (OH-) as well as unusual complexes of iron and oxygen atoms. These intermediates would be harmful to the cell if they escaped from the reaction center, but they do so only rarely. 

Oxygen reduction uccurs through the formation or peroxide intermediates. Vetier14 has reviewnd oxygen redox reactions at metal electrode surfaces, On platinum die current for calhedic reduction is... 

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