Peroxide pathway

Whilst PUFAs can be oxidised enzymatically within cells by the above mentioned reactions involving free radicals to yield prostaglandins and leukotrienes, it is important to stress that they can also be oxidised non-enzymatically to yield a variety of carbonyls. This latter mechanism involves the formation of acyclic fatty-acyl hydroperoxides through a radical-mediated peroxidative pathway. 

The relative importance of the various heterogeneous oxidation pathways depends on pH. At pH values below —4.5 the hydrogen peroxide pathway typically dominates. In urban areas hydrogen peroxide may not be abundant enough to be the most important oxidant. Here transition metal catalysts can enhance the rates considerably, especially if there are alkaline materials from fly ash or ammonia to neutralize the growing acidity of droplet phases, which otherwise limits SO2 solubility. 

Two-electron peroxide pathway Four-electron pathway ... 

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]. 

Criegee, R. (1962). Peroxide pathways in ozone reactions. In Peroxide Reaction Mechanisms" (J. O. Edwards, ed.), pp. 29-39. Wiley (Interscience), New York. 

A variety of compounds such as hydrocarbons, alcohols, furans, aldehydes, ketones, and acid compounds are formed as secondary oxidation products and are responsible for the undesirable flavors and odors associated with rancid fat. The off-flavor properties of these compounds depend on the structure, concentration, threshold values, and the tested system. Aliphatic aldehydes are the most important volatile breakdown products because they are major contributors to unpleasant odors and flavors in food products. The peroxidation pathway from linoleic acid to various volatiles is determined in several researchs, - by using various techniques (Gas chromatography mass spectrometry, GC-MS, and electron spin resonance spectroscopy, ESR), identified the volatile aldehydes that are produced during the oxidation of sunflower oil. In both cases, hexanal was the major aldehyde product of hydroperoxide decomposition, whereas pentanal, 2-heptenal, 2-octenal, 2-nonenal, 2,4-nonadienal, and 2,4-decadienal were also identified. 

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... 

In alkaline solution the peroxide pathway is dominant and relatively fast. Organic or inorganic impurities at the surface favor this pathway. Generally, Pt is the most active catalyst. The reduction kinetics are faster in concentrated KOH or NaOH than in concentrated H3PO4 or H2SO4. At highly dispersed Pt on carbon support, the reduction occurs predominantly via the four-electron pathway. 

As observable from figures 5 and 6, the amide specific band absorptions for proteins, amide I band around 1654 cm-i and amide II band around 1541 cm" (Firth et al. 2008 Banuelos et al. 1995) are not changed when LDL was deposited on the gold support. This observation is important because it proved that the secondary structure of protein is preserved subsequent deposition therefore it can be concluded that the deposition on solid support did not affect theLDL functionality, and, consequently, that deposed LDL is expected to react with free radicals according to the same pathway as free LDL. Moreover, it should be mentioned that this argument is consistent with the data published by Paker (Paker, 1991) where it is mentioned that LDL ex vivo peroxidation pathway is similar as in vivo peroxidation pathway. 

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... 

In the so-called series two-electron or peroxide pathway, the reaction is represented by... 

The O2 reduction may be carried out in either alkaline or acidic solution. Depending on the electrode surface the reduction proceeds either predominantly by FIO or by both the 4e- and peroxide pathways. These mechanisms are discussed elsewhere... 

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]. 

In fact, the ORR is an extremely complicated reaction that is dependent upon the electronic structure and number of the active sites on the catalyst surface, which in turn are influenced by many factors including the size and shape of the catalyst particle, the presence of any contaminant species competing for reaction sites, the presence of oxides which may be formed during potential cycling, the effect of the catalyst support, etc. In addition to the 4-electron reaction, there is the possibility for the 2-electron peroxide pathway [3],... 

In low-temperature PEMFCs, those that operate in the range of 65-80°C, the peroxide pathway is problematic because if the peroxide is not quickly decomposed into water, it can form radicals that chemically attack the membrane, causing premature failure. 

---