Hydrogen peroxide decomposition hydroxyl radicals

Yields and kinetics depend on the type and number of Ti species and the crystal size of the catalyst used. Ti distribution between lattice (selective) and extra-lattice (unselective) sites is, in turn, closely linked to synthesis and characterization procedures, both of which require special thoroughness [4]. Inadequate characterization and, therefore, the impossibility of clear assessment of siting of Ti in the catalyst, is a frequent obstacle to a correct evaluation of the literature, especially early publications. These considerations are of general value, but are central to the hydroxylation of phenol where extra-framework species are a major source of hydrogen peroxide decomposition and radical chain oxidations. The hydroxylation of phenol was indeed proposed by three different groups as an additional test to assess the purity of TS-1 [2, 9, 11]. Van der Pool et al. estimated from Weisz... 

The free radicals that form upon decomposition also possess great oxidizing power and are very reactive with organics present in the pool or spa environment. Ozone will oxidize bromide ion to bromine and bromate, chloride ion to chlorine and hydrogen peroxide forming hydroxyl radicals. 

Metal-ion catalysis of hydrogen peroxide decomposition can generate perhydroxyl and hydroxyl free radicals as in Scheme 10.26 [235]. The catalytic effects of Fe2+ and Fe3+ ions are found to be similar [235]. It is not necessary for the active catalyst to be dissolved [237], as rust particles can be a prime cause of local damage. The degradative free-radical reaction competes with the bleaching reaction, as illustrated in Scheme 10.27 [237]. Two adverse consequences arise from the presence of free radicals ... 

The superoxide anion radical and hydrogen peroxide are not particularly harmful to cells. It is the product of hydrogen peroxide decomposition, the hydroxyl radical (HO ), that is responsible for most of the cytotoxicity of oxygen radicals. The reaction can he catalyzed hy several transition metals, including copper, manganese, cohalt, and iron, of which iron is the most ahimdant in the human body (Reaction 2 also called the Fenton reaction). To avoid iron-catalyzed reactions, iron is transported and stored chiefly as Fe(III), although redox active iron can be formed in oxidative reactions, and Fe(III) can be reduced by semiquinone radicals (Reaction 3). 

The entire complex situation is untypical of hydrogen peroxide. Decomposition of H202 starts from dissociation by O—O-bond to hydroxyl radicals, which then in gas-phase (refer to Chapter 4) and liquid-phase (refer to Chapter 6) decomposition lead to final products H20 and 02. Here one deals with a single complex reaction with a definite set of subsequently proceeding elementary reactions, but not with several sets as for decomposition of organic peroxides. 

Similar results were obtained with vinyl acetate 89). They confirm the endothermal character of hydrogen peroxide decomposition into two hydroxyl radicals. A higher concentration of free radicals leads to a more intensive initiation and termination. 

It is possible that some acetate radicals are formed by the direct discharge of the ions as, it will be seen shortly, is the case in non-aqueous solutions but an additional mechanism must be introduced, such as the one proposed above, to account for the influence of electrode material, catalysts for hydrogen peroxide decomposition, etc. It is significant that the anodes at which there is no Kolbe reaction consist of substances that are either themselves catalysts, or which become oxidized to compounds that are catalysts, for hydrogen peroxide decomposition. By diverting the hydroxyl radicals or the peroxide into an alternative path, viz., oxygen evolution, the efficiency of ethane formation is diminished. Under these conditions, as well as when access of acetate ions to the anode is prevented by the presence of foreign anions, the reactions mentioned above presumably do not occur, but instead peracetic acid is probably formed, thus,... 

In non-aqueous solutions the Kolbe electrosynthesis takes place with high eflSciency at platinized platinum and gold, as well as at smooth platinum, anodes increase of temperature and the presence of catalysts for hydrogen peroxide decomposition, both of which have a harmful effect in aqueous solution, have relatively little influence. The mechanism of the reaction is apparently quite different in non-aqueous solutions and aqueous solutions in the former no hydroxyl ions are present, and so neither hydroxyl radicals nor hydrogen peroxide can be formed. It is probable, therefore, that direct discharge of acetate ions occurs at a potential which is almost independent of the nature of the electrode material in a given solvent. The resulting radicals probably combine in pairs, as in aqueous solution, to form acetyl peroxide, which subsequently decomposes as already described. ... 

A paper by Gierer et al. [63] probably resolves the mechanism. These workers found that a number of hydroxy- or methoxy-substituted stilbenes would react with hydroxyl radicals, or with a combination of superoxide and hydroxyl radicals. The formation of superoxide and hydroxyl radicals by hydrogen peroxide decomposition is hard to prevent and incomplete chelation of metal ions might explain the inconsistent results reported by the earlier workers. 

Scheme 3.26 Pictorial illustration of hydroxyl radical bovmdto iron oxide surface after hydrogen peroxide decomposition.

In fact, this overall reaction does not reflect the radical nature of hydrogen peroxide decomposition, which is accompanied by the formation of significant concentrations of hydroxyl radicals. It is the secondary reactions of the hydroxyl radical. 

The resulting hydrogen peroxide undergoes metal-mediated decomposition to yield hydroxyl radical (Eq. 4, Scheme 8.36). Metals such as Cu(I) and Ee(II) can... 

The evolution of fluoride ions in actual fuel cell effluent and during laboratory accelerated life studies has been reported. One common example of radical generation from peroxide decomposition is in the Fenton test, where peroxyl or hydroxyl radicals can be formed through the reaction of hydrogen peroxide with Fe(II) (Scheme 3.2). 

Thus, antioxidant effects of nitrite in cured meats appear to be due to the formation of NO. Kanner et al. (1991) also demonstrated antioxidant effects of NO in systems where reactive hydroxyl radicals ( OH) are produced by the iron-catalyzed decomposition of hydrogen peroxide (Fenton reaction). Hydroxyl radical formation was measured as the rate of benzoate hydtoxylation to salicylic acid. Benzoate hydtoxylation catalyzed by cysteine-Fe +, ascorbate - EDTA-Fe, or Fe was significantly decreased by flushing of the reaction mixture with NO. They proposed that NO liganded to ferrous complexes reacted with H2O2 to form nitrous acid, hydroxyl ion, and ferric iron complexes, preventing generation of hydroxyl radicals. 

The other important property affecting lipid oxidation is the chelating effect of chlorogenic acids. It is important to keep in mind that the influence of biometals (Fe, Cu etc.) on lipid free radical oxidation is essential. It is well known that iron can react with hydrogen peroxide by the Fenton reaction (Equation 3). The hydroxyl radical formed in the Fenton reaction is capable of reacting with lipid and PUFA as the initiation stage. Iron can also participate in alkyl peroxide or lipid peroxide decomposition. Therefore, the nature of iron chelation in a biological system is an important aspect in disease prevention. 

Martin et al. (1989) studied the oxidation of HMSA by Fenton s reagent and investigated the decomposition of both hydrogen peroxide and HMSA. They determined an estimate of the absolute rate of reaction between HMSA and hydroxyl radicals. The decomposition of hydrogen peroxide follows the first-order kinetics and can be described as follows ... 

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