Sulphur oxidation mechanism

Mechanism and sulphur oxidation Apart from its intrinsic interest the economic importance of acid corrosion and more lately interest in ore leaching, has stimulated considerable work on the oxidation of sulphur, Fe and Mn. It must be stressed that the Thiobacilli are obligate aerobes, i.e. that depend on molecular oxygen as a terminal electron acceptor. Possible reactions for the oxidation of sulphur are... 

The sulphite aftertreatment is particularly important with permonosulphuric acid treatment. Evidence for the underlying mechanism is available from analysis of sulphur oxidation products formed in the various processes (Table 10.34). It is evident from these results that the concentration of RSS()5 anionic groups necessary to change the hydration of the fibre surface is achieved by the reaction of bisulphite with cystine monoxide residues to give the required cysteine-S-sulphonate groups [311]. 

In the case that any of the mechanical elements of this section presented a failure, the formation of a concentrated acid puddle would occur with the simultaneous release of sulphur oxides. Subsequent to this release, and in the absence of mitigation systems, an acid cloud could form (Greenberg, 1991), extremely dangerous due to its high toxicity (LC50 = 320 mg/m3/2hr) (McAdams, 2006). 

Katoh, H., Kuniyoshi, I., Hirai, M., and Shoda, M. (1995). Studies of the oxidation mechanism of sulphur-containing gases on wet activated carbon fibre. Appl. Catal. B Environ., 6, 255-62. 

As has already been seen, these a priori lumped kinetic models account for the macroscopic properties of the reactions, such as cool flames, the delays of autoignition... but without being able to relate them to the chemical structure of the reactants. In other words, they are incapable of describing both qualitatively and quantitatively the formation of individual molecules, such as the toxic substances (carbon monoxide, aldehydes, butadiene, aromatics, PAHs, soot) or the tropospheric pollutants (nitrogen and sulphur oxides, unburnt hydrocarbons, various oxygenated compounds), produced by the burning of fuels. There is therefore a strong requirement to develop detailed reaction mechanisms, likely to predict both the kinetic and chemical characteristics of these reactions. 

The corresponding complexes with cysteine do not follow the same oxidation mechanism. In their reactions with H2O2, there is oxidation at the co-ordinated sulphur leading first to the highly reactive sulphenato-derivative (32) which is stabilized by the metal ion. Subsequent oxidation leads to the sulphinato-species (33). 

The effect of sulphur oxidation on the transmission mechanism of Vhh couplings in a series of 1,3-dithiane derivatives has been studied by Gauze et The results have been rationalized in terms of the hyperconjugation interactions involving the S=0 group the compounds studied were 1,3-dithiane-1-oxide, cj5-l,3-dithiane-l,3-dioxide, 1,3-dithiane-1,1,3-trioxide and 1,3-dithiane-l, 1,3,3-tetraoxide. 

The oxidation of an aldose to the next lower aldose with ammonium vanadate in sulphuric acid or perchloric acid has been mentioned in Chapter 2. Isbell and his co-workers have extended their studies of peroxide-cleavage reactions to include such carbocyclic compounds as 2,3,4,6/5-pentahydroxycyclohexanone (426) and x> /o-trihydroxycyclohexane-l,2,3-trione, which gave DL-glucaric acid (see Scheme 145) and xylaric acid, respectively, on oxidation. Mechanisms... 

A spectrophotometric study of the oxidation of aldoses by Ce(IV) in dilute sulphuric acid showed that the reactions are first order in both Ce(IV) and sugar. Similar kinetics for D-glucose, D-ribose and D-erythrose indicate a common oxidation mechanism involving cyclic (furanoid or pyranoid) forms in the rate determining radical formation step. Dissimilar kinetic and thermodynamic data for glyceraldehyde, on the other hand, point to a different oxidation pathway for the triose. Formation of an intermediate free radical species in the rate determining step has also been proposed for the Ce(IV) oxidation of L-rhanmose. ... 

It has been tentatively suggested that one mechanism underlies the Willgerodt reaction and the Kindler modification of it. A labile intermediate is first formed which has a carbon—carbon bond in the side chain. The scheme is indicated below it postulates a series of steps involving the addition of ammonia or amine (R = H or alkyl), elimination of water, re addition and eUmination of ammonia or amine until the unsaturation appears at the end of the chain then an irreversible oxidation between sulphur and the nitrogen compound may occur to produce a thioamide. 

NaCl, interact with the sulphur and vanadium oxides emitted from the combustion of technical grade hydrocarbons and die salt spray to form Na2S04 and NaV03- These conosive agents function in two modes, either the acidic mode in which for example, the sulphate has a high SO3 thermodynamic activity, of in the basic mode when the SO3 partial pressure is low in the combustion products. The mechanism of coiTosion is similar to the hot coiTosion of materials by gases widr the added effects due to the penetration of tire oxide coating by tire molten salt. 

As with c -polyisoprene, the gutta molecule may be hydrogenated, hydro-chlorinated and vulcanised with sulphur. Ozone will cause rapid degradation. It is also seriously affected by both air (oxygen) and light and is therefore stored under water. Antioxidants such as those used in natural rubber retard oxidative deterioration. If the material is subjected to heat and mechanical working when dry, there is additional deterioration so that it is important to maintain a minimum moisture content of 1%. (It is not usual to vulcanise the polymer.)... 

The early work of Kennerly and Patterson [16] on catalytic decomposition of hydroperoxides by sulphur-containing compounds formed the basis of the preventive (P) mechanism that complements the chain breaking (CB) process. Preventive antioxidants (sometimes referred to as secondary antioxidants), however, interrupt the second oxidative cycle by preventing or inhibiting the generation of free radicals [17]. The most important preventive mechanism is the nonradical hydroperoxide decomposition, PD. Phosphite esters and sulphur-containing compounds, e.g., AO 13-18, Table la are the most important classes of peroxide decomposers. 

As indicated above, when a positive direct current is impressed upon a piece of titanium immersed in an electrolyte, the consequent rise in potential induces the formation of a protective surface film, which is resistant to passage of any further appreciable quantity of current into the electrolyte. The upper potential limit that can be attained without breakdown of the surface film will depend upon the nature of the electrolyte. Thus, in strong sulphuric acid the metal/oxide system will sustain voltages of between 80 and 100 V before a spark-type dielectric rupture ensues, while in sodium chloride solutions or in sea water film rupture takes place when the voltage across the oxide film reaches a value of about 12 to 14 V. Above the critical voltage, anodic dissolution takes place at weak spots in the surface film and appreciable current passes into the electrolyte, presumably by an initial mechanism involving the formation of soluble titanium ions. 

Tantalum is severely attacked at ambient temperatures and up to about 100°C in aqueous atmospheric environments in the presence of fluorine and hydrofluoric acids. Flourine, hydrofluoric acid and fluoride salt solutions represent typical aggressive environments in which tantalum corrodes at ambient temperatures. Under exposure to these environments the protective TajOj oxide film is attacked and the metal is transformed from a passive to an active state. The corrosion mechanism of tantalum in these environments is mainly based on dissolution reactions to give fluoro complexes. The composition depends markedly on the conditions. The existence of oxidizing agents such as sulphur trioxide or peroxides in aqueous fluoride environments enhance the corrosion rate of tantalum owing to rapid formation of oxofluoro complexes. 

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