Surface radical chain oxidation

Whatever the initial step of formation of surface silyl radicals, the mechanism for the oxidation of silicon surfaces by O2 is expected to be similar to the proposed Scheme 8.10. This proposal is also in agreement with the various spectroscopic measurements that provided evidence for a peroxyl radical species on the surface of silicon [53] during thermal oxidation (see also references cited in [50]). The reaction being a surface radical chain oxidation, it is obvious that temperature, efficiency of radical initiation, surface precursor and oxygen concentration will play important roles in the acceleration of the surface oxidation and outcome of oxidation. 

Vitamin E and the highly substituted phenol derivatives BHA and BHT function as inhibitors of the radical-chain oxidation of lipids. Vitamin C also is an antioxidant, capable of regenerating vitamin E at the surface of cell membranes. 

Like most other engineering thermoplastics, acetal resins are susceptible to photooxidation by oxidative radical chain reactions. Carbon—hydrogen bonds in the methylene groups are principal sites for initial attack. Photooxidative degradation is typically first manifested as chalking on the surfaces of parts. 

A PP sample after ozonization in the presence of UV-irradiation becomes brittle after 8 hrs of exposure, whereas the same effect in ozone is noticeable after 50-60 hours.Degradation of polymer chain occurs as a result of decomposition of peroxy radicals. The oxidation rapidly reaches saturation, suggesting the surface nature of ozone and atomic oxygen against of PP as a consequence of limited diffusion of both oxygen species into the polymer. Ozone reacts with PP mainly on the surface since the reaction rate and the concentration of intermediate peroxy radicals are proportional to the surface area and not the weight of the polymer. It has been found that polyethylene is attacked only to a depth of 5-7 microns (45). 

Catalyst accelerates the decomposition of hydroperoxide to free radicals on the surface. The free radicals then diffuse into the reactant bulk and initiate the chain oxidation of the oxidized substance. 

Catalytic surface is active toward hydroperoxide and decomposes it to free radicals. The free radicals initiate the chain oxidation of RH in the liquid phase. 

Such a dependence was interpreted within the scope of the model of chain oxidation with diffusionally controlled chain termination on the surface of solid antioxidant (for example, Mo or MoS2). According to the Smolukhovsky equation, the diffusion velocity of radical R02 at the distance //2 is v = 0.2DkS13, where S is the surface of the solid inhibitor and k is the coefficient of proportionality between the surface and number n of the solid particles (S=k x ). The function F for such diffusionally controlled chain termination is the following ... 

Since a radical is consumed and formed in reaction (3.3) and since R represents any radical chain carrier, it is written on both sides of this reaction step. Reaction (3.4) is a gas-phase termination step forming an intermediate stable molecule I, which can react further, much as M does. Reaction (3.5), which is not considered particularly important, is essentially a chain terminating step at high pressures. In step (5), R is generally an H radical and R02 is H02, a radical much less effective in reacting with stable (reactant) molecules. Thus reaction (3.5) is considered to be a third-order chain termination step. Reaction (3.6) is a surface termination step that forms minor intermediates (T) not crucial to the system. For example, tetraethyllead forms lead oxide particles during automotive combustion if these particles act as a surface sink for radicals, reaction (3.6) would represent the effect of tetraethyllead. The automotive cylinder wall would produce an effect similar to that of tetraethyllead. 

In the development of effective catalytic oxidation systems, there is a qualitative correlation between the desirability of the net or terminal oxidant, (OX in equation 1 and DO in equation 2) and the complexity of its chemistry and the difficulty of its use. The desirability of an oxidant is inversely proportional to its cost and directly proportional to the selectivity, rate, and stability of the associated oxidation reaction. The weight % of active oxygen, ease of deployment, and environmental friendliness of the oxidant are also key issues. Pertinent data for representative oxidants are summarized in Table I (4). The most desirable oxidant, in principle, but the one with the most complex chemistry, is O2. The radical chain or autoxidation chemistry inherent in 02-based organic oxidations, whether it is mediated by redox active transition metal ions, nonmetal species, metal oxide surfaces, or other species, is fascinatingly complex and represents nearly a field unto itself (7,75). Although initiation, termination, hydroperoxide breakdown, concentration dependent inhibition... 

Effect of S/V Ratio. The reactor wall plays two important roles in this oxidation. First, the reactor surface promotes the initiation of radical chain (12), but if it is exceedingly large, it adversely affects the total rate of the oxidation reaction (6, 12). Second, it accelerates heterogene-... 

The mixed behaviour of such catalysts, in terms of oxo-type and allylic oxidation, was also confirmed in the oxidation of a-pinene, yielding a mixture of the epoxide and the allylic oxidation product (D-verbenone). The epoxide stems from the existence of a high valent Ru(V)=0 intermediate, while D-verbenone formation points to the presence of a radical chain involving peroxoruthenium as intermediated128,1291 The activity of encapsulated H, Cl, Br, nitro-substituted Ru on and Co(salophen) (structure of ligand see insert also known as saloph) is always at least a factor of two higher than in solution. Comparable Co/Si ratios are obtained from XPS and TGA, indicated no significant amounts of complex at the external surface. 

A similar kinetic expression was found by Hong et al. [132] for the catalytic, photochemical oxidation of S(IV) on Ti02. In this case, for k < 385 nm, quantum yields in excess of unity (e.g., 0.5 < free-radical chain reactions (i.e., reactions 79 to 84). The observed quantum yields, which ranged between 0.5 and 300, depended on the concentration and nature of free-radical inhibitors present in the heterogeneous suspension. 

Specihcally with regard to the pyrolysis of plastics, new patents have been filed recently containing variable degrees of process description and equipment detail. For example, a process is described for the microwave pyrolysis of polymers to their constituent monomers with particular emphasis on the decomposition of poly (methylmethacrylate) (PMMA). A comprehensive list is presented of possible microwave-absorbents, including carbon black, silicon carbide, ferrites, barium titanate and sodium oxide. Furthermore, detailed descriptions of apparatus to perform the process at different scales are presented [120]. Similarly, Patent US 6,184,427 presents a process for the microwave cracking of plastics with detailed descriptions of equipment. However, as with some earlier patents, this document claims that the process is initiated by the direct action of microwaves initiating free-radical reactions on the surface of catalysts or sensitizers (i.e. microwave-absorbents) [121]. Even though the catalytic pyrolysis of plastics does involve free-radical chain reaction on the surface of catalysts, it is unlikely that the microwaves on their own are responsible for their initiation. 

It has been observed that the activation energy for thermal decomposition of ethyl nitrate is substantially reduced in the presence of lead oxide or copper surfaces. Above 200° C approximately, the thermolysis of ethyl nitrate becomes much more complex and detonation in the gas phase is common. In the range 242-260° C the reaction was found to be half-order, with an overall rate coefficient" k = 10 - exp(—46,800)/J 7 mmole. l . sec apparently the initiation step is unaltered but the subsequent radical chain mechanism affects the overall rate of decomposition. 

One of the few cases in which hydrogenated surface complexes have been suggested to be active involves the decomposition of nitrous oxide.The reaction was suggested to involve hydrogen in the carbon, although the reaction mechanism was written in terms of gas phase hydrogen involved in a free radical chain reaction. As a result, the importance of hydrogenated complexes is open to question. 

Although ozone is quite reactive, most photochemical oxidations in the atmosphere (as in surface waters) involve still more reactive free radicals. Free radicals are species that contain an unpaired electron therefore, the reaction of any free radical with a chemical other than another free radical still leaves an electron unpaired. The newly formed free radical may readily react with another chemical, forming yet another free radical, and so on. An example of such a free-radical chain reaction is a combustion flame. A free radical is destroyed only when reaction with another free radical causes the two unpaired electrons to pair with each other. 

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