Methane radiolysis

The radiolysis of methane in the solid phase has been examined in several studies. ESR studies by Smaller and Matheson and by Wall eta/. have shown that methyl radicals and hydrogen atoms are formed in nearly equal quantities and that Gchj = 0.9. A product analysis in a study by Ausloos et shows that hydrogen and ethane are almost the only products of solid methane radiolysis at 20 or 77 °K. It seems that ethane is formed by both methylene insertion and by methyl radical combination, while hydrogen is formed by direct elimination and by bimolecular processes. A small quantity of ethylene formed at 20 °K is absent at 77 °K this has been attributed to the reaction of hydrogen atoms with ethylene at 77 °K (but not at 20 °K) to give ethyl radicals and finally higher products which are observed in increased yield at 77 °K. 

We have carried out methane radiolysis experiments under widely varying conditions (9). The most important results are obtained either by homogeneous gamma ray irradiations with a Co source or by heterogeneous irradiations with this same gamma ray source or with fission fragments. Alumina (CAU), silica (CSU), and uranium impregnated charcoal (ECU) have been used as microporous solids. 

In any case the existence of a large surface is a requisite for a rate increase. Nevertheless, this condition is not sufficient in itself because inhibition occurs in the presence of active charcoal. This inhibition by active charcoal has been mentioned previously in the case of methane radiolysis. This example illustrates very well the influence of the nature of the surface on the course of the reaction. Similar observations have been made in the case of thermal and catalytic polymerization of styrene, acrylonitrile, and ethylene (4 -44) ... 

The production of unsaturated hydrocarbons as final products in methane radiolysis, is also controversial. The results of numerous workers (10, 11, 18, 20, 21, 25, 30, 31, 40, 42, 48, 49), who have reported unsaturated hydrocarbon products from the radiolysis of pure methane, show no general agreement as to the products or their yields. 

Figure 4. Effect of electric field in the radiation cell on the production of Hz and from methane radiolysis...

Though it was clear from general considerations that the neutralization of ions must contribute to the radiolyses yield, direct evidence has only recently been obtained [262] on the basis of studies on the electric field effect on methane radiolysis. It has been found that 30% of the whole amount of hydrogen are formed by recombination of positive ions with electrons or with negative ions. The latter appear either in the interaction of electrons with methane molecules, e -f- CH4 = CHj + H (or CHg -j- H ), or owing to direct attachment of electrons to particles possessing positive electron affinity. 

The above methane radiolysis products can also be produced in radical reactions. The contribution from these reactions to radiolysis yields has been investigated by the acceptor method, i.e. by introducing species capable of capturing radicals. From the data on the radiolysis of CH4—CD4 mixtures it has been concluded that ethane is generated as a result... 

The yield of excited particles in methane radiolysis is 2.3 which is comparable to that of ions [303]. However, the mechanisms of processes involving excited particles and producing methane radiolysis products have still not yet completely been elucidated. 

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