Hydrogen from methane radiolysis

Fig. 64 Gas amounts evolved during radiolysis of polyether-polyurethane (EB irradiation electron energy 20 MeV). The data were taken from [07W1]. (A) carbon dioxide ( ) hydrogen (O) methane.

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 effect of electrical fields on the radiolysis of ethane has been examined by Ausloos et and this study has shown that excited molecules contribute a great deal to the products. The experiments were conducted in the presence of nitric oxide, and free-radical reactions were therefore suppressed. The importance of reactions (12)-(14) was clearly demonstrated by the use of various isotopic mixtures. Propane is formed exclusively by the insertion of CH2 into C2H6 and the yield is nearly equal to the yield of molecular methane from reaction (14). Acetylene is formed from a neutral excited ethane, probably via a hot ethylidene radical. Butene and a fraction of the propene arise from ion precursors while n-butane appears to be formed both by ionic reactions and by the combination of ethyl radicals. The decomposition of excited ethane to give methyl radicals, reaction (15), has been shown by Yang and Gant °° to be relatively unimportant. The importance of molecular hydrogen elimination has been shown in several studies ° °. ... 

The radiolysis of propane has been studied extensively in experiments that have included a wide range of techniques. The gas-phase radiolysis in the absence of inhibitors yields the products hydrogen, ethane, propene, 2,3-dimethylbutane, methane, ethylene, isobutane, acetylene, isopentane and n-butane as well as small quantities of butene-1, -pentane, 2-methylpentane and -hexane ° ° . At high conversions the yield of ethylene, propene, 2,3-dimethylbutane and isobutane are all reduced. The reduction in ethylene arises from hydrogen atom addition, while the reduction in the other products may arise from the reaction of propyl ions with propene to remove both C3H6 and the source of isopropyl radicals. 

In the y-radiolysis of water-isopropyl alcohol mixtures in the vapor phase, the total yields of hydrogen, methane, and carbon monoxide are linear over the whole concentration range, in the presence and absence of electron scavengers. The majority of the hydrogen produced from energy absorption by both water and isopropyl alcohol is formed by H-atom abstraction from isopropyl alcohol. Part of the H-atom yield is formed by electron-positive ion neutralization and part by processes not involving electrons. As far as the formation of H atoms is concerned, both electron-positive ion and ion-ion neutralizations appear to be independent of the composition of the positive ion cluster. A yield of molecular hydrogen is also present in both water and isopropyl alcohol. 

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