γ-Radiolysis of methane adsorbed on γ-alumina. Part 1.—Development of sites active in energy transfer

Abstract

The influence of surface sites on the transfer of γ-radiation energy between γ-alumina and adsorbed methane has been studied by analysing the yields and desorption temperatures of radiolysis products as a function of the γ-alumina pretreatment temperature. On γ-alumina outgassed below 570 K irradiation causes surface hydration to decompose but direct energy transfer to methane does not occur. On γ-alumina outgassed above 570 K however methane is efficiently radiolysed to products subsequently desorbable as C₂ and C₃ alkanes and alkenes together with hydrogen. As the outgassing temperature rises above 750 K the yields of the higher hydrocarbon products decline although the quantity of methane becoming strongly adsorbed continues to increase. Methane also undergoes a slow reaction with γ-alumina in the absence of radiation, and changes in reactivity and product selectivity similar to those in irradiated experiments are observed at the same two outgassing temperatures. Both 570 and 750 K moreover correspond to regions of particularly rapid weight loss during outgassing. These results are used to derive a coherent model of the successive stages in dehydration of the γ-alumina surface and to deduce the main mechanistic features of the radiolysis of adsorbed methane.