The influence of surface sites on the transfer of y-radiation energy between y-alumina and adsorbed methane has been studied by analysing the yields and desorption temperatures of radiolysis products as a function of the y-alumina pretreatment temperature. On y-alumina outgassed below 570 K irradiation causes surface hydration to decompose but direct energy transfer to methane does not occur. On y-alumina outgassed above 570K 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 y-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 y-alumina surface and to deduce the main mechanistic features of the radioiysis of adsorbed methane.
An earlier study showed that 7-alumina surfaces outgassed above 570 K contain sites involving exposed lattice ions at which methane is chemisorbed during 7-irradiation. When the species so formed are heated they decompose yielding C1--Cf alkanes and alkenes together with hydrogen. The present study investigates the kinetics of the reactions occurring during irradiation. These reactions are shown to be the activation of surface sites and the dissociative chemisorption of methane, in accord with the mechanism previously suggested. Overall product yields are chiefly determined by the rate at which excited charge carriers reach the surface, the highest rate observed being G(-CH,)= 2.0 but declining when fewer than -3 x lo1* m-2 chemisorption sites remain unoccupied. A kinetic scheme is proposed to account for the variation in yields with methane coverage, radiation dose and dose rate, and specific surface area of the y-alumina. It is also shown that the individual products formed when the precursors decompose depend on the configuration of the methane chemisorption sites, and so on the origin of the 7-alumina and the outgassing temperature used.Two subsidiary reactions are identified. The first of these resembles normal radiolysis but occurs at sites less accessible to methane. In the second, however, new surface species are formed when irradiation continues after either the methane or the chemisorption sites have been exhausted. These scavenge part of the adsorbed hydrocarbon material.
Previous studies showed that methane adsorbed on y-alumina undergoes radiolysis to form chemisorbed precursor species. These decompose when heated to give C1-C3 alkane and alkene products together with hydrogen. The present study uses nitric oxide, nitrous oxide, sulphur hexafluoride, oxygen and carbon dioxide as additives to interfere with product formation, and so allows probable structures to be deduced for each precursor. Both alkane and alkene precursors involve alkylaluminium groups which decompose by homolytic fission of the Al-C bond. The alkane precursor has an accessible hydroxide ion from which a hydrogen atom can be extracted during desorption, whereas the alkene precursor does not.Earlier parts of this study ' * showed that y-alumina surfaces outgassed above 570K contain exposed ion sites at which methane becomes chemisorbed during y-irradiation at 77 K. All this hydrocarbon material is recovered as C1-C3 alkanes and alkenes, together with hydrogen? when the y-alumina is reheated to the original outgassing temperature. The rate of methane consumption is determined by the rate at which excited charge carriers arrive at the surface and activate chemisorption sites, but the ratios between each product depend chiefly on the site configurations formed during outgassing. The present paper describes the effect of chemical additives on the formation and desorption of these products, with the aim of elucidating the structure and decomposition reaction of each product precursor. Additives chosen include scavengers previously used in radiolysis of homogeneous phases as well as poisons known to interfere with hydrocarbon reactions on oxide surfaces.Nitric oxide has frequently been used to scavenge free radicals in hydrocarbon radi~lysis.~ It is readily adsorbed at exposed aluminium ion sites on y -a l ~m i n a , ~ including those at which but-1 -ene undergoes isomerisation. Oxygen also scavenges radicals in homogeneous phase radi~lysis.~ It is adsorbed by y-alumina slightly in the absence and readily in the presence 5 * 6 of radiation to give -0; and -0ions at exposed aluminium ion sites. Many adsorbed hydrocarbons react with oxygen when irradiated.' Sulphur hexafluoride and nitrous oxide both scavenge electrons in homogeneous phase radiolysis. Nitrous oxide decomposes on y-alumina at high temperatures or under irradiation,1° and as on other oxides the products probably include e 0 -ions. Carbon dioxide is readily adsorbed by y-alumina l 2 and efficiently poisons the sites which cause isotopic exchange between normal and deuterated hydrocarbons or deuterium.Under irradiation extra carbon dioxide is adsorbed but very little Alkenes irradiated on y-alumina in the presence of carbon dioxide polymerise to long chain carboxylic acids.As in earlier papers, " physisorbed " products are arbitrarily defined as those desorbed at up to 313 K, and " chemisorbed " products as those desorbed at higher temperatures. The following definitions are also continued : total product carbon (TPC) = chemisorbed CH4+2(C2H4 + C2Hs) + 3(C...
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