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VINCENZO AQUILANTI
Rare Gas Sensitized Radiolysis of Methane
by Vincenzo Aquilanti Laboratorio di Chimica delle Radiazioni e Chimica Nucleare del C.N.E.N., Istituto di Chimica Generale e Inorganica, Uniuersith d i Roma, Rome, Italy (Received April IS, 1966)
This paper reports the results of a systematic investigation of the xenon-, krypton-, and argon-sensitized radiolysis of methane and of CH4--CD4mixtures in the gas phase. The results are interpreted in terms of nonionic mechanisms for xenon sensitization and of ionic processes for krypton and argon sensitization.
Introduction Although the radiolysis of methane has been investigated by several authors,lt2 some major questions remain unanswered. I n particular, the relative roles of ions and free radicals in the mechanism of formation of hydrogen and higher hydrocarbons is not clear. Following previous studies from this laboratory of charge-exchange processes between rare gas ions and methane3 and ion-molecule reactions in methane,4 it was thought of interest to investigate systematically the radiolysis of mixtures of rare gases and methane. If the rare gas (Xe, Kr, or Ar) is in large excess, practically all the radiation is absorbed by it, and the energy is then transferred to the methane by different mechanisms, depending on the nature of the rare gas itself. It should therefore be possible to obtain evidence about some of the reaction paths which occur in the radiolysis of pure methane, i.e., nonionic processes in xenonsensitized experiments, and the reactions of particular ions in the radiolysis sensitized by krypton or argon. The ionic nature of Kr-sensitized and Ar-sensitized radiolysis was first proposed by Meisels, Hamill, and Williamss; a few experiments are reported by Ausloos and Lias,Zb but only with free-radical scavengers and at relatively low concentrations of rare gases. Other studies6 were made at very high conversions, and it is difficult to establish a correlation between the elementary processes and the over-all observed behavior. Present experiments include studies of the general features of sensitized radiolysis in pure methane and in the presence of small amounts of NO, 0 2 , and hydrocarbons, and the isotopic spectrum of hydrogen produced in the radiolysis of mixtures containing a rare gas, methane, and methane-&. The Journal of Physical Chemistry
Experimental Section All the hydrocarbons used in the present study were Phillips research grade. Methane was purified by collecting the vapor at - 196”; this procedure produced a purity better than 99.95%. The rare gases, obtained from Air Liquide Co., were used without further purification. Matheson NO was purified by bulb-to-bulb distillations. Methane-d4 was obtained by Merck Sharp and Dohme of Canada, and contained 4.5% CHD3. The gaseous mixtures were prepared manometrically and their composition was checked by mass spectrometry and gas chromatography. The cells used were cylindrical Pyrex vessels (about 73 cc.), and the base, which was directly exposed to radiation, consisted of an aluminium plate sealed to the Pyrex with Araldite 121B. The source of radiation was a Machlett EG-60 X-ray tube, operated a t 50 kv., with tungsten as anticathode; plate currents to the tube were adjusted in the 5-30-ma. range. The dose rate was measured with a ferrous sulfate dosimeter, taking into (1). For a review, see S. C. Lind, “Radiation Chemistry of Gases,” Remhold Publishing Gorp., New York, N. Y., 1961; F. Williams, Quart. R m . (London), 17, 101 (1963). (2) More recent papers include: (a) J. Maurin, J . chim. phys., 5 9 , 15 (1962); (b) P. Ausloos, et al., J. Chem. Phys., 38, 2207 (1963); 39, 3341 (1963); 40, 1854 (1964); (c) L. W. Sieck and R. H. Johnsen, J . Phys. Chem., 67, 2281 (1963); (d) R. W. Hummel, Discussions Faraday SOC.,36, 75 (1963); (e) F. Fayard, J . chim. phys., 60, 651 (1963); (E) W. P. Hauser, J . Phys. Chem., 68, 1576 (1964). (3) A. Galli, A. Giardini-Guidoni, and G. G. Volpi, Nuovo Cimento,
31, 1145 (1964). (4) G. A. W. Derwish, A. Galli, A. Giardini-Guidoni, and G. G. Volpi, J . Chem. Phys., 40, 5 (1964). (5) G. G. Meisels, W. H. Hamill, and R. R. Williams, Jr., J . Phys. Chem., 61, 1456 (1957). (6) (a) R. W. Hummel, Nature, 192, 1178 (1961); (b) K. Yang, U. S. Patent 898,015 (1962); (c) R. Cipollini, A. Guarino, and G. Perez, Gam. chim. ital., 9 5 , 43 (1965).
RAREGASSENSITIZED RADIOLYSIS OF METHANE
account the absorption coefficients of the various gases in the X-ray spectral range and the geometrical conditions of irradiation. Under typical radiolysis conditions (rare gas pressure 100 mm., plate current 15 ma.), the absorbed dose rates were calculated to be: xenon, 7.6 X lo1’ e.v./min.; krypton, 6.3 X lo1’ e.v./ min.; argon, 8.5 X 10l6 e.v./min. However, these values are not expected to be very accurate, and since similar uncertainties affect the absolute G values, the yields of the pyoducts are referred to methane conversion. After irradiation, the samples were analyzed by mass spectrometry for methane conversion, inorganic gas partial pressures, and isotopic ratios. CrC6 hydrocarbons were measured by gas chromatography, using two columns in series (15-m. dimethylsulfolane on Celite, plus 1-m. dinonyl phthalate on Celite),’ nitrogen as carrier gas, and a hydrogen-flame ionization detector. Calibrations were made by means of mixtures of known composition.
Results and Discussion General. The radiolysis of mixtures of CH4 and rare gases was studied by measuring the methane conversion and the distribution of the main products a t different irradiation times. Experiments a t different mixture compositions (methane from 3to7%), diff erent totalpresmres (from 50to 200 mm.) ,anddifferent dose rates (plate currents from 5 to 30 ma.) were also carried out. However, both the methane conversion and the product distribution were found to depend only on the total dose absorbed. I n all cases, methane conversion (up to about 20y0) was found to be a linear function of the total absorbed dose. The G (:-CHI) values shown in Table I were calculated from the slopes of the straight lines so obtained. The hydrogen yields were also found to be proportional to the absorbed doses and are reported in Table I as ratios G(Hz)/G(-CHr). Table I : Cocversion Rates and Hydrogen Yields in the Rare Gas Sensitized Radiolysis of Methane
a
Rare gas
G ( - CH4)
G(H2)’G(-CH4)
G(H2)’G(-cH4)a
Xenon Krypton Argon
6
1.00 0.75 0.72
0.46 0.52
7
22
0.50
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saturated hydrocarbons were found, but for the argonsensitized experiments ethylene was rather more abundant: a t 3% conversion, it amounted to 0.05% of the initial methane. In the experiments where NO was added to the gaseous mixtures (as much as 30% of the initial methane), no change in conversion rate was observed; however, the hydrogen yields decreased (Table I) and no gaseous hydrocarbons werefound, apart from ethylene in the experiments with krypton and argon. Figures 1, 2, and 3, which report typical results for the production of GHs, C3He, i-C.;Hl0, and n-C4Hlo, show that all hydrocarbon yields decrease with increasing methane conversion. A study of the yields at low conversion is therefore necessary if one wishes to investigate the initial processes. Figure 4 gives the ratio C3Hs/C2Hs obtained at the lowest conversion values allowed by present analytical methods. This ratio is constant for the krypton and argon mixtures, but shows a marked initial increase when xenon is used as sensitizer. Therefore, in the latter case propane is a typical secondary product. I n Figure 5, the ratios HD/DZ, obtained in the radiolysis of rare gas-CHrCD1 mixtures, are plotted against conversion. This ratio extrapolates towards zero at zero conversion for the xenon experiments, while for krypton and argon it seems to maintain a finite value. Therefore it appears that in these latter systems HD is a primary radiolysis product, namely, that hydrogen atoms are initially produced; conversely, in xenon-sensitized radiolysis hydrogen is mainly produced by a process of molecular detachment. Figure 5 also shows that the exchange reactions leading to HD formation are strongly inhibited in the presence of NO. These results, together with the observation (Table I) that NO decreases hydrogen yields, seem to indicate that, besides primary processes leading to the formation of Hz,more hydrogen is produced during the radiolysis v i a the products themselves; this latter source of hydrogen is inhibited by NO because of its effect in decreasing the yield of hydrocarbons. It seems altogether unjustifiable to identify the yields obtained in the presence of NO as “molecular yields” (as suggested by several authors1b,8)because NO should not act as a scavenger of hydrogen atoms, but as a catalyzer in their recombination. Xenon-Sensitized Radiolysis. I n their lower state 2PSl2, Xef ions do not transfer their charge to methane,
With added NO.
Ethane, propane, iso- and n-butane are the main hydrocarbons produced in the radiolyses. only traces (
