G. G. MEISELS,W. H. HAMILL AND R. R. WILLIAMS, JR.
1456
cm.a/molecule sec. that differs by less than the experimental uncertainty from the k for the A+ Hz reaction, 1.8 X 10-9 cm.a/molecule/sec. I n view of the assumptions and approximations involved in the assessment of the IC’s, it is quite possible that there are systematic errors that could cause the rates to be in error by a factor of two (in either direction). Such systematic errors would not affect the relative accuracy which is believed to be in the range flo%, and certainly would not affect the principal thesis of this paper, that such reactions must be considered in formulating radiation chemical reaction mechanisms. A secondary conclusion is implicit in the argument developed above, that is, that accuracy of measurement of ion-molecule reaction cross sections is not of great importance as far as the formulation of radiation chemical reactions is concerned. If a secondary (ionic) reaction can be identified in a mass spectrum obtained with a mass spectrometer of the type used for conventional analytical pur-
+
Vol. 61
poses (Westinghouse Type LV, Consolidated Electrodynamics Model 21-103 or -620) at pressures the order of 2 4 times normal operating pressure, it follows that the reaction cross section is of the order of 100 A.2 Hence, at atmospheric pressure the half-life of the ion with respect to this reaction will be the order of lo-* times its half-life with respect to recombination with an electron (for reasonable radiation dose rates). The characteristics of conventional analytical mass spectrometers are such that for such reactions to be easily observable the reaction cross section must be very large. Even an increase by a factor of lo3 of the dynamic range of measurement by the mass spectrometer would result in the slowest secondary reaction observable with a mass spectrometer still being fast in comparison with the rate of recombination of ions. Acknowledgments.-The author is indebted to Drs. G. Gioumousis, D. 0. Schissler and C. D. Wagner for stimulating discussions of various aspects of the subject considered in this paper.
THE RADIATION CHEMISTRY OF METHANE1 BY G. G. MEISELS,W. H. HAMILL AND R. R. WILLIAMS, JR. Contribution from the Department of Chemistry, University of Notre Dame, Notre Dame, Indiana Received June 10, 1867
The radiation chemistry of methane in mixtures with argon or krypton has been studied using 50 kev. X-rays or 1 MeV. electrons. For A-CH, a t 25” ion-pair yields, M / N , were: Hz, 2.0; CzHs, 0.7; CaHa 0.1; C4H10, 0.05. Smaller amounts of C6Hll and C a l l were also present. Use of Dz and propylene gives yields of 1.0 for H. and for Hz. In mixtures of ACH4-12 relative yields were Hz, 100; CZH4, 14; CZHe, 0.6; CaHs, 0.0. C4”la, 0.0; CHaI, 34. CzHaI, 18. CaHJ, 0.5. C4He1, 0.1; CH21s, 9. In Kr-CH4-I2 murtures the relative ylelds were: kz, 100; CzHe, 0.0; &Hs, 0.0; bHJ, 200; .&HJ, 5; CHJz -50. Mass spectrometric measurements, of the type reported b Stevenson, demonstrate the reaction steps: A + +’CHa = CH3+ A +.H; CHa+ CH4 = C2H6+ Hz; K r + &4 = CH4+ Kr; CHI+ CHl = CHsf CHs. In addition the followmg reactions may occur in gas atmos heres: CH3+ CH4 M = CsH7+ M; CzHs+ CH4 = C3H,+ Hz. Important uncharged intermediates are H, 8H3, C Z H ~CZH,. , Higher hydrocarbons are formed from combination of free radicals, some of which form from addition to ethylene.
+
+
+
+
+
+
++
+
+
+ +
The effects of high energy radiation upon meth- tion chemistry has been emphasized by Stevenson’ ane have been observed by Lind and co-workers2 and confirmed by the authors of this paper.8 who established the pattern of products in this and Experimental other hydrocarbons. Similar further studies have Materials.-All hydrocarbon employed were rebeen reported by Honig and Shepparda and by search grade; they were furthergases purified by trap to trap L a m ~ e .As ~ yet no adequate reaction mechanism distillation on a vacuum line. Matheson argon wasDpnssed has been proposed. Neither Lind’s ion-cluster over copper a t 700” and then through a trap a t -78 . Hytheory6 nor a free radical mechanism analogous to drogen iodide was prepared by the reaction of phosphoric and potassium iodide. Resublimed iodine was deone proposed by Eyring, Hirschfelder and Taylora acid assed before use. Deuterium from the Stuart Oxygen can account for the important facts, in particular ompany and tetradeuteromethane from Tracerlab, Inc., the formation of propane, butane and ~ e n t a n e . ~ -by ~ allocation from the U. S. Atomic Energy Commission, The present study is an attempt to discover such a were used as received The former contained 0.5% H D the latter 0.8% CDaH. mechanism and a proposal based directly upon andApparatus and Procedure.-One source of radiation was a ion-molecule reactions whose importance in radia- Machlett AEG-50-T X-ray tube with a beryllium window
8
operated a t 47.5 kv. When operated a t 50 kv. this tube (1) Contribution from the Radiation Project of the University of Notre Dame, supported in part under Atomic Energy Cornmission gives a continuous X-ray spectrum from 0.25 to 4.4 A. Maximum plate current for the tube is 50 ma. but in this contract AT(l1-11-38 and Navy equipment loan contract Nonr-06900. work it never exceeded 20 ma. A Van de Graaff accelerator Taken in part from a dissertation submitted by G. G. Meisels to the Graduate School of the University of Notre Dame in partial fulfillment (High Voltage Engineering Corporation) was used also. Electrons of 1.5 MeV. were employed a t target currents of of the requirements for the Ph.D. degree, June, 1955. (2) S. C. Lind, D. C. Bardwell and J. H. Perry, J . Am. Chem. SOC., 2-20 pamp. The irradiation cell for the X-ray runs permitted the 48, 1556 (1926); S. C. Lind and D. C. Bardwell. ibid., 48,2335 (1926). determination of the saturation ionization current. Two (3) R. E. Honig and C. W. Sbeppard, THIBJOURNAL.60,119 (1946). aluminum windows of the 6 mil thickness were attached to (4) F. W. Lampe, J . Am. Chem. SOC..79, 1055 (1957). (5) 6. C. Lind, “The Chemical Effeota of Alpha Particles and Electrona,” Reinhold Publ. Corp., New York, N. Y.,1935. (7) D . 0. Schissler and D. P. Stevenaon, ibid., B4, 926 (1956). ( 8 ) G. G. Meisels, W. H. Hamill and R. R. Williams, Jr., ibid., 26, (6) H. Eyring, J. 0. Hirschfelder and H. 8. Taylor, J . Chem. Phys., 790 (1956). 4, 479 (1936).
RADIATION CHEMISTRY OF METHANE
Nov., 1957
the ground flanges of a cylindrical Pyrex vessel by means of Gelva V-7 thermoplastic resin. The efficiency of the attached cold-finger was tested with a mixture of argon ,and ethane. The half time for freezing out the ethane was found to be 8 sec. For the Van de Graaff runs all-glass Pyrex cells with spherical windows were employed. A high voltage power supply with a maximum of 12,000 v. was used for ionization current measurements. The output voltage was varied by continuously increasing the input from a motor driven Variac. The current was measured on a calibrated Brown recorder a t the beginning of every X-ray run and never required more than 5% of the total irradiation time. Measurements of chemical decomposition were made by mass spectrometric analysis. Yields of methane consumed could not be measured reliably because they corresponded to a very small fractional change in composition. Hydrogen yields are relatively reliable because it is the largest single product. Yields of higher hydrocarbons are inaccurate because they are very small. Yields of alkyl iodides are very inaccurate because of an unsuitable inlet system on the mas8 spectrometer. After analysis of the total sample the product hydrocarbons were frozen out with liquid nitrogen and the rare gas, methane and hydrogen collected in a SaundersTaylor apparatus.9 The trap was then brought to room temperature and the rest of the gases transferred to a small sample bulb for analysis. Evidence for some of the reactions was obtained from mas8 spectrometric work under conditions similar to those used by Stevenson.' A C.E.C. 21-103-A mass spectrometer was modified to permit variation of the repeller (pusher) from 0-10 v., of the ionizing voltage between 5-50 v., and the use of magnetic scanning. I n runs with added hydrogen iodide its pressure was kept constant at 0.1 mm. by a cold-finger a t - 152'. Iodine was added by subliming it onto the inside surface of the radiation cell.
Results Table I summarizes the results of radiolysis of methane-argon mixtures. All values are given as ion-pair yields ( M I N ) . The direct determination of N was not possible when 1.5 Mev. electrons were used and the hydrogen yield in methane-argon mixtures was therefore used as an internal dosimeter. I n the absence of a cold-finger initial composition seems to have no effect on the products and unsaturated hydrocarbons are virtually absent. With the cold-finger in liquid nitrogen three changesin ( M / N ) can be noted: that for hydrogen increases by 0.2, that for ethane decreases by 0.13 and those for ethylene and acetylene appear at approximately 0.07 each. This corresponds stoichiometrically to an over-all reaction of C Z H ~ CZHZ 3H2 = 2CzHa. Because of the efficiencyof the cold-finger the higher hydrocarbons cannot be attributed to recycling of products. Some polymer of the approximate composition (CH1.,), was produced also with ( M / N ) for the monomer about 0.04.
+
+
TABLE I RADIOLYSIS OF ARGON-METHANE MIXTURESWITH 50 Kv.
X-RAYS AT 25" Product
Ht CzHe CaHs
.
..
CZH4 CzHt (9) K.
M/N with ooidfinger
M/N. without oold-finger
2.2 0.50 .10 .05
2.0 0.65 * 10 .05
traces
traces
.07 .08
.00 .00
W. saundera and H. A. Taylor, J . Chem. Phya.. 9,616 (1941).
1457
The yield of methane consumed was found to be independent of radiation intensity but dependent on temperature. It increased approximately linearly with temperature from 2.5 at 200°K. to 3.0 at 570°K. (Table 11). In one experiment an allglass cell was completely immersed in liquid nitrogen. The ion-pair yields were estimated by comparison with the radiation yield of hydrogen at room temperature in the same cell under water, all other conditions being identical. Approximate yields were: H2, 1.5; CZH6, 0.7; C3Hs, 0.8; C4H10,0.4; C ~ Hand I ~ C6H14,0.3; CzH2and CzH4,0.06. TABLE I1 ARQON-METHANE MIXTURES IRRADlaTED ELECTRONS Temp., 'C.
Current,
amp.
CnHs
-CHI
Yields" CsHs
WITH
1.5 MEV.
CdHm
CaandCs
-60 30 2.6 -60 30 2.4 0.42 13 2 .84 0.11 0.03 0.04 2.7 15 2.7 .64 30 .13 .OS .OS 30 30 2.7 .ll .OS .ll .84 30 100 2.7 .24 .10 -05 .09 150 30 .55 2.9 .09 .06 .06 200 30 3.0 .67 .07 .09 .10 200 30 3.0 .67 .09 .03 .07 a Yields of hydrocarbons are expressed relative to ( M / N ) . HZ = 2. Combined yields of ethylene and other unsaturates approximate 0.06. Other yields not reported were not measured.
The addition of hydrogen to the system at room temperature does not change the products of radiolysis. With increasing deuterium the maximum yield of H D is 2.0 and for H2 the minimum is about 1.0. These runs can be summarized in the equations (M/N)Hz/HD
(M/N)H2
=E
0.5 f 0.7CHa/Dz = 2.0
+ O.S(M/N)HD
(1)
(2)
CHsD is produced also but its yield is small when there is less deuterium than methane. At large concentrations of deuterium ( M / N )CH,D increases rapidly to 1.7 for CH4/D2 = 0.07, the lowest in this series of runs (Figs. 1 and 2). The addition of small amounts of propylene to argon-methane-deuterium mixtures left (M/N)Hz unchanged but (M/N)HD decreased t o 1.0. In mixtures of argon-methane-d4-propylene ( d 4 / N )Dz = 1.0 while HD disappears (Table 111). In radiolyses of mixtures of argon-methane-carbon monoxide no ketene could be found in the products. Since the reaction of CHZwith CO to form CH2CO occurs in the photolysis of ketene,l0 it appears that CH2 is not present as an intermediate. Additional evidence is found in the radiolysis of argon-rnethar~emethane-d~mixtures for which the yield of CH2Dz was small although the reactions CH2
+
CH2 D2 = CHzDa CDI = CHzDe CD2
+
+
(3) (4)
appear to take place readily.11J2 (10) G. B. Kistiakowsky and W. L. Marshall, J . Am. Chsm. SOC., 74, 88 (1952). (11) J. Chanmugam and M. Burton, ibid.. 78, 509(1956). (12) H. Wiener a n d M. Burton, ibid., 76, 6815(1953).
G. G. MEISELS,W. H. HAMILLAND R. R. WILLIAMS, JR.
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TABLEI11 AND/OR HYDROGEN ARGON-METHANE RUNSWITH ADDEDPROPYLENE CsHs Hn or Da
System
CD4-CaHe CD4-CaHs cDrcsH6-H~ CD4-CaHg-Hz CHrDz-CaHg CH4-Dz-CaHg CH4-Dz-CzHe
0.005 * 008 .035 .011 ,005
CIHE CHI or CDI
_.__
(WN)H2
...
0.039 .019 .23 .03 .034 .045 .013
- - -02.0
2 C&/Dz.
3
13
Fig. l.-Ion-pair yields in ar On methane-deuterium mixtures for: 0 ,H2; 0 ,HD; 0 , 8 H 5 as functions of CH4/Da.
1.0
... ... 1.1 0.8 1.2
1.1 0.3 0.5
... ... ...
TABLE IV PRODUCTS IN THE RADIOLYSIS OF A-CHI, A-CH4-HI AND A-C&-Iz MIXTURES WITH 1.5 MEV.ELECTRONB A-CHI
Relative yields" A-CHI-HI
A-CHcIn
8.5 0.05 CzHe 0.00 CaHs CHio ' 0.00 1.22 CzH4 ' 0.14 CzHz 0.00 CaHe ; CHaI 2.9 1.51 C2Ht.I 0.04 CaHd CJLJ
