ABSENCE OF H ATOMPRODUCTION IN RADIOLYSIS OF SOLID HYDROCARBONS
Composition of 0-Type. We have discussed the literature concerning the likely composition of “reduced COz”-similar to O-type11,30-in recent publicat i o n ~ . ~The ~ , ~basic ~ controversy concerns whether that species is 33-35 or is nota1J6similar to adsorbed CO. The present data provide evidence on the structure of 0-type. Thus, we have found that OnCOl is 3 and, previously,ls that [e] ‘v 1. A species which would satisfy these findings is 3C-OH. This composition has been suggested to correspond to CH30Hads.37-39 The rate-limiting step in CHI oxidation at low potentials would then be +C-OH
+ HzO +C02 + 3H+ + 3e-
(7)
Summary and Conclusions (1) The species on Pt in presence of CH, (0-type) appears to lie ca. five-eighths of the way between CHI and COz. One electron per site is released when this species is oxidized. A composition of >C-OH fits these observations. (2) The oxidation current of this species at 0.30 V is ca. three-eighths of the oxidation current of CHI. This suggests that it lies in the path of the main CH, --t COz reaction. (3) The rate-limiting step of the over-all reactmionat
2403
low potentials (50.35 V) is the oxidation of this species,
viz. +C-OH
+ H 2 0 +COz + 3H+ + 3e-
(4) At higher potentials (20.40 V), the rate-limiting step is the rate of CH4adsorption.
Acknowledgments. We are pleased to acknowledge that this work was supported by the U. S. Army, Mobility Engineering Research and Development Center, Fort Belvoir, Virginia, on Contract DA-44-009AMC-l408(T). (30) J. Giner, Paper presented at 15th C.I.T.C.E. Meeting, London, 1964. (31) S. B. Brummer and K. Cahill, Discussions Faraday SOC.,45, 67 (1968). (32) 5. B. Brummer and K. Cahill, J . Electroanal. Chem., in press. (33) M. W. Breiter, Electrochim. Acta, 12, 1213 (1967). (34) T . Beigler and D. F. A. Koch, J . Electrochem. Soc., 114, 904 (1967). (35) T. Beigler, J . Phys. Chem., 72, 1571 (1968). (36) B. J. Piersma, T . B. Warner, and S. Schuldiner, J . Electrochem. Soc., 113, 841 (1966). (37) B. I. Podlovchenko and E. P. Gorgonova, Dokl. Akad. Nauk S S S R , 156, 673 (1964). (38) 0. A. Petrii, B. I. Podlovchenko, A. N. Frumkin, and H. Lal, J . Electroanal. Chem., 10, 253 (1965). (39) 0. A. Khnzova, Y . B. Vasiliev, and V. S. Bagotskii, Elektrokhimiya, 2, 267 (1966).
Absence of Hydrogen Atom Production in Radiolysis of Solid Hydrocarbons] by Dietrich Timm and John E. Willard Department of Chemistry, University of Wisconsin, Madison, Wisconsin (Received January 7, 1969)
Radiolysis of ethane, n-hexane, 3-methylpentane, 3-methylpentane-dlc, or methylcyclohexane at 4°K produces the expected esr spectra of trapped free radicals, but no evidence of trapped hydrogen atoms, although the H atom doublet is present with the CH, radical spectrum when CHI is irradiated. Radiolysis of 3-methylpentane-& at temperatures in the range from 20 to 50°K produces no trapped D atoms, although it has been shown that D atoms from the photolysis of DI in this matrix can be trapped in this temperature range. The presence of 0.3 mol yoisobutene in CHI during irradiation at 4°K does not reduce the trapped H signal. Irradiation of 1 mol % ’ CzHBin Ar at 4°K produces little or no H atom yield compared t o that from 1 mol % CH, in Ar. These observations lead to the conclusion that the free radicals formed by radiolysis of solid hydrocarbons are not produced by elimination of H atoms. Ion-molecule reactions are, therefore, the probable mode of formation.
Introduction The assumption that hydrogen atoms are produced in the radiolysis of alkanes is consistent with extensive evidence.2 However, in each such case, an ionmolecule mechanism can be written which would produce the observed results. Only for CHI is there unambiguous evidence for H atom production in alkane
radiolysis, as far as we are aware. H atom esr lines have been observed from liquid CH4 during steady(1) This work has been supported in part by U. S. Atomic Energy Commission Contract AT(l1-1)-1715 and by the W. F. Vilas Trust of the University of Wisconsin. (2) Examples and references are given by M. C. Sauer and I. Mani, J . phys. Chem., 72, 3856 (1968).
Volume 73, Number 7 July 1069
2404 state radiolysis at 100"Ii,3 and from solid CH, at 4°K,4-6following radiolysis. The fact that trapped H atoms have not been observed from the radiolysis of solid hydrocarbons' other than methane, whereas trapped electrons* and trapped radicalsga are produced, may be the result of the temperatures (usually 77 or 4°K) and matrices used. If the temperature is too high, or the packing of the particular matrix molecules chosen is not favorable for trapping, H atoms may diffuse rapidly, and combineeb with other H atoms or radicals. If the temperature is too low, caging effects may preclude escape from the parent radical, as they reduce H atom escape in the photolysis of H I in hydrocarbon matrices.1°-12 We have, therefore, made a search for trapped H atoms in a variety of irradiated organic solids at a variety of temperatures. The choice of compounds and temperatures has been influenced by the following information from other investigations: (1) H atoms are produced and trapped at 4°K by radiolysis of CH4 but not by radiolysis of methyltetrahydrofuran, 2-methylpentene1, 3-methylpentane (331P), or C2HsOH;13 ( 2 ) H atoms produced in 3MP or 3-1'IP-dld at 77°K by photolysis of H I can diffuse to be captured by an olefin, but are not trapped by the matrix; (3) trapping of H and D atoms from the photolysis of H I and D I takes place in 3MPd14(but not 3MP-h14)at 20 to 50"K.12
Experimental Section Materials. Matheson research grade methane and prepurified argon were used as received. Phillips research grade ethane and n-hexane were degassed on the vacuum line. 3-Methylpentane (3MP) and methylcyclohexane (MCHx), both Phillips pure grade, were passed through freshly activated silica gel, degassed by freeze-pump-thaw cycles, and stored on the vacuum line over sodium mirrors. The 3MP showed less than mole fraction (mf) impurity and the RlCHx less than loq3 mf by gas chromatographic analysis. The spectra, using a l-cm light path, showed typically 0.01 0.d. for 3MP and 0.02 0.d. for MCHx at 200 nm. Commercial paraffin was recrystallized from 3MP and degassed as a liquid under vacuum at elevated temperature. Aldrich cycloheptane, distilled from freshly activated silica gel on the vacuum line, was >99.5% pure as shown by gas ~ REer& chromatography. Perdeuterated 3 1 , ~from Sharp and Dohme of Canada was used as received, except for degassing. The deuteration was 99% as indicated by mass spectrometric analysis. Samples for irradiation were condensed into esr tubes a t 77°K from the Storage Volumes On the VaCUUm line and sealed. Samples containing two components were mixed in the gas phase before condensation to minimize the possibility of segregation in the matrix, except the methane-isobutene and 3MP-isobutene mixtures which were mixed in the esr tubes in the liquid state before The Journal o j Physical Chemistry
DIETRICHTIMM AXD JOHN E. WILLARD final freezing at 77°K. The solids were presumably crystalline, except 3MP and 3?tIP-d14, which freeze as glasses, and MCHx, which sometimes freezes as a glass and sometimes as the polycrystalline form. Sample tubes were 3-mm or 2-mm i.d. quartz. Ordinary fused quartz was used when it was desired to observe the esr hydrogen lines since the hydrogen signal induced in it by irradiation is less intense than that induced in high purity synthetic silica (Suprasil). With most ordinary quartz, the hydrogen signal from the tube could be avoided completely if the tube was heated to redness while evacuated. This treatment had no effect on Suprasil. When signals in the region of the free electron g value were to be examined, Suprasil was used since it shows less interfering signal in this region. Irradiations. y irradiations were made with a 6oCo source14 which provided a dose rate to the samples of 5.0 X lo1*eV g-' min-I under the conditions normally used for irradiation at 77°K and 3.6 X 10'' eV g-I min-' at 4°K. Samples of tritiated 3MP-hl4 and 3MP-d14, containing about 14 Ci ml-l, which were investigated for H atom growth during self-irradiation, were prepared by Dr. Mervyn Long of our laboratory, by shaking 3MP with Raney nickel in the presence of 3H2gas.15 For y irradiations at 77°K the small, high specific activity 6OCo source was positioned adjacent to the sample, under liquid nitrogen. For irradiations at 4°K the source was positioned adjacent to the liquid helium dewar in the liquid nitrogen which surrounded it. Esr Measurements. Esr measurements were made with a Varian 4500 X-band spectrometer equipped with Fieldial, using 100-kc modulation. The output was recorded either as the first derivative of the esr (3) R. W. Fessenden and R. H. Schuler, J . Chem. Phys., 39, 2147 (1963). (4) B. Smaller and M. S. Matheson, ibid.,28, 1169 (1958). (5) (a) L. A. Wall, D. W. Brown, and R. E. Florin, J. Phys. Chem., 63, 1762 (1959); (b) D. W.Brown, R. E. Florin, and L. A. Wall, ibid., 66, 2602 (1962). (6) (a) W. V. Bouldin, R. A. Patten, and W. Gordy, Phus. Rev. Lett., 9,98 (1962); (b) W. Gordy and R. Morehouse, Phys. Rea., 151, 207 (1966). (7), A review of investigations on the radiation chemistry of organic solids is given by J. E. Willard in "Fundamentals of Radiation Chemistry," P. Ausloos, Ed., John Wiley & Sons, Inc., New York, N. 19681 Chapter 9, (8) See for example: (a) M. Skirorn and J. E. Willard, J . Amer. Chem. Soc., 90, 2184 (1968); (b) K. Tsuji and F. Williams, J . Phys. Chem., 72, 3884 (1968). (9) (a) W. G. French and J. E. Willard, ibid.,72, 4604 (1968); (b) R. Klein, M,D. Scheer, and R. Kelley, ibid., 68, 598 (1964). (10) J. R. Nash, R. R. Williams, Jr., and W. H. Hamill, J . Bmer. Chem, sot., 8 2 , 5974 (1960). (11) s. Aditya and J. E. Willard, ibid., 88, 119 (1966). (12) D. Timm and J. E. Willard, ibid., in press. (13) D. R. Smith and J. J. Pieroni, Can. J . Chem., 45, 2723 (1967). (14) Using an irradiator similar in design to that described by R. J. Hanrahan, Int. J . Appl. Radiat. Isotopes, 13, 254 (1962). Acta, (15) M. A. Long, A. L. Odell, and M, Thorp, I , 174 (1963).
J.
ABSENCE OF H ATOMPRODUCTION IN RADIOLYSIS OF SOLIDHYDROCARBONS signal or, with the aid of an integrator, as the esr absorption curve. For investigations at 4°K the sample tubes were irradiated and measured in an esr cavityI6attached to a 20-cm length of stainless steel waveguide soldered to a metal lid fitted to a 2-1. dewar, in which the cavity was positioned under liquid helium. A Styrofoam box, filled with liquid nitrogen, held the helium dewar while in the 6oCoirradiation chamber or between the poles of the Varian magnet. The dewar lid, which supported the cavity, held a vacuum-tight microwave pressure window which closed the end of the waveguide, a stainless steel tube, with removable vacuum-tight cap for inserting sample tubes into the cavity, and a valve for evacuating the cavity and flushing it with gaseous helium at 77°K while inserting samples which had been externally frozen at 77°K. Following an irradiation, the assembly was attached to the waveguide of the esr microwave bridge by connection at the pressure window. The height of the assembly was limited to 40 cm by the vertical dimension of the irradiation chamber, precluding a long neck to reduce heat leakage, such as usually used in liquid helium dewars. When the metal cover was insulated on the lower side by a 10-em thick layer of ~olyurethane,'~ molded to fit snugly in the opening of the dewar so that the cold gas passed out through the narrow annulus between the insulation and the dewar walls, experiments could be continued for up to 5 hr with a 1.25-1. filling of liquid helium. The sensitivity of this cavity at 77°K was about tenfold lower than that of the Varian cavity for a sample in the Varian liquid nitrogen dewar, for the modulation amplitudes used. At 4°K the sensitivity was a factor of 2 lower than that of the Varian cavity at 77°K. Liquid helium levels were determined by the change in electrical resistance when a carbon resistor on a dipstick'* passed through the gas-liquid interface. For measurements in the range from 10 to 77°K samples were cooled in a stream of helium gas produced by boiling liquid helium in the commercial 25-1. storage container in which it was received. The gas was conducted through a flexible metal liquid helium transfer tube to a specially designed'g Pyrex and quartz dewar tube in which the sample was mounted in the esr cavity, in a manner similar to that used with the Varian variable temperature device for temperatures above 77" K. Measurements were made at power levels from 38 to 0.01 mW, using a reference cavity at powers below 0.4 mW. The H atom doublet from H atoms in CH, at 4°K does not show saturation effects up to at least 1 mW. Alkyl radical signals can be observed at 4°K without severe distortion from saturation effects at powers up to at least 40 mW. The signal near the free electron G value which is induced in quartz at 4°K by y irradiation is highly saturated at 1 mW and above, but at lower powers it begins to dominate over alkyl radical signals. I n general, there is considerable line
2405
broadening in radical signals at 4"K, except for CH3. Radical spectra at 4°K sometimes have the appearance of a dispersion curve, presumably due to high values of T I at this temperature which result in fast passage conditions.2O
Results Radiolysis of CH, at 4°K. Following irradiation of solid CH, to a dose of 3.6 X lo'* eV g-l at 4°K we obtain an esr spectrum (Figure 1)essentially identical with that reported earlier from Q band measurements,6b including the hydrogen doublet, the methyl radical quartet, and the four quartets (with somewhat different spacing) attributed6b to exchange coupled H and CHs pairs. RIeasurements were generally macle at a micro-
Figure 1. Esr spectrum of CHa following y dose of 3.6 X IO1* eV g-l at 4°K.
wave power of 1.5 mW. G(CH3) and G(H) were estimated by comparison of their integrated signals to that of a pitch sample which had been standardized by comparison with a galvinoxyl solution of known concentration at room temperature; both were G = 0.7 f 0.2. The intensity of the H atom lines was unchanged over 3 hr at 4°K. After removal of the liquid heliutn and warming, it dropped to ca. 40% of its initial 4°K value at 12"K, 30% at 20°K) and had disappeared at 30°K. Methyl radicals could still be detected at (16) We are indebted to Professor R. N. Dexter of the University physics department for the design of this cavity which is described in detail in the Ph.D. thesis of 8. V. Filseth, University of Wisconsin (1962). (17) Made by mixing liquid plastic foam A and liquid plastic foam B obtained from the United States Plastic Corp., 1550 Elida Rd., Lime, Ohio 45805. This was cast in place in the mouth of the dewar around the wave guide and sample-introduction tube, using a polyethylene bag as a form. (18) A. C. Rose-Innes, "Low Temperature Techniques," D. Van Nostrand Co., New York, N. Y., 1964. (19) D. Timm and J. E. Willard, Reu. Sci. Instrum., in press. (20) C. K. Jen, S. N. Foner, E. L. Cochran, and V. A. Bowers, Phys. Rev.,112, 1169 (1958).
Volume 7.9, Number 7 July 1969
2406
DIETRICHTIMM AND JOHN E. WILLARD
33°K after a 25-min period of gradual warming from 4°K. The presence of 0.3 mol % isobutene in CH, during radiolysis at 4°K does not reduce the trapped H signal or alter the methyl radical signal produced in pure CH4. Radiolysis of Other Hydrocarbons at 4°K. Polycrystalline ethane irradiated to the same dose and observed under the same conditions as noted for methane in the preceding section gave a radical signal of approximately the same size, but no hydrogen atom signal even when the region of the hydrogen lines was measured at a sensitivity 40 times that required to give a full-scale radical signal. Assuming G(H) from CHI is about unity, G(H) from is less than At 4°K the C2H; radical signal is a poorly resolved six-line spectrum (Figure 2 ) . As the sample is warmed, the resolution improves until, at 77"I
