Radiolysis of Aqueous Methane Solutions
3863
shown in Figure 5 , the development of the absorption of the solvated electron in ethanol and 1-propanol lags behind the production of eay- in water. The delayed formations correspond to t~12 = 2-4 and 50 psec respectively for ethanol and 1-propanol. This may be due to a solvation time of the electron in these alcohols. Slower formation times have been observed in low-temperature ethanol glasses,8 and alcohols a t temperatures below -100°.31 Here the relaxation is observed by a decrease in the spectrum in the infrared which parallels an increase in the yellow-red part of the spectrum, and is associated with the breaking of hydrogen bonding with subsequent orientation of the solvent dipoles to form deeper traps for the solvated electron. The present data may also be explained in this way, although a full spectrum was not observed. Microwave absorption measurements in liquid alcohols32 show that the relaxation time for the rotation of free monomeric molecules is 21.9 psec in 1-propanol at 20°, which is close to that observed in the present work, while the relaxation time for H bond breakage is 430 psec. Figure 5 also shows that the formation of the benzene excimer, B2*, is delayed 10 psec behind that of the eaqin water. This is of the order of the time taken to form the excimer from the ground state and the first excited singlet state. The shape of the curve which shows a large initial delay cannot be calculated by the analog device used for the other curves. It would appear that the formation of the first excited singlet state is delayed with respect to
Radiolysis of A
U ~ O U SMethane
cay- in water and that the delay in the excimer formation is the resultant of this process together with the reaction of the excited singlet with ground state benzene. The initial delay in the formation of the excited singlet state could be due to a prior ion neutralization reaction or to the direct formation of a higher excited state with subsequent cascade to the first excited singlet state. These processes are short compared to the excimer formation time of 10 psec. It is concluded that excited singlet states are formed rapidly in both cyclohexane and benzene in a time that is short compared to 10 psec. Rapid ion neutralization or direct excitation could explain this result. The differing subsequent chemistry in the two liquids lies in the different nature of the two excited states. In cyclohexane the state i s short lived, t1/2 = 0.2 nsec, has high energy, -7 V, and transfers energy rapidly, k = 2-4 X 1011 M - 1 sec-1. In benzene the first excited singlet state has a t1,2 = 20 nsec, an energy of 4.7 V, and a transfer constant k 2 X 1O1O. Unlike benzene the excited singlet state of cyclohexane is very reactive with conventional electron scavengers such as N20, SFs, and C02, a feature which must enter into the treatment of the scavenging data in these systems.
-
(31)J. H. Baxendale and P. Wardman, Nature (London:, 230, 449 (1971). (32)S. K. Garg and C. P. Smyth, J. Phys. Chem., 6$31294 (1965).
Solutions'
. C.,Stevens, Robert M, Clarke, and Edwin J. Hart* Chemistry Division, Argonne National Laboratory, Argonne, iliinois
60439 (Received May 8, 1972)
Publication costs assisted by the Argonne National Laboratory
The y-ray and electron pulse irradiation of aqueous methane and some ethane solutions is reported. The absorption spectra of the CH3 and CzHs free radicals have been measured in the wavelength range 210-270 nm. At 210 nm 4CH3) = 850 M-1 cm-1 and c(C2H5) = 520 M-I cm -1. The bimolecular recombination rate constants are 1.24 f 0.2 x l o 9 and 0.96 f 0.2 x 109 M-1 sec-1 for CH3 and CzHb, respectively. The rate constant k(QH CH4) = 1.21 f 0.4 x 108 M-1 sec-1; k(CH3 Hz02) = 3.5 X 107 M - l sec-1. The yields G( -CHI), G(C&), G(H2), G(H202), and G(N2) for NBO-CH~solutions are reported for some acid, neutral, and alkaline solutions. A radiolysis mechanism is also given.
+
Introduction Whereas Inany investigators have studied the radiolysis of gaseous methane as was brought Out in a recent review,2 relatively few papers have appeared on the radiolysis of aqueous methane solutions.3-~ The results of studies on ~ x y g e n - f r e e ~and , ~ oxygen-saturated solutions4~6demonstratethat only the OH free radical reacts with methane to any appreciable extent. By relative rate constant measure-
+
ments k(CH4 + OH) has been established as 1.4 x 108 M-1 sec-l.? As expected, CH3 is the product radical and its re(1) Work performed under the auspices of the U.S. Atomic Energy Commission. (2) G. G. Meisels in "Fundamentai Processes in Radiation Chemistrv." P. Ausloos. Ed.. Interscience. New York. N . Y . . 1968.Chaater
6;p 347. (3) E. J. Hart, J. K. Thomas, and
(1964).
s. Gordon, Radiat.
Res. Suppi., 4, 74
(4) G.R. A. Johnson and J. Weiss, Chem. Ind., 13,358 (1965). The Journal of Physical Chemistry, Voi. 76, No. 25. 1972
3864
G . C. Stevens, R. M. Clarke, and E, J. Hart
activity, with 0 2 , 4 CO2,S CH30H,g acetone,lOJl and glycine12 has been reported. Utilizing pulse radiolysis techniques we measured the absorption spectrum of the CH3 free radical and its rate of formation and decay in order to establish k ( 0 H CH4) and k(CH3 CH3). Product yields have also been determined by y-ray radiolysis in order to assist in interpreting the mechanism of reaction. Some preliminary results are also reported on the radiolysis of aqueous ethane,
-+
+
Experimental Section Matheson research grade CH4 and CzH6 were used without further purification. Research grade NzO was purified by freezing, thawing, and pumping, followed by refreezing and pumping again. For the NzO experiments deoxgenated water was saturated with NzO a t 740 mm pressure giving a concentration of 0.025 M.High-pressure methane solutions were prepared by introducing a 1.0 m M NzO solution into the high-pressure optical cell13 against a back pressure of CE-I.1. The cell was then closed, CH4 admitted to the desired pressure, and the solution equilibrated by shaking for 10-15 min. Methane solutions of pH 3 and 9 were irradiated by 6Wo y-rays a t dose rates between 0.027 and 10 krads/min. The initial rates of 6 2 & formation were measured at the lower dose rates, whereas G(C&OH), G(-CH4), G(Hz02), and G(Hz) were measured at the higher dose rates. In the Linac radiolysis experiments, the pulse length of the 13.5-Mev electron beam was varied between 50 nsec at 5 A and 8 psec a t 0.2 A. Transient spectra were observed down to 220 nm, using an apparatus in which the analyzing light beam was split on a prism after passing through the irradiation cell. This permitted the study of optical changes simultaneously a t two vvaveiengths. A special apparatus designed for work in the short ultraviolet was used at wavelengths below 220 n m ~ t The * signal-to-noise ratio was improved by using a pulsed analyzing light source, having a pulse duration of 2-3 msec. a time appreciably longer than the time scale for transient decay. Observations were made with CH4 solutions saturated at atmospheric pressure and under pressures up :o 400 psi in a high-pressure optical cell.13 For product analysis the electron pulse irradiations were carried out by irradiating 40 ml of solution in a 100-ml syringe, The high-pressure irradiations were carried out in a 30-ml high-pressure cell designed so that solutions could be withdrawn without opening the cell.15 In all electron pulse irradiations the dose was measured by collection of charge from the cell and Faraday cup in a manner previously described.l6 All "Co and electron pulse dosimetry was standardized by the Fricke dosimeter using G(Fe3+) =
yM_ CIiqOM
Figure 1. Calibration curve for
methanol analysls
500-fold excess of CH4 relative to CHsOH is present, further oxidation of CH30H to HCHO or CHzOH-CHzOH is unlikely. Therefore we consider C02 a reasonably good measure of the CH30H present. Figure l displays a calibration curve showing C02 development in a synthetic solution containing up to 60 pM CH30H. The methanol of the irradiated samples fell within this concentration range. Ethane was measured on a 5750 F & M Scientific research chromatograph. An 8-ft column of 80-100 mesh Porapak Q was used to separate the methane, ethane, propane, and butane. The column temperatures used were 24 and 157". A calibration curve for ethane was prepared by saturating triply distilled water with ethane and measuring the concentration on a Van Slyke apparatus. Then IO-ml samples of 1-10 pM of ethane were prepared and extracted with 10 ml of helium. A 0.5-cc sample of the resulting gas was injected into the chromatograph and the area of the ethane curve was measured. A plot of area (cm2) us. KM ethane in the aqueous solution provided the calibration curve. The irradiated samples were analyzed for ethane by an identical procedure. The elution times for methane and ethane a t room temperature were 1 and 8 min, respectively. Propane and butane eluted in 2.5 and 3 min a t 167". No attempt was made to measure the methane, propane, and butane quantitatively for these runs.
Results and Discussion A . Transient Spectra. Exploratory experiments demonstrated that under our conditions of electron pulse and y-ray irradiation only the OH radical reacted directly with CHI and no reaction between eaq- or the H atom and CH4 was detected. Consequently our goal was to measure the
15.6.17
The dissolved gases, CH4, H2, Nz,and 0 2 , were separated from the aqueous phase in a Van Slyke apparatus and analyzed on a Chromatograph with a molecular sieve column. A residual nongaseous product assumed to be methanol remained in the solution after the removal of gases. For analysis of this water soluble product, the irradiated sample was neutralized, degassed, and forced into a 50-ml syringe to which 500 fi.M of H202 was injected. The product was then oxidized to COz by irradiation and the concentration of COz was measured on the Van Slyke apparatus. Clearly this method of analysis is not specific for CH3OH since H C H 0 and HCOON: as well as higher molecular weight alcohols, aldehydes, and acids would yield COz on y-ray irradiation. However, under our conditions where a 100- to The Journal of Physical Chemistry, Vol. 76, No. 25, 1972
(9) (10) (11) (12) (13) (14) (15) (16) (17)
R. M . Clarke and E. J. Hart, Abstracts of the 160th National Meeting of the American Chemical Society, Chicago, Ill., Sept 1970. M. Ahmad, S c i l n d . ( K a r a c h i ) ,6, 57 (1968). M. Anbar, D. Meyerstein, and P. Neta. J. Chem. Soc. B, 742 (1966). A. Appleby, J. Holian. G . Scholes, M. Sirriic, and J. J. Weiss, Int. Congr. Radiat, Res., 2nd, 65 (1962). W. V. Sherman, J. Phys. Chem., 71, 4245 (1967). D. H. Volman and L. W. Swanson, J. Amer. Chem. Soc.. 8 2 , 4141 (1960). I. A. I . Tahaand R. R . Kuntz, J. Phys. Chem., 73, 4406 (1969). G. Moger and 0 . Dobis, Magy. Kern. Foly., 75, 4406 (1969). E. J. Hart and M. Anbar, "The Hydrated Electron," Wiley, New York, N. Y., 1970, p 195. B. D. Michael and E. J. Hart, manuscript in preparation, M. H. Studier and E. J. Hart, J. Amer. Chem. SOC., 91, 4068 (1969). B. D. Michaei, E. J. Hart, and I
