J. Phys. Chem. 1995,99, 707-711
707
Temperature and Pressure Dependence of the C2H4 Yield from the Reaction C a s + 0
2
E. W. Kaiser Ford Motor Company, Research Laboratory, Chemistry Department, Mail Drop 3083/SRL, Dearbom, Michigan 481 21 -2053 Received: August 24, 1994; In Final Form: October 31, I994@
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The yield of CZ& from the reaction CZHS 0 2 CzH502 (1) has been investigated as functions of temperature (260-530 K)and pressure (50-1500 Torr)at nonambient temperature using a relative rate technique. This yield has a non-Arrhenius temperature dependence, increasing slowly at a gas density of 4.9 x 10l8molecules cm-3 with an apparent activation energy of 1.1 f 0.25kcal in the temperature range 250-400 K and then increasing sharply as reaction 1 becomes reversible. These results are consistent with the formation of C2H4 via rearrangement of an excited adduct (C2Hs0z*) with an energy barrier less than 1.1 cal. The yield follows a dependence at temperatures below 400 K. The pressure dependence becomes less pronounced at higher temperature at 529 f 10 K). Rate constants of the coupled mechanism for CZ& formation developed by Wagner et al. provide reasonable agreement with these data to within the experimental error. The overall rate constant of reaction 1 was determined as a function of temperature (260-460 K) at 580 Torr relative to that of C2H5 Clz CzHsCl C1 (3). No temperature dependence is observed in the range 260-380 K [kl = (8.1 f 0.3) x cm3 molecule-' s-l]. At 460 K, the rate constant decreases -35%.
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Introduction The reaction of alkyl radicals with 02 is of central importance in combustion and atmospheric chemistry. The smallest alkyl radical whose 02 reaction shows the major characteristics of larger alkyl radicals is C2HS. As reviewed by Wagner et al.,l the products of this reaction were initially thought to be the peroxy radical CZHSOZ at low temperatures C2H5
+0 2 +M
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C2H5O2
+M
(1)
and ethylene at higher temperatures formed by an abstraction reaction with an activation energy of 5-10 kcal, C2H5
+ 02
e
+
C2H4 HOZ
(2)
A mechanism based on reactions 1 and 2 would show a CZ& yield which is inversely dependent on total pressure as long as reaction 1 is in the low-pressure limit and no pressure dependence after the high-pressure limit is approached (P w 50 Torr at 298 Kz). However, experiments at room temperature3 have shown that the CzH4 yield is inversely dependent on total pressure for pressures between 1 and 60oO Torr,well into the high-pressure region. This indicates that CZ& is formed by a pressure-dependent addition-elimination process via rearrangement of the excited C&-Is&* adduct, which produces a pressuredependent CZ& yield to infinite total pressure: CzH5
+0 2
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C2H5Oz*
This addition-elimination mechanism has been analyzed theoretically as functions of pressure and temperature using both RRKM' and QRRK4 theory. The energetics of this reaction has also been the subject of recent detailed theoretical calculat i o n ~ .However, ~~~ the data available for testing the resulting mechanisms are limited. The current research seeks to fill this @
Abstract published in Advance ACS Abstracrs, December 15, 1994.
gap by determining the CzH4 yield as functions of pressure (501500 Torr) and temperature (253-529 K) using the relative rate technique. h u i n g these experiments, the overall rate of reaction 1 is also measured:
Experimental Section The experimental technique used in the determination of the CZ& yield from reaction 1 has been described previ~usly.~ In the current studies, two reactors were used. One reactor (for temperatures 250-385 K) was a Pyrex cylinder (200cm3 x 5 cm diameter) enclosed in a Pyrex, temperature-controlling (f2 K) jacket through which a 50-50 mixture of ethylene glycol and water f l ~ w e d .Higher ~ temperature experiments ('450 K) were performed in a second cylindrical reactor (80cm3 x 2.2 cm diameter) which was placed in a tube oven. This reactor had a much larger temperature nonunifonnity (a decrease of 20 K from the center to the extreme end of the reactor) as determined by a thermocouple placed into a well down the center of the reactor. The average temperature based on the measured temperature profile is quoted for experiments in this reactor. Reactant mixtures containing C&, Clz, 02,and M (Nzor He) were irradiated for a chosen time by a Sylvania F6T5 BLB fluorescent lamp, and the entire contents of the reactor were removed for gas chromatographic (GC)analysis after the experiment was complete. The W radiation from the lamp dissociates Clz to form C1 atoms which react with CZ& yielding CZHSradicals. The ethyl radicals react predominantly with 02 to form either CZHSOZ or C z b . The CZHSOZ radicals produce stable products (e.g.. CH3CH0, CZH~OH, and C350zH) which are not measured in these experiments but do not affect the determination of the CzH4 yield. The yield of CzH4 is obtained from the final CZ& concentration and the amount of C2H6 consumed. Small corrections are made for secondary consumption of C f i and reaction of CZH5 with Clz. Simiiar experiments were performed to measure the rate constant ratio for the reaction of C a s radicals with 02 relative to that with Clz in order to determine the temperature and pressure dependence of the overall CZHS 02reaction. In these
0022-3654/95/2099-0707$09.00/0 0 1995 American Chemical Society
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708 J. Phys. Chem., Vol. 99, No. 2, 1995
TABLE 1: Selected C2& Yields during the C1 Atom Initiated Oxidation of CzHs temp (K) time (s)” (C~H~)/(CZ&)O c2H4 yield (%)” 0.892 0.31 253b 120 0.386 360 0.682 253b 0.906 0.442 29gb 120 29gb 140 0.88 0.374 29gb 360 0.71 0.39 0.008 0.5 0.77 298‘ 29gh 120 0.887 0.355 60 0.887 0.37 298”‘ 29gdf 7200 0.938 0.35 0.937 0.39 29gdf 7200 298”g 60 0.34 0.37 298”s 7.5 0.904 0.37 0.868 0.63 373h 120 0.884 0.733 384b 120 360 0.704 0.748 384b 513‘ 120 0.954 13.9 12.9 720 0.741 513’
Irradiation time; CJ& yields corrected for secondary consumption (typically
