Reaction of OH Radicals with Aromatic Hydrocarbons
1763
Rate Constants for the Reaction of OH Radicals with a Series of Aromatic Hydrocarbons D. A. Hansen, R. Atkinson, and J. N. Pttts, Jr.' Chemistry Department and StatewMe Air follutbn R e k r c h Center, University of California,Riverside, California 92502 (Received Aprll9, 7975) Publlcatbn costs assisted by the Unlversyl of Cellfornie, Rivers&
Absolute rate constants for the reaction of OH radicals with a series of aromatic hydrocarbons have been determined a t room temperature using a flash photolysis-resonance fluorescence technique. The rate constants (kl X 10l2 cm3 molecule-' sec-l) obtained are as follows: benzene, 1.24 f 0.12; toluene, 5.78 f 0.58; o-xylene, 15.3 f 1.5; m-xylene, 23.6 f 2.4; p-xylene, 12.2 f 1.2; 1,2,3-trimethylbenzene, 26.4 f 2.6; 1,2,4-trimethylbenzene, 33.5 f 3.4; 1,3,5-trimethylbenzene, 47.2 f 4.8. These absolute rate constants are in good agreement with those determined recently for benzene and toluene and with those derived for a series of aromatic hydrocarbons from a relative rate study in an environmental chamber.
Introduction In recent years the OH radical has been shown to be an important and reactive species in a large variety of oxidation processes, including combustion reactions,' photochemical air p o l l u t i ~ n , ~and . ~ stratospheric chemistry,4g5 and has recently been detected in ambient air.6 While much work has been reported for the reactions of OH radicals with alkanes7-13 and a l k e n e ~ , ~ - " J there ~ - ~ ~are few data available for aromatic h y d r o c a r b ~ n s . ~ JRecent~J~ ly the absolute rate constants for the reaction of OH radicals with benzene and toluene have been determined at 298OK over the pressure range 3-100 Torr of helium using a flash photolysis-resonance fluorescence technique.18 Rate constants for the reaction of OH radicals with a series of aromatic hydrocarbons have been c a l ~ u l a t e d 'from ~ the initial rates of disappearance of the aromatic hydrocarbons relative to that of n-butane in an environmental chamber a t 304 f 1°K and atmospheric pressure. Besides being of fundamental interest, absolute rate constants for the reaction of OH radicals with aromatic hydrocarbons are needed to model the chemistry occurring in polluted urban atmospheres,20 particularly because of the increased use of aromatics in unleaded gasoline in the U.S.A.21 and the inherently high aromatic content of gasoline in Europe and Japan.22 In this work the absolute rate constants for the reaction of OH radicals with a series of aromatic hydrocarbons have been determined using a flash photolysis-resonance fluorescence technique. Experimental Section The apparatus and techniques used have been described p r e v i ~ u s l y and , ~ ~ hence only a brief summary will be given here. OH radicals were produced by the pulsed vacuum ultraviolet photolysis of H20 a t wavelengths longer than the LiF cutoff (21050 A). OH radical concentrations were monitored as a function of time after the flash by resonance fluorescence using a cooled EM1 9659QA photomultiplier fitted with an interference filter transmitting the 3064-A band of OH (A%+, u' = 0 X2n, u" = 0). The intersection of the detection system aperture and the resonance radiation beam defined a fluorescence viewing zone whose cross section was -2 cm in diameter at the center of
-
the reaction vessel. This region was well separated from the reaction vessel walls, thus minimizing the contribution of wall losses to the observed OH radical decays. The flash lamp was operated a t discharge energies of 25-50 J per flash a t repetition rates of one flash every 3 sec. Signals were obtained by photon counting with multichannel scaling. OH radical decay curves, such as that shown in Figure 1, were accumulated from 45 to 1620 flashes, depending on the signal strengths. OH half-lives ranged from 1.22 to 133 msec and the OH radical concentrations were followed over at least 3 half-lives. In all cases the flash duration was negligible in comparison to these OH radical half-lives. All experiments were carried out under flow conditions so that the gas mixture in the reaction vessel was replenished every few flashes in order to avoid the accumulation of photolysis or reaction products. The partial pressure of HzO in the reaction cell ranged from 0.01 to 0.03 Torr. The argon used had a purity level of 199.998%, according to the manufacturer, while gas chromatographic analysis showed the aromatic hydrocarbons to have the following impurities:24 benzene, 0.5% toluene and 10.5% xylene; toluene, 0.1% benzene and 0.05% rn-xylene; o-xylene, 10.2% impurity; m-xylene, 1.0% p-xylene and 0.4% ethylbenzene; p-xylene, 0.3% rn-xylene; 1,2,3-trimethylbenzene, 0.2% of the 1,3,5 isomer and 2.2% of the 1,2,4 isomer; 1,2,4-trimethylbenzene, 1.0%of the 1,3,5-trimethylbenzene; 1,3,5-trimethylbenzene, 0.2% of the 1,2,4 isomer and 0.1% ethyltoluene. A known fraction of the total argon flow was saturated with the aromatic vapor a t 255-293'K, depending on the aromatic hydrocarbon used. Aromatic hydrocarbon partial pressures in this fraction of the argon flow were determined by their ultraviolet absorption using a 9.0-cm path length cell and a Cary 15 spectrophotometer. The absorption cell was calibrated using known pressures of the aromatic hydrocarbons as measured by an MKS Baratron capacitance manometer. All flows were monitored by calibrated flowmeters and the gases were premixed before entering the reaction vessel.
Results The reaction of OH radicals with a series of aromatic hydrocarbons was studied a t room temperature with argon as The Journal of Physical Chemistry, Vol. 79, No. 17, 7975
Hansen, Atkinson, and Pitts
1764 600r
I
TOLUENE
/
400k I
w
\
;k...l oo-
"0
5 TIME
IO (msec)
15
[AROMATIC] 2 molecule c m3-I 3
I
Figure 2. Plots of OH decay rate against aromatic concentration for benzene and toluene: total pressure -100 Torr of argon.
20
Figure 1. Time dependence of the OH resonance fluorescence signal intensity accumulated from 377 flashes of an H20 (0.010 Torr)toluene (0.00145 Torr)-argon (99.7 Torr) mixture with a multichannel scaler channel width of 100 psec and a flash energy of 50 J per flash. the diluent gas. Under the experimental conditions used, the decay of the OH radical concentration, [OH], is given by the integrated rate expression [OH]o/[OH]= So/S = exp[(ko
-
I
2 4 -T R I ME TPY L BE N Z E NE
/
/-
0 m XYLENE
+ kl[aromatic])(t - t o ) ]
(1) where [OHIOand [OH] are the concentrations of OH at times t o and t , respectively, SOand S are the corresponding resonance fluorescence intensities, ko is the first-order rate constant for removal of OH in the absence of added reactant (primarily attributed to diffusion out of the viewing zone and to reaction with impurities), and kl is the rate constant for the reaction
OH + aromatic
-
products - --
(1)
In all experiments exponential decays of the resonance fluorescence signal were observed and the measured decay rate, defined as R = ( t to)-' In (So/S), was found to depend linearly on the concentration of added reactants. The decay rates obtained from OH radical decay curves such as shown in Figure 1 typically had error limits of f2-3%. Equation I was thus obeyed and rate constants kl were accordingly derived from the slopes of plots of the decay rate R against the reactant concentration. In the absence of added reactant, the OH radical decay rates, R = ko, ranged from 5.2 to 10.3 sec-l and were similar to those reported previouslyz3 from this laboratory. Figures 2-4 show plots of the OH radical decay rate against the aromatic hydrocarbon concentration for the aromatic hydrocarbons studied at a total pressure of -100 Torr, while Table I gives the rate constants kl obtained from such plots. For benzene and toluene, total pressures were varied over the ranges 50-600 and 100-620 Torr, respectively. Within the experimental errors, no dependence of the rate constants k l on total pressure was observed over these pressure ranges, and hence rate constants k l for the other aromatic hydrocarbons were determined a t -100 Torr total pressure only. In all cases, a variation of a factor of 2 in the flash energy (from 25 to 50 J per flash) produced no change in the rate constant within the experimental errors, indicating that secondary reactions were negligible under these conditions. The Journal of Physical Chemistry, Vol. 79, No. 17, 1975
~------7,0'3 2
00
[AROMATIC] molecule cm-3
Flgure 3. Plots of OH decay rate against aromatic concentration for rn-xylene, gxylene, and 1,2,etrimethylbenzene; total pressure -100 Torr of argon. 600-
-
1.3 5-TRIMETHYLBENZENE I
/
1.2 3-TRIMETHYLBENZENE
[AROMATIC] molecule
ern-'
Figure 4. Plots of OH decay rate against aromatic concentration for o-xylene, 1,2,3-trimethylbenzene, and 1,3,5-trimethylbenzene;total pressure -100 Torr of argon.
Discussion The initial OH radical concentration after the photolysis flash can be estimated from the flash energies and H20 concentrations used and from previous ~ o r k l to ~ ,be~ ~ -10" molecules cm-3. From these initial OH radical concentrations and the aromatic hydrocarbon concentrations
Rsaction of OH Radicals with Aromatic Hydrocarbons
1765
TABLE I: Rate Constants kl for the Reaction of O H Radicals with a Series of Aromatic Hydrocarbons h ; , cm3 molecule-l sec-'
Aromatic hydrocarbon
Temp, "C
Total pressure, Torr
0
a
Benzene
50.4 -t 0.4 24.8 f 0.5 (1.28 i 0.04) x 10-l2 (1.24 f 0.12) x 24.3 f 0.4 100.0 f 0.2 (1.22 f 0.06) x 10-l' 24.5 f- 0.5 199.9 i 0.6 (1.20 f 0.06) x lo-'' 24.6 f 0.3 600.0 f 0.4 (1.24 i 0.11) x lo-'' Toluene 25.0 f 0.5 99.7 f 0.3 (5.90 i 0.16) x 10-l2 249.7 f 0.6 24.6 f 0.2 (5.69 f 0.27) x (5.78 * 0.58) x 10-l' 618.6 f 0.5 24.7 f 0.5 (5.75 f 0.16) x lo-'' (1.53 i 0.15) x lo-" 24.8 f 0.2 o-Xylene 101.3 f 0.2 99.9 f 0.3 m -Xylene (2.36 f 0.24) x 24.1 f 0.5 p-Xylene 99.6 * 0.3 (1.22 0.12) x 10-11 24.1 f 0.6 1,2,3 -Trimethylbenzene (2.64 + 0.26) x lo-" 23.9 f 0.4 99.9 f 0.3 1,2,4-Trimethylbenzene 100.3 i. 0.3 23.7 f 0.6 (3.35 * 0.34) x lo-" 23.9 0.3 (4.72 -t 0.48) x lo-'' 1,3,5-Trimethylbenzene 100.4 f 0.3 0 Error limits are the least-squaresstandard deviations of plots such as those shown in Figures 2-4. b Error limits shown are the estimated overall error limits which include the least-squares standard deviations as well as estimated accuracy limits of other parameters such as total pressure and aromatic concentrations.
*
TABLE 11: Comparison of Room Temperature Rate Constants, kl,for the Reaction of O H Radicals with Aromatic Hydrocarbons from the Present Work with Literature Values a n d with the Room Temperature Rate Constants for the Reaction of O(3P) Atoms with Aromatic Hydrocarbons
OH radical cm3 molecule-l sec-'
kl x
Aromatic hydrocarbon
This worka
0 ( 3 atom ~) I? x 1013, cm3 molecule-1 sec-l
Ref l g b
Ref 18'
Ref 9d
Ref 24
Benzene 1.24 i 0.12 53.8 1.59 f 0.12 0.24 + 0.033 4.2 f 1.5 6.11 f 0.40 0.75 * 0.075 Toluene 5.78 * 0.58 12.8 f 3.8 1.74 i: 0.18 o-Xylene 15.3 f 1.5 m -Xylene 23.6 f 2.4 23.2 f 1.7 18.7 3.52 * 0.35 / I -Xylene 12.2 f 1.2 12.3 i 2.5 1.81 * 0.18 23 f 5 11.5 * 1.2 1,2,3-Trimethylbenzene 26.4 i 2.6 1,2,4-Trimethylbenzene 33.5 * 3.4 33 i 5 10.0 i 1.0 1,3,5-Trimethylbenzene 47.2 * 4.8 51.5 f 6.5 27.9 i 3.3 0 Total pressure 250 Torr ( A r ) . Total pressure 1 atm of air; rates relative to OH + n-butane placed on an absolute basis using t h e rate constant for OH + n-butane as 3.0 X 10-l2 cm3 molecule-' sec-1,8-10-1*Total pressure 100 Torr (He).d Total pressure -1 Torr ( H e ) , mixture of isomers.
1
used it can be estimated that errors in the measured rate constants due to the reaction of OH radicals with reaction products would be typically
