3416
J. Phys. Chem. 1982, 86, 3416-3418
A Kinetic Study of the HO,
+ H02 Reaction
R. Slmonaltls and Jullan Helcklen* Department of Chemistry and Ionosphere Research Laboratoty, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received November 19, 1981; In Final Form: Mey 3, 1982)
-
The kinetics of the reaction HOz + HOz products (1) were studied in the pressure range of 5-770 torr at 296 K with flash photolysis of Clz in the presence of CH30H and 02 as a source of H02radicals. These radicals were monitored by ultraviolet absorption at 220 nm. The rate coefficientkl was found to be pressure dependent with a low-pressure limiting value of klo = (1.4 f 0.2) X cm3s-l. Above about 50 torr pressure the rate coefficient increases and becomes (2.5 f 0.1) X cm3s-l at 1 atm of Nz. Thus this reaction proceeds both by direct abstraction as well as addition.
Introduction The reaction of HOP with itself
Pyrex cylindrical cell provide the photolysis radiation. For low-temperaturestudies the reaction vessel was surrounded by a jacket through which cooled CH30H was circulated HOz + HOz HzOz + 02 (1) for temperature control and a second evacuated jacket for insulation. The analysis radiation at 220 nm is provided is of importance in the atmosphere since it is the most by a 30-W deuterium source. The differential absorption important source of atmospheric H202and leads to chain signal due to HOz is detected with Hamamatsu 1P28 termination due to HO, removal. The kinetics of this photomultipliers placed at the exit slits of two monoreaction have now been extensively studied and have been chromators. The analysis monochromator (Bausch and reviewed recently.1*2 Lomb) was set for most runs at 220 nm with a spectral The present picture shows that reaction 1 is complex bandwidth of 3.0 nm. For a few runs a band width of 1.5 since it shows a pressure d e p e n d e n ~ e , ~a -negative ~ temnm was employed in order to test the effect of the bandperature ~oefficient,~.~ and a humidity dependen~e.~.'-~ width on the determination of kl. To improve the sigThese observations have led to the conclusion that reaction nal/noise (S/N) ratio, signals from several flashes, when 1is not a bimolecular abstraction reaction but is instead necesssary, were accumulated in a Tracor-Northern (NS a three-body reaction leading to the formation of a rela570) signal averager. The detection sensitivity for HOz at tively long-lived H204complex. 220 nm is -0.1 mtorr and at 1mtorr S / N N 10 with 30 Recently Sander et al.l0 studied the H02-HO, reaction flashes. between 100 and 700 torr pressure and found a pressure The chlorine (Matheson Research Purity) was purified effect in this region. Their data lead to a third-order rate by distillation. Methanol (Baker Analyzed Reagent Abcoefficient of 5 X cm6 s-l. Their extrapolated data solute) was dried with molecular sieves and distilled before also suggest a limiting low-pressure direct abstraction reuse. The oxygen and N2 was purified by passing them action with a rate coefficient of -1.5 X cm3 s-l. through a trap maintained at -196 "C. In order to avoid There is still no systematic study of reaction 1 over a any possible reaction between the Clz and CH30H, they wide range of pressure, and the value of kl is still uncertain were stored separately and were mixed just prior to enby a factor of two.2 In this paper we report a study of tering the reaction vessel. reaction 1using the flash photolysis-ultraviolet absorption technique (FP-UV) over a pressure range of 5.0-770 torr Results at 296 K. H 0 2 radicals are produced by flash photolysis of ClzCH3OH-OZmixtures. The reactions leading to HOz forExperimental Section mation and removal are The flash photolysis-ultraviolet absorption apparatus Clz + hv 4 2C1 (2) has been described before." Two 1-m long flash lamps C1+ CH3OH HCl + CHZOH (3) placed at opposite sides of a 1-m long and 2.5-cm diameter CHzOH + 0 2 HOP CHzO (4) (1)C. J. Howard. "Proceedines of the NATO Advanced Studv InstiHOz + HO2 HzO2 + 0 2 (1) tu& on Atmospheric Ozone", U.g Department of Transportation-Report No. FAA-EE80-20. 1979. Reactions 5 (k5 = 2 X cm3sec-' 2, and 6 (k6 = 2 X (2)'Chemical Kinetic and Photochemical Data for Use in StratoC1+ HOz HC1+ 0 2 (5) spheric Modelling", National Aeronautics and Space Administration, Jet Propulsion Laboratory Publication 81-3,1981. CHpOH + HOP CH30H + 02 (6) (3)R. A. Cox and J. P. Burrows, J. Phys. Chem., 83, 2560 (1979). (4)B.A. Thrush and J. P. T. Wilkinson, Chem. Phys. Lett., 66,441 cm3 s-l, assumed maximum possible value) are not im(1979). portant for the [H02]/[CH30H]and [HOZ]/[0,] ratios (5)J. P. Burrows, D. I. Cliff, G. W. Harris, B. A. Thrush, and J. P. T. London, Ser. A, 368,463 (1979). Wilkinson, R o c . R. SOC. employed, since reactions 3 and 4 are fast: k3 = 6.33 X lo-" (6)R.-R.Lii, R.A. Gorse, M. C. Sauer, and S. Gordon, J.Phys. Chem., cm3 s-l l2 and k , = 2 X cm3~4-I.l~Thus it follows that 83,1803 (1979). reactions 3,4, and 1 are the only fate of C1, CHzOH, and (7)J. E. Hamilton and R.-R. Lii, Int. J . Chem. Kinet. 9,875 (1977). (8)W. B. Demore, J. Phys. Chem., 83,1113 (1979). HOz radicals, respectively. -+
-
-+
+
-+
+
(9)C. J. Hochanadel, T. J. Sworski, and P. J. Ogren, J.Phys. Chem., 84,3274 (1980). (10)S. P. Saunder, M. Peterson, R. T. Wataon, and R. Patrick, J. Phys. Chem., 86,1236 (1982). (11)R.Simonaitis and J. Heicklen, J. Phys. Chem., 85,2946 (1981).
0022-385418212086-3416$0 1.2510
(12)J. V. Michael, D. F. Nava, W. A. Payne, and L. J. Stief, J. Chem. Phys., 70,3652 (1979). (13)H.E.Radford, Chem. Phys. Lett., 71, 195 (1980).
0 1982 American
Chemical Society
Kinetic Study of the HOP
+ HO, Reaction
The Journal of Physical Chemistry, Vol. 86, No. 17, 1982 3417
TABLE I: Flash Photolysis o f Cl,-CH,OH-O, Mixtures, Rate Coefficients for the HO,
+ HO, Reaction at 296 K
. , . ... ' .. . .' ...' , . . ' .. .,.' .. . . . ....' ,
.'.... .
95c
I 0
'. I
I
4.0
I
80
TIY. m M
J
I
12.0
16.0
Figure 1. Transmittance vs. time for run with [MI = 6.0 torr: [HO,], = 1.64 X lo1' ~ m - k~, ;= 1.4 X lo-'* cm3 s- : two flashes.
oov
IO
20
30
40
io
60
io
810
90
Inert Gas N, 2.5 2.00 3.73 3.26 3.60 2.4 2.00 3.73 2.5 1.71 3.93 2.6 2.2 2.12 3.28 1.94 3.35 2.3 2.39 3.00 2.0 2.52 2.98 2.0 1.45 2.63 1.8 1.45 2.60 1.8 2.0 1.47 2.93 2.1 1.26 3.13 1.5 2.49 2.21 1.6 1.73 2.42 1.5 1.90 2.20 1.6 2.05 2.35 1.3 1.45 1.88 1.4 2.73 2.13 1.4 1.64 2.10 1.3 1.98 2.00 Inert Gas He 500 1.58 2.40 1.6 1.44 2.85 1.8 500 1.44 2.73 1.8 500 1.8 1.50 2.63 500 1.52 2.68 1.8 500 1.56 2.18 1.5 52 51 1.58 2.13 1.4 51 1.7 1.58 2.45 50 2.3 2.25 1.5 1.4 50 2.8 2.08 The partial pressure of the C1,-0,-CH,OH mixture was
