J. Phys. Chem. 1986,90, 441-444
Pressure Dependence of the Rate Constant for the Reaction HO, HOPNO* M (M = N,, 0,) a t 298 K
+
+ NO, + M
-
441
Michael J. Kurylo* and Philip A. Ouellette Center for Chemical Physics, National Bureau of Standards, Gaithersburg, Maryland 20899 (Received: July 26, 1985)
The pressure dependence at 298 K of the rate constant for the combination reaction between H 0 2 and NO2 was investigated by flash photolysis kinetic absorption spectroscopy. Measurements were made at N2 and O2pressures ranging from 25 to 600 torr and the data were fit to an expression suitable for describing the pressure dependence of reactions in the falloff region. Potential sources of error in these measurements are discussed. The present results are compared to earlier measurements of this reaction system and their importance with respect to atmospheric chemistry is detailed.
Introduction
The formation of pernitric acid ( H 0 2 N 0 2 )in the troposphere and stratosphere, its subsequent photochemistry, and its reactions play important roles in the catalytic cycles involving odd-hydrogen radicals (HO,) and oxides of nitrogen (NO,).’ The identification of H 0 2 N 0 2as a primary product in the termolecular reaction between hydroperoxy radicals and nitrogen dioxide2” underscores the need for a detailed understanding of the kinetics of this reaction. HO2 NO2 M H02N02 M (1)
+
+
+
+
This detailing is complicated by the fact that reaction 1 is in the falloff region between second- and third-order kinetics at tropospheric and stratospheric pressures. The only direct measurements of the rate constant for reaction l in the third-order region are the discharge flow laser magnetic resonance experiments of Howard6 at 300 K in the range 0.5-3 torr. Cox and Patrick’ extended the experimental data base on the rate constant kl to the transition region (40-600 torr) using molecular modulation spectrometry for measurements at 283 K. A simple Lindemann-Hinshelwood analysis of their data yields a value for the limiting third-order rate constant in reasonable agreement with that measured by Howard6 at low pressure. More recently, Sander and Petersod have measured the temperature dependence (229-362 K) of kl over the pressure range 50-700 torr for several diluent gases. However, the values predicted at 283 K by these flash photolysis/ultraviolet absorption measurements lie systematically about 50% above the kl values from ref 7. A calculation of the falloff parameters for reaction 1 using the data of Sander and Peterson8 and the expression recommended by Troe9 results in third-order rate constants which agree reasonably well with those of ref 6 and a high-pressure limit which differs only slightly from that calculated in the RRKM model of Baldwin and Golden.’O The most recent atmospheric modeling recommendations for kl by the NASA Panel for Data Evaluation” are based on the data of ref 8. However, in view of the importance of this reaction and the difference in the measured rate constants from the two ~~
~~
(1) National Research Council, ‘Causes and Effects of Stratospheric
Ozone Reduction: An Update”, National Academy Press, Washington, DC, 1982. (2)
R. Simonaitis and J. Heicklen, J. Phys. Chem., 80, 1 (1976). (3) H. Niki, P. C. Maker, C. M. Savage, and L. P. Breitenbach, Chem. Phys. Lett., 45, 564 (1977). (4) P. L. Hanst and B. W. Gay, Enuiron. Sci. Techno]., 11, 1105 (1977). (5) S. Z. Levine, W. M. Uselman, W. H.Chan, J. G. Calvert, and J. H. Shaw, Chem. Phys. Lett., 48, 528 (1977). (6) C. J. Howard, J . Chem. Phys., 67, 5258 (1977 (7) R. A. Cox and K. Patrick. Int. J. Chem. Kinet., 11, 635 (1979). (8) S.P. Sander and M. E. Peterson, J . Phys. Chem., 88, 1566 (1984). (9) J. Troe, J . Phys. Chem., 83, 114 (1979). (10) A. C. Baldwin and D. M. Golden, J. Phys. Chem., 82, 644 (1978). (11) W. B. DeMore, J. J. Margitan, M. J. Molina, R. T. Watson, D. M. Golden, R. F. Hampson, M. J. Kurylo, C. J. Howard, and A. R. Ravishankara, Evaluation No. 7 of the NASA Panel for Data Evaluation, J.P.L. Publication 85-37, 1985.
high-pressure investigations, there is a need for further study under conditions directly applicable to atmospheric modeling. In this manuscript we report the results of our recent investigation (using flash photolysis kinetic absorption spectroscopy) of the room temperature pressure dependence of reaction 1 for M = N2 and 02.As we will detail later, the data analysis is most susceptible to systematic error for low values of kl, i.e., at pressures below 100 torr. In view of the importance of measurements in this region (particularly for measurements of the temperature dependence of kl in the falloff region) we devoted considerable effort to the development of appropriate mathematical procedures for the data analysis. Experimental Section
The flash photolysis kinetic absorption spectroscopy apparatus has been described in detail in the preceding rnanuscript.l2 The same Pyrex concentric-cylinder flash lamp/reaction cell equipped with quartz analysis optics was used in this study. As before,12 HOz radicals were produced via the flash photolysis of mixtures of C12, CH,OH, and O2 in the presence of the desired amount of diluent gas (either N 2 or 0 2 )Reactant . concentrations were chosen such that radical production by the reaction sequence Cl2
C1
+ hv
+ CH30H
CH2OH
-+
+0 2
-
2c1
+ HCl CH2O + HO2
CH2OH
+
was complete within 10-20 p s after the flash. The addition of N O 2 a t concentrations far in excess of the initial H 0 2 concenallowed for essentially pseudo-first-order kinetic tration, [HO,],,, decay curves (although corrections for deviations from this behavior had to be made under certain conditions and will be discussed later). Following their production, H 0 2 radicals were monitored in real time by their ultraviolet absorption of light from a xenon arc lamp at 225 nm by using eight passes of the White optics (resulting in an effective path length of 450 cm as determined in our earlier investigation12). The analysis wavelength was selected to minimize the interference due to light absorption by product H 0 2 N 0 2 without sacrificing very much sensitivity in H 0 2detection. For this analysis, a monochromator resolution of 3 nm was employed. Decay profiles from 50 to 150 flashes were averaged to achieve an enhanced signal-to-noise ratio prior to mathematical reduction of the data. The use of the mylar reflector, the flash lamp operation, and the cell temperature regulation were all as described in our earlier H 0 2 + H 0 2 study.12 Two important considerations restrict the experimental conditions for studying reaction 1 from those of ref 12. These considerations also dictate the mathematical analysis of the experimental absorption decay curves. The first of these is the potential (12) M. J. Kurylo, P. A. Ouellette and A. H. Laufer, J. Phys. Chem., preceding paper in this issue.
This article not subject to US.Copyright. Published 1986 by the American Chemical Society
Kurylo and Ouellette
I
7 5 . 0 \
0.0
300 . 0
' . . . '......
,
I
.....
360 . 0
340 .E¶
Waveiensth
\
'120.0
Cnm3
Figure 1. The solid line represents transmission of a BrCl filter. Equilibrium filter mixture contained 88 torr of Br2,88 torr of C12, and 224 torr of BrCI. Path length = 0.5 cm. Resolution = 1 nm. The dotted line shows the Cl, absorption cross section (taken from ref 11). The dashed line is the product of the NO2 absorption cross section and photolysis quantum yield as given in ref 11. Ordinate units are 3'% transcmz for the CI2and NO2 absorption mission for the BrCl filter and curves.
for significant error in the measured H 0 2 decay rates due to the rapid reaction HO2 NO OH + NO2 (2)
+
+
where N O is produced from the photolysis of NO2 in the 300cm3 molecule-' 400-nm region. Since, at 298 K, k2 = 8.3 X s-l, the influence of this reaction can be kept below 10% under conditions where k , > 1.7 X lo-', cm3 molecule-' s-I and NO2 photolysis is less than 0.2%. The latter can be accomplished (as in earlier s t u d i e ~ ~ . by ' ~ )filtering the photolysis flash with an appropriate equilibrium mixture of C12/Br2/BrC1. In so far as we wanted to minimize N O 2 photolysis without dramatically affecting the C12 photolysis rate, we selected a filter mixture which had its major absorption in the 340-400-nm region. Unlike the filter selected in ref 8 which contained excess C12, we used an equilibrium mixture which contained near equal amounts of C12and Br2. The transmission characteristics of this filter ( B e l = 224 torr, Br, = 88 torr, C12 = 88 torr, cell length = 0.5 cm) are shown along with the C12and NO2 absorption spectra in Figure 1 indicating its utility in curtailing NO2 photolysis above 340 nm with noticeably less of an effect on C12 photolysis in the region around 320 nm. It should be noted that, while this equilibrium mixture appears optimum, its use is restricted to temperatures above 280 K due to Br, vapor pressure limitations. At lower temperatures a filter mixture containing less Br2 would have to be used. With the flash lamp filter chamber filled with the indicated mixture, the extent of NO2 photolysis was examined as a function of flash energy, Le., discharge voltage. This was accomplished by monitoring the N O 2 in absorption at 400 nm before and after multiple flashes. Based on these results, flash photolysis conditions were selected under which N O production was limited to less than 0.15% of the NO2 present. Initial H 0 2 concentrations were then varied by changes in [Cl,]. The second consideration in performing these kinetic studies centers around the initial concentration of H 0 2 and thus on the level of second-order component (due to reaction 3) in the H 0 2 H02 + HO2 -+ H202 + 0 2 (3) pseudo-first-order decay curves. Appreciable occurrence of reaction 3 would lead to erroneously high values of k, calculated from a purely first-order analysis. This second-order contribution becomes increasingly important with decreasing pressure (decreasing k,) and decreasing [NO,]. (Although k, is also pressure dependent, its dependence is not nearly as great as that of kl.) To quantitatively examine this problem, we generated (via computer modeling)14 numerous H 0 2 decay profiles covering a wide (13) S. P. Sander and R. T. Watson, J . Phys. Chem., 84, 1664 (1980).
range of [HOzlO,[MI, and [NO,]. For these calculations, k3 was chosen as the mean of our value12 and that reported by Sander et al.15as a function of pressure and kl was taken from the NASA recommendation." These decay curves (of the form optical density vs. time) were then analyzed by a nonlinear least-squares exponential procedure (to be described) and the calculated first-order decay rates were compared to the pure first-order components due solely to reaction 1. As expected, the calculated rates were faster, the percent increase being a function of the [N02]/[H02], ratio and [MI. The absolute value of this difference, however, remained essentially constant for a given [H02], and varied linearly with k3. Similarly, for a given value of k3 (Le., at a given total pressure and constant temperature) this difference varied linearly with [HO2lo. We were therefore in a position to correct H 0 2 decays in the study of reaction 1 for contributions due to reaction 3 over a wide range of operating conditions thereby enabling us to carry out our investigations at lower pressures where the effects from the latter reaction would be most noticeable. For the majority of the experiments reported here, such corrections were typically less than 5%. However, at low [NO2](Le., low first-order decay rates) and particularly at the lowest pressure of inert gas (25 torr) corrections as high as 25% were required. As a verification of this correction procedure, several experiments were conducted over a very wide range of [HO,], at fixed [MI and fixed [NO,]. The corrected decay rates agreed well within their standard deviations. The desired gas mixtures were prepared by combining calibrated flows of the appropriate gases in a mixing chamber just upstream from the reaction cell. The total flow rate at all pressures was such that complete sample replacement occurred every three to five flashes ( 30 s). Approximately 250 experiments (each signal averaged over 50-150 flashes) were conducted with the following ranges of concentrations (expressed in molecules/cm3): C12, (8-1 6) X 10l6; C H 3 0 H , (2-8) X and NO2, (4-50) X The desired total pressures were then achieved through the addition of ultrahigh-purity N2 or 02. For the N 2 runs (1-3) X 10" molecules/cm3 of O2was always present. The variation in [Cl,], coupled with changes in flash energy from 100 to 200 J, resulted in a range of [H0210from 2 X lOI3 to 6 X lOI3 molecules/cm3. The reactant gases had the following stated purities: C12, 99.96%, used after redistillation; C H 3 0 H , Spectral Grade, used after vacuum drying and redistillation; NO2, 99.5% minimum, redistilled and stored as a dilute mixture in 0,; 02,99.99%; and N,, 99.999%. N
Results and Discussion The most common analysis of first-order kinetics involves a linear regression fit of In (concentration) vs. time. Because of the potential for such semilogarithmic plots to bias one end of the concentration spectrum without proper weighting, we have always employed a nonlinear least-squares exponential analysis. The modification of first-order absorption decay data to account for residual absorption has been discussed by othersI3 and is applicable in the present study due to absorption of the analysis light by product H 0 2 N 0 2 . In a manner similar to our analysis of kinetic data on reaction 3,12 we have used a nonlinear least-squares fitting of the modified equation In ( I / I J = (lCou) exp(-kt) (1) thereby selecting "best fit" values for I , (the post flash, postreaction analysis light intensity), (ICou),and k . For the specific reaction system at hand, u is a composite absorption cross section and equals UHO,- aHO2NO2+ UNO,. A slight additional term can also be included to account for any H202formed via reaction 3. Thus, Co = [HO,], can be calculated from the mathematical analysis (knowing the absorption path length I) and can be used to determine the magnitude of any correction to be applied to k (the first-order decay rate). In this computation, the values of the absorption coefficients at 225 nm were taken from ref 11. The bimolecular rate constants (values of k l expressed as a second(14) R. L. Brown, National Bureau of Standards Report, NBSIR 81-77, May 1981. (15) S. P. Sander, M. Peterson, R. T. Watson, and R. Patrick, J . Phys. Chem., 86, 1236 (1982).
The Journal of Physical Chemistry, Vola90, No. 3, 1986 443
Pressure Dependence of Reaction between H 0 2 and NO2
