Rate constants for the reaction of Cl atoms with O 3 at temperatures from 298 to 184K.

Transcription

1 Rate constants for the reaction of Cl atoms with O 3 at temperatures from 298 to 184K. S.D. Beach, I.W.M. Smith* and R.P. Tuckett Int. J. Chem. Kinetics., (2002) 34, DOI: /kin This is the author s version of a work that was accepted for publication in International Journal of Chemical Kinetics. Changes resulting from the publishing process, such as editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. A definitive version was subsequently published in the reference given above. The DOI number of the final paper is also given above. Professor Richard Tuckett (University of Birmingham) / July

2 Rate Constants for the Reaction of Cl Atoms with O 3 at Temperatures from 298 to 184 K. SIMON D. BEACH, IAN W.M. SMITH* AND RICHARD P TUCKETT School of Chemistry, The University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K. Received June 2001; accepted 2001 ABSTRACT: Using the standard, low pressure, discharge-flow technique, with resonance fluorescence in the vacuum ultraviolet to observe Cl atoms, rate constants have been determined for the reaction of Cl atoms with O 3 at temperatures down to 184 K. The measured rate constants for K fit the Arrhenius expression: k(t) = (3.6 ± 0.7) exp(( ± 50 K) / T) cm 3 molecule -1 s -1. The results extend the data on this key atmospheric reaction to slightly lower temperatures. The data are in fairly good agreement with those currently in the literature but suggest that the rate constant approximately 15% lower than that given by currently recommended rate expressions at the lowest temperatures found in the stratosphere. * Correspondence to: I. W. M. Smith (i.w.m.smith@bham.ac.uk) 2

3 INTRODUCTION The role of the reaction o -1 Cl + O 3 ClO + O 2 H = kj mol (1) r 298 in the catalytic destruction of stratospheric ozone has been recognised since the seminal work of Molina and Rowland [1]. It is now clear [2] that this reaction has a central role in all the catalytic cycles involving ClO x, not just that proposed by Molina and Rowland in which reaction (1) and that between ClO radicals and O( 3 P) atoms serve as the two chainpropagating steps. In 1994, Wennberg et al. [3] published the results of the SPADE (Stratospheric Photochemistry, Aerosols and Dynamics Expedition) campaign in which simultaneous measurements were made of the concentrations of a number of radical species in the lower stratosphere. In the interpretation of their results, Wennberg et al. suggested that the contributions of the various catalytic cycles to ozone destruction needed re-assessment and, in particular, that new laboratory measurements were required of the rate constants for the key reactions in the cycles down to the lowest temperatures found in the lower stratosphere. Largely because of its importance in stratospheric chemistry, the kinetics [4 12] of reaction (1) and the nascent states [13] of the reaction products have been the subject of numerous experimental studies. Rate constants have been measured both by pulsed photolysis and discharge-flow techniques. The methods used in these kinetic measurements and the results from them are summarised in Table 1. Although there have been a large number of investigations, only the experiments of Nicovich et al. [9] have extended to temperatures below 200 K. In the present examination of the kinetics of reaction (1), we employed a conventional discharge-flow apparatus with resonance fluorescence in the vacuum ultraviolet to follow 3

4 changes in the concentration of Cl atoms. The flow tube could be operated to temperatures as low as 184 K. EXPERIMENTAL METHOD The kinetic experiments were carried out in a Pyrex flow tube (length 1.5 m and internal diameter 2.2 cm) that was equipped with a sliding injector so that molecular species could be introduced at different points along the reactor. The central part of the flow tube (length 1.3 m) was surrounded by a double jacket. The outer jacket was evacuated and refrigerants were passed through the inner jacket to provide temperatures down to 184 K. For temperatures of 210 K and above ethanol was circulated through the inner jacket, whereas for lower temperatures n-pentane was used. In all cases, the circulating fluid was passed through a bath containing ethanol cooled by appropriate additions of liquid nitrogen. Relative concentration of Cl atoms were observed via their resonance fluorescence, mainly in the (4s 1 3p 4 ) 4 P 3/2 (3p 5 ) 2 P 3/2 transition at nm, at an observation station, located at the downstream end of the flow tube. Signals from the solar-blind photomultiplier tube (Electron Tubes, type 9403, sensitive between 120 and 210 nm) used to detect the fluorescence from Cl atoms were amplified using a time constant of 10 s and recorded on a chart recorder. Further details of the flow tube, as well the design of the resonance lamp and the observation station have been given elsewhere [14]. Cl atoms were generated upstream of the main flow tube by passing a mixture of ca. 0.5% Cl 2 in helium through a microwave discharge (EMS, Microtron 200). This gas flow constituted ca. 25% of the total, the remainder being largely the main flow of He carrier gas, together with the smaller flow of gas introduced through the moveable injector. In order to estimate the flow, and hence the initial concentration, of chlorine atoms, as well as their rate of loss on the flow tube walls, in preliminary experiments they were titrated with ClNO introduced through the moveable injector (see below) [15]. ClNO was prepared by allowing NO and Cl 2 to react and then freezing out the ClNO and pumping on it. In these experiments. 4

5 Ozone was generated by passing O 2 through a commercial ozoniser, storing the eluting gases on silica gel cooled to ca. 175 K, as described previously [16]. Mixtures of ozone, diluted in He, were then prepared in a 10 Pyrex storage bulb, which was transferred to the gas handling apparatus that was connected to the flow tube. Absolute concentrations of ozone entering the flow tube were determined by measuring its absorption of nm radiation in a 10 cm long absorption cell. Radiation from a pen ray mercury passed through the cell and an interference filter (Corion) with peak transmittance at nm. The intensity of the transmitted radiation was measured with and without O 3 flowing through the cell. This cell was placed so that the diluted ozone mixture could be diverted through it just before entering the sliding injector. In order to optimise these measurements, by ensuring that between 10% and 90% of the nm radiation was absorbed, it was necessary to control the flow of the mixture through the cell so that its pressure and thus the ozone concentration were higher than those in the flow tube.. In converting the measured absorbances to ozone concentrations, and hence mole fractions of O 3 in the flowing gas, we used the Beer-Lambert law and an absorption cross-section of cm 2 molecule -1 [12]. It was estimated that this procedure allowed us to estimate the O 3 concentrations introduced into the flow tube to ± 5%. In experiments at reduced temperatures, the tip of the injector through which O 3 was admitted was at least 5 cm inside the cooled section of the flow tube for all measurements. Temperatures were measured, essentially simultaneously with the kinetic measurements, by inserting a calibrated thermocouple in the moveable injector. These measurements demonstrated that the temperatures along the flow tube were constant to ± 2 K during the kinetic measurements. All experiments were carried out with helium as the main carrier gas. Linear flow velocities between 11.6 m s -1 and 17.5 m s -1 were employed. Gas flows were monitored using calibrated gas flow controllers (Bronkhurst Hi-Tec and MKS Instruments) and the total pressure in the flow tube was measured with a 0 10 Torr Baratron pressure gauge. High purity helium (Air Products, GC grade; %) and argon (BOC Ltd., %) were used, without further purification, as the carrier gases in the main flow tube and in the resonance lamps, respectively. 5

6 RESULTS AND DISCUSSION Before measurements on the reactions between Cl atoms and O 3, we carried out two sets of measurements designed (a) to determine the fractional dissociation of Cl 2 in the microwave discharge under the conditions of our experiments, and (b) the rate of loss of Cl atoms on the walls of the flow tube over the range of temperatures used in our experiments. Both these sets of experiments involved titrations of the Cl atoms with ClNO [15]. In the first series of measurements, the flow tube was maintained at room temperature and ClNO was introduced through the central injector well upstream of the observation point. This allowed sufficient time for the second-order titration reaction Cl + ClNO Cl 2 + NO (2) (k 2 = cm 3 molecule -1 s -1 [11]) to go to completion, as long as the initial concentration of Cl atoms was sufficiently high. In practice, titrations of Cl atoms with ClNO were performed through a range of Cl 2 flows passing through the discharge, which would have corresponded to Cl 2 concentrations in the flow tube of molecule cm -3 in the absence of the discharge. The results of these titrations with ClNO indicated that, throughout this range of added Cl 2, ca. 15% of the Cl 2 was dissociated in the discharge. Although these measurements were made at higher initial concentrations of Cl 2 than in the kinetic measurements on the Cl + O 3 reaction, extrapolation to lower initial Cl 2 flows indicated that similar fractional dissociation was likely at lower Cl 2 flows. This inference of a constant fractional dissociation of Cl 2 in the discharge was confirmed by measuring, in the absence of ClNO, the intensity of the resonance fluorescence signal (I RF ) as the flow of Cl 2 through the microwave discharge was varied. The variation of the I RF with the flow of Cl 2 was linear, down to flows of Cl 2 corresponding to those employed in the experiments on the Cl + O 3 reaction. From these experiments, it was also possible to estimate that the minimum detectable concentration of Cl atoms in our system, with an integration time of 10 s and a 6

7 signal-to-noise of 1:1, was ca molecule cm -3. Most of our kinetic experiments were performed with an initial Cl atom concentration of ca molecule cm -3. In a second series of experiments, Cl atoms were titrated against ClNO which was introduced though the moveable injector at various points within the constant temperature portion of the flow tube. In this way, we could estimate the rate of loss of Cl atoms on both the inner wall of the main flow tube and the outer wall of the central injector. The results were analysed assuming first-order heterogeneous loss and yielded rate constants that, within the fairly large experimental errors (typically ca. 30%), varied from about 10 s -1 at 298 K to about 4 s -1 at 210 K. This decrease in the rate of wall loss as the temperature decreased was unexpected. It indicates that, at least for reactions of Cl atoms, it is unnecessary to adopt the turbulent flow technique [10] to reduce the effects of heterogeneous loss of the atoms at low temperatures to acceptable limits. The main experiments on the Cl + O 3 reaction were carried out by the well-established flow tube method under pseudo-first-order conditions, [Cl] 0 << [O 3 ] 0. For a given addition of ozone, corresponding to [O 3 ], the intensity of resonance fluorescence, I RF, from Cl atoms was measured as a function of the distance (x) between the observation zone and the point at which O 3 was added to the flow. In agreement with the findings of many others, measurements on Cl atoms in the absence of O 3 showed that the resonance fluorescence signal at the observation point increased slightly as the sliding injector was withdrawn. This effect is due to the reduction in the loss of atoms or radicals on the outside wall of the sliding injector as less of it is exposed to the flow of atoms or radicals. For such measurements plots of n (I RF ) x versus x, where (I RF ) x is the intensity of the resonance fluorescence signals from Cl atoms at the distance x from the tip of the injector to the observation point, were linear with a positive slope. Under pseudo-first-order conditions, the data obtained could again be analysed by plotting n (I RF ) x versus x, but now the slope of the observed line was negative as reaction with O 3 lowered the concentration of Cl atoms reaching the detector. A number of such plots are shown in Fig. 1. The initial estimate of the pseudo-first-order rate constant (k 1st,exp ) for reaction between Cl atoms and a chosen concentration of O 3 was determined by taking the difference in the slopes 7

8 of the lines in the presence and absence of O 3, like those shown in Fig. 1, and multiplying the result by the linear flow velocity. The resultant values of k 1st,exp now corrected for the change in heterogeneous loss as the injector was withdrawn, were further corrected to allow for deviations from plug-flow as recommended by Kaufman [17]. The diffusion coefficient for Cl atoms in helium was estimated for the pressure of the experiments (between 1.2 and 2.1 Torr) by assuming it to be the same as that for Ar atoms in helium. The resultant correction for the effects of diffusion changed the first-order rate constants by less than 5%. All the values of these corrected rate constants (k 1st ) obtained at a given temperature were then plotted against [O 3 ] to obtain k 1 for that temperature from the gradients of these plots. Examples are given in Fig. 2, and the values obtained for k 1 are listed in Table 2. The errors cited correspond to 95% confidence limits. Fig. 3 shows an Arrhenius plot of the second-order rate constants determined in the present work. The best fit straight line yields the Arrhenius expression: k 1 (T) = (3.6 ± 0.7) exp (( 310 ± 50 K) / T) cm 3 molecule -1 s -1 Although our actual measurements gave a value of k 1 (298 K) somewhat higher than those recommended by the NASA [11] and IUPAC [12] evaluation panels, the above Arrhenius expression yields a value in good agreement with those recommended. In terms of temperature range, our experiments are closest to those undertaken by Nicovich et al. [9]. Their data are shown on Fig. 3 and it can be seen that they are in good agreement with our data at the higher end of the common temperature range but that there is significant disagreement below ca. 220 K. They chose to fit their data to two Arrhenius expressions covering different ranges of temperature. The parameters from these fits, those from other experiments over similar ranges of temperature, and our own results are shown in Table 1. We also include in Table 1 the Arrhenius expressions from the two latest evaluations [11,12]. It can be seen that our data are quite consistent with the recommendations. However, our data yield a value of k 1 (180 K) = cm 3 molecule -1 s -1, whereas extrapolation of 8

9 the NASA and IUPAC recommendations outside their stated ranges gives k 1 (180 K) = cm 3 molecule -1 s -1 and cm 3 molecule -1 s -1, respectively. In summary, our experiments demonstrate that conventional flow tube experiments on reactions of Cl atoms can be extended to temperatures very close to the lowest found in the Earth s stratosphere. We have employed such measurement to extend kinetic measurements on the reaction between Cl atoms and O 3 down to 184 K. Our value at 180 K, the lowest temperature found in the stratosphere, is only ca 9% lower than the value calculated from the Arrhenius expression currently recommended by the IUPAC panel [12]. However, it is ca. 17% lower than that based on the present recommendation by the NASA panel [11] and ca. 25% lower than the values determined by extrapolation of the results of Seeley et al. [10] Nicovich et al. [9]. All the ClO x catalytic cycles that contribute to the destruction of stratospheric ozone start with the Cl + O 3 reaction. The overall chain length is generally taken to be the ratio of the rate of this reaction to that of the reaction between Cl atoms and CH 4 [10] Cl + CH 4 HCl + CH 3 (3) which converts atomic chlorine to the reservoir compound HCl. Seeley et al. [19] have measured rate constants for reaction (3) down to 181 K. Together with our measurements and those of Nicovich et al. [9] on reaction (1), and estimates of the concentrations of O 3 and CH 4, it now is possible to estimate the ClO x chain length for all temperatures that are found in the stratosphere. 9

10 ACKNOWLEDGEMENTS We are grateful to NERC for a research grant and a research studentship (S.D.B.) in support of this work. We also thank Dr Kevin Hickson for experimental advice throughout the period of this work. BIBLIOGRAPHY [1] Molina, M. J.; Rowland, F. S. Nature 1974, 249, 810. [2] Wayne, R. P. Chemistry of Atmospheres (3rd Edition), Oxford University Press, [3] Wennberg, P. O.; Cohen, R. C.; Stimpfle, R. M.; Koplow, J. P.; Anderson, J. G.; Salawitch, R. J.; Fahey, D. W.; Woodbridge, E. L.; Keirn, E. R.; Gao, R. S.; Webster, C. R.; May, R. D.; Toohey, D. W.; Avalone, L. M.; Profitt, M. H.; Loewenstein, M.; Podolske, J. R.; Chan K. R.; Wofsy, S. C. Science 1994, 266, 398. [4] Kurylo, M. J.; Braun, W. Chem Phys Lett 1976, 37, 232. [5] Zahniser, M. S.; Kaufman, F.; Anderson, J. G. Chem Phys Lett 1976, 37, 226. [6] Clyne, M. A.A.; Nip, W.S. J Chem Soc Faraday Trans , 72, 838. [7] Watson, R. T.; Machado, G.; Fischer, S.; Davis, D. D. J Chem Phys 1976, 65, [8] Leu, M.-T. ; DeMore, W. B. Chem Phys Lett 1976, 41, 121. [9] Nicovich, J. M.; Kreutter, K. D.; Wine, P. H. Int J Chem Kinet 1990, 22, 359. [10] Seeley, J.V.; Jayne, J.T.; Molina, M. J.; J Phys Chem 1996, 100, [11] DeMore, W. B.; Sander, S. P.; Friedl, R. R.; Golden, D. M.; Hampson, R. F., Jr.; Kurylo, M. J.; Huie, R. E.; Moortgat, G. K.; Ravishankara, A. R.; Kolb, C. E.; Molina, M. J. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling, Supplement to Evaluation 12: Update of Key Reactions, Evaluation Number 13; NASA JPL-Publication (2000). [12] Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Kerr, J. A.; Rossi, M. J.; Troe, J. J Phys Chem Ref Data, 1997, 26, 521. [13] (a) Baumgärtel, S.; Gericke, K.-H.; Chem Phys Lett 1994, 227, 461; (b) Baumgärtel, S.; Delmdahl, R. F.; Gericke, K.-H.; Tribukait, A.; Eur Phys J D, 1998, 4,

11 [14] Beach, S. D.; Hickson, K. M.; Smith, I. W. M., Tuckett, R. P. Phys Chem Chem Phys 2001, 3, [15] Clyne, M. A. A. In: Physical Chemistry of Fast Reactions; Levitt, B. P., Ed.;. Plenum Press, New York and London, 1973, chap. 4. [16] Hickson, K. M.; Sharkey, P.; Smith, I. W. M.; Symonds, A. C.; Tuckett, R. P.; Ward, G. N. J Chem Soc Faraday Trans 1998, 94, 533. [17] Kaufman, F. J Phys Chem 1984, 88, [18] De More, W. B. J Geophys Res 1991, 96,

12 Table 1. Comparison of kinetic parameters from different studies of the reaction between Cl atoms and O 3 Reference Exptl. range of T/K k(298 K) / A / method a cm 3 molecule -1 s -1 cm 3 molecule -1 s -1 (E act / R) / K Kurylo and Braun [4] FP RF ± ± ± 40 Zahniser et al. [5] DF RF ± ± 30 Nip and Clyne [6] DF RA ± ± ± 30 Watson et al. [7] FP RF ± ± ± 25 Leu and DeMore [8] DF MS ± 0.3 Nicovich et al. [9] PLP RF ± ± ± ± ± 46 Seeley et al. [10] TFT-RF ± ± ± 61 DeMore et al. [11] evaluation ± ± 100 Atkinson et al. [12] evaluation ± ± 100 this work DF RF ± ± 50 a FP RF: flash photolysis, resonance fluorescence; DF RF: discharge-flow, resonance fluorescence; DF RA: discharge-flow, resonance absorption; DF MS: discharge-flow, mass spectrometry; PLP RF: pulsed laser photolysis, resonance fluorescence; TFT-RF: turbulaent flow tube, resonance fluorescence. 12

13 Table 2. Rate Constants (k 1 / cm 3 molecule -1 s -1 ) for the Reaction between Cl Atoms and O 3 T / K k 1 / cm 3 molecule -1 s ± ± ± ± ± ± ± ± ± ± ±

14 Figure Captions Figure 1 Plots of the intensity of resonance fluorescence from Cl atoms against the distance between the observation region and the end of the injector through which O 3 is admitted. (a) 298 K: [O 3 ] / molecule cm -3 = zero ( ); 0.82 ( ); 1.82 ( ); 3.52 ( ); 5.16 ( ); 11.2 ( ); and 12.4 ( ). (b) 242 K: [O 3 ] / molecule cm -3 = zero ( ); 2.53 ( ); 6.57 ( ); 5.70 ( ); 17.0 ( ); 17.5 ( ); and 16.4 ( ). (c) 222 K: [O 3 ] / molecule cm -3 = zero ( ); 1.38 ( ); 5.24 ( ); 6.64 ( ); 8.95 ( ); 12.4 ( ); and 14.8 ( ). (d) 184 K: [O 3 ] / molecule cm -3 = zero ( ); 2.14 ( ); 5.01 ( ); 11.6 ( ); 13.5 ( ); 13.7 ( ); and 16.7 ( ). Figure 2 Plots of the pseudo-first-order rate constants, k 1st, against [O 3 ] for (a) 298 K, (b) 242 K, (c) 222 K and (d) 184 K. Figure 3 Arrhenius plot of the rate constants for the reaction between Cl atoms with O 3 : filled circles are results from the present work, open circles those from Nicovich et al. [9]. 14

15 Figure 1 4 (a) (b) 3 2 n ( I RF / Arb. Units ) x 2 n ( I RF / Arb. Units ) x x / cm x / cm (c) 3 (d) 2 n ( I RF / Arb. Units ) x 1 n ( I RF / Arb. Units ) x x / cm x / cm 15

16 Figure (a) 200 (b) k 1st / s k 1st / s [O 3 ] / molecule cm [O 3 ] / molecule cm (c) 150 (d) k 1st / s k 1st / s [O 3 ] / molecule cm [O 3 ] / molecule cm -3 16

17 Figure 3 2x10-11 k 1 / cm 3 molecule -1 s x x x x x x (1000/T) / K -1 17