J. Phys. Chem. 1981, 85, 1896-1899
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Rate Constant for the Reaction of Atomic Bromine with Formaldehyde from 223 to 480 K D. F. Naval* J. V. Michael,* and L. J. Stleft Astrochemistry Branch. Laboratory for Extraterrestrial Physics, NASA/Goddard Space Flight Center, GreenbeR, Maryland 20771 (Received: February 18, 198 1)
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The absolute rate constant for the reaction Br + H2CO HBr + HCO has been determined from 223 to 480 K by using the technique of flash photolysis-resonance fluorescence (FP RF). Bromine atoms were produced by flash photolysis of CHBr3at X > 195 nm. For each of the five temperatures at which the reaction was studied, the rate constant was shown to be invariant for substantial variations in [H2CO],total pressure (Ar), and flash intensity (Le., initial [Br]). A linear least-squares treatment of the data in Arrhenius form yields the expression k = (1.44 f 0.31) X lo-" exp(-750 f 56/T) cm3 molecule-'s-' with indicated error being at the one standard deviation level. The results are compared with the only previously published determination, this being a measurement at 298 K with the discharge flow-ESR technique. The rate constant is theoretically considered and the implication of these results for stratospheric chemistry is discussed.
Introduction The role of bromine compounds in stratospheric chemistry and the contribution of such compounds to the catalytic destruction of ozone have been considered in recent modeling studiee.'p2 The catalytic cycle is initiated by the reaction Br + O3 BrO + O2 (1)
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Interruption of the cycle has been suggested to be due to the formation of the reservoir molecule HBr through the reaction of atomic bromine with H02,1g2H202,1~2 and HPC0.2 Br H 0 2 HBr O2 (2)
+
--
+ H202 Br + H2C0 Br
+ HBr + HOP HBr + HCO
(3)
(4)
Rate constants for reaction 1 have been determined as a function of temperature by three different technique^,^-^ but rate data for reactions 2-4 have been nonexistent until very recently. Thus the most recent calculations by Yung et ale2used estimated values for k z and k4 and an upper limit for k3 reported recently by Leu6 Yung et a1.2 escm3 molecule-' s-l and stated timated that k4 5 1 x that, for an HzCO mixing ratio of -0.1 part per billion by volume (ppbv), k4 would have to be greater than 1 X cm3 molecule-' s-' at stratospheric temperatures in order for reaction 4 to be comparable in magnitude to reaction 2 as a loss process for Br. Subsequently, LeBras et al.' reported the first measurement of k4. Using the discharge flow-ESR technique, they obtained k4 = (1.6 f 0.3) X cm3 molecule-' s-' a t 298 K. It is clear that an evaluation of the importance of reaction 4 in the chemistry of atmospheric bromine depends on a determination of k4 over a range of temperatures, including those appropriate for stratospheric modeling. Because of this particular need and because of the paucity of absolute rate data for bromine atom reactions in general. we undertook the present measurement of the &solute rate constant for the reaction Br + HzCO at five temperatures from 223 to 480 K.
*
Visiting Professor of Chemistry, Catholic University of America, Washington, DC 20064. 'Adjunct Professor of Chemistry, Catholic University of America, Washington, DC 20064.
Experimental Section The present study employed the technique of flash photolysis-resonance fluorescence (FP RF). Apparatus, including a new improved reaction cell, and general procedures used for the kinetic experiments have been described p r e v i o ~ s l y . Flash ~ ~ ~ photolysis of CHBr, at X > 195 nm (Homosil cutoff) was used in the experiments reported here to produce Br atoms. Preliminary experiments employed photolysis of CH3Br at X > 165 nm (Suprasil quartz) as used in our study of the Br + O3reaction! Measurements at 298, 254, and 220 K showed poor reproducibility, curvature in the first-order decay plots, and generally lower values of the derived bimolecular rate constant than observed in the final experiments. Experiments at 298 K with CH3Br replaced by CHBr, but still employing Suprasil quartz gave somewhat higher values of k and somewhat better reproducibility. The final experiments, as mentioned above, employed CHBr, and Homosil quartz and gave the best reproducibility and the least evidence for curvature in the first-order decay plots. Interference from products of H2C0 photolysis could have contributed to the difficulties in the experiments employing Suprasil quartz (X > 165 nm) since there is a very intense absorption peak in the H2C0 spectrum at 175 nm.lo This strong absorption is eliminated in the experiments employing Homosil quartz (X > 195 nm). Resonance radiation of bromine atoms was generated by flowing a premixed Br2-He mixture (XBr= 0.0002) through a microwave discharge lamp at 0.3 torr. Previous experience4p5had shown that this low molecular fraction of bromine in the resonance lamp prevents any substantial (1)Wofsey, S. C.; McElroy, M. B.; Yung, Y. L. Geophys. Res. Lett. 1975,2,215. ( 2 ) Yung, Y. L.; Pinto, J. P.; Watson, R. T.; Sander, S. P. J. Atmos. Sci. 1980,37, 339. (3)Leu, M. T.;DeMore, W. B. Chem. Phys. Lett. 1977,48, 317. (4)Michael, J. V.;Lee, J. H.; Payne, W. A.; Stief, L. J. J. Chem. Phys. 1978.68.4093. (5) M'ichael, J. V.; Payne, W. A. Int. J . Chem. Kinet. 1979,11, 799. (6)Leu, M. T.; Chem. Phys. Lett. 1980,69,37. (7)LeBras, G.; Foon, R.; Combourieu, J. Chem. Phys. Lett. 1980,73, 357. (8)Klemm. R. B.: Stief. L. J. J. Chem. Phvs. 1974.61.4900: . . . Michael. J. V.f Lee, J. H. J. Phys. Chem. 1979,83,16. (9)Stief, L. J.; Nava, D. F.; Payne, W. A.; Michael, J. V. J. Chem. Phys. 1980,73, 2254. (10)Mentall. J. E.: Gentieu. E. P.: Krauss., M.:, Neuman. D. J. Chem. Phys.'1971,55,'5471.'
This article not subject to US. Copyright. Published
1981 by
the American Chemical Society
Reaction of Br with H,CO
line reversal in the resonance source, thus preventing any subsequent effect of producing nonlinearity in first-order decay plots caused by the resonance source. Detection of the resonantly scattered fluorescent photons was accomplished a t right angles to both the photolyzing and resonance light sources through a CaF2lens (focal length = 1.5 in. for h 2 125 nm) while flowing dry N2 in front of the photomultiplier. These photons were recorded in repetitive flashes by a multichannel analyzer operating in the multiscaling mode. The handling of H,CO in this work was similar to that in earlier studies from this laboratory of reactions involving this m ~ l e c u l e . ~ J ~ Experiments were carried out at five temperatures from 223 to 480 K over substantial ranges of total pressure and [H2CO]. For the experiments conducted at room temperature and above, it was necessary to use somewhat low flash intensities in order to avoid curvature effects in the decay plots. These effects are due to a secondary process, probably atomic bromine re-formation. Nevertheless, the range of flash intensities was substantial. In order to obtain sufficient fluorescence signal at lower temperatures, factors such as the vapor pressures of bromoform necessitated the use of somewhat higher flash intensities. We believe, however, that the absorbed flash intensities under the conditions at these lower temperatures are probably equivalent to absorbed intensities at the higher temperatures. Argon (Matheson, 99.9995%) and helium (Airco, 99.9999%) were used without further purification. Bromine (Baker, 99.9%) was further purified by bulb-to-bulb distillation at 240 K with the middle third being retained. Bromoform (Eastman, 96%) was similarly purified at 77 K. Formaldehyde was prepared from paraformaldehyde as described previously." Analysis by mass spectrometry showed the sample to be 99.7% pure, with the only measurable impurity being HzO.
The Journal of Physical Chemistry, Vol. 85, No. 13, 1981 1897 1
8
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I
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+
Figure 1. Flrstsrder decay plots for the reaction Br H&O. The lines are determined from a linear least-squares analysis of the data points; errors in the decay constants are one standard deviation. (0) 298 K, [HpCO] = 31.3 mtorr, [CHBr,] = 12.5 mtorr, [Ar] = 50 torr, flash energy = 110 J, ,k = 1056 f 35 s-'. (A)223 K, [H2CO] = 28.1 mtorr, [CHBr,] = 12.5 mtorr, [Ar] = 50 torr, Rash energy = 127 J, , k = 705 f 26 s-'. I
I
I
I
1
I
I
I
Results All of the reported experiments were carried out under pseudo-first-order conditions with [H2CO]>> [Br]. The decay of [Br] is represented by In [Br] = -kobdt + In [Br], (5) while the pseudo-first-order decay constant is given by kobsd = ~ ~ [ H z C + O Ik d (6) k4 is the reaction rate constant for reaction 4 and k d is the decay constant for Br atom diffusion from the analytical
viewing zone. Linear least-squares methods were used to obtain k o b d from plots of the logarithm of accumulated counts against time where accumulated counts are proportional to [Br]. k d was measured in separate experiments a t the various conditions of the reactant kinetic measurements, except that [H2CO]= 0, thereby enabling determination of k4 from eq 6. Under the experimental constraints needed to prevent interference from secondary processes, the observed pseudo-first-order Br atom decay plots were linear as required by eq 5. Typical examples are shown in Figure 1. Within experimental errors, the invariance of the decay constant over substantial time intervals is evidence that only reaction 4 and diffusional loss contribute to the decay. The diffusional corrections were typically much less than 10%. Results obtained from the experiments at each of the five temperatures are presented in Table I. Figure 2 shows an Arrhenius plot of the mean k4 values obtained at the (11) Michael, J. V.; Nava, D.F.;Payne, W. A.; Stief, L.J. J. Chem. Phys. 1979, 70, 1147.
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The Journal of Physical Chemistty, Vol. 85, No. 13, 7987
Nava et al.
TABLE I: Rate Data for the Flash Photolysis-Resonance Fluorescence Study of the Reaction Br [H,CO I, [CHBr, I, [&I, T,K mtorr mtorr torr flash energy, J no. of expt
1 0 ' * l ~ ,cm3 ,~ molecule-' s-'
223
12.0 14.1 12.0 24.1 28.1 31.3 42.2 46.9 24.1 56.3 62.5
12.5 6.3 12.5 25.0 12.5 18.8 18.8 28.1 25.0 25.0 37.5
25 25 50 50 50 50 75 75 100 100 100
110-27 2 81-163 182, 298 110-272 81-182 195 127-272 110,182 182, 298 81-182 127,182
3 3 2 3 3 1 3 2 2 3 2 27
0.55 f 0.02 0.59 i 0.04 0.56 f 0.09 0.56 f 0.03 0.55 * 0.05 0.57 0.58 ?: 0.10 0.56 f 0.05 0.56 f 0.04 0.52 f 0.03 0.57 + 0.12 0.56 f 0.05b
2 54
15.6 14.1 31.3 25.8 46.9 39.8 62.5 58.6
9.4 12.4 18.8 22.7 21.1 35.1 37.5 51.6
25 30 50 55 75 85 100 125
56-110 20-56 36, 81 20-56 36-110 20-56 36-95 20, 36
3 4 2 3
5.6 11.3 25.0 31.3 22.5 46.9 37.5 50.0 62.5
7.5 15.0 18.8 12.5 30.0 18.8 50.0 37.5 25.0
15 30 50 50 60 75 100 100 100
110 56-203 36-81 56-1 82 56-1 82 36-163 36-182 36-81 36-203
36
0.67 i: 0.04 0.70 f 0.08 0.73 f 0.09 0.71 f 0.08 0.72 i 0.04 0.65 f 0.06 0.74 f 0.01 0.66 f 0.01 0.70 f 0.06b 1.13 1.09 f 0.11 1.08 t 0.10 1.11f 0.09 1.07 f 0.07 1.01 f 0.05 1.13 f 0.10 1.11f 0.12 1.00 f 0.11 1.08 t O . l O b
2 4 4 3 4 2 19 2 2 2 2 2 2 2 4 18
1.68 f 2.01 f 1.94 * 1.54 f 1.93 + 1.87 * 1.85 * 3.05 t 3.36 f 3.06 f 3.32 * 3.51 f 3.29 f 3.56 ?: 3.34 f 3.31 f
298
a
+ H,CO
373
11.3 18.8 28.1 21.3 37.5 31.3
28.1 25.0 37.5 53.1 50.0 78.1
45 50 75 85 100 125
20, 36 36-144 36-144 36-110 36-144 36, 56
480
6.3 7.8 10.9 12.5 15.6 18.8 23.4 21.9
15.6 15.6 25.0 31.3 31.3 46.9 46.9 50.0
25 25 50 50 50 75 75 100
144,182 110,182 56, 81 144, 182 110,182 127,182 127, 203 20-81
Error limit is one standard deviation.
-+
3 3 2 24 1 4 5 3 4 4 6 5 4
0.06 0.15 0.09 0.04 0.08 0.02 O.lgb 0.27 0.07 0.12 0.56 0.24 0.39 0.41 0.14 0.2fjb
Mean value of rate at that temperature.
centrations of Br are required and it becomes more difficult to isolate the primary reaction Br + HzCO HBr + HCO (4) from subsequent secondary reactions such as Br + HCO HBr + CO (8) If both reactions contribute fully to Br loss at the [Br] levels in the ESR study,' then the rate constant determined from the loss of Br can be equal to twice k4. The substantially larger value for k4 observed in the ESR work7 may therefore be due in part to a contribution from a secondary Br loss process such as reaction 8. This possibility was considered by LeBras et al.7 but was not deemed to be significant for the conditions of their experiments except if reaction 8 has a rate constant near the collision frequency. There are no published data on the temperature dependence of k4 with which the present results may be compared. The significant exponential temperature factor suggests that a potential energy barrier exists for reaction 4. This is not surprising since the reaction is only slightly
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4
exothermic (-0.5 kcal mol-l). We note that reaction 4 is one of very few H atom abstraction reactions by Br that is exothermic. This is due to the low C-H bond strength (87 kcal mol-l) in H2C0. The present results can be theoretically rationalized in terms of activated complex theory. We have carried out calculations for various assumed activated complex models. In all cases the Br atom approach is assumed to be colinear with the C-H bond in HzCO, and moment of inertia ratios, (111213)t/ (111213)Hzc0, are then calculated as a function of the rBrH distance. It is further assumed that two low valued degenerate bends make the greatest contribution to the vibrational partition function ratio, R, = q v i / q v H z c o . We find that any complex configuration between 1.5 IrBrH I3.0 A is possible for respective 250 IV b I450 cm-' bending vibrations. We present in Figure 2 the results for the tight complex, rBrH = 1.5 A and +, = 250 Cm-'. The calculated line is given by k4 = 6.68 X 10-11T1~zR,(250cm-') exp(-580/T) (9) in molecular units. This result and that for the loose
J. Phys. Chem. lS81, 85, 1899-1906
Flgure 3. Comparison of the atomic bromine removal rate in the stratosphere for reaction with H202,H,CO, and HO,. The Br removal rate for reaction with H,02 represents an upper limit; see text and ref 6.
complex, rBrH = 3.0 and Vb = 450 cm-', both predict curvature in the Arrhenius plot. Unfortunately the temperature range of the present study does not allow a clear choice to be made between the two extreme cases. The major point to be made, however, is that curvature is not unexpected, and the slight curvature exhibited by the data is probably real. Such curvature in the related C1+ CHI system12J3has been explained in terms of quantum mechanical tunneling12 and/or differing reactivity of the two J states of Cl.13 Such explanations for the present case are clearly not necessary since either extreme model is reasonable. Also the splitting between J = 112 and 312 levels in Br is 3685 cm-'. Thus, the 2Brlj2equilibrium population at the highest temperature of the present investigation is very small thereby suggesting that the significant reactant is the 2Br3/2state. The implication of these results for stratospheric chemistry may be evaluated by comparing the removal rates for (12) Whytock, D. A,; Lee, J. H.; Michael, J. V.; Payne, W. A.; Stief,
L.J. J . Chem. Phys. 1977, 66, 2690.
(13) Ravishankara, A. R.; Wine, P . H. J. Chem. Phys. 1980, 72, 25.
1899
atomic bromine by reactions 2-4. The result for the first two reactions are taken from Leu6 who used his own upper limit for k3, an estimated value for k2 of 4.5 X lo-'' cm3 molecule-' s-l, and concentration profiies for H02and H202 from the calculations of Yung et al.2 The result for reaction 4 is based on the Arrhenius expression for k4 given in eq 7 and a calculated concentration profile for H2C0 from Stief et a l . 1 4 Recent observations by Barbe et al.16 are not significantly different from the calculated profile. The resulting comparison of removal rates for atomic bromine is summarized in Figure 3. This shows that the Br removal rate due to reaction with H2C0, while some three orders of magnitude larger than that for removal by H202, is still only of the order of a few percent of the Br removal rate by the dominant H 0 2 reaction. It may be noted that the contribution from the Br + H2C0 reaction is greatest in the lower stratosphere and it is for this same region that Yung et ala2suggest the maximum effect of the synergistic coupling of chlorine and bromine chemistry through the C1 + Br + 02. reaction C10 + BrO
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Note Added in Proof, Posey et al.16 have recently reported a value for k2 = 2.2 x 10-13cm3 molecule-' s-l. This value substantially reduces the removal rate for Br by H02,and makes the removal rate of Br by H 0 2 slightly lower than that by H2C0 as shown in Figure 3. Such s-') would make HBr a low Br removal rates relatively unimportant reservoir for odd bromine in the stratosphere. Acknowledgment. J.V.M. acknowledges support by NASA under Grant NSG 5173 to Catholic University of America. (14) Stief, L. J.; Michael, J. V.; Payne, W. A.; Nava, D. F.; Butler, D. M.; Stolarski, R. S. Geophys. Res. Lett. 1978, 5 , 829. (15) Barbe, A,; Marche, P.; Secroun, C.; Jouve, P. Geophys. Res. Lett. 1979, 6, 463. (16) Posey, J.; Sherwell, J.; Kaufman, M. Chem. Phys. Lett. 1981, 77, 476.
Electrode Kinetics of the Br,/Br- Couple Israel Rublnsteln' Institute of Chemistry, Tel-Aviv University, Ramat Aviv, Israel (Received: November 4, 1980: In Final Form: February 20, 1981)
The electrode kinetics and mechanism of the Br-/Br, couple on platinum electrodes was investigated by the coulostatic method. The kinetic parameters were calculated from the overpotential decay curves, taking into account partial mass-transport control for a multistep process. The results were interpreted in terms of the combined adsorption isotherm, which is dependent on the size of the adsorbed intermediate. The rate-determining step (rds) was found to be charge transfer from a Br- ion to form an adsorbed bromine atom. On oxide-covered platinum this ion is discharged from solution, while on nominally oxide-free ("reduced") electrodes the ion is adsorbed on the surface in a step preceding the rate-determining step. On reduced electrodes the exchange current density io was found to decrease with time after immersion of the electrode in the test solution. This was shown to result from a slow deactivation of the surface due to the formation of adsorbed bromine molecules.
Introduction The Br-/Br2 system has been the subject of several electrochemical investigations, some relating to adsorption (1) Department of Chemistry, University of Texas, Austin, TX 78712.
phenomena2* and others mainly concerned with evaluation Of the kinetics and mechanism.7-'0 (2) M. W. Breiter, Electrochim. Acta, 8, 925 (1963). (3) V. S. Bagotzky, Yu. B. Vassilyev, T. Weber, and J. N. Pirtskhalava, J . ElectroanaL Chem., 27, 31 (1970).
0022-3654/81/2085-1899$01.25/00 1981 American Chemical Society
