J. Phys. Chem. 1986, 90, 4131-4135 R1 1) to finally yield iodine once again. Computer Simulations. In an independent study, Citri and Epstein,26using Hindmarsh’s version3’ of the Gear algorithm32 for stiff differential equations, simulated the bromate-iodide reaction in both batch and flow conditions. The same 13 elementary reactions were found sufficient to simulate both conditions satisfactorily. Their model only fails to predict in batch the pH-dependence of the iodine disappearance at the instant of clocking (reaction R2). The major reactions they proposed are the same as ours. They used our value of kl for reaction R6. One difference between their mechanism and ours is their inclusion of the reaction
+ I2
+ IBr
lo7, k, = 1 X loz (R21) They considered reaction R21 as the starting point for the oxidation of iodine. Closer examination shows that reaction R21 is a composite of reaction R10 and the reverse of R7 (hydrolysis of iodine), both of which are included in our mechanism. Their study showed that the most important reactions in the mechanism are R5, R7, R11, and R21, which concurs with our findings. HOBr
HOI
kf = 8
X
Conclusion Our experiments have shown that, though the bromate-iodide reaction displays simple kinetic behavior in the production of iodine, it, however, possesses a built-in clock mechanism which can separate the production of iodine from the oxidation of iodine to iodate. Our preliminary investigations on the iodine-bromate reaction indicate that the reaction is very complex with at least one autocatalytic step.I9 In a flow reactor, however, we expect the major species, bromate, iodide, bromide, iodine, bromine, and iodate to coexist, the relative quanti.ties of each varying under (30) Faull, J. H. Jr. J . Am. Chem. SOC.1934, 56, 522-526. (31) Hindmarsh, A. C. GEAR: Ordinary Differential Equation Solver; UCID-30001, Rev. 3, Lawrence Livermore Laboratory, 1974. (32) Gear, C. W. Commun. ACM 1971, 14, 176.
4131
bistable conditions from one steady state to the other.z6 The biggest shortcoming in our experimental work is our inability to quantitatively assess IBr which is a very important species in our mechanism. Under batch conditions we expect this species to rise during the course of the reaction, but in the later stages of the reaction it should hydrolyze before giving iodine and bromide (R10 R6). Under flow conditions the IBr will maintain a finite concentration throughout. The role of R13 was examined and found to be insignificant under batch conditions. Our reasoning is based on the fact that since R3 is faster than R4, the final reaction would be R4 + R3
+
2Br03-
+ I2 + 101- + 12H+
-
Br2 + 61z + 6 H z 0
(R22)
which would be autocatalytic in iodine. Such a scenario would yield a distortion of our exponential kinetics in the later stages of the reaction. Our experimental data analysis did not show this. It appears then that the kinetic control is so efficient that reaction R16 which leads to higher oxidation states of iodine (>+1) is insignificant in the presence of iodide ions. In general, any higher oxidation state of iodine is thermodynamically unstable with respect to iodine in the presence of iodide. There are two points of note from our study. Firstly, there is no global evidence of autocatalysis in the first stage of the reaction. The nonlinearity which is responsible for the oscillatory behavior comes from a combination of the two reactions (R4 R2). Secondly, the mechanism we propose does not involve any freeradical mechanisms as most bromate reduction mechanism often do.
+
Acknowledgment. We thank Prof. K. Kustin, Prof. I. R. Epstein, and Ofra Citri for helpful discussions and giving us access to the stopped-flow spectrophotometer. We also thank Prof. J. G. Sheppard who critically read this manuscript and offered some useful suggestions. This work was supported by research grants 2543 RB 49/83 and 2:2538:1 from the University of Zimbabwe Research Board. Registry No. Br03-, 15541-45-4; I-, 20461-54-5.
Mechanism for the Reaction of CH200 with SO2 Shiro Hatakeyama,* Hiroshi Kobayashi,+ Zi-Yu Lin,: Hiroo Takagi, and Hajime Akimoto Division of Atmospheric Environment, The National Institute for Environmental Studies, P.O.Tsukuba-gakuen, Ibaraki 305, Japan (Received: November 13, 1985; In Final Form: March 17, 1986)
The gas-phase reaction of C H 2 0 0 produced by the reaction of ozone with ethylene was investigated. The pressure dependence of the yield of HzSO4 formed in the reaction of C H 2 0 0 with SOzwas studied in the pressure range of 10-1 140 Torr. The H2SO4 yield (0.40 at the high-pressure limit) decreased as the total pressure decreased but did not fall to zero even at low pressure, which implies that stabilized ground-state CHzOO species are formed at low pressure as a primary product. Vibrationally excited C H 2 0 0 *as well as another excited CH200*, which does not suffer collisional stabilization even at the highest pressure studied, is also produced in the reaction. The formation ratio was obtained as CHz00:CH200t:CH200* = 0.20:0.20:0.60. The ratio of the rate constant for collisional deactivation of C H 2 0 0 t with air to that for decomposition for CH,OOt was determined to be (1.0 f 0.9) X cm3/molecule. As for the reaction mechanism for the reaction of C H 2 0 0 with SO2,the formation of adduct between CHzOO and SO2 was concluded. The adduct is proposed to react with another SO2 molecule to produce HCHO and SO3 or to decompose to HCOOH and SOz with the rate constant ratio of (4.9 2.0) x 1 0 - l ~cm3/molecule.
Introduction It is well recognized that the Criegee intermediates (RR’COO, peroxymethylene-type biradicals) are formed in the reactions of ozone with olefins, although they have not as yet been detected t Present address: Environment Pollution Control Center, Co., Ltd., Higashikojiya, Tokyo 144, Japan. *Present address: Chinese Research Academy of Environmental Sciences, Beijing, China.
0022-3654/86/2090-4131$01.50/0
spectroscopically. Recently, much attention has been given to this intermediate in view of acid rain, since oxidation of SOz with Criegee intermediates is indicated to be a ‘potential source of atmosperic H2S04.’ Calvert and Stockwel12reported on the basis of computer simulation that under highly polluted atmosphere (1) Calvert, J. G., Ed. SO2, NO, and NO2 Oxidation Mechanism: Atmospheric Considerations; Butterworths: Boston, MA, 1984. (2) Calvert, J. G.; Stockwell, W. R. Enuiron. Sci. Technol. 1983, 17, 428A.
0 1986 American Chemical Society
4132 The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
the reaction of the Criegee intermediates can have a significant contribution (up to 50% of total oxidation of SO2) if humidity is low. However, reactivity of these intermediates has not been well studied so far. Recently, it has been suggested that a hot intermediate is formed in the first step of ozone-olefin reactions and only the collisionally deactivated intermediate can undergo bimolecular reactions with aldehydes, SO2,or other reactant^.^" In our recent study’ a remarkable pressure effect has been observed on the reaction of C H 3 C H O 0 formed in the ozonetram-2- butene reaction with SO2forming H2S04.Thus, the yield of H2S04falls down to zero as the total pressure decreases, supporting the contention suggested above. As for the reaction mechamism forming H2S04, a simple bimolecular reaction, RR’COO + SO, RR’CO SO3,has been assumed in the previous w0rks,’9~9’whereas Martinez and Herrons proposed a “quasi-concerted” mechanism which invokes the formation of an adduct between SO2and the Criegee intermediate. Since such an adduct was detected by a high-resolution mass spectroscopy technique9 from the reaction of 03-1 -butene-SO,, it has been desired to investigate the reaction pathway in detail. In the work reported here, the reaction of C H 2 0 0 formed in the ozone-ethylene reaction has been studied. The effect of total pressure on the C H 2 0 0 SO2 reaction is reported, and the experimental evidence for the addition mechanism for the reaction of C H 2 0 0 with SO, is presented.
-
Hatakeyama et al. TABLE I: Yield of SO2 in C2H,(- 175 mTorr)-03 (- 122 mTorr)-S02 (-670 mTorr) Reactions under Various Total Pressures of Dry Air
AH,SOa/AOl total Dress., Torr AH,SO,/AO,
total press., Torr
0.204 0.191 0.212 0.217 0.209 0.216 0.193 0.222 0.212 0.225 0.237 0.215 0.221 0.240
10 10 20 22 35 40 40 47 50 53 90 90 90 91
+
94 160 160 160 250 250 250 360 360 570 760 760 1140 1140
0.244 0.226 0.284 0.253 0.266 0.265 0.313 0.364 0.320 0.386 0.382 0.396 0.407 0.410
+
Experimental Section
Ethylene (Takachiho, >99.5%) and SO2 (Matheson, >99.98%) were used without further purification. Ozone was prepared from O2 (Nippon Sanso,>99.9%) by the use of a silent discharge ozone generator. Pure ozone was obtained by passing the 0 3 - 0 2 mixture through a trap cooled with liquid nitrogen. Isotope labeled ozone (1803) was prepared with I8O2(Nippon Sanso, atomic purith 99%) by silent discharge. H C H O and C H 3 C H 0 monomers were prepdared by heating the corresponding polymers (paraformaldehyde, Wako; paraldehyde, Tokyo Kasei) and stored at liquid nitrogen temperature. Two kinds of experiments were carried out. One was done by use of a cylindrical Pyrex cell of 3-L volume as a reactor. In the presence of a large excess of SO2(-670 mTorr) , ethylene (- 175 mTorr) and ozone (- 122 mTorr) were introduced into the cell and the total pressure was adjusted to 10-1 140 Torr with research grade air (Takachiho). The reactor was allowed to stand for 3 h for the completion of the reaction and the sedimentation of product aerosols. Gaseous compounds remaining in the reactor were then purged with a stream of N2. Sedimentary products were washed out with 50 mL of ether. The resulting solution was concentrated, treated with diazomethane, and diluted to a constant volume (5 mL). Quantitative analysis was performed by gas chromatography and the G C / M S (Neva TE-600) gas chromatographs used were Shimadzu GC-bA and GC-4BM with a flame ionization detector and a flame photometric detector, respectively. A 2-m column of 5% DEGS on Chromosorb was employed. The column oven temperature was kept at 120 “C. The other type of experiment was carried out in a larger volume (-6 m3) evacuable smog chamber,I0 whose inner surface is coated with PFA (perfluoroethylene-perfluoroalkyl vinyl ether copolymer). In the presence of 1 atm of purified air, 0-6 mTorr (3) Su, F.; Calvert, J. G.; Shaw, J. H. J . Phys. Chem. 1980, 84, 239. (4) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. J. Phys. Chem. 1981,85, 1024. (5) Kan, C. S. L.; Calvert, J. G.; Shaw, J. H. J. Phys. Chem. 1981, 85, 2359. (6) Herron, H. T.; Martinez, R. I.; Huie, R. E. Int. J . Chem. Kinet. 1982, 14, 201. ( 7 ) Hatakeyama, S.; Kobayashi, H.; Akimoto, H. J . Phys. Chem. 1984, 88, 4736. ( 8 ) Martinez, R. I.; Herron, J. T. J . Environ. Sci. Health, Part A 1981, A16, 623. (9) Schulten, H.-R.; Schurath, U. J . Phys. Chem. 1975, 79, 51. (10) Akimoto, H.; Hoshino, M.; Inoue, G.; Sakamaki, F.; Washida, N.: Okuda, M. Environ. Sci. Technol. 1979, 13, 471.
“‘I L
I
I
I
I
t
200
400
600
800
1000
I
[ A i r ] / Torr Figure 1. Change in the yield of H2S04in C2H, (-175 mTorr)-03 (- 122 mTorr)-SO2 (-670 mTorr) reactions at various total pressures
of air.
of SO2,4 mTorr of ozone, and 1.5 or 3.3 mTorr of ethylene were introduced into the chamber in this order with purified air as carrier gas. Reactants and products were monitored by means of long path (221.5 m) FT-IR (Block Engineering-JASCO International, FTS-496s). Resolution was 1 cm-’ and 128 scans were accumulated (scanning time - 5 min). The temperature was controlled a t 30 OC. The reaction of ozone (5 mTorr) and ethylene (3 mTorr) in the presence of excess H C H O (9 mTorr) under 760 and 55 Torr of total pressure was also studied. Competitive reactions between C H 3 C H 0 and SO, against C H 2 0 0 were carried out for CzH4 (3 or 4 mTorr)-03 (4 mTorr)C H 3 C H 0 (13 mTorr)-S02 (0-1.6 mTorr) systems in the smog chamber under 1 atm of air. To clarify the mechanism for the reaction of the Criegee intermediate with SO2,the C2H4 (8 mTorr)-1803 (14 mTorr)-SO, (1 mTorr) reaction was carried out in an 11-L quartz cell which is equipped with multireflection mirrors for FT-IR analysis. Results
Pressure Effect on the Yield 0fH2S04. Formation of aerosol was clearly observed when a reaction of ozone with ethylene proceeded in the presence of excess SO2in the glass reactor. From the products deposited on the wall, sulfuric acid was identified by means of G C and GC/MS after esterification with diazomethane. No sulfur-containing compound other than sulfuric acid was detected. The yield of HzSO4 (AH2S04/A03)in ozone-ethylene-SO, reactions under atmospheric pressure was already reported7 to be 0.390 0.053. The ozone-C2D4-S02 reaction gave the same yield (0.393 f 0.063), thus showing no isotope effect for the yield of sulfuric acid under atmospheric pressure. Yields of H2SO4 at various total pressures are shown in Table I and Figure 1. It should be noted that the yield of H2SO4 at the extrapolated zero total pressure does not come down to zero. In order to confirm that the positive intercept of HzS04yield is
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 4133
Mechanism for the Reaction of CHzOO with SOz TABLE II: Yields (in mTorr) of HCHO and That of the Consumption of SO2 in the Reaction of C2H4-O3in Air after 1 h4 [C,H,I, [O,lO [SO210 A[HCHOI -A[SO21 AA[HCHOl 1.58 1.50 1.58 1.58 1.53 1.67 1.55 1.59 1.55 1.48 1.57 1.63 1.60 1.65
3.6 3.8 3.7 3.6 3.8 3.6 3.6 3.6 3.7 3.9 3.5 3.7
4.0 3.6
0.14 0.14 0.19 0.20 0.23 0.23 0.24 0.35 0.30 0.32 0.38
0.14 0.16 0.12 0.18 0.23 0.22 0.27 0.26 0.29 0.29 0.32
4AA[HCHO]/A[C2H4] is calculated by subtracting the value of A[HCHO]/A[C2H4] without SO2 from the value of A[HCHO]/ A[C2H,] with SO,. A[S02] means decrease of [SO,] only due to the reaction (contribution of wall loss is corrected).
I
__ - - -
I
__--*
-4r
-2
-10
-A---
1.13 1.15 1.11 1.17 1.22 1.21 1.26 1.25 1.28 1.28 1.31
1
I
I
I
I
0.95 0.97 1.06 av 0.99 0.18 0.19 0.26 0.30 0.43 0.48 0.61 1.06 1.50 2.00 3.10
I
1
I
2 [S0210/mTorr
3
Figure 2. Yields of SO2consumption and increase of HCHO formation vs. added SO2concentration in C2H4(- 1.6 mTorr)-O, (-4 mTorr)SO2 (0-3.1 mTorr)-air systems: A, AS02/AC2H4;TOP,AAHCHO/ ACzH4.
ascribed to the reaction of CHzOO with SOz,and not with other free radicals, ozone-ethylene reactions in the presence of excess HCHO were carried out in the smog chamber under 760 and 55 Torr, since the reaction of C H 2 0 0with HCHO is known to give a characteristic compound assigned to HCOOCHzOH as a bimolecular reaction prod~ct.~"The infrared absorption bands at 1765, 1170, 1120, and 925 cm-' which are ascribed to HCOOCHzOH were observed under both 760 and 55 Torr by means of FT-IR, in agreement with the report of Niki et al." The ratio of the IR absorbance at 1765 cm-' due to HCOOCHzOH at 55 Torr to that at 760 Torr was 0.58 (normalized by the consumption of C2H4),which agreed well with the ratio of the H2S04yield at 55 Torr to that at 760 Torr, 0.58. In the presence of excess CH3CH0, propene ozonide (POZ, CH3CHOCHz00) was clearly detected at 760 Torr but was not observed at 55 Torr. Unimolecular decomposition of POZ under the low pressure would explain the result. Reaction of SO2with the Criegee Intermediate. In the system C2H4 (1.5 or 3.3 mTorr)-03 (4 mTorr)-SOz (0-6 mTorr), addition of SOzincreased the yield of HCHO as depicted in Table 11. The consumption of SOz balances with the increment of HCHO. Changes in ASO2/ACZH4and AAHCHO/ACzH4as a function of SO2concentration are plotted in Figure 2, where S O 2 and AC2H4are the decrease of SO2and CZH4, respectively, and AAHCHO is the additional increase of HCHO in the presence of SOz which is calculated by subtracting the yield in the absence of SO2. As shown in Figure 2, ASO2/AC2H4agrees with AAHCHO/ACzH4,which indicates that the consumption of one molecule of SO2 yields one additional HCHO molecule. The addition of SO2 in the C2H4-03 system affected the yield of formic acid. The yield of HCOOH increased sharply with the (11) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L.P. Environ. Sci. Technol. 1983, 17, 3 12A.
0
ou5
-
-6
0
r-l
u-4 -2 I
I
I
I
I
3 4 5 6 [ SO2lO/ mTorr Figure 3. Yield of HCOOH (0and solid line) and the inverse of the yield of HCOOH (A and broken line) vs. added SO, concentration in C2H4(-3.3 mTorr)-O, (-4 mTorr)-SO, (0-6 mTorr)-air systems. 2
1
301
I
0
I
I
I
I
02
04
I
I
I
1
08 06 [SO2Io/rnTorr
I
I
10
I
I
12
Figure 4. Inverse of the yield of propene ozonide vs. added SO2concentration in C2H4(3 or 4 mTorr)-03 (-4 mTorr)-CH3CH0 ,.,( 13 mTorr)-SO, (0-1.6 mTorr)-air systems.
addition of 10.5 mTorr of SOz. The yield maximized at ca. 0.5 mTorr of the initial concentration of SOzand decreased gradually with the increase of SO2 as shown in Figure 3. In the CzH4-'803-SOz reaction, HC'*OOH and HCO'*OH were formed in addition to HC'80180H,while only HC'80180Hwas formed in the absence of SO2.lZ Isotopic scrambling in SO2 was not seen in this experiment since the added concentration of SOz was too low to be analyzed by FT-IR. Competitive reactions of SO2and CH3CH0with CHzOOwere studied in the presence of a constant concentration of CH3CH0 (13 mTorr), ethylene (3 or 4 mTorr), and ozone (4mTorr) and varying amounts of SOz (0-1.6 mTorr). Addition of SOz decreased the yield of POZ. The inverse of the yield of POZ is plotted against the initial concentration of SOz in Figure 4.
Discussion Pressure Effect on the Yield of H#04. Figure 1 indicates that H2S04is formed even under the lowest pressure of 10 Torr. This fact is in marked contrast with our previous result7for the reaction of CH3CHO0produced in the reaction of tranr-2-butene-O3-SCJ2. In the latter case, the yield of HzSO4 falls down to zero when the total pressure was decreased ca. 10 Torr. This fact verifies the hypothesis that only collisionally stabilized Criegee intermediates can undergo bimolecular reactions with SO2 and other reactants. Oxidation of SOz under the low total pressure in the C2H403-S02 system may be either due to the OH radical produced in the secondary reaction^'^ or due to the stabilized CHzOO which can be formed even at the low total pressure. The formation of HCOOCH20H in the same ratio as the yield of H2S04 under (12) The assignment of lsO-labeled formic acids in the IR spectrum was based on the data in the following: Hatakeyama, S.; Bandow, H.; Okuda, M.; Akimoto, H. J. Phys. Chem. 1981,85, 2249. (13) Finlayson, B. J.; Pitts, J. N., Jr.; Atkinson, R. J . Am. Chem. SOC. 1914, 96, 5356.
4134
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
Hatakeyama et al.
the low pressure indicates that the latter should be the case. Niki et al.” also reported that a large fraction of C H 2 0 0 was produced initially “cold”. Thus, we can conclude that the effect of O H radical on the SO2 oxidation should be minimall4 under our experimental conditions and the stabilized C H 2 0 0 is formed at a substantial fraction in the primary step of the reaction of C,H, and 0, even under the lowest total pressure studied. From these results the reaction mechanism in eq 1-9 is proposed.
C~H+ , o3
---*
CH200*
k3
+ HCHO C H 2 0 0 *+ H C H O CHzOO + H C H O CH~OO*
/o
CCHz 1 1 ‘0
C H 2 0 0 *+ M C H 2 0 0 + SO2
-k5
-
a
(la)
p (1b) 1 - (Y - p ( l c )
dec products
-% C H 2 0 0 + M
(3)
(4)
products (SO3,HCHO, etc.) ( 5 )
+ H C H O --% HCOOCH,OH ki C H 2 0 0 + CH3CHO POZ /”1 1 dec products C H 2 0 0 - CCl+ CHzOO
&/Torr-’
for C2H4 (-175 Figure 5. 1/[@PH2S04 - (1 - a - @)]vs. mTorr)-0, (- 122 mTorr)-S02 (-670 mTorr) systems.
(6)
The intercept of Figure 1 gives 1 - a - p to be 0.20 f 0.03. The plot of eq 10 by using this value is given as Figure 5. From the (7) intercept of the plot in Figure 5 , ,8 is estimated to be 0.20 f 0.14. Then a is 0.60 f 0.17. From the intercept and slope, the ratio R8 (8) of the rate constants k 4 / k 3is given as (1 .O f 0.9) X mol‘0 ecule/ cm3. SO3 H 2 0 H2S04 (9) The value k 4 / k 3is supposed to be determined by the height of the barrier to the isomerization of the initially formed perReaction of ozone with SO2,HCHO, and C H 3 C H 0 can be to dioxirane and the amount of the internal energy neglected since the rate constants of these reactions are k n ~ w n ’ ~ ~oxymethylene ~ contained in the intermediate. Karlstrom and RoosI9calculated
