Environ. Scl. Technol. 1986, 20,387-393
(11) Atkinson, R.; Darnall, K. R.; Lloyd, A. C.; Winer, A. M.; Pitts, J. N., Jr. Adv. Photochem. 1979, 11, 375-488. (12) Atkinson, R. Chem. Rev., in press. (13) Atkinson, R.; Carter, W. P. L.; Winer, A. M. J. Phys. Chem. 1983,87,1605-1610. (14) Hoshino, M.; Akimoto, H.; Okuda, M. Bull. Chem.SOC. Jpn. 1978,51, 718-724. (15) . , Darnall. K. R.: Atkinson, R.; Pitts, J. N., Jr. J. Phys. Chem. 1979,83, 1943-1946. (16) Takagi, H.; Washida, N.; Akimoto, H.; Nagasawa, K.; Usui, Y.; Okuda, M. J. Phys. Chem. 1980,84,478-483. (17) Bandow, H.; Washida, N.; Akimoto, H. Bull. Chem. SOC. Jpn. 1985,58, 2531-2540. (18) Bandow, H.; Washida, N. Bull. Chem. SOC.Jpn. 1985,58, 2541-2548. (19) Bandow, H.; Washida, N. Bull. Chem. SOC.Jpn. 1985,58, 2549-2555. (20) Besemer, A. C. Atmos. Environ. 1982, 16, 1599-1602. (21) Takagi, H.; Washida, N.; Akimoto, H.; Okuda, M. Spectrosc. Lett.-1982, 15, 145-152. (22) Winer, A. M.; Graham, R. A.; Doyle, G. J.; Bekowies, P. J.; McAfee, J. M.; Pitts, J. N., Jr. Adv. Environ. Sci. Technol. 1980,10,461-511. (23) Plum, C. N.; Sanhueza, E.; Atkinson, R.; Carter, W. P. L. Pitts, J. N., Jr. Environ. Sci. Technol. 1983, 17, 479-484. (24) Tuazon, E. C.; Atkinson, R.; Carter, W. P. L. Environ. Sci. Technol. 1985,19, 265-269. (25) Lin, C.-Y.; Lin, M. C., presented at the Fall Technical Meeting, Eastern Section, Combustion Institute, 1984.
title of article, names of authors, journal issue data, and page numbers Prepayment, check or money order for $13.50 for photocopy ($15.50 foreign) or $6.00 for microfiche ($7.00 foreign), is required and prices are subject to change. Acknowledgments We t h a n k William D. Long a n d Sara M. Aschmann for assistance in conducting these experiments. Registry No. NO,, 11104-93-1; OH, 3352-57-6; glyoxal, 10722-2; methylglyoxal, 78-98-8; biacetyl, 431-03-8; benzene, 71-43-2; toluene, 108-88-3; o-xylene, 95-47-6; m-xylene, 108-38-3;p-xylene, 106-42-3;1,2,34rimethylbenzene,526-73-8; 1,2,4-trimethylbenzene, 95-63-6; 1,3,5-trimethylbenzene, 108-67-8. L i t e r a t u r e Cited (1) Black, F. M.; High, L. E.; Lang, J. M. J. Air Pollut. Control ASSOC. 1980, 30,1216-1221. (2) Carter, W. P. L.; Ripley, P. S.; Smith, C. G.; Pitts, J. N., Jr. "Atmospheric Chemistry of Hydrocarbon Fuels"; Experiments, Results and Discussion, USAF Final Report ESL-TR-81-53, NOV1981; Vol. 1. (3) Nelson, P. F.; Quigley, S. M.; Smith, M. Y. Atmos. Environ. 1983, 17,439-449. (4) Leone, J. A.; Seinfeld, J. H. Int. J. Chem. Kinet. 1984,16, 159-193. ( 5 ) Shepson, P. B.; Edney, E. 0.;Corse, E. W. J. Phys. Chem. 1984,88,4122-4126. (6) Dumdei, B. E.; OBrien, R. J. Nature (London) 1984,311, 248-250. (7) Atkinson, R.; Lloyd, A. C. J. Phys. Chem. Ref. Data 1984, 13, 315-444. (8) Tuazon, E. C.; Atkinson, R.; Mac Leod, H.; Biermann, H. W.; Winer, A. M.; Carter, W. P. L., Pitts, J. N., Jr. Environ. Sci. Technol. 1984, 18, 981-984. (9) Leone, J. A.; Flagan, R. C.; Grosjean, D.; Seinfeld, J. H. Int. J. Chem. Kinet. 1985, 17, 177-216. (10) Atkinson, R.; Carter, W. P. L.; Darnall, K. R.; Winer, A. M.; Pitts, J. N., Jr. Int. J. Chem. Kinet. 1980,12,779-836.
Received for review April 25,1985. Accepted August 19, 1985. The authors gratefully acknowledge the financial support of the US. Environmental Protection Agency under Cooperative Agreement CR-810964-01 (ProjectMonitor, Marcia C. Dodge). Although the research described in this article has been funded by the Unites States Environmental Protection Agency, it has not been subjected to the Agency's required peer and policy reuiew and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred.
Formation of Methyl Nitrite in the Surface Reaction of Nitrogen Dioxide and Methanol. 1. Dark Reaction Hlroo Takagl, Shlro Hatakeyama, and Hajlme Aklmoto" Division of Atmospheric Environment, The National Institute for Environmental Studies, P.O. Tsukuba-gakuen, Ibaraki 305, Japan
Sellchlro Koda Department of Reaction Chemistry, Faculty of Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan
T h e reaction of nitrogen dioxide and methanol, which has been known in the gas phase to proceed thermally as CH,OH 2N02 CH30N0 "OB, was found t o proceed heterogeneously on various surfaces as well, in an 11-L Pyrex cell and a 6065-L evacuable smog d m n b e r . An FTIR study has revealed that t h e reaction products a r e t h e same as in t h e homogeneous reaction, Le,, methyl nitrite a n d nitric acid. Neither methvl nitrate nor nitrous acid was observed. Among t h e surfaces studied, uncoated stainless steel, Pyrex, and t h e smog chamber wall surfaces were found to be the most active to induce t h e surface reaction. T h e apparent activation energy of t h e surface reaction in t h e smog chamber was determined t o be -11.9 f 3.3 k J mol-I.
+
-
+
Introduction T h e r m a l reaction of methanol and NOz 0013-936X/86/0920-0387$01.50/0
CH,OH
+ 2N02
-
CH30N0 + HN03
(1)
is of much interest t o t h e atmospheric chemist in t h a t (i) i t could be a possible major source of methyl nitrite in t h e polluted atmosphere particularly when methanol is used as automobile fuel and (ii) it is an analogue t o the reaction of water a n d NOz,
H 2 0 + 2NOZ
-
HONO
+ HN03
(2)
which would be an important reaction of nitrous acid observed in the atmosphere (1). Since both methyl nitrite nitrous acid accelerate efficiently phobchemical smog formation by providing OH radicals under the irradiation of sunlight, it is of much significance to characterize reactions 1 a n d 2 under t h e simulated atmospheric conditions. The classical gas-phase reaction of methanol a n d NOz, eq 1, has recently been reinvestigated by Niki e t al. (2)and
0 1986 American Chemical Society
Envlron. Sci. Technol., Vol. 20, No. 4, 1986 387
Koda et al. ( 3 ) ,and the stoichiometry and basic kinetics have been established. Niki et al. (2) confirmed the stoichiometric reaction 1by direct observation of all the reactants and products, using the FTIR method, and showed the kinetics to follow the rate law, d[CH,ONO]/dt = k[N0z]z[CH30H] a t the reactant pressures of 0.1-1.0 torr being consistent with earlier studies ( 4 , 5 ) . Koda et al. (3)found that the kinetic order with respect to methanol decreased with the increase of the methanol pressure in the range of ca. 1-10 torr, and proposed that asym-N20, rather than sym-Nz04might be involved as intermediates of reaction 1. The termolecular rate constants of reaction 1 were reported to be (5.7 f 0.6) X cm6 molecule-z s-l a t 25 OC (2) and (9.0 f 0.2) X cm6 molecule-2 s-l a t 21 "C ( 3 ) . On the other hand, the kinetics of the reaction of NOz and water in the gas phase have been studied by England and Corcoran (6), who reviewed the earlier studies and showed that the initial rate of disappearance of NOz was first order with respect to the water vapor concentration and greater than first order with respect to NOz. More recently, the importance of the heterogeneous surface reaction of NOz and water vapor was pointed out by Sakamaki, Hatakeyama, and Akimoto (7) in relevance to the unknown radical supply (8, 9) in the smog chamber experiments. The kinetics of the surface reaction have been studied extensively by Sakamaki et al. (7) and by Pitts et al. (10) in air at the NOz concentration of 1-23 ppm and 0.1-20 ppm, respectively, and shown that the HONO formation follows the rate law, d[HONO]/dt = k [NO,] [H,O] rather than d[HONO]/dt = k[N0z12[HzOl, which is expected from the stoichiometry. However, gaseous HN03 could not be detected in any of the smog chambers employed at the National Institute for Environmental Studies (NIES) and at the Statewide Air Pollution Research Center (SAPRC). Thus, although the kinetic law has been established, the stoichiometry and the mechanism of the surface reaction of water vapor with NO2 remain unresolved. From the analogy of the reactions, the reaction of methanol and NOz may also proceed heterogeneously on the reactor surface to emanate products into the gas phase. The study of the heterogeneous reaction of methanol and NOz might give better insight into the nature of the reaction of water vapor and NOz also. In the present study, the dark reaction of methanol and NO2 was studied at the pressure of 50-200 mtorr of reactants in 1atm air in 11-L cells with different surface materials, and the heterogeneous component of the reaction was elucidated. The reaction was also studied in a 6056-L evacuable and bakable smog chamber, where the surface reaction was found to be predominant at the NOz pressure of 20-60 mtorr. This paper (Paper I) describes the kinetic study of the dark reactions of methanol and NO2 on surfaces, and the following paper (Paper 11)reports the observed enhancement of the rate of the methanol-NOz reaction in the smog chamber under the photoirradiation. Experimental Section Experiments were conducted by using a quartz or Pyrex cell (120-mm i.d., 1000-mm length, and 11-L volume) and an evacuable smog chamber (1400-mm diameter, 3500-mm length, and 6065-L volume). Details of the smog chamber system have been described elsewhere (11). Both the cells and the chamber were equipped with gold-coated, multireflection mirrors inside for FTIR measurement. The surface area of the quartz and the Pyrex cell is 3770 cmz, and the total surface-to-volume ratio is ca. 35 m-l. A series of runs were carried out in the quartz cell inserting 388
Environ. Sci. Technol., Vol. 20, No. 4, 1986
either a PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer) coated or uncoated roll of stainless steel (SUS 304) plate (0.1 mm thickness, 700 mm X 380 mm rolled into a cylindrical form to contact closely to the quartz wall, 2660 cmz one side surface area) in order to enhance the heterogeneous surface reaction. The inner wall of the smog chamber is PFA coated 17 pm thick, and each end flange of the chamber holds 19 windows (250-mm diameter each) made of quartz and Pyrex. The surface area of the PFA-coated wall is ca. 39.4 mz, and the quartz and Pyrex window surfaces are 0.9 and 1.0 m2, respectively. Other surface materials in the smog chamber are gold (multireflection mirrors), aluminum (mirror covers), stainless steel (a part of mirror holders), and invar (mirror supporting rods). The total area of these metals is ca. 0.2 m2. Thus, the total surface-to-volume ratio of the smog chamber is 6.9 m-l including the contents. The PFA coating on the smog chamber wall and the SUS plate specimen, which was inserted into the quartz cell, contains primer coating underneath. The primer coating is polytetrafluoroethylene (PTFE) containing TiOz and carbon as pigments and Crz07and to obtain better adhesiveness to the stainless-steel surface. Temperature dependence of the reaction was studied in the smog chamber in the range 0-40 OC by varying the temperature of the wall, which is jacketed with thermostated heat transfer oil. Both end flanges of the chamber are not thermostated, and the temperature of the gas deviates from that of the wall to the extent of 4 "C at the most as the difference between the wall and the room temperature increases. The average gas temperature was measured by a thermometer fed through into the chamber. All the runs in the quartz and Pyrex cell were carried out at 22 f 1 "C. The concentrations of the products were measured by means of the long-path FTIR. The path lengths were 40.0 m and 221.5 m for the cells and the smog chamber, respectively. The same spectrometer was used for both systems by switching the optical path. The spectrum was typically obtained with 128 scans (scanning time -4 min) at a resolution of 1.0 cm-l. The absorptivities used were CH,ONO, 0.161 (991 cm-l Q-branch peak), and HN03, 1.38 (1326 cm-l &-branch peak) torr-l m-l. The initial concentrations of methanol and NO, were determined by pressure reading in a known volume before flushing into the reactor. For a few runs the decrease of CH30H and NO2 was monitored a t the peaks 2846 and 2918 cm-', respectively. In the quartz and Pyrex cell runs, NO2 was first introduced into the cell with purified air up to ca. 400 torr, and then methanol was flushed with air to fill the cell at 760 torr. In the smog chamber runs, the chamber was first filled with the purified air at ca. 760 torr, and then NO2 and methanol were introduced successively by using air as the carrier gas. The reaction mixture was stirred by a pair of fans during the run. Results Reaction in the Blank Quartz Cell. The dark reaction of methanol and NOz was first studied in a quartz cell at the pressure of methanol of ca. 38 mtorr, NOz of 35-150 mtorr, and 1 atm of air in order to assure the gas-phase reaction 1, which has been established in the 0.1-10-torr region ( 2 , s ) . Figure 1shows typical data of CH30N0 and HNO, formation in low conversion (open symbols). As shown in Figure 1, HN03 is observed in the gas phase right after the mixing of CH30H and NOz though the yield is only a fraction of CH30N0 after the reaction proceeds. When the initial rates of CH30N0 formation (R) are
-1
I
0
50
100
Reaction
Time
I
I
I
I
200
150 (min)
I
10
Flgure 1. Time profiles of the formation of CH,ONO (0,0)and HNO, (A,A)for the reaction of CH,OH and NO, in the blank quartz cell (open symbols) and in the quartz cell with a PFA-coated plate (solid symbols): [CH,OH], = 35.9, 38.1 mtorr and [NO,], = 113.8, 121.5 mtorr, respectively.
40
20 NO2
100
200
(mTorr)
Flgure 2. Log-log plot of the initial formation rate (R')of CH,ONO due to the surface reaction vs. [NO,], for the reaction of NO, and methanol in the quartz cell with the PFA-coated plate (0)and uncoated SUS (+), in the Pyrex cell (m), and in the evacuable smog chamber (A): [CH,OH], 38 mtorr.
-
Table I. Kinetic Data for CH3OH-NO2 Mixtures in 760 torr of Air in the Quartz Cell (22 f 1 "C)
[CH,OH], mtorr
[NO,], mtorr
d[CH,ONO] / d t , mtorr min-'
37.1 38.1 38.4 35.9 38.1
35.8 76.2 113.8 133.0 152.4
0.0051 0.0133 0.0297 0.0441 0.0566
L
k"'
b
cm6
X
I-
-
molecule-2 s-l
E
CI
1.94 1.06 1.09 1.26 1.18
20-
4 I
Y
8
R I
a P 1.15 f 0.09
Average of four runs with [NO,] 2 76.2 mtorr. Error limit cit-
10-
V
Y
ed is 1 ~ . W
0
plotted against [NO2]:, a linear relationship was obtained confirming the major contribution of the termolecular reaction (eq 1). The termolecular rate constant ( P I ) as defined by d[CH,ONO]/dt = k"'[N02]2[CH30H] can be derived from an integrated equation 1
1
1
b(a - x )
1
= kIIIt
(3)
where a and b are the initial concentrations of CH30H and NOz, x is the concentration of CH30N0 at time t, and the stoichiometry of -ACH30H = -2AN02 is assumed. The rate constants obtained from eq 3 are given in Table I, and the average value was determined to be (1.15 f 0.09) X cm6 molecule-2 s-l. This value is appreciably higher than the homogeneous reaction rate constant reported before by Farlie et al. ( 4 ) ,(0.45 f 0.08) X (25 "C), and by Niki et al. (2), (0.57 f 0.06) X (25 "C), but only slightly higher than the value, (0.90 f 0.02) X cm6 molecule-2 s-l (21 "C), by Koda et al. (31, all of which were obtained a t much higher reactant pressures (0.1-10 torr). In view of the results of the following experiments, the rate constant obtained in the quartz cell under the low reactant pressures in the present study may be subjected to a contribution of the heterogeneous surface reaction. Reaction in the Quartz Cell with Extra Surfaces. When a cylinder of PFA-coated stainless steel was inserted into the quartz cell, the initial formation of CHBONOwas also nearly linear with reaction time, but the rate was significantly enhanced as compared to the reaction in the blank cell (solid symbols in Figure 1). On the contrary, the formation of gaseous HN03 was suppressed in the
25
50
Reaction Time
75 (ppm)
100
Figure 3. Time profiles of the formation of CH,ONO (0,0)and HNO,
-
-
(A,A)in the quartz cell with a SUS plate (solid symbol) and in the Pyrex cell (open symbols): [CH,OH],
38 mtorr and [NO,],
114 mtorr.
presence of the PFA surface. No new product other than CH30N0 and HN03 was observed in the presence of the foreign surface. The heterogeneous component of the CH,ONO formation rate (R3 due to the PFA surface was evaluated by subtracting the CH30N0 formation rate due to the homogeneous reaction using the termolecular rate constant obtained in the blank cell. Figure 2 shows the log-log plot ( 0 )of the heterogeneous reaction rate thus obtained vs. the initial concentration of NOz. The slope of the plot is 1.11f 0.17, and thus, the heterogeneous component was found to be linearly dependent on NOz in contrast to the second-order dependence for the homogeneous reaction. The apparent bimolecular rate constant of the heterogeneous reaction due to the PFA surface was defined by d[CH,ONO]/dt = .Iz1'[N02][CH30H] and was calculated from the plot of In {b(u - x ) / a ( b - 2x))/(2a - b) = h 9 . Here, a and b are the initial concentrations of CH30H and NO2, and the stoichiometry of -ACH30H = -2AN02 was assumed as before. The obtained kinetic data are summarized in Table 11. When the uncoated SUS cylinder of the same dimension was substituted for the PFA-coated one, the formation rate of CH30N0 was enhanced drastically (see solid symbols in Figure 3). The observed initial rate of CH,ONO formation was more than an order of magnitude larger than the homogeneous reaction rate estimated by using the rate constant given above, even at the highest NOz pressures Environ. Sci. Technol., Vol. 20, No. 4, 1986 389
Table 11. Kinetic Data for CH30H-N02Mixtures in 760 torr of Air in the Quartz Cell with PFA-Coated and Uncoated SUS Cylinder and in the Blank Pyrex Cell
mtorr 38.2 38.0 38.4 38.0 38.3 38.1 38.2 38.0 38.0 38.1 38.2
Quartz Cell (with PFA-Coated SUS) 56.5 0.016 0.33 56.8 0.012 0.41 68.7 0.012 0.24 75.5 0.023 0.44 91.4 0.025 0.38 106.2 0.024 0.32 121.5 0.024 0.28 134.1 0.029 0.31 144.7 0.039 0.38 153.0 0.054 0.52 153.2 0.038 0.36 av 0.36 10.08
37.4 37.9 38.0 38.1 38.0
Quartz Cell (with Uncoated SUS) 23.3 0.08 5.0 38.1 0.21 7.7 61.5 0.32 7.9 70.4 0.54 9.5 114.1 0.77 11.9 av 8.4 f 2.5
38.5 37.3 37.8 38.1
Pyrex Cell (Blank) 37.9 0.08 56.8 0.12 72.6 0.23 113.3 0.37
3.0 3.1 4.7 4.9 av 3.9 I 1.0
a The initial formation rate of CH,ONO due to the heterogeneous surface reaction.
employed. Nitric acid was observed as before, although the gaseous concentration shows a strong saturation. The initial rate of CH30N0 formation was obtained by a curve fitting of a third-order polynomial to the time profile typified by Figure 3 and taking an initial slope a t t = 0. The log-log plot of the initial heterogeneous reaction rate vs. NO2 pressure reveals that the apparent reaction order is 1.43 f 0.15 as shown in Figure 2 ( 6 ) . Nevertheless, the second-order rate analysis was made in the present work to make a comparison with the rates for the other surfaces easier. The second-order rate constant, kI1, given in Table I1 was obtained by taking an initial slope of the plot of In (b(a- r ) / a ( b- 2r)]/(2a- b ) vs. t and is slightly dependent on the reagents' partial pressure as expected. The average heterogeneous rate constant is 25 times as large as that for the PFA surface. Reaction in the Pyrex Cell. The reaction in a blank Pyrex cell was found to proceed much faster than that in the blank quartz cell but slower than that in the quartz cell containing the SUS cylinder as shown in Figure 3 (open symbols). Figure 2 depicts that the initial rate of CH30N0 formation was proportional to the 1.48 f 0.18 degree to the initial concentration of NOz, being close to the case of the reaction with the SUS surface in the quartz cell. The apparent second-order rate constant of the initial surface reaction was obtained by the curve-fitting method as before and is given in Table 11. Thus, the observed rate constant is about 10 times larger than that in the quartz cell containing the PFA surface. The relative yield of HNO, is higher than in the case of other surfaces. Reaction in the Smog Chamber. The dark reaction of methanol and NOz in the smog chamber also gave CH30N0 and HN03 as reaction products, which increase nearly linearly with reaction time as depicted in Figure 4. Note that the initial concentration of CH30H employed in these runs is about 10 times smaller than those em390
Environ. Sci. Technol., Vol. 20, No. 4, 1986
Reaction Time ( m i n )
Figure 4. Time profiles of the formation of CH,ONO (0)and HN03 (A) in the evacuable smog chamber: [CH,OH], = 3.7 mtorr and [NO,], = 36.5 mtorr.
Table 111. Kinetic Data for CH30H-N02Mixtures in 760 torr of Air in the Evacuable Smog Chamber
[CH,OHI, mtorr
[Nod, mtorr
temp, K
R /a, mtorr mi&
3.8 3.8 3.9 3.7 3.5 3.9
19.1 28.1 30.4 36.5 45.4 56.7
303 303 303 303 303 303
0.0031 0.0043 0.0058 0.0045 0.0070 0.0093
6.8 9.2
38.3 38.3 38.1 38.4
303 303 303 303
0.0045 0.0071 0.0079 0.0055
12.2
15.5
k" x 10-20 cm3
molecule-' 2.4 2.2
2.8 1.7
2.7 2.5 av 2.4 f 0.4
Rb
3.9 3.6 3.6 3.7 3.9
38.4 38.1 38.0 36.5 38.1
277 285 294 303 311
0.0068 0.0072 0.0057 0.0054 0.0038
"The formation rate of CHsONO due to the heterogeneous surface reaction. *The formation rate of CH30N0 including the homoeeneous reaction.
ployed in the quartz and the Pyrex runs. The formation rate of CH30N0 by the homogeneous reaction as calculated by using the termolecular rate constant is only less than 10% of that observed experimentally in Figure 4, indicating that the heterogeneous reaction is predominant also in the smog chamber under the present experimental conditions. The observed rate of CH30N0 formation was found to be nearly proportional to the first order of NO2 as in the case of the quartz cell with PFA surfaces (see (A) in Figure 2). An apparent bimolecular rate constant of the surface reaction was calculated after subtracting the contribution of the homogeneous reaction and the results are given in Table 111. The dependence of the CH30N0 formation rate on the CH30H concentration was also studied in the smog chamber. The results are given in Table I11 and shown in Figure 5. The effect of agitating the reaction mixture was checked by terminating the stirring fan a t the middle of the run. The difference in the CH30N0 formation rate before and after the stopping of the fan was within experimental error. This result indicates that the reaction rate is not determined by the transportation of species in the bulk gas phase.
Table IV. Material Balance of the Gaseous Species in the Surface Reaction of NO2 and CH,OH
surface
[CH30H], mtorr
[NO,], mtorr
-ACH,OH
-AN02
ACH3ONO
37.4 37.9 38.5 37.3
23.3 38.1 37.9 56.8 38.4
1.69 f 0.18' 1.14 f 0.09
1.00
0.93 i 0.08
0.20 f 0.02
1.00
1.01 f 0.15 0.76 i 0.10 0.94 f 0.17
n.d.b 0.12 0.05 0.13 i 0.04 0.13 f 0.01
quartz/SUS Pyrex smog chamber/PFA
15.5
1.00
1.31 f 0.30 1.13 0.12
*
1.00 1.00
0.51 f 0.08
0.41 f 0.03
AHNO3
*
Error range is la. Not determined due to the strong saturation of the gaseous HN03. I
g
V
I
I
*
o
5 [CH30H J
10 (rnTorr)
15
Flgure 5. Formation rate of CH,ONO vs. [CH,OH], for the reaction of NO, and methanol in the smog chamber: [NO,], = 38 mtorr.
Temperature dependence of the overall reaction studied in the smog chamber is shown in Table 111. Although the data are rather scattered, the formation rates of CH30N0 and HN03 are slightly negatively dependent on temperature. An Arrhenius plot gives the apparent activation energy of -11.9 f 3.3 kJ mol-l. Material Balance. The material balance of the gaseous species in the surface reaction of NOz and CH30H was studied for a few runs employing comparable initial concentrations of NOz and CH30H. For each run, -ACH30H, ACH30N0, and AHN03 were plotted against -ANOz at different reaction time, and the slopes were determined from least-squares plots. The results are summarized in Table IV. Only for the smog chamber run is the material balance of -ACH30H, -ANOz, and ACH30N0 close to the stoichiometric relation of CH30H + 2N02 = CH30N0 + "OB. In other cases, the apparent ratio of -ACH30H, -AN02, and ACH30N0 in the gas phase is closer to l:l:l, and the consumption ratio of ACH30H/AN02 exceeds unity particularly when the ratio of [NOz]/[CH30H] gets lower. The increment of ACH30N0 + AHN03 in the gas phase exceeds -ANOz in some cases in the smaller cells.
Discussion Surface Reaction of NO2 with Methanol. The characteristics of the surface reaction can be summarized as follows: (i) The reaction products are the same as those in the gas phase, Le., CH30N0 and HN03, and neither CH30N02 nor HONO was detected. (ii) The surface reaction rate follows the first-order kinetics with respect to NO2 in the smog chamber and in the quartz cell with a PFA surface, in contrast to the gas-phase reaction, which follows the second-order dependence ( 2 , 3 ) . In the quartz cell containing SUS surface and in the Pyrex cell, the reaction rate was proportional approximately to the 1.5 power of NO2 pressure. (iii) The formation rate of CH3ON0 increases with the concentration of CH30H a t the lower concentration range but tends to saturate as the concentration increases. (iv) The rate of the surface reaction is very much dependent on the materials. The observed heterogeneous reaction rate constants per geometric S/Vratio are 1.5 X 3.5 X and 1.1x cm3 molecule-'^-^ m, for the PFA, SUS, and Pyrex sur-
faces, respectively ([CH30H] = 38 mtorr, [NO,] = 30-150 mtorr). Among the surfaces employed the SUS surface is the most active, followed by the Pyrex and PFA-coated plate, and the quartz surface is the most inactive. However, since the dependence of the reaction rate on the CH30H concentration was not studied for each surface, the relative rate obtained above may not be representative of the catalytic activity under different conditions. The smog chamber surface was also found to be very reactive giving the normalized reaction rate of 3.5 X cm3 molecule-I s-l m for the runs with 3.8 mtorr of CH30H. The above findings indicate that the reaction of NO2 and methanol, CH30H 2NOZ CH30N0 + HN03 AHo= -65 kJ mol-l (1)
-
+
which has been well-known in the gas phase ( 2 , 3 ) ,is enhanced catalytically on various surfaces. The material balance of the gaseous species has shown that the ratio of -ACH,OH, -ANOz, and ACH30N0 is approximately 1:2:1 as expected from the stoichiometry of eq 1 in the case of the smog chamber. The yield of gaseous HNO, is ca. one-third of that of CH30N0. These results suggest that the overall surface reaction of NO2 and CH30H proceeds following the stoichiometric reaction 1as in the gas phase, but most of HNO, formed remains on the surface. In the quartz cell with the SUS surface and in the Pyrex cell, however, the ratio of -ACH30H/ (-AN02) exceeds unity as opposed to the stoichiometry of eq 1. The ratio tends to increase as the concentration ratio of CH30H to NOz increases. In these smaller cells, unreactive adsorption of CH30H and NO2 may be considerable, which would give the deviation of the ratio from the stoichiometry. As the ratio of CH,OH to NO2 decreases, most of adsorbed CH30H would react with nitrogenous species on the surface to emit a nearly stoichiometric amount of CH30N0 to that of CH30H consumed. The fact that the ratio of ACH30NO/(-AN02) is close to unity rather than to 0.5 as expected from eq 1may be ascribed to the contribution of the reaction of NO2 preadsorbed on the surface right after the reactant is introduced into the cell. Relation of NOz and Water Reaction. It would be worthwhile to compare the present results for reaction 1 with those of the reaction of water and NO2, HzO + 2N02
-
HONO
+ HN03 AHo = -37 kJ molW1(2)
which has been reported earlier (6, 7,101. Both reactions proceed on the gas-solid interface as well as homogeneously in the gas and liquid phase (12, 13). The kinetic dependence with respect to the NO2 pressure is the same, i.e., the second order in the homogeneous gas-phase reaction and the first order in the heterogeneous surface reaction. The heterogeneous second-order rate constants of the methanol-NO2 and the water-NO2 reactions in the evacuable smog chamber a t NIES are (2.4 f 0.4) x and (7.0 f 0.9) X cm3 molecule-ls-l (7), respectively, Thus, the surface reaction of the methanol and NO2 is ca. Environ. Sci. Technol., Vol. 20, No. 4, 1986
391
3000 times faster than that of water and NO2. The surface reaction of H 2 0 and NO2 has been written as a nonstoichiometric reaction,
NO2 + H 2 0 2HONO
(4) in our previous study (7) due to the failure of detecting HNO, in the gas phase. In view of the similarity of the reaction to that of methanol and NO2,the surface reaction of water and NO2 may also proceed as written by eq 2, but the reason for the stay of HNO, on the surface in this case remains unresolved. Mechanism of the Surface Reactions. From the kinetic dependence on NO2 and CH,OH as observed in this study, the surface kinetics can be conceived, e.g., CH,OH($) NO2(g)
+
+
(CH,OH)ad
(5)
--
(6)
(NO2)ad
(NO2)p.d + NO2(g)
[(N02)2Iad
(7)
[(N02)21ad + (CH3OH)ad [CH3OH.(NO2)2]ad (8) [CH30H*(N02)21ad-.+ (CH3ONO.HNOJad (9) (CH,ONO*HNOJ,d
-
CH,ONO(g)
+ HNO,(g)
(10) Although the present study does not give any details on these steps, the dependences on NO2 (smaller than second order) and CH30H (smaller than first order) seem to be consistent with the above mechanism. The observed negative temperature dependence of the overall reaction suggests that the steps of eq 5-8 rather than the rearrangement step, eq 9, are rate determining. Being relevant to the reactive nitrogenous intermediate, Koda et al. (3)have noted that the alternative pathways of the methanol-NO2 reaction, CHSOH + 2N02 CH3ON02 + c~s-HONO A H 0 3 0 0 = -61 kJ mol-l (11) -+
CH,OH
+ 2N02
-
CH30N02+ trans-HONO A H O 3 0 0 = -64 k J mol-l (12)
have comparable exothermicity to that of reaction 1, but the reaction pathway to form CH,ONO and HNO, has been observed exclusively in the gase phase. From this fact and the kinetic analysis they proposed a six-membered ring transition state involving asym-N204,which is known only in low-temperature matrices (14-16) and also postulated in the liquid phase (17, 18). On the other hand, Cooney and Tsai (19) have observed by Raman spectroscopy that a self-ionization of N02/N204occurs to a far greater extent than in the liquid phase to generate nitrosonium (NO+) and nitrate (NO,-) ions on the alkali cation-exchanged zeolites. Such a self-ionizationhas also been reported in a low-temperature solid of N 0 2 / N 2 0 4(15), while usym-N2O4is known to be predominant in diluted matrices (14,16). Although it is premature to assess the reaction intermediate involved in the surface reaction, the fact that the exclusive reaction channel (eq 1)is also the case for the surface reaction as in the gas phase is suggestive that for similar type of transition state or reaction intermediate might be involved in the surface reaction. It would be interesting to note here that in our previous study (7) of the surface reaction of NO2 and water, the use of H2180 gave HlsONO exclusively, which could be interpreted by the same mechanism as proposed (3) for the case of methanol. The difference in the surface activity as observed in the present study may be related to the chemical form of the adsorbed nitrogenous species as well as the acidity of the hydroxyl group of the adsorbed CH30H. The NO2-cata392
Envlron. Sci. Technol., Vol. 20, No. 4, 1986
lyzed surface reaction of the double bond migration of 2-butenes on porous Vicor and several types of zeolites has been studied by Wada, Otsuka, and Morikawa (20) and is ascribed to the formation of acidic hydroxyl group in the presence of surface NO2. Atmospheric Implications. Methanol is a potential candidate of the alternative liquid fuel to petroleum, in particular as an automobile fuel in the near future. In fact fleet tests for pure methanol engines have been already performed in a number of countries. Methyl nitrite has been reported as one of the new emissions from practical engines (21,22) as well as from more fundamental constant-volume combustion vessels (23). It is now established that the nitrite is not produced during the bulk combustion but is produced through a certain postcombustion reaction during the exhaust as well as analytical processes. The most probable reaction is that between NO2 and the unburnt methanol emitted from the combustion process, but the rate of the homogeneous reaction 1 seems to be too slow for explaining the relatively large production of methyl nitrite under several performance conditions. For example, methyl nitrite production of more than 100 ppm was reported (22) a t 20 s after sampling the exhaust gas from the engine exhaust line of an ordinary 4-cycle S.I. engine. Thus, the heterogeneous reaction seems to play a decisive role in these practical systems, though any quantitative estimation is not yet possible due to the unresolved temperature dependence over the necessary wide range and also due to the difficulty in characterizing the surface condition in these practical systems. Several workers (24, 25) have recently discussed the effect of methanol replacement of a part of gasoline on the photochemical smog formation. The methanol substitution was stated to help decrease the calculated ozone level in some cases. But they did not take into account the dark reaction (eq 1)(both of homogeneous and heterogeneous nature) to yield methyl nitrite, which is an important source of radicals when photolyzed. The heterogeneous rate constant for the methanol reaction is about 3000 times as large as that of the water reaction with NO2, at least as far as the heterogeneous reactions are concerned; thus, although it is not possible to assess quantitatively the heterogeneous rate of CH30N0 formation in the atmosphere, the nitrite-forming reaction should be as important as the HONO-forming reaction from water and NO2 if the concentration of methanol exceeds the order of 1 ppm. The monitoring of methanol and nitrite concentrations in the atmosphere as well as the detailed estimation of the fate of the emitted methanol is considered to be important if methanol is to be used practically. Acknowledgments We thank A. Morikawa of Tokyo Institute of Technology and H. Bandow for helpful discussions. Registry No. CH,OH, 67-56-1; CH,ONO, 624-91-9; 7697-37-2; stainless steel, 12597-68-1.
"03,
Literature Cited (1) Harris, G. W.; Carter, W. P. L.; Winer, A. M.; Pitts, J. N., Jr.; Platt, U.; Perner, D. Enuiron. Sci. Technol. 1982,16, 414-419. (2) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Int. J. Chem. Kinet. 1982,14,1199-1209. (3) Koda, S.; Yoshikawa, K.; Okada, J.; Akita, K. Enuiron. Sci. Technol. 1985, 19, 262-264. (4) Fairlie, A. M., Jr.; Corberry, J. J.; Treacy, J. C. J. Am. Chem. SOC.1953, 75, 3786-3789. (5) Silverwood, R.; Thomas, J. H. Trans. Faraday SOC.1967, 63, 2476-2479.
Envlron. Scl. Technol. 1986, 2 0 , 393-397
England, C.; Corcoran, W. H. Znd. Eng. Chem. Fundam. 1974, 13, 373-384. Sakamaki, F.; Hatakeyama, S.; Akimoto, H. Int. J. Chem. Kinet. 1983, 15, 1013-1029. Carter, W. P. L.; Atkinson, R.; Winer, A. M.; Pitts, J. N., Jr.; Znt. J. Chem. Kinet. 1981,13, 735-740. Carter, W. P. L.; Atkinson, R.; Winer, A. M.; Pitts, J. N., Jr.; Znt. J . Chem. Kinet. 1982, 14, 1071-1103. Pitts, J. N., Jr.; Sanhueza, E.; Atkinson, R.; Carter, W. P. L.; Winer, A. M.; Harris, G. W.; Plum, C. N. Znt. J. Chem. Kinet. 1984, 16, 919-939. Akimoto, H.; Hoshino, M.; Inoue, G.; Sakamaki, F.; Washida, N.; Okuda, M. Environ. Sci. Technol. 1979, 13, 471-475. Schwartz, S. E.; White, W. H. Adv. Environ. Sci. Technol. 1983,12, 1-46. Schwartz, S.E. In “SO2, NO and NOz Oxidation Mechanisms: Atmospheric Considerations”, Calvert, J. G., Ed.; Butterworth: Boston, MA, 1984; pp 173-208. Fateley, W. G.; Bent, H. A.; Crawford, B., Jr. J. Chem. Phys. 1959, 31, 204-217. Bolduan, F.; Jodl, H. J.; Loewenschuss, A. J. Chem. Phys. 1984,80, 1739-1743. Bandow, H.; Akimoto, H.; Akiyama, S.; Tezuka, T. Chem. Phys. Lett. 1984, 111, 496-500. Anbar, M.; Taube, H. J. Am. Chem. SOC.1955, 77, 2993-2994.
(18) White, E. H.; Feldman, W. R. J . Am. Chem. SOC. 1957, 79, 5832-5833. (19) Cooney, R. P.; Tsai, P. J. Raman Spectrosc. 1980,9,33-38. (20) Wada, Y.; Otsuka, K.; Morikawa, A. J . Catal. 1981, 71, 136-143. (21) Jonsson, A.; Persson, K. -A.; Bertilsson, B. M. “Organic Nitrites in the Exhaust Emissions from Alcohol Fueled Vehicles”, Proceedings of the Fifth International Alcohol Fuel Technology Symposium, Auckland, New Zealand, 1982; pp 3-179-3-186. (22) Ito, K.; Yano, T.; Takahashi, F. “Methyl Nitrite Formation in Exhaust Gases Emitted from a Methanol Fueled S.I. Engine”, Proceedings of the Fifth International Alcohol Fuel Technology Symposium, Auckland, New Zealand, 1982; pp 3-171-3-178. (23) Okada, J.; Koda, S.; Akita, K. Fuel 1985, 64, 553-557. (24) Whitten, G. Z.; Pullman, J. B. “Methanol Fuel Substitution Can Reduce Urban Ozone Pollution”, Proceedings of the Sixth International Alcohol Fuel Technology Symposium, Ottawa, Canada, 1984; pp 2-61-2-65. (25) Wilson, K. W.; McCormack, M. C. “Methanol As an Ozone Control Strategy in the Los Angeles Area”, Proceedings of the Sixth International Alcohol Fuel Technology Symposium, Ottawa, Canada, 1984; pp 2-68-2-74. Received for review April 29,1985. Accepted November 5,1985.
Formation of Methyl Nitrite in the Surface Reaction of Nitrogen Dioxide and Methanol. 2. Photoenhancement Hajime Aklmoto” and Hiroo Takagl Division of Atmospheric Environment, The National Institute for Environmental Studies, P.O. Tsukuba-gakuen, Ibaraki 305, Japan
The surface reaction of NO2 and methanol described in the preceding paper was studied in the evacuable smog chamber under the irradiation of UV-visible light (2290 nm). Kinetic analysis of the gaseous reactant and products with the aid of computer modeling revealed that the surface reaction to yield CH30N0 and H N 0 3 is enhanced by the irradiation. The apparent second-order rate constant of the surface reaction increases linearly with light intensity, and the photoenhancement factor ranged from 1.8 to 5.9 for the kl value (primary NO2 photolysis rate) of 0.16-0.38 min-l and for the NO2 and methanol initial concentrations of 5 and 30 ppm, respectively.
Introduction In the preceding paper (I) (referred to as paper l ) , a thermal surface reaction of NO2 and methanol
+
(1) CH30H 2 NO2 ?!!+ CH30N0 + H N 0 3 has been characterized in dark in the smog chamber. In the present study, the kinetics of the surface reaction of NO2 and methanol have been studied under UV-visible irradiation. The observed evidence that the apparent rate of reaction 1 is enhanced under the irradiation will be reported in the present paper. This result may be of interest being relevant to the analogous reaction of NO2 and water,
wall
HzO + 2N02 HONO + HNOB (2) which has been discussed ( 2 , 3 )in relation to the unknown radical source issue for the smog chamber studies raised by Carter et al. (4, 5 ) . 0013-936X/86/0920-0393$01.50/0
Experimental Section Several runs of CH30H (5-20 ppm)-NO2 (37-60 ppm)-dry air (1atm) were carried out at 30 “C using the evacuable and bakable smog chamber at the National Institute for Environmental Studies (NIES). Full accounts of the chamber system have been reported previously (6), and a brief description has been given in paper 1. In each run, the chamber was first filled with purified dry air, and then methanol was injected using air as the carrier gas. After a reference spectrum had been recorded, NO2 was next introduced with the carrier gas, and the IR measurement was made approximately every 10-20 min. The formation of CH30N0 in the dark was monitored for the first ca. 2 h after the injection of NO2, and then the irradiation by a solar simulator (6) was commenced and continued for another ca. 2 h. The reaction mixture in the chamber was mixed with the two fans during runs. The light intensity was measured by primary NO2 photolysis rate (6) and was varied in the range of 0.16-0.38 min-l. Concentrations of the reactants and the products were measured by means of a long-path (221.5-m) FTIR set in the chamber. Spectral resolution of 1 cm-l was employed, and 128 scans were accumulated to obtain a spectrum. Absorption peaks and absorption coefficients (base 10, torr-l m-l) used for the quantitative measurement are as follows: CH,OH (2847 cm-l; 0.0711, Q-branch), CH30N0 (1445 cm-l; 0.0408, peak only), HCHO (1745 cm-l, 0.690, &-branch), and CH30N02 (1292 cm-l; 0.476, Q-branch peak to P-branch valley). Computer Simulation and Reaction Model. Computer simulation for the reaction system studied was carried out by using the reaction model given in Table I.
0 1986 American Chemical Society
Environ. Sci. Technol., Vol. 20, No. 4, 1986
393
