2653
vex?e
erature Range 200-346°K
lege Park, Maryland
SO740 and National Bureau of Standards, Washington, D.C. S.0234
Nalknal Bureau of Standards, Washington, D. C. $OM4
Ghemiatry Department, University of Maryland, College Park, Maryland
ROY40
(Received March do, 1972)
Publication costs ossisted by The Petroleum Research Fund
Using the technique of flash photolysis-resonance fluorescence, absolute rate constants have been measured for the reaction O 0 2 M -+ O3 M. For the case of M = Ar the temperature range covered was 20034B°K, and the total pressure was varied from 50 to 500 Torr. Over the indicated temperature range, an expl(1014 =t46 cal mol-’)/ = (6.57 i 0.59) X Arrhenius plot of the data yielded the expression E T ] em6 molecule-” sec-1, A comparison of the third-order rate constants for M = He, Ar, and KZgave the relative efficiencies for these three gases as 0.92: 1.0:1.6 at 2 9 P K . At 218”K, the efhieneies of Ar La 32 were in the ratio of 1.0:1.7. The reported rate measurements indicate that the rate of production of stratospheric ozone could br nearly a factor of 2 lower than that estimated from previously reported values of the third-order sate constant.
+ +
+
The reaction of atomic oxygen, 0(3P),with molecular oxygen is the only important ozone forming reaction in both the troponphere and stratosphere. In order to understand e4 ther photochemical air pollution4 or the rate conthe ozone balance in the stant for this reaction must be known accurately. Thc need for kiiietic irformation on reactions important in stratospheric chcmistry has been emphasized by the recent suggestion that nitrogen oxides emitted by supersonic aircraft might significantly reduce the ozone concentration in the ‘The atomic oxygen-molecular oxygen reaction has been the subject of many studies in recent year^,^-^^ thc data p i o r to mid-1967 having been reviewed by A t room temperature, several techniques have been applied t o the study of this reaction and a wide range of experimental parameters covered. The temperature dependence of this reaction, however, has been determined only in experiments using flow systems at low total pressure, with the atomic oxygen decay being f o l ~ o w ~by d the air afterglow
0
-f-
YO +NO2
+ hv
I n both of these studies, care was taken to prevent the introduction of extraneous active species into thr: reaction zone which artight contribute to the decay of atomic oxygen. As is the case in nearly all flow systems, however, the measured 0 atom decay might have been influenced by reactions occurring on the walls of the reactor.
Since
0
+ 0, -2%0 3
is of such importance in stratospheric chemistry and since its rate constant needs to be well eststbIished a t (I) This research carried out at the University of Maryland is part of a thesis t o be submitted to the Faculty of the University of Maryland in partial fulfillment of the requirements for the degree of Doctor of Philosophy. (2) Supported in part by the Clim~dticImpact Assessment Program, Office of the Secretary, Department of Transportation. (3) Acknowledgment is made by thia author to the donors of The 1”etroleum Research Fund, administered by the Anierican Chemical Society, for support of thia research. (4) P. A,. beight,on, “Photochemistry of Air Pollution,” Academic Press, New Pork, N. Y., 1961. (5) M. Nicolet, J . Geophys. Res., 70, 679 (1966). (6) P. J. Crutaen, Quart. J . Roy. Meteorol. AYOC., 96, 320 (1970). (7) P. 3 . Crutaen, J . Geophus. PZes., 76, 7311 (1971). (8) H. Johnston, Science, 173, 517 (1971). (9) M. A. A. Clyne, D. J . McKenney, and B. A. Thrush, T ~ a n s . Faraday Soc., 61, 2701. (1965). (10) M. P.R. Mulcahy and D. J. Williams, ibid., 64, 59 (1968). (11) G. M. Meaburn, D. Perner, J. LeCaXve, and M. Bourene, 1.Phys. Chem., 72, 3920 (1968). (12) F. Raufman and J. R.Kelso, J . C h e m Phys., 44, 4541 (1967). (13) F. Stub1 and H. Niki, ibid., 5 5 , 3943 (1971). (14) It. J. Donovan, D. Husain, and L. J. Mirsch, Trans. Faraday Soc., 66, 2551 (1970). (15) N.Hippler and J. Troe, Ber. Bunsnges. P h ~ s .Chcm., 75, 27 (1O71). (16) T. G. Slanger and G. Black, J . Ch,em. Phys., 53, 3717 (1970). (17) P. D. Francis, Brit. J . A p p l . P h w , 1717 (1969). (18) M . C . Bauer, J . Phys. Chem., 71, 3311 (1967).
The Journal of Physical Chemistry, Vol. 78, No. 19,1972
R. E. RUIE,5. T. HERRON, AND I?. D. DAVIS
2654 temperatures and pressures corresponding to stratospheric conditions, 11e have undertaken a flash photolysis-resonance fluorescence study of this reaction over a wide temperature and pressure range and under conditions in which neither secondary reactions nor wail reactions were important,.
Z 700 C t
2600
Experimental Se ctitilen
0
The apparatus and technique have been discussed in detail previously.2132 In these experiments a mixture of a few Torr oi' molecular oxygen and a diluent gas (helium, argon, or nitrogen) was flash photolyzed above X I050 8, producing on the order of 0.05 to 0.005 mTorr of a,bloo) were used to generate one kinetie decay curve (see Figure 1). The reaction mixture n a s changed several times in the course of an experimeiit to prevent ozone accumulation. The first-order rate constant was determined from a plot of bhe logarithm of the count rate, corrected for background, against channel number, or time. A good linear fit wacj found over at least a full decade change in the count rate. The second-order rste constant at a particular total pressure was Calculated from the slope of a plot of this first-order rate constant against the molecular oxygen pressure as shown in Figure 2. In a few cases, when only one or two data points were taken at a given temperature and presswc, a zero intercept was assumed. Finally, thc third-order rate constant was derived by dividing the second-order rate constant by the total pressure of the diluent gas. With argon and nitrogen as third bodies, the third-order rate constant at 298°K and around 220°K was also calculated from the slope of B plot of the second-order rate constant against total pressure, a8 shotvn in Figure 3 , When the inert gas pressure was In0 Tors or less, the contribution to the measured rate From i h t reaction 0 0 2 0 2 40 8 (I2became f3igKdicallr. In those cases, the rate of the ~eactionnith oxygen as the third body was calculated using the rate expression for argon at higher pressures from thp presmt work, and the relative third body efficiencies given by Kaufman and Relso.12 This value xt as then subtracted from the total measured first-order rate. At morst, this correction amounted to about 1,377, of the total first-order rate constant. The gases used in this study were Matheson Gold Labei'3 helium, argon, and oxygen, and Air Products Ultra-high Purityz3 nitrogen (all better than 99.9% stated purity). All mere used without further purification. I n tlw prr:paration of the gas mixtures, low pressures (1-6 Torr) mere measured on a dibutyl
+ +
The Journal of Phypbal Chemistry, VOL76, IVO.1.9, 1979
+
'c =; 500 c
2400 0
300
200
100 IO 20 30 40 5 0 60 70 80 90
Channel Number (Time base i07'ys/channe Figure 1. Typical O(3P)atom decay curves: flash intensity, 31 J and total pressure, 250 Torr.
0 + O2 + Ar -+ O3 +At : 800 v) I
u
x
600 400
200 1.0 2 . 0 3.0 4 0 5 0 6 0 7 0 80 Poz(torr)
Figure 2 . Plot of the experimentdly measured first-order rate constant as a function of the 0 2 pressure: total pressure, 250 Torr.
phthalate manometer and higher pressures were measured on a two-turn Bourdon gauge. The precision to which various gas mixtures could be made up was determined to be -3%.
Results and Discussion The results of the present study are presented in Tables 1-111. The stated error limits were derived (19) M. C. Sauer and L. M. Dorfman, J. Amer. Chem. SOC.,87, 3801 (1965). (20) €1. S. Johnston, Nut. Stand. Ref. Data Sei., .Vat. Bur. Stand. ( U . S . ) , 20 (1968). (21) D. D. Davis, A. M. Bass, and W. Braun, Inl. J . Chem. Kinet.,
101 (1970). (22) D. D. Davis, R. E. Huie, J. T. Herron, M. 3. Iiurylo, and W. Braun, J . Chem. Phys., 56, 4868 (1972). (23) Certain comniercial instruments and materials arc identified in this paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the Xational Bureau of Standards, nor does it imply that the instruments or materials identified are necessarily the best available for the purpose. 2,
Table IIE : Rate Measurements on the 69 02 Ar .-+Oa Ar
+ +
+
Third-order rate constant
T, O K
221
le I : Rate Measurements on the Reaction 0 Oz He -+ Os -t-I& a t 298°K
+
+-
02,
Be,
Torr,a
Torrb
1.00 1.00 1.25 2.00 2.53 5.00 5.00
251) 250 250 250 250 250 230 250
6.20
F'laah energy,' J
247
Third-order rate constant x 1034 molecule-2 aec-1
aec-1
92 100 39 100 39 92 100 39
225
227
Firstorder rate constant,
113 113 t 14 182 258 428 456 578
265
298
+-
.
i
"
-
~
a).
~
le 19: Kate Measurements on the Reaction 0 +- Qz -t- Na 4 8 s -t 32
T, OK
218
221. 298
02,
Nz,
Torrb
Torr3
1.54 4.12 7.1 1.31 2.38 2.88 1.03 1.42
2.79 4.31 7.1 1.11 1.94 2.49 4.43 a-c
50 50 60 250 250 50 50 50 c50 50 50 25CI 250 250 250
Flash energy,' J
39 45 37 45 4.5 51 48 26 37 28 37 45 48 39 39
Firstorder rate aonstant, sec-1
210 645 1037 667 1390 334 40 59 117 165 261 169 288 326 654
See corresponding footnotes in Table I.
Third-order rate constant
x
Torra
Tor@
J
2.08 3.67 1.00 2.00 2.06 2 . 12 2.20 5.20 6.20 7.66 7.66 8.50 3.80 2.57 3.20 2.60 4.88 7.71
1 Torr = 9.65;T X 1018 molecules em-3. b Pressures greater than 100 Toir are the total pressure M 02, others are A fiash energy of 80 J corresponds to pressures of M aPonc, about 1.5 X photon3/cm2 a t the reaction cell (A < and 6.36 rt: 0.72 X IOv3* cm6 sec-l zib 298°K and about 224°K. A fit, of the third-order rate constants from Table 111 to the Arrhenias equaiim gsve the expression k~~ = (6.57 f 0 . S ) X 1EB-3h e?:p[(1014 =k 46 cal mol-')/RT] cm6 molecule-2 see - I " TBie stated uncertainties are the standard errors of the least-squares fit to the data. From a consideration of the maximum and minimum slopes which could lw fit through the error bars of each point in Figure 4, the indicated uncertainties in A and E were :kIbO cal/rnsl and 9 ~ 2 . 1 5cmb molecule-2 sec-', respectivel:y. h coinparison of the third-order rate constants for ;\I = Hr, Ar, and R'zgave the relative efficiencies of thcee tlirec gases for deactivat>ion as &92: 1.0: I 6 at 298°K At 218"K, the efficiencies of Arto X2tvrreinthcratio 1.&1:1.7. At the IOU er oxygen pressures used in this work, the absorption of the flash lamp radiation by molecular oxygen in the reaei ion zone was low, resulting in only a small initial concentration gradient for the oxygen atoms A t higher oxy-gen pressures, this gradient bec~imelarger, thus possibly leading to spurious kinetic results As illustrated in Figure 2 , however, the first-order rate constants were directly proportional to the 0, pressure over the entire experimental range, dcmonstrating that no effects on the rate measurements resulted from oxygen atom concentration gradients in the reaction zone. Another possible source of error in this study, which has been discussed previously,22concerns the production of electronically excited oxygen atoms in the flash photolysis of inolecular oxygen. I n these experiments, a minimum of a 200-psec delay was employed betweerr the flash photolysis of the reaction mixture and the atom concentration measurements used in determining the rate constants. At the pressures used in this study, the 200-pxec delay nrould ensure that all atoms
*
4
i
!
i
+ +-
+
+ -+
50 tori
0
100 tori 500 lorr
V
+
The Journal of Physical Chemistry, Vol. 76?No. 19,1972
X
0 I 5 0 lorr
?-.L_/ 0
Y
1
2
l
l
3 I / Tl x
4
5
6
1
lo3 (4) l
Figure 4. Arrhenius plot of the third-order rate constxmt for the reaction of atomic oxygen with 0 2 : (- - - -) represents values taken from ref 10 and (----) represents values taken from ref 9.
produced by the flash were in the ground electronic state and at thermal equilibrium. In the experiments employing He or Ar as the inert gas, most of the O(lD) produced by the flash would be quenched by 02, producing 0 2 ( '8,+) .24 The excited molecular oxygen could then react with the ozone produced by the 0 0 2 31 reaction to regenerate atomic oxygen.25 Since the concentrations of both 0 3 and S,('Z,+) would be proportional to the flash intensity, any contribution from this reaction would be strongly dependent upon the flash intensity. To investigate the possibility that this reaction could have influenced the observed 0 atom decay, or that any other secondary reactions could have been important, the Aash intensity was varied over about a factor of 10 at both 298 and 225°K (see Table I11 and Figure 2). The observed invariance of the first-order rate constant with flash intensity demonstrates that the rate of atom loss was not being influenced by secondary reactions involving O,('&+) and OB or any other secondary processes such as 0 0 3 -+2 0 , and 0 0 M-+-0 2 4-h4. Since the third-order rate constant was obtained from the slopes of the first-order rate against the molecular oxygen partial pressure at a fixed total pressure, the results are independent of any impurities in the inert gas. Additionally, lack of significant. dependence of the third-order rate constant on total pressure indicates that no impurity capable of a rapid secondorder reaction with atomic oxygen was present in the molecular oxygen. (As indicated earlier, ultrahigh purity gases were used throughout this study.) Other possible systematic errors associated with the use of the
+
+
+
+ +
(24) R. J. Donovan and D. Husain, Chem. Rev., 70, 489 (1970). (25) T. . ' l Izod and 1%.P. Wayne, Proc. Roy. SOC.,Ser. A , 308, 81 (1968).
k, oms molecule-:!
3.33 3.03 6.5'7 X 2.44 X 4.7 X
x x
Method
scc-1
Flash photolysis-resonance fluorescence
1.0-34 10-34
exp(l.014 cal mol-l/IRT) 11.7 x 10-84 5 . 1 ~x 3 10-94 exp(1.800 c d niol-l/RT) exp(1680 cal mol-l/RT) 23 x 10-3* 9.0 6.33 10.2 8.84 4.42 4 4 6.5 5.6
x
Ref
This work
Discharge flow Thermal decomposition-fiow
9 10
Pulse radiolysis
11
Thermal decomposition-flow
12
Flash photolysis-chemi-
13
10-84
X IOWs4 x 10-34 X '% :Lo-ad x 10-94
x
10-34 X IFa4
:
c 4.6 x 5.0 :x 4.9 x 8 x
10-54
10-54 10-84
luminescence
10-34 10-34
IO+*
Flash photolysis-absorption
14.
Photolysis relative to 0 3-
15
%0.-34 1.0-34 1.0-34
NO2
4.63 X 1.16 X 8.14 X
4 . 4 x 10-a4 7 . 0 x 10-84 1.24 X 10-a4 2.25 x 1 0 - 8 4 2.48 x 1 0 - 3 4 11.5 x 1 0 - 8 4 9.6 x 10-54 1.92 x 1 0 - 3 4 exp(2100 cal mol-l/RT) exp(2100 ea1 mol-l/RT) exp(890 cal mol-l/ET)
resonance fluorescence technique have been discussed more fully elsewhere.** In Table XV, i h t results of the present work are summarized along with rate immurements reported by other workers. Excluded are those discharge flow measuremeiits which were carried out under conditions where the effects uf active impurities and metastable molecular oxygen species could have significantly influenced the final r e s u k 2 6 Most of the earlier kinetic investigations on the system 0 0 2 1M -+ 0, 4- M were rawied out at a single temperature, 298°K. In three ol thesc studies, oxygen atoms were produced via flash photolysis and loss of atomic oxygen was then followed either by 0 K O chemiluminescence (Stuhl and Kiki),13 kinetic absorption spectroscopy (Donovan, Husaai?, and Kirsch) , I 4 or resonance fluorescence (Slanger and lack).16 These results are seen to
+ +
+
Flash photolysis-resonance fluor escen ce Discharge flow Pulse radiolysis Pulse radiolysis
16
IC,, (review)
'do
K,, (review)
28
17 18 19
agree with the present data to within 30%. I n two reported pulse radiolysis studies by S a u e P and Sauer and Dorfrnan,'g the reaction was followed by monitoring the formation of ozone. The rate constants determined at 298°K were about 50% lower than those measured in this work. I n another study,oHippler and Troei5 used the photolysis of SO2 at 3660 A as a means of determining the rate constant for the reaction 0 4O2 Kz -3c 03 E2 relative to the process 0 -iSO2 - -+ KO 02. I n this study the extent of the ozone-producing reaction was determined by measuring the quantum yield for destruction of NO, with and without added 0,. The results from this investigation were about 40% higher than reported here for N, as a third body. I n both of the latter studies, the experimental
+ +
+
(26) F. Kaufman and J. R. Kelso, J . Chem. Phys., 40, 1162 (1964).
The Journal of Physical Chemistry, VoL, 76, N o . 19, 1976
R. E. HUIE,J. T. RERRBN, AND D. D. DAVE
2658 uncertainties associated with the techniques were sufficiently large as to explain the difference between their results and the present measurements. I n an extensive study by Kaufman and ICelsolz a flow system 'vi as employed in which the thermal decomposition of ozone was the atom source and detection of the atoms was by means of chemiluminescence. The agreement befween the results of this study and those reported here for the third body gases He, Ar, and KZis seen to be within 1 0 % ~ As noted in the introduction, the only previous direct data on the temperature dependence of the rate of reaction of atomic oxygen vith molecular oxygen are derived from flow system studies. Clyne, NcKenney, and Thrushg studied the reaction in a plug-flow reactor, using a micromaw discharge in oxygen as an atom source. The decay of the oxygen atoms down the reactor was determined by monitoring the chemiluminescence froin the reaction 0 -tKO 4 NOz hv. At room temperature, the results of these authors are in reasonable agreement; with the present work ( i150j0). At lower teniperatures, however, their data diverge from the present results and are about a factor of 3 higher at 220°K 3IuTcahy and T.Villiams10 also used S O (*hemiluminescenceto follow the atom the 0 concentration, but used the thermal decomposition of ozone as an atow sourcc and carried out the study in a bulb reactor. These authors report a third-order rate constant for Hr 17 hich is a factor of 2 higher than the present results st 298°K and a factor of 3 higher at 220°K. In neitl-er study were sufficient data given t o allow a detailed discussion of thc result;. Although specific reasons for the discrepancies between the above studies of the temperature dependence of the 0 0 2 Peaction and the results of this work cannot be given at present, some pomibilities can be considered. I n both studics, heterogeneous reactions involving the walls of 1,he reaction vessels are possible sources of error; whereas, in the present work, the reaction time is much shorter than the time! for diffusion to the u d l s . Since wall reactions would be expected to be more important at low temperature^,^' this could also contribute the stronger ~ e i ~ p e r ~ t udependence re observed in these flow studies;. Additionally, the results of !\lulcahy and WilliarnslO depend on the assumption of perfect mixing i n their bulb reactor, which if in error could possibly
+
+
+
The Journal of Physical Chemistry, Vol. 76, No. 19, 2972
explain why their results were also higher than those of Clyne, et aL3 In the review of Johnstonz0on the kinetics of neutral oxygen species, the recommended value for the reaction of atomic oxygen with molecular oxygeii was derived from the equilibrium constant and the recornmended rate constant for the reverse reaction. The preexponential term obtajned by this method is a factor of 5 lower than from the present study, and the exponential term is a factor of 2 higher. At 220"11, t h i s results in a rate constant a factor of 2 higher than reported here. This discrepancy, however, is probably not as unreasonable as it first appears. For example, the ozone bond strength used by Johnston was 24.8 k c d and the activation energy used for the reverse reaction was 22.7 kcal mol-I. An error of 5% in either of these values would be sufficient to account for the observed difference in the exponential factors. Benison and Axn.orthy,2Sin an earlier revieiv of ozone decomposition, also derived a value for the recombination rate constant. Their value of the rate constant Qa XI reaction was also based for the 0 0 2 ~!\l-+= on the rate constant for the reverse reaction and the equilibrium constant. The Arrhenius parameters so derived &renot extremely different from those reported here, although at stratospheric tcrnperatures tbc calculated rate constant is a factor of 1.5 lower than that measured in this work. In the present work, the rate constant for the reaction of atomic oxygen with molecular oxygen as a function of temperature has been determined in a static system free of the possible uncertainties associated with wall reactions, Rate constants have been measured at temperatures and pressures corresponding to stratospheric conditions. The reported rate measurements indicate that the rate of production of stratospheric ozone couid be nearly a factor of 2 lower than that estimated from previously reported values of the thirdorder rate constant. This could be of considerable importance in estimating the magnitude of possible ozone destruction mechanisms necessary to explain the observed ozone profiles.
+ +
+
(27) J. T. Herron and R. E. Huie, J.Phys. Chem., 73, 3327 (1869). (28) 8. W. Bcnson and A. E . Axworthy, J . Chem. Phys., 42, 2614 (1965).
