Kinetics of the reaction Cl + O3 → ClO + O2

Volume

37, number

KINETICS

Received 20

CHEMICAL

2

OF THE REACTION

tiCtOl,Cr

PHYSICS

LETTERS

15 Januvy

1976

Cl + OJ -j CIO + O2

1975

over the temperature range 210 to The bimolecular rate constant for the reaction Cl + 03 - Cl0 + O2 is measured 360 K in it dischare flow system using Cl nfom rcsonzncc fluorescence at 134-72 nm to monitor the decay of Cl under pseudo fist order conditions. Results, expressed in the form k, = (2.35 i 0.5) X 1O-11 esp[-(171 + 30)/T], arc compared with other recent studies of this reaction. Stratospheric implications are discussed briefly including the possl%ilitY of a concurrent thee-body mechanism with 02 forming ClOs.

1. Introduction The

importance

chemistry Cicerone ct+o,

of

chlorine

species

in atmospheric

was first emphasized by Stolarski and [ I] who suggested that the reactions )

-+CIOiO~

Cl0 + 0 + Cl + 0,

(1)

,

(3

comprise a catalytic qrcle for removal of odd oxygen in the stratosphere. The possibility of future ozone depletion by this cycle with chlorine atoms released by the photolysis of chlorofluorocarbons was postulated by Molina and Rowland [2] and supported by several model calculations [3-j] I The subject has been extensively reviewed elsewhere [6]. These calculations used the only avaiIable rate constant for reaction (1). that at 298 K by Clync and Watson [7] _ A fuil assessment of this problem requires tempcrature dependent rate constants for this and several * ctner rea; *:WIS. Of particular importance is the tem-

Sciences,

address: Department of‘ Atmospheric University of Michipn, Ann Arbor,

48105,

USA;

.* Present

and Oceanic Michipn

to perature range from 240 to 270 K corresponding altitudes around 35 km, where mxtimum ozone depletion by Cl-species has been predicted to occur [a]. Because of the sensitivity of these calculations to Xr, ~additional work on this reaction is desirable. Several studies using different techniques have recently been completed [&lo]. The work reported here utilizes a discharge flow system with resonance fluorescence detection of chlorine atoms to measure k, in the temperature range 210 to 360 K.

2. Experimental The fast flow reactor used in this study (fig. 1) has been described previously as applied to rate constant measurements for the OH + HCl reaction [ 1 I]. The 110 cm long, 2.54 cm i.d:, Pyrex flow tube is attached by an O-ring seal to a stainless steel section containing illumination and observation ports for atom or radical detection by either resonance fluorescence or absorption. Both sections are coated with phosphoric acid and are baked at 25@‘C to inhibit wall recombination. Electric heating mantles and copper coiis through which cold nitrogen may be circulated provide tern.

VoIumc 37,

number

15 January

CHEXIICAL PHYSICS LETTERS

Z

1976

SILICA TEMPERATURE

CARRIER MANIFOLD

MICROWAVE RESONANCE LAhlP

GA5

/\

INJECTOR VACUUM

U.V.

VACUUM PUNP

SPECTROMETER

Fig. 1. Diagamofapparatus. perature control over the range 200 to 450 K with stability of +2 K along a 50 cm long reaction zone as verified by a movable iron-constanlan thermocouple probe. Additional pumping speed, up to linear velocities of about 3000 cm s-l, has been obtained with a Roots blower (Heraus Model RG350) backed by a forepump (Stokes 148 Microvac). Chlorine atoms are formed upstream in a microwave discharge of helium with a trace amount of Cl, (2 0.00 1%) added through a Monel fine metering valve (Nupro Type S) from a one liter bulb containing 10% Cl? (Matheson High Purity) in helium (= 500 torr). Impurity atom concentrations, mainly H and 0, are minimized by passing the helium carrier gas (Matheson High Purity) through a silica gel trap at 77 K and then dividing the gas flow SO that the major portion bypasses the discharge in a method similar to that described by Clyne and Cruse [12]. Resulting H and 0 concentrations are estimated by resonance fluorescence to be < 10” cm-‘_ As described in greater detail previously [ 131, ozone generated in a Welsbach ozonizer (Model T408) and adsorbed on silica gel at -78°C is eluted with a measured flow of helium and passes through an optic~I cell to determine its concentmtion by absorption at 253.7r.m at 2 measured t&al pressure (-700 torr) of helium. It enters the flow tube through an O-ring sealed, 3 mm o.d., sliding pyrex.injector tube whose axial position may be varied within the reaction zone. Absorption measurements in the flow tube at 253.7 nm and with ozone concentrations higher than used in these experiments but und& similar flow

conditions, have verified that ozone decomposition is negligible during passage throu$ the stainless steel! teflon, and Pyrex tubing between the first absorption cell and the flow tube. Chlorine atoms are detected at the downstream end of the reaction zone by observing fluorescence from the 3p44s 7P3,2 * 3~’ ‘P3,* resonance transition at 134.72 nm excited by a microwave discharge lamp (20 W forward power) of a trace of Cl, in helium at 0.5-1.0 torr. ‘The optical arrangement consists of an evacuated baffle region on both lamp and detection sides of the stainless steel cell to reduce scattered light, magnesium fluoride windows and lens, a 0.3 meter vacuum monochromator (IMcPherson Model 218) with a 2400 line per mm grating blazed at 150.0 run: and photomultiplier (E.M.R. 541-G-O& 18). The latter was operated in the analog mode with output to a picoammecer (Keithley 610BRj and strip chart recorder. In similar work with chlorine fluorescence Clyne and Cruse [ 121 and Bemand and Clyne [ 141 preferred to use the weaker 3~~4s ‘P,,, + 3p5 2P,,Z transition at 137.95 nm for both its freedom from line reversal at higher Cl concentrations and its greater sensitivity under. their conditions of lamp intensity and kc reversal. Under our experimental conditions, however, fluorescence from the 134.72 nm transition is up to five times stronger than that at 137.95 nm and has a lower scattered background intensity than the latter. The difference may be attributed to a less reversed source in this work as indicated in table 1 by cornparing lina intensity ratios for transitions from a -. 227

Volume Table

37, number

CHEMICAL

2

PHYSICS

1976

1

Comparison

of chlorine lamp characteristics

Intensity ratio

He + tract CIz, pxsent tvork, decreasing CL addition

~137.95~~139.65

6.7

6.7

7.3 a)

7.3

0.46

0.97

1.4a)

2.8

I

1.5 January

LETTERS

L34.72JI136.35

He+0.1%6!~ ref. [14], fig. la

Ratio of Einstein A-coefficients

7

4.3

4

0.19

7.3 [ 151 10 [14] 5.6 [IS] 7.1 [IS]

a) Approximate

lamp characteristics

used in fluorescence csperiments.

common upper state to both the 3p5 3P,,2 ground state and the much less populated 3p5 2P,,2 excited state (OE = 881 cm-‘). Also shown in table 1 are the theoretical values from the ratios of EinsteinAcoefficients for the corresponding transitions as tabulated by Wiese et al. [15] and experimental values of Bemnnd end Clyne [ !4] from fluorescence intensity ratios under unreversed conditions. The 137.95 to 139.65 nm line ratio for transitions from the 4P312 upper level is near its unreversed value of 7.3 for the varying amounts of Cl2 added to the helium in the lamp discharge, yet appreciably lower (4.3) in fig. la of ref. 1131, indicating some reversal even for the relatively weak 137.95 nm line. For the more strongly allowed transitions from the common upper ?P 3/z state to the same two lower states, the ratio, 113‘:.77/ I,,, 35 is much more sensitive to the amount of Cl; added. Compared with ;I theoretical 5.6 [IS] (7.1 according to ref. [ 141) we observe from about 0.4 to 4 for decreasing Cl? additions, where the highest 5:alues are reached at the expense of greatly reduced total lamp intensity. A mtio of about 1.3 here represents our normal operating condition where the fluorescent sipal at 134.72 nm is maximized relative to scattered backsounti signal al that same wavelength.

from an rf discharge static lamp [ 16) with heated calcium hypochloite as a chlorine source. For absorption nmeasuremenls this lamp is operated nearly unreversed With the ratio f134.72/1136.35 > 4.0. Optically thin conditions in the flow tube are confirmed by observing the fluorescence intensity ratio for these same lines. The limiting value of I ?$ ,/ I f13rj3 = 5.0 + 0.5 for [Cl] < 2 X 1O1l cm ~-is in good agreement with that expected from the ratio of A-values tibulated by Wiese etal. [115],-4,~,,,/A,~,,, = 5.6, but considerably lower than that observed by Bemand and Clyne [14] oflf134.7dff136.35 =;7.1 + 0.6. However, we have not attempted to correct our values for possible transmission or quantum efficiency differences between the two wavelengths in our detection system. The Bemand and Clyne value includes a correction of about 10%. Initial experiments were done with a spectral band width of 0.3 nm. Most of the data, however, were taken with larger slits giving a bandpass of 7 nm full width at half maximum, centered at 134.7 nm and in&ding some contributions (x 20%) from other lines of the 2P,,-‘PJoo manifoid. Higher resolution scans through this region indicated the absence of fluorescence other than from chlorine lines. Wit! the

Bemand

widz;

slits

cm

at

i

and Clyne’s

[Id:]

corresponding

134.52.

to

36.35 lamp ratio is 0.19, very much lower, indicating strong reversal, as would be expected for their composition ofO.l% Cl, in He, where [Cl] is probably greater than lOi cme3. To avoid the problem of line reversal of the fluorescent emission when using the stronger transltlon, initial chlorine atom co;lcentrations in the flow tube are <2 X 10” cm_37 corresponding to optical depths of< 0.3. The absolute concentration is determined by reSonance absorption using the 134.73 nm line 228

the

detection

limit

was

[Cl]

z 2 X

10

a signal to noise ratio of unity.

3. Results and discussior! Reaction (1) was studied at torr by dbs-eruing the decrease injector position was varied to time. A first order rate constant was determined from a plot of

pressures from 2 to 5 in Cl signal as the ozone increase the reaction for each experiment In [Cl] versus injector

Volume 37, number

position,

2

CHEhlICAL

PHYSICS LETTERS

x:

~l=(dln[Cl]/a-)F, where V is the average flow velocity in the reaction zone. Values for kr ranged from 90-400 s-l. The valid. ity of the plug flow approximation for similar experimental conditions has been discussed previously [17] _ Concentrations in the ranges 2 X IO9 < [Cl] < 2 X loll crnm3 and 8 X 101’ < [O,] <‘4 X lOI crnm3 assured pseudo first order conditions. Linearity of the decay plots over one to one and a half orders of magnitude for [Cl] confirmed the absence of interfering side reactions, particularly of reaction (2) by product Cl0 with oxygen atom impurity from the discharged

carrier

gas. Without

t!le purification

steps

described above, 0 atom concentrations of = 10” cm -3 were observed, causing curvature in the first order decay plot as Cl, regenerated by reaction (I), approached a steady state concentration. The regeneration of Cl when known amounts of either 0 or NO were purposely added to the reaction zone indicates that Cl0 is indeed the product of reaction (1) and that both reaction (2) and the reaction ClO+NO+Cl+NO,

(3)

are fast, as had been reported previously [ 18,7]. Quantitative measurements of reactions (2) and (3) using this steady state method are now in progress. The bimolecular rate constant, k,, was obtained for each experiment by dividing X-’by the measured , ozone concentration. At any given temperature the average value of kl obtained in this manner agreed to within 10% with the value obtained from the unitweighted least squares slope of a plot of k’ versus [03] (fig. 3). An intercept in such a plot may be attributed to a change in first order wall removal in the presence of added reactant as noted by Margitan et al. [ 191 in a previous study. In the present work, howefir, since the small intercept was not reproducible between sets of experiments under similar conditions and was usually less than one standard deviation in magnitude, it is uncertain whether a heterogeneous contribution or experimental imprecision was responsible. Reported values for X-l are calculated from the individual experiments as k’/ [O,].The values calculated from the slopes of k’ versus [03] plots, usually somewhat lower, are within the stated errors.

Fig. 2. Plot of kI = (dln[Cl]/dr)Fversus

03 concentration

at room temperature with unit weighted least squares fit. The 28 room temperature (296 K) experiments give an average value of (1.30 + 0.13) X 10-l’ cm3 S --! . The total of 65 experiments, represented on an Arrhenius plot in fig. 3, are fitted ty a unit-weighted least squares method to the expression kl = (2.35 f 3.20)X

IO-‘t

exp[-(171

2 U)/;“i

,

T/K 350 I

L

300 ,

k = 2.35

0.5

II

250 I

200 I

1

lo-” en’3 (=+) cm’ set-’

I

I

I

3

4

5

T-'/IO-3

I

K-’

Fig. 3. Arrhenius pint. The number of runs at each temperatux are given in parentheses. Error bars represent one standard deviation. Other values shown are (a) Clyne and 1Yatson [7], cb) Davis and Watson [8], Cc)Nip and Clyne [ IO], and (d) Kurylo and Braun [ 91.

229

Volume

37,

number 2

CHEMICAL

In both c&s *he stated error, one standard deviation, indicates the precision of t:le method. Including estimates of systematic errors in pressure, flow rate temperature and ozone concentration measurements, the overal accuracy of the rate constant is 120%. The vahz ;tir kl from this study is considerably lower than the previous value of (I 8.5 f 0.36) X lo- 1 l cm3 s-l .at 298 .K by Clyne and Watson 171. They also used a discharge-flow technique, but monitored.the decay of ozone by mass spectrometryin the presence of excess Cl atoms as measured by a somewhat indirect methcd. Other recent studies are in good agreement with the lower value for k, at 298 K but oniy in fair agreement on the temperature dependence. The values of Davis and Watson [S], (3.6 2 0.4) X 10-l’ exp(-3 18/Y-), and Kurylo and Braun [9], 2.94 X IO-” exp(-298/T), both by flash photolysis-resonance fluorescence methods, and that byNipandClyne [lo] of(5.18*0.5)X 10-llexp (-418/Z) by discharge flow-C! resonance absorption indicate values for k, at 220 K 20 to 30% lower than this study. It now appears that the value for k, at stratospheric temperatures is 40 to 50% lower than that used in early calculations which mc!deled ozone destruction by CIO, i3-61, but newly measured values of several of the other important elementary rate constants need to be introduced into the model calculations for an improved estimate of the overall stratospheric effect. The rate parameters for the Cl + 0, reaction may be compared with those of a few other, spin-allowed atom- or radical-ozone reactions. Little can be said about the Arrhenius activation energies except that they are larger for O-atom transfer to another 0 as in 0 t 0, (= 4.4 kcal mole-‘) [20], HO, + O3 (== 3 kcal mole-‘) [20], or HO •t 03 (” 1.9 kcal mole-‘) [13] than toIIasinH+03,whereanew temperature dependence study [21] reports 1 .l kcal mole-‘, and for Cl + 03, where values range from our 0.34 to about 0.83 kcal mole-‘. Pre-exponential factors show the expected decrease from 1.2 X lo-” cm3 s-l for H+ 0, to 1 to 5 X 10-l’ for 0 or Cl + 03, and I to 4 X IO-” for OH, NO, or HO, f OS_ The Cl + 0, r,:action is thus seen to’ ” fall well within the range of kinetic behavior for similar elementary steps. ‘,_ Finally, the possibility of a concurrent three-body 230 :

.

PHYSICS

LETTERS

15 January

1976

reaction, Cl + 0, + M + ClO, f M, needs to be considered. This was recently suggested by Simonaitis and Heicklen [22] as a possible precursor step to stratospheric lK!IO, formation and rain-out 3s an additional removal step for stratospheric ClO, species These authors suggest the possibility of a rate constan ash&has3 to6X 1O-3o cm6sp1 forM=02 even though the reaction (i) is only weakly exothcrmic; (ii) is sterically improbable, since it requires insertion of Cl into 03 ; and (iii) competes with the rapid twobody process examined in this work whose transition state must involve Cl-attack on either end of the 0, molecule and which is therefore unlikely to be a long-lived complex since reaction (1) is 39 kcal exothermic. The formation, by an alternate path, of a Clog complex which is sufficiently strongly bound and which will survive colllsional stabilization must therefore be considered highly improbable. In the present work, if He were an equally efficient third body, the effective two-body rate constant for Cl + 0, f M at 5 torr pressure would be about 1 X 10-‘2‘ cm3 s-l or 7% of the measured rate constant. No pressure dependence was ever seen, but such a would admittedly have been so small as dependence to be within experimental -iror in our study. The alternate, more plausible supposition that ClOa is formed by way of Cl + 0, Cl00

+ M + Cl00

+ 0,

+ M )

+ OClOO + 0,

(4) ,

(5)

leads to an effective three-body rate constant, k’” = Kak,. Watson 1231 has estimated Kd to be about 9.3 X lo-” cm3 at 245 K which would require kj to be in the range 3 to 6 X IGel cm’ s-l, or ex-

cessively large for a reaction of ozone with a triatomic molecule at such a iow temperature. It is interesting to note that the reaction

(6) which is very similar to (5) is extremely slow, its upper limit having recently been reduced from 5 X lo-l5 cm3 s-l [24] to about lo-‘” cm3 s-l [25] i The proposed three-body Cl + 0, recombination rate ‘constant must therefore be judged implausibly ,bigh and the reaction’s stratospheric importerice,

marginal even at that high rate, may be considered extremely unlikely.

Volume

37, number

2

CHEMICAL WYSICS

Note added in proof

[I21 M.A. Clyne and H.W. Crux,

Due to a recently discovered, consistent Galibration

error,

all ozone

concentrations

were

8% larger

than reported and k, should therefore be reduced by 7.570at all temperatures, i:e. k, = (2.17 ir 0.5) X lo-l1

exp[-(l71

5 30)/T]

cm3 s-l.

References

J. Chem.

Trans. II 66 (1972) 1281. 1131 J.G. Andersor! gd F. Kaufman,

Sot.

1976

Faraday

Cticm. Phys.

Lcttcrs

16 (1972) 375. 1141 P.P. Bemand ;Ind A!.A.A. Clync, J. Chem. Sot. Faraday Traps_ II 71 (1975) 1132. [15l 1V.L. Wese. R1.W. Smith and B.hl. Milts, Atomic Transition Probabilities. Vol. 2, NSRDS-NBS 72 (!_JS Govt. Printing Office, Washington, 1969). f161J.G. Anderson, Geophys. Rcs. Letters 2 (1975) 231. and F. Lufmnn. J. Chcm. [171 J.G. Anderson, J.J. krgitan [la]

[ 1; R.S. Stolarski and R.J. Cicerone. Can. J. Chem. 52 (1974) 1610. PI h1.J. hlolinn and F.S. Rowland, Nature 249 (1974) 810. R.S. Stolarski and S. Walters, ScienLr 131 R.J. Cicerone, 185 (1974) 1165. Geophys. Res. Letiers 1 (1974) 20.5. [41 P.J. Crutzen, 187 [51 S.C. Wofsy. MB. McElroy and N.D. Sze, Science (1975) 535. 161F.S. Rowland and M.J. hlolina, Rev. Geophys. Space Phys. 13 (1975) 1. [71 X1.A.A. Clyne and R.T. Watson, J. Chem. Sot. Faraday Trans. I70 (1974) X50. 181D. Davis and R.T. Watson, to be published. PI 5l.J. Kurylo and W. Braun, Chcm. Phys. Letters 37 (1976) 232. 1101 W. Nip and &!.A.& Clyne, to bc published. 1111 MS. Zahniser, F. Kaufman and J.C. Anderson, Chem. Phys. Letters 27 (1974) 507.

15 January

LETTERS

[I91 [2Oi

Phys. 60 (1974) 3310. P.P. Bemand, M.A.A. Clyne and R.T. Watson,

J. Chcm. Sot. Faraday Trans. I 69 (1973) 1356. J.J. Margitan, F. Kaufman and J.G. Anderson. Intern. I (1975) 281. J. Chem. Kinetics Symposium R.F. Hampson Jr. and D. Garvin, Chemical Kinetic and Photochemical Data for Modelling Atmospheric

Chemistry, N.B.S. Technical Note 866 (US Govt. Printing Office, Washington, 1975). 1211 h1.A.A. Clyne, to be published. and J. Hcicklcn, to be ptiblished. 12.21 R. Simona::is Kinetics Data Survey VIII. 1x31 R.T. Watson, Chemical Rate Conshnts of Cl0 al‘ Atmospheric Interest, NBSIR 74-516 (US Govt. Printing Office, Washiqton, 1975). and R.T. Walson, J. 1241 h1.A.A. Clyne, D.J. McKenney Chem. Sot. Faraday Trans. I (1975) 327. I-251C.L. lin, S. Jaffc and W.B. DcMore, paper prcscntcd 169th National hlccting, Americ;rn Chcmicnl Society (1975).