J . Phys. Chem. 1986, 90, 159-165 i
H
s
i
100 +%A& .{.,I,>?i
v) v)
2 95.0
.
..:.....4 . ..i?. ...’I... .:. . .d.:.. .3: .#- . . .. .. . . . . . . . * . . . .. . ... . . .. .. ..... . ,.. ... ... .. . .‘.. ... .... 1 :: . . . .. : , j
*
~
.
I
9 9
Ul
*
90.0 p
* :
t
..
*
e. e.
**
3
I
1
85.0 i.,-.
L4
-10.0
-5.00
1_.L_*
- L -
...A
c.Al.c.-e---L.-~*
0.00
5.00
10.0
MM/S
Figure 6. Full Mossbauer spectrum at room temperature of the residue of the hydrogenation of DPE in the presence of 10% H2S.
of pyrite.4 Actually, they are almost identical with the pyrite dimorph marcasite.I8 We identify this doublet as Fa2. However, the parameters are not those of the cubic-phase pyrite. We believe we have obtained an iron disulfide, but the crystallographic structure is not that of pyrite and is closer to that of orthorhombic FeS2. X-ray diffraction was not sufficiently sensitive to detect this phase. It is reemphasized that this phase is absent when the partial pressure of HIS is reduced. The full Mossbauer spectrum of the residues of this reaction is shown in Figure 6. The conversion of DPE to benzene and phenol is not reduced by the presence of FeS2in the reactor. This is in contrast to our earlier observation in the study of hydrocracking of diphenylmethane.” This is perhaps not so surprising since all iron sulfides show great reactivity toward oxygen.I5 The present results suggest that the presence of a nonstoichiometric iron sulfide surface is necessary in order to obtain a higher conversion of DPE. The presence of H2S in the reactor is necessary in order to maintain such a nonstoichiometric surface. It is noted that in the absence of pyrrhotite and in the presence of H2Sthe conversion of DPE is dramatically reduced (less than (17) S. Kawasumi, M. Egashira, and H. J. Katsuki, J. Catal., 68, 237 (1981). (18) P. A. Montan0 in ‘Mossbauer Spectroscopy and Its Chemical Applications, J. G. Stevens and G. K. Shenoy, Eds., American Chemical Society, Washington, DC, 1981, Adv. Chem. Ser. No. 194, Chapter 7.
159
20%). The fact that some conversion occurs in the presence of H2S and in the absence of the catalyst is related to the presence of iron sulfides formed on the stainless steel reactor walls, a fact often ignored in the literature. It is interesting to compare the I S observed in the present experiments with those obtained in the in situ Mossbauer measurements in The IS is significantly more negative than for runs carried out with pure FeS2 or pyrrhotite (no other compound present). Similar results are observed here; however, in the present study we were able to identify two phases-one with an IS around 0.21 mm/s at 440 OC and the other with parameters close to those of FeS2. The last phase appears only when the hydrogen sulfide partial pressure in the reactor is high. It is clear that a great degree of interaction is taking place on the iron sulfide surfaces. In a model compound study such as the one reported here, it is more feasible to identify some of the species present. In a complex material such as coal, it is very difficult to identify the roles of vastly differing organic functional groups and their interaction with pyrrhotite or HzS. Still, in general, we can propose that maintaining a nonstoichiometric surface enhances the reactivity of the iron sulfides. There is evidence of a synergistic effect of the pyrrhotite and H2S in the catalytic hydrogenation of diphenyl ether, very similar to the results obtained for diphenylmethane. In summary, pyrrhotites are shown to play a catalytic role in the hydrogenation of diphenyl ether. The present results are indicative of a probable catalytic role of iron sulfides in the cleavage of oxygen bonds in coal. The only real problem presently encountered with the use of these catalysts is their low surface area. One can try to support the catalyst on standard oxide materials (alumina, silica); however, care must be taken not to oxidize the catalysts during preparation. The second problem is more fundamental. The pyrrhotites have a rather complicated crystallographic structure,18 consisting of a large unit cell with many molecules. Consequently, stabilizing such a structure on a support, as well as being a high dispersion, is an extremely difficult task. However, we hope that new ideas for special supports which retain the activity of the sulfides will make this compound more attractive in industrial applications, specifically in direct coal liquefaction. Acknowledgment. We thank the U S . Department of Energy for their support. Registry No. DPE,101-84-8;Fe7S8,12063-67-1; H2S,7783-06-4; iron sulfide, 11 126-12-8;pyrrhotite, 1310-50-5.
Rate Constant and Branching Ratio for the Reaction of OH with CIO G. Poulet,* G. Laverdet, and G. Le Bras Centre de Recherches sur la Chimie de la Combustion et des Hautes TempCratures, 45045 Orleans Cedex. France (Received: May 8, 1985) The rate constant and branching ratio for the reaction OH + CIO products [HOz + C1 (la) and HCI + O2(1b)] have been measured in a discharge flow system at 298 K. OH was monitored via the laser-induced fluorescence technique. CIO and HCI were analyzed by means of a modulated molecular beam mass spectrometer. The results are kl = (1.94 f 0.38) X 10-” cm3molecule-I s-l and kl,/kl = 0.98 0.12. This implies that channel l b cannot be a significant process for converting C10 into inactive HCI in stratospheric ozone-chlorine chemistry. +
Introduction The reaction between OH and C10 radicals can play an important role in stratospheric chlorine chemistry since one of its two possible channels converts the active species C10 into the reservoir molecule HCI. Such a sink would reduce the catalytic effect of the C10, chain carriers in the ozone destruction cycles. 0022-3654/86/2090-0159$01.50/0
The two possible branches of this reaction are O H + C10 H02 + C1 AHozs8 N -1.7 kcal mol-’ O H + C10 HC1+ 0, AH0298 N -55.6 kcal mol-] -+
-
0 1986 American Chemical Society
160 The Journal of Physical Chemistry, Vol. 90, No. 1, 1986 O U A D . MASS FILTER
0 PUMP
Poulet et al. chemical system used could be simultaneously detected by a molecular beam quadrupole mass spectrometer (V.G. Micromass Q 50). The sampling orifice was located 5.5-cm downstream from the center of the LIF cell. The modulation of the molecular beam allowed for a synchronous detection of the ion signal. A low energy of the electrons (-20 eV) in the ionization source was generally used for the detection of C10 radicals or the other species of The detection limit (S/N interest (Cl, HCl, 03,C102, NOz, = 2) for HC1, observed during the branching ratio experiments. was approximately 5 x 1010 molecules ~ m - ~ . The main reactor was a quartz tube which was 80 cm long and 2.4 cm i.d. O H radicals were prepared via the reaction H + NO2 O H NO at a fixed position in a sidearm tube numbered 1 in the figure. A trace of molecular hydrogen in helium was dissociated by a microwave discharge (2450 MHz) into H atoms which reacted with excess NOz as checked by mass spectrometry. C10 radicals were used in excess to ensure pseudo-first-order kinetics in OH. They were prepared by two methods in the upstream part (1.2 cm 0.d.) of the central injector (number 2). Chlorine atoms, produced by a microwave dissociation of Cl,, reacted at 10 cm downstream from the cavity with 0, or C10, to generate CIO radicals: .e.).
-
Figure 1. Schematic diagram of the apparatus
Uncertainties in the values of both the rate constant k , ( k , = kla klb)and the branching ratio a ( a = kla/kl) could significantly influence the model calculations for ozone depletion. Four kinetic studies of the O H + C10 reaction have been reported.I4 All of these studies used the discharge flow technique which is the most convenient method for the kinetic investigation of such a radical-radical reaction. The first work of Leu and Lin' cm3 molecule-l s-I yielded for k , a value of (9.1 f 1.3) X at 298 K and a lower limit of 0.65 for a. Ravishankara et al.2 found a temperature-independent value of (1.17 f 0.31) X lo-" cm3 molecule-l s-I for k , and did not make measurement of a. Since the present work was initiated, two other studies have been recently p u b l i ~ h e d . ~Hills . ~ and Howard3 measured kl between 219 and 373 K: k , = (8.0 f 1.4) X exp[(235 f 46)/Tl cm3 molecule-l S-I. At 298 K, they reported the values (1.75 f 0.31) X lo-" cm3 molecule-'^-^ for k , and (0.86 f 0.14) for a. Finally, Burrows et aL4 found that both k , and a were temperature independent over the range studied (243-298 K) with average values of k l = (1.19 f 0.09) X lo-" cm3 molecule-l s-l and a = (0.85 f 0.07). Although the branching ratios obtained in these two last studies are in excellent agreement, they were both obtained from the measurement of H 0 2 formed in channel la either by laser magnetic resonance detection3 or by indirect detection after chemical conversion to OH in a resonance fluorescence systems4 But, as stated by Hills and H ~ w a r dan , ~ accurate determination of the effect of reaction 1 on stratospheric O3requires further study with emphasis on the detection of HC1 produced in channel 1b. Concerning the magnitude of k l , it appears that there is a good agreement between three of the previous ~ t u d i e s , ] ,the ~ , ~other determination3 being somewhat higher and showing a slightly negative temperature dependence. In the present work we report the measurement of k l with various conditions of production and absolute titration of C10 radicals. We also measured the branching ratio of reaction 1 from the direct detection of HC1.
+
Experimental Section The apparatus and the experimental principle used for this work (Figure 1) have been described in detail p r e v i o ~ s l y . ~ -Briefly, ~ a discharge flow system was used with laser-induced fluorescence (LIF) detection of OH. The fluorescence was excited from a frequency-doubled dye laser (Lambda Physik FL 2000) pumped by means of a XeCl laser (Lambda Physik EMG 101). The signal was collected through an interference filter centered at 309 nm with IO-nm fwhm and detected by use of a photomultiplier (Hamamatsu R 922) whose output was connected to a boxcar integrator (PAR 162 + 164). The other reactive species of the (1) M. T. Leu and C. L. Lin, Geophys. Res. Let?., 6, 425 (1979). (2) A. R. Ravishankara, F. L. Eisele, and P. H. Wine, J . Chem. Phys. 78, 1140 (1983). ( 3 ) A. J. Hills and C. J. Howard, J . Chem. Phys., 81, 4458 (1984). (4) J. P. Burrows, T. J. Wallington, and R. P. Wayne, J . Chem. SOC., Faraday Trans. 2, 80, 957 (1984). (5) G. Poulet, G. Le Bras, and .I.Combourieu, J . Chem. Phys., 69, 767 (1978). ( 6 ) G. Poulet, G. Laverdet, and G. Le Bras, Chem. Phys. Lett., 94, 129 (1983). (7) G. Poulet, G. Laverdet, and G. Le Bras, J . Chem. Phys., 80, 1922 (1984).
+
C1
+ O3
-
k6 = 1.2 X Cl
C10
+ O2
(6)
cm3 molecule-' s-l
-
+ ClO*
ClO
+ c10
(ref 8) (15)
kl5 = 5.9 X lo-" cm3 molecule-' s-'
(ref 8)
Once formed, C10 radicals were allowed to flow to the main reactor through the sliding central injector (6.0 mm i.d.). Flow rates of He( 1) and He(2), used in the production of O H and C10, respectively, were adjusted in order to optimize C10 production and to reduce flow perturbation at the injection point 2. Typical 4 with a total flow of about flow conditions were He(l)/He(2) 30 L/h (STP). At 298 K, in the pressure range 0.5-0.9 torr, the flow velocity in the main reactor ranged from 17 to 31 m s-I. Total pressure was measured with a Baratron through port 2 at the middle of the observation zone used for the kinetic experiments. During this pressure measurement, all gases flowing through the central injector were deviated through lateral port 4. In the upstream region of the C10 source inlet, a higher pressure could be measured (up to 5 torr) giving a mean flow velocity in this injector 2 or 3 times lower than in the main reactor. Under these conditions a loss of C10 concentration was observed inside this injector. This observation resulted from the absolute titration of C10 in the main flow tube via the NO C10 reaction, excess NO being introduced through port 3:
-
+
NO
+ C10
4
NO2
+ C1
(5)
k5 = 1.7 x lo-" cm3 molecule-' s-'
(ref 8)
The concentration of C10 entering the main reactor at point 2 ([C1OIz)was deduced from the mass spectrometric calibration of NOz formed in reaction 5 and could be compared to the initial concentration of the C10 precursors O3 or C102. This titration was done with an excess of C1 over O3or C102 in order to avoid complication from the reaction of C1 formed in reaction 5 with O3or CIOz. Furthermore, side reactions of NO with 0, or CIOz could not occur: NO O3 NO2 O2 (1 1)
+
k l l = 1.8 NO
X
-
+ CIOz
k I 6 = 3.4 x
+
cm3 molecule-'
s-I
NO2 + CIO
cm3 molecule-'
(ref 8) (16)
SKI
(ref 9)
The values obtained in preliminary experiments for [Cl0J2were always found lower than the values [CIO]o = [OJOor 2[C102]o predicted from the stoichiometry of reaction 6 or reaction 15, respectively. For the highest [CIO]z used in this work (-3 X l O I 3 molecules ~ m - ~a) C10 , loss as high as 50% was observed
OH
+ C10 Reaction
The Journal of Physical Chemistry, Vol. 90, No. 1, 1986 161
inside the C10 injector. Relatively high pressure and flow velocity gradients inside the C10 source did not allow us to accurately establish whether this C10 loss process was only due to the second-order homogeneous recombination: C10 C10 products (12)
+
-
for which k12= 1.5 X cm3 molecule-' s-I is the generally accepted value for the rate constant in the range of pressure used However, the concentration of C12 formed in the C10 loss process ([C12]rec)was measured from a calibration of mass 70 and was compared to the concentration of recombined C10 ([ClO],,,) measured from the titration experiment. Within experimental uncertainties, it was found that [ClO],, = 2[C12],,, which would agree with a stoichiometric production of C12 in the bimolecular recombination reaction 12. The C10 recombination in the main reactor was also measured when the NO2 signal was again observed as a function of the position of the movable inlet 2 during the titration experiments. A negligible C10 loss was observed in this part of the apparatus, which is consistent with the value of kI2and the low heterogeneous C10 recombination generally reported. The rate constant for this wall loss in the main flow tube was found to be lower than 2 s-l in every conditions of the present work. But the magnitude of the ClO loss in the central injector, as observed in the preliminary experiments described above, made it necessary for us to perform an absolute titration of C10 during the course of each kinetic investigation of the reaction OH + C10. All the surfaces of the reactor were treated specially to reduce the OH heterogeneous loss. The discharge tube 1 was only cleaned with a 10% HF solution and rinsed with distilled water. The main flow tube and the outer surface of the central injector were coated with Halocarbon wax. The rate constant for the OH loss on the surface of the main reactor was measured by introducing NO2 through the sliding injector 2 instead of its normal introduction downstream from the microwave cavity 1. Then, the decay of O H concentration, observed from the LIF signal level, was plotted as a function of injector position. A mean value of 10 s-' was found for kI4: O H wall products (14)
+
+
-
+
Results As in the other kinetic investigations of reaction 1, kl was measured under pseudo-first-order conditions [ClO] >> [OH]. The previous determinations of kl were done either with or without an excess of C1 concentration over the concentration of C10 precursor. In the present study, both conditions of excess initial concentration of these reactants were used, specially in order to account for the partial regeneration of OH radicals due to secondary chemistry. Indeed, with excess C1 a correction for this O H regeneration must be applied to the rate coefficient measurement, since C1 reacts with H02 formed in channel l a via two ways as recently observed:13 C1+ H02 HC1+ 0 2 (2a)
-
-
k2a = 3.2 X lo-'' cm3 molecule-' C1
+ HO2
k2b = 9.1
X
C10
s-I
+ OH
cm3 molecule-' s-'
(ref 8) (2b) (ref 8)
OH produced in channel 2b could lead to an underestimation in the measurement of k l . Therefore, a correction must be done to calculate the actual value for kl in the experiments where C1 was used in excess. This correction could be obtained from the analytical solution of the kinetic system as suggested by Ravishankara et aL2 Provided [Cl] is greater than [ClO], a simplified expression of the correction factor (cp) is obtained: kl(corrected) = cpkl(measured) with
-
The inner surface of the injector was also coated with Halocarbon wax in order to reduce the loss of C1 atoms produced in discharge tube 2. The discharge zone of this tube was cleaned with a 10% H F solution, rinsed with distilled water, and treated with a 30% H3P04solution to promote C12 dissociation. The absolute concentration of C1 atoms ([Cl],), which was needed for the computer simulations of the experiments made with excess C1, was measured by mass spectrometry in two ways: [Cl], was deduced either from the discharge on/off method at m / e 70 or from the fast titration reaction: C1 C2H3Br C2H3Cl Br (17)
+
gel trap at 196 K. After excess oxygen had been pumped off, O3 was stored in a 10-L bulb. The ozone concentration was obtained from the measurement of the pressure drop and from the absolute titration in the reactor by converting excess NO to NO2 via the reaction O3 NO NO, O2 (1 1)
-
+
kI7 = 1.43 X lo-', cm3 molecule-'
s-I
(ref 12)
The purity of gases and the preparation of reactants were as follows: He, >99.9995% (helium was passed through liquid N2 trap before entering tubes 1 and 2); H2, >99.9995%, C12, >99.9%; HCl, >99.99%; C2H,Br, >99.5%; NO2, >99.5%; NO, >99.0% (the NO2contained was measured during the titration experiments of C10 or 0 3 ) .C102 was prepared under vacuum from the addition of excess H2SO4 to KC103 and stored in the dark at 273 K. The purity and the lack of decomposition were periodically checked from mass spectrometric analysis. O3was prepared by using a commercial ozonizer (Trailigaz) and collected in a silica (8) W. B. Demore, M. J. Molina, R. T. Watson, D. M. Golden, R. F. Hampson, M. J. Kurylo, C. J. Howard, and A. R. Ravishankara, J.P.L. Publication No.83-62,Pasadena, CA, 1983. (9) P. P. Bemand, M. A. A. Clyne, and R. T. Watson, J . Chem. SOC., Faraday Trans. 1, 69, 1356 (1973). (10)R. A. Cox,R. G. Derwent, A. E. J. Eggleton, and H. J. Reid, J . Chem. Sot., Faraday Trans. 1,175, 1648 (1979). (11) N. Basco and J. E. Hunt, Int. J . Chem. Kinet.,11, 649 (1979). (12) J. Y.Park, I. R. Slagle, and D. Gutman, J . Phys. Chem., 87, 1812 (1983).
At 298 K, for klb/kl ranging from 15% to 0%, the values of cp range from 23% to 28%, which shows that the O H regeneration in channel 2b is significant under the experimental conditions used. Another procedure for the calculation of this correction without any approximation consists of a complete numerical integration of the rate equations of the chemical mechanism. The mechanism used in this computer model is described further since simulation was also needed for the evaluation of the importance of other secondary reactions and for the measurement of the branching ratio of reaction 1. 1. Relative Determination of kl. The first conditions chosen for the measurement of k , were to use an excess of C102 over C1 for C10 production. This C10 source (reaction 15) had not been used in previous studies of reaction 1. However, a complication occurred since C102 was found to react rapidly with OH: O H + C102 products (18)
-
No kinetic data for reaction 18 have yet appeared. Therefore, an independent kinetic study of this reaction was undertaken. The detailed results of this study are to be published. The rate constant measured at 298 K is k18 = (6.9 f 0.5) X
cm3 molecule-' s-'
Such a relatively high value for kI8 allowed for a relative determination of kl in experiments where reaction 18 could be used as a reference reaction. The measurement of the ratio kI/kl8 was done for a fixed reaction time by measuring the O H signal level in the presence of C102 and C10 successively introduced through injector 2 of the reactor described in Figure 1. An excess amount of C1 over C102 was used in the C10 source. The observed (13) Y.P. Lee and C. J. Howard, J . Chem. Phys., 77,756 (1982)
162 The Journal of Physical Chemistry, Vol. 90, No. 1, 1986
0 l 0
I 1
2
[CIO] (10'3 molecule c m - 3 )
3
molecule cmm3) Figure 2. Plot obtained in the determination of k,(OH + C10) relatively to &]*(OH+ C10,); [OH],,, [OH],, and [OHIl8are the OH concentrations without reactant, with CIO, and with CIO,, respectively. The slope of this plot is the ratio kl/k18. [CIO]
O H signal in the absence of any reactant ([OH],) represented the difference between the O H concentration initially produced in injector 1 and the OH loss at the walls of the outer surface of injector 2 and of the main flow tube. One advantage of the fixed injection of reactant C102 or C10 was to suppress any variation in OH loss on the surface of injector 2. Thus, this relative kinetic method consisted of the successive measurement of [OH],, [OH],,, and [OH],, these two last concentrations being observed in the presence of C102 and C10, respectively. The value of k l / k , , was calculated from the simple expression
L=
Poulet et a].
In ([OHlo/[OHl,) = -k- , [ClOI In ([oHIO/~oHll~) k18 [c1021
Figure 3. Plot of pseudo-first-order rate constants vs. C10 concentration with excess C1 for CIO production: B, [CI] > [OJ; 0 , [CI] > [C102]. (Data are uncorrected for the OH re-formation in reaction 2b, see text.)
TABLE I: Reactions Used in Computer Simulations'
-
reaction (la) OH + CIO H 0 2 + CI (lb) OH + C10 -* HCI + 0 2 (2a) H 0 2 + CI HCI + O2 (2b) H02 + C1- CIO + OH (3) H02 + OH H20 + 0 2 (4) HO2 + NO NO2 + OH (5) C10 + NO -* NO2 + C1 (6) CI + 03 CIO + 0 2 (7) CIO + H02 -* ClOH + 0 2 (8) OH + 03 HO;! + 0 2 (9) HO2 + 03 OH + 202 (10) H02 + H02 H202 + 0 2 (11) NO + 03 NO2 + 0 2 (12) CIO + CIO products (13) OH + OH H2O + 0 (1 4) OH + wall products
-
-
--
---
+
+
rate constant at 298 K 1.9 X lo-" 3.2 X 10'" 9.1 X lo-'* 7.0 X lo-"
8.3 X 1.7 X lo-" 1.2 x lo-"
5.0 X 6.8 X
2.0 x 10-15 1.7 X 10'l2 1.8 x 10-14 1.5 x 10-14 1.9 X lo-'' 10 s-'
A plot of [C102]L as a function of [ClO] is given in Figure 2 "Rate constants are from ref 8 except for kI2 which is the best acwhich summarizes the data obtained under the following concepted value from ref 10 and 11; k , (= kl, + klb) and k I 4were meaditions: T = 298 K, P = 0.51 - 0.85 torr, [OH] C 3 X 10" molecules ~ m - and ~ , [ClO] = (0.54-3.12) X lOI3 molecules ~ m - ~ . sured in this work. As mentioned earlier, [ClO] was obtained from the absolute All data obtained for the pseudo-first-order rate coefficient as a titration with NO. [C10]/[C102] ranged from 1.6 to 1.0 in these function of [ClO], with the two sources for ClO production, are experiments. The fixed reaction time for the observation of the given in Figure 3. These data, which do not include the correction OH levels was in the range (3-7) X s. The slope of the for OH re-formation, lead to the mean value straight line of Figure 2 gives k l / k 1 8= 2.00 f 0.26. The unk,(uncorrected) = (1.53 f 0.16) X lo-]' cm3 molecule-' s-l certainty represents two standard deviations. The combination of this ratio with our direct measurement of kI8leads to the value The good linearity observed in the plot of Figure 3 indicates that (1.38 f 0.26) X lo-" cm3 molecule-' s-, for k l . Due to the use this correction does not strongly depend on the experimental of excess C1 ([Cl]/[ClO] was in the range 2-5) this value had conditions of excess C1. Nevertheless, another procedure was to be corrected by multiplying it by the correction factor cp = 1.28, applied for this correction from a simulation of the detailed considering that channel 1b is negligible. Finally these relative mechanism which is given in Table I. A computer model was experiments yield the value used to solve the integration of the system of differential equat i o n ~ . , The ~ excess C1 concentration measured for each kinetic k , = (1.77 f 0.33) X 10-l' cm3 molecule-'^-^ at 298 K experiment was taken into account, as well as the initial conThe error includes uncertainties in both the measurements of the centrations of all the other species, giving a calculated OH decay rate constant k l , and the ratio kl/k18. as a function of reaction time. This computer simulation was made 2. Absolute Determination of k , . Since the relative method for each experiment, leading to a best fit value kl(sim). For this of determining k l may not be as accurate as an absolute one, two series of experiments, the ratio [Cl]/[ClO] ranged from 1.58 to other types of absolute experiments were carried out. In the first 30.4 and the corresponding correction factors k,(sim)/k,(expt) one, the OH decay was observed as a function of reaction time were in the range 1.15-1.29. This agrees well with the first by using an excess of C1 atoms over C102or 03.At 298 K and calculation of cp = 1.28. P = 0.67-0.79 torr, with [OH] < 3 X lo1' molecules cm-3 and The experimental data of Figure 3 were individually corrected ~ , O H decay was [CIO] = (0.23-2.20) X loi3molecules ~ m - this which gave for k l the corrected value always pseudo-first order. A correction for axial diffusion was k , = (1.99 & 0.25) X lo-" cm3 molecule-' s-' done, as in all the experiments of this work, by taking for the diffusion coefficient D(OH, He) = 5 0 7 P ' cm2 s-' (Pin torr).14 The uncertainty is twice the standard deviation. These corrected (14) R. S. Yolles and H. Wise, J . Chem. Phys., 48, Si09 (1968).
( i s ) J. L. Jourdain, Thsse de Doctorat d'Etat, OrlCans, 1982.
OH
+ C10 Reaction
0
The Journal of Physical Chemistry, Vol. 90, No. 1, 1986 163
2
1
0
I
I
1
0.5
1.0
1.5
I
2 .o
3
[o3] ( 1 0 ' ~molecule c m S 3 )
Figure 4. Typical mass spectrometriccalibration plot of C10. T = 298
K, P = 0.72 torr, v = 2150 cm s-'. [CI] > (reaction 6 ) ; A, [CIO] = [NO,] (reaction 5).
[o,];0 , [CIO] =
[03]
[CIO] molecule cm-3 ) Figure 5. Plot of pseudo-first-order rate constant vs. C10 concentration
with the condition [0,]> [Cl] for C10 production,
data obtained with C1 in excess over either C102or O3 are also reported in Figure 6. It may be noticed that, in the above experimental procedure, no complication occurred from the possible secondary reactions of OH with C12 or C1 which were not considered in the simulated mechanism:
+ C1, C1 + HOC1 OH + C1 -.0 + HCl
OH
-+
(19) (20)
These two reactions were found to he negligible under every experimental conditions of this study, since the OH signal remained constant when C1, or C1 were introduced into the reactor with the same concentrations as in the kinetic experiments. For reaction 19 this observation is consistent with the published cm3 molecule-' s-l. No values1,2.16of k19= (5.5-7.4) X kinetic data exist for reaction 20. Considering that the concentrations of C1 and C1, were in the same range of magnitude in the present experiments, the value of k19can be regarded as an upper limit for k20. In the second series of absolute measurement of k l , ozone was used in excess over C1 atoms. This excess of O3was experimentally adjusted by ensuring that the secondary reaction 8 was kept negligible:
OH
k8 = 6.8
+ 03
X
-
HOz
+0 2
cm3 molecule-' s-'
(8) (ref 8)
Identical conditions could not be achieved with C102 in excess over C1 atoms since, as seen earlier, C102 would react with OH with a rate constant kI8 100 times higher than k8. Before and after each kinetic experiment a new procedure was used for the C10 titration. The NO C10 titration method, which was used in the previous experiments, could not be used in the presence of an excess of O3 over C1 since perturbating secondary reactions 5 and 11 would occur, as discussed in the Experimental Section. Therefore a mass spectrometric calibration of C10 was done, C10 being monitored at m / e 51 during the kinetic experiments. This calibration was obtained by increasing C1 concentrations, 0,being directly introduced into the reactor (through port 4) in order to make negligible the C10 recombination. The excess C1 condition was verified from the disappearence of O3 observed at m l e 48. Thus, the C10 concentration could be taken equal to the O3 Concentration. With C1 kept in excess, C10 signal levels were measured for several O3concentrations. A typical calibration plot
+
(16) L. M. Loewenstein and J. G. Anderson, J . Phys. Chew., 88, 6277 (1984).
[CIO]
(
1013molecule cm-3 )
Figure 6. Composite plot of pseudo-first-order rate constant vs. C10 concentration with the following conditions for CIO production: B, [CI] > [OJ;0 , [Cl] > [ClOz] (both series of data are corrected for the interference of reaction 2b); A, IO3] > [Cl]. The slope yields the mean value of the rate constant for the OH + CIO reaction at 298 K.
is shown in Figure 4. The accuracy of this C10 titration method was ensured by making a direct titration from the NO C10 reaction. C10 was again produced inside the central tube with an excess of C1, and the C10 concentration in the reactor was obtained from the calibrated NO2 concentration. The corresponding signal of C10 at m / e 5 1 allowed us to compare this C10 concentration with the concentration resulting from the calibration plot. As shown in the example in Figure 4, these two calculations were in good agreement. The experimental conditions for this last series of kinetic measurements of kl were T = 298 K, P = 0.70-0.92 torr, [OH]
