J. Phys. Chem. 1991,95,8745-8748
8745
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Kinetics of the o(~P) C6H6 Reaction over a wide Temperature Range Taebo KO, George Yaw Adusei, and Arthur Font@* High- Temperature Reaction Kinetics Laboratory, The Isermann Department of Chemical Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180-3590 (Received: April 15, 1991; In Final Form: June 11, 1991) Rate coeficients for the 0 + C6H.j reaction have been measured by using the HTP (high-temperaturephotochemistry)technique
with 193-nm laser photolysis of SO2to generate ground-state oxygen atoms and atomic resonance fluorescence to monitor relative concentrations of the 0 atoms. The data cover the 600-1310 K range and are best represented by k(7')= 5.35 X
IO-" exp(-261 IK/O cm3 molecule-' s-' with a precision of better than *IO% and corresponding accuracy of *23%, both at the 2a statistical confidence level. The present data are in excellent agreement with those from several other investigations; together, they can be represented by k(T) = 4.0 X lo-" exp(-2349K/T) cm3 molecule-' s-' for the 300-1450 K range, again with an estimated accuracy of about f23%. The results indicate that the 0 atom addition mechanism dominates over this entire temperature range. Introduction
Aromatic compounds play important roles in combustion. On the one hand, aromatics improve the knock resistance of gasoline, while on the other, they are linked to soot generation.' Information on the reaction between ground-state oxygen atoms with benzene O('P) + C6H6 products (1) is needed to improve the understanding of such processes.2 For temperatures up to 870 K, rate coefficient measurements on reaction 1 have been made by using the discharge flow,'g4 phase-shift?v6 and flash photolysis-resonance fluorescence7 techniques. The data from those techniques are in good agreement with each other, except for those of Bonanno et al.' Within experimental uncertainty, the kl data may be seen4-' to vary according to the Arrhenius equation. The initial step for the reaction in this temperature range is thought to be the electrophilic addition of the 0 atom to the benzene ring to form a triplet biradi~al.~** The dominant channels through which this adduct can dissociate have been investigated by Sibener et al. using crossed molecular beam experiments at collision energies of 27 and 36 kJ mol-'? They found evidence for the product channels C6H50 H and C6HsOH, while a potential CSH6+ CO channel a p parently does not contribute observably. Similarly, Nicovich et aL7 using the flash photolysis-resonance fluorescence technique did not detect CO. The barrier in the entrance valley of the potential energy surface is 11 kJ mol-' as determined by Gonzalez Urena et al. in crossed beam experiment^.^ A priori, abstraction of a ring hydrogen O('P) C & 5 O H + C6H5 (2) which is 36 kJ mol-' endothermic at 298 K,l0 could be expected -b
+
+
-.
(1) Brezinsky, K. Prog. Energy Combust. Sci. 1986, 12, 1. (2) Rotzoll, G. Inr. J . Chem. Kinet. 1985, 17, 637. (3) Bonanno, R. A.; Kim, P.; Lee, J. H.; Timmons, R. B. J . Chem. Phys. 1972, 57, 1377. (4) Tappe, M.; Schliephake, V.; Wagner, H. Gg. 2.Phys. Chem. 1989, 162, 129. ( 5 ) Atkinson, R.; Pitts, J. N., Jr. J . Phys. Chem. 1975, 79, 295. (6) Colussi, A. J.; Singleton. D.L.; Irwin, R. S.;Cvetanovic, R. J. J . Phys. Chem. 1975. 79. 1900. (7) NiGich,' J. M.; Gump, C. A.; Ravishankara, A. R. J . Phys. Chem. 1982. 86. 1684.
Chem. Phys. 1980, 72, 4341. (9) Gonzalez Urena. A.; Hoffmann, S.M. A.; Smith, D. J.; Grice, R. J . Chem. Soc., Faraday Trans. 2 198682, 1531. (10) To arrive at the PIPmK for this reaction, we used heats of formations for C6H6and C6H5 from: Pedley. J. B.; Naylor, R. D.;Kirby, S.P. Thermochemical Data of Organic Compounds, 2nd ed.; Chapman and Hall: London. 1986; p 91. McMillen, D.F.; Golden, D. M. Annu. Rev. Phys. Chem. 198Z33.493. The values for 0 and OH are from: Chase, M. W., Jr.; Davis, C. A.; Downey, J. R., Jr.; Frurip, D.J.; McDonald, R. A.; Syverud, A. N. JANAF Thermochemlcal Tables, 3rd 4.; Journal of Physical and Chemical Reference Data; American Chemical Society: Washington, DC, 1985; Vol. 14, Supplement 1.
at elevated temperatures. A direct observation of that reaction was reported by Barry et al." They detected OH radicals by laser-induced fluorescence in a crossed beam study at a collision energy of 69 kJ mol-', i.e. at an energy much larger than corresponds to the endothermicity. The present work was initiated to extend the temperature range of the kl measurements and to investigate the possibility of abstraction at elevated temperatures. While this work was in progress, Leidreiter and WagnerI2reported kinetic measurements in the 1200-1450 K range made by using a shock-tube technique with resonance absorption monitoring of 0 atoms. Their results are in good agreement with the extrapolation of the low-temperature measurements; i.e. no curvature in the Arrhenius plot is observed. Since that work involved some modeling of the absorption profiles, the present resonance fluorescence technique allows a check on their results, as well as provides the first rate coefficient measurements in the 870-1 200 K temperature range.
Experimental Technique The experiments were carried out in an HTP (high-temperature photochemistry) reactor of the basic design B of Mahmud et al." However, preliminary experiments in which 0 atoms were produced by photolysis of O2or C 0 2 using a flash lamp with various filters led to nonexponential decays, cf. also Nicovich et al.' No such problem was encountered with 193-nm excimer laser photolysis of SO2,I4which was therefore used. Otherwise, the general operational procedures followed were those we recently described.15J6 Therefore, only a brief description for the present work is given here. The reactor consisted of an alumina reaction tube, surrounded by S i c resistance heating elements, enclosed in a water-cooled steel vacuum chamber. The temperature was measured by an axially placed retractable Pt-Pt/ 13% Rh thermocouple, doubly shielded to minimize radiation effects. An off-axis thermocouple without radiation shields was used to check on the performance of the former and the significance of radiation effects. In the course of these experiments, the difference in the temperature readings from the two thermocouples was always less than 5 K. The temperature of the reaction zone was measured before and after each experiment. Temperature drift in the course of an experiment was typically less than 5 K. Flow rates of all gases were determined by using calibrated mass flow meters and flow controllers, and the pressure was measured with an MKS Baratron pressure transducer. (1 1) Barry, J. N.; Fletcher, I. W.; Whitehead, J. C. J. Phys. Chem. 1986, 90, 4911. (12) Leidreiter, H. I.; Wagner, H. Gg. Z . Phys. Chem. Neue Forge 1989, 165, 1 . The individual data were sent to us by H. Gg. Wagner. (13) Mahmud, K.; Kim, J.-S.; Fontijn, A. J. Phys. Chem. 1990,942994. (14) Felder, P.; Effenhauser, C. S.; Haas, B. M.; Huber, J. R. Chem. Phys. Lert. 1988. 148, 417. (15) KO,T.; Marshall, P.; Fontijn, A. J . Phys. Chem. 1990, 94, 1401. (16) Marshall, P.; KO, T.; Fontijn, A. J . Phys. Chem. 1989, 93, 1922.
0022.365419 112095-8745$02.50/0 0 1991 American Chemical Society
8746 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991
KO et al.
TABLE I: Sumarrry of R8C CoMdent M I u r c r r a b 01 the 0 + C f i Reaction 1 O'* [MI, 1o"[s02], 1Oi4[ C & . ] mI, T," K 598b 615' 6176 660b 660b 716
:I'
763 78 1 807 810 818 835 866 869 885 900 917 926 963 989 994 1007 1036 1045 1063 1077 I1816 I 194b I 203' I 234b 1252* 1253' I 274b I 3056 I 308b
P, mbar
cm-3
240 390 390 240 240 240 530 340 270 290 500 490 250 240 340 540 270 450 350 690 330 270 280 270 580 340 560 540 690 470 460 460 460 640 290 350 400
3.0 4.7 4.7 2.7 3.0 2.5 5.3 3.3 2.6 4.7 4.6 4.5 2.3 2.2 2.9 4.6 3.1 3.7 3.4 5.5 2.5 2.0 2.0 2.0 4.2 2.4 3.9 3.7 4.4 2.9 2.8 2.8 2.7 3.8 1.7 2.0 2.2
cm-3 3.4 3.4 3.4 2.1 2.6 6.7 15 1.3 7.8 24 14 22 23 5.6 4.5 17 3.3 14 13 12 4.7 4.7 1.4 2.6 9.1 2.1 13 6.1 1.1 2.9 4.6 1.9 1.9 2.5 3.1 3.7 3.5
cni3 2.91 6.15 3.20 5.49 2.92 1.84 5.47 7.22 1.31 5.99 4.3 1 5.76 3.46 2.70 2.73 2.01 2.12 4.20 1.59 4.31 2.64 2.16 2.15 3.05 2.18 0.87 2.94 3.72 2.62 0.24 0.30 0.40 0.30 0.21 0.30 0.25 0.28
z, cm 28' 28 28 15 15 IO
IO 15
IO IO 15 15 15
IO 6.0 6.0
IO IO IO 5.0 5.0 6.0 6.0 6.0 4.5 15 4.5 3.0 12 12 12 12
IO IO IO 7.0 5.0
4 cm s-I
10-'*(k f q), LE, mJ
cm3 molecule-' s-l
9.2
IO IO IO IO 13 8.6 12 8.7 6.9 6.9 6.9 7.2 13 16 15
12 12 12 5.2 9.8 17 16 9.7 9.0 15
9.6 3.8 17 29 12 30 31 24 24 23 24
0.74 f 0.07e 0.82 f 0.1 1 1.00 f 1.06 1.11 f 0.14 1.15 f 0.89 1.47 f 0.06 I .59 f 0.03 1.79 f 0.20 1.61 f 0.05 2.08 f 0.16 1.74 f 0.12 1.77 f 0.07 2.36 f 0.43 2.35 f 0.87 2.57 f 0.36 2.39 f 0.10 2.88 f 0.13 2.67 f 0.21 2.93 f 0.23 2.23 f 0.03 3.44 f 0.09 3.88 f 0.02 3.72 f 0.50 3.88 f 0.16 3.70 f 0.03 4.17 f 0.19 5.37 f 0.15 4.10 f 0.26 6.09 f 0.35 6.01 f 0.46 6.67 f 0.22 6.49 f 0.16 7.05 f 0.41 6.79 f 1.05 6.95 f 0.32 7.29 f 1.94 7.06 f 1.51
OuT/T = 2%. '462 ppm C6H6/Ar was used, in all other ex riments 5009 ppm C6H6/Ar was used. ' z = 28 cm indicates that SO2and C6H6 were injected with bath gas; i.e. the cooled-inlet was not used. 6 h e number in the parentheses indicates the number of neutral density filters used. 'Should be read as (0.74 f 0.07) X cm3 molecule-' s-I.
SO2and C6H6 were introduced through the moveable cooled inlet, while the bath gas Ar flowed in from the bottom of the reactor. The distance from the tip of the cooled inlet to the center of the reaction zone was adjusted such that the reaction gases were at least 95% mixedI6 with the bath gas. The cooled inlet was not used in some experiments at low temperatures where thermal decomposition would be negligible (see Table I). In those experiments the reaction gases were premixed with the bath gas before flowing into the reactor. The gases used and their purities were Ar (from the liquid of 99.998% purity) from Linde, and mixtures of 1.07% SOz (99.98%), 462 ppm C6H6 (99.9%), and 5009 ppm C6H6 (99.9%) in Ar (99.999%) from Matheson. The photolysis source was a Questek Series 2000 excimer laser operated at about 1 Hz;the energy of the laser pulse was reduced by a beam splitter and neutral density filters made of overlapping metal gauzes. The cross section of the laser beam was 2.8 cm2. The relative concentrations of the O(3P) atoms were monitored by fluorescence of the 130.2-1 30.6-nm resonance triplet, induced by a CW (continuous wave) microwave discharge lamp through which flowed 99.999% He. The 0 atom fluorescence was detected by a solar-blind photomultiplier tube (PMT) placed at right angles to both the laser beam and the beam of light from the flow lamp. The signal from the PMT was fed to a multichannel scaler signal averager via an amplifier/discriminator. The experiments were carried out under pseudo-first-order conditions, [O]
