12809
J. Phys. Chem. 1995, 99, 12809-12813
Kinetics of the Reaction of Ground State Oxygen Atoms with Trichloroethylene from 295 to 1127 K: Comparison to Reactions with Some Other Substituted Ethylenes Jasmina Hranisavljevic and Arthur Fontijn* High-Temperature Reaction Kinetics Laboratory, The Isermann Department of Chemical Engineering, Rensselaer Polytechnic Institute, Troy, New York 121 80-3590 Received: March 17, 1995; In Final Form: June 6, 1 9 9 9
+
Rate coefficient measurements for the reaction O(3P) trichloroethylene have been obtained in two types of high-temperature photochemistry (HTP) reactors. Ground state oxygen atoms were produced by either flash photolysis or laser photolysis of SO2 or 0 2 , and their relative concentrations were monitored by resonance fluorescence. The data are well fitted by k(295-1127 K) = 1.1 x exp(-695 WT) 1.5 x exp(-7508 WT)cm3 molecule-' s-I, with 2 0 precision limits varying from f3% to f 1 2 % and 2 0 accuracy limits of %&22%. The rate coefficients are compared to those of 0 atom reactions with other chloro- and alkyl-substituted ethylenes; a unified semiempirical k(T) estimation method is established for lower temperatures and suggested for higher temperatures. The heat of formation of C2Cl3 is discussed.
+
1. Introduction Chlorinated hydrocarbons and their incomplete combustion products represent a severe health hazard from waste incinerators.' To help evaluate the individual processes, which may be involved, we have presented results on the reactions of O(3P) atoms with three chloroethylenes,CH24HC1, CHz-CC12, and trans-CHCl=CHCl, in the 300- 1200 K temperature rangee2 Here we extend the work to O(3P)
+ CHCl=CCl,
-
products
(1)
Trichloroethyleneis among the most frequently present of these compound^.^ Its combustion in rich flames has been modeled? but measured kinetic data at realistic temperatures and pressures are lacking. In other recent work we have presented results on the 0 atom reactions with the four isomeric butenes? Le., with a series of alkyl-substituted ethylenes. It was shown5 that the rate coefficients below about 500 K correlate well with the respective substituent constants. Those at higher temperatures can be calculated as the sum of (i) an extension of a classical transition state theory expression for the rate coefficients for electrophilic 0 atom addition to the double bond below 500 K and (ii) estimated rate coefficients for H abstraction. The present work was undertaken in part to see whether these approaches can be extended to chloroethylenes.
2. Experimental Technique The measurements were performed in two different HTP (high-temperature photochemistry) reactors. The older type, reactor A, has most recently been described by KO et a1.,6 and the newer, reactor B, has been discussed by Mahmud et a1.l Both reactors consist of an alumina reaction tube surrounded by S i c heating elements, insulation, and a water-cooled steel vacuum chamber. To prevent thermal decomposition of thermally unstable reactants at higher temperatures, a movable cooled inlet is used to introduce the reactant gases. After emerging from the cooled inlet, the gases are mixed with the heated Ar bath gas at the upstream (bottom) side of the reactor. The mixing time for reactant gases is calculated from the @Abstractpublished in Advance ACS Abstracts, August 1, 1995.
0022-365419512099-12809$09.0010
distance from the tip of the cooled inlet to the center of the observed reaction zone and selected such that the mixing better than 95% is achieved prior to entering the reaction zone.8 The temperature of the observed reaction zone is measured by a doubly thermally shielded thermocouple placed on the reaction tube axis. Checks are performed using off-axis thermocouples. Pressures are measured by an MKS Baratron pressure transducer located downstream of the reaction zone. Flow rates of gases are determined by precalibrated flow meters and controllers. Ground state oxygen atoms were produced by flash photolysis (FP) of SO2 or 0 2 , or 193 nm laser photolysis (LP) of S02, through MgF2 or Suprasil quartz windows, respectively. In reactor A only FP was used, whereas in reactor B both FP and LP were used. The relative concentrations of the oxygen atoms were monitored by fluorescence of the 130.2-130.6 nm resonance triplet, detected by a photomultiplier tube through a CaF2 window (A > 125 nm). The source of resonance radiation was a microwave discharge lamp through which He flowed at 2.0 mbar. Bath gas Ar (99.998%) was obtained from Linde. The photolytes used were 0 2 (99.6%), anhydrous SO2 (99.98%), and 1.07% SO2 (99.98%) in Ar (99.999), all from Matheson. C2HCl3 obtained from either of three sources, Aldrich (99%), Fisher (99.9%), and J.T.Baker (loo%), was vaporized on line into 99.998% Ar. The compositions of such prepared mixtures, calculated from the saturated vapor pressures, were checked by gas chromatographic analysis. The operating procedures have been s~mmarized.~ Briefly, the experiments were carried out under pseudo-first-order conditions [O]
