New technique for generating high concentrations of gaseous OH radicals in relative rate measurements

Abstract

We have developed a technique for generating high concentrations of gaseous OH radicals in a reaction chamber. The technique, which involves the UV photolysis of O 3 in the presence of water vapor, was used in combination with the relative rate method to obtain rate constants for reactions of OH radicals with selected species. A key improvement of the technique is that an O 3 /O 2 (3%) gas mixture is continuously introduced into the reaction chamber, during the UV irradiation period. An important feature is that a high concentration of OH radicals [(0.53-1.2) × 10 11 radicals cm −3 ] can be produced during the irradiation in continuous, steady-state experiment. Using the new technique in conjunction with the relative rate method, we obtained the rate constant for the reaction of CHF 3 (HFC-23) with OH radicals, k 1 . We obtained k 1 (298 K) = (3.32 ± 0.20) × 10 −16 and determined the temperature dependence of k 1 to be (0.48 ± 0.13) × 10 −12 exp[−(2180 ± 100)/T] cm 3 molecule −1 s −1 at 253-328 K using CHF 2 CF 3 (HFC-125) and CHF 2 Cl (HCFC-22) as reference compounds in CHF 3 -reference-H 2 O gas mixtures. The value of k 1 obtained in this study is in agreement with previous measurements of k 1 . This result confirms that our technique for generating OH radicals is suitable for obtaining OH radical reaction rate constants of ∼10 −16 cm 3 molecule −1 s −1 , provided the rate constants do not depend on pressure. In addition, it also needed to examine whether the reactions of sample and reference compound with O 3 interfere the measurement when selecting this technique. C

Kinetics of the gas‐phase reaction of CF3OC(O)H with OH radicals at 242–328 K

“…The total pressure increased as the O 3 /O 2 mixture was introduced; the final total pressure was 238-313 Torr in all the measurements. By this technique, a constant OH radical concentration can be produced during the irradiation [8]. To determine whether the O 3 /O 2 flow rate interfered with the measurement of k 1 , we measured the values of k 1 at 298 K at flow rates of 6, 12, and 20 cm 3 min −1 ; the values were the same within experimental uncertainty (Table I) (5) The parameter ␣ is the yield of CF 3 OC(O)H from the reaction of CF 3 OCH 3 with OH radicals (␣ = 0-1).…”

“…Experiments were performed in an 11.5-dm 3 cylindrical quartz chamber (diameter, 10 cm; length, 146 cm) with an external jacket [8]. The temperature in the reaction chamber was controlled by circulating heated water or a coolant (PF-5060; Sumitomo 3M Ltd.) through the external jacket.…”

Rate constants for the gas‐phase reaction of CF3CF2CF2CF2CF2CHF2 with OH radicals at 250–430 K

“…An O 3 /O 2 (3%) gas mixture, which was generated from pure O 2 with a silent-discharge ozone generator (ECEA-1000; Ebarajitsugyo, Japan), was continuously introduced into the chamber at a flow rate of 10-50 cm 3 min −1 during the UV irradiation. A steady-state high OH radical concentration of (0.53-1.2) × 10 15 molecule cm −3 can be produced in the chamber by using this technique [7]. The flow rate of the O 3 /O 2 gas mixture was controlled by a mass flow controller.…”

“…Experiments were performed in a previously described 11.5-dm 3 quartz cylindrical chamber (10-cm i.d., 146 cm long) with an external jacket [7]. The chamber temperature (253-328 K) was controlled by circulating heated water or a coolant (PF-5060; Sumitomo 3M Ltd.) through the external jacket.…”

“…The partial pressure of the reactants decreased by 0.2% for each GC-FID analysis. The uncertainties in the measured concentrations of CF 3 CF 2 CF 2 CF 2 CF 2 CHF 2 and the reference compounds were <2% and generally in the range of 0. is a parameter correcting for the nonreactive decay of 0.2% of reactants as the reactants were taken for each GC-FID analysis during the sampling process (D n = n ln(0.998), where n is the sampling number of the GC-FID analysis [7]); and k 1 and k r are the rate constants for reactions 1 and 2, respectively.…”