Rate and mechanism of the atmospheric degradation of 2H‐heptafluoropropane (HFC‐227)

Abstract

The atmospheric chemical behaviour of 2H‐Heptafluoropropane (CF3CHFCF3, HFC‐227) with respect to its rate and mechanism of degradation in the troposphere has been investigated. The rate coefficient for the reaction of HFC‐227 with OH radicals has been determined in direct time resolved experiments using a combination of laser‐pulse initiation and LIF. In the temperature range T=298‐463 K the Arrhenius expression k(T) = (3.8±0.8) 10−13 exp(‐1596±77K/T) cm3/s, which corresponds to k (298 K) = (1.8±0.3) 10−15cm3/s, was found. The mechanism of the atmospheric degradation of HFC‐227 has been investigated using both, laser‐pulse initiated/time resolved and UV photolysis/FTIR product studies. It is concluded that the major carbonyl products are CF3COF and CF2O which result from the decomposition of the oxy radical CF3CF(O)CF3 by C‐C bond fission. The rate coefficient for this decomposition process has been determined to be k = (2±1) 103s−1 at 298K. Based on the reactivity of HFC‐227 with respect to the reaction with OH radicals, the atmospheric lifetime and the GWP value for HFC‐227 are estimated to be ∼40 years and 0.6±0.2, respectively.

Environmental assessment of CFC alternatives

“…Although the present value agrees well to Nelson et al [9], Zhang et al [10], and Zellner et al [11] within about 10% at 430 K, the results of these three groups are 20-35% larger than that of ours at 298 K. This deviation of temperature dependency leads to a difference in the activation energy; the values of E/R reported by three groups are 12-13% smaller than that of ours. These three groups have measured OH rate constants by using the absolute rate technique.…”

“…Thus, if it is the case the contribution of the impurities on the measured rate constant is negligibly small. Zellner et al [11] have not mentioned the purity of their sample.…”

“…Nelson et al [9] have measured the reaction rate constant by using the discharge flow combined with the laser induced fluorescence technique over the temperature range 294-369 K. Zhang et al [10] have measured the rate constant by using the flash photolysis combined with the resonance fluorescence method over the temperature range 270-365 K. Zellner et al [11] have measured the rate constant by using the laser photolysis combined with the resonance fluorescence method over the temperature range 298-463 K. In their study, OH radicals were generated by excimer laser photolysis of HNO 3 at l = 193 nm. Hsu and DeMore [12] have measured the rate constant by using the relative rate technique over the temperature range 296-367 K. CH 4 and CH 3 CHF 2 were used as the reference compounds.…”

Environmental effects of large igneous province magmatism: a Siberian perspective

“…OH + CF 3 CHFCF 3 (HFC-227ea). The recommended rate expression is derived from a combined fit to the data (below 400 K) from the absolute studies of Nelson et al [949], Zellner et al [1513], and Zhang et al [1524] and the relative rate studies of Hsu and DeMore [615] (two determinations which have been recalculated based on the current recommendations for the rate constants for the reference reactions OH + CH 4 and OH + CHF 2 CF 3 ). (Table: [911] and Barry et al [88] (renormalized to the current recommendation for the rate constant for the reference reaction OH + CH 3 CCl 3 ).…”

Environmental effects of ozone depletion: 1998 assessment

“…Atmospheric oxidation of halothane (Bilde et al, 1998), isoflurane, and HCFC-123 (Edney et al, 1991;Tuazon and Atkinson, 1993a;Hayman et al, 1994) gives CF 3 C(O)Cl of which approximately 60% undergoes hydrolysis to give CF 3 C(O)OH. Oxidation of HCFC-124 (Tuazon and Atkinson, 1993a;Edney and Driscoll, 1992) and HFC-227ea (Zellner et al, 1994;Møgelberg et al, 1996) gives CF 3 C(O)F which is then converted into CF 3 C(O)OH. Current and projected use of HFC-227ea is minor.…”