Time Resolved Studies of Intermediate Products in the Oxidation of HCFC 141b (CFCl_2CH_3) and HCFC 142 b (CF_2ClCH_3)

摘要

The oxidative degradation of HCFC 141 b (CFCl_2CH_3) and HCFC 142b (CF_2ClCH_3) in the presence of NO at T = 293 K and p = 50 mbar has been studied using laser pulse initiation combined with cw laser long path absorp-tion/LiF for the detection of OH and NO_2. The absolute yield and temporal behaviour of these products have been found to be sensitive indicators for the reaction behaviour of the alkoxy radicals CFCl_2CH_2O and CF_2ClCH_2O, respectively. The rate coefficients for the reactions of the alkoxy radicals with O2 were determined to be k(CFCl_2CH_2O + O_2 → CFCl_2CHO + HO_2) = (2.0±1.0) x 10~(-15)cmVs and k(CF_2ClCH_2O + O_2 → CF_2ClCHO + HO_2) = (2.5±1.5) x 10~(-15)cm~3/s. Upper limits for the rate coefficients for unimolecular decomposition of the alkoxy radicals by C-C-bond fission of k(CFCl_2CH_2O + M → CFCl_2 + CH_2O + M) ≤ 1.0 x 10~3 s~(-1) and k(CF_2ClCH_2O + M → CF_2Cl + CH_2O + M) ≤ 1.5 x 10~3s~(-1) were derived. We conclude from these data that CFCl_2CHO and CF_2ClCHO will be formed as the dominant primary carbonyl products in the oxidation of HCFC 141 b and 142 b throughout the troposphere. Moreover, the rate coefficients for the reactions of the HCFCs with OH radicals at T= 293 K have been measured. The values k(CFCl_2CH_3+OH → CFCl_2CH_2 + H_2O) = (4.6 ± 0.8) x10~(-15)cm~3/s and k(CF_2ClCH_3 + OH →CF_2ClCH_2 + H_2O) = (2.6 ± 0.4) x 10~(-15)cm~3/s were obtained.