Mechanism of atmospheric oxidation of 1,1,1,2-tetrafluoroethane (HFC 134a)

Abstract

The chlorine-initiated photooxidation of hydrofluorocarbon 134a (CF₃CH₂F) has been studied in the temperature range 235–318 K and at 1 atm total pressure using UV absorption. Trifluoroacetyl fluoride [CF₃C(O)F] and formyl fluoride [HC(O)F] were observed as the major products. IR analysis of the reaction mixture also showed carbonyl fluoride [C(O)F₂] as a product. By measurement of the yields of [HC(O)F] from the photooxidation as a function of [O₂] and temperature, the rate of the unimolecular decomposition of the oxy radical, CF³CHFO, reaction (5), was determined relative to its reaction with O₂, reaction (4): CF₃CHFO + O₂→ CF₃C(O)F + HO₂(4), CF₃CHFO → CF₃+ HC(O)F (5) The results were treated using both an arithmetic derivation and numerical integration with a detailed reaction scheme. Inclusion of other recently published kinetic data leads of the following recommended rate expression for reaction (5) at 1 atm k₅= 7.4 × 10¹¹ exp[(–4720 ± 220)/T] s^–1 The errors are 1σ.

The observation of enhanced product yields in the present work is attributed to the reaction of the CF³O radical with HFC 134a leading to further peroxy radical formation. The results have been incorporated into a 2D atmospheric model to assess the environmental implications of HFC 134a release in the troposphere.