Kinetics of the self-reaction of CF2radical and its reaction with H2, O2, CH4and C2H4over the temperature range 295–873 K

Abstract

Laser flash photolysis, coupled with time-resolved UV absorption detection, has been used to measure rate coefficients for the self-reaction of CF2: CF2+ CF2→ C2F4(1) and its reaction with the molecular species H2, O2, CH4 and C2H4 over the temperature range 295–873 K. The CF2 radicals were produced by the 193 nm laser photolysis of C2F4. At room temperature, values of k1=(4.01 ± 0.51)× 10–14 cm3 molecule–1 s–1 were obtained for the rate coefficient of reaction (1) and σ249=(1.61 ± 0.21)× 10–17 cm2 molecule–1 for the absorption cross-section of CF2 at 249 nm, where the spectral resolution was 0.5 nm (FWHM). The value of k1 is in good agreement with previous room-temperature measurements, but the value of σ249 is almost a factor of two lower, reflecting the lower spectral resolution used in this study. The absorption cross-section, σ249, was found to decrease with increasing temperature, whereas the rate coefficient, k1, exhibits a positive temperature dependence. The temperature dependence of k1 at 760 Torr N2, when combined with earlier shock-tube determinations at 1200 and 1400 K, could be fitted to a modified Arrhenius expression: k1=(1.39 ± 0.20)× 10–19T2 exp[(347 ± 56)/T] cm3 molecule–1 s–1for the temperature range 295–1400 K. The uncertainties refer to random error in the parameters at the 95% confidence level. Systematic errors are estimated to be ± 10%. However, the variation of k1 with pressure at 873 K indicated that the reaction had not quite reached its high-pressure limit at 760 Torr, so this expression can only be regarded as an empirical expression for the temperature dependence at atmospheric pressure. At lower temperatures, k1 had clearly reached its high-pressure limiting value at 760 Torr. No evidence could be found at any temperature for products other than C2F4, implying that recombination is the only channel occurring.No reaction was observed between CF2 and any of the molecular reagents studied at temperatures up to 873 K. Upper limits were derived for the rate coefficient of CF2 with each reagent at several temperatures. AT 873 K, the values are (in units of cm3 molecule–1 s–1) < 5 × 10–19 for H2, < 3 × 10–17 for O2, < 4 × 10–19 for CH4 and < 2 × 10–16 for C2H4.Experiments performed using the 193 nm photolysis of C3F6 as a source of CF2 suggested another reactive and absorbing species is generated in rather higher yield than CF2, at variance with earlier studies. The results could be rationalised by assuming the species to be C2F3 which reacts with CF2 with a rate coefficient estimated to be ca. 2 × 10–13 cm3 molecule–1 s–1 at 295 K.The implications of the very low reactivity of CF2 with molecular species, compared with its reactivity with radical species, O, H and OH, are discussed in the context of the role played by the CF2 radical in the chemistry of flame inhibition by fluorinated hydrocarbons considered as potential halon replacements.