Reflected Shock Tube and Theoretical Studies of High-Temperature Rate Constants for OH + CF₃H ⇆ CF₃ + H₂O and CF₃ + OH → Products

Abstract

The reflected shock tube technique with multipass absorption spectrometric detection of OH radicals at 308 nm, using either 36 or 60 optical passes corresponding to total path lengths of 3.25 or 5.25 m, respectively, has been used to study the bimolecular reactions, OH+CF3H-->CF3+H2O (1) and CF3+H2O-->OH+CF3H (-1), between 995 and 1663 K. During the course of the study, estimates of rate constants for CF3+OH-->products (2) could also be determined. Experiments on reaction -1 were transformed through equilibrium constants to k1, giving the Arrhenius expression k1=(9.7+/-2.1)x10(-12) exp(-4398+/-275K/T) cm3 molecule(-1) s(-1). Over the temperature range, 1318-1663 K, the results for reaction 2 were constant at k2=(1.5+/-0.4)x10(-11) cm3 molecule(-1) s(-1). Reactions 1 and -1 were also studied with variational transition state theory (VTST) employing QCISD(T) properties for the transition state. These a priori VTST predictions were in good agreement with the present experimental results but were too low at the lower temperatures of earlier experiments, suggesting that either the barrier height was overestimated by about 1.3 kcal/mol or that the effect of tunneling was greatly underestimated. The present experimental results have been combined with the most accurate earlier studies to derive an evaluation over the extended temperature range of 252-1663 K. The three parameter expression k1=2.08x10(-17) T1.5513 exp(-1848 K/T) cm3 molecule(-1) s(-1) describes the rate behavior over this temperature range. Alternatively, the expression k1,th=1.78x10(-23) T3.406 exp(-837 K/T) cm3 molecule(-1) s(-1) obtained from empirically adjusted VTST calculations over the 250-2250 K range agrees with the experimental evaluation to within a factor of 1.6. Reaction 2 was also studied with direct CASPT2 variable reaction coordinate transition state theory. The resulting predictions for the capture rate are found to be in good agreement with the mean of the experimental results and can be represented by the expression k2,th=2.42x10(-11) T-0.0650 exp(134 K/T) cm3 molecule(-1) s(-1) over the 200-2500 K temperature range. The products of this reaction are predicted to be CF2O+HF.

Thermal Decomposition of CF3 and the Reaction of CF2 + OH → CF2O + H

“…Note that the reaction CF 3 + OH (k 12 in the table) shows substantial sensitivity. This rate constant was recently measured in this laboratory and shown to be in excellent agreement with theoretical calculations 5 and therefore cannot be varied in the simulations. Hence, in large measure, the profiles are mostly dominated by CF 3 decomposition and CF 2 + OH.…”

Catalytic pyrolysis of CHF3 over activated carbon and activated carbon supported potassium catalyst

“…It can be seen that the carbon-based material treated with HNO 3 contains large amounts of carboxyl (-COOH), carboxylic anhydride (-C 2 O 3 -) and lactonic (-COO-) groups evolving as CO 2 (decomposition temperatures are 473-673 K and above 673 K respectively). Also, numerous phenolic (-C 5 H 5 -OH), quinine and carbonyl (-CO-) groups were detected, whose decomposition temperatures were between 873 K and 1173 K in the CO evolution [21][22][23][24][25].…”

Hydrolysis of Formyl Fluoride Catalyzed by Sulfuric Acid and Formic Acid in the Atmosphere

“…Formyl fluoride (HFCO) is a key molecule, produced via atmospheric degradation of hydrofluorocarbons (HFCs) 1−7 and hydrofluoroolefins (HFOs). 8−10 HFCs have been used as a refrigerant instead of CFCs and HCFCs to protect the ozone layer.…”

“…Formyl fluoride (HFCO) is a key molecule, produced via atmospheric degradation of hydrofluorocarbons (HFCs) − and hydrofluoroolefins (HFOs). − HFCs have been used as a refrigerant instead of CFCs and HCFCs to protect the ozone layer . Here, HFC-134a is one of the highest concentrations of HFCs in the atmosphere, which mainly reacts with the OH radical; this leads to the formation of the dominant intermediate product HFCO. − In recent years, experimental results have shown that the concentration of HCF-134a has been increasing year by year due to anthropogenic emission; ,− this leads to the increase of the concentration of HFCO in the atmosphere.…”