High-Temperature Fluorocarbon Chemistry Revisited

Abstract

The thermal dissociation reactions of C2F4and C2F6were studied in shock waves over the temperature range 1000-4000 K using UV absorption spectroscopy. Absorption cross sections of C2F4, CF2, CF, and C2were derived and related to quantum-chemically modeled oscillator strengths. After confirming earlier results for the dissociation rates of C2F4, CF3, and CF2, the kinetics of secondary reactions were investigated. For example, the reaction CF2+ CF2→ CF + CF3was identified. Its rate constant of 1010cm3mol-1s-1near 2400 K is markedly larger than the limiting high-pressure rate constant of the dimerization CF2+ CF2→ C2F4, suggesting that the reaction follows a different path. When the measurements of the thermal dissociation CF2(+Ar) → CF + F (+Ar) are extended to temperatures above 2500 K, the formation of C2radicals was shown to involve the reaction CF + CF → C2F + F (modeled rate constant 8.0 × 1012(T/3500 K)1.0exp(−4400 K/T) cm3mol-1s-1) and the subsequent dissociation C2F (+Ar) → C2+ F + (Ar) (modeled limiting low-pressure rate constant 3.0 × 1016(T/3500 K)−4.0exp(−56880 K/T) cm3mol-1s-1). This mechanism was validated by monitoring the dissociation of C2at temperatures close to 4000 K. Temperature- and pressure-dependences of rate constants of reactions involved in the system were modeled by quantum-chemistry based rate theory.