High-Temperature Fluorocarbon Chemistry Revisited.

Area of Science:

• Chemical Kinetics
• Physical Chemistry
• Combustion Science

Background:

• Understanding the high-temperature decomposition of fluorocarbons like tetrafluoroethylene (C2F4) and hexafluoroethane (C2F6) is vital for combustion and atmospheric chemistry.
• Previous studies have established primary dissociation rates, but secondary reaction kinetics at extreme temperatures remain less understood.

Purpose of the Study:

• To investigate the thermal dissociation reactions of C2F4 and C2F6 at high temperatures (1000–4000 K).
• To identify and quantify the kinetics of secondary reactions involving radical species.
• To develop a validated mechanism for C2 radical formation at high temperatures.

Main Methods:

• Shock wave experiments were employed to achieve temperatures between 1000 and 4000 K.
• UV absorption spectroscopy was used to monitor the concentrations of various species, including C2F4, CF2, CF, and C2.
• Quantum-chemically modeled oscillator strengths were used to derive absorption cross sections.
• Rate constants were modeled using quantum-chemistry based rate theory.

Main Results:

• Absorption cross sections for C2F4, CF2, CF, and C2 were determined.
• Earlier dissociation rate results for C2F4, CF3, and CF2 were confirmed.
• The reaction CF2 + CF2 → CF + CF3 was identified with a rate constant of 10^10 cm^3 mol^-1 s^-1 at 2400 K.
• A mechanism for C2 radical formation involving CF + CF → C2F + F and subsequent C2F dissociation was proposed and validated.

Conclusions:

• The study elucidated key secondary reaction pathways in the high-temperature dissociation of C2F4 and C2F6.
• The identified reaction CF2 + CF2 → CF + CF3 proceeds via a pathway distinct from CF2 dimerization.
• A validated mechanism for C2 radical formation at temperatures up to 4000 K was established, improving kinetic models for fluorocarbon systems.