Methane pyrolysis in preparation of pyrolytic carbon: Thermodynamic and kinetic analysis by density functional theory

The density functional theory has been successfully applied in analyzing pyrolytic carbon deposition by methane pyrolysis from the view of thermodynamics and kinetics based on a total number of 39 elementary reactions. M06-2X/def2-TZVP level was employed to optimize species structures and locate the transition states. The enthalpy changes and Gibbs free energy changes of all the reactions in the temperature range of 298.15–1800 K were derived with optimized species. Results show that the reacting temperature should be above 1200 K based on the equilibrium constant analysis, which is consistent with the typical reaction temperature adopted in experiments. Potential energy surface profiles report that radical attacking reactions have lower energy barriers than those direct decomposition reactions, especially hydrogen radical attacking reactions. The energy barriers of the first steps, dehydrogenations of methane and ethylene, are 272.4 kJ·mol⁻¹ and 288.9 kJ·mol⁻¹ at 1200 K, which are very close to the experimental activation energy for methane pyrolysis. The most favorable decomposition reaction path is the path of hydrogen radical attacking reactions. The highest energy barrier of the path at 1200 K is 185.7 kJ·mol⁻¹ presented by the C–H bond breaking in ethynyl attacked by hydrogen radical.