Simulation of the thermal decomposition of methane at constant volume and temperature using methods of molecular dynamics and thermodynamics

The thermal decomposition of methane at constant temperatures and densities ranging from $0.05$ to $0.524$ g/cm$^3$ has been simulated using methods of molecular dynamics and equilibrium thermodynamics. The molecular-dynamics simulation of the initial stage of methane decomposition has been performed using the reaction force field ReaxFF-lg at temperatures of $2500$–$4000$ K. The simulation results showed that the methane decomposition consists of the successive formation and decay of radicals and light hydrocarbons and their replacement with polyatomic hydrocarbons of increasing complexity, similar to polycyclic aromatic hydrocarbons, whose decomposition and aggregation leads to generation of condensed-carbon nucleation centers. In turn, the results of the thermodynamic calculations indicate that the methane decomposition begins and occurs at lower temperatures as compared with the results of ultrashort nonequilibrium calculations by the molecular dynamics method. Thus, the application of molecular dynamics and thermodynamics methods for the same process presents extreme versions of possible sequences of states in the cases of ultrashort nonequilibrium and long-term, similar to equilibrium, processes of methane thermal decomposition.