Formation mechanism of polycyclic aromatic hydrocarbons in benzene combustion: Quantum chemical molecular dynamics simulations

High temperature quantum chemical molecular dynamics simulations on the polycyclic aromatic hydrocarbon (PAH) formation during combustion of benzene were performed using the density-functional tight-binding (DFTB) method. Systems with varying H/C of 0.8, 0.6, 0.4, and 0.2 and temperatures of T_n =2500 K and T_n =3000 K were employed for the study of the PAH formation and growth mechanism, and trajectories were analyzed by recording average C:H compositions, common elementary reactions and molecular species, ring count, and other characteristic quantities as functions of time. We found that at H/C=0.8 mostly short polyacetylenic hydrocarbons were formed, and no significant PAH growth was found. At lower H/C ratio, longer polyacetylenic chains started to form and new five- and six-membered rings were created due to chain entanglement. Significant PAH growth forming only pericondensed PAHs was observed at lower H/C ratios of 0.4 and 0.2. In addition, smaller hydrocarbon species, such as C₂ H₂, C₂ H, and C ₂, are constantly produced by fragmentation of hydrocarbons (unimolecular reactions) and remain common species, although they are simultaneously consumed by the H-abstraction- C₂ H₂ -addition growth mechanism. Hydrogen is found to have a clear inhibitive effect on PAH and carbon cluster growth in general, in agreement with recent experimental observations.