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

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 C(2)H(2), C(2)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-C(2)H(2)-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.