Pyrolysis and oxidation of benzene and cyclopentadiene by NO_x: a ReaxFF molecular dynamics study

Benzene (C₆H₆) and 1,3-cyclopentadiene (c-C₅H₆) are critical intermediate species in the combustion of fossil fuel and the formation of polycyclic aromatic hydrocarbons (PAHs). This study investigates the underlying mechanisms of pyrolysis and oxidation of C₆H₆ and c-C₅H₆ in the presence of O₂, NO and NO₂, respectively, under combustion conditions via ReaxFF molecular dynamics simulations. The size growth in the pyrolysis system is accompanied by an amorphous nature as well as an increase in the C/H ratio. In the oxidation sytems, NO₂ is the most effective in the oxidation of both C₆H₆ and c-C₅H₆, followed by NO and O₂. In the presence of NO_x, O and N radicals generated in the high-temperature decomposition reactions of NO and NO₂ are actively involved in the addition and H-abstraction reactions of C₆H₆ and c-C₅H₆. Remarkably, the decomposition of NO₂ dramatically increases the number of O radicals in the system, which significantly accelerates the ring-opening of C₆H₆ and c-C₅H₆ by O-addition and forms linear-C₆H₆O and C₅H₆O species, respectively. Afterwards, the formation of -CH₂- by H-transfer plays an essential role in the decomposition of linear-C₆H₆O and -C₅H₆O. Reaction pathways of O and N radicals with C₆H₆ and c-C₅H₆ are reported in detail. The O and N-addition of C₆H₆ facilitate the decomposition to resonance-stabilized cyclopentadienyl radicals after the restructuring of the C-C bond.