A ReaxFF molecular dynamics study of polycyclic aromatic hydrocarbon oxidation assisted by nitrogen oxides

Fossil fuel-derived soot poses a persistent problem. A joint reduction is conducted via the reaction between NO_x and soot. The underlying reaction mechanisms of large polycyclic aromatic hydrocarbons (PAHs), as well as the interactions between O₂, NO and NO₂, have been extensively investigated by ReaxFF molecular dynamics simulations for the first time. The pyrolysis and oxidation of coronene are conducted at 2500 K and 50 atm. Coronene in the pyrolysis system first experiences dehydrogenation reactions and subsequently undergoes recombination reactions, leading to the formation of large PAHs. Among the three oxidizers, NO₂ is the strongest in coronene oxidation, followed by NO and O₂. Meanwhile, the O radical is identified as the key species for PAH oxidation. In the presence of O₂, the formation of O radicals requires the assistance of H radicals that are formed by the dehydrogenation of PAHs, which retards the oxidation process. In the presence of NO, O radicals can be directly formed via reactions of NO → N+O and N + NO ⇌ N₂ + O. NO₂ first undergoes a decomposition reaction: NO₂ → NO + O, followed by NO decomposition. O-addition and N-addition, as well as subsequent fragmentation reactions including the generation of CO_x and (H)CN are important routes in PAH oxidation. Furthermore, compared to NO, NO₂ provides more O radicals that significantly accelerate PAH oxidation. This study provides fundamental insight into PAH oxidation that may help to design strategies to inhibit soot and NO_x emissions at the source.