Role of ammonia addition on polycyclic aromatic hydrocarbon growth: A ReaxFF molecular dynamics study

Ammonia (NH₃), one of the most promising carbon-free fuels, has received great research interest. In particular, NH₃ is often blended with hydrocarbon fuels to achieve desired combustion characteristics. However, NH₃ addition could affect soot formation, which has not been adequately understood. In this study, the effect of NH₃ on the growth of polycyclic aromatic hydrocarbons (PAHs) is investigated with the reactive force field molecule dynamics (ReaxFF MD) simulations and quantum chemistry calculations. The simulation results indicate that NH₃ addition slows down the growth of large carbon-containing species in the C₂H₄/O₂ system. Novel path with HCN addition is discovered in the PAH growth, which inhibits the PAH growth compared to the conventional Hydrogen-Abstraction-Carbon-Addition (HACA) path via C₂H₂ addition. Moreover, quantum chemical calculations verified the rationality of this addition path and explained its inhibition on PAH growth by calculating the heat of reactions and reaction energy barriers. According to the present study, NH₃ also has an inhibitory effect on both the HCN addition path and C₂H₂ addition path by providing H atoms to promote the generation of PAH radicals back to PAH molecules, and this effect is more significant for the C₂H₂ addition path. Results from the study provide a fundamental insight for the inhibition of PAH growth due to the NH₃ addition from an atomistic insight, which helps to improve the understanding of the combustion of blends of NH₃ and hydrocarbon fuels.