A theoretical and modeling study of nitrogen chemistry in polycyclic aromatic hydrocarbons growth process

Ammonia blended with hydrocarbon fuels inhibits soot formation due to the chemical effect of NH3 in polycyclic aromatic hydrocarbons (PAHs) growth process. Nitrogenous species are highly concentrated intermediates in combustion of fuels doped with NH3, while their interaction with PAHs remains unclear for the lack of experimental data. In this work, hydrogen cyanide (HCN) reacts with benzene and naphthalene as isoelectronic species of C2H2 to complete the hydrogen-abstraction-acetylene-addition (HACA) chemistry reaction in PAHs growth process. The reaction pathways for HCN-benzene and HCN-naphthalene were investigated at G3(MP2, CC)// B3LYP/6-311G(d,p) level, and the rate coefficients were calculated using Rice-Ramsperger-Kassel-Marcus (RRKM) theory to analyze the yield distribution of various reaction products. Cyano-substituted species or NPAHs may form in HCN-benzene and HCN-naphthalene reaction systems. The kinetic results show that the formation of cyano group dominates the HCN addition at high temperatures, which indicates that the Frenklach HACA route is more favorable in flame conditions. The branching ratios results show that HCN addition to benzene and naphthalene competes with C2H2 addition within the temperature range of 1000-2000 K. Besides, the yield of HCN reacting with naphthalene is considerable at low temperatures. The simulation results confirm the contribution of HCN addition to benzene by detecting the mole fraction of benzonitrile. Theoretical study and kinetic simulation identify the interaction between HCN and PAHs, blocking the formation of larger PAHs. These findings provided a deeper insight into the chemical effect of nitrogenous species on PAHs growth, which helps to explain the inhibitory effect of NH3 on soot formation.