Probing the influence of hydrogen cyanide on PAH chemistry

Hydrogen cyanide (HCN) is an important and highly toxic intermediate in coal, nitrogen-rich biomass, and ammonia combustion. It may play an important role in polycyclic aromatic hydrocarbons (PAH) and soot reduction, but the influence of HCN on PAH chemistry is unclear due to the lack of experimental data. With this motivation, the experimental and numerical investigations were performed in this work. Key intermediates were detected, identified, and quantified by the combination of Linear Trap Quadropole (LTQ) Velos Orbitrap mass spectrometer and gas chromatography-mass spectrometer in a jet-stirred reactor fueled by C₂H₂/HCN/N₂ in the temperature range of 800-1200 K. The results reveal that the formation of benzene increases with the addition of HCN, but the PAH formation decreases. PAH reduction can be attributed to the formation of N-containing PAH (NPAH) via HCN-PAH interaction reactions, which were investigated by quantum chemistry and the Rice–Ramsperger–Kassel–Marcus theory with solving the master equation (RRKM-ME). Reaction rate comparison suggests that the HCN addition pathways to NPAH compete with the C₂H₂ addition pathways to PAH. The product yields in the systems of 1-naphthyl radical + C₂H₂ and 1-naphthyl radical + HCN were evaluated (T=800-2500 K and p=0.1-100 atm). The results indicate that the increasement of a new aromatic ring via cyclization is difficult in the 1-naphthyl radical + HCN system due to the high energy barrier, and the growth of larger NPAH is limited since the saturated N atoms in the heterocycle rings and C[tbnd]N functional group inhibit further carbon addition.