A novel route for PAH growth in HACA based mechanisms

It is widely accepted that the hydrogen abstraction acetylene addition (HACA) mechanism accelerates the formation of benzenoid polycyclic aromatic hydrocarbons (PAHs), like pyrene, from naphthylene via phenanthrene and stops at the formation of cyclopentaring-fused products like acenaphthylene/pyracyclene. In this experimental study it was found that PAH growth initiated/carried by the addition of any C2Hx radical/neutral species can continue beyond acenaphthylene resulting in cyclopentaring-fused PAHs (CP-PAHs) like fluoranthene more preferably compared to the HACA pathways forming benzenoid-PAHs like pyrene. Similarly, production of naphthalene from benzene via HACA was also found to be less probable. On the other hand, hydrogen abstraction and vinyl radical addition (HAVA) pathways seem to be more promising for producing PAHs during aliphatic hydrocarbon pyrolysis, except polyacetylenes, at least up to moderate temperature (∼1300 K). These interesting outcomes were extracted from the gas-phase products of pyrolysis of acetylene and ethylene detected by using an in situ direct sampling vacuum ultraviolet (VUV) single photon ionization (SPI) time of flight mass spectrometry (TOFMS) technique. Small aliphatic products to large PAHs up to m/z = 324 were observed. Observed products mass spectra showed a remarkable sequence of mass peaks at regular intervals of mass number 24 in acetylene pyrolysis products and at intervals of mass number 26 in the case of ethylene pyrolysis products, indicating the role of particular corresponding radicals such as C2H and C2H3 in products formation. Inclusion of reaction pathways reported here in existing kinetic models will be helpful to better predict especially fuel rich combustion phenomena for which existing models are inadequate. It will also be useful for understanding carbonaceous dust formation in the interstellar atmosphere.