Evidence on the formation of dimers of polycyclic aromatic hydrocarbons in a laminar diffusion flame

Abstract

The role of polycyclic aromatic hydrocarbons (PAHs) in the formation of nascent soot particles in flames is well established and yet the detailed mechanisms are still not fully understood. Here we provide experimental evidence of the occurrence of dimerization of PAHs in the gas phase before soot formation in a laminar diffusion methane flame, supporting the hypothesis of stabilization of dimers through the formation of covalent bonds. The main findings of this work derive from the comparative chemical analysis of samples extracted from the gas to soot transition region of a laminar diffusion methane flame, and highlight two different groups of hydrocarbons that coexist in the same mass range, but show distinctly different behavior when processed with statistical analysis. In particular, the identified hydrocarbons are small-to-moderate size PAHs (first group) and their homo-and heterodimers stabilized by the formation of covalent bonds (second group).

Experimental study of the influence of hydrogen as a fuel additive on the formation of soot precursors and particles in atmospheric laminar premixed flames of methane

“…Although the mechanism of formation of nascent soot particles from PAHs precursors is still discussed in the community, a large number of studies [43,47,[64][65][66][67][68][69] mentioned that moderate sized PAHs as pyrene could play an important role in the nucleation process. More specifically, the formation of dimers of such PAHs has been suggested as a potential key step allowing the formation of nascent soot particles [47,[66][67][68][69][70][71][72][73]. However, because of the weak energy of the Van der Waals bonds characterizing dimers of PAHs, especially at flame temperature [74,75], this hypothesis has often been considered as a numerical tool for kinetic modelling rather than a real physical process actually involved in sooting flames.…”

Quantification of PAH concentrations in premixed turbulent flames crossing the soot inception limit

“…Tielens et al [19] and Mao et al [20] concluded that weak intermolecular interactions and van der Waals forces are responsible for bond formation arising from the majority of binary collisions between PAH radicals. However, experimental investigations can lead to a contrary conclusion for different flame conditions [21,22]. Furthermore, numerous theoretical studies suggest that closed-shell PAH dimers have very short lifetimes under flame conditions except for species much larger than normally observed in flames [23][24][25][26][27][28].…”

PAH Growth in Flames and Space: Formation of the Phenalenyl Radical

“…Polycyclic aromatic hydrocarbons (PAHs) are early intermediates in the soot formation process in both terrestrial [1][2][3] and interstellar environments [4][5][6][7] . Soot and PAHs formed during the incomplete combustion of fossil fuels are detrimental to human health due to their toxic and carcinogenic properties [8][9][10][11] .…”