Nucleation of soot: experimental assessment of the role of polycyclic aromatic hydrocarbon (PAH) dimers

Abstract

The irreversible dimerization of polycyclic aromatic hydrocarbons (PAHs) – typically pyrene (C16H10) dimerization – is widely used in combustion chemistry models to describe the soot particle inception step. This paper concerns itself with the detection and identification of dimers of flame-synthesized PAH radicals and closed-shell molecules and an experimental assessment of the role of these PAH dimers for the nucleation of soot. To this end, flame-generated species were extracted from an inverse co-flow flame of ethylene at atmospheric pressure and immediately diluted with excess nitrogen before the mixture was analyzed using flame-sampling tandem mass spectrometry with collision-induced fragmentation. Signal at m/z = 404.157 (C32H20) and m/z = 452.157 (C36H20) were detected and identified as dimers of closed-shell C16H10 and C18H10 monomers, respectively. A complex between a C13H9 radical and a C24H12 closed-shell PAH was observed at m/z = 465.164 (C37H21). However, a rigorous analysis of the flame-sampled mass spectra as a function of the dilution ratio, defined as the ratio of the flow rates of the diluent nitrogen to the sampled gases, indicates that the observed dimers are not flame-born, but are produced in the sampling line. In agreement with theoretical considerations, this paper provides experimental evidence that pyrene dimers cannot be a key intermediate in particle inception at elevated flame temperatures.

Experimental and numerical investigation of the transition from non sooting to sooting premixed n-butane flames, encompassing the nucleation flame conditions

“…In the reference parameters set, pyrene is used as the only precursor for nucleation and condensation. As mentioned in several studies [3,[89][90][91][92][93], pyrene dimerization is very unlikely to be the source of actual particles. However, soot inception is still not understood.…”

Phenalenyl growth reactions and implications for prenucleation chemistry of aromatics in flames

“…Summarizing, the pA2A biradical can grow an extra five-membered ring on its zig-zag edge by adding acetylene thus propagating the chain of p-radicals from pA2A to pA2AR5 via the HACA mechanism. It is pertinent to note that the 165 and 189 m/z signals have been commonly seen in mass spectra in numerous flame studies [64][65][66] and, in particular, they were hypothesized to correspond to pA2A to pA2AR5. 67 The rate constants for reaction (4) calculated at 1 atm are depicted in Fig.…”

Elucidating the polycyclic aromatic hydrocarbons involved in soot inception

“…Several obstacles hinder experimental research on soot inception. First, the process from the gas phase to particles is transitory, and it is challenging to pinpoint the actual transition point 20 , 32 – 35 . In addition, particles generated during and post sampling may cause significant perturbation to the experimental results.…”

“…In addition, particles generated during and post sampling may cause significant perturbation to the experimental results. For example, Adamson et al 35 , 36 detected pyrene dimers in an ethylene nonpremixed flame using a flame-sampling tandem mass spectrometer. However, these were generated in the sampling line.…”