Reactive polycyclic aromatic hydrocarbon dimerization drives soot nucleation

Abstract

Nucleation is an important, yet poorly understood step in soot formation. Here, the importance of reactive PAH dimerization in reducing soot nucleation reversibility is investigated by simulating soot formation in a so-called "nucleation" flame (P. Desgroux et al., Combust. Flame, 2017, 184, 153-166). There, inception of soot particles is prolonged at minimal subsequent growth. With only reversible PAH dimerization, the simulated soot concentration is negligible. Accounting however for PAH chemical bond formation after physical dimerization, stabilizes dimers by covalent bonds and increases the soot concentration by four orders of magnitude, in good agreement with Laser Induced Incandescence measurements. In particular, dimers of benzene with benzene, phenylacetylene, naphthalene, toluene, acenaphthylene and cyclopentapyrene make significant contributions to the total soot concentration. The abundance of dimers with small PAHs highlights the dominant role of PAH concentration over their size and dispersion forces on dimer formation. Higher collision factors are used for irreversible dimerization models using larger PAHs because of their lower concentrations and not their larger dispersion forces leading to reduced reversibility and more stable dimers. The qualitative trend of main peaks agrees well with stochastic simulations and aerosol mass spectra measured in the above "nucleation" as well as premixed flames highlighting the abundance of PAHs with five-membered rings and substituted aliphatic chains in incipient soot. The predicted number of trimers is very low, i.e. less than 3% of the total soot nuclei formed, indicating that covalently bonded PAH dimers can be the main contributors to soot nucleation.

Dimers of polycyclic aromatic hydrocarbons: the missing pieces in the soot formation process

“…Moreover, we show that the contributions of several dimers are necessary to properly reproduce the experimental spectra. These results consequently support the hypothesis of kinetic rather than thermodynamic control frequently used in emerging soot formation mechanisms 15,[19][20][21] . For instance Eaves et al 15,19 introduced in their models the concept of the reversibility of the nucleation, implying that nucleation may not be governed by equilibrium processes as already suggested by the pioneering work of Dobbins et al 51 .…”

“…Aubagnac-Karkar et al 20 used the hypotheses of the reversibility of the nucleation and pyrene dimerization to successfully reproduce soot volume fraction profiles experimentally measured in five premixed flames characterized by different equivalence ratio, fuels and pressure conditions leading to a variation of seven orders of magnitude between the numerical results and the experimentally obtained soot volume fractions. More recently, Kholghy et al 21 used the dimerization of small to moderate-sized PAHs in their kinetic model, that also considers the reversible formation of PAH clusters by van der Waals forces. In particular, they advocate dimer stabilization through the formation of covalent carbon-carbon bonds following two hydrogen abstractions from the monomers in a dimer.…”

Unveiling trends in soot nucleation and growth: When secondary ion mass spectrometry meets statistical analysis

“…However, this step of molecular clustering could be a crucial one for understanding the transition from the gas to the condensed phase, and therefore each chemical insight we can bring has paramount importance. To this purpose, the role of PAH dimerization in the formation of NSPs is worth of particular attention being a currently much debated topic [82,83,33]. The formation of Van der Waals dimers in flames is indeed questionable because of the very weak bond energies, which do not theoretically enable them to survive at the flame temperature.…”

“…The formation of Van der Waals dimers in flames is indeed questionable because of the very weak bond energies, which do not theoretically enable them to survive at the flame temperature. However, different authors regard as physically realistic the formation of dimers in flames [5,83,84]. For instance, by using a semi-empirical molecular dynamic approach, Schuetz et al [84] showed that dimers of pyrene can be stabilized by the development of free internal rotors in the colliding PAHs.…”

“…For instance, by using a semi-empirical molecular dynamic approach, Schuetz et al [84] showed that dimers of pyrene can be stabilized by the development of free internal rotors in the colliding PAHs. More recently, Kholghy et al [83] proposed a model in which soot nucleation is not completely reversible and involves a stabilization pathway of the dimers by the formation of covalent bonds through a sequence of hydrogen abstraction and carbon-carbon bond formation. In this last work, which is also based on data obtained from flame PRE-N, the authors conclude that only the formation of dimers of small PAHs can provide soot quantities comparable to the available measurements.…”