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1. The Molecular Composition of Soot

   https://doi.org/10.1002/anie.201914115  

   Jacobson, Rachelle S.; Korte, Andrew R.; Vertes, Akos; Miller, J. Houston (January 2020, Angewandte Chemie International Edition)

   Abstract Soot (sometimes referred to as black carbon) is produced when hydrocarbon fuels are burned. Our hypothesis is that polynuclear aromatic hydrocarbon (PAH) molecules are the dominant component of soot, with individual PAH molecules forming ordered stacks that agglomerate into primary particles (PP). Here we show that the PAH composition of soot can be exactly determined and spatially resolved by low‐fluence laser desorption ionization, coupled with high‐resolution mass spectrometry imaging. This analysis revealed that PAHs of 239–838 Da, containing few oxygenated species, comprise the soot observed in an ethylene diffusion flame. As informed by chemical graph theory (CGT), the vast majority of species observed in the sampled particulate matter may be described as benzenoids, consisting of only fused 6‐membered rings. Within that limit, there is clear evidence for the presence of radical PAH in the particulate samples. Further, for benzenoid structures the observed empirical formulae limit the observed isomers to those which are nearly circular with high aromatic conjugation lengths for a given aromatic ring count. These results stand in contrast to recent reports that suggest higher aliphatic composition of primary particles. 

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2. Soot-based Global Pathway Analysis: Soot formation and evolution at elevated pressures in co-flow diffusion flames

   https://doi.org/10.1016/j.combustflame.2021.01.007  

   Zhou, Dezhi; Yang, Suo (May 2021, Combustion and Flame)
3. Detailed Study of the Formation of Soot Precursors and Soot in Highly Controlled Ethylene(/Toluene) Counterflow Diffusion Flames

   https://doi.org/10.1021/acs.jpca.2c06538  

   Gleason, Kevin; Gomez, Alessandro (January 2023, The Journal of Physical Chemistry A)

   We perform spatially resolved measurements of temperature, gaseous species up to three-ring Polycyclic Aromatic Hydrocarbons (PAHs), and soot in atmospheric pressure counterflow diffusion flames. First, we characterize fully a baseline ethylene flame and then a toluene-seeded flame in which an aliquot of ethylene in the feed stream is replaced with 3500 ppm of prevaporized toluene. The goal is twofold: to investigate the impact of a common reference fuel component of surrogates of transportation fuels and bypass the main bottleneck to soot formation from aliphatic fuels, that is, the formation of the first aromatic ring. The composition of the fuel and oxidizer streams are adjusted to maintain a constant stoichiometric mixture fraction and global strain rate, thereby ensuring invariance of the temperature–time history in the comparison between the two flames and decoupling the chemical effects of the fuel substitution from other factors. Major combustion products and critical radicals are fixed by the baseline flame, and profiles of critical C2–C5 species precursors to aromatic formation are invariant in both flames. On the other hand, doping with toluene boosts the aromatic content and soot volume fraction, increasing the mole fraction of benzenoid structures and soot volume fraction by a factor of 2 or 3, relative to the baseline ethylene flame. This finding is consistent with the expectation that the formation of the first aromatic ring is no longer a bottleneck to soot formation in the doped flame. In addition, toluene bypasses completely benzene formation, opening a radical recombination pathway to soot precursors through the production of C14H14 (via dimerization of benzyl radical) and pyrene (through dimerization of indenyl radical). 

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4. Discrete element method model of soot aggregates

   https://doi.org/10.1103/PhysRevE.110.054902  

   Demidov, Egor V; Gor, Gennady Y; Khalizov, Alexei F (November 2024, Physical Review E)
5. Small aromatic hydrocarbons control the onset of soot nucleation

   https://doi.org/10.1016/j.combustflame.2020.08.029  

   Gleason, Kevin; Carbone, Francesco; Sumner, Andrew J.; Drollette, Brian D.; Plata, Desiree L.; Gomez, Alessandro (January 2021, Combustion and Flame)

   The gas-to-particle transition is a critical and hitherto poorly understood aspect in carbonaceous soot particle formation. Polycyclic Aromatic Hydrocarbons (PAHs) are key precursors of the solid phase, but their role has not been assessed quantitatively probably because, even if analytical techniques to quantify them are well developed, the challenge to adapt them to flame environments are longstanding. Here, we present simultaneous measurements of forty-eight gaseous species through gas capillary-sampling followed by chemical analysis and of particle properties by optical techniques. Taken together, they enabled us to follow quantitatively the transition from parent fuel molecule to PAHs and, eventually, soot. Importantly, the approach resolved spatially the structure of flames even in the presence of steep gradients and, in turn, allowed us to follow the molecular growth process in unprecedented detail. Noteworthy is the adaptation to a flame environment of a novel technique based on trapping semi-volatile compounds in a filter, followed by off-line extraction and preconcentration for quantitative chemical analyses of species at mole fractions as low as parts per billion. The technique allowed for the quantitation of PAHs containing up to 6 aromatic rings. The principal finding is that only one- and two-ring aromatic compounds can account for soot nucleation, and thus provide the rate-limiting step in the reactions leading to soot. This finding impacts the fundamental understanding of soot formation and eases the modeling of soot nucleation by narrowing the precursors that must be predicted accurately. 

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