Xavier Mercier

Detailed information on the chemical and physical properties of the precursors of incipient carbon nanoparticles (CNPs) in flame combustion provides clues on the reaction pathways for CNP formation and growth. Therefore, this data is needed by the modeling community who seeks to understand inception at a fundamental level. However, identifying and isolating the precursors in a reactive environment at high temperature remains a challenging task. The present work reports a multi-diagnostic approach to identify the molecular species involved in the inception of CNPs based on the comparative analysis of surface morphology (scanning electron microscopy), chemical composition (time of flight secondary ions mass spectrometry), chemical state (X-ray photoelectron spectroscopy), and structure (Raman spectroscopy) of samples extracted from a nitrogen-diluted ethylene laminar diffusion flame stabilized on a Yale burner. Statistical analysis enables the reduction of the pool of species to be considered by showing, for instance, that large polyaromatic molecules are not required for the CNP inception to occur. Several low m/z species are identified as likely candidates that are consistent with polycyclic aromatic hydrocarbons (PAHs) and their derivatives. Among them, this work stresses the importance of species slightly above the curve representing the maximally condensed aromatics and of non-benzenoid PAHs (containing 5-member aromatic rings for instance). This new experimental evidence reveals trends consistent with the “combined physical and chemical inception” group of hypotheses, according to which small clusters (typically dimers) of PAHs initially bound by physical forces are rapidly stabilized by the formation of C–C covalent bonds according to various postulated mechanisms (extended HACA, spin localization).

Understanding the molecular mechanisms governing soot inception is critical for developing accurate models of particulate formation in combustion systems. Polycyclic aromatic hydrocarbons (PAHs) are well established as key precursors/intermediates that participate to soot formation, but the growth reactions towards the particulate phase are still the subject of active debate. Among the proposed chemical mechanisms, aromatic resonance-stabilized radicals (RSRs) offer a promising alternative pathway, but their in situ detection in flames has proven challenging, limiting further mechanistic insight. Here, we report the direct observation and identification of several polycyclic aromatic RSRs, using a combination of synchrotron-based mass-selected vacuum ultraviolet photoelectron spectroscopy and quantum chemical calculations. The existence of both delocalized and localized π-radicals and their specific implication on the different stages of soot formation, via radical-driven PAH reaction mechanisms, is discussed in the context of combustion chemistry.

Carbonaceous particles are widespread in combustion, atmospheric, extraterrestrial, and nanomaterials environments. Resonance-stabilized radicals (RSRs) are commonly identified in fuel combustion and pyrolysis processes and play an essential role in carbonaceous particle formation. Despite their importance, comprehensive experimental and mechanistic understanding of particle inception through RSR reactions is lacking. This work investigated particle size distribution, chemical composition, and thermal behavior of soot particles generated by the flow reactor pyrolysis reactions of typical RSRs, in particular, 1-indenyl, 1-methylnaphthyl, and 2-methylnaphthyl radicals, and by the pyrolysis of hydrocarbons with a variety of structures. Particle size distributions show soot particles with mobility diameters in an incipient-particle range of 1.3 to 1.6 nm. Laser desorption/ionization mass spectrometry results suggest that soot products consist of much larger covalently bound clusters (CBCs) than those observed in the gas phase. Under our experimental conditions, the CBCs exhibit a phase transition for particles with calculated molecular diameters of around 1.5 nm. Evaporation experiments and thermogravimetric analysis of the soot products reveal distinct thermal characteristics for small and large CBCs. These results implicate CBCs as bridges between gas-phase species and soot particles. The present work provides a soot-inception mechanism called RSR clustering (RSRC) that is characterized by the reactive clustering of RSRs. The RSRC mechanism contrasts with conventional soot formation models that attribute soot inception primarily to the aggregation of large-size polycyclic aromatic hydrocarbons.

In order to tackle the climate emergency, it is imperative to advance cleaner technologies to reduce pollutant emission as soot particles. However, there is still a lack of complete understanding of the mechanisms responsible for their formation. In this work, we performed an investigation devoted to the study of persistent radicals potentially involved in the formation of soot particles, by continuous wave and pulsed electron paramagnetic resonance. This work provides experimental evidence of the presence in nascent soot of highly branched, resonance-stabilized aromatic radicals bearing aliphatic groups, linked together by short carbon chains, and reinforced by non-covalent π-π interactions. These radicals appear to be highly specific of nascent soot and quickly disappear with the increasing soot maturity. Their presence in nascent soot could represent an underestimated health risk factor in addition to the already well documented effect of the high specific surface and the presence of harmful adsorbates.