Alessandro Faccinetto

Détection ultra-sensible d'hydrocarbures aromatiques polycycliques adsorbés à la surface des suies par désorption laser/ionisation laser/spectrométrie de masse à temps de vol : une approche pour étudier les processus de croissance des suies dans les flammes: Présent dès les premiers instants de la combustion, les Hydrocarbures Aromatiques Polycycliques (HAP) jouent un rôle prépondérant dans la formation des suies avant d’être adsorbés par les particules ainsi formées. Les HAP influencent ainsi la quantité, la composition et la morphologie des suies émises pendant la combustion. L'étude de l'évolution de la phase adsorbée de ces particules au sein de la flamme doit permettre de mieux comprendre leur mécanisme de formation ainsi que leur impact sur la santé et l’environnement. Une expérience originale dédiée à une telle étude et basée sur le couplage des techniques de Désorption Laser, Ionisation Laser et Spectrométrie de Masse a été développée pendant cette thèse. Des suies collectées dans des flammes sont irradiées par un laser à 532 nm permettant ainsi la libération des HAP adsorbés. Les HAP sont ionisés par un deuxième laser à 266 nm avant d'être finalement analysés dans un spectromètre de masse à temps de vol. L’interprétation des spectres ainsi obtenus a fait l’objet d’une analyse très approfondie afin de caractériser les étapes de désorption et d’ionisation. Les analyses, réalisées dans différents types de flammes permettent d’identifier les HAP présents à la surface des suies à différents stades de leur formation, l’étendue des spectres variant fortement dans la zone de nucléation des suies. Afin de distinguer les HAP gazeux de ceux adsorbés, une nouvelle méthode de collecte basée sur le dépôt des suies et des HAP gazeux sur des substrats différents a été développée. Par ce biais, l’implication de réactions hétérogènes entre les HAP gazeux et les suies dans la croissance de ces dernières a pu être mise en évidence.

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.

Rationale: This study has developed a data processing protocol based on mass defect analysis for the automatic construction of unique peak lists addressing the need for the fast and efficient treatment of databases of mass spectra with limited mass resolution. Methods: The data processing protocol, implemented in MATLAB, is tested on a database of 126 mass spectra obtained from time‐of‐flight secondary ion mass spectrometry analysis of the exhaust of a laboratory diesel miniCAST burner deposited on Ti substrates. Results: The data processing protocol converts the mass spectra into a data matrix suitable for chemometrics (peak list) by combining mass defect analysis and multivariate analysis. In particular, the role of the mass defect analysis is expanded to improve mass calibration and automate the construction of the peak list. Conclusions: In this context, mass defect analysis becomes an invaluable technique for the efficient processing of databases of mass spectra with limited mass resolution by allowing the fast and automated construction of a peak list common to all mass spectra, by improving the mass calibration, and finally by reducing the number of molecular formulae consistent with a given accurate mass, thus facilitating the identification of unknown ions.

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.