Ex situ characterization of the precursors of incipient nanoparticles in a laminar diffusion flame of ethylene

Abstract

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).