Role of radicals in carbon clustering and soot inception: A combined EPR and Raman spectroscopic study

Some aspects of soot formation from gas phase molecules at high temperatures are still unclear. Aromatic π-radicals may be key elements for a change in the model of carbon clustering and particle formation. However, experimental investigations are still needed on the radical nature of molecules and particles in flames and on their roles in the transition from molecules to incipient molecular clusters and their further evolution to mature soot. In this paper, we present electron paramagnetic resonance (EPR) and Raman spectroscopy measurements of particles collected in an ethylene-rich premixed flame at various residence times during nucleation. The experimental results, combined with measurement of the particle size distribution by a differential mobility analyzer, are used to investigate the role of radicals in particle nucleation and rearrangement of aromatic molecules in just-nucleated particles. For all the sampled particles, an EPR signal typical of persistent carbon-centered aromatic radicals is measured. An abrupt change in the EPR signal intensity, which is characteristic of stronger supramolecular interactions, is observed when the size distribution changes from monomodal to bimodal, as confirmed by Raman spectroscopy. Our experimental results indicate strong involvement of π-radicals during particle nucleation and growth. The paramagnetic/radical nature of the sampled particles is discussed on the basis of recent studies on the role of resonantly stabilized radicals in soot nucleation. The presence of localized π-electrons on the edge of aromatic soot constituents and their role in carbon clustering are also discussed.