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The Role of Polynuclear Aromatic Hydrocarbon in Soot Formation: A Theoretical and Experimental Study Open Access

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In the combustion fuels under rich conditions, some fraction of the fuel carbon is converted into particulate carbon. This carbonization process most often leads to "soot", a form of amorphous carbon characterized by small (10 - 30 nm) primary particles, with both crystalline and amorphous domains, and aggregated into fractal structures. While our understanding of early chemistry of soot formation as well as the aggregated structure and oxidation of large particles is fairly complete, little is known about the mechanism in which the 2-D polynuclear aromatic hydrocarbons (PAH) form 3-D structures.There are multiple theories of soot formation however, the majority revolve around aggregation of small, planar PAH occurring through non-bonded, pi-electron interactions or continual growth of a single molecule that may invoke aliphatic linkages as well as both curved and planar PAH. Even though the theories have been supported by a large number of theoretical calculations, none are unequivocally supported by experimental evidence. In this work, results of both theory and experiment are presented to explore the idea that the most critical step in the transition from two- to three-dimensional structures is the agglomeration of PAH of modest molecular size. Atom-pair calculations of intermolecular potentials for homo-molecular dimers of several PAH were conducted and it was found that binding energies rise rapidly with molecular size, asymptotically approaching the experimental exfoliation energy for graphite (5.02 kJ/mol/C). Experimentally, Raman spectroscopy is employed to analyze thermophoretically sampled flame particulate. Using the two most dominant features in the Raman spectrum of carbon particles, the "G" band, near 1580 cm^-1 and a series of bands due to defects collectively known as the "D" bands with the first, "D1", appearing in the upper 1300 cm^-1 range, exploration of the differences in the Raman spectra for samples collected in different flame regions will be discussed. In particular, the ratio between the intensities of the D and G band will be explored as indicators for graphitization. The analysis presented supports the theory that clusters of aromatic molecules averaging 1.0 - 1.2 nm in size are being formed in the flame and change little in size throughout the flame.

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