No abstract

Soot particles formed in a system of non-premixed liquid fuel flames supported on a wick-fed, smoke point test burner (ASTM D1322-08) were characterised by in-situ visible light extinction and thermophoretically-sampled high-resolution transmission electron microscopy measurements, HRTEM.

Visible light extinction was measured in a series of nitrogen-diluted, ethylene/air, non-premixed flames and this data was used to determine the optical band gap, OBG, as a function of flame position. Collimated light from a supercontinuum source is telescopically expanded and refocused to match the f- number of a dispersing monochromator. The dispersed light is split into a power metering channel and a channel that is periscoped and focused into the flame. The transmitted light is then recollimated and focussed onto a silicon photodiode detector. After tomographic reconstruction of the radial extinction field, the OBG was derived from the near-edge absorption feature using Tauc/Davis-Mott analysis. A slight evolution in OBG was observed throughout all flame systems with a consistent range of OBG observed between approximately 1.85 eV and 2.35 eV. Averaging over all positions the mean OBG was approximately 2.09 eV for all flame systems. Comparing these results to previously published computational results relating calculated HOMO-LUMO gaps for a variety of D2h PAH molecules to the number of aromatic rings in the structure, showed that the observed optical band gap is consistent with a PAH of about 14 rings or a conjugation length of 0.97 nm. This work provides experimental support to the model of soot formation where the transition from chemical to physical growth starts at a modest molecular size; about the size of circumpyrene.

A technique for linking the physical composition of polynuclear aromatic hydrocarbon (PAH) stacks and clusters to their electronic properties is reported. Kohn-Sham HOMO-LUMO gaps are reported for a series of monomers, stacks, and clusters of six, high-symmetry PAHs (pyrene, coronene, ovalene, circumpyrene, circumcoronene, and circumovalene) generated by DFT calculations with the 6-31G* basis set and a B3YLP exchange correlation functional in NWChem. A previously published, atom-pair minimization algorithm was used to optimize the geometries of the PAH stacks and clusters. HOMO-LUMO gaps decrease with an increase in monomer size; homogeneous stacks and clusters indicate substantial lowering of the HOMO-LUMO gap because of agglomeration effects with the formation of dimers and formation of clusters (two or more stacks) being the most dominant contributions. Heteromolecular particulates had HOMO-LUMO gaps that were strongly influenced by the larger components in the system. The HOMO-LUMO gaps of homogeneous clusters approached a maximum agglomeration effect because of the localization of electronic interactions among adjacent stacks. Previously published, experimentally determined optical band gaps (OBG) from Tauc/Davis-Mott analysis of extinction spectra in various laminar, non-premixed flames had an average OBG of 2.1eV. Based on the computations presented here, this work suggests that clusters with this OBG are comprised of modest molecular size PAH, about the size of ovalene.