PAH structure analysis of soot in a non-premixed flame using high-resolution transmission electron microscopy and optical band gap analysis

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 characterized by in-situ visible light extinction and thermophoretically-sampled high-resolution transmission electron microscopy measurements, HRTEM. The fuels studied were heptane, toluene and their iso-volumetric mixture (H50T50), given their relevance as surrogate fuels. Extinction measurements were used to calculate the soot volume fraction, Fv, and determine the optical band gap (OBG) as a function of flame position. The OBG was derived from the near-edge absorption feature using Tauc/Davis-Mott analysis. A direct band gap (r = 0.5) was selected for this analysis assuming that the electronic properties of soot are dominated by the molecular structure of the PAHs. For the HRTEM analysis, soot samples were collected at different locations in the flame using thermophoretic sampling and a fast-insertion technique. The images were then analyzed using a ‘lattice-fringe’ algorithm, to determine important parameters such as the fringe length. Polycyclic aromatic hydrocarbon (PAH) sizes were estimated from conjugation lengths obtained from OBG measurements and fringe lengths from HRTEM measurements. Across all studied flames, the peak Fv ranged from 3.4 ppm in the heptane flame to 17.6 ppm in the toluene flame. Despite this wide range, the average OBG across the different flames only varied from 2.05 eV in the H50T50 to 2.10 eV in the toluene flames, which is consistent with molecule lengths of between 0.96 nm and 0.99 nm. Lattice fringe analysis yielded slightly lower average fringe lengths between 0.91 and 0.99 nm throughout the different flames. Results of in-situ and ex-situ characterization of soot suggests that flames of different fuel nature and sooting stage undergo the transition from chemical to physical growth at a similar size, about the size of circumpyrene.