Soot Nanostructure: Definition, Quantification and Implications

Randy  Vander Wal, NCSER c/o NASA-Glenn

Soot is ordinarily considered as a carbonaceous material with environmental and health consequences highly dependent upon particle size. Though to-date unexplored, the nanostructure of the soot, i.e. the degree of atomic level order in carbon lamella comprising the soot primary particle can have profound consequences for soot reactivity and associated environmental and health effects. The talk will define soot nanostructure, describe its quantification by image analysis of high resolution transmission electron microscopy images and illustrate its impact on oxidation rate.

Differences in soot nanostructure based upon formation and growth conditions will be presented first. Fuel structure effects can be masked or accentuated depending upon both temperature and rate of increase. Low temperature yields an amorphous soot for all fuels studied here, regardless of flow rate. High temperature yields different results depending upon the rate of increase. A rapid increase in temperature, as realized by a high flow rate, emphasizes pyrolysis kinetics that favor polyaromatic hydrocarbons (PAHs) with 5-membered rings leading to soots with many shells and capsules; a highly curved nanostructure. Slower rates result in a different pyrolysis chemistry leading to graphitic soot, as characterized by long graphitic segments, oriented parallel to each other.

To quantify these differences in nanostructure, a lattice fringe analysis program has been developed to quantify the data conveyed by HRTEM images. The robustness of this program is demonstrated by using a series of carbon blacks possessing different levels of graphitic structure, prepared at different heat treatment temperatures. Its credibility is benchmarked against a traditional measure of graphitic structure, as provided by Raman analysis. Lattice fringe length is found to be monotonic with the level of graphitic structure as provided by the ratio of the integrated intensities of the G/D spectral peaks in the Raman spectra.

We further explore the relationship between soot nanostructure and reactivity towards oxidation by measuring the oxidation rates of laboratory synthesized soots with “model” nanostructures as governed by the synthesis conditions of temperature, time and initial fuel identity. Structural variations in the graphene layer plane dimensions necessarily alters the ratio of basal plane versus edge site carbon atoms. A corresponding variation in the overall reactivity, reflecting an average of the different reactivities associated with these specific atomic sites arises. This variation is illustrated here between a disordered soot derived from benzene and a graphitic soot derived from acetylene. Their oxidation rates differ by nearly 5-fold. Curvature of layer planes, as observed for an ethanol derived soot, is found to substantially increase oxidative reactivity. Relative to fringe length as a manifestation of graphitic structure, curvature more effectively increases reactivity towards oxidation. Larger variations in oxidation behavior may be expected, depending upon the soot synthesis conditions. Other physical properties may similarly be affected. Related implications due to differences in nanostructure will be discussed.

Download invited presentation: