Effect of nanostructure and BET surface area on the oxygen reactivity of soot filter cakes

The oxygen reactivity of propane flame soot in a filter cake was systematically investigated, considering the morphological properties of the particles. A scanning mobility particle sizer (SMPS), high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy were used to determine the initial aggregate size, the primary particle size, and the nanostructure of the soot particles. The samples were analyzed using an experimental setup, which enables oxidation and in-situ measurement of the BET surface, revealing its development as the reaction progresses. Particles with five different mobility diameters were generated using a combustion aerosol standard (CAST) soot generator. While the nanostructure of the particles was similar at four operating points, the particles with the smallest mobility diameter differed as they were more amorphous, which was consistently seen in HRTEM images and Raman spectra. The results showed that for the particles with similar nanostructure, the mobility diameter, the size of the primary particles and the initial surface area correlate. These particles have similar oxygen reactivity. The amorphous particles instead, showed a significantly higher reactivity, despite having the lowest specific surface area. The nanostructure appears to be decisive for the reactivity. Further the results clearly showed that surface area development affects the soot conversion profiles. Surface-normalized reaction kinetics for soot at each operation point were formulated.