Effect of nanostructure, porosity and extent of oxidation on soot reactivity

Oxidation evolutions of structure, porosity and reactivity properties of four model carbons (PU, S160, N330 and R250) with different initial reactivity were studied. Results showed that changes in the porosity and nanostructure properties instead of initial properties significantly affected the soot reactivity during the soot oxidation process. Two high-reactivity soot surrogate samples, i.e., PU and S160, initially presented turbostratic disordered crystallites, resulting in higher reactivity at the early oxidation stage. Oxidation mainly proceeded inwardly in a peeling fashion, and more disordered internal crystallites were exposed after oxidized, slightly increasing the reactivity of PU and S160. Two low reactivity soot surrogate samples, i.e., N330 and R250, initially exhibited the typical core-shell structure. The oxidation rates of N330 and R250 were lower than that of PU and S160 due to more ordered exterior crystallites and less surface oxygen content at the early oxidation stage. Once oxygen penetrated the particle core, many pores were generated. Next, the particle became hollow through internal burning of the more reactive internal carbon at the late oxidation stage. After 40% burnoff, these hollow structures promoted oxidation in both outward and inward directions. Therefore, oxidation rates of N330 and R250 significantly increased.