Nanostructural and morphological characteristics of single soot aggregates during low-temperature oxidation

Abstract

In this paper the evolution of morphological and nanostructural properties of soot primary particles during low-temperature oxidation with oxygen is presented. Soot primary particles and aggregates from three soot samples with varying morphological and nanostructural initial properties were investigated by high-resolution transmission electron microscopy (HRTEM) at consecutive steps of oxidation. Soot samples were deposited on a TEM grid and then oxidized repetitively. After each oxidation step labelled identical aggregates were scanned and reanalyzed by HRTEM. Size distributions of primary particles and their nanostructure was tracked using an image analysis algorithm. The oxidation conditions (temperature 850 K, O2 concentration 5% by volume) correspond to those of the real application, e.g. for regeneration of particle filters.The tracking of different aggregates revealed that the morphological changes of the primary particles are determined by the predominant oxidation mode, which is governed by the nanostructure of the primary particles. Extended graphene layers favor internal oxidation, while bent, short and reactive layers favor both, external and internal oxidation. In surface oxidation mode, primary particles do not shrink homogeneously, but rather form a blackberry-like cluster of smallest nucleated particles.While a partly expansion of graphene layers is detected in the early stages of oxidation, the number density of graphene layers gradually decreases with increasing oxidation time. Additionally, graphene layers are oxidized by fragmentation until final degradation. Tracking of selected primary particles at different positions within the aggregate brought about that each primary particle is subjected to its own oxidation history. Therefore, each observed and analyzed primary particle possibly exhibits a different oxidation stage at the same oxidation time.