Quantification of nanostructure changes by HRTEM and fringe analyses during NO₂-O₂ oxidation

Development of the regeneration process on diesel particulate filters (DPF) requires a better understanding of soot oxidation phenomena, especially its relation to soot nanostructure. Nitrogen dioxide (NO₂) is known to play an essential role in passive regeneration by partially oxidizing soot at low temperatures, in the presence of oxygen (O₂) in the exhaust. However, change in soot nanostructure due to oxidation by NO₂-O₂ mixtures has not received much attention. Highresolution transmission electron microscopy (HRTEM) reveals nanostructure of model carbons and changes during oxidation. This work focuses on passive regeneration of the DPF by oxidation of soot at normal exhaust gas temperatures (300-400°C). HRTEM imaging of partially oxidized model carbons (R250, M1300, arc-generated soot, graphitic carbon onion, and flame soot) under NO₂-O₂ mixtures is done to investigate physical changes in nanostructure to test for correlation with the oxidation behavior. Each model carbon presents a specific type of nanostructure-As identified in varied diesel soots. Fringe analysis of HRTEM images of carbon nanostructure is performed to reveal the difference in the structural metrics of fringe lengths, curvature, spacing and tortuosity of the resultant structures. The variation in oxidation rates of different samples highlights the dependence of the oxidation on the nanostructure. These results are compared with the oxidation and nanostructure of a diesel soot.