Nanostructure changes in diesel soot during NO₂–O₂ oxidation under diesel particulate filter-like conditions toward filter regeneration

Abstract

Development of the regeneration process on diesel particulate filters 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 oxidizing soot at low temperatures, especially 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. This work focuses on nanostructure evolution during passive regeneration of the diesel particulate filter by oxidation of soot at normal exhaust gas temperatures (300°C–400°C). High-resolution transmission electron microscopy of partially oxidized model carbons (R250, M1300, arc-generated soot) and diesel soot under NO2–O2 mixtures is used to investigate physical changes in nanostructure correlating with the material’s behavior during oxidation. Microscopy reveals the changing nanostructure of model carbons during oxidation while fringe analysis of the images points to the differences in the structural metrics of fringe length and tortuosity of the resultant structures. The variation in oxidation rates highlights the inter-dependence of the material’s reactivity with its structure. NO2 preferentially oxidizes edge-site carbon, promotes surface oxidation by altering the particle’s burning mode with increased overall reactivity of NO2+O2 resulting in inhibition of internal burning, typically observed by O2 at exhaust gas temperatures.

Nanostructure Quantification of Carbon Blacks

“…Offering representative nanostructure, many carbon blacks are also used as surrogates for soot in oxidation studies, activated carbon in absorption studies, etc. [24,[26][27][28][29][30][31][32]. An illustration of edge and basal plane sites is shown in Figure 11.…”

Reduction of Diesel Engine’s Particulate Matters using Retrofit CeO2 Diesel Oxidative Catalyst and Partial Flow Diesel Particulate Filter System

“…HC and CO are eliminated due to catalyst. During the process, NO will be oxidized to NO2, and it plays an essential role in soot regeneration in particulate filters [12,13,14,15].…”

“…Fig. (14) represents the smoke intensity of exhaust gas measured from the smoke meter. The result is like opacity percentage from exhaust gas analyzer; both are shown over 50% soot reduction with an after-treatment system.…”

Quantitative high-resolution transmission electron microscopy nanostructure analysis of soot oxidized in diesel spray flame periphery

“…32,35 Vander Wal and colleagues 36,37 have been developing an analytical methodology for these nanostructures and applying it to varieties of diesel soot sampled from exhaust or diesel particulate filter (DPF) in order to examine, for example, fuels and engine operation impacts on the nanostructures. [38][39][40][41][42][43] However, these studies are mainly focusing on the soot oxidation by oxygen at relatively low temperature corresponding to the regeneration of DPF, where more oxidized and less reactive nanostructures are often evidenced by increased size, reduced tortuosity, and smaller layer separation of carbon crystallites or fringes. Similar nanostructure analysis has also been applied to diesel in-flame soot 18,33 and in-cylinder soot.…”