Towards clean exhaust, nanostructure changes in diesel soot during NO2–O2 oxidation

Diesel particulate matter is the particulate component of diesel exhaust, which includes diesel soot and aerosols such as ash particulates, metallic abrasion particles, sulfates, and silicates. Noteworthy studies have investigated and reported that release of these particulate matter to the atmosphere poses health related problems. Consequently, diesel engines are fitted with Diesel particulate filters (DPFs) whose efficient working is subject to periodic filter regeneration by oxidation of the accumulated soot. Ideally, this depends on the oxidative reactivity of soot which, in turn, is influenced by fuel formulation, combustion conditions and soot nanostructure. 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. Yet, largely lacking from relevant literature is the effect of change in soot nanostructure due to oxidation by NO₂–O₂ mixtures.

A thorough review of published studies reveals that significant insight into the reaction (NO₂–O₂) mechanism and its kinetics of oxidation are prevalent; regardless, there a need to understand the physical features underlying the reaction mechanism. To this end, researchers from The Pennsylvania State University, USA: Dr. Madhu Singh and Professor Randy Vander Wal in collaboration with Mek Srilomsak and Dr. Katsunori Hanamura at the Tokyo Institute of Technology and Dr. Yujun Wang at the Cummins, Incorporation proposed to investigate the nanostructure evolution during passive regeneration of the diesel particulate filter by oxidation of soot at normal exhaust gas temperatures (300 °C–400 °C). Their work is currently published in International Journal of Engine Research.

In their work, carbon blacks were used to model soot for the proposed analysis, along with soot from a diesel engine; so as to understand nanostructure evolution as it would take place in a DPF. The researchers analyzed the oxidative reactivity of a diesel soot collected from a DPF by subjecting it to oxidation under gas, oxidant staging and temperature conditions similar to those occurring during passive regeneration. Lastly, its reactivity to oxidation was analyzed qualitatively based on changes observed in its nanostructure using transmission electron microscope and quantified using fringe analysis of the transmission electron microscopy (TEM) images.

The researchers’ microscopy results revealed the changing nanostructure of model carbons during oxidation, while fringe analysis of the images pointed to the differences in the structural metrics of fringe length and tortuosity of the resultant structures. Moreover, the variation in oxidation rates highlighted the inter-dependence of the material’s reactivity with its structure. Specifically, NO₂ was seen to preferentially oxidize edge-site carbon and promote 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.

In summary, the study investigated three model carbons along with a diesel-engine generated soot for their reactivity toward oxidation by a gas mixture comprising NO₂ + O₂, under conditions typically encountered during passive regeneration of a DPF. Generally, the study revealed that NO₂ appeared to act as an accelerator, increasing the oxidative attack of edge-site carbon atoms but not basal plane sites. In a statement to Advances in Engineering, Professor Randy Vander Wal, the corresponding author highlighted that their study contributes to the foundation for optimizing current DPF soot regeneration strategies.