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.

A membrane filter was coated on conventional filter made of SiC. The first and second layers consisted of aluminaparticles with diameter of 0.5 and 1.5 micrometer, respectively; because small particles have higher filtration efficiency to prevent soot penetration into the membrane layer while large particles with a higher permeability can achieve lower pressure drop. Nanoscale silverpalladium particles were dispersedly deposited on the surface of the alumina particles layer as a top layer. The novel membrane filter demonstrated lower pressure drop while keeping high trapping efficiency during the soot trapping and presented lower soot oxidation temperature compared to the conventional filter.

<div class="section abstract"><div class="htmlview paragraph">Development of the diesel particulate filter (DPF) aims to attain fast oxidation of accumulated soot at low temperature. Numerous researchers have explored the characteristics of soot oxidation under ambient conditions of simulated exhaust gas using thermogravimetric analysis or a flow reactor. In this study, temperature programmed oxidation (TPO) experiments were carried out for soot entrapped in miniaturized DPFs, cut-out from practical particulate filters, yielding wall-flow features typically encountered in real-world DPFs. Furthermore, when using the miniaturized samples, highly accurate lab-scale measurements and investigations can be facilitated. Examining different temperature ramping rates used for the TPO experiments, we propose a rate of 10°C/min as the most effective in analyzing soot oxidation in the practical filter substrates. Then, wash-coated catalyzed filters (CDPFs) were benchmarked with bare-type DPFs to clarify their effects on soot oxidation in a practical wall-flow system. According to the Arrhenius expression, differences in soot cake thickness in CDPFs reflect various values of estimated activation energy. This is due to the soot-catalyst proximity. With presence of 450 ppm nitrogen dioxide (NO₂) in a reactant gas mixture, the soot oxidation range was extent to a lower temperature. Moreover, a reduction in the estimated activation energy was achieved, even in the case of using bare-type DPFs. The thick soot cake layers in bare-type DPF result in a significant amount of soot mass remaining after treatment at 600°C, a typical active regeneration temperature. Subsequently, soot residuals were traced and characterized after a complete active regeneration process. For these reasons, thickness of a soot cake layer was proposed to be a new factor to define an updated regeneration strategy.</div></div>

The agglomeration and growth of ash during regeneration of a diesel particulate filter was investigated through time-lapse visualization utilizing a field-emission scanning electron microscope and energy dispersive spectroscopy. The primary and large scale ash particles contain calcium, zinc, phosphorous and sulfur were dispersed in Particle Matters (PMs) cake layer. During the shrinkage of PMs Cake layer in regeneration, these ash components were gathered together to form various types of large agglomeration, as well as aggregation ash particles. They were nearly uniform deposited around the perimeters of surface pores.