NO₂ Oxidation Reactivity and Burning Mode of Diesel Particulates

The NO₂ oxidation kinetics and burning mode for diesel particulate from light-duty and medium-duty engines fueled with either ultra low sulfur diesel or soy methyl ester biodiesel blends have been investigated and are shown to be significantly different from oxidation by O₂. Oxidation kinetics were measured using a flow-through packed bed microreactor for temperature programmed reactions and isothermal differential pulsed oxidation reactions. The burning mode was evaluated using the same reactor system for flowing BET specific surface area measurements and HR-TEM with fringe analysis to evaluate the nanostructure of the nascent and partially oxidized particulates. The low activation energy measured, specific surface area progression with extent of oxidation, HR-TEM images and difference plots of fringe length and tortuosity paint a consistent picture of higher reactivity for NO₂, which reacts indiscriminately immediately upon contact with the surface, leading to the Zone I or shrinking core type oxidation. In comparison, O₂ oxidation is shown to have relatively lower reactivity, preferentially attacking highly curved lamella, which are more reactive due to bond strain, and short lamella, which have a higher proportion of more reactive edge sites. This preferential oxidation leads to Zone II type oxidation, where solid phase diffusion of oxygen via pores contributes significantly to slowing the overall oxidation rate, by comparison.