The kinetics of oxidation of Diesel soots by NO2

The kinetics of oxidation of three soots, from a Diesel engine fuelled by either Ultra-Low Sulphur Diesel (ULSD) or biodiesel, by NO2 have been measured in a packed bed at various temperatures (300–550 °C) and [NO2] (20–880 ppm) relevant to regenerating a Diesel Particulate Filter. Adsorbed hydrocarbons and oxygen accounted for a significant fraction (∼20% by mass) of the otherwise carbonaceous material. After pre-treatment (heating up to 550 °C in a flow of pure Ar and holding the temperature at 550 °C for 1 h) to ensure consistency between samples, they were subsequently burned at a fixed temperature in a flow of NO2 + Ar. For this, a balance on oxygen atoms entering and leaving the packed bed showed that during oxidation in NO2 any oxygen remaining in a soot after pre-treatment was not rapidly liberated as CO or CO2. A mass balance on the element nitrogen demonstrated that no N2 or N2O was formed below 550 °C; mass balances on carbon and oxygen demonstrated that all the carbon ended up as CO or CO2 and below 550 °C the nitrogen yielded only NO. The oxidation of soot in NO2 was found to be first-order with respect to NO2. Also, the soot derived from biodiesel was more reactive than soot from ULSD; nevertheless, the apparent activation energies for oxidation by NO2 were the same (70 ± 18 kJ mol−1) for each carbon. When the distribution of diameters of the individual spherules of soot was taken into account, it was not possible to tell whether there was internal burning of porous spherules or, on the other hand, non-porous, solid spherules were burning on their exteriors.

► Rates of oxidation by NO2 of Diesel soot were measured: 300–550 °C, [NO2] 20–880 ppm.
► Adsorbed hydrocarbon and oxygen formed a significant fraction (∼20 wt%) of the soot.
► Soots were pre-treated at 550 °C in pure Ar before reaction with NO2/Ar mixtures.
► Sole gaseous products <550 °C were NO, CO and CO2: rate was 1st order in [NO2].
► Rates for soot from different fuels varied, but values of E were equal ∼70 kJ mol−1.