Reactivity of Soot With Nitrogen Oxides in Exhaust Stream

Exhaust stream from automotives and trucks will be subjected to new and more stringent emissions standards. In the case of Diesel engine the lowering of carbon particulates emission (soot) is based on a filtration system in which trapped soot is periodically removed by controlled oxidation with O₂ at 550°C or above. It is known that in presence of NO₂ the carbon-O₂ reaction is significantly speeded-up. Since NO₂ may be formed in the gas stream by catalytical oxidation of NO present in the exhaust stream, it is thought that small amounts of NO₂ (500 ppm by vol) will promote the continuous oxidation of the deposited carbon particulates in the temperature range 200 - 500°C. The behaviour of the CRT^™ system is based on this principle.

Very little is known about the reactivity of soot with NO₂. Moreover, the effect of other species present in the exhaust stream like O₂ and H₂O is also not well established. The present study investigates the reactivity of carbon black particles having properties similar to soot with small amount of NO₂ (450 ppm by volume) in the gas phase. The rate of carbon consumption in presence of NO₂ becomes significant at 300°C. Moreover, the reactivity is significantly improved in the presence of O₂ which is not reacting alone with carbon at that temperature. The simultaneous presence of NO₂ and O₂ leads to an enhanced gasification rate of the carbon. The investigation of the reaction mechanism suggests the build-up of a transient NO₂ complex on the carbon surface which desorbs as NO and CO₂ or CO. Furthermore, it appears that the main effect of NO₂ is to promote the desorption rate below 400°C of oxygen complexes built-up on the carbon surface.