The simultaneous reduction of nitric oxide and soot in emissions from diesel engines

Recent studies demonstrate that the decomposition of nitric oxide on a soot molecule forms surface nitrogen and oxygen. The surface nitrogen can be recombined to gaseous N₂ while the surface oxygen desorbs from the soot molecule as CO. This non-catalytic conversion of gaseous NO into N₂ is investigated using density functional theory, transition state theory and a kinetic Monte-Carlo (kMC) simulation. The results are validated against experiments. A mechanism for the conversion of NO to N₂ on a soot surface is explored. The geometries of the intermediate stable species as well as the transition states were optimized to identify the different reaction steps. The forward and backward reaction rate of each intermediate reaction is calculated applying transition state theory. A kMC simulation using the current rates and intermediate species demonstrates feasible mechanisms for the conversion of NO to N₂ on a soot surface. It is also suggested that a portion of NO is trapped on the soot surface and this increases during the reaction and blocks the active carbon sites inhibiting further reactions. By combining different theoretical techniques in a multi-scale model, we are able to describe the conversion of soot in the presence of NO accurately.