Technical Report 57, c4e-Preprint Series, Cambridge

Authors: Markus Sander, Abhijeet Raj, Oliver R. Inderwildi, Markus Kraft*, Sven Kureti, and Henning Bockhorn

Reference: Technical Report 57, c4e-Preprint Series, Cambridge, 2008

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In this study the non-catalytic interaction between soot and nitric oxide (NO) is investigated. Recent studies demonstrate that the decomposition of nitric oxide at radical carbon sites 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 (DFT), Transition State Theory and a kinetic Monte-Carlo simulation (kMC) at 560°C on a radical zigzag soot surface. The results are validated against experimental observations. The DFT calculations have been performed with DMoL3 utilizing the HCTH exchange functional. A mechanism for the conversion of NO to N₂ on a zigzag 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 vibrational frequencies, geometry, energy, spin population and bond population were computed in these DFT calculations to evaluate the forward and backward reaction rate of each intermediate reaction applying Transition State Theory. A kinetic Monte-Carlo simulation employing 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. The amount of trapped NO 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 for the first time able to describe the conversion of soot in presence of NO accurately.

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