Experimental and modeling studies of lean and rich exhaust conditions for selective catalytic reduction of nitrogen oxides with ammonia

To assist with SCR catalyst sizing and optimization of LNT-SCR aftertreatment systems, a predictive kinetic model is needed. In this dissertation, an iron based zeolite SCR catalyst has been used in a reactor bench to simulate cycling between lean rich exhaust conditions across the catalyst. Steady-state lean SCR kinetic mechanisms were studied with special attention given to quantifying the effects of oxygen on the lean SCR reactions. From these studies, a prominent reaction between NH3 and N2O under lean conditions has been discovered. Additionally it was found that kinetic rate forms from literature are inadequate due to their inability to capture the strong non-linear relationship between NOx conversion and the amount of stored NH3 on the catalyst. Due to the switch from lean to rich exhaust from the regeneration of the LNT, the effects of varying H2O and CO2 concentrations have been studied for the purposed lean SCR reactions. It was found that H 2O strongly affects the storage capacity of NH3 on the SCR catalyst. From these results an additional reaction has been proposed to capture the effect of H2O on NH3 storage. The affects of H 2, CO, and C3H6 were studied to understand their roles in rich exhaust conditions. Experimental results have allowed the water gas shift reaction, the oxidations of H2 and CO, and reactions between NO and H2, CO, or C3H6 to be disregarded. It was observed that H2, CO, and C3H6 have strong reactions with NO2 yielding the need to include a reaction for each reductant with NO2 in the kinetic mechanism. It was determined that these reactions with NO2 can inhibit NOx conversion on the SCR catalyst by affecting the NO to NOx ratio at the catalyst, which can be accounted for using NO2 reactions with CO, H 2, and C3H6. It was also observed that the oxidation of C3H6 will strongly increase the oxidation of NH 3 which have been accounted for in a global reaction. CO and H 2 storage on the catalyst is negligible while C3H6 has been found to store on the catalyst by a non-traditional method to determine the storage of C3H6.