The reduction of nitrogen oxides (NO and NO 2 ) in the exhaust of lean burn gasoline engines and diesel engines are a challenge with the main reason being the presence of excess oxygen. Starting from 1996, monolith lean NOx trap (LNT) catalysts containing both noble motel sites (usually Pt) and NOx adsorption sites (BaO or K 2 O) are used industrially for NOx abatement. This technology requires the engine run in cyclic mode between a long fuel lean phase and a short fuel rich phase. Under lean condition, NO oxidation and NO/NO 2 adsorption take place to form nitrites and nitrates on the catalyst. The surface NOx are subsequently reduced to N 2 by reductants under rich condition. In spite of the industrial success, detailed mechanisms of NOx storage and reduction are not fully understood and robust kinetic models are sparse in the literature. At the same time, experimental approaches in studying NOx traps face difficulties in interpreting the data due to the dynamics of the system and the coupling between simultaneous reactions happening on different sites.
In this work, a kinetic simulation by combining a reactor model (heat and mass balance) with global reaction mechanisms was carried out to better understand the performance of the Pt/BaO/Al2O3 catalyst. The following major assumptions were made: (1) radial gradients are negligible compared to axial gradients; (2) axial diffusion/conduction is negligible compared to convection, and (3) fully developed laminar flow along the monolith channel. The resulting time dependent differential equation set is in hyperbolic type, characterized by the absence of the second order derivatives. Finite difference method with upwinding scheme is found effective in solving the partial differential equations without oscillation. Numerical diffusive error caused by upwinding can be minimized by using finer grids and higher order schemes.
By solving the NO oxidation model, differences in spatial NO conversion curve were found between NO 2 inhibition and non-inhibition cases. First, with no NO 2 in the feed, the inhibition model predicts a fast build-up of NO 2 close to the inlet than non-inhibition models. Second, upon the addition of NO 2 in the inlet NO/O 2 mixture, the conversion of NO was decreased significantly while non-inhibition models are not affected. The above model predictions fitted well with experimental observations. An adsorption model assuming NO 2 disproportionation and direct NO surface reaction was used for NOx storage with NO 2 /O 2 and NO/O 2 as the inlet. The two time scale mechanism was found necessary to describe NO 2 adsorption on the 20wt% BaO catalyst. The model and parameters required to fit the NOx breakthrough curves suggest that CO 2 and H 2 O in the feed reduce the number of sites for NO adsorption. The rate constants for both fast and slow NO 2 uptake are decreased in the presence of CO 2 and H 2 O, but the total capacity remains the same. NO 2 inhibition in NO oxidation makes the interaction between NO oxidation and NO/NO 2 adsorption profound. Under reaction conditions, H 2 reduction of surface NOx is reductant supply limited with NH 3 as the reducing intermediate. The confined reduction front moving along the channel localizes the heat generation leading to a surface temperature in the reduction front about 35°C higher than the inlet gas temperature at our reaction conditions.
