(292b) Modeling and Simulation of Multilayer NSR/SCR Monolithic Catalyst

NO_x storage and reduction (NSR) and selective catalytic reduction (SCR) are the two commercialized exhaust aftertreatment technologies used for the abatement of NO_x emission from the lean burn gasoline and diesel engines. The combination of NSR and SCR offers an opportunity to integrate the functionalities of both technologies and in so doing overcome the shortcomings associated with the individual systems. The use of integrated NSR/SCR technology can lower precious metal usage in lean NO_x traps and eliminate the need for a separate NH₃ system to provide the reductant in the SCR. Substantial work has been done in dual-bed LNT/SCR system while only few studies have appeared in the literature focusing on the dual-layer architecture [1]. Two of the several advantages afforded by the dual-layer design are its compactness and the proximal yet segregated ammonia generation and consumption catalytic functionalities.

In this study, we build on our recent experience in modeling the lean NO_x trap [2] and in experimentally studying [1] the dual-layer LNT/SCR catalyst.  We employ a spatially 2D mathematical model of the catalytic monolith together with global kinetics to study the complex spatiotemporal phenomena occurring in the LNT/SCR. The global kinetic models for both LNT and SCR reactions provide more efficient computation yet capture the main reaction pathways and associated kinetics.  Various transient experimental methods (such as TPD, TPO, cycling with and without reductants, cycling under different reacting conditions etc.) were used to develop a robust kinetic model for Pt/Rh/BaO/Al₂O₃ LNT catalyst. NH₃ oxidation is found to have a detrimental effect on the overall performance of dual layer catalyst [1]. Thus, the adsorption of NH₃ on the LNT catalyst during the rich phase and subsequent oxidation during the lean phase are included in the LNT kinetic model. Similarly, we use recently developed kinetic model for Cu-ZSM5 catalyst for the SCR reactions.

Our simulation results show significant reduction of NH₃ selectivity in the dual-layer catalyst (SCR is layered on top of LNT). However, the NO_x conversion may be slightly reduced by the addition of the SCR layer in the absence of H₂O and CO₂ at 300^oC, consistent with the experimental results [1]. The SCR activity (NO_x reduction) is lower than LNT activity (NO_x storage) at 300^oC. Therefore, the reduction in NO_x conversion can be partly attributed to the diffusional barrier caused by the additional SCR layer. The performance of the combined LNT/SCR system can be improved by using low Pt dispersion LNT catalyst which is found to yield higher NH₃ selectivity over a wide range of temperature [2, 3].    

Various multilayer configurations and the effects of operating parameters (temperature, cycle times etc) were studied by means of numerical simulations. These simulation results along with comparisons to experimental results will be presented.

References:

[1]       Y. Liu, M.P. Harold and D. Luss, Applied Catalysis B: Environmental.

[2]       B.M. Shakya, M.P. Harold and V. Balakotaiah, Catalysis Today, 184 (2012) 27.

[3]       R.D. Clayton, M.P. Harold, V. Balakotaiah and C.Z. Wan, Applied Catalysis B-Environmental, 90 (2009) 662.