Numerical Modeling Study of Detailed Gas Diffusivity into Catalyst Washcoat for Lean NOx Catalyst 2019-01-0993
Lean burn technology is an approach for increasing the fuel efficiency of internal combustion engines. The technology often employs NOx Storage Catalysts(NSC) in contrast to three-way catalysts. However, since NSCs require thicker washcoats, there are concerns that mass transfer limitations will result in insufficient purification performance for a given mass of catalyst. To increase the performance, it is necessary to control the pore properties of the washcoat, including particle diameter, macro and meso pore diameters, and others. In this study, an elementary reaction model was combined with a pseudo-2D diffusion model to describe both the reaction kinetics and mass transfer. The calibrated model was utilized to design pore parameters in order to increase the performance.
Kinetic rate constants of the NSC reaction model were described by Arrhenius equations and the gas diffusion equations of macro and meso pores were described by a pseudo-2D model. The kinetic model was composed of 24 reactions. Arrhenius parameters were optimized by catalytic activity measurements from 6 catalysts with different pore properties. Good agreement was achieved between the measured and calculated values.
Using the calibrated reaction model combined with the pseudo-2D diffusion model, a sensitivity analysis for NOx concentration was performed to identify key parameters of the NSC. In summary, parameters with the highest sensitivity were the loading amount and particle diameter of NOx storage material, CeO2, and washcoat thickness. Consequently, to increase the NOx storage performance, these parameters should be increased. With these changes, the number of NOx storage sites will be increase, and the gaseous NOx will easily move to the catalyst surface and into the storage site of meso pore. Therefore, it will lead to better performance. This prediction is supported by previous experimental reports, demonstrating the constructed model can be utilized for the design of pore parameters of catalyst washcoats.