Numerical Modeling Study of Catalyst Surface Reactivity and Gas Diffusivity with Lean NO
Catalyst simulation, which can analyze the complicated reaction pathway of exhaust gas purifications and identify the rate-determining step, is an essential tool in the development of catalyst materials. This requires an elementary reaction model which describes the detailed processes, i.e. adsorption, decomposition, and others. In our previous work, the elementary reaction model on Pt/CeO2 catalyst was constructed. In this study, we focused on extending the Zeolite catalyst and including the gas diffusivity through the catalyst layer.
The reaction rate of a Zeolite catalyst was expressed by an Arrhenius equation, and the elementary reaction model was composed of 17 reactions. Each Arrhenius parameter was optimized by the catalytic activity measurements. The constructed model was validated with NOx conversion in cyclic experiments which were repeated with Lean phase (NOx adsorption) and Rich phase (NOx reduction). We were able to obtain good agreement between calculated and measured values.
The reaction model was extended to the honeycomb catalyst with two layers, Pt/CeO2 as the bottom layer and Zeolite as the top layer. The honeycomb catalyst performance was determined by gas diffusivity through the Zeolite into the Pt/CeO2, in addition to reactivity. For the Zeolite layer, the effective diffusion constant was expressed by Fick's law, and bulk diffusivity was estimated from the difference of reactivity with or without Zeolite. In some Zeolites, the calculation results showed that Beta (BEA) Zeolite had the highest NH3-SCR activity and gas diffusivity, and almost matched the experimental results. We were able to obtain an accurate elementary reaction model.