Selective oxidation of NH3 in a Pt/Al2O3@Cu/ZSM-5 core-shell catalyst: Modeling and optimization

Abstract

Modeling and optimization is presented of the Pt/Al2O3@Cu/ZSM-5 core–shell (CS) catalyst introduced in our recent study in which we successfully demonstrated the synthesis and application of the CS catalyst as an Ammonia Slip Catalyst (ASC). The commercial ASC converts NH3 to N2 using a dual-layer architecture comprising a bottom oxidation catalyst layer (supported Pt) and a top selective catalytic reduction (SCR) of NOx layer (Fe- or Cu-exchanged zeolite). The Pt/Al2O3@Cu/ZSM-5 particle, which emulates the dual-layer architecture, was shown to have superior performance due in part to an enhanced Pt activity and dense zeolite shell. A 1 + 1 D heterogeneous fixed-bed reactor containing the core–shell catalyst is developed in this study to predict the CS catalyst performance data, advance the understanding, and enable optimization. Incorporating independent kinetic models for Pt-catalyzed NH3 oxidation and Cu/ZSM-5 catalyzed NOx reduction, the reactor model predicts all of the experimental trends. The Cu/ZSM-5 kinetics are tuned using independently measured data for NH3 uptake/desorption, NH3 and NO oxidation, and standard, fast, and NO2 SCR. The particle-scale model segregates the SCR reactions from the parallel process of intracrystalline diffusion of reacting species using independently-estimated species diffusivities from diffusion-limited NH3 oxidation on a Pt/Al2O3@Na/ZSM-5. The reactor model is validated for CS catalysts having a 0.5 and 1.2 μm thick Cu/ZSM-5 shell. The tuned model is used to determine the minimum Pt loading and Cu/ZSM-5 shell thickness that achieves prescribed NH3 conversion and N2 selectivity targets. A Pt loading as low as 0.02 wt% along with 0.83 μm thick Cu/ZSM-5 is predicted to give a NH3 conversion of 70% at 250°C and a N2 selectivity at 500°C of 90%.