Experimental and modeling study of selective ammonia oxidation on multi-functional washcoated monolith catalysts

• Mixed and dual-layer monoliths were compared at various feed compositions.
• Mixed catalyst performs better at low temperature (150–300 °C) with lower N2O yield.
• Dual layer catalyst performs better at T > 300 °C giving lower NOx yield.
• 1+1 D monolith model combine reaction kinetics on Pt catalyst and Fe–ZSM-5 catalyst.
• Model confirms the experimentally observed NH3 conversion and product selectivity.

An experimental and modeling study of the selective oxidation of ammonia to N2 on washcoated multi-functional Pt/Al2O3 + Fe–ZSM-5 monolith catalysts is reported. The catalysts are compared for a range of temperature, space velocity, and feed composition in the presence of H2O and CO2. The addition of the Fe–ZSM-5 is effective in converting the NOx produced by the poorly selective Pt/γ-Al2O3 catalyst. In the absence of feed NO, the dual-layer catalyst has a high selectivity to N2 due to selective reaction in the Fe–zeolite layer between NH3 from the bulk and counter-diffusing NOx formed in the underlying Pt/Al2O3. For T < 300 °C the mixed Fe–ZSM-5/Pt/Al2O3 catalyst has a lower N2O yield than the dual-layer Fe–ZSM-5/Pt/Al2O3 catalyst. This is attributed to NO migration and reduction on the Fe–ZSM-5. The mixed washcoat gives a lower N2 yield than the dual-layer due to Pt-catalyzed NH3 oxidation to NOx, which diffuses to the gas phase before its reduction. The mixed catalyst is less selective to N2 because the Pt catalyst in proximity to the SCR catalyst leads to the oxidation of NH3 back to NOx. Most of the trends are predicted by a 1+1 D reactor model that includes the Pt/Al2O3 ammonia oxidation and Fe–ZSM-5 SCR kinetics and the relevant transport processes. Kinetic parameters are estimated from single catalytic component experiments while the effective diffusivities in the layers are estimated from the simulation of multi-layer experiments in which inert top layers of γ-Al2O3 and Na–ZSM-5 were deposited onto an underlying Pt/Al2O3 layer. With the help of the model, explanations of the trends are provided.

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