Experimental and kinetic modeling study of NH3-SCR of NOx on Fe-ZSM-5, Cu-chabazite and combined Fe- and Cu-zeolite monolithic catalysts

A comprehensive experimental and modeling study of selective catalytic reduction of NOx with NH3 was carried out on Fe-ZSM-5 and Cu-chabazite (CHA) catalysts. The experiments reveal that Cu-CHA catalyst has a higher NH3 storage capacity and activity for NH3 oxidation and standard SCR compared to Fe-ZSM-5. The NOx reduction activity on the Fe-ZSM-5 catalyst was found to be strongly dependent on the NO2 feed fraction in contrast to Cu-CHA catalyst for which NOx conversion was much less sensitive to NO2. In the presence of excess NO2, both N2O and ammonium nitrate were produced on both catalysts although Fe-ZSM-5 catalyst had a higher selectivity towards these byproducts compared to Cu-CHA. For different feed conditions (NO2/NOx=0–1), Cu-CHA was a more active NOx reduction catalyst at lower temperatures (<350 °C) while Fe-ZSM-5 was more active at higher temperatures (>400 °C). Global kinetic models were developed to predict the main features of several SCR system reactions investigated experimentally. The models account for NH3 adsorption, NH3 oxidation, NO oxidation, standard SCR, fast SCR, NO2 SCR, ammonium nitrate formation and its decomposition to N2O, N2O decomposition and N2O reduction by NH3. The 1+1 dimensional reactor model accounts for potential washcoat diffusion limitations. The model accurately predicts the steady state NOx and NH3 conversions and the selectivity of the different products formed during these reactions. The model was used to predict the performance of standard and fast SCR reactions on combined systems of Fe- and Cu-zeolite monolithic catalysts which were found to have higher NOx conversion activity over a wider temperature range than with individual Fe- and Cu-zeolite catalysts as reported in our earlier study ( Metkar et al., 2012b). Among various configurations of the combined catalysts, either a single brick made up of a dual-layer catalyst with a thin Fe-zeolite layer on top of a thick Cu-zeolite layer or a sequential arrangement of short Fe-ZSM-5 brick followed by longer Cu-CHA brick resulted in high NOx removal efficiency over a wide temperature range of practical interest.

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► First experimental and kinetic modeling study of Cu-chabazite SCR catalyst.
► 1+1 dimensional monolith model combines multi-reaction global kinetics and mass transport.
► Model predicts conversion and selectivity trends for standard, fast, and NO2 SCR.
► Model confirms expanded high NOx conversion temperature window for combined Fe/Cu catalysts.