N2O Formation during NH3-SCR over Different Zeolite Frameworks: Effect of Framework Structure, Copper Species, and Water

The formation characteristics of N2O were investigated with respect to copper-functionalized zeolites, i.e., Cu/SSZ-13 (CHA), Cu/ZSM-5 (MFI), and Cu/BEA (BEA) and compared with the corresponding zeolites in the H form as references to elucidate the effect of the framework structure, copper addition, and water. Temperature-programmed reduction with hydrogen showed that the CHA framework has a higher concentration of Cu2+ (Z2Cu) compared to MFI and BEA. The characterizations and catalyst activity results highlight that CHA has a framework structure that favors high formation of ammonium nitrate (AN) in comparison with MFI and BEA. Moreover, AN formation and decomposition were found to be promoted in the presence of Cu species. On the contrary, lower N2O formation was observed from Cu/CHA during standard and fast SCR reactions, which is proposed to be due to highly stabilized AN inside the zeolite cages. On the other hand, significant amounts of N2O were released during heating due to decomposition of AN, implying pros and cons of AN stability for Cu/CHA with possible uncontrolled N2O formation during transient conditions. Additionally, important effects of water were found, where water hinders AN formation and increases the selectivity for decomposition to NO2 instead of N2O. Thus, less available AN forming N2O was observed in the presence of water. This was also observed in fast SCR conditions where all Cu/zeolites exhibited lower continuous N2O formation in the presence of water.