Effect of Cu species on N₂O formation in the NH₃-SCR reaction over Cu-SSZ-13

This work systematically investigates the effect of Cu/Al ratio on NO _x conversion, N₂O formation, NH₃ oxidation, and hydrothermal stability over Cu-SSZ-13 catalysts under simulated diesel-exhaust NH₃-SCR conditions. Multiple characterization techniques, including XRD, H₂-TPR, NH₃-TPD, UV-vis DRS, XPS, EPR, and in situ DRIFTS, were employed to clarify the relationship between Cu-species evolution, surface intermediate reactivity, and N₂O formation. The results show that the Cu/Al ratio strongly regulates the distribution of Z[Cu²⁺(OH)]⁺, Z₂Cu²⁺, and CuO _x -like species. An insufficient Cu loading limits the number of redox-active Cu species required for low-temperature SCR, whereas excessive Cu loading promotes the formation of CuO _x -like species and less selective Cu environments, leading to enhanced NH₃ oxidation and N₂O formation. After hydrothermal aging at 800 °C for 16 h, dealumination of the zeolite framework disrupts the coordination bonds between active Cu²⁺ and framework Al, driving the transformation to CuO _x . When the Cu/Al ratio exceeds 0.3, it further generates inert CuAlO _x that lack SCR activity and strongly promote N₂O formation. Since low-temperature N₂O formation is closely associated with the formation and decomposition of NH₄NO₃-related intermediates, these results indicate that N₂O suppression over Cu-SSZ-13 is not governed simply by Cu loading, but by maintaining an appropriate relative distribution of Cu species.