Effective reduction of nitric oxide over a core–shell Cu-SAPO-34@Fe-MOR zeolite catalyst

Abstract

In this study, a core–shell catalyst of Cu-SAPO-34@Fe-MOR was successfully prepared through a silica-sol adhesion method, and its performance for selective catalytic reduction of nitric oxide by NH₃ (NH₃-SCR) was evaluated in detail. The Fe-MOR coating has not only increased the high-temperature activity and broadened the reaction temperature window of Cu-SAPO-34 to a large extent, but also increased the hydrothermal stability of Cu-SAPO-34 markedly. It is demonstrated that a strong synergistic interaction effect exists between Cu²⁺ and Fe³⁺ ions and promotes the redox cycle and oxidation–reduction ability of copper ions, which greatly accelerates the catalytic performance of the core–shell Cu-SAPO-34@Fe-MOR catalyst. Abundant isolated Cu²⁺ ions and Fe³⁺ ions on the ion exchange sites performing NO_x reduction at low and high temperature region lead to the broad reaction temperature window of Cu-SAPO-34@Fe-MOR. In addition, more weakly adsorbed NO_x species formed and the increased number of Lewis acid sites may also contribute to the higher catalytic performance of Cu-SAPO-34@Fe-MOR. On the other hand, the better hydrothermal ageing stability of Cu-SAPO-34@Fe-MOR is related to its lighter structural collapse, fewer acidic sites lost, more active components (Cu²⁺ and Fe³⁺) maintained, and more monodentate nitrate species formed in the core–shell catalyst after hydrothermal ageing. Last, the mechanism study has found that both Langmuir–Hinshelwood (“L–H”) and Eley–Rideal (“E–R”) mechanisms play an essential role in the catalytic process of Cu-SAPO-34@Fe-MOR, and constitute another reason for its higher activity compared with that of Cu-SAPO-34 (only “L–H” mechanism).