Enhanced activity of CuO/K2CO3/MgAl2O4 catalyst for lean NOx storage and reduction at high temperatures

Abstract

Herein, we designed a new NO_x storage and reduction CuO/K₂CO₃/MgAl₂O₄ catalyst operating within the high temperature region of 350–550 °C. Compared with the Al₂O₃ supported catalyst with the same Cu and K loading, it exhibits superior NO_x storage and reduction performance. The NO_x reduction percentage (NRP) of the CuO/K₂CO₃/MgAl₂O₄ catalyst remains above 90% over a wide temperature range (400–550 °C), and reaches the highest NRP of 99.9% at 450 °C with the N₂ selectivity of 99.7%. Uncovered CuO particles with better reducibility exist on the CuO/K₂CO₃/MgAl₂O₄ catalyst, with the high NO_x oxidation and reduction ability above 400 °C. Potassium carbonates on the CuO/K₂CO₃/MgAl₂O₄ catalyst mainly exist in three forms, including free ionic carbonate, bridging bidentate carbonate and chelating bidentate carbonate. Under lean-burn conditions, most of carbonates on the CuO/K₂CO₃/MgAl₂O₄ catalyst can store NO_x to form nitrates, but only parts of them participate in NO_x storage on the CuO/K₂CO₃/Al₂O₃ catalyst. The MgAl₂O₄ support offers additional sites for NO_x adsorption, while the formed nitrate on it shows low thermal stability. So, NO_x is mainly stored on K₂CO₃ at high temperatures, because MgAl₂O₄ can enhance the thermal stability of the supported K₂CO₃ on it. Our results show that the thermal stability of K₂CO₃ directly determines the thermal stability of the formed nitrates. Accordingly, the CuO/K₂CO₃/MgAl₂O₄ catalyst shows the high NSR activity because of the efficient redox ability of CuO and high thermal stability of K₂CO₃ at high operating temperatures.