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

Liu, Yaoyao, Guo, Lihong, Zhao, Dongyue, Li, Xingang, Gao, Zhongnan, Ding, Tong, Tian, Ye, Jiang, Zheng
RSC advances 2017 v.7 no.44 pp. 27405-27414
adsorption, aluminum oxide, catalysts, copper, cupric oxide, nitrates, nitrogen, nitrogen oxides, oxidation, potassium carbonate, temperature, thermal stability
Herein, we designed a new NOₓ 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 NOₓ storage and reduction performance. The NOₓ 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 NOₓ 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 NOₓ to form nitrates, but only parts of them participate in NOₓ storage on the CuO/K₂CO₃/Al₂O₃ catalyst. The MgAl₂O₄ support offers additional sites for NOₓ adsorption, while the formed nitrate on it shows low thermal stability. So, NOₓ 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.