Exploring the influence of chemical state of Cu species on CO-SCR performance in spinel-type CuM2O4 (M = Co, Mn, Fe, Ni, and Cr): The synergy between Cu2+and surface oxygen vacancy

The influence of the chemical state of Cu species in spinel-type catalysts on the selective catalytic reduction (SCR) of NO with CO has not been well understood. A series of CuM2O4 (M = Co, Mn, Fe, Ni, and Cr) were synthesized and studied in this work. Notably, CuCo2O4 demonstrated the best CO-SCR performance, exhibiting the de-NOx activity exceeding 90 % in a broad temperature window of 143-381 degrees C. Furthermore, CO conversion surpassed 90 % above 225 degrees C. Characterizations revealed that the synergy between Cu2+ and surface oxygen vacancy in the Cu2+-Ov-M8+ sites (8 represents the M cations in the lower valence states) played a vital role in enhancing the CO-SCR performance of the CuM2O4 catalysts. The superior CO-SCR performance of CuCo2O4 was also attributed to the optimal interplay between Cu and Co cations, resulting in superior Cu2+/Cu+ redox cycle and improved lattice oxygen mobility. In situ DRIFTS verified that Cu2+ was the primary site during the CO-SCR reaction, and two possible mechanisms (oxygen vacancy-assisted and lattice oxygen-assisted routes) over CuM2O4 were carefully proposed. This study unravels the nature of the CO-SCR activity of the CuM2O4 catalysts and provides novel perspectives on the development of Cu-based catalysts for CO-SCR applications.