Engineering CeZrOx-Cu/SSZ-13 coupled catalysts to synergistically enhance the low-temperature NH3-SCR activity

Abstract

Improving the low-temperature (<200 °C) NOx conversion of Cu/SSZ-13 NH3-SCR catalysts is crucial for achieving zero NOx emissions from diesel engines. This is primarily due to the limited oxidation capacity of active Cu species limits NOx conversion at low temperatures, particularly following the hydrothermal treatment at high temperatures (>650 °C) and high water vapor (10 vol.%). In this study, CeZrOx-Cu/SSZ-13 (CZ/CSZ) coupled catalysts were synthesized to construct additional active sites for NO oxidation on the CSZ surface, thus improving the low-temperature NOx conversion through synergistic interaction. The results reveal that when the CSZ and CZ mass ratio is 1:3, the NOx conversion at 150 °C is improved by 11% after hydrothermal treatment at 800 °C for 16 h. The coupled CZ oxidizes NO at low temperatures and generates abundant HONO species on CZ/3CSZ. These HONO species formed on CZ transfer to CSZ and rapidly react with BAS-NH3 to produce easily decomposable NH4NO2. This efficient synergistic mechanism circumvented the rate-limitation caused by Cu species, thus enhancing the low-temperature activity. Additionally, the coupled CZ obviously reduces the amount of H2O adsorption on CZ/3CSZ and effectively mitigates the damage of Si-OH-Al sites by H2O molecules during the hydrothermal treatment. The improved structural stability enables the maintenance of isolated Cu2+ and stable acid sites, ensuring the rapid response of synergistic reactions, ultimately resulting in a significantly enhanced in the low-temperature SCR activity even after the hydrothermal treatment. This work would provide a novel strategy for designing the next generation of commercial NH3-SCR catalysts.