Improving Copper Active Site Speciation on Cu–Ce/SSZ-13 for Ammonia Oxidation via Si/Al Ratio Modulation

• Published in: ACS applied materials & interfaces 2024-05, Vol.16 (20), p.26088-26098
• Main Authors: Zhao, Yang, Yi, Xiaokun, Dou, Baojuan, Kang, Running, Bin, Feng
• Format: Article
• Language: English
• Subjects: Energy, Environmental, and Catalysis Applications
• ISSN: 1944-8244; 1944-8252
• DOI: 10.1021/acsami.4c01898

Catalytic oxidation is a promising purification technique for ammonia (NH3) emission. However, high ignition temperatures and NO x peroxide generation limit its effectiveness due to a lack of active sites. Herein, the effects of Si/Al ratio (SAR) modulation on the speciation of copper active sites and the reaction mechanism at different acidic sites were investigated by loading CuO–CeO2 onto SSZ-13 with different SARs (Cu–Ce/SAR15, 20, and 30). Among them, Cu–Ce/SAR20 exhibits the lowest induction temperature (T 20 = 180 °C) and the highest nitrogen selectivity (above 95%), attributing to a higher number of Cu2+ exchange sites. In situ IR spectroscopy and isotopic (18O2) transient response experiments indicate that more active Cu2+ in Cu–Ce/SAR20 provides sufficient Lewis acidic sites for NH3 adsorption and favors the stability of Si–OH–Al structures (Brønsted acid sites). NH3 adsorbed at Lewis acidic sites tends to form peroxide byproducts (NO x ), while the NH4 + adsorbed at Brønsted acidic sites generates the key intermediate NH4NO2, which decomposes to N2 at high temperatures, thus enhancing nitrogen selectivity. The whole process mainly follows the Mars–van Krevelen (M-K) mechanism, with the Langmuir–Hinshelwood (L-H) mechanism playing a supporting role. Z2Cu2+ coordinates with adjacent Al atoms within the six-membered ring (6MR) and undergoes a slight deformation at high temperatures, facilitating the migration of the lattice oxygen. SAR plays a crucial role in local environmental speciation of reactive Cu2+, where the sufficient isolated Al provided in SAR20 pulls Cu2+ into the eight-membered ring (8MR), allowing it to come into contact with NH3 more readily.