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

Catalytic oxidation is a promising purification technique for ammonia (NH 3 ) 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–CeO 2 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 Cu 2+ exchange sites. In situ IR spectroscopy and isotopic ( 18 O 2 ) transient response experiments indicate that more active Cu 2+ in Cu–Ce/SAR20 provides sufficient Lewis acidic sites for NH 3 adsorption and favors the stability of Si–OH–Al structures (Brønsted acid sites). NH 3 adsorbed at Lewis acidic sites tends to form peroxide byproducts (NO x ), while the NH 4 + adsorbed at Brønsted acidic sites generates the key intermediate NH 4 NO 2, which decomposes to N 2 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. Z 2 Cu 2+ 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 Cu 2+, where the sufficient isolated Al provided in SAR20 pulls Cu 2+ into the eight-membered ring (8MR), allowing it to come into contact with NH 3 more readily.