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 (NH₃) emission. However, high ignition temperatures and NO 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 (T₂₀ = 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), 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.