Local Structures and Catalytic Ammonia Combustion Properties of Copper Oxides and Silver Supported on Aluminum Oxides

The local structures and catalytic NH₃ combustion properties of copper oxides (CuO_x) and silver (Ag) catalysts supported on aluminum oxides (Al₂O₃) were studied. In order to achieve high catalytic NH₃ combustion activity and high N₂ (low N₂O/NO) selectivity, the preparation conditions for impregnated binary catalysts were optimized. In comparison with the single CuO_x/Al₂O₃ and Ag/Al₂O₃, binary CuO_x–Ag supported on Al₂O₃ showed high performance for catalytic NH₃ combustion. Among the binary catalysts, sequentially impregnated CuO_x/Ag/Al₂O₃ exhibited the highest activity and N₂ selectivity. Because the combustion activity is closely associated with the Ag–Ag coordination number estimated from Ag K-edge XAFS, highly dispersed Ag nanoparticles supported on Al₂O₃ are considered to play a key role in the low-temperature light-off of NH₃. CuO_x/Ag/Al₂O₃ also showed higher N₂ (lower NO) selectivity for temperatures at which NH₃ conversion reached approximately 100%, indicating that the N₂ is directly produced from the NH₃ combustion reaction over CuO_x/Ag/Al₂O₃. Based on several analyses, a reaction mechanism for catalytic NH₃ combustion over CuO_x/Ag/Al₂O₃ was finally suggested.