Catalytic oxidation of ammonia: A pre-occupied-anchoring-site strategy for enlarging Ag nanoparticles at low Ag loading and achieving enhanced activity and selectivity on Ag-CuOx/Al₂O₃ catalyst

The Ag nanoparticles (Ag-NPs) in Ag/Al2O3 catalysts play a crucial role in the selective catalytic oxidation of NH3 (NH3-SCO). To enhance NH3-SCO activity, Cu, which has stronger anchoring ability than Ag, is introduced onto Al2O3, reducing available anchoring sites for Ag. As Ag cannot displace anchored Cu species, Ag species agglomerate into larger Ag-NPs even with low Ag loading. Consequently, these enlarged Ag-NPs become more active centers for NH3-SCO. The optimal Ag:Cu molar ratio is confirmed as 2:3. This 'pre-occupied-anchoring-site' strategy decreases Ag loading to 1/5 of the original, reducing catalyst costs while maintaining activity. In situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) studies reveal that NH3-SCO on 2Ag1.8Cu/Al (weight ratio) catalyst follows the hydrazine mechanism below 200 degrees C, coexisting with the imide mechanism from 200-250 degrees C, and solely the imide mechanism beyond 250 degrees C. This strategy is applicable to various transition metals, including Mn, Co, Ni, and Fe, promoting cost-effective Ag-NPs formation.