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 °C, coexisting with the imide mechanism from 200–250 °C, and solely the imide mechanism beyond 250 °C. This strategy is applicable to various transition metals, including Mn, Co, Ni, and Fe, promoting cost-effective Ag_NPs formation.