Silver confined on the outer surface of HZSM-5 support for NOx reduction by NH3 (NH3-SCR): decoding the influence of Ag particle size and Ag0/Ag+ ratio on the reaction performance

Adopting effective methods to identify clearly the active surface sites is crucial to engineer high-performance catalysts. To develop cost-effective and applicable NH 3 -SCR catalysts, the distribution states of Ag on HZSM-5 support have been identified by XRD and XPS extrapolation methods. As quantified, Ag dispersion on HZSM-5 has a threshold of 0.12 mmol Ag/100 m 2 . Above the threshold, bigger Ag 0 particles with better capability to activate O 2 and NO x molecules can be formed, thus achieving Ag/ZSM-5 catalysts with much better reaction performance below 300 °C. In contrast, below the threshold, Ag is mainly present in the form of highly dispersed nano-clusters/micro-crystallites, and partly as Ag + ions, thus being unfavorable to the reaction at low temperature. DFT calculation has testified that the aggregated Ag 0 particles with bigger sizes (>5 nm) have weaker interfacial interaction with the support, thus showing better capability to activate the reactant molecules. The optimal NH 3 -SCR performance is achieved on 8% Ag/ZSM-5 with a mean Ag size of 8.6 nm and a Ag 0 /Ag + ratio of 1.72, on which 100% NO conversion with a wide window from 250 to 400 °C can be achieved with H 2 assistance. In conclusion, both the Ag particle size and Ag 0 /Ag + ratio are the two main factors determining NH 3 -SCR performance on Ag/ZSM-5 catalysts.