Exploring the Improvement of NO _x Conversion and N ₂ Selectivity by Tandem Layer Cu/Fe–Zeolite Catalysts for NH ₃ –SCR

Abstract Achieving high NO x conversion and N 2 selectivity over a wide temperature range of NH 3 –SCR system is critical to meeting stringent NO x emission standards. Cu– and Fe–containing zeolites with different frameworks (CHA, MOR, beta) were prepared and evaluated for NH 3 –SCR performance in single–layer and tandem–layer configurations. The framework structure strongly influenced the activity by affecting metal dispersion, accessibility of active sites, and acidity. Among them, beta–based catalysts which possess a three–dimensional channel system formed by 12R pores showed the best overall performance. Cu–zeolites exhibited high low–temperature NO x conversion but led to higher N 2 O formation, while Fe–zeolites demonstrated lower N 2 O formation but lower NO x conversion at low temperatures. Importantly, this work reveals the underexplored potential of MOR–based catalysts for NH 3 –SCR. Tandem layer configurations using MOR catalysts (thin Fe–MOR layer followed by thick Cu–MOR layer) can suppress N 2 O formation with slight sacrifice in NO x conversion. This enhancement was attributed to the efficient utilization of NH 3 through the tandem layers. These findings suggest that MOR–based tandem catalysts represent a valuable direction for further investigation, offering a strategy to balance high NO x conversion and low N 2 O formation in practical NH 3 –SCR applications.