Tailoring Cu-based small-pore zeolites towards NH3-SCR for NOx abatement

Abstract

Selective catalytic reduction with ammonia (NH₃-SCR) stands as the most effective technology for mitigating nitrogen oxide (NO_x) emissions from diesel engines and industrial sources. Over the past few decades, Cu-based small-pore zeolites have emerged as leading catalysts for NH₃-SCR owing to their broad operational temperature window, exceptional N₂ selectivity, superior low-temperature activity, and robust hydrothermal stability. This review systematically provides the structural and mechanistic aspects governing the performance of Cu-based small-pore zeolites. First, it introduces the speciation of Cu active sites (e.g., Cu²⁺, [Cu(OH)]⁺, CuO_x oligomers, and CuO_x clusters), their introduction methods (ion exchange, one-pot synthesis, and impregnation methods), and their distinct roles in standard and fast SCR reaction mechanisms. Subsequently, the influence of zeolite topologies, including CHA, AEI, AFX, ERI, LTA, KFI, and intergrowth and cocrystallized structures, on catalytic performance is addressed. The impact of synthesis strategies (traditional hydrothermal synthesis, interzeolite transformation synthesis, solvent-free synthesis, and seed-assisted methods) on catalytic activity and framework stability is also described. Furthermore, this review discusses the dual roles of the framework Si/Al ratio: a lower framework Si/Al ratio enhances low-temperature activity by increasing Brønsted acid sites and active Cu loading, albeit at the expense of declined hydrothermal stability due to framework instability, while a higher framework Si/Al ratio improves hydrothermal stability by reducing dealumination susceptibility, though this comes with decreased low-temperature activity. The strategies to reconcile this trade-off, such as Al distribution optimization, defect passivation, secondary cation incorporation (e.g., Sm³⁺, Ce³⁺, and La³⁺), core–shell architecture design, and use of metal oxide composites, are comprehensively presented. This review also addresses chemical poisoning (sulfur, phosphorus, alkali metal, and hydrocarbon poisoning) and mitigation strategies, as well as N₂O formation and control. Finally, this review highlights key opportunities and persistent challenges for NH₃-SCR catalysts in meeting future emission standards (such as Euro 7, United States Environmental Protection Agency 2027) and addressing exhaust from carbon-neutral internal combustion engines, providing a future research direction in this field.