A comprehensive review of Cu speciation in Cu-CHA for NH3-SCR: EPR spectroscopy bridging synthesis–structure–performance relationships

Copper-substituted chabazite (Cu-CHA) zeolites have been extensively developed through various synthetic strategies aimed at improving their catalytic activity and stability for the ammonia-assisted selective catalytic reduction (NH 3 -SCR) of nitrogen oxides (NO x ). A comprehensive understanding of the synthesis–structure–performance relations is crucial for the rational design of more efficient catalysts. In this review, we present an overview of bottom-up and top-down synthetic approaches for Cu-CHA zeolites, with particular emphasis on the aqueous ion-exchange and one-pot hydrothermal synthetic methods. We demonstrate how Electron Paramagnetic Resonance (EPR) spectroscopy elucidates the copper species in two types of Cu-CHA (Cu-SSZ-13 and Cu-SAPO-34) synthesized by different methods, and also examines their hydrothermal stability, thereby correlating the relationships among catalyst synthesis, active site structure and catalytic performance. Cu-SSZ-13 catalysts synthesized via one-pot methods show superior copper dispersion and NH 3 -SCR activity but suffer from poor high-temperature hydrothermal stability due to uneven aluminum distribution and CuAlO x formation, which can be partially mitigated by post-treatments. In contrast, Cu-SAPO-34 catalysts exhibit excellent hydrothermal stability but are limited by suboptimal copper speciation and framework vulnerability at low temperatures, highlighting the need for tailored synthetic strategies that optimize both copper and framework structures. Notably, Cu 2+ coordinated with extra-framework Al and framework-associated Al contribute significantly to the detrimental effects on NH 3 -SCR activity and stability performance. By examining copper speciation under reaction conditions and considering the influence of different preparation methods, we aim to elucidate key relationships that can guide the development of highly efficient Cu-CHA catalysts. • Summarizes bottom-up and top-down Cu-CHA synthesis focusing on aqueous ion exchange and one-pot hydrothermal methods. • Elucidates Cu species in Cu-CHA from different syntheses for NH 3 -SCR by EPR spectroscopy and examines their hydrothermal stability. • Bridges the synthesis–structure–performance relationships of Cu-CHA catalysts in NH 3 -SCR using EPR techniques. • Highlights CuAlO x species and framework-associated Al greatly contribute to detrimental effects on NH 3 -SCR activity and stability.