Widening the ammonia selective catalytic reduction window over copper-based SSZ-39 zeolites: Synergy of copper(II)...

Ammonia selective catalytic reduction (NH₃-SCR) is currently constrained by a trade-off between sluggish low-temperature Cu⁺ reoxidation and high-temperature parasitic NH₃ oxidation. We hypothesized that a dual-strategy-integrating zeolite morphology engineering with precise active site construction-could maximize isolated Cu²⁺ sites while suppressing CuO aggregation. This design aims to electronically couple these species to balance the reduction and oxidation half-cycles, thereby widening the operational temperature window.

Copper-based SSZ-39 catalysts were synthesized via a heat-enhanced impregnation method on small-sized zeolite crystals with reduced thickness. Subsequent catalytic evaluations encompassed both the primary NH₃-SCR and competitive ammonia oxidation (NH₃-SCO) reactions to systematically decouple and map the temperature-dependent reaction pathways. To elucidate structure-activity relationships, a suite of characterization techniques, combined with density functional theory (DFT) calculations, was employed to probe the distribution, electronic structure, and synergistic reaction mechanism of the distinct copper species.

The optimized Cu_4%-S39-N-HEI catalyst demonstrated exceptional performance, achieving >90 % NO_x conversion at 200 °C and maintaining robust activity (> 80 %) at 500 °C, coupled with minimal N₂O emissions (< 10 ppm). Mechanistic studies revealed a critical synergy: isolated Cu²⁺ sites function as high-affinity NH₃ anchors (E_ad = -2.68 eV), preventing high-temperature oxidation, while proximal CuO nanoclusters facilitate oxygen activation for low-temperature reoxidation. Theoretical calculations confirmed that electronic coupling between these species narrows the bandgap (2.51 → 2.39 eV), harmonizing the redox cycles and establishing a robust paradigm for wide-temperature efficient deNO_x catalysts.