Influence of Si/Al ratio on NH3-SCR activity over Cu-exchanged SAPO-34

Abstract

Euro VII emission standards demand advanced deNO_x catalysts for lean-burn exhaust aftertreatment. Cu-SAPO-34 is a leading NH₃-SCR catalyst, yet its low-temperature activity and hydrothermal stability require improvement. This study establishes a quantitative structure–activity relationship linking framework Si/Al ratio to catalytic performance. SAPO-34 supports with varying Si/Al ratios (0.20–0.40) were synthesized, and corresponding Cu-SAPO-34 catalysts were prepared via ion exchange. Characterization (XRD, XRF, NH₃-TPD, H₂-TPR) revealed that optimal Si/Al ratio (0.361) yields highly dispersed Cu²⁺ ions and balanced Brønsted/Lewis acidity, achieving nearly 100% NO_x conversion at 235 °C and >90% NO_x conversion between 210–540 °C (550 ppm NO_x, 550 ppm NH₃, 6 vol% O₂, 10 vol% H₂O, GHSV = 120 000 h⁻¹). Critically, after severe hydrothermal aging (800 °C, 4 h, 10 vol% H₂O), this catalyst maintains >90% activity at 200–400 °C, demonstrating exceptional durability. Excessive silicon (Si/Al = 0.406) promotes Cu_xO_y cluster formation, degrading performance. These findings demonstrate that intermediate Si/Al ratios synergistically optimize activity and stability by balancing acidity, Cu dispersion, and structural integrity. This work provides actionable design principles for developing robust Cu-SAPO-34 catalysts meeting stringent Euro VII standards.