Improving the hydrothermal stability of Al-rich Cu-SSZ-13 zeolite via Pr-ion modification

Abstract

Al-rich (Si/Al = 4–6) Cu-SSZ-13 has been recognized as one of the potential catalysts to replace the commercial Cu-SSZ-13 (Si/Al = 10–12) towards ammonia-assisted selective catalytic reduction (NH₃-SCR). However, poor hydrothermal stability is a great obstacle for Al-rich zeolites to meet the catalytic applications containing water vapor. Herein, we demonstrate that the hydrothermal stability of Al-rich Cu-SSZ-13 can be dramatically enhanced via Pr-ion modification. Particularly, after high-temperature hydrothermal aging (HTA), CuPr_1.2-SSZ-13-HTA with an optimal Pr content of 1.2 wt% exhibits a T₈₀ (temperature window of NO conversion above 80%) window of 225–550 °C and a T₉₀ window of 250–350 °C. These values are superior to those of Cu-SSZ-13-HTA (225–450 °C for T₈₀ and no T₉₀ window). The results of X-ray diffraction Rietveld refinement, electron paramagnetic resonance (EPR) and spectral characterization reveal that Pr ions mainly located in the eight-membered rings (8MRs) in SSZ-13 zeolite can inhibit the generation of inactive CuO_x during hydrothermal aging. This finding is further supported by density functional theory (DFT) calculations, which suggest that the presence of Pr ions restrains the transformation from Cu²⁺ ions in 6MRs into CuO_x, resulting in enhanced hydrothermal stability. It is also noted that an excessive amount of Pr ions in Cu-SSZ-13 would result in the production of CuO_x that causes the decline of catalytic performance. The present work provides a promising strategy for creating a hydrothermally stable Cu-SSZ-13 zeolite catalyst by adding secondary metal ions.