Relationships between copper speciation and Brønsted acidity evolution over Cu-SSZ-13 during hydrothermal aging

Abstract To investigate possible relationships between copper speciation and Bronsted acidity evolution during hydrothermal aging, a series of aged Cu-SSZ-13 catalysts were prepared by varying temperatusres from 550 to 850 °C. Besides selective catalytic reduction of NOx by NH3 (NH3-SCR), NH3 oxidation was also adopted as a probe reaction. The catalysts were characterized by H2 temperature-programmed reduction (H2-TPR), in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and magic angle spinning nuclear magnetic resonance (MAS NMR). Isolated Cu ions protect Si-O(H)-Al sites against hydrothermal cleavage, but aggregated Cu species are found to destroy the zeolite framework. Due to the instability of ZCuOH (Z represents negatively charged zeolite [Si-O-Al]- site) at 8-member ring sites, mild hydrothermal aging induces the migration of metastable [Cu(OH)]+ ions to unsaturated 6-member ring sites to form Z2Cu, and excess [Cu(OH)]+ ions start to aggregate at 700 °C. The formation of two types of aggregated Cu species, CuAlOx and CuOx, and their roles in SCR reaction were distinguished. The former one originates from the combination of Cu with extraframework Al and is inert, while the latter forms upon more severe aging and catalyzes over-oxidation of NH3 toward NO, which degrades high-temperature SCR activity.