Destructive and Protective Effects of NH3 on the Low-Temperature Hydrothermal Stability of SAPO-34 and Cu-SAPO-34

The influences of gaseous, weakly adsorbed, and strongly adsorbed NH 3 on the low-temperature (<100 °C) hydrothermal stability of SAPO-34 and Cu-SAPO-34 were investigated. NH 3 temperature-programmed desorption (NH 3 -TPD), 1 H magic angle spinning nuclear magnetic resonance (MAS NMR), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were adopted to characterize the adsorption states of NH 3 and H 2 O in SAPO-34, and the destruction of the SAPO-34 framework was revealed by direct and cross-polarization 29 Si, 27 Al, and 31 P MAS NMR. Gaseous NH 3 coadsorbed with H 2 O inside SAPO-34 micropores and induced the hydrolysis of framework P–O–Al and Si–O(H)–Al bonds. Weakly adsorbed NH 3 was released during aging and played a similar negative role to gaseous NH 3 . When being combined with hydrolyzed Al species from the framework, active Cu ions transformed to inactive CuAl 2 O 4 -like species, leading to deactivation in low-temperature SCR of Cu-SAPO-34. Strongly adsorbed NH 4 + via 200 °C preadsorption protected the framework integrity of SAPO-34 and the SCR activity of Cu-SAPO-34.