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 NH₃ on the low-temperature (<100 °C) hydrothermal stability of SAPO-34 and Cu-SAPO-34 were investigated. NH₃ temperature-programmed desorption (NH₃-TPD), ¹H magic angle spinning nuclear magnetic resonance (MAS NMR), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were adopted to characterize the adsorption states of NH₃ and H₂O in SAPO-34, and the destruction of the SAPO-34 framework was revealed by direct and cross-polarization ²⁹Si, ²⁷Al, and ³¹P MAS NMR. Gaseous NH₃ coadsorbed with H₂O inside SAPO-34 micropores and induced the hydrolysis of framework P-O-Al and Si-O(H)-Al bonds. Weakly adsorbed NH₃ was released during aging and played a similar negative role to gaseous NH₃. When being combined with hydrolyzed Al species from the framework, active Cu ions transformed to inactive CuAl₂O₄-like species, leading to deactivation in low-temperature SCR of Cu-SAPO-34. Strongly adsorbed NH₄⁺ via 200 °C preadsorption protected the framework integrity of SAPO-34 and the SCR activity of Cu-SAPO-34.