Remarkable self-degradation of Cu/SAPO-34 selective catalytic reduction catalysts during storage at ambient conditions

Abstract

A model Cu/SAPO-34 SCR catalyst with all Cu species maintained as isolated Cu(II) ions is synthesized. Following lengthy storage on the shelf under ambient conditions, this catalyst completely and irreversibly deactivates upon any heat treatments above ∼100 °C. Via detailed characterizations with surface area/pore volume analysis, XRD, H2-TPR, and 27Al, 29Si and 31P solid-state NMR, as well as continuous wave and pulsed EPR studies, it is concluded that over the course of storage, the SCR active sites [Cu(OH)]+ and Brønsted acid sites ≡Si-O(H)-Al≡ are attacked by H2O molecules trapped in the SAPO-34 framework pores, and undergo hydrolysis to form copper hydroxide and terminal Al sites. Upon thermal treatment, these species interact with each other to form copper-aluminate-like species, leading to irreversible deactivation of this catalyst. This deactivation mechanism does not require or necessarily lead to extensive degradation (i.e., crystallinity loss) of the SAPO-34 support.