Identifying the Factors Governing the Early-StageDegradation of Cu-Chabazite Zeolite for NH3-SCR

摘要

To understand the degradation mechanism of the copper-ion-exchanged SSZ-13 (Cu-SSZ-13) is of high significance for rationally designing a zeolitic catalyst for ammonia-selective catalytic reduction of NOx (NH3-SCR). In this work, we focused on an Al-rich Cu-SSZ-13 and studied its structural degradation under hydrothermal conditions through a set of characterization techniques, including in situ X-ray diffraction (XRD), pair distribution function analysis and transmission electron microscopy–energy dispersive X-ray analysis (TEM–EDX). The results indicated that the chabazite structure tends to contract in the c direction upon hydrothermal treatment and consequently leads to the collapse of the four-membered ring. Such a structure change then results in the movement of isolated Cu²⁺ species from the face of the double six-membered ring to its center, which damages the structure further. However, the larger rings (6MRs and 8MRs) partially remain during the structure degradation, which possibly explains that some of the isolated Cu²⁺ species are alive even when the XRD-detectable crystallinity completely loses. The particle-by-particle observations through TEM–EDXanalysis suggested that the occurrence of structural degradation differsremarkably from one individual particle to another. In general, particleswith smaller size, having a lower Si/Al ratio and a higher Cu/Al ratio,tend to degrade easily. These results offer a thorough understandingof the structural degradation of Cu-SSZ-13 from the microscopic pointof view and point out that the uniformity in composition and particlesize of the zeolites plays a critical role in the early-stage degradation.