Improved high-temperature hydrothermal stability of Cu-SSZ-13 by an ammonium hexafluorosilicate treatment

Abstract

The hydrothermal stability of selective catalytic reduction (SCR) catalysts is a critical factor in the design of an after-treatment system for diesel engine exhaust. In the present study, Cu-SSZ-13 catalysts were treated with ammonium hexafluorosilicate (AHFS) to improve their hydrothermal stability. Modified and unmodified Cu-SSZ-13 catalysts were then hydrothermally aged at 850 °C in a simulated exhaust gas containing water for 6 and 12 h. Their catalytic activities were tested in a fixed-bed reactor system. X-ray diffraction (XRD), transmission electron microscopy (TEM), N₂-adsorption, inductively coupled plasma-auger electron spectroscopy (ICPAES), ²⁷Al and ²⁹Si solid state nuclear magnetic resonance (NMR), vacuum Fourier transform infrared spectroscopy (FT-IR), and Raman spectroscopy were employed to elucidate the effects of AHFS treatment. The results showed that the modified Cu-SSZ-13 catalyst maintained its high SCR activity after hydrothermal aging whereas the unmodified Cu-SSZ-13 catalyst exhibited remarkably lower NO conversions at all temperatures tested. The results consistently indicated that the chabazite (CHA) structure of the modified Cu-SSZ-13 catalyst was preserved after hydrothermal aging whereas the CHA structure of unmodified Cu-SSZ-13 collapsed. The observed collapse of the CHA structure of the unmodified Cu-SSZ-13 catalyst was determined to be the result of dealumination of the Si(OSi)₂(OAl)₂ and Si(OSi)₃(OAl) structures during hydrothermal aging. Moreover, the copper ions were converted into CuO particles. However, AHFS treatment induced the Si(OSi)₂(OAl)₂ structure to transform into the Si(OSi)₃(OAl) and Si(OSi)₄ structures with better hydrothermal stabilities, which better maintained the CHA structure after high temperature hydrothermal aging. Meanwhile, the OH⁻ ions of the surface Si-OH groups could be exchanged with F⁻ ions to form a highly stable hydrophobic surface, which prevented steam from eroding the Cu-SSZ-13 catalyst.