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

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 degrees 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), N-2-adsorption, inductively coupled plasma-auger electron spectroscopy (ICPAES), Al-27 and Si-29 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.