Effects of the Si/Al Ratio on the Improvement of SO₂ Tolerance of the Cu-SSZ-16 Catalyst for NH₃–SCR at Low Temperatures

Sulfur poisoning is an inevitable challenge for Cu-based zeolite catalysts on diesel vehicles due to the residual sulfur in diesel fuel. In this work, a high-silica Cu-SSZ-16-6.7 (6.7 is the Si/Al ratio as measured by inductively coupled plasma (ICP)) catalyst was examined for sulfur poisoning and desulfation to assess its application potential. For comparison, a conventional Al-rich Cu-SSZ-16-3.0 (a Si/Al ratio of 3.0 by ICP) catalyst was also tested. Through comparing these two Cu-SSZ-16 catalysts with varying Si/Al ratios, it was found that the generation of H2SO4 and Cu-sulfate with relatively low decomposition temperatures was the main reason for the deactivation of the sulfated Cu-SSZ-16-6.7 catalyst, while these sulfate species could be decomposed by desulfation. Furthermore, [Cu(OH)]+-Z species in the Cu-SSZ-16-6.7 catalyst were more sensitive to SO2 than the Cu2+-2Z species. However, the [Cu(OH)]+-Z species could be fully recovered after desulfation, and the low-temperature activity was recovered completely to equal that of the fresh sample, which cannot be achieved for the Cu-SSZ-16-3.0 catalyst and most other Cu-based small-pore zeolites. Therefore, the Cu-SSZ-16-6.7 catalyst with an increased Si/Al ratio, which shows significant improvements in SO2 tolerance in the NH3–SCR reaction, can be a qualified candidate for the SCR catalyst on diesel vehicles.