Low-Cost CuX Catalyst from Blast Furnace Slag Waste for Low-Temperature NH₃-SCR: Nature of Cu Active Sites and Influence of SO₂/H₂O

A series of Cu-exchanged zeolite NaX (derived from blast furnace slag) catalysts were synthesized by an ion-exchanged method. The low-temperature catalytic activity and SO₂or/and H₂O resistance of the catalysts were studied as well. The results found that the Cu(N)X catalyst exhibited excellent low-temperature selective catalytic reduction (SCR) activity, whose NO conversion was nearly 100% at ca. 175 °C and remained stable, while the Cu(C)X catalyst showed the poorest catalytic performance. When SO₂and H₂O was introduced, the NO conversion of the Cu(N)X catalyst declined from 100% to 60%, and it could recover to ca. 63% after cutting off SO₂and H₂O. More isolated Cu²⁺species and surface chemisorbed oxygen species on the Cu(N)X catalyst could provide more active sites for SCR reaction, and more surface oxygen vacancies accelerated the oxidation of absorbed NH₃species and NO species. Besides, the higher surface acidity and redox ability on the Cu(N)X catalyst promoted the SCR reaction performance. In addition, after the poisoning of SO₂/H₂O, the surface acidity decreased, redox ability became poor, and the Cu active species was less than that of the Cu(N)X catalyst, indicating that the Cu active sites were consumed during the reaction. Furthermore, it could also be found that the generated sulfate species occupied the main active sites and reduced the varieties of nitrate species and intermediates, thereby reducing the SCR activity of the catalyst.