Effects of sulfation on hematite for selective catalytic reduction of nitrogen oxides with ammonia.

Hematite (α-Fe2O3) is a promising candidate for NH3 selective catalytic reduction (NH3-SCR) of NOx due to its good sulfur resistance. However, the activity of pure α-Fe2O3 is very low. In this work, α-Fe2O3 obtained excellent N2 selectivity and medium-high temperature activity via a simple surface sulfation method. The α-Fe2O3-350 (sulfated at 350 °C) sample showed an NO conversion rate of ~ 100% in the range of 275-350 °C and exhibited excellent H2O and SO2 resistance ability at 300 °C. Furthermore, pure α-Fe2O3 was used as a model catalyst to fully uncover the effect of sulfation on FeOx-based catalysts in NH3-SCR reactions. Structural characterization indicated that the degree of surface sulfation of the catalyst would be deepened with increasing temperature, and the states of sulfate species on α-Fe2O3 changed from surface sulfates to bulk-like sulfates. Although sulfation treatment reduced the redox properties of α-Fe2O3, it significantly increased its surface acidity and thus the activity. Excessive bulk-like sulfates induced a decrease in activity. Sulfation inhibited the adsorption of NOx on the α-Fe2O3 catalyst surface and reduced the thermal stability of nitrates at medium-high temperature. Thus, the Langmuir-Hinshelwood (L-H) mechanism was inhibited, and the reaction mainly followed the Eley-Rideal (E-R) mechanism.