Improvement of the Activity of γ-Fe2O3 for the Selective Catalytic Reduction of NO with NH3 at High Temperatures: NO Reduction versus NH3 Oxidization

Abstract

Lignite is widely used as the fuel for coal-fired power plants, and its flue gas temperature is about 50–100 °C higher than others. V2O5–WO3/TiO2 is extremely restricted in the selective catalytic reduction (SCR) of NO from the coal-fired power plants burning lignite due to the drop of NOx conversion, low N2 selectivity, and volatility of vanadium pentoxide at high temperatures. Therefore, a more environmental-friendly SCR catalyst with excellent SCR activity and better N2 selectivity at 350–450 °C should be developed for this application. In this work, sulfated Fe–Ti spinel catalyst was developed for the SCR of NO from the coal-fired power plants burning lignite. The drop of NOx conversion at high temperatures was mainly related to the simultaneous occurrence of the catalytic oxidization of NH3 to NO during the SCR reaction. Ti was incorporated into γ-Fe2O3 to decease the oxidization ability of Fe3+ on the surface, and the sites for −NH2 adsorption and the active components for −NH2 oxidization were separated after the sulfation to decrease the probability of the collision between −NH2 adsorbed and Fe3+ on the surface. They both inhibited the catalytic oxidization of NH3 to NO over γ-Fe2O3. However, the SCR reaction over γ-Fe2O3 was simultaneously restrained after the incorporation Ti and the sulfation. Therefore, NOx conversion over γ-Fe2O3 at high temperatures depended on the ratio of NH3 conversion through the catalytic oxidization of NH3 to NO to that through the SCR reaction. This ratio decreased after the incorporation of Ti, and it further decreased after the sulfation, resulting in an obvious promotion of NOx conversion at high temperatures. Therefore, sulfated Fe–Ti spinel showed excellent SCR activity, N2 selectivity, and H2O+SO2 durability at 300–450 °C, which was suitable for the application in the coal-fired power plants burning lignite.