Enhancement of activities by sulfation on Fe-exchanged TiO2-pillared clay for selective catalytic reduction of NO by ammonia

Fe-exchanged TiO2-pillared clay (Fe-TiO2-PILC) has been previously found to be more active than vanadia-based catalysts for selective catalytic reduction (SCR) of NO by NH3. The present work shows that its SCR activity is further improved (by ca. 50%) after sulfation by SO2+O2 at 450°C in the absence of water. Although, H2O decreased the SCR activity slightly, the activities on the sulfated catalysts were still 53–57% more active than the fresh catalyst. The enhancement is attributed to an increase in surface acidity due to sulfation. In situ FTIR spectra and gas product analyses show that both NH4+ ions and coordinated NH3 are highly active in reacting with NO+O2 to produce N2+H2O at high temperatures (>200°C), but they are not amenable for oxidation by O2. Gaseous NH3 contributes to the oxidation of NH3 to NO+N2 by O2, a competitive reaction for SCR. Sulfation of Fe-TiO2-PILC increases ammonia adsorption due to the increase in surface acidity and, thus, decreases its gaseous concentration. Therefore, on the sulfated catalysts, NH3 oxidation to NO+N2 is inhibited and the SCR activity is enhanced. However, sulfation also decreases the activity of NO oxidation to NO2 by O2, which is an important step for NO reduction on Fe-TiO2-PILC. Hence, the highest SCR activity is obtained on the catalyst with an optimal amount of sulfur (e.g. 0.7 wt.%).