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.%).