Enhancement of activities by sulfation on Fe-exchanged TiI2-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 reduct ion (SCR) of NO by NH3. The present work shows that its SCR activity is fur ther improved (by ca. 50%) after sulfation by SO2 + O-2 at 450 degreesC in the absence of water. Although, H2O decreased the SCR activity slightly, th e activities on the sulfated catalysts were still 53-57% more active than t he fresh catalyst. The enhancement is attributed to an increase in surface acidity due to sulfation. In situ FTIR spectra and gas product analyses sho w that both NH4+ ions and coordinated NH3 are highly active in reacting wit h NO + O-2 to produce N-2 + H2O at high temperatures (>200 degreesC), but t hey are not amenable for oxidation by O-2. Gaseous NH3 contributes to the o xidation of NH3 to NO + N-2 by O-2, a competitive reaction for SCR. Sulfati on of Fe-TiO2-PILC increases ammonia adsorption due to the increase in surf ace acidity and, thus, decreases its gaseous concentration. Therefore, on t he sulfated catalysts, NH3 oxidation to NO + N-2 is inhibited and the SCR a ctivity is enhanced. However, sulfation also decreases the activity of NO o xidation to NO2 by O-2, which is an important step for NO reduction on Fe-T iO2-PILC. Hence, the highest SCR activity is obtained on the catalyst with an optimal amount of sulfur (e.g. 0.7 wt.%). (C) 2001 Elsevier Science B.V. All rights reserved.