Ion-exchanged pillared clays for selective catalytic reduction of NO by ethylene in the presence of oxygen

Ion-exchanged pillared clays (PILCs) were studied as catalysts for selective catalytic reduction (SCR) of NO by ethylene. Three most important pillared clays, Al₂O₃-PILC (or Al-PILC), ZrO₂-PILC (or Zr-PILC) and TiO₂-PILC (or Ti-PILC), were synthesized. Cation exchanges were performed to prepare the following catalysts: Cu-Ti-PILC, Cu-Al-PILC, Cu-Zr-PILC, Cu-Al-Laponite, Fe-Ti-PILC, Ce-Ti-PILC, Ce-Ti-PILC, Co-Ti-PILC, Ag-Ti-PILC and Ga-Ti-PILC. Cu-Ti-PILC showed the highest activities at temperatures below 370°C, while Cu-Al-PILC was most active at above 370°C, and both catalysts were substantially more active than Cu-ZSM-5. No detectable N₂O was formed by all of these catalysts. H₂O and SO₂ only slightly deactivated the SCR activity of Cu-Ti-PILC, whereas severe deactivation was observed for Cu-ZSM-5. The catalytic activity of Cu-Ti-PILC was found to depend on the method and amount of copper loading. The catalytic activity increased with copper content until it reached 245% ion-exchange. The doping of 0.5 wt% Ce₂O₃ on Cu-Ti-PILC increased the activities from 10% to 30% while 1.0 wt% of Ce₂O₃ decreased the activity of Cu-Ti-PILC due to pore plugging. Cu-Ti-PILC was found to be an excellent catalyst for NO SCR by NH₃, but inactive when CH₄ was used as the reducing agent. Subjecting the Cu-Ti-PILC catalyst to 5% H₂O and 50 ppm SO₂ at 700°C for 2 h only slightly decreased its activity. TPR results showed that the overexchanged (245%) PILC sample contained Cu²⁺, Cu⁺ and CuO. The TPR temperatures for the Cu-Ti-PILC were substantially lower than that for Cu-ZSM-5, indicating easier redox on the PILC catalyst and hence higher SCR activity.