FTIR and Kinetic Studies of the Mechanism of Fe3+-Exchanged TiO2-Pillared Clay Catalyst for Selective Catalytic Reduction of NO with Ammonia

A series of FTIR spectroscopic and kinetic studies of the selective catalytic reduction (SCR) of nitric oxide by ammonia were conducted on Fe3+-exchanged TiO2-pillared clay (Fe-TiO2-PILC) catalyst. It was found that NO molecules were adsorbed on the fresh Fe-TiO2-PILC catalyst and then oxidized by O2 to adsorbed NO2 and nitrate species. These NOx adspecies could be reduced by NH3 at high temperatures. NH3 molecules could also be adsorbed on the Brønsted acid and Lewis acid sites on the Fe-TiO2-PILC catalyst to generate, respectively, NH+4 ions and coordinated NH3 species. These NH3 adspecies were active in reacting with NO, NO+O2 and NO2, but the reaction rates of NH3+NO+O2 and NH3+NO2 were much higher than that of NO+NH3. However, under reaction conditions, the surface of Fe-TiO2-PILC was mainly covered by NH+4 ions and coordinated NH3, and no NOx adspecies were detected. This is in agreement with the zero-order for the SCR reaction with respect to NH3. A possible reaction scheme for the SCR reaction on Fe-TiO2-PILC was proposed. NO reduction initially involved the reaction between NO2 and pairs of NH3 adspecies to form an active intermediate, which finally reacted with gaseous or weakly adsorbed NO to produce N2 and H2O.