The promoting role of rare earth oxides on Fe-exchanged TiO2-pillared clay for selective catalytic reduction of nitric oxide by ammonia

Abstract

Fe3+-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 promoting role of rare earth oxides with variable valence (i.e. CeO2, PrO1.83 and TbO1.75) on the Fe–TiO2–PILC was studied in this work. The catalysts were also characterized by XRD, FT-IR and NO oxidation experiments. It was found that the addition of a small amount (≤2wt.%) of CeO2 and PrO1.83 to Fe–TiO2–PILC increased the SCR activity (by 35%) both in the absence and presence of H2O+SO2, but TbO1.75 did not improve the SCR activity significantly. XRD analysis showed that a cerium oxide phase was not observed on CeO2/Fe–TiO2–PILC when the amount of CeO2 was below 2wt.%. However, when more cerium oxide (5wt.%) was doped on the Fe–TiO2–PILC, the SCR activity was decreased and a cubic CeO2 phase was detected. This suggests that well dispersed cerium oxide is beneficial to the improvement of SCR activity. FT-IR spectra of NH3-adsorbed samples indicated that the addition of cerium oxide did not increase the surface acidity of Fe–TiO2–PILC. The improvement of SCR activity was attributed to an increase in the oxidation activity of NO to NO2 by O2 since NO2 is an intermediate for the SCR reaction on the Fe–TiO2–PILC catalysts. By comparison, the SCR reaction on vanadia catalysts does not involve the adsorption and oxidation of NO for and as a result, the addition of CeO2 to V2O5/TiO2 does not increase their SCR activities.