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

Fe3+-exchanged TiO2-pillared clay (FeTiO2-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 (less than or equal to 2 wt.%) of CeO2 and PrO1.83 to FeTiO2-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 2 wt.%. However, when more cerium oxide (5 wt.%) 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 O-2 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. (C) 2000 Elsevier Science B.V. All rights reserved.