Selective Reduction Of NO With C₃H₆ Over Cu-Ti-PILC Catalysts

Nitrogen oxide (NOx) as one of air pollutants has very harmful effect on the human health and ecological environment. Therefore, the efficient removal of NOx has been one of the main topics in environmental science and technology. Various wet and dry processes have been put forward. Among these processes, the selective catalytic reduction of NOx with propylene (C3H6-SCR) is believed to be one of the most effective methods. The catalysts used in this reaction are zeolite based catalysts, noble metal catalysts, metal oxide catalysts and pillared clays (PILCs) based catalysts. The PILCs-based catalysts show their performance at lower reaction temperature in C3H6-SCR reaction. Moreover, water and SO2 have little effect on their catalytic activity. However, the active sites of PILCs-based catalyst and the role of metal oxide pillar in C3H6-SCR reaction are not clear. In addition, their catalytic activity is not high. Thus, Cu-ion-exchanged titanium-pillared clays (Cu-Ti-PILCs) are used as catalyst in C3H6-SCR reaction to study the active sites and the role of TiO2 pillar. And some promoters are added into Cu-Ti-PILC catalysts to improve the catalytic activity. The obtained results are listed below:(1) Cu-Ti-PILC were prepared using ion exchange strategy and used for selective catalytic reduction (SCR) of NO by propylene. The influences of pH value and pretreatment condition on the catalytic performance and the nature of copper species were investigated. The obtained results showed that more Cu2+ ions existed over the catalysts prepared with weak acidic solution or pretreated with Ar. The Cu2+ species exhibited higher activity than other copper species. Moreover, the Cu2+ species improved the catalyst for adsorbing NO to form nitrate species, which cuased the catalyst with most Cu2+ ions to obtain the highest low-temperature activity.(2) A series of Cu-Ti-PILC catalysts with different Ti/clay rates and Ti-PILC calcination temperatures were used for the SCR of NO by propylene to examine the role of titania pillar in these catalysts. Based on XRD, BET, ICP, TPR and NO-TPD techniques, it was confirmed that the incorporation of Ti into clay significantly decreased the cation exchange capacity (CEC) of Ti-PILC support and improved the interlayer spacing, the surface area and the amount of isolated Cu2+ ions. These features make NO adsorbed more easily by copper species to form the intermediate of nitrate species, which increased the conversion of NO into N2 from 14.0% to 55.4% and decreased the reaction temperature from 320℃to 280℃. Moreover, the copper hydroxyl cations (the precursor of copper species) bound to anatase phase and the clay are apt to form the amorphous CuO species, however, the one connected with TiO2 pillar by hydroxyl is prone to the formation of the Cu2+ species.(3) A series of bimetallic catalysts were prepared by adding Mn, Ru, Pd or Ag into Cu-Ti-PILC and used for SCR of NO by propylene. Among these bimetallic catalysts, the Ag/Cu-Ti-PILC and Pd/Cu-Ti-PILC catalyst showed higher activity than Cu-Ti-PILC catalyst. The 0.2 wt.% Pd/Cu-Ti-PILC catalyst showed the highest yield on N2 of 63.5% and 4 wt.% Ag/Cu-Ti-PILC catalyst showed the highest yield on N2 of 75.0%. The obtained results showed the incorporation of Ag or Pd into Cu-Ti-PILC catalyst did not destroy the interlayer structure, the dispersion of copper species and the coordination structure of copper species. The addition of Ag increased the amount of Cu2+ and CuO over Cu-Ti-PILC catalyst, however, the addition of Pd decreased the amount of Cu2+ and CuO over Cu-Ti-PILC catalyst. Adding Ag or Pd into Cu-Ti-PILC catalysts improved the catalyst for adsorbing NO to form nitrate species, which caused Ag/Cu-Ti-PILC and Pd/Cu-Ti-PILC catalysts to exhibit higher conversion of NO into N2 than Cu-Ti-PILC catalyst in C3H6-SCR reaction. Moreover, for Ag/Cu-Ti-PILC catalyst, its catalytic activity decreased in present of SO2 or in present of SO2+H2O, while this activity was still higher than that of Cu-Ti-PILC catalyst. For Pd/Cu-Ti-PILC catalyst, its catalytic activity decreased slightly in present of SO2, which was still higher than that of Cu-Ti-PILC catalyst. However, in present of SO2+H2O, the catalytic activity of Pd/Cu-Ti-PILC catalyst remarkably decreased, which was lower than that of Cu-Ti-PILC catalyst.Based on above research, it was concluded that isolated Cu2+ ions connected with TiO2 pillar with hydroxyl was active site of Cu-Ti-PILC catalyst in C3H6-SCR reaction. Moreover, the addition of Pd or Ag to Cu-Ti-PILC catalyst improved its catalytic activity.