| NOx is one of the main air pollutants and the selective catalytic reduction (SCR) is the most effective and widely used method to remove NOx in the flue gas from stationary sources. Recently, as NOx emmsions become more and more severe in China, the clear cut goal for the nitrogen oxides control during the "twelevth five-year" has been proposed by our government. As such, the SCR technology received more and more attentions. According to the Chinese situations, the development for vanadium-free SCR catalysts with wide temperature window and low-temperature SCR catalysts are both the hotspots in this area. In this dissertation, the modifications of these two types of SCR catalysts have been carried out, to obtain better catalytic performances under various flue gas conditions.Firstly, surface sulfation modifications by SO2for CeO2catalysts were undertaken. From activity tests and characterization results, it was found that surface sulfation could greatly enhance the NO removal efficiency in the temperature range of200-570℃and showed very good selectivity to N2. The surface sulfation could improve the concentration of surface active oxygen and promote the adsorption and activation of NH3on catalyst surface. Based on these studies, the catalysts were further studied by the mean of DRIFT. It showed that the sulfation could make an abundance of Bronsted acid sites formed. And the amount and acidity of Lewis acid sites could be also elevated. As a result, the SCR reaction rate was obviously promoted.Moreover, it was confirmed that SCR reaction over sulfated CeO2should follow the E-R model.Secondly, the effects of Ca modification by impregnation method on the low temperature SCR activity over MnOx/TiO2catalysts were investigated detailedly. It was observed that with the increase in Ca content, the SCR activity and NO oxidation efficiency were firstly obviously decreased and then partly recovered. It was attributable to the fact that when Ca content was low, the formation of CaO would neutralize the surface acidity and when Ca content was higher, the generation of CaTiO3could suppress the formation of CaO, retarding the neutralization and making the acidity recovery. Afterwards, the effects of Ca modification by sol-gel method were also investigated. It was found that the introduced Ca would have strong interactions with Mn and Ti, resulting in better dispersibity and great increase in specific surface area and pores volume, leading to the enhancement of SCR activity.Finally, low temperature SCR activities and selectivities for different metal modified MnOx/TiO2catalysts were investigaed. Ca0.04Ce0.02Mn0.04/TiO2catalyst was selected as its best combination property in activity and selectivity. DRIFT studies indicated that Ca could effectively inhibit the formation of N2O. This could be attributed to the following two points:(1) Ca doping could retard the formation of NO2during SCR reaction as NO2could easily oxidize the NH3species to form N2O.(2) Ca modification could suppress the process that adsorbed NH3species was excessively dehydrogenation to form NH species and thereby benefit to the selectivity, as NH species was active to form N2O.In all, the sulfation modification CeO2was developed, which possessed the advantages that it was easy to be prepared, and had a wide active temperature window, high SCR activity, and high selectivity, providing useful guidances for SCR catalysts development with wide temperarue window. Futhermore, the effects of alkaline earth onto the low temperature SCR catalysts was investigated. It was concluded that different modified methods would affect the MnOx catalysts in different ways. And the optimal catalyst with both good denitration efficiency and selectivity was obtained by Ca doping. |
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