| Currently, the technology of selective catalytic reduction (SCR) of NOxwith NH3under oxygen-rich conditions not only has been widely used to thestationary pollution source, but also has been recognized as the most effectiveand promising method for mobile flue gas denitrogenation in the future, likediesel engine exhaust purification. In NH3-SCR system, variety of zeolitessuch as ZSM-5, MOR, BEA and FER have been extensively applied assupports. However, mesoporous materials are rarely used. Compared withtraditional microporous molecular sieve, SBA-15has a lot of advantages, suchas large pore diameter, uniform pore size distribution, thick pore wall and highsurface area. However, the low acidity of SBA-15limits its application in theNH3-SCR reaction. Based on this, modified Al-SBA-15was synthesized by ahydrothermal synthesis method and a series of Mn supported Al-SBA-15catalysts were prepared. The effect of the content of Mn, calcinationtemperature have been investigated to the catalytic performance of Mn-Al-SBA-15catalysts. Fe as a second active component was added to the Mn-Al-SBA-15catalyst to modify the catalyst with a view to developing a newly andhighly efficient catalyst for the diesel engine exhaust purification. First of all, Al-SBA-15was synthesized through one step method, and theeffect of precursor reaction temperature, the pH value, the ratio of Si/Al on thestructure of Al-SBA-15materials have been investigated. The results showedthat the modified Al-SBA-15support maintained the ordered mesoporousstructure when the reaction temperature was40℃, the pH value was1.5andthe ratio of Si/Al was20.Then, Mn/AS catalysts were prepared through impregnation method andthe influences of Mn content, different ratio of Si/Al, calcination temperatureon its SCR activities, physical structure, MnOxstates, surface atomconcentration were researched. The content of Mn played an important role inthe structure of catalysts. When the Mn content was under10%, the catalystmaintained the perfect mesoporous structure. While, the pore structure wasdestroyed to some extent in the catalyst with20%Mn content, mainlyreflected in the decrease of specific surface area, pore diameter, pore volume.The SCR activity was affect by AS support with different ratio of Si/Al. Withthe increase of the content of aluminum, catalytic activity had a certain degreeof improvement, and then stayed the same. Regarding the calcinationtemperature, the catalyst with10%Mn content and Si/Al=20calcinated at300℃exhibited the best activity for NO removal, with more than95%NOxconversion in the range of210~360℃. In all the catalysts, MnO2and Mn2O3species both existed on the surface of catalysts, but the relative contents ofMnOxwas different. When the catalyst was calcinated at200℃and300℃,the Mn species mainly existed in the form of MnO2, and when the calcinationtemperature increased at400℃or500℃. In order to know more about thefresh and used Mn/AS catalysts. TEM and XPS were used to analyse thestructure and the rusults showed that the pore arragements became worm-like after the NH3-SCR reaction, parts of Mn species aggregated. This may bethe reason that the SCR activities of used Mn/AS decreased. At last, Fe was added as a second active component to modified the Mn/AScatalysts. And N2adsorption-diffraction desorption, X-ray, X-ray photoelectronspectroscopy, FT-IR, H2temperature-programmed reduction and activity testswere used to approach the properties of catalysts. The limitation of stability inthe presence of H2O and SO2and the lifetime were also discussed. The resultsshowed that the Mn-Fe/Al-SBA-15catalyst exhibited a higher SCR activity thanMn/Al-SBA-15or Fe/Al-SBA-15. Mn-Fe/AS(300) exhibited the best activityfor NOxremoval, with almost100%NOxconversion in the range of180~390℃,which could be attributed to the synergistic effect of MnO2-Mn2O3, theappropriate Mn4+/Mn3+ratio and the enhanced redox activities. After theaddition of Fe, the binding energy of Mn on Mn-Fe/Al-SBA-15catalystdecreased about0.4eV, and the ratio of Mn4+/Mn3+dropped from3.32to1.10,resulting in an increase in the Mn outer electron cloud density, whichcontributed to the catalytic reaction. Fe species mainly existed in the form ofFe2O3and FeO. |
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