Research On The Main Factors In Low-temperature NH₃-SCR Over Mn Based Catalysts

The technology of selective catalytic reduction of NOxby NH₃（NH₃-SCR） iswildly used to abate NOxfrom stationary sources （power plants, and et al.）, and theresearch on the NH₃-SCR reaction at low temperature （＜200°C） is a hot topicaround the world. Manganese based catalysts are wildly studied for the highlow-temperature NH₃-SCR activities. In this work, MnO₂of different crystal phasesand manganese based catalysts supported on complex supporterswere synthesized,and then the main effect facors in low-temperaturev NH₃-SCR over the catalysts werestudied. Characterizations such asBrunauer-Emmett-Teller （BET） surface area, X-raydiffraction （XRD）, X-ray photoelectron spectroscopy （XPS）,thermal gravimetric（TG）analysis and temperature programmed desorption （TPD） were carried out to findout the effects of the crystal phase of MnO₂and the complex supporters. The mainresults were summarized as follows:The crystal phase had a great effect on the low-temperature NH₃-SCR activityover MnO₂nanorods. Tunneled α-MnO₂had much higher catalytic activity thanlayered δ-MnO₂under the same reaction conditions. For α-MnO₂, the [2×2] tunnelstructure, longer Mn-O bonds and Lewis acid sites on the mainly exposed crystalplane are all beneficial for the NH₃adsorption; moreover, the surface property ofα-MnO₂is highly capable to activate NH₃and NO, which also accounts for the highactivities of α-MnO₂nanorods.The tunnel size also had effects on the low-temperature NH₃-SCR activities overthe tunneled MnO₂nanorods. Below200°C, α-MnO₂with a [2×2] tunnel structurehad the highest activity and N₂selectivity, followed by γ-MnO₂with a [1×2] tunnelstructure and β-MnO₂with a [1×1] tunnel structure. MnO₂with the tunnel structuresshow molecule-sieve properties. The [1×1] tunnel structure, high crystallinity, smallsurface area and stable atom arrangement on the mainly exposed plane account for thelow activity and low N₂selectivity of β-MnO₂; meanwhile, the [2×2] tunnel structureand unstable atom arrangement on the mainly exposed plane are the reason for higheractivity of α-MnO₂. By the addition of Zr, the low-temperature NH₃-SCR activity and N₂slectvityover Mn/TiO₂was enhaced. The NOxconversion for Mn/Ti-Zr catalysts could be90%at150°C and nearly100%at200°C; further more, the N₂selectivity for Mn/Ti-Zrcatalysts could be90%at150°C, and80%at200°C. With the addition of Zr, thesurface area and acid sites increased, which may account for the higher activity ofMn/Ti-Zr; moreover, the addition of Zr decreased the crystallinity and surfaceoxidative ability, which may account for the higher N₂selectivity of Mn/Ti-Zr.