Low-Temperature Selective Catalytic Reduction Of NO_X By NH₃ Over Manganese Oxide Octahedron Molecular Sieve Catalysts

According to the current state of flue gas emission from the stationary sources in our country, low-temperature selective catalytic reduction of NOx by ammonia (NH₃-SCR) is an economical way to removal NOx in the power plant for it matches domestic boiler conveniently and possesses high efficiency and low cost. For the reasons above, there have been strong interests to develop highly active and performance stable catalysts for low-temperature NH₃-SCR. In this work, manganese oxide octahedral molecular sieves (OMS-2) were prepared to investigate the low-temperature NH₃-SCR activities under the simulated emission conditions of stationary sources. The research mainly focused on OMS-2 with K+ in the tunnel (K-Hol) and vanadium doped ones (V-OMS-2). The catalysts were characterized by X-ray Diffraction (XRD), N2 adsorption/desportion (BET), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, thermogravimetry (TG), temperature-programmed reduction by H₂ (H₂-TPR), Transmission Electron Microscopy (TEM), etc. The structure–activity relationship of the OMS-2 catalysts were built on the basis of results of phychemical properties and catalytic activities. The main results were summarized as follows:The prepared K-Hol had high NH₃-SCR activities and obtained nearly 90% NOx conversion rate at 100 oC. The SEM and TEM observations showed that K-Hol was tetragonal prism-shaped nanorods with exposed the {110} plane, and the atoms in the {110} planes arranged to form semitunnel structures. The BET, TG, NH₃+O2 transition reactions and atomic structure model analyses indicated that active surface lattice oxygen atoms of K-Hol, which favored activation of NH₃, were around 1.6% with respect to total lattice oxygen atoms. The structure-activity relationship research results revealed that both efficient semitunnel structured external surfaces and high active surface lattice oxygen atoms predominantly accounted for the high catalytic activities of K-Hol.V doping could have significantly influence on catalytic activities of OMS-2. H₂-TPR, TEM and NH₃ adsorption/desportion results revealed that both surface defect sites (Lewis acid) and redox abilities were efficiently controlled by amounts of V doping, and more V dopings resulted in more Lewis acid sites and weaker bulk redox abilities. The results of catalytic tests indicated that the 2%V-OMS-2 catalyst showed the highest catalytic activities among the V-OMS-2 catalysts. In fact, the desired doping amount of V can not only provide more Lewis acid sites for adsorption of NH₃, but also maintain excellent surface redox abilities for activation of NH₃ and inhibit the formation of thermal stable NO3- species which always deactivate the catalysts. Also, The N₂ selectivity and water & sulfur resistance of OMS-2 could all be promoted by V doping.