Preparation And Performance Of Novel Mn-based Low-temperature DeNO_x Catalysts

Nitrogen oxides (NOx) emitting from the combustion of fossil fuels are currentlyconsidered as one of the dominant sources of atmospheric pollution, which has givenrise to a variety of health and environmental-related issues, such as acid rain,photochemical smog, ozone depletion. The selective catalytic reduction (SCR) ofNOxwith NH3is nowadays well established as the most effective technology toeliminate NOx. The development of low-temperature deNOxcatalysts with highcatalytic activity, SO2-tolerance and stability is highly desirable but remainschallenging. Recently, Mn-based catalysts have been the rising star as thelow-temperature deNOxcatalysts due to the desirable low-temperature activities,inherently environmentally benign characteristics and outstanding physical andchemical properties. Unfortunately, Mn-based catalysts are very sensitive to thepresence of H2O and SO2in the feed gas and are severely deactivated by traceamounts of SO2. To address these problems, in this dissertation, three novel kinds ofMn-based deNOxcatalysts have been prepared via in-situ supported method,construction of core-shell structure, design of binary metal oxides with thehierarchical structures, respectively. The main study contents are listed as follows.(1) The catalysts with highly dispersed MnOx-CeOxmixed oxides are synthesizedby in-situ supported MnOx-CeOxmixed oxides on carbon nanotubes (CNTs) via aPSS assisted reflux method. The morphological structure, surface properties andcatalytic performance of catalysts prepared by different method are investigated. Thein-situ prepared catalysts display better NH3-SCR activity, more extensive operating-temperature window, higher SO2tolerance and improved water resistance than thatof the catalysts prepared by impregnation or mechanically mixed method. Theexcellent catalytic performance of the in-situ supported catalysts can be attributed tothe good dispersion of the active component on the surface of CNTs, the strong interaction among the manganese oxide, cerium oxide and CNTs. The gooddispersion of the active component not only provides more active sites, but alsopromotes the chemisorption and activation of the reactants, resulting in a bettercatalytic activity. The strong interaction among the manganese oxide, cerium oxideand CNTs enhances the stability and SO2tolerance of the catalysts.(2) By coating the mesoporous TiO2layers on CNTs-supported MnOxand CeOxnanoparticles, we obtained a core–shell structural deNOxcatalyst with high catalyticactivity, good SO2-tolerance and enhanced stability. The porous feature of TiO2sheaths provides a larger surface area to adsorb reagents, resulting in a highercatalytic activity. Moreover, the meso-TiO2sheaths serve as an effective barrier toprevent the aggregation of metal oxide NPs and thus enhance the thermal stability.More importantly, the meso-TiO2sheaths can not only prevent the generation ofammonium sulfate species from blocking the active sites but also inhibit theformation of manganese sulfate, resulting in a higher SO2-tolerance. The studyindicates that the design of a core–shell structure is efective to promote theperformance of deNOxcatalysts.(3) Starting with shape-controlled synthesis of metal-organic frameworks(Mn3[Co(CN)6]2·nH2O) nanocubes as precursors, we rationally designed andoriginally developed a high-performance deNOxcatalyst based on MnxCo3-xO4nanocages with hollow and porous structures derived from Mn3[Co(CN)6]2·nH2Onanocubes via an annealing treatment in air. As compared with conventionalnanoparticles, MnxCo3-xO4nanocages possess a much better catalytic activity atlow-temperature regions, higher N2selectivity, more extensive operating-temperature window, higher stability and SO2-tolerance, due to their hollow andporous structures and the uniformly distributed active sites. The feature of hollowand porous structures provides a larger surface area and more active sites to adsorband activate reagents, resulting in the higher catalytic activity. Moreover, the uniformdistribution and the strong interaction between manganese and cobalt oxide species not only enhance the catalytic cycle but also inhibit the formation of manganesesulfate, resulting in high catalytic cycle stability and improved SO2-tolerance. Thesynthesis approach in the present study is conducive to the development ofhigh-performance catalysts.