Preparation Of Mn-based Catalysts For Low-temperature Selective Catalytic Reduction Of NO With NH₃

Nitrogen oxides?NO_x,mainly NO and NO₂?are one of the major atmospheric pollutants,which have caused serious environmental problems,such as haze,acid rain,photochemical smog,affecting human health and the growth of plants and animals.Selective catalytic reduction?SCR?of NO_x with NH₃ is considered as an efficient denitration technology and has been widely used in industrial applications.V₂O₅-WO₃/TiO₂ is a typical commercial catalyst because of its high catalytic activity.However,there are some unavoidable problems for V₂O₅-WO₃/TiO₂ catalyst,such as high activity temperature?200²50 ??,toxic vanadium species recycle difficulties,low N2 selectivity at high temperatures and the huge use cost.Appropriate SCR reactors are usually placed at the downstream of desulfurization device and electrostatic precipitator to reduce the harm of fly ash and SO₂ to the catalyst.Because the temperature of the inlet flue gas is about 150 ?,it is necessary to undergo further heating treatment before entering the denitration device to ensure the high NO conversion.Therefore,it is of great economic and practical significance to research environment-friendly catalysts with high catalytic activity and long life under low-temperature conditions.Manganese oxides?MnO_x?as a alternative catalyst has attracted great attention due to its various valence states,because the transfer between different valence states can improve its redox performance,endowing its good SCR activity below 300 ?.However,the lowtemperature?<200 ??catalytic activity of MnO_x catalysts remains unsatisfactory,and the sulfur and water resistance is poor.In this thesis,several efforts have been made to improve the NH₃-SCR performance of the Mn-based catalysts,which includes preparing nanostructured MnO_x,increasing surface acidity,doping with cerium and loading by reduced graphene oxide?rGO?,etc.Accordingly,mesoporous MnO_x/MnCO₃,MnO₂/rGO,and MnO_x-CeO_x/rGO catalysts were prepared and the corresponding reaction mechanism and sulfur and water resistance characteristic were explored.I.The mesoporous MnO_x microcube catalysts were prepared via a homogeneous precipitation method followed by calcination at 300⁵00 ? in air by using Mn?NO₃?₂·4H₂O and urea as raw materials for low-temperature NH₃-SCR.The results show that the calcined product obtained at 350 ??C-350?exhibits the highest SCR activity,reaching 97% of NO conversion at 100 ?.The C-350 product is composed by MnO₂ and MnCO₃ with a mesoporous structure,in which the particle size of MnO₂ is c.a.14 nm.In addition,CO32-provides more acid sites and surface chemically adsorbed oxygen,promoting the transfer between active oxygen species,thereby effectively improving the low-temperature SCR activity of MnO_x catalyst.II.MnO_x nanoparticles were prepared by the reaction of KMnO₄ with ascorbic acid in different molar ratios?1:0.2¹:1?at room temperature for low-temperature NH₃-SCR.When the molar ratio of KMnO₄ to ascorbic acid is 1:0.5,the prepared MnO_x?0.5?catalyst shows good low-temperature activity,reaching 64% of NO conversion at 60 ? and 89% of NO conversion at 70 ?,due to its nanoparticle size?c.a.15 nm?and high content of Mn⁴⁺.The MnO₂/rGO catalysts were further fabricated by adding graphene oxide?GO?during above synthesis process,followed by a pre-oxidation treatment at 160 ?.It was found that the SCR activity of MnO₂/rGO?5%?catalyst is higher than that of MnO_x?0.5?,achieving 83% of NO conversion at 60 ?.In addition,the MnO₂/rGO?5%?catalyst shows better sulfur and water resistance than that of MnO_x?0.5?P-O?sample MnO_x?0.5?after pre-oxidation treatment?.When 100 ppm of SO₂ and 5 vol% of H₂O vapors are simultaneously passed in for 4 h at 160 ?,the NO conversion for MnO_x?0.5?P-O and MnO₂/rGO?5%?reduces to 66.4% and 82.1%,respectively.Then the NO conversion of MnO_x?0.5?P-O and MnO₂/rGO?5%?returns to 90% and 99.5% after cutting off the injection of SO₂ and H2 O,respectively,indicating that the promotional effect of GO on the sulfur and water resistance of MnO_x catalysts.It is shown that the introduction of GO can disperse the MnO_x particles,and the pre-oxidation treatment further increases the content of Mn⁴⁺ and Mn³⁺,thereby improving the low-temperature SCR activity of MnO_x catalyst.III.A series of MnO_x-CeO_x/rGO catalysts prepared by a redox reaction of KMnO₄ and Ce?NO₃?₃·6H₂O under hydrothermal condition with different dose of graphene oxide?GO?were investigated for NH₃-SCR.The results indicate that the obtained catalysts exhibit excellent SCR activity at the temperature range of 60¹80 ?,especially for the MnO_x-CeO_x/rGO?10%?catalyst,showing nearly 100% of NO conversion at 80 ?.The introduction of GO in a suitable amount can enhance the specific surface area of the catalyst,and the oxygen functional groups on GO surface not only can act as the anchor points to disperse the MnO_x and CeO_x nanoparticles,but also can provide the acid sites to adsorb NH₃ and increase the surface chemisorbed oxygen with higher mobility,meanwhile,the reducibility of MnO₂ species is improved,which are all beneficial to the low-temperature SCR activity.In addition,MnO_xCeO_x/rGO?10%?catalyst exhibit excellent sulfur and water resistance.When 100 ppm of SO₂ and 5 vol% of H₂O vapors are simultaneously passed in for 3 h at 160 ?,the NO conversion of MnO_x-CeO_x/rGO?10%?catalyst drops to 84%,which then recovers to 100% when the SO₂ and H2 O vapors are cut off.Whereas the NO conversion of MnO_x-CeO_x reduces to 58% after introduction of SO₂ and H2 O vapor,and the activity only return to 65% after cutting off the injection of SO₂ and H2 O.The enhanced sulfur and water resistance of the resultant catalyst should be attributed to the synergistic effects resulted from the doping of MnO_x by CeO_x and the introduction of GO.