Preparation And Performance Of Hierarchical MnO_x/ZSM-5 Catalyst For The Low-temperature NH_<sub>3-SCR

Selective catalytic reduction of NO_xby NH₃is the most widely applied technology to remove NO_xfrom stationary sources,with high stability and efficiency,and the key factor is the catalysts.The most commercially used catalyst V₂O₅-WO₃?Mo O₂?/Ti O₂have a narrow active temperature window concentrated on 300-400?.In order to obtain higher NO_xconversion,the SCR reactor is usually placed before the dust removal and desulfurization device,which makes the catalysts poisoned in the high concentration of dust and SO₂,if it is placed after the dust removal and desulfurization device,it needs to be reheated to achieve higher NO_xconversion.Therefore,in order to solve the above problems,it is important to develop a high-performance low-temperature NH₃-SCR catalyst.Mn-based catalysts exhibits excellent low-temperature SCR activity due to its rich variable valences and redox capacity,which is easily poisoned in an atmosphere containing SO₂and H₂O,the development of an efficient low-temperature catalyst with good resistant to SO₂and H₂O is the key to the application of SCR technology.Adjustment of the structure is an important method to improve the low temperature catalytic performance and SO₂&H₂O tolerance.The catalyst with hierarchical pore structure exhibits good research prospect in terms of improving catalytic activity and SO₂tolerance due to its higher specific surface area and larger pore size.The ZSM-5 zeolite has adjustable surface acidity,shape selectivity and a stable framework structure,which has been widely applied as carrier in the SCR reaction.In this paper,hierarchical ZSM-5 was used as support and a series of hierarchical Mn O_x/ZSM-5catalysts were synthesized by ethanol dispersion method.The main content of this paper is to explore the preparation parameters and reactivity of hierarchical pore structure catalyst,the structure and properties of the hierarchical Mn O_x/ZSM-5 catalyst,and the important influence of the hierarchical pore structure on low temperature activity and SO₂&H₂O resistance.First of all,a hierarchical ZSM-5 zeolite was successfully prepared via a desilication-recrystallization method using tetraethyl ammonium hydroxide?TEAOH?and cetyltrimethylammonium bromide?CTAB?as the desilication and structure-directing agents,and hierarchical Mn O_x/ZSM-5 catalysts were synthesized by ethanol dispersion method and applied for the NH3-SCR reaction.The effect of preparation parameters on catalytic performance was investigated.The results show that Mn O_x/ZSM catalyst exhibited the best performance on the preparation conditions of hydrothermal temperature of 150??hydrothermal time of 24 h,TEAOH concentration of 1M?Mn loading of 15wt.%and calcination temperature of 400?,and can maintain nearly 100%NO_xconversion in the temperature range of 120–240°C with N₂selectivity over 90%.Secondly,the effects of TEAOH concentrations and Mn loading on the reaction performance and structural characteristics of the hierarchical Mn O_x/ZSM-5 catalysts were analyzed through various characterization methods.The results show that the appropriate concentration of TEAOH has a decisive effect on the formation of the mesoporous structure of the catalyst,when the concentration of TEAOH is 1M,the catalyst forms the most mesoporous structure.When the loading of manganese is 15wt.%,15%-Mn O_x/ZSM-5-1M catalyst has largest specific surface area and mesopore volume,which is beneficial to mass transfer and diffusion of reactants,best dispersion of Mn O_xand more active Mn sites,and there are more strong acid sites on the surface of the catalyst.At the same time,the higher concentration of high valence Mn species(Mn³⁺+Mn⁴⁺)and chemisorbed oxygen species on the catalyst surface are also the reasons why the catalyst showed the best low-temperature catalytic activity.Finally,the hierarchical Mn O_x/HZ-ET catalyst and non hierarchical Mn O_x/Z-P catalyst were compared to explore the effect of micropore-mesoporous coexistence structure on the performance and structural properties of the catalyst.The results show that hierarchical pore structure can effectively improve the catalyst's low-temperature SCR activity and SO₂&H₂O resistant.Mn O_x/HZ-ET catalyst can maintain about 60%NO_xconversion after 1h addition of100ppm SO₂and 10%H₂O.The regular or irregular mesopores are exhibited in Mn O_x/HZ-ET with different size range,the larger micropore is about 0.78 nm,the mesopore is about 3.2 nm,and there also exhibits mesopores with a pore size of 4-15 nm.Expanded micropores and newly generated mesopores structure all promote the dispersion of Mn O_xparticles on the surface and mesopores ZSM-5,and the Mn O_xparticle size is smaller.Higher concentration of Mn⁴⁺and chemisorbed oxygen species and as well as more surface strong acid sites make Mn O_x/HZ-ET catalyst show better low temperature catalytic activity than Mn O_x/Z-P.At the same time,XRD and TG results show that less deposition of?NH₄?₂SO₄or NH₄HSO₄on the surface of Mn O_x/HZ-ET catalyst in an atmosphere containing SO₂due to the existence of mesoporous structure,making the catalyst have better resistance to SO₂?...