Study On The Application Of MnO_x Catalysts In NH₃-SCR And Its Promotion Mechanism Of SO₂ Resistance

Designing and constructing low-temperature,high-efficiency,SO₂-resistant de-NO_x catalysts has become a key research direction in the field of flue gas denitrification.Mn-based catalysts has been discovered a promising catalytic system in low temperature NH₃-SCR.As the merit of Mn O_x,the multiple valence states distributions（VSDs）endowed the catalyst with excellent low temperature de-NO_x activity.Clarifying the effect of Mn O_x multiple valence states on its catalytic activity and SO₂resistance has important practical significance for the design and development of high-efficiency Mn-based catalysts suitable for low-temperature denitrification.This paper focued on Mn O_x catalysts（Mn O₂,Mn₂O₃,Mn₃O₄）,the effect of Mn O_xvalence distribution on its catalytic activity and SO₂ resistance was study.Mn O_xcatalysts with excellent performance are selected to investigate the key factors affecting their sulfur resistance performance at different temperatures.On this basis,by introducing the synergistic function component Fe into Mn-based catalysts to optimize the valence distribution of the catalyst and SO₂ resistance.By means of a series of characterization and analysis methods（TG,XRD,BET,XPS,NH₃-TPD,CO₂-TPD,H₂-TPR,In-situ DRIFTs,penetration experiments,etc.）to obtain the redox properties of each catalyst,surface structure,adsorption capacity of NO_x/SO_x,to explore the internal relationship between the physicochemical properties of catalysts and de-NO_xperformance,and provide a theoretical basis for the preparation of Mn-based catalysts with high activity and strong sulfur resistance.The specific research contents and main conclusions are as follows:（1）Explore the catalytic activity and SO₂ resistance of Mn O₂,Mn₂O₃,Mn₃O₄catalysts,and understand the intrinsic properties of Mn O_x catalysts with different valences distributions in NH₃-SCR applications.The results showed that the NO_xconversion rate of Mn O₂ catalysts was the highest in the low temperature range（90-150°C）,the Mn₃O₄ catalyst has the best catalytic activity in the medium and high temperature range（150-330°C）,Besides,the Mn₃O₄ catalyst has the best SO₂resistance at 100°C.A series of characterizations found that a single Mn O_x catalyst exhibited the similar irreversible deactivation（～49%）in the SO₂ resistance test for up to 7 h,but the high valence distribution of Mn O₂ easily oxidized SO₂ to SO₃ and generated more NH₄HSO₄,showing a high proportion of reversible deactivation（～24%）,but the low valence distribution of Mn₃O₄ can alleviate the rapid poisoning of the catalyst to a certain extent because of its suitable redox ability,and then generate a small amount of ammonium sulfate,so the proportion of reversible deactivation is relatively low（～6%）.（2）The SO₂ resistance performance of Mn₃O₄ catalyst at different temperatures was deeply explored,and the essential reasons for its excellent SO₂ resistance performance were understood.The study found that the order of the total deactivation amount of the catalyst within 7 h is as follows:Mn₃O₄-150（～98%）>Mn₃O₄-200（～78%）>Mn₃O₄-100（～54%）>Mn₃O₄-250（～52%）.The characterization found that the competitive adsorption of SO₂ and NO on the surface of Mn₃O₄ catalyst only occupies part of the active sites at 100°C,and the NH₃-SCR reaction can still proceed smoothly.The formation of a large number of sulfate species at 150°C severely inhibited the NO_x adsorption and hindered the progress of the NH₃-SCR reaction,resulting in a rapid reduction of the NO_x conversion rate to the lowest（～5%）after the introduction of SO₂ for 2 h.（3）The Mn Fe catalyst was designed and prepared,the effect of the introduction of the synergistic functional component Fe on the valence state,physicochemical properties and de-NO_x performance of the Mn₃O₄ catalyst was explored,and the mechanism of the improvement of the SO₂ resistance of the Mn Fe catalyst was revealed.The results show that the introduction of Fe can not only regulate the valence state distribution of Mn,build a more suitable redox cycle,which is conducive to the occurrence of the reaction of Mn²⁺+Fe³⁺?Fe²⁺+Mn ³⁺on the catalyst surface,but also can increase the specific surface area and surface of the catalyst,but also increase the specific surface area of the catalyst and the relative proportion of oxygen adsorbed on the surface.Thus,it exhibits excellent de-NO_x activity（the NO_x conversion rate is above 90%in the range of 90-250°C,and the N₂ selectivity is also maintained above70%）.By comparing the SO₂ resistance test results of Mn₃O₄ and Mn Fe catalysts at different temperatures,it is found that the large specific surface area of Mn Fe catalysts helps the dispersion of active components,so the total deactivation of NO_x conversion rate is significantly reduced.Besides,the introduction of Fe can preferentially combine with SO₂ to inhibit the formation of Mn SO₄,alleviate the direct poisoning of the active component Mn,improving the SO₂ resistance of the catalyst.