Study On Controllable Preparation And NH₃-SCR Performance Of Manganese-based Molecular Sieve Flue Gas Denitration Catalyst

The problem of atmospheric nitrogen deposition seriously affects the ecosystem environment as well as humannormal life.Emissions of nitrogen oxides from industryfluegases is one of the important causes of atmospheric nitrogen deposition.Excessive atmospheric nitrogen deposition not only leads to a series of ecological and environmental problems,but also poses a serious threat to human health.Therefore,effectively control nitrogen oxides emissions from industrial sources is of very far-reaching significance for the improvement of environmental quality in China.By putting NH₃-SCR technology into industrial flue gas denitrification practice on a large scale is the main method to effectively control nitrogen oxides emissions in flue gas in recent years.Currently,the conventional commercial V-W-Ti catalysts have limited their widespread use in China and abroad due to their high activity temperature,narrow temperature window（350～450°C）and the biological toxicity of vanadium.Therefore,there is an urgent need to develop a new,non-vanadium SCR catalyst with a wide temperature window to effectively control nitrogen oxides emissions.SAPO-18molecular sieve is favored by researchers because of its regular and good pore structure as well as abundant surface acidic sites.The active component transition metal manganese is widely used in the NH₃-SCR reaction due to its rich resources and good low-temperature activity.By studying the conformational relationship of manganese-based catalysts in the reaction,it can provide a scientific basis for improving the activity of existing catalysts and designing and preparing new catalysts.In this paper,a series of transition metals Mn-modified SAPO-18 catalyst was prepared by in-situ hydrothermal synthesis,and transferred denitration catalyst having excellent SCR performance were prepared by modulating a series of different synthesis conditions,and a series of characterizations were performed on the morphological structure,physical and chemical properties and catalytic activity of the catalysts.This paper used in-situ hydrothermal synthesis method and changed the manganese source precursors（manganese acetate,manganese sulfate,manganese nitrate）,crystallization time（12h,24h,48h）,catalyst types（DIPEA,DIPA,TETA）and manganese content n（Mn O）/n（Al₂O₃）molar ratio（0,0.1,0.2,0.3,0.4）and other influencing factors were used to prepare the corresponding transition metal Mn modified SAPO-18 catalyst.And through the catalyst activity test,X-ray diffraction（XRD）,scanning electron microscope（SEM）,specific surface area analysis（BET）,ultraviolet visible diffuse reflectance（UV-Vis）,electron paramagnetic resonance（EPR）,X-ray photoelectron energy Spectroscopy（XPS）and a series of characterization methods to investigate the structure properties of Mn-SAPO-18（such as grain size,acid content,etc.）and catalytic performance（NO_x conversion rate and N₂selectivity,etc.）.The results of this paper show that:（1）The physical phase characterization of Mn-SAPO-18catalysts illustrates that when the manganese source precursor is manganese acetate,molecular sieves with AEI-type topology can be obtained,and all of them have cubic crystal structures.According to the SEM spectra,the molecular sieves have a uniform grain size distribution between 3 and 5μm and smooth crystal surfaces.The crystallinity of the molecular sieves increased and then decreased when the crystallization time was varied,and when the crystallization time was 24h,the synthesized molecular sieves had a cubic structure and a smooth surface.The Mn-SAPO-18 catalyst with cubic crystal structure and homogeneous morphology was synthesized when the catalyst was DIPEA with different types of templating agents.When different manganese dosages are applied,the appropriate amount of Mn atoms can participate in the molecular sieve skeleton synthesis by competing with Al and P atoms for substitution.The optimal combination of metal Mn-modified SAPO-18 molecular sieves was manganese acetate precursor,24 h crystallization time,DIPEA as templating agent and manganese content n（Mn O）/n（Al₂O₃）was 0.2 to prepare a molecular sieve catalyst with an AEI-type structure and a cubic structure with good crystallinity.（2）The activity test of the Mn-SAPO-18 catalyst showed that when the synthesis conditions of the metal Mn-modified molecular sieve were manganese acetate as the manganese source precursor,crystallization time of 24 h,DIPEA as the templating agent,and manganese content n（Mn O）/n（Al₂O₃）of 0.2,the NOx removal rate was consistently above 90%in the temperature window of275-375℃.The N₂ selectivity was also consistently maintained above 90%in the temperature window range of 150～375℃as characterized by the N₂ selectivity test.（3）The Mn-SAPO-18 catalysts through a series of characterizationresults such as UV-Vis and XPS indicated that the amount of manganese added to the molecular sieve catalysts may affect the distribution and valence of manganese.When the catalyst is noncalcined,it is mainly present in the skeleton of the molecular sieve as Mn²⁺.After calcining,Mn is mainly present on the catalyst skeleton as highly valent Mn³⁺or Mn⁴⁺species.And the presence of highly valent Mn³⁺or Mn⁴⁺species on the catalyst skeleton is an important reason for the high denitrification activity of the catalyst in the SCR reaction.