Study On The Promotion Effects Of Phosphorus On Carbon Supports For MnO_x-CeO₂ Catalysts In Low-temperature Selective Catalytic Reduction Of NO With NH₃

Nitrogen oxides?NO_x=NO+NO₂?as one of the main sources of air pollution.The discharge of large amounts of NO_x gas has caused serious environmental pollution such as acid rain,haze,ozone holes and photochemical smog.Although V₂O₅-WO₃?Mo O₃?/TiO₂ catalysts have been commercialized,such catalysts are very harmful to the environment and can only maintain high activity at high temperatures?300-450 oC?.Therefore,it is of great significance and value to research and develop a low-temperature SCR catalyst with high denitration activity,environmental friendliness,and non-toxicity.Manganese-cerium metal oxide has gradually become a research hotspot for lowtemperature denitration catalysts at home and abroad due to its good low-temperature activity,its own environmental friendliness and excellent physical and chemical properties.As a traditional non-metallic heteroatom,phosphorus atom has a larger atomic radius and higher electron-donating ability.It is a common dopant that changes the surface properties of carbon materials.Therefore,in this thesis,based on phosphorus-doped carbon aerogel,a series of catalysts were prepared for the research of NH₃-SCR by means of loading manganese-cerium metal oxide on the surface of the carbon aerogel and treatment at different carbonization temperatures.The activity of NH₃-SCR of phosphorus-doped carbon aerogel and phosphorusdoped carbon aerogel supported by manganese-cerium metal oxide and phosphorus doped carbon aerogel and the mechanism of phosphorus doping to promote the activity were also explored.I.Explored the use of phenol,m-cresol and furfural as precursors and H₃PO₄ as the source of phosphorus.After one-pot sol gel,aging,atmospheric drying and carbonization process,phosphorus-doped carbon aerogel was obtained.Mn?NO₃?2·4H₂O and Ce?NO₃?3·6H₂O were used as metal oxide precursors,and MnO_x-CeO₂/P-CA catalyst was prepared for NH₃-SCR reaction after calcination at 450 oC.The results showed that with the increase of the metal oxide loading,the NH₃-SCR activity of the catalyst showed a trend of increasing first and then decreasing.The mass of Mn element is 10 %,when Mn:Ce=1:1,the activity of 10%MnO_xCeO₂/P-CA catalyst was the highest,and the denitration efficiency can reach almost 100 % at 160 oC.Through characterization,it was found that phosphorus doping effectively increased the content of Mn4+,Ce3+ metal ions and chemisorbed oxygen that promoted catalyst activity.P doping provided more acid sites and active sites on the surface of the catalyst,which was more conducive to NH₃ adsorption,and also effectively improved the redox capacity of the catalyst,thereby promoting the NH₃-SCR reaction.II.Explored the use of phenol,m-cresol and furfural as precursors,and H₃PO₄ as the source of phosphorus.After a pot of sol gel,aging,atmospheric drying,and P doping carbon aerogel?P-CA?as a metal-free catalyst for NH₃-SCR.The results showed that the NH₃-SCR activity of the P-CA catalyst was related to the carbonization temperature and the P content on the catalyst surface.When the carbonization temperature is 800 oC,the P content on the surface of the PCA-800 catalyst is as high as 2.50 %,and the catalytic activity was the highest.The denitration efficiency at 200 oC could reach 60.6 %.After 8 hours of vacuum heat treatment at 380 oC,the P₂O₅ clogged in the micropores was successfully removed,which further increased the activity of the P-CA-800 vac catalyst.The denitration efficiency at 200 oC could reach 76.8 %.Due to the masked P₂O₅ acid sites were exposed on the surface,the NH₃ adsorption capacity of the P-CA-800 vac catalyst increased,which was more conducive to the NH₃-SCR reaction.At the same time,the P-CA-800 vac catalyst was analyzed before and after changes in the content of phosphorus and oxygen-containing functional groups on the surface after NH₃-SCR reaction at different temperatures.After that,the C3-P=O content in the P-CA-800 vac catalyst was less,indicating that the C3-P=O groups were the active site in the P-CA-800 vac catalyst.III.By comparing the conversion rate of P-CA-800 vac catalyst in different atmospheres,the introduction ofO₂ could improve the conversion rate of NO,while the introduction of NH₃ would further increase the conversion rate of NO.It showed that the coexistence of NH₃ andO₂ played a crucial role in the high conversion rate of NO on the surface of P-CA.The catalytic reduction of NO on the surface of phosphorus-doped carbon aerogel catalyst?P-CA?follows the dimer mechanism.Dimer?NO?2 decomposes into N₂ O and active oxygen O*.Then N₂ O can also be directly decomposed into N₂ and active oxygen O*,NO and generated active oxygen O*.The reaction forms NO₂.Based on the three-coordinate P-doped graphene?PC3G?structure,the electronegativity?2.19?of P is less than that of carbon?2.55?,so the C3-P=O groups of positively charged P.The group is considered to be the catalytically active site during the NH₃-SCR reaction.Due to the activation ofO₂ molecules on the surface of the phosphorus-doped carbon aerogel,as well as the presence of acidic groups for NH₃ adsorption and active sites for NO₂ generation,the phosphorus-doped carbon aerogel has good NH₃-SCR activity.And a preliminary evolution mechanism from C3-P=O to C-O-P structure is proposed.IV.It is worth considering that although the P-CA-800 catalyst has lower apparent activation energy and higher activity at low temperatures,its GHSV is only 500 h-1,which is significantly lower than other inorganic metal oxide-based catalysts.This may be due to the small number of active species on the surface of the catalyst.Therefore,further work is required to increase the number of active centers of the catalyst system.In addition,more experimental and theoretical work is needed to demonstrate and improve the proposed reaction mechanism and the oxidation resistance of the C-P bond must also be improved to improve the durability of the P-CA catalyst.