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

The atmospheric pollutants produced by the combustion of fossil fuels are nitrogen oxides（NO_x）,and the main components of atmospheric pollutants are nitric oxide（NO）and nitrogen dioxide（NO₂ ）,which cause serious environmental problems such as acid rain,optical smog and ozone destruction.NH₃selective catalytic reduction of NO（NH₃-SCR）is a widely used technology to control the emission of NO from fixed sources.In industrial production,V₂O₅-WO₃/Ti O₂ is used as commercial catalyst.However,the main active component of the catalyst has disadvantages such as strong toxicity and high price of V₂O₅,and it is difficult to recycle the waste of V₂O₅-WO₃/Ti O₂ catalyst,which is easy to cause secondary pollution to the environment.Therefore,the development of low-cost and environmentally friendly catalysts has become a research hotspotCarbon material carrier has the advantages of large specific surface area,high porosity and easy recovery of metal oxides in deactivated catalyst.There are abundant functional groups on the surface of carbon materials.Carbon itself can also act as a catalyst.The surface chemical properties of carbon materials have an important effect on the catalytic activity of carbon based catalyst for NO reduction.It has been reported that oxygen-containing groups or nitrogen-containing groups are beneficial to the direct catalytic decomposition of NO on carbon.The oxygen-containing functional groups on carbon can improve the surface acidity,Mn⁴⁺/Mn^x+ratio and the number of adsorbed oxygen species of Mn-Ce O_xcatalyst supported by carbon materials,and thus improve the NH₃-SCR activity of Mn-Ce O_xcatalyst supported by carbon materials.I.Phosphorus-doped carbon aerogels（P-CA）was prepared from phenol,m-cresol,furfural and phosphoric acid MnO_x-Ce O₂ /P-CA catalysts were prepared via incipient wetness impregnation of Mn（NO₃）₂·4H₂O and Ce（NO₃）₃·6H₂O over phosphorus-doped carbon aerogels（P-CA）for selective catalytic reduction of NO by NH₃（NH₃-SCR）.The results show that P doping on carbon support significantly promotes the NH₃-SCR performance of MnO_x-Ce O₂ /P-CA catalysts.P modification can improve the hydrophilicity of carbon support and facilitate the dispersion of MnO_x-Ce O₂ components on carbon surface,which enhances the electronic interactions between MnO_xand Ce O₂ .The optimal catalyst10%MnO_x-Ce O₂ /P-CA shows higher surface acidity,more Mn⁴⁺and Ce³⁺,and a larger amount of surface chemisorbed oxygen（O_α）than that of 10%MnO_x-Ce O₂ /CA catalyst.As a result,even though in the company of SO₂ ,more NO can be adsorbed and oxidized over the10%MnO_x-Ce O₂ /P-CA catalyst to generate more NO complexes,alleviating the suppression of SO₂ on the SCR reaction via Langmuir-Hinshelwood（L-H）mechanism,and thus the SO₂ resistance is enhanced.II.H₃PO₄as phosphorus source,phenol,m-cresol and furfural were prepared through a pot of sol-gel and aging to obtain phosphorus-doped wet aerogel,and then phosphorus-doped carbon aerogel（P-CA）was obtained through atmospheric drying and carbonization process.Phosphorus-doped carbon aerogel was ground and used as a metal-free catalyst to remove NO_x.The results show that the carbonization temperature of phosphorus doped carbon aerogels and the content of P on the surface of P-CA catalyst are related to the catalytic activity of NH₃-SCR.When the carbonization temperature is 800℃,the NOx removal efficiency of p-CA catalyst is the highest,and the NOx removal efficiency of P-CA catalyst is 60.6%at 200 ^oC.The higher the content of P,the higher the NO_xremoval efficiency of phosphorus doped carbon aerogel catalyst.However,the phosphoric acid compound will have thermal decomposition and release at high temperature,and part of the functional groups containing phosphorus will be converted into P₂O₅particles,resulting in the pore of carbon aerogel catalyst being blocked and covering the surface of the active site at the same time.The P-CA-800 catalyst was subjected to vacuum heat treatment for 8 hours.When P₂O₅particles were removed,the NO_xremoval efficiency of P-CA-800_vaccatalyst was increased to 76.8%at 200^oC.Through characterization,it was found that phosphorus atom doping on the surface of carbon aerogel material could produce more acid sites and active sites conducive to NH₃-SCR reaction.The functional groups of phosphorus on the surface of carbon aerogel materials are C₃P,C₃-P=O,C-O-P and P₂O₅.The presence of C₃-P=O and C-O-P functional groups is conducive to the NH₃-SCR reaction.In addition,the NOx removal efficiency of p-CA-800_vaccatalyst is low under the passage of NO or NO+O₂ in different atmosphere,indicating that the passage of NH₃is very important for NH₃-SCR reaction.In-situ DRIFTs test shows that the adsorption peak of NH₃disappears after the passage of NO+O₂ in p-CA-800_vaccatalyst in advance.After NO+O₂ is entered in advance,NH₃is entered.However,the strength of NO adsorption peak will not disappear with NH₃entering.The results showed that Eley-rideal（E-R）mechanism was the main reaction path of NH₃-SCR reaction over P-CA-800_vaccatalyst.DFT calculation was carried out for the main phosphorus-containing functional groups on the surface of P-CA-800_vaccatalyst.The presence of COP was beneficial to the adsorption of NH₃.The C₃PO site is the most active site for NO molecule adsorption.However,the structure of NO molecule is relatively stable.The COP site is the most active site for O₂ molecular dissociation.At the same time,the adsorption of NH₃is exothermic reaction,and the adsorption of NO and O₂ is endothermic reaction.