Study On The Degradation Of Nitric Oxide In Coal-fired Flue Gas By Functionalized MIL-125(Ti) Catalyst

Rapid development of national economy benefits from fossil fuels consumption（especially coal）.Therefore,it is necessary to study on the removal of the nitrogen oxides（NO_x）,which not only cause great damage to environment,but also seriously threaten people’s health leading to many diseases.The traditional denitrification technologies,such as selective catalytic reduction（SCR）,still have the problem of secondary environmental pollution.Through efficient,safe and environmentally friendly photocatalytic technology and adsorbent adsorption technology,it provides a new solution for removing NO_x.Metal-organic framework（MOFs）materials are widely used in the field of photocatalysis or adsorption.Among them,MIL-125（Ti）has good photocatalytic and adsorption ability after amino functionalization,but there are few studies on the combined effect of the two abilities in the process of nitrogen oxide removal.In this thesis,based on density functional theory（DFT）and Grand Canonical Monte Carlo（GCMC）calculation methods as well as photocatalytic experiment methods,the mechanism of NO removal by NH₂-MIL-125（Ti）was investigated on a multi-scale.The research carried out in this thesis is as follows:The primitive cell model of NH₂-MIL-125（Ti）was optimized by DFT,and the electronic structure of the optimized structure was calculated.It was found that its photocatalytic reaction ability mainly came from the new donor energy level introduced by the-NH₂ group,which replaced the top of the original valence band and reduced the band gap,thus obtaining a wide range of visible light response.The optimal exposed crystal surface（001）was then determined by surface energy,and the active site of the reaction（the surface Ti atom）was determined by the orbital at the conduction band and the surface electrostatic potential.On this basis,the adsorption behaviors of several gas molecules on exposed crystal planes were calculated,and the degradation mechanisms of NO on NH₂-MIL-125（Ti）surface were investigated.The results showed that superoxide radical（·O₂^-）was the main active substance in the photocatalytic degradation of NO,and the main product of degradation was NO₂.On the contrary,the reaction energy of HNO₂ and HNO₃ generated by hydroxyl radical（·OH）was too large,and the reaction was difficult to carry out,so·OH contributed little to the degradation efficiency of NO.At the same time,it was found that the adsorption energy of SO₂,an important component in coal flue gas,was small at the active site of the catalyst,which would not inhibit the degradation efficiency of NO.Subsequently,GCMC was used to investigate the adsorption behavior of NH₂-MIL-125（Ti）relative to NO,NO₂ and SO₂ gases at 298K and 1bar,and the calculation showed that NH₂-MIL-125（Ti）had the ability to adsorb and store NO₂ and SO₂.Then,three adsorption regions in the NH₂-MIL-125（Ti）cavity were determined by the adsorption density distribution and radial distribution function,and the adsorption energies of three gases in three regions were calculated by means of the DFT.The results show that the adsorption capacity of NO₂ is the highest in the area of tetrahedral pore and metal center,while the adsorption capacity of SO₂ is stronger than that of NO₂ and NO near the organic connector of octahedral pore.Combined with the calculation results of the photocatalysis and adsorption capacity of NH₂-MIL-125（Ti）,it can be inferred that the mechanism of NO removal by NH₂-MIL-125（Ti）is that NO is first photooxidized by O₂ to produce NO₂,and then the produced NO2 is adsorbed by the catalyst bulk phase.Finally,the photocatalytic degradation efficiency of NO was studied by NH₂-MIL-125（Ti）photocatalytic degradation of NO under the conditions of dry aerobic,wet anaerobic,wet aerobic and SO₂ are involved.The results showed that the degradation efficiency of NO reached 81.1%when dry aerobic,while the removal rate of NO was only 12.2%when wet anaerobic,which verified the conclusion that·O₂^-was the main active substance in the photocatalytic degradation of NO and·OH degradation efficiency was low in the calculation of DFT.The oxygen content in the wet aerobic experiment was the same as that in the dry aerobic experiment,but the degradation efficiency of NO in the presence of water was 35.0%,indicating that H₂O would hinder the photocatalytic degradation of NO,which was due to the decrease of photocatalytic efficiency due to the competitive adsorption of·O₂^-and·OH produced by H₂O.The increase of photocatalytic efficiency with time under wet aerobic conditions may be due to the auxiliary effect of Termolecular Eley-Rideal（TER）degradation mechanism.NO₂ was not observed at the exit when NO was degraded in dry aerobic condition,and the concentration of SO₂ dropped sharply when the catalyst was loaded and maintained for a considerable period of time,indicating that NH₂-MIL-125（Ti）could adsorb and store NO₂ and SO₂.In the experiment of dry aerobic presence of SO₂,the degradation efficiency of NO did not decrease,but further increased to 96.2%.The results indicate that NH₂-MIL-125（Ti）catalyst has a promising application in simultaneous desulfurization and denitrification.There are 29 figures,7 tables and 129 references in this thesis.