Preparation And Photocatalytic Nitrogen Fixation Of MIL-125(Ti) Composites

Ammonia is an important basic chemical raw material in the field of fertilizer and fiber manufacturing,as well as an emerging high-efficiency energy storage material.At present,the industrial synthetic ammonia process has huge energy consumption,high raw material cost,and accompanied by a large amount of greenhouse gas emissions.Therefore,it is necessary to find a green,clean,low-energy,and low-cost ammonia synthesis process.In recent years,photocatalytic ammonia synthesis technology has received widespread attention.This technology uses nitrogen and water as raw materials（N₂+3H₂O→2NH₃+3/2O₂）to synthesize ammonia and oxygen using the primary energy of solar energy under ambient temperature and pressure.It provides new hope for the development of a new green ammonia synthesis technology.MIL-125（Ti）,as a classic MOFs material,has a large specific surface area,adjustable band structure and pore structure,coordination unsaturated metal active center,which can effectively adsorb and activate nitrogen molecules.It is a potential high-efficiency photocatalytic nitrogen fixation catalyst.This research is based on MIL-125（Ti）,while giving full play to its advantages in nitrogen adsorption and activation,it also addresses the problems of low carrier separation efficiency and poor light absorption performance.By rationally designing and modifying the components of MIL-125（Ti）（ligand and metal center）,more active sites of Ti metal center were exposed,the light response range was broadened,the excited state charge concentration was increased,the effective separation of photogenerated e^--h⁺pairs were promoted,and the migration of photogenerated carriers to the active site of N₂ reaction was accelerated.The specific work is as follows:（1）The ligand-like functionalization strategy was developed by the co-grinding assisted synthesis method.The ligand terephthalic acid（BDC）was functionalized with Ti₃C₂T_x（T）as a modifier,and Ti₃C₂T_xfunctionalized T-MIL-125（Ti）photocatalyst was synthesized.The photocatalytic performance of the photocatalyst was evaluated by the photocatalytic ammonia synthesis test under simulated sunlight（320-780 nm）.The results showed that the dinitrogen fixation activity of T-MIL-125（Ti）was better than that of MIL-125（Ti）(31.68μmol·g^-1_cat·h^-1).Among them,10%-T-MIL-125（Ti）showed the best nitrogen photofixation activity of 103.02μmol·g^-1_cat·h^-1,which was 3.25 times that of MIL-125（Ti）,and the activity did not significantly decrease after three cycles.The improvement of T-MIL-125（Ti）photocatalytic nitrogen fixation performance is mainly due to the fact that T with a larger molecular weight leads to an increase in the steric hindrance of BDCT,which leads to T-MIL-125（Ti）with more ligands missing defects,exposing more unsaturated coordinated Ti metal active sites;T,which has excellent light absorption and conductivity,enhances the effective use of light energy by T-MIL-125（Ti）,accelerates the photo-generated electrons transmission from the ligand of BDCT to the active site of the unsaturated coordinated Ti metal,inhibits the recombination of photogenerated e^--h⁺pairs and realizes the rapid and effective transmission and utilization of photogenerated carriers.（2）The MIL-125（Ti,Fe）bimetallic MOFs photocatalyst was synthesized by one-step hydrothermal method using Fe Br₃ as a modifier for the metal center of MIL-125（Ti）to solve the problem of poor visible light absorption of MIL-125（Ti）.XRD,FT-IR,Raman,BET characterization results showed that the appropriate introduction of Fe has little effect on the crystalline structure,surface groups,specific surface area and pore structure of MIL-125（Ti）.The results of UV-Vis DRS and visible light（420-780 nm）nitrogen fixation activity tests showed that the addition of Fe significantly improves the light absorption performance and the nitrogen fixation performance of the catalyst under visible light irradiation.Among them,MIL-125（Ti,Fe）-0.10 has the highest visible nitrogen fixation activity of 15.26μmol·g^-1_cat·h^-1,and the extension of MIL-125（Ti,Fe）light response range to visible light region is mainly due to:The Fe³⁺in MIL-125（Ti,Fe）in-situ replaces part of the Ti⁴⁺metal nodes in MIL-125（Ti）to form Fe-O_x clusters with visible light response,which greatly enhances the absorption of visible light,improves the band structure of the photocatalyst,increases the concentration of excited state charge,and finally enhances the photocatalytic nitrogen fixation performance.