| Ammonia is one of the most important chemicals in modern society.At present,NH3 is mainly produced by the"Haber-Bosch"method using N2 and H2 as raw materials under extremely high temperature and pressure.Photocatalytic nitrogen fixation uses water and N2 as raw materials to produce ammonia under ambient environment.The core of photocatalytic nitrogen fixation technology is the design and preparation of highly efficient photocatalytic materials.A variety of semiconductor photocatalytic materials have been reported for nitrogen fixation research,but the photocatalytic nitrogen fixation activity of these materials is still not ideal.TiO2 has attracted much attention in the field of photocatalysis due to its structural stability and high utilization rate of ultraviolet light.However,the wide band gap?3.0-3.2 eV?limits the research of TiO2 in the field of photocatalytic nitrogen fixation.Therefore,it is of great research significance to further enhance the activity of TiO2 photocatalytic nitrogen fixation and elucidate the mechanism of semiconductor photocatalytic nitrogen fixation.In this paper,through the construction of TiO2 surface defects,the material's photogenerated charge separation efficiency and N2 adsorption and activation ability are optimized.In addition,by constructing g-C3N4,Ti3C2with TiO2 to form heterojunction composite materials,it can promote the migration and separation of photogenerated carriers,significantly improve the photocatalytic nitrogen fixation activity,and the possible reaction mechanism was revealed.The specific research contents are as follows:First,the TiO2 precursor was prepared by solvothermal method,and then the temperature-boosting method was used to assist the reduction of NaBH4 to prepare defect-rich TiO2?TiO2-OV?nanoparticles.X-ray diffraction,FT-IR,Raman spectroscopy and transmission electron microscopy were used to analyze the structure,composition and morphology of the prepared TiO2 materials.Electron paramagnetic resonance technology confirmed that oxygen vacancies have been constructed on the surface of TiO2.The results of electrochemical impedance and photocurrent proved that the defect structure is conducive to the migration of photogenerated electrons which improves the efficiency of photo-generated carrier separation.At the same time,the linear voltammogram showed that TiO2-OV is easier to activate N2 than TiO2.The photocatalytic nitrogen fixation experiments showed that TiO2-OV has excellent photocatalytic nitrogen fixation activity(38.2 mol·L-1),which was about 2 times higher than TiO2(22.8 mol·L-1).The ultraviolet-visible diffuse reflectlance spectrum results showed that the introduction of defect structure can effectively optimize the light absorption performance of the material.The results of electrochemical impedance and photocurrent proved that the construction of the defect structure is beneficial to improve the efficiency of photogenerated carriers separation.At the same time,the linear voltammogram suggested that TiO2-OV is easier to activate N2 than TiO2.Therefore,surface defect modification coul achieve triple optimization of catalyst light absorption performance,carrier separation efficiency and N2 activation,and significantly improved the nitrogen fixation activity of the photocatalyst.Secondly,TiO2-OV and g-C3N4/TiO2-OV composite materials were prepared in H2/Ar atmosphere by the temperature-programming method.The composition,structure and morphology of the prepared materials were analyzed by X-ray diffraction and transmission electron microscopy.Electron paramagnetic resonance results proved that both TiO2-OV and g-C3N4/TiO2-OV composites have oxygen defect structures.The photocatalytic nitrogen fixation experiment results proved that the construction of heterostructure can effectively improve the photocatalytic nitrogen fixation performance of g-C3N4/TiO2-OV,in which the activity of 1 wt%g-C3N4/TiO2-OV after 4 h of photocatalytic nitrogen fixation reaction was 31.6 mol·L-1.The results of steady-state fluorescence spectroscopy,photocurrent and impedance certificated that the construction of heterostructure can effectively promote the separation of photogenerated electron-hole pairs.And the possible photocatalytic reaction mechanism was clarified.Finally,in order to further optimize the photoconversion efficiency of the catalyst and improve the utilization rate of the photogenerated electron-hole pair,the Ti3C2/TiO2-OV heterojunction composite material was constructed by ultrasound.The results of photocatalytic nitrogen fixation experiments showed that Ti3C2/TiO2-OV composites have significantly enhanced photocatalytic nitrogen fixation activity,in which,Ti3C2/TiO2-OV-2 had the best photocatalytic nitrogen fixation activity.After 2 h photocatalytic nitrogen fixation reaction,it could produce 67.9 mol·L-11 NH3,which is about 3 times the activity of TiO2 monomer(20.9mol·L-1).The ultraviolet-visible diffuse reflectlance spectrum results indicated that the construction of the composite structure can effectively enhance the light absorption performance of the catalyst.The impedance and photocurrent results demonstrated that the interface structure of Ti3C2 in close contact with TiO2-OV can promote the photogenerated electron transfer,and the possible mechanism was proposed. |
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