| The Haber-Bosch method for synthesizing ammonia is considered to be one of the greatest reactions of the 20th century,but the environmental pollution problems exposed due to its high energy-consuming production process are becoming more and more serious.Photocatalytic nitrogen fixation uses solar energy to achieve ammonia synthesis at normal temperature and pressure,which greatly improves the energy-consuming process.It is considered as a green and clean environment-friendly technology and provides theoretical guidance for ammonia synthesis under mild conditions.However,due to the extremely stable nitrogen-nitrogen triple bond,photocatalytic nitrogen fixation technology has a low conversion rate from solar energy to chemical energy.Therefore,the development of photocatalysts that can efficiently activate nitrogen-nitrogen triple bonds needs to be solved urgently.Compared with traditional photocatalysts,MOFs materials are considered as an ideal nitrogen-fixing photocatalyst due to their large specific surface area,adjustable band structure,and unsaturated metal sites.In addition,transition metal iron plays a vital role in both industrial ammonia synthesis and biological nitrogen fixation.Based on this,we speculate that among many MOFs materials,Fe-MOFs materials can be used as efficient nitrogen-fixing photocatalysts and the nitrogen fixation property can be improved by further modification.Specific research contents are as follows:（1）A series of Fe-MOFs materials were prepared by solvothermal method and their photocatalytic nitrogen fixation activity was investigated.The experimental results show that:MIL-88（Fe）-40.035μmol·L-1h-1,MIL-100（Fe）-46.532μmol·L-1h-1,MIL-101（Fe）-50.355μmol·L-1h-1,indicating that Fe-MOFs can be used as efficient nitrogen-fixing photocatalysts;Further comparison of MIL-101（Fe）and MIL-101（Cr）with the same structure,the nitrogen fixation activity test results show that MIL-101（Cr）has no nitrogen fixation activity,which proves that transition metal iron can effectively activate the nitrogen-nitrogen triple bond to generate ammonia.Combining a series of characterizations（PC,EIS,PL,DRS,in situ FT-IR）and theoretical calculations further proves that the rate-controlling step of the photocatalytic nitrogen fixation process is the activation of the nitrogen-nitrogen triple bond,and the transition metal iron is used as Fe-MOFs metal catalytic center can effectively activate the nitrogen-nitrogen triple bond,so it has good nitrogen-fixation activity.（2）Based on the research in the previous section,the catalyst was modified by copper ion doping to improve its nitrogen fixation performance.Cu/MIL-101（Fe）with different contents was prepared by solvothermal method,and the nitrogen fixation activity of the composite catalyst was evaluated.The experimental results show that copper ion doping has a certain effect on nitrogen fixation,as the doping amount increases,the activity gradually increases,and 6-Cu/MIL-101（Fe）has the best performance of 157.6μmol·L-1h-1.Compared with pure catalyst,the performance is improved by 3 times.Combined with a series of characterization analysis,it can be seen that on the one hand,the increase in activity is due to copper ion doping,the specific surface area of the catalyst becomes larger,so it will expose more reactive sites,and the increased specific surface area will adsorb more nitrogen and thus provide sufficient nitrogen source;On the other hand,copper ion doping makes the band gap of the catalyst wider and the conduction band potential more negative,thus enhancing the reducing ability of electrons;In addition,copper ion doping can form certain defects and improve electron-space by effectively capturing electrons,the separation of the hole pairs prolongs the life of the carriers,therefore,there are more electrons involved in the activation of the nitrogen-nitrogen triple bond under the excitation of light conditions,thus further promoting the reduction of nitrogen. |
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