Preparation Of MOFs And Their Derivatives And Their Electrocatalytic Nitrogen Fixation Properties

Ammonia plays an important role in the national economy as an important agricultural fertilizer,industrial raw material,future fuel substitute and hydrogen storage molecule.At present,harbor-bostch process is mainly used for industrial ammonia production,which has the problems of high energy consumption and producing greenhouse gas CO₂.Electrochemical ammonia synthesis has the advantages of high efficiency,environmental protection and energy saving,and can realize distributed production,etc.At present,the focus of research is the high efficiency catalyst and the best synthesis process.The combination of high efficiency catalyst and electrochemical synthesis route can realize the efficient conversion of N₂to NH₃.In this paper,MOFs,their derivatives and NFO@MOFs composites were prepared by hydrothermal method.The effects of potential,temperature and magnetic field strength on the electrocatalytic nitrogen reduction performance of materials were studied respectively.The main research results are as follows:（1）All the four MOFs had complete crystal structure.The results showed that Fe-BTC had the highest ammonia yield(3.63×10^-10mol s^-1cm^-2)and Faraday efficiency（0.31%）at-0.376V（vs.RHE）and 80^oC.The mechanism study shows that the NH₃generation is due to the NRR process of N₂on Fe-BTC.After the addition of AC magnetic field,the ammonia yield(3.61×10^-9mol s^-1cm^-2)and Faraday efficiency（5.67%）of Fe-BTC material were increased by about 10 times and 18 times,respectively.This phenomenon was mainly due to the increase of the adsorption capacity of N₂on Fe-BTC surface by AC magnetic field.And the superposition of the induced electromotive force generated by the AC magnetic field and the electric potential of the electric field itself provides additional energy for the NRR reaction.（2）Fe₂O₃,Co₃O₄,Ni O and Cu O were obtained by thermolysis of the four MOFs precursors,respectively.The morphology of the derivatives could maintain the organic framework of the MOFs precursors.Fe₂O₃showed the best activity,with the highest ammonia yield(1.62×10^-10mol cm^-2s^-1)and Faraday efficiency（0.055%）at-0.276V（vs.RHE）.The mechanism study shows that NH₃is produced because of NRR process of N₂on Fe₂O₃.With the addition of AC magnetic field,the ammonia yield of Fe₂O₃material(3.67×10^-10mol cm^-2s^-1)is about 2.26 times higher than that without the application of magnetic field.When the magnetic field intensity is196m T～3.43m T,the ammonia yield of NRR decreases.The main reasons are as follows:First,the magnetic field of 0m T～1.96m T inhibs HER and promotes NRR,so the NH₃yield increases.However,when 1.96m T～3.43m T,the magnetic field does not further inhibit HER and therefore does not further promote NRR.Second,the Lorenz force and Kelvin force generated by the external magnetic field make the electrolyte on the working electrode produce additional convection,resulting in the magnetic current;Thirdly,the magnetic field enhances the energy state of electrons in Fe₂O₃,and meanwhile induces the magnetofluid effect near the electrode,which promotes the charge transfer in the catalytic process,and promotes the transfer of H protons generated at the anode to the cathode and the combination of activated N,thus promoting the ammonia production.（3）NFO@Fe-BTC composite material was successfully synthesized by hydrothermal method,and the NRR properties of the material were tested.The results showed that five kinds of materials with different loading loads all had NRR activity,among which the NRR activity of N was the highest.The highest ammonia yield(1.91×10^-10mol s^-1cm^-2)and Faraday efficiency（1%）were found at-0.376V（vs.RHE）,due to NFO@Fe-BTC-1 having the highest specific surface area and therefore the most active sites.And the mechanism shows that all NH₃is generated by N₂on the catalyst surface by NRR.The effect of AC magnetic field on the catalyst performance was studied.The results showed that when the magnetic field intensity was lower than0.406m T,the AC magnetic field promoted the NRR behavior of the catalyst.When the magnetic field intensity was 0.406m T,the NRR behavior of the catalyst was promoted.The highest ammonia yield(2.45×10^-10mol s^-1cm^-2)and Faraday efficiency（1.02%）were 1.2 times and 1.1 times of those without AC magnetic field,respectively.This phenomenon is mainly due to the first,the AC magnetic field inhibs HER,and then promotes NRR;The second magnetic field promotes the oxidation reaction of the anode,which increases the number of H protons produced by the anode,and thus increases the chance of reaching the cathode and combining with activated N to form NH₃.Third,the magnetic field promotes the charge transfer,so the H proton movement rate in the electrolyte is accelerated,and thus the NH3 yield is increased compared to that without the magnetic field.When the magnetic field intensity increased from 0.406m T to 2.840m T,ammonia yield and Faraday efficiency decreased with the increase of magnetic field intensity,ammonia yield decreased from2.45×10^-10mol s^-1cm^-2to 1.02×10^-10mol s^-1cm^-2.Faraday efficiency decreased from1.02%to 0.15%.This is mainly because the AC magnetic field still promotes the oxidation reaction of the anode,which increases the number of H protons and speeds up the movement rate.However,the H protons arriving at the cathode through the proton exchange membrane form more H₂,thus promoting HER and thus inhibiting NRR,so that the NH₃yield decreases.