Study On Preparation Of Modified Carbon Nitride/iron Matrix Composites And Its Nitrogen Photofixation Performance

Nitrogen is one of the most important elements in living organisms,and it is also one of the basic components of nucleic acids and amino acids.Its importance to life on Earth goes without saying.Nitrogen gas（N₂）is the main form of nitrogen and is abundant in the atmosphere.However,due to the highly stable nitrogen nitrogen triple bond（N≡N,945 k J/mol）in nitrogen gas,its chemical properties are very stable and it is difficult for organisms to use directly.Only a small amount of nitrogen-fixing microorganisms can convert nitrogen gas into nitrogen-containing compounds that can be used by other organisms.Nitrogen photofixation is a new nitrogen fixation technology driven by solar energy.Compared with traditional nitrogen fixation processes,nitrogen photofixation does not require external energy input,and only uses light energy to initiate reactions,which has the advantages of energy conservation and environmentally friendliness.Graphitic carbon nitride is a polymer composed of carbon and nitrogen elements.Due to its non-toxic and excellent band structure,it has great potential in the field of nitrogen photofixation.However,in the nitrogen photofixation reaction,the limitations of graphitic carbon nitride,such as small surface area,few reactive sites,weak nitrogen gas adsorption and activation performance,low efficiency of photo-generated charge carrier transfer,and high rate of photo-generated electron-hole recombination,have hindered its applications in nitrogen photofixation.In this study,three different composite photocatalysts based on graphitic carbon nitride were prepared using template method,alkali-assisted method,supramolecular self-assembly method,and hydrothermal loading,including porous carbon nitride/zinc ferrite（PCN/ZFO）,nitrogen-deficient carbon nitride/iron oxide（GCN_x/FO）,and nitrogen-deficient 3D hollow spherical carbon nitride/cobalt ferrite（3DV_NPCN/CFO）,and their nitrogen photofixation performance was studied.The specific work is as follows:In this study,large surface area porous carbon nitride（PCN）was prepared using monodisperse silica spheres as hard templates.Then,zinc ferrite was loaded on PCN through a simple hydrothermal method to form a porous carbon nitride/zinc ferrite（PCN/ZFO）composite material with a Z-Scheme heterojunction structure.The large surface area of its porous structure significantly enhanced the catalyst’s ability to adsorb N₂,while the Z-Scheme heterojunction structure inhibited the recombination of photo-generated electron-hole pairs and effectively enhanced the nitrogen photofixation activity.In the evaluation of nitrogen photofixation performance,the highest nitrogen fixation activity of PCN/ZFO could reach 73.66μmol/L/h.In this study,nitrogen-deficient carbon nitride/iron oxide（GCN_x/FO）composite photocatalyst with a Z-Scheme heterojunction structure was prepared by a one-step calcination method.The concentration of nitrogen defects was regulated by adjusting the amount of KOH,and the crystal phase and magnetic properties of iron oxide were controlled by the amount of ferrous oxalate.The transformation between hematite（α）and magnetite（γ）of Fe₂O₃ was analyzed by X-ray diffraction,and the magnetic property of GCN_x-0.1/FO product was tested by vibrating sample magnetometer.Introducing nitrogen defects reduced the band gap and extended the light absorption spectrum range,while enhancing the ability to adsorb and activate N₂,and nitrogen defects can capture electrons to become new reactive sites.The experimental results of nitrogen photofixation performance showed that the nitrogen fixation activity of GCN_x/FO samples could reach 69.7μmol/L/h.In this study,3D hollow spherical carbon nitride with nitrogen defects was successfully prepared by supramolecular self-assembly method combined with alkali-assisted method,and cobalt ferrite was loaded on its surface to obtain a nitrogen-deficient 3D hollow spherical carbon nitride/cobalt ferrite（3DV_NPCN/CFO）composite material with a Z-Scheme heterojunction structure.The hollow spherical structure expanded the surface area and enhanced the physical adsorption capacity of the photocatalyst to N₂,and provided more reactive sites,while the nitrogen defect structure introduced by the alkali-assisted method improved the chemical adsorption and activation abilities of nitrogen gas,and captured electrons to become new reactive sites.The experimental results of nitrogen photofixation performance showed that the nitrogen fixation efficiency of 3DV_NPCN/CFO could reach 73.32μmol/L/h.