Study On Photocatalytic Nitrogen Fixation Of Metal Oxide/Metal-organic Framework Derivatives

Nitrogen is abundant in nature and plays an important role in life on earth.Plants can use nitrogen to synthesis chlorophyll for photosynthesis.The survival of animals also depends on nitrogen,which is an important component of protein and nucleic acid.Nitrogen element mainly presents in the form of nitrogen molecule in nature.Most organisms cannot directly convert nitrogen into compounds such as ammonia or nitrate.Haber-Bosch method is employed to convert nitrogen into ammonia in industry,which consumes a lot of energy and causes serious environmental pollution in the process of operation.Therefore,people are looking for a clean way to achieve nitrogen fixation.Photocatalytic nitrogen fixation can complete catalytic reactions at room temperature and atmospheric pressure with H₂O and N₂acting as reactants and solar energy as the driving force.Although several research results have been achieved after decades of efforts by researchers,photocatalytic nitrogen fixation is still useless in industrial production.The main problems are the low adsorption capacity of N₂,the low activation of N₂,the low utilization rate of light energy and the low carrier separation efficiency.In order to solve these problems,the following aspects are emphasized in this paper:（i）abundant active sites of photocatalytic nitrogen fixation are constructed in catalysts to facilitate the adsorption and activation of N₂;（ii）expanding the specific surface area of the catalyst makes more active site exposed on the surface of the catalyst to contribute the occurrence of catalytic reaction;（iii）heterojunction catalyst can integrate the advantages of various semiconductor materials at the same time,such as expanding the absorption range of light and improving the separation efficiency of electron-hole pair.We have completed the research on the performance of several catalysts in the exploration of the performance of photocatalytic nitrogen fixation,which has certain inspiration for the preparation of more outstanding and stable nitrogen fixation catalyst.Type Ⅱ heterojunction was constructed by combining g-C₃N₅and Fe doped W₁₈O₄₉(Fe-W₁₈O₄₉)for photocatalytic nitrogen fixation.The surface of g-C₃N₅is porous and fluffy.The band gap energy of g-C₃N₅is relatively narrow,which can adjust the absorption range of the hybrid and improve the utilization efficiency of light energy.Fe-W₁₈O₄₉nanowires are rich in oxygen vacancies（OVs）,which provides sufficient active sites for nitrogen adsorption and activation.The generated complex Fe-W₁₈O₄₉/g-C₃N₅has excellent electron and hole separation ability,and the photogenerated carriers can be separated efficiently,which seriously elevates the nitrogen fixation capacity of the catalyst.When Fe-W₁₈O₄₉/g-C₃N₅is used as the catalyst and Na₂SO₃is used as the hole-sacrificing reagent for nitrogen fixation,the formation rate of ammonia is significantly increased compared with pure g-C₃N₅or Fe-W₁₈O₄₉.In order to further improve the catalytic performance of the catalyst,Z-scheme heterostructure Mo O_3-x/Fe-W₁₈O₄₉was prepared by two-step solvothermal method.In the Z-scheme heterojunction system,the weak reduction electrons and weak oxidation holes generated by photoexcitation can be quenched,which makes the catalyst exhibit strong redox ability.The prepared Mo O_3-xis a flower-like structure formed by the stacking of nanosheets,which can be used as the attachment carrier of nanowires Fe-W₁₈O₄₉to effectively inhibit the aggregation of nanowires.Hence,more oxygen vacancies in Fe-W₁₈O₄₉will appear on the surface of the catalyst.The vacancies in Mo O_3-xcan serve as the composite center of weak reduction electrons and weak oxidation holes to promote the occurrence of nitrogen fixation reaction.The nitrogen fixation performance of heterojunction Mo O_3-x/Fe-W₁₈O₄₉is improved than Mo O_3-xand Fe-W₁₈O₄₉.Under the conditions of simulated sunlight,the ammonia production rate of Mo O_3-x/Fe-W₁₈O₄₉reaches 137.5μmol g^-1h^-1without adding any sacrificial reagent.In addition,Mo O_3-x/Fe-W₁₈O₄₉shows good stability in the photocatalytic process.Nitrogen-doped graphite carbon（NGC）nanocages hybrid loaded with WC and Co nanoparticles（WC-Co/NGC）were synthesized by calcination of H₃PW₁₂O₄₀(PW₁₂)encapsulated in ZIF-67 in N₂atmosphere.WC-Co/NGC-2 with the most outstanding photocatalytic effect has a NH₃formation rates of 142μmol g^-1h^-1and 157μmol g^-1h^-1under visible light and simulated sunlight.The formation rate approaches sixfold higher than individual Co/NGC.Due to the synthesized WC-Co/NGC still maintains a 3D configuration after calcination,its specific surface area is larger than that of Co/NGC.At the same time,the content of pyridine N in WC-Co/NGC-2 is higher than Co/NGC,which can effectively act as electron operation center to assist the photocatalytic reaction.Thus,WC-Co/NGC-2 has achieved excellent catalytic effect.Here,the core-shell structure of MIL-125@Ti O₂was prepared by post solvothermal method for photocatalytic nitrogen fixation.Linker defects that can be acted as active sites appeared in the residual MIL-125 after TAA etched.In addition,the mesoporous structure accelerates electron/mass exchange and promotes the activation of N₂in MIL-125@Ti O₂.The shell structure constructed by thin Ti O₂nanosheets possesses a large specific surface area.The existing oxygen vacancies active sites can be fully exposed to achieve N₂adsorption,which enhances the occurrence of catalytic reaction.A O-Ti-N covalent bond is established between the titanium-oxygen clusters in the peripheral of Ti O₂and the linker in MIL-125,which improves the effective separation of electron-hole pairs.In nitrogen fixation performance tests,the ammonia formation rate of MIL-125@Ti O₂-2h is 8 times higher than that of MIL-125alone.It provides a novel idea for the prepared of photocatalytic nitrogen fixation materials with controllable structure and multiple active sites.