ZIF-67 Derivatives And Their Composites For High Efficiency Photocatalytic Nitrogen Fixation

Ammonia（NH₃）is one of the main raw materials for synthetic nitrogen fertilizer and chemical production in agriculture.Moreover,due to its advantages of easy liquefaction,high energy density,high hydrogen content,convenient transportation and safety,ammonia is also a good zero-carbon fuel and hydrogen energy carrier.The main nitrogen element for synthetic ammonia comes from nitrogen in the air.Nitrogen is about 78%of the total air content,is a cheap and easily available raw material,inexhaustible.But the triple bond N≡N in nitrogen is very stable and is not easily activated to take part in the reaction.In today’s society,ammonia synthesis is still widely dependent on the Haber-Bosch process with high pollution and energy consumption.Therefore,researchers have conducted a lot of research to find environmentally friendly and sustainable artificial synthesis technology of ammonia.Photocatalytic nitrogen fixation,which can reduce nitrogen to ammonia by solar energy in a clean and sustainable way under mild conditions,has attracted wide attention.With the unremitting efforts of researchers,important progress has been made in improving the performance of catalysts to achieve efficient photocatalytic nitrogen fixation.However,photocatalytic nitrogen fixing has not been widely used in industrial production due to its poor adsorption capacity of N₂,low separation efficiency of photogenerated carriers and holes,and poor utilization efficiency of light energy.Among all kinds of semiconductor catalysts,MOFs have attracted wide attention due to their abundant pore size structure,high specific surface area and easy adjustment of structural functions.Based on the metal-organic framework ZIF-67,the photocatalytic nitrogen fixation performance of several catalysts was investigated by preparing its derivatives and forming heterojunctions with other semiconductor materials.Some methods and evidences are provided for designing and optimizing the structure of catalyst and improving the photocatalytic nitrogen fixing efficiency of catalyst.The main contents of this paper are as follows:（1）Construct Type II heterojunction by combining ZIF-67 with Cu-doped TiO₂ to improve photocatalytic nitrogen fixation efficiency.Compared with other MOF materials,ZIF-67 is easy to synthesize,and ZIF-67 has rhomboidal dodecahedron structure and large specific surface area,which makes ZIF-67 an excellent catalyst support.Cu-doped TiO₂ has abundant oxygen vacancies,which can be used as active sites to enhance the adsorption and activation ability of the catalyst for N₂.Heterojunction ZIF-67/Cu-TiO₂ has a large specific surface area and excellent ability to separate photoelectrons from holes,which significantly improves the photocatalytic nitrogen fixation efficiency of the catalyst.In pure water,the photocatalytic nitrogen fixation efficiency of ZIF-67/Cu-TiO₂ is 120.6μmol g^-1h^-1 and 9.8 times that of pure ZIF-67 under the irradiation of simulated sunlight.（2）Zn-Co₃O₄ with different content of oxygen vacancy（OVs）was prepared by low temperature oxidation method with Zn-doped ZIF-67 as precursor by adjusting the doping ratio of Zn for photocatalytic nitrogen fixation.Zn-Co₃O₄ still maintains the skeleton structure of porous rhomboidal dodecahedron,and has abundant oxygen vacancy and excellent electron transport capacity.These characteristics make Zn-Co₃O₄ show good photocatalytic nitrogen fixation effect.In visible light irradiation and pure water without sacrificing reagents,the nitrogen fixing efficiency of Zn-Co₃O₄ doped with zinc is 96.8μmol g^-1 h^-1,much higher than that of pure Co₃O₄,which provides a new idea for the design of photocatalysts based on metal-organic skeleton materials.（3）In order to further explore the application of metal skeleton materials in photocatalytic nitrogen fixation and enhance the photocatalytic nitrogen fixation performance of the catalyst,we encapsulated H₃PW₁₂O₄₀(PW₁₂)in Zn-doped ZIF-67 as the precursor.The composite porous nanocages（Zn-Co₃O₄/WO₃）with abundant oxygen vacancy were also synthesized by low temperature oxidation method.Compared with Zn-Co₃O₄,the synthesized Type II heterojunction Zn-Co₃O₄/WO₃ has larger specific surface area and higher oxygen vacancy content,as well as better light absorption performance,which enables Zn-Co₃O₄/WO₃ to better adsorb and activate N₂ and make full use of light energy,thus enhancing the photocatalytic nitrogen fixation activity of the material.In pure water irradiated by visible light,the ammonia formation rate of Zn-Co₃O₄/WO₃ is 231.9μmol g^-1 h^-1,which is significantly higher than that of Zn-Co₃O₄.