Fabrication Of 2D MOFs Based Metal-organic Frameworks Nanosheets For Photocatalytic NH₃ Synthesis And Their Mechanisms

Ammonia（NH3）,a key intermediate in the process of nitrogen cycle,plays a vital role in agriculture,biological,and industrial fields.Currently,more than 90%of NH3production relies on the Haber-Bosch process,which is operated at high reaction temperature and pressure（300-500℃,150-300 atm）,leading to more than 1%of the global annual energy consumption and 1.6%of total global CO2 emissions.Thus,accelerating the development of solar energy for NH3 synthesis and clean energy conversion is a vital opportunity.However,the rate-limiting step of the NH3 synthesis process is breaking the N≡N bond,due to the extremely stable structure of N₂(bond energy of 940.95 k J mol^-1).Thus,how to design a new catalytic system to enhance the performance of NH3 synthesis,accelerating the N2 activation and hydrogenation on the surface is still a great challenge.To dress the problem,the two-dimensional MOFs-based nanosheets（OVR-Cu Co-MOFs and OVs-Co Bi-MOFs nanosheets）with defect engineering were synthesized,and applied in exploring the properties of designing the novel catalytic system for direct NH3 synthesis employing air（N2 and O2）under visible light.In situ Fourier Transform Infrared（FTIR）and quantum chemical calculation method was adopted to explore the mechanisms of photocatalytic NH3 synthesis.Additionally,the separation and migration of charges on the nanosheets of defects sites,its dynamic behaviors,and reaction mechanism had also been analyzed thoroughly.The main achievements are as follows:A novel pathway of direct air reduction reaction（ARR）to ammonia is developed over an effective catalyst of oxygen-vacancy-rich bimetallic organic framework ultrathin nanosheets（OVR-Cu Co-MOFs NS）.The morphological and atom feature was further studied by electron microscope from which the plate structure and metal atoms can be observed,and the nearly transparent nature suggests the ultrathin thickness.By the structure characterizations,the results demonstrated OVR-Cu Co-MOFs NS with rich oxygen vacancies were synthesized.From photo/electro characterization,high concentration of oxygen vacancies on the OVR-Cu Co-MOFs NS can not only be used as N2 chemisorption and reaction sites,but also can prolong the lifetime of photogenerated carriers（τ=1.22 ns）,increase the utilization rate of sunlight（14.21%）,enhance the photocurrent intensity（60μA）and so on.The catalyst with unique oxygen defective sites shows an excellent NH3 synthesis efficiency with air(280.26μmol g^-1·h^-1),which is 4.6-fold higher than that with pure N₂.Moreover,the experiments and theoretical calculations indicate that the transformation of air mainly follows a redox pathway,in which N2 and O2 could be trapped at the oxygen vacancies to generate nitric oxide（*NO）and further transformed into ammonia.Compared with traditional nitrogen reduction（*N2→*NH→NH3）,the ARR process（*N₂→*NO→NH₃）shows lower free energies of the onset activation step（*N₂→*N-NO）and rate-limiting step（*NO→*NHO）.This work provides a new and sustainable pathway for photo-driven ammonia synthesis.To further improve the performance of NH3 synthesis,two-dimensional MOFs nanosheets（OVs-Co Bi-MOFs NS）with oxygen vacancies were synthesized via a simple one-step hydrothermal method.The morphology and elemental composition were obtained by the microscopic characterization.At the same time,rich oxygen vacancies are confirmed by the structural characterization.From photo/electro characterization,oxygen vacancies can enhance visible light response and improve photogenerated carrier migration.Especially,visible light absorption peak at 675 nm wavelength and photogenerated carrier recombination peak at 475 nm wavelength could be attributed to rich oxygen vacancies.At visible light（λ>420 nm）,OVs-Co Bi-MOFs nanosheets exhibits a best NH₃ yield(173μmol·gcat^-1·h^-1),which is1.3-fold higher than AM 1.5,and 1.5-fold higher than the performance of simulated air.Using in situ Fourier Transform Infrared（FTIR）and density function theory calculation,the mechanism of NH3 synthesis was explored:*NO,as the intermediates,could enhance the efficiency of photocatalytic NH3 synthesis.In all,designing the novel catalytic system for direct NH3 synthesis employing air via couple redox in this paper.*NO,as the intermediates,could decrease the free energy of N2-activation and hydrogenation.This work provides a new and sustainable pathway for photo-driven ammonia synthesis.