Modification Of MIL-88A And Photocatalytic Nitrogen Fixation Performance Study

Photocatalytic nitrogen fixation is an ideal route for ammonia synthesis that enables sustainable nitrogen conversion.However,the low N₂ solubility and high carrier complexation rate are still the difficulties of the current nitrogen fixation process.Therefore,the design of novel catalytic reaction systems and the construction of high-efficiency photocatalysts are important means to achieve green and efficient photocatalytic nitrogen fixation.Based on this,the preparation process of the material was firstly optimized in this paper,and MIL-88A and its composites were synthesized in situ by providing iron source directly from iron mesh,and the seamless contact between the conducting substrate and catalyst could accelerate the charge transfer.Meanwhile,in order to ensure the sufficient contact between the catalyst and N₂,a gas-phase nitrogen fixation system with water vapor as the proton source was designed to address the problems of low solubility of N₂ in water and poor mass transfer efficiency,and the photoactivity of the catalyst was measured under visible light irradiation.In addition,the photoelectric properties,structural morphology,and elemental valence of the composites were analyzed by various characterization tools,and then the charge interaction mechanism and nitrogen fixation mechanism of the reaction process were inferred.The main studies are as follows:（1）A MIL-88A（Fe/Mo）/Fe mesh bionic photocatalyst was prepared by introducing Mo into the backbone structure of MIL-88A by a simple hydrothermal method inspired by the unique structure and mechanism of action of nitrogen fixation enzymes.The performance was evaluated in a nitrogen-fixing system in the gas phase,and the photocatalytic activity of the material was tuned by varying the Mo doping concentration.The photoactivity experiments demonstrated that the photocatalytic nitrogen fixation performance of MIL-88A(Fe/Mo_0.017)/Fe mesh(2.30×10^-2μmol·cm^-2·h^-1)under visible light irradiation was improved by about 9.6 times compared with that of pure MIL-88A/Fe mesh.Combined with the analysis of a series of characterization results,it can be reasonably inferred that the presence of Fe³⁺/Fe²⁺and Mo⁶⁺/Mo⁴⁺redox pairs can accelerate the transport and separation of photogenerated charges through single-and two-electron redox reactions.The formation of the mimetic"Mo Fe cofactor"serves as a new N₂ capture and binding site,which can more effectively weaken N≡N and activate nitrogen under photoexcitation.（2）A thin-film La/Ti O₂/MIL-88A/Fe mesh composite photocatalyst was designed by coupling the MIL-88A/Fe mesh with semiconductor phase to extend the lifetime of photogenerated carriers more effectively.The photoactivity was further investigated by adjusting the photodeposition time of La.The results showed that the 15La/Ti O₂/MIL-88A/Fe mesh(2.775×10^-2μmol·cm^-2·h^-1)had the best nitrogen fixation performance and the ammonia yield was 11.6 times higher than that of the MIL-88A/Fe mesh,which greatly improved the photoactivity of the catalyst.Combining with the characterization results for further analysis,presumably the reasons for the increased activity can be attributed to:the construction of the heterojunction promotes the charge transfer between the interfaces;the unsaturated 5d orbital of La optimizes the nitrogen activation ability of the carrier and the ability to photoexcite electrons;Furthermore,the abundant oxygen vacancies can act as electron acceptors and active centers for nitrogen reduction,which promote the surface chemistry and also provide more pathways for electron migration,thus effectively inhibiting the complexation of photogenerated electron-hole pairs.