Electronic Structure And Interface Engineering To Co-regulate The Bi₂MoO₆ Photocatalytic Nitrogen Fixation Properties

Ammonia（NH₃）and nitric acid（HNO₃）are important chemical raw materials in chemical production and widely used in the industrial production of synthetic fiber and chemical fertilizer.In 2017,global NH₃and HNO₃production reached 176 and500 billion tons,respectively.In addition,NH₃has the advantages of low liquefaction pressure,large hydrogen capacity,and high energy density,making it a very promising hydrogen energy carrier.Although nitrogen（N₂）accounts for 78%of the atmosphere,it is difficult to convert N₂to NH₃due to the strong bond energy of N≡N.In industrial production,ammonia is mainly synthesized by the Haber-Bosch process with high purity N₂and high purity（H₂）as raw materials in the presence of high temperature（673-873 K）,high pressure（15-25 MPa）and iron catalyst conditions,resulting in large energy consumption and a large amount of greenhouse gas emissions.Nitric acid production is mainly prepared by ammonia oxide with a platinum and rhodium alloy as the catalyst under high temperature conditions（760-840℃）with high energy consumption.In addition,large chemical plants and others have resulted in a serious waste of fossil fuels in the transportation of NH₃and HNO₃.Therefore,the economic,environmental,and non-renewable resource problems of the traditional methods limit their future applications.Therefore,against the background of a"dual-carbon target"and"dual control of energy consumption",it is of great theoretical and practical significance to explore the green synthetic NH₃and HNO₃technologies with low energy consumption and low emission.The photocatalytic synthesis of NH₃uses N₂as the nitrogen source and H₂O as the direct proton source.At normal temperature and pressure,the light energy is used to drive the semiconductor catalytic activation of stable N≡N bonds,which provides a new idea for the ammonia industry.In this paper,Bi₂MoO₆is the main catalyst,which can effectively regulate the electronic structure and activate the catalyst active site,so as to significantly reduce the activation energy barrier of N₂molecules on the surface of bismuth-based catalysts and improve the photocatalytic performance of nitrogen fixation.Specifically,in two aspects of the research work:（1）A simple solvent thermal method was used to rationally design and synthesize an In³⁺doped Bi₂MoO₆photocatalyst to achieve"overall nitrogen fixation".The density functional theory（DFT）calculation results show that the Bi³⁺part of the[Bi₂O₂]²⁺layer in the Bi₂MoO₆structure is replaced by In³⁺.The density of states（DOS）further proves that In³⁺doping can regulate the d-band center of Bi₂MoO₆,reduce the energy barrier,and promote the chemisorption/activation of N₂molecules at the H₂O/In-Bi₂MoO₆interface,so as to improve the photocatalytic nitrogen fixation（PNRR）performance of the catalyst.Experiments show that,compared compared to Bi₂MoO₆,5%In-Bi₂MoO₆is 1.4 times higher;with N₂and H₂O,the generation rate of NH₃/NH₄⁺is 53.4μmol·g^-1·h^-1,which is 13 times that of Bi₂MoO₆(4.1μmol·g^-1·h^-1).In³⁺doping reduces the thermodynamic energy barrier for adsorption/activation of N₂molecules on the Bi₂MoO₆surface,increasing the formation rate of NH₃/NH₄⁺by 13 times;interestingly,the resulting NH₃/NH₄⁺can be further oxidized by·O₂^-to NO₃^-,the formation rate of NO₃^-can reach 54μmol·g^-1·h^-1,and achieving the"overall nitrogen fixation"reaction（N₂→NH₃/NH₄⁺→NO₃^-）through relay catalysis under mild conditions.（2）Electrostatic self-assembly is used to construct the heterojunction of MoS₂/In-Bi₂MoO₆.The built-in electric field formed at the interface between MoS₂additives and In-Bi₂MoO₆can effectively promote the separation and transfer of optical electron/hole in the catalytic system and generate additional Mo active sites,thus strengthening the reaction performance of NRR.Importantly,the introduction of MoS₂additives effectively inhibited the oxidation of NH₃/NH₄⁺to NO₃^-（AOR）and improved the selectivity of photocatalytic nitrogen fixation.3%MoS₂/In-Bi₂MoO₆photocarrier density compared to Bi₂MoO₆at 22.48 times higher,PNRR generated NH₃/NH₄⁺at a rate as high as 130μmol·g^-1·h^-1,43 times higher than Bi₂MoO₆,while the amount of NO₃^-generated was negligible.The above work developed the basic theory of selective photocatalytic nitrogen fixation,which provides important references for the clean production of NH₃and HNO₃.