The Construction Of Ti₃C₂ MXene-based Composites And Its Application Of N₂ Photofixation

Ammonia（NH₃）,as a typical green renewable clean energy,it is not only an important chemical raw material,which is crucial to the develop of industry and agriculture,but also an important energy storage intermediate and carbon-free energy carrier.NH₃ will be the core and hub of the future energy cycle,and it plays a vital role in the green and sustainable development of the future economy.Currently,the energy consumption of industrial ammonia synthesis is high and the carbon emission is huge.Therefore,it is extremely urgent to explore and develop an environmentally friendly N₂reduction process under mild conditions.As energy and environmental advantages,photocatalytic N₂ reduction to synthesize NH₃ with solar energy as the driving force has been widely concerned and studied.The key to improve the efficiency of photocatalytic N₂ reduction of synthetic ammonia lies in the design and construction of high-efficient photocatalysts.Traditional semiconductor photocatalysts have some problems,such as low carrier separation efficiency,poor electron transfer efficiency and limited solar energy capture capacity,and thus it is very significant to explore and build new photocatalyst materials for promoting the development of N₂ photofixation technology.As the new member of the two-dimensional material family,MXenes have excellent optical absorption capacity,adjustable band gaps and remarkable electron transfer efficiency,making it an ideal photocatalyst.In view of the limitations of photocatalytic N₂ reduction technology,combining the unique optical properties of the new two-dimensional MXenes material,this thesis takes MXenes as the research subject,and constructs a series of MXenes-based photocatalysts with different structures,defect types and composite material.It will provide theoretical reference and new direction for promoting the application of MXenes in photocatalysis.The main results are as follows:（1）Layered Ti₃C₂ with abundant low-valence Ti(Ti^（4-x）+)were prepared by H₂thermal reduction process,and these Ti^（4-x）+sites act as the active sites for N₂ capture and activation.Then,the sandwich-like r-Ti₃C₂/Au was built via embedding Au nanospheres into the interlayers of Ti₃C₂ under solvent driving force.This unique sandwich structure increases the contact area between Au and r-Ti₃C₂,which is conducive to improving the contact probability between carriers and the reaction solution.Meanwhile,the unique sandwich-like architecture prevents the self-stacking of the Ti₃C₂ layers,which favors the exposure of the active sites for utilization.Under the room temperature and pressure,the yield of NH₄⁺is～326μmol L^-1and the yield rate of NH₄⁺is 33.8μmol h^-1 g^-1_cat under white light irradiation in pure water.Importantly,r-Ti₃C₂/Au exhibits an excellent catalytic cycle stability.Combining X-ray photoelectron spectroscopy（XPS）and electron paramagnetic resonance（EPR）to analyze the surface chemical properties of r-Ti₃C₂,the valence state changes of Ti and the existence of oxygen vacancies（OVs）were investigated.Meanwhile,the key intermediates at the catalytic interface were detected and analyzed by in situ diffuse-reflectance infrared Fourier transform spectroscopy（DRIFTS）and N₂ isotope labeling techniques,revealing the catalytic mechanism and reaction process of photocatalytic N₂ reduction.Based on the experimental characterization data and theoretical calculation analysis,the mechanism of N₂ photoreduction over r-Ti₃C₂/Au was revealed.（2）Ultrathin Ti₃C₂ nanosheets with rich surface Ti defects had been prepared by solution reduction route,and reported the preparation of stabilized single-atom Fe via a simultaneous self-reduction stabilization process.Single-atom Fe was reduced in situ by the low-valence Ti sites,and simultaneously stabilized on the surface of Ti₃C₂ by forming Fe-C bonds with adjacent C atoms.Hot carriers generated from plasmonic Ti₃C₂ were effectively separated,and improved the utilization efficiency of hot carriers.Meanwhile,the existence of hot carriers induced by plasmonic Ti₃C₂ is demonstrated singlet oxygen detection and analysis method.By means of XPS analysis,the reaction process of Ti₃C₂ self-reduced anchoring single atom Fe had been proved.Based on the analysis results of N₂ isotope labeling techniques and theoretical calculation analysis,the mechanism of N₂ near infrared photoreduction was revealed.（3）We constructed a Schottky junction photocatalyst made of mesoporous hollow carbon nitride（C₃N₄）spheres decorated with partially reduced Ti₃C₂ quantum dots（r-Ti₃C₂ QDs）via electrostatic self-assembly.The Schottky junction is formed at the interface between C₃N₄ spheres and r-Ti₃C₂ QDs which enables the spatial separation of photogenerated electrons and holes,resulting in suppression of charge carrier recombination.Such double defect sites of Ti³⁺and OVs facilitate the capture and activation of N₂ molecules,leading to efficient reduction of preactivated N₂ molecules to NH₃ by the trapped electrons transferred from the photoexcited C₃N₄ hollow spheres.Combining with XPS and EPR analysis techniques,the active sites and surface defects of the catalyst were confirmed.The key intermediates at the catalytic interface were detected and analyzed by in situ diffuse-reflectance infrared Fourier transform spectroscopy（DRIFTS）and N₂ isotope labeling techniques,revealing the catalytic mechanism and reaction process of photocatalytic N₂ reduction.