Fabrication Of Two Dimensional Niobium-based Catalysts And Their Applications In Photocatalytic N₂ Fixation

As one of the largest annual production of chemical products in the world and the carrier of carbon-free clean energy,ammonia plays an extremely important role in industry and agriculture fields.At present,the synthesis of ammonia mainly relies on the industrial Haber-Bosch process,which not only meets the global demand for NH₃,but also brings great pressure to energy and environment on account of the harsh reaction conditions.In view of the current situation of energy shortage and environmental pollution,it is of great strategic significance to develop sustainable clean energy to synthesize ammonia efficiently under mild conditions.Among the many feasible strategies,photocatalytic nitrogen fixation is considered to be one of the most promising technologies for directly producing NH₃ from nitrogen and water under ambient conditions.However,due to the narrow absorption range of photocatalysts,low photogenerated charge separation efficiency and slow surface dynamics,the solar energy conversion efficiency is low.Herein,this paper takes two-dimensional niobic acid（2D HNb₃O₈）as the research object and systematically studies the light absorption characteristics,carrier separation efficiency and surface dynamics for niobium-based catalysts,through a series of modification strategies,such as,morphology,surface defect,elements doping and load cocatalysts.After then,the photocatalytic performance of niobium-based catalysts for nitrogen reduction could be optimized.（1）Niobic acid and two dimensional niobic acid nanosheets with oxygen defects（V_o-HNb₃O₈）were prepared by solvothermal and acid etching.Then,Au/V_o-HNb₃O₈ catalyst was obtained by in-situ photoreductive loading precious metal co-catalyst Au on V_o-HNb₃O₈.According to the analysis of UV-Vis diffuse reflectance spectrum,the light absorption band of V_o-HNb₃O₈ had a red shift in comparison with single phase HNb₃O₈.Besides,the light absorption band edge of Au/V_o-HNb₃O₈ was widened to 600 nm because of the plasma resonance effect of Au.Fluorescence spectroscopy combined with electrochemical analysis showed that the existence of oxygen defects could reduced the recombination of electrons and holes,then improved the charge separation efficiency.The separation efficiency of photoelectric charge was improved greatly for Au/V_o-HNb₃O₈ due to the surface plasma resonance,and then,the photocatalytic nitrogen fixation efficiency of Au/V_o-HNb₃O₈ was improved,and the ammonia production rate reached 102μmol g^-1 h^-1.（2）B-V_o-HNbO₃ catalysts with boron doping and oxygen defect were prepared by thermal treatment.According to the result of UV-Vis diffuse reflectance spectroscopy analysis,the introduction of doping and defects can effectively regulate the band structure of the catalyst.Furtherly,the light absorption range of B-V_o-HNb O₃ is widened to the visible light region.Fluorescence spectra and electrochemical analysis indicated that the existence of B doping and oxygen defect could effectively improve the separation efficiency of photogenerated electron hole pairs.Density functional theory（DFT）calculations showed that the introduction of B doping and oxygen defects could adsorb and activate N₂ effectively,and greatly reduce the potential barrier during the photocatalytic reaction.By exploring the photocatalytic nitrogen reduction under mild conduction,it is revealed that the B-V_o-HNb O₃has the best photocatalytic nitrogen fixation effect.Under the full spectrum（λ>400 nm）light irradiation,the ammonia production rate reached 170μmol g^-1 h^-1;moreover,the ammonia production rate of 12.25μmol g^-1 h^-1 was detected for B-V_o-HNb O₃ under theλ>700 nm light irradiation.