Enhanced Photocatalytic Nitrogen Fixation Performance And Reaction Mechanism Over Photo-Induced Dynamic Halogen Vacancies On Bismuth Oxyhalide

Ammonia is one of the important inorganic chemical products and plays an important role in national economy.The current global ammonia synthesis technology is based on the Harber-Bosch method,which uses a catalyst to produce ammonia from H₂and N₂ under high temperature and pressure conditions.The production of ammonia by the Harber-Bosch method consumes about 1-2%of the world’s energy and accounts for1-2%of global CO₂ emissions.Recently,photochemical nitrogen fixation technology using airborne N₂ molecules and abundant H₂O can undergo ammonia synthesis reaction at room temperature and atmospheric pressure,which is considered as a promising technology to alleviate energy and environmental problems simultaneously because of its low energy consumption and low CO₂ emission.However,due to the high bond energy of the N≡N triple bond,the current yield of photocatalytic ammonia synthesis is still low,resulting in a method that is far from replacing the Huber method.Therefore,an in-depth study of the reaction mechanism of multiphase catalytic photocatalytic ammonia synthesis and the development of highly active and stable photocatalysts guided by this theory are essential to overcome the above problems.Bismuth oxyhalide（Bi OX（X=C1,Br,I））,a widely used material in photocatalysis,possesses excellent physicochemical properties and a unique layered structure.Previous studies have shown that the energy band structure of bismuth oxyhalide-based photocatalysts can be changed when the content of Bi elements in bismuth oxyhalide-based photocatalysts is changed,which lays the foundation for the synthesis and further modification of bismuth-rich bismuth oxyhalide-based Bi_xO_yX_z photocatalysts（the ratio of Bi atoms,O atoms and halogen atoms is x:y:z）.In view of the above research background,the systematic study on the in situ construction mechanism of photogenic halogen vacancies in bismuth halide oxygen-rich Bi_xO_yX_z photocatalysts and their photocatalytic nitrogen fixation performance enhancement mechanism was carried out in this dissertation,and the main research contents and conclusions are as follows.The rapid-scan in situ FT-IR spectroscopy platform provides insight into the mechanism of heterogeneous photocatalytic nitrogen fixation reactions.The rapid-scan in situ FT-IR platform consists of a Vertex 70v FT-IR vacuum spectrometer（Bruker）equipped with an in situ ATR reaction cell and an ultra-high vacuum piping system.The photocatalyst is vacuum coated and vacuum degassed on the surface of ATR crystals（diamond,monocrystalline silicon）.Water is dropped onto the sample surface and a program is written to control the shutter to regulate the light process.The reaction process was scanned rapidly at millisecond time-resolved scale to obtain FT-IR spectra at different times.By introducing isotopes(D₂O,¹⁵N₂)under different reaction conditions,the spectra were analyzed and combined with other experimental and computational data to deeply analyze the non-homogeneous photocatalytic reaction process.A strategy to adapt diurnal alternation of nitrogen photoimmobilization for ammonia synthesis is proposed by surface Br cycling on ultrathin Bi₄O₅Br₂ nanosheets.Rapid-scan in situ ATR FT-IR spectroscopy verifies that the photocatalytic N₂ reduction proceeds via an alternating association mechanism on the Bi₄O₅Br₂ surface with Br vacancies.DFT and AIMD calculations demonstrate that surface bromine vacancies can reasonably promote N₂ adsorption and lower the energy barrier for N₂ reduction.During the dark reaction,Br^-ions can reoccupy the surface vacancy sites,leading to photocatalysts that protect the vacancy active sites from being occupied by groups such as H₂O molecules and hydroxyl groups in the dark and aerobic environment,which is a key point to ensure the stable occurrence of the photoreaction and dark reaction cycles during ammonia synthesis in the liquid phase.An overall strategy is proposed to advance sustainable ammonia production by combining photocatalytic reaction promotion enhancement for light reactions with catalyst surface regeneration and protection processes for dark reactions.The activation of N₂ and photocatalytic reduction conversion were promoted by introducing surface tensile stress in Bi₅O₇I to construct high concentration of type I vacancies at the step-edge sites.The spherical differential electron microscopy results demonstrate the presence of more intrinsic regional vacancies on the surface of Bi₅O₇I nanotubes due to the presence of surface tensile stress.The presence of I at the step edges reduces the formation energy of I vacancies at this site.Rapid-scan in situ FT-IR analysis in the liquid phase elucidated that the reduction energy of photocatalytic N₂ was further promoted on the I-vacancy-rich surface by an alternating association mechanism during the illumination process.Ultimately,a new strategy to increase the concentration and rate of I vacancy generation is proposed and enables the advancement of sustainable and efficient ammonia production.The reaction mechanism of photodynamic halogen vacancies for photocatalytic ammonia synthesis was investigated in depth by Bi₄O₅Cl₂ nanosheets,which promote the activation of N₂ and photocatalytic reduction conversion by partial dissociation of H₂O molecules in the liquid phase system to provide active protons through the in situ construction of halogen vacancies.Rapid-scan in situ FT-IR in the liquid phase verified the reaction process of hydroxylation on the catalyst surface and the participation of H₂O as a proton donor in the nitrogen fixation reaction by isotope experiments during the light illumination.The in-depth reaction mechanism in the photocatalytic nitrogen fixation reaction facilitated by dynamic Cl vacancies generation and extraction of protons from H₂O molecules was elucidated,which finally allowed to inspire and provide new ideas for the design of new materials for sustainable and efficient ammonia production.The differences of different kinds of photo-induced halogen vacancies generation and action mechanism were compared by preparing Bi₄O₅Cl₂,Bi₄O₅Br₂,Bi₄O₅I₂ series samples.By detecting the dissolution rate of halogen ion concentration during the photo-induced process,the rate of halogen vacancy generation was proved in the order of Cl,Br and I and gradually increased.A comprehensive analysis with the nitrogen fixation performance was conducted to elucidate the general rule of different bismuth oxyhalide in the process of building halogen vacancies.In addition,a new method for in-depth analysis of the NRR reaction mechanism by isotopically labelled in situ FT-IR was developed to further contribute to the in-depth mechanistic study.In this dissertation,the above series of studies have elucidated the mechanism and in-depth reaction mechanism for the enhancement of the heterogeneous photocatalytic nitrogen fixation performance of ultrathin bismuth-rich bismuth oxyhalide materials.It expands the understanding of the reaction mechanism of photocatalytic ammonia synthesis and enriches the design mechanism of photocatalytic materials guided by the reaction mechanism.The potential of photocatalytic nitrogen fixation technology for future applications in achieving the carbon peaking and carbon neutrality goals is further explored.