Construction Of ZnO Nano Heterojunction And Photocatalytic Synthesis Of Ammonia

Ammonia（NH₃）is not only an important nitrogen source for artificial nitrogen fertilizer,but also an important hydrogen storage carrier.The nitrogen content of the earth’s atmosphere is close to 80%,and the nitrogen source of synthetic ammonia is very rich.However,efficient and green ammonia synthesis is a challenge because of the strong bond energy between the nitrogen and nitrogen triple bonds and the considerable activation barrier.At present,industrial synthetic ammonia still uses the traditional Haber synthetic ammonia method,which requires high temperature and high pressure.What’s more,a large amount of CO₂ is generated in both the hydrogen production and the ammonia synthesis process.Therefore,it is of great significance to find a cheap,efficient,green and energy-saving ammonia technology.Compared with Haber industrial ammonia technology,the use of sunlight as a driving force,the use of semiconductor materials as a photocatalyst,the reaction of nitrogen with water,photocatalytic synthesis of ammonia under mild reaction conditions.For increasing the absorption of light by semiconductor photocatalysts,accelerating the mobility of photogenerated carriers,promoting the separation of photogenerated electrons and holes,inhibitting the separation of photogenerated electrons and holes,and increasing the efficiency of photocatalytic immobilization of nitrogen,two semiconductor catalysts were designed and synthesized,and the following research work was carried out:（1）Photocatalytic synthesis of ammonia is a sustainable and energy-saving synthetic ammonia technology.Rich oxygen vacancies and heterostructures are important to increase the photocatalytic nitrogen to ammonia.In this paper,Fe₂O₃/ZnO nanocomposites were synthesized by hydrothermal method using ethylene glycol as the reducing agent.The X-ray diffraction（XRD）,transmission electron microscopy（TEM）,electron paramagnetic resonance（EPR）,N₂-TPD,UV-vis DRS and photoluminescence（PL）were used to characterize the Fe₂O₃/ZnO catalysts.The performance of photocatalytic synthesis of ammonia of Fe₂O₃/ZnO catalysts was tested at room temperature.The results show that heterostructure was formed between ZnO nanorods and Fe₂O₃ nanoparticles while abundant surface oxygen vacancies were produced over the Fe₂O₃/ZnO catalysts.Fe₂O₃ nanoparticles not only provided rich N₂ chemical adsorption sites,but also promoted the light absorption of ZnO in the visible region.The heterostructure between ZnO nanorods and Fe₂O₃ nanoparticles effectively inhibited recombination of photogenerated electrons and holes.The4%Fe₂O₃/ZnO catalysts exhibited enhanced photocatalytic nitrogen fixation efficiency with a NH₃ rate of 2059μmol·L^-1·g^-1·h^-11 with better stability.The formation of abundant oxygen vacancies and rapid photo-induced carrier separation efficiency are key factors for boost photocatalytic performance.（2）Furthermore,sunlight-driven photocatalytic reduction of nitrogen using earth-abundant semiconductor-based materials offers one green strategy to produce ammonia.However,it is still a bottleneck stemming from the difficulties of N₂chemisorption and activation for NH₃ yield.Herein,we report a facile and controllable in situ reduction strategy to design a plasmatic Bi/ZnO heterojunction nanostructure with rich surface oxygen vacancies,showing excellent photocatalytic nitrogen fixation performance.Semimetallic Bi NPs over the surface of photocatalyst provides additional nitrogen activation sites except for oxygen vacancies.The heterojunction formed between Bi NPs and ZnO nanorods effectively inhibits the recombination of electrons and holes by decreasing the bulk defects of ZnO.Taking further,the SPR effect of the Bi NPs plays a key role in promoting the yield of NH₃ in photocatalytic nitrogen fixation performance.It exhibits enhanced photocatalytic nitrogen fixation efficiency with a NH₃ rate of 2500μmol·L^-1·g^-1·h^-1（almost which is nearly 4 times higher than that of pure ZnO nanoparticles）.