Application Of Iron-based Catalysts In Electrocatalytic Nitrogen Reduction Reaction

Ammonia（NH₃）plays a vital role in social and economic development and industrial production,widely used in chemical industry,agriculture,medical and other fields.Over 90%of NH₃ has been produced by the Haber-Bosch process over the past 100 years.However,this method requires nitrogen（N₂）and hydrogen（H₂）to synthesize the required product NH₃ under the conditions of high temperature（400-600℃）and high pressure（150-300 atm）.At the same time,this process consumes more energy and emits more greenhouse gases.Therefore,it is of great significance to develop an economical,efficient and environmentally friendly method to gradually replace the original Haber-Bosch process.Electrocatalytic nitrogen reduction reaction（NRR）can convert N₂ rich in air into NH₃ at normal temperature and pressure by using the electric energy from wind,light,tide and other clean energy as the driving force.However,the process involves the breaking of the triple bond N≡N,which is chemically inert,and also faces the competition of the side reaction hydrogen evolution.Therefore,it is of great significance to design and develop non-precious metal catalysts with high activity and good selectivity for electrocatalysis of NRR.In this paper,a series of iron based catalysts with good performance are designed and prepared and used for electrocatalytic NRR.Meanwhile,the structure-activity relationship between their structural characteristics and electrocatalytic performance is systematically studied.The main research contents include the following three parts:1.Synthesis of Ce-Fe₃C QDs/N-C material and its application in electrocatalytic nitrogen reduction reaction.In this section of work,a kind of Ce-doped Fe₃C quantum dots（QDs）（Ce-Fe₃C QDs/N-C）catalyst supported on nitrogen-doped carbon was prepared by sol-gel method for electrocatalytic NRR.When the doping amount of Ce was adjusted to 6 wt.%,the catalyst obtained better ammonia production rate(35.59μg h^-1 mg_cat.^-1)and Faraday efficiency（7.9%）at-0.4 V vs RHE potential in 0.1 M KOH electrolyte.The good catalytic performance of the catalyst can be attributed to the fact that the doping of Ce effectively regulates the surface electronic structure of Fe₃C quantum dots and promotes the adsorption and activation of N₂molecules on the catalyst surface.At the same time,the introduction of N-C can prevent the agglomeration of Fe₃C QDs,so that it has a large specific surface area,which greatly promotes the performance of NRR catalyst.At the same time,the catalyst can operate continuously at-0.4 V vs RHE for more than 20 h,showing good NRR electrocatalytic durability.2.Synthesis of FeMoO₄ nanomaterial and its performance in electrocatalytic nitrogen reduction synthesis of ammonia.In the process of biological nitrogen fixation,ferritin and molybdenum ferritin in biological nitrogen fixation play important catalytic roles in the process of NRR reaction.Inspired by biological nitrogen fixation,FeMoO₄ catalyst containing Fe and Mo was prepared by solvothermal method and applied to electrocatalytic NRR.The results show that FeMoO₄ has better NRR performance than mono-metal oxides（Fe₂O₃ and Mo O₃）,and the maximum ammonia production rate is 33.55μg h^-1 mg_cat.^-1in 0.1 M Na₂SO₄electrolyte at-0.6 V vs RHE potential.The good NRR performance of the catalyst can be attributed to the synergistic enhancement of NRR activity by Mo and Fe.In addition,the catalyst also showed over 20 hours of electrocatalytic NRR durability.3.Study on the synthesis of Fe-Cu₃N material and its electrocatalytic nitrogen reduction performance.In this section of work,Fe-doped Cu₃N（Fe-Cu₃N）catalyst with nanovesicle structure was prepared by electrochemical,calcination-etching method and applied to electrocatalytic NRR.Fe-Cu₃N-3 catalyst with the best NRR activity（deposition potential 3.5 V vs RHE）was selected by adjusting the deposition potential during electrochemical preparation.The catalyst showed high NRR activity in 0.1 M Na₂SO₄electrolyte,and the ammonia production rate reached32.48μg h^-1 mg_cat.^-1at-0.3 V vs RHE.Through the characterization of the physical and chemical properties of these catalysts and the comparison of the catalytic performance of NRR,it can be concluded that the good electrocatalytic NRR activity of Fe-Cu₃N-3 is mainly attributed to:（1）Cu₃N catalyst itself has certain catalytic activity on electrocatalytic NRR;（2）the introduction of Fe³⁺can effectively regulate the active site on the surface of Cu₃N catalyst,enhance the adsorption of N₂ molecules,promote the reaction of NRR,and finally achieve a higher ammonia production rate.In addition,the catalyst also demonstrated over20 hours of electrocatalytic NRR durability.