Preparation Of Znfe-based Composite Metal Oxide Catalysts And Their Electrocatalytic Performance For Nitrogen Reduction Reaction

Electrocatalytic nitrogen reduction（NRR）has attracted extensive attention due to its mild operating conditions,environmental protection and low energy consumption.However,the reaction has the problem that the N≡N triple bond is difficult to crack.In addition,protons and electrons tend to have competitive hydrogen evolution reactions（HER）during the reaction process,resulting in a very low Faraday efficiency（FE）of NRR.Although noble metal catalyst materials show good NRR activity,however,its scarcity and high cost limit the possibility of application;the catalytic activity of non-noble metal catalyst materials（such as iron based materials）is relatively low.The research and development of novel NRR catalyst materials with high activity and selectivity has become a hot topic in this field.Hydrotalcite（LDHs）,as a two-dimensional functional material,its composition,morphology and electronic structure are adjustable.It has shown excellent performance in several electrocatalytic reactions.In this paper,based on the unique structural characteristics of LDHs laminates with tunable and uniformly distributed elements,using Fe-based LDHs as the precursor,two kinds of composite metal oxide catalyst materials were prepared under control:α-Fe₂O₃/ZnO and Ru/ZnFeO_x.The composition,morphology and structure of the samples were characterized and analyzed,discusses the mechanism of material formation,the NRR reaction performance of the samples was tested.the structure-activity relationship of the materials was investigated and the reaction mechanism of NRR was speculated.The main results are as follows:（1）α-Fe₂O₃/ZnO composite oxide materials were prepared by controlling calcination temperature using ZnFe-LDHs as precursor.Through characterization,the formation mechanism ofα-Fe₂O₃/ZnO is speculated:when the calcination temperature is below 200°C,the process of hydroxyl and carbonate removal takes place.ZnO crystal phase was formed due to partial collapse of ZnFe-LDHs laminates.When the calcination temperature rises to 300°C,the LDHs structure is completely destroyed andα-Fe₂O₃/ZnO composite structure is formed,the crystal phases of ZnO andα-Fe₂O₃ are interlaced and have obvious interfaces;with the increase of temperature,ZnO andα-Fe₂O₃ continue to grow,and the particle size of the phase begins to increase.The electrocatalytic NRR test showed that theα-Fe₂O₃/ZnO samples prepared by calcination at 300°C showed the best electrochemical activity.The yield of ammonia gas reached 11.1μg h^-1mg_cat,Faraday efficiency is2.13%,which is twice higher than ZnFe-LDHs precursor.This may be attributed to the synergistic effect of ZnO andα-Fe₂O₃.On the one hand,ZnO has a high HER overpotential,which may inhibit the occurrence of competitive HER in the reaction process.On the other hand,the interface between ZnO andα-Fe₂O₃ crystals produces abundant surface oxygen vacancies.Thus,more nitrogen adsorption sites are provided,andα-Fe₂O₃ acts as the reactive site to live dissolve and dissociate N₂.（2）Using ZnFe Ru-LDHs as precursor,Ru/ZnFeO_x catalyst material was prepared by controlled hydrogen reduction.Research on material preparation process shows that,,as the reduction temperature increases from 100°C to 200°C,ZnFe Ru-LDHs structure is gradually destroyed,and Ru⁰ is completely reduced to the surface of the catalyst.Ru/ZnFeO_xrich in oxygen vacancies was prepared.With the reduction temperature increasing to 400°C,Ru⁰ particles gradually grow up;at the same time,Fe⁰ is reduced,Ru/ZnFeO_x-Fe structure was formed.The electrocatalytic test showed that the Ru/ZnFeO_x electrocatalyst prepared at 200°C showed the highest catalytic activity,the yield of ammonia gas is16.1μg h^-1mg_cat and the Faraday efficiency is 4.65%.The high activity of Ru/ZnFeO_x can be attributed to more oxygen vacancies on the surface to adsorb nitrogen and the electron-rich effect of Ru⁰;at the same time,Ru nanoparticles have high activation ability for N₂.