Preparation Of Iron-Based Metal Catalysts And Their Electrocatalytic Nitrogen Reduction Performance

As one of the most important chemicals around the world,ammonia（NH₃）is not only widely used in many fields such as medicine,agriculture,and textiles,but also is expected to become a significant green energy carrier in the future.Currently,large-scale synthesis of NH₃in the industry mainly relies on the Haber-Bosch process,which is characterized by harsh conditions,high energy consumption,and high pollution.In recent years,researchers have been exploring alternative strategies that are more green and sustainable.In this regard,electrocatalytic nitrogen reduction reaction（NRR）can be driven by renewable energy without generating greenhouse gases,providing a green and environmentally friendly synthesis process for NH₃ production under normal temperature and pressure.However,due to the high bond energy of N≡N and competitive hydrogen evolution reaction（HER）,the catalytic performance of NRR is difficult to meet the needs of industrial production.Therefore,developing electrocatalysts with high catalytic activity and selectivity is the key to the current field of electrocatalytic NRR.Iron-based catalysts are the most economical and abundant material in the world,and Fe is an important active center in biological nitrogen fixation enzymes,which is conducive to the adsorption and activation of N₂ molecules.Therefore,iron-based catalysts have potential application prospects in the large-scale development of NRR electrocatalysts.In this study,the NRR electrocatalytic performance of iron-based materials was systematically studied by combining material characterization,electrochemical testing,and density functional theory（DFT）calculations.The specific research contents are as follows:（1）Popcorn-shaped Fe₃O₄ nanospheres with small sizes and rich oxygen vacancies were synthesized by classical hydrothermal methods.The popcorn-shaped Fe₃O₄ nanospheres catalyst showed good NRR performance,achieving an NH₃ yield of 20.7±0.9μg h^-1 mg_cat.^-1and Faradaic efficiency（FE）of 40.0±1.7%at-0.3 V vs.RHE.In addition,combined with characterization techniques before and after stability testing,the electrochemical stability of the Fe₃O₄ catalyst was confirmed.Further characterization techniques and electrochemical experimental analysis indicate that the prepared popcorn-shaped Fe₃O₄ nanospheres are small in size and rich in a large number of oxygen vacancies,which can increase the active surface area of the material to expose more active sites,while adjusting the electronic structure of the material to promote electron transfer,thereby enhancing the catalytic performance of NRR.（2）A unique Fe-Fe₃O₄ composite material was prepared by a one-step green synthesis method under environmental conditions.The results showed that the Fe-Fe₃O₄ catalyst achieved a high FE of 53.2±1.8%and an NH₃ yield of 24.6±0.8μg h^-1 mg_cat.^-1at-0.4 V vs.RHE.In addition,the Fe-Fe₃O₄ catalyst exhibits excellent stability and durability,and its NRR selectivity is significantly superior to commercial Fe and Fe₃O₄ nanoparticles,indicating that Fe-Fe₃O₄ is an efficient and stable NRR electrocatalyst at room temperature.Characterization techniques and DFT calculations show that Fe-Fe₃O₄ exhibits fast electron transport capabilities and inhibits competitive HER processes.（3）Fe-TiO₂ catalysts were prepared by developing a general one-pot phase change engineering strategy.The engineering strategy demonstrates the advantages of high phase transformation efficiency and low material cost.Experimental results and DFT calculations indicate that surface oxygen vacancies and newly introduced Fe sites are potential electrocatalytic NRR active sites in the Fe-TiO₂ catalysts,which can effectively reduce the NRR reaction energy barrier,thereby promoting the performance of NRR.The optimized anatase type Fe-TiO₂ has an NH₃ yield of 30.9±0.4μg h^-1 mg_cat.^-1and FE of 40.4±1.1%at-0.4 V vs.RHE,which is superior to most previously reported Ti-based catalysts.In addition,5 times repeated experiments and 48 h long-term stability tests both proved that Fe-TiO₂ catalyst is a stable and efficient NRR electrocatalyst.Meanwhile,M-TiO₂（M=Mn,Co,Ni,Cu）exhibited similar phase transition and catalytic performance to Fe-TiO₂,indicating the scalability of this method and opening up a new method for designing transition metal oxide catalysts.