Preparation Of FeMo Composites And Investigation Of Its Role In Electrocatalytic Nitrogen Reduction

The fixation of N₂ to NH₃ is a critical chemical process,as ammonia is a globally important chemical for the production of a variety of economically beneficial products.The current dominant ammonia production method in industry is the Haber-Bosch process,however,this approach imposes a significant burden on global energy consumption and CO₂ emissions.The electrocatalytic nitrogen reduction reaction（e NRR）is viewed as a promising substitute to Haber-Bosch for nitrogen fixation,due to its environmental compatibility and integration.However,the strong hydrogen evolution reaction（HER）and the difficult dissociation of N≡N in this procedure lead to low catalytic activity and selectivity of e NRR catalysts,thus it is meaningful to develop efficient e NRR catalysts.Biological nitrogenase in rhizobacteria in nature provide a considerable amount of of nitrogen fertilizer to plants through biological nitrogen fixation.On this basis,we attempted to synthesize a series of electrocatalysts similar to the active components of biological nitrogenase from the perspective of simulating biological nitrogenase,and carried out a detailed NRR performance study on them.The detailed research is structured into the following three sections.（1）By simulating the active composition of FeMo nitrogenase,a one-step hydrothermal method was used to achieve FeS/Mo S₂ nanomaterials with ammonium molybdate tetrahydrate as the source of molybdenum,ferric chloride as the source of iron and the introduction of cofactors containing sulphur.The morphology,composition and structure were specifically analyzed with the assistance of characterization equipment.After that,the ammonia yield reached 20.87μg·h^-1·mg_cat.^-1in 0.1 M Na₂SO₄ in ambient conditions,with Faraday efficiency of 13.6%.Moreover,the catalyst maintained excellent electrochemical stability after several cycle tests and 24 h long time tests,and showed good selectivity with no by-product hydrazine generation in the reaction.（2）The high specific surface area and adjustable porosity of MOFs,combined with the introduction of heterogeneous metals,can lead to an increase in electrical conductivity and an enhancement of e NRR performance by creating more active sites.Accordingly,we synthesized bimetallic FeMo-MOFs using the solvothermal method and then annealed at different temperatures to achieve the bimetallic FeMo-MOFs derivatives Fe₂Mo₄O₈/Fe_1.67Mo_1.33O₄ composites（denoted as FeMo O-x,x stands for annealing temperature）.with further investigation of their electrocatalytic nitrogen fixation performances.In a neutral electrolyte of 0.1 M Na₂SO₄ at testing potential of-0.6 V（vs.RHE）,the highest annealing temperature among the three sets of FeMo O materials synthesized,FeMo O-900 displayed relatively high ammonia yield(20.35μg·h^-1·mg_cat.^-1)and the Faraday efficiency of 11.3%was achieved,while the e NRR performance test also verifies that the performance of the nitrogen fixation is gradually increasing as the annealing temperature increases.（3）The current researches have indicated that self-supported electrocatalysts with high specific surface area and abundant metal sites are favourable for electrocatalytic reactions.Fe,Mo and S are the constituent elements of nitrogen-fixing enzymes and there are abundant Fe resources.We were inspired to synthesize Fe₃O₄/Mo₇S₈ composites on iron foam surfaces via anodic oxidation and solvothermal methods.Measurements showed that Fe₃O₄/Mo₇S₈ as e NRR electrocatalyst yielded up to 5.38×10^-11mol^-1·s^-1·cm^-2 of ammonia in 0.1 M Na₂SO₄ electrolyte,peaking at 17.3%FE at-0.3 V（vs.RHE）.Moreover,electrochemical stability and excellent selectivity were demonstrated in long cycle stability tests up to 48 h.