Design,Preparation And Electrocatalystic Ammonia Synthesis Performance Of MoSe₂-based Catalyst

Ammonia is one of the most important chemical raw materials and occupies an important position in industrial and agricultural production and national defense industry.At present,industrial-scale ammonia synthesis mainly relies on the traditional Haber-Bosch process（reaction conditions:300～500℃,200～300 atm）.According to statistics,the world’s annual synthetic ammonia production is as high as 100 million tons,and its average annual energy consumption accounts for nearly 2%of the world’s total energy consumption.At the same time,it emits large amounts of greenhouse gases such as CO₂.Compared to Haber-Bosch process,Electrocatalytic Nitrogen Reduction（NRR）is considered to be a potential replacement technology for ammonia synthesis because it can directly synthesize ammonia from nitrogen and water under mild conditions.At present,the main challenge for electrochemical synthesis of ammonia is that the Faraday efficiency and ammonia production rate are too low.In response to the above challenges,this paper intends to learn from the composition of nitrogenase,and to improve the efficiency of electrocatalytic ammonia synthesis by designing a nitrogen reduction catalyst with a special valence electron structure.The main research results obtained in this paper are as follows:（1）Molybdenum（Mo）was used as the active center of the catalyst,and the electronic structure of Mo was adjusted by introducing a lower electronegative Se,and a Mo Se₂ catalyst with better nitrogen reduction activity was designed and prepared.Theoretical research believes that Mo,as the main metal activity center of nitrogenase,exists in the form of+3 valence.,but since the+3 valence state Mo-based catalyst is difficult to prepare,there are few studies paying attention to the effect of its valence state.In view of this,this paper studied the Mo Se₂ nanosheet array catalyst which was prepared by vapor deposition method,in order to use Se defects and relatively small electronegativity to modulate the surface electronic structure of Mo,and then make it have the characteristics of+3 valence.Characterization results such as EDS and XPS proved that there are a small amount of Se defects in the Mo Se₂ nanosheet array catalyst,and the addition of Se makes the binding energy of Mo⁴⁺significantly negatively shifted（a negative shift of 0.34 e V）.Further electrochemical test results show that in 0.01 M HCl solution,compared with the complete+4 valence state of Mo O₂,the highest ammonia yield of the Mo Se₂nanosheet array catalyst can be increased by 3.5 times,reaching 3.22μg cm^-2 h^-1.This experimental result confirmed the feasibility of using lower electronegativity elements to control the valence electron structure of Mo to improve its catalytic activity for nitrogen reduction.（2）On the basis of the above work,a more efficient Co-Mo Se_2-x nitrogen reduction catalyst was prepared by introducing Co element into the Mo Se₂ catalyst to modulate the electronic structure of Mo.At the same time,considering that the dissolution and adsorption of nitrogen in the aqueous electrolyte is limited,this paper also studied the influence of the different morphology and structure of the catalyst on the nitrogen adsorption（nitrogen reduction selectivity）.The research results show that the hollow Mo Se₂ nanosphere catalyst prepared by the solvothermal method had better nitrogen adsorption capacity than the Mo Se₂ nanosheet array catalyst prepared by the vapor deposition method,and the Faraday current efficiency of catalyzing the reduction of nitrogen to ammonia was 4.3 times of the latter.Its highest Faraday efficiency reached 3.23%,which may be due to the hollow sphere structure enhancing the residence time of nitrogen on the catalyst surface,thereby increasing the selectivity of the catalyst.On this basis,this paper further used solvothermal method to prepare Co-doped hollow Co-Mo Se_2-x nanospheres.The XPS analysis results show that the valence state of Mo was more inclined to+3 by Co doping,and its binding energy exhibited a negative shift of 0.2 e V,compared to Mo Se₂ without Co.The electrochemical test results show that,compared with the Mo Se₂ nanosphere catalyst,the Co-Mo Se_2-x nanosphere catalyst achieved the highest ammonia yield of 8.63μg mg^-1_cat h^-1（an increase of about 70%）.At the same time,the Faraday current efficiency also reached 4.41%.