Fabrication Of Molybdenum-based Electrocatalysts And Their Performance In Ammonia Synthesis

As an important raw chemical material,ammonia（NH₃）is widely used in various aspects of humankind,such as agricultural production,industrial manufacture,and energy.At present,the industrial production of NH₃ counts heavily to the Haber-Bosch process by utilizing hydrogen and nitrogen as raw materials.To maintain a satisfactory ammonia production efficiency,this reaction requires extremely harsh reaction conditions（high temperature and high pressure）.At the same time,the process is complicated and consumes a lot of energy,and emits a large amount of carbon dioxide,causing serious environmental problems.Electrocatalytic synthesis of ammonia has become a hot topic in ammonia production due to its mild reaction conditions,low energy consumption and low pollution.It is a potential ammonia synthesis process to convert high-purity nitrogen or nitrate ion in aqueous solution into NH₃ by using electrochemical method.Nevertheless,the development of electrocatalytic ammonia synthesis has been plagued by low reaction yield rate and selectivity,which is caused by the more favorable hydrogen evolution reaction（HER）.Whether nitrogen or nitrate reduction reaction,the efficiency of NH₃ synthesis mainly depends on the design and regulation of catalyst.Therefore,we need to design reasonable catalysts to improve the NH₃ performance.In this paper,molybdenum-based materials are used as catalysts for electrocatalytic NH₃synthesis.The molybdenum-based materials are modified through defect engineering,interface engineering and surface modification strategies to enhance their catalytic performance.The research contents are as follows:（1）Firstly,Fe-Ni Mo O₄ nanorod catalysts containing enriched oxygen vacancies（OVs）were synthesized by hydrothermal and annealing method,and used to synthesize NH₃ by electrocatalytic nitrogen reduction.Iron-doping was used to modulate the Ni Mo O₄ nanorods to induce more OVs,which can absorb and polarize N₂ molecules and weaken the bond energy of N≡N to facilitate NH₃ synthesis.BET test shows that the introduction of Fe increases the specific surface area of Ni Mo O₄,thus exposing more active sites for the adsorption of N₂.The transient photovoltage（TPV）test shows that Fe-Ni Mo O₄ is more favorable for electron transfer than pure Ni Mo O₄,which is helpful for electrocatalytic synthesis of NH₃.In N₂-saturated 0.1 M Na₂SO₄solution,5%Fe-Ni Mo O₄ catalyst obtained an NH₃ yield rate of 15.36μg h^-1 mg_cat.^-1and the Faraday efficiency（FE）of 26.85%at-0.5V vs.RHE.Besides,the 5%Fe-Ni Mo O₄ catalyst exhibited good durability.（2）Then,Co S/Mo S₂ nanocomposites for electrocatalytic nitrogen reduction were synthesized by two-step hydrothermal method.The electrophilic ability of Mo is stronger than that of Co,and the electrons around Co S transfer to Mo S₂ during bonding,forming an electron-deficient region around Co S to accept lone pairs of electrons from N₂ and facilitate the adsorption of N₂.The formation of an electron-rich region around Mo S₂ can provide electrons to the anti-bonding orbitals of N₂,which is beneficial to the fracture of the N≡N and promote the hydrogenation of N₂.In addition,Co S nanoparticles were loaded on the Mo S₂ nanosheets,which increased the specific surface area of the catalyst and provided more active sites for N₂ adsorption and activation.The Co S/Mo S₂ catalyst gained the maximum NH₃ yield of 23.23μg h^-1 mg_cat.^-1and the FE of 12.63%at-0.45V vs.RHE in N₂-saturated 0.1 M Na₂SO₄ solution.Moreover,the catalyst has good electrochemical stability.（3）Finally,for electrocatalysis of nitrate reduction,Mo O₂ nano-block catalyst coated with carbon layer and containing a large number of oxygen vacancies was synthesized by water bath and annealing process by using polyvinylpyrrolidone（PVP）as the carbon source.The introduction of the carbon layer prevents the oxidation of Mo O₂ during the annealing process and forms more OVs,which is beneficial to the adsorption of NO₃^-.The appropriate adding amount of PVP can obtain a suitable thickness of carbon layer,which can help the electron transfer,enhance the conductivity of catalysts,and be favorable for the adsorption of NO₃^-and hydrogenate to form NH₃.The Mo O₂@C-2 catalyst acquired an NH₃ yield of 902.17 mmol h^-1g_cat.^-1and the maximum FE of 81.49%at-0.5V vs.RHE in 1 M Na OH+0.1 M Na NO₃ solution and the catalyst has good electrochemical stability.