Preparation Of Molybdenum-based Catalyst And Their Performance In Electrocatalytic Ammonia Synthesis

Ammonia（NH₃）is regarded as one of the most valuable chemicals for human beings and ecological system.Industrial NH₃production is still dominated by the traditional Haber-Bosch process.However,harsh operating conditions and the production of high-purity H₂lead to a large amount of CO₂release and energy consumption.Electrochemical N₂reduction reaction（NRR）under ambient conditions is an environmentally friendly method,because it can use electricity obtained from renewable hydropower,wind,solar or nuclear power.But this process is limited by nonpolar N≡N triple bond and competitive hydrogen evolution reaction（HER）.Therefore,it is necessary to develop the electrocatalyst with high activity and selectivity for NRR.In this paper,a series of morphological control and catalytic property modification were carried out on molybdenum-based catalyst and functional metal-organic framework（MOFs）,and the application of high efficiency electro-catalytic NH₃synthesis was evaluated.The mechanisms of the two electrocatalysts for improving the performance of NH₃synthesis were discussed.The main research contents and experimental results are as follows:（1）The MoP@P-doped porous carbon octahedral catalyst（MoP@PPC）was prepared by using the molybdenum-based MOF NENU-5 as an ideal sacrificial template for the preparation of porous carbon supports.In detail,the molybdenum-based metal organic framework NENU-5 octahedron was prepared by co-precipitation method using copper acetate,L-glutamic acid,phosphomolybdic acid and trimesic acid.The NENU-5was calcined to prepared Cu-Mo O₂@C octahedron under Ar atmosphere,and then Mo O₂@PC octahedron was prepared by chemical etching.Finally,a subsequent thermal conversion process was carried out in Ar atmosphere to form porous MoP@PPC octahedron,where Na H₂PO_2·H₂O served as the phosphorus source.XRD and EDX analysis confirmed that the Cu particles were completely removed by Fe³⁺chemical etching.The temperature conditions of the two annealing stages are optimized through a large number of experiments and a variety of characterization methods.MoP@PPC with good element distribution and high purity crystal phase was obtained.MoP@PPC octahedron exhibits excellent electrocatalytic NRR activity in acid electrolyte under ambient conditions.At-0.3 V（vs.RHE）potential,the NRR test showed that the ammonia yield reached 28.73μg h^-1mg^-1_cat.,and the highest Faraday efficiency（FE）was 2.48%.The synergistic effect of MOF-derived phosphorus-doped porous carbon and in situ generated MoP nanoparticles can improve NRR performance.（2）We achieve the model catalyst@MOF interface（MoS₂@ZIF-71）by encapsulating ball-like MoS₂nanoflower electrocatalyst with a layer of zeolitic-imidazole framework-71.In this model catalyst@MOF interface,ZIF-71 concentrates N₂molecules at the MoS₂-ZIF interface to enhance MoS₂-N₂interactions and serves as a hydrophobic barrier to suppress HER,ball-like MoS₂nanoflower with the unique ultrathin subunits provides the rich active edges and sites for NRR.This molecular-level interface of pseudo high-pressure states can efficiently boost the collision rates and interaction strength of reactants under ambient conditions,and then that can be further improve NRR performance.In detail,Mo G nanosphere was prepared by hydrothermal method.The thiourea and Mo G nanosphere were placed in a Teflon-lined autoclave for hydrothermal reaction to prepare ball-like MoS₂nanoflower.The MoS₂/PC electrode was placed in a ZIF growth solution to prepare MoS₂@ZIF-71 electrocatalyst.MoS₂@ZIF-71electrocatalyst exhibits excellent NRR activity in neutral electrolyte.At-0.2 V（vs.RHE）potential,the NRR test showed that the NH₃yield reached 56.69μg h^-1mg^-1_MoS2,and the FE was 30.91%.This highly active catalyst@MOF interface can be used to design low-cost NRR electrocatalysts.This design is universal and can allow the facile integration with emerging active catalyst to further boost NRR activity and FE.