Preparation Of Electrocatalyst Based On Mn₁₂ Monomolecular Cluster And Study Of Nitrogen Fixation Performance

Ammonia is not only an important energy carrier,but also an important chemical raw material,which is indispensable in agriculture,chemical industry,medicine,aerospace and other fields.Since the advent of the Haber-Bosch ammonia synthesis process,it has become the main method of ammonia synthesis.Today,when the energy problem is increasingly prominent,this high-energy,low-efficiency ammonia synthesis method is contrary to the green and energy-saving production and lifestyle.Therefore,it is imminent to develop a gentle,efficient,energy-saving and sustainable way to fix nitrogen.Electrocatalytic nitrogen reduction is considered a very promising method for ammonia synthesis because of clean power and available reactants（H₂O,N₂）.Electrocatalytic nitrogen fixation has huge development potential.There are four main types of catalysts that have been developed:noble metals,non-noble metals,nonmetals and single-atoms.Transition metals have the characteristics of high reserves,low prices,and low toxicity,which have attracted the attention of researchers.In this topic,for the first time in the field of electrocatalytic nitrogen fixation,a single molecular cluster compound containing manganese with two valence states—Mn₁₂ is introduced.Mn₁₂ has a small molecular size,a simple preparation process,and the ability to substitute carboxylic acid-based ligands.The different valence states of manganese from high to low endow it with a rich redox process,which lays the foundation for its application in the electrochemical field.Based on the characteristics of this compound,carboxylated carbon nanotubes are used for functional design through ligand exchange,and then used as a catalyst to investigate the electrocatalytic nitrogen fixation performance.Mn₁₂-CNTs has high electrocatalytic nitrogen reduction activity.In the 0.05 M H₂SO₄ electrolyte,the NH₃ yield of Mn₁₂-CNTs is the highest,reaching 21.4μg h^-1mg^-1_cat,and the FE is 33%.Through the Mn₁₂synthesis reaction mechanism,the doped cobalt atoms are designed to synthesize Co-Mn₁₂ crystals.The Co-Mn₁₂ was subjected to the same reaction as Mn₁₂,the influence of introduction of heteroatoms on the morphology and structure of the catalyst was explored,and the influence of the introduction of heteroatoms on the electrocatalytic nitrogen fixation performance was further analyzed.The results confirmed that the introduction of cobalt atoms effectively adjusted the electronic structure of Mn₁₂-CNTs,which significantly improved the performance of electrocatalytic nitrogen reduction to produce ammonia.With 0.05 M H₂SO₄ electrolyte,the NH₃ produced by Co-Mn₁₂-CNTs reaches 34.07μg h^-1mg^-1_cat,and the FE is 21.98%.Then the Co-Mn₁₂crystals were used to synthesize highly efficient and stable Co-Mn O₂ nanomaterials under the control of hydrothermal conditions.Compared with the pure Mn O₂ phase,the heteroatom-introduced Mn O₂ has almost no difference in morphology and structure.Further proves the introduction of defects can effectively adjust the active sites of transition metals and enhance the electrocatalytic nitrogen reduction activity.Co-Mn O₂ is used as an electrocatalyst for nitrogen fixation.In the 0.1 M Na₂SO₄electrolyte,the NH₃ output is the highest,11.7μg h^-1mg^-1_cat,and the FE is 11%.Compared with Mn₁₂-CNTs and Co-Mn₁₂-CNTs materials,as a single-valence oxide,it exhibits excellent stability in the electrochemical process,and its stability is significantly enhanced compared with Mn₁₂-CNTs and Co-Mn₁₂-CNTs.