Preparation Of Derivatized Carbon-based Catelyst Based On MIL-101(Fe) And Research On Performance Of Electrocatalytic Hydrogen/Ammonia Production

Seeking sustainable clean energy is the main way to alleviate the energy crisis and environmental problems.Hydrogen（H₂）is regarded as the most ideal clean energy because of its renewability and zero carbon emissions during the combustion process.At present,industrial production of H₂ is mainly derived from fossil fuels such as coal and natural gas,thus it is still an unsustainable energy source.In contrast,the use of light,electricity and other technologies to catalyze the decomposition of water to produce hydrogen is expected to achieve true green hydrogen production and clean recycling of hydrogen energy.However,due to the limitation of hydrogen production rate and energy efficiency,these new hydrogen production technologies are still difficult to compare with traditional catalytic reforming hydrogen production technologies in terms of scale and cost.In addition,for the large-scale application of hydrogen energy in the future,its efficient storage is also highly challenging.At present,there is no way of storing and transporting hydrogen that is low-cost,safe,and can meet the requirements of high-quality and volumetric hydrogen storage density.In response to the above-mentioned challenges,based on the development of a high-efficiency water electrolysis catalyst for hydrogen production and taking the high hydrogen content（17.6 wt%）and easy liquidification of NH₃,this thesis describes the research on the direct electrochemical synthesis of ammonia from nitrogen and water.By combination of hydrogen production by electrolysis of water and hydrogen storage by ammonia,clean and highly efficient recycling of hydrogen energy is expected to be achieved by construction of an ammonia nitrogen cycle.The main research work and progress of the thesis are as follows:（1）A carbon-based catalyst derived from metal-organic frameworks（MOFs）with a rich pore structure was prepared by a confined heat treatment strategy of recrystallization salt encapsulation.MOFs materials are considered to be carbon-based catalyst precursors with high potentials because of their ordered pore structure and highly dispersed metal centers.However,in the actual pyrolysis preparation process,collapse of the pore structures and sintering and agglomeration of the metal components can be of great challenge.In response to the above problems,in this paper,MIL-101（Fe）is used as the precursor of the iron-carbon-based hydrogen evolution catalyst.It is completely encapsulated in Na Cl by recrystallization from a saturated Na Cl solution.After freeze-drying,it is pyrolyzed in a reducing atmosphere.Fe/Fe₃O₄@C with porous structure was obtained.The study found that due to the confinement effect of the recrystallization salt encapsulation,the iron-based metal particles can achieve a uniform distribution in the carbon,and have a larger specific surface area(390.0 m2g^-1,doubled compared to the direct pyrolysis of MOF).The study revealed the important role of recrystallized salt encapsulation in the pyrolysis process,that is,it can effectively prevent the agglomeration of metal components,retaining a large amount of carbon and maintain the stability of the pore structures.（2）A series of M-Fe@C hydrogen evolution catalysts（M is a metal with metal activity weaker than Fe）are prepared by using the iron-carbon-based materials derived from the above-mentioned MIL-101（Fe）pyrolysis as raw materials.Although metal centers are easy to modulate in MOF materials,there are still big differences in the preparation of MOF-based materials with different metal centers.The metal replacement strategy proposed in this paper provides a simple way to prepare MOF-derived carbon materials with different metal active centers.Easy universal method.Taking the replacement of Ru as an example,Ru_x-Fe@C（x is replacement time）series catalysts with different Ru contents were prepared by controlling the reaction time for the study of electrocatalytic hydrogen evolution performance.The study found that the substitution reaction between Ru³⁺in the solution and Fe exposed on the surface of the carbon layer in Fe@C resulted in a Ru-Fe catalyst with excellent hydrogen evolution catalytic activity,and greatly improved the utilization of Ru active sites and stability of the catalyst.Experimental tests show that Ru_{60 min}-Fe@C with a Ru content of only 1.25 wt%corresponds to an overpotential of only 20 m V at a current density of 10 m A cm^-2 in 1 M KOH,which is less than commercial Pt/C（21 m V）,and the Tafel slope is 40.8 m V dec^-1,and the overpotential decays by only 3 m V after 1000 cycles of CV.（3）On the basis of the Fe@C catalyst obtained above,a bifunctional catalyst with both hydrogen evolution and nitrogen reduction activity was prepared by introducing active sites for nitrogen reduction through direct phosphating.An important challenge for the electrocatalytic nitrogen reduction process of ammonia synthesis using water as a raw material is that hydrogen evolution is not only a competitive reaction,but also an essential step for ammonia synthesis.This makes it difficult to obtain an efficient nitrogen reduction catalyst.Considering that phosphorus and nitrogen are in the same main group and may have a"similar compatibility"phenomenon,so it is proposed that the introduction of phosphorus in the hydrogen evolution catalyst can achieve a balance between the reduction of nitrogen and the competitive reaction of hydrogen evolution.As a verification test,MIL-101（Fe）was used as the precursor to synthesize Fe P@C catalyst material through pyrolysis and phosphating strategy.The research results show that the incorporation of P element not only helps retain the inherent three-dimensional pore structure of MIL-101（Fe）and enlarge the electrochemical active area,but also exhibits good nitrogen activation performance while enhancing the hydrogen evolution activity.In 0.1 M Li₂SO₄ electrolyte,the highest ammonia yield can reach1.52μg h^-1 cm^-2（@-0.4V vs.RHE）.This research result also confirmed the feasibility of using a higher activity hydrogen evolution catalyst to achieve nitrogen activation.