Preparation And Electrocatalytic Ammonia Synthesis Study Of Two-Dimensional MXene:Ti₃C₂T_x Based Composite Materials

Ammonia is one of the largest chemical products in the world,and plays an important role in industrial and agricultural production and energy storage and conversion.At present,the global ammonia production is about 150 million tons per year,which is mainly derived from the traditional Haber-Bosch ammonia synthesis process:that is,by using iron-based catalysts under high temperature and high pressure conditions.The Haber method for ammonia synthesis has high energy consumption and serious pollution:the energy consumption of the entire process accounts for 1-2%of the world’s total annual energy consumption.The high-purity hydrogen used is derived from the natural gas reorganization of fossil fuels,and the annual CO₂emissions are as high as 450 million tons.Therefore,the energy crisis and environmental problems are becoming increasingly prominent under the circumstances,it is of great significance to find a green,environmental-friendly and efficient ammonia synthesis process for the sustainable development of the national economy.The electrochemical ammonia synthesis method can make the thermodynamic non-spontaneous ammonia synthesis reaction driven by electric energy free from or less restricted by thermodynamic equilibrium,and realize the synthesis of ammonia at room temperature and pressure,so it has become a hot research field that has attracted much attention.In view of this,we have carried out a series of research work,mainly including the following parts:（1）Accordion-shaped multilayer Ti₃C₂T_x nanosheets were successfully prepared by HF etching of Ti₃Al C₂.Subsequently,Au nanoparticles were evenly loaded on the surface of Ti₃C₂T_x nanosheets through the ethylene glycol reduction process,and high-efficiency electrocatalytic nitrogen reduction to ammonia synthesis was achieved at room temperature and pressure.Electrochemical tests of CV,LSV,CA,etc.showed that Au/Ti₃C₂T_x nanosheets exhibited excellent catalytic activity in 0.1 M HCl electrolyte,and the ammonia production rate at a potential of-0.30 V was 18.5μg h^-1 mg^-1.The efficiency is 8.75%.At the same time,the current density of the catalyst has little attenuation during the continuous polarization process for up to 20 hours,and it has good electrochemical stability.（2）Inspired by the thermodynamic metastable transition metal Ti atoms on the surface of the two-dimensional Ti₃C₂T_x MXene material,Ti O₂ nanoparticles with rich oxygen vacancies were grown in situ on the surface of the Ti₃C₂T_x nanosheets through a cost-effective one-step oxidation method for use at room temperature Electrocatalytic synthesis of ammonia catalyst under normal pressure.XRD,TEM,XPS,ESR and other physical and chemical characterization prove that Ti O₂ is uniformly distributed on the surface of Ti₃C₂T_x nanosheets in the form of nanoparticles,and rich oxygen vacancies are formed during in-situ growth.Electrochemical tests show that the catalyst has excellent catalytic activity and good selectivity in acidic electrolyte（0.1 M HCl）,and an ammonia production rate of 24.0μg h^-1 mg^-1 is obtained at-0.40 V vs.RHE potential.And achieved a Faraday efficiency of 6.5%at-0.35 V vs.RHE,while also having excellent stability and durability.The rich oxygen vacancies on the surface of the multi-layer Ti O₂/Ti₃C₂T_x nanosheets are the main active sites for ammonia synthesis,while the internal unoxidized Ti₃C₂T_x not only has excellent conductivity,which is conducive to electron transport,but also can effectively avoid the self-aggregation of Ti O₂nanoparticles.The synergy between the two promotes the transition from N₂to NH₃.（3）Using the unique structure and surface properties of two-dimensional Ti₃C₂T_x,combined with the vacancy engineering strategy,a two-dimensional Au/Ti O₂ composite material derived from Ti₃C₂T_x and enriched with oxygen vacancies was designed and prepared for electrocatalytic synthesis of ammonia catalyst at room temperature and pressure.In 0.01 M HCl electrolyte,the catalyst can obtain an ammonia production rate of 64.6μg h^-1 mg^-1 at a potential of-0.40 V and achieve a Faraday efficiency of up to 29.5%at-0.30 V,while maintaining the good electrochemical stability for long-term test.The isotope tracing experiment qualitatively confirmed that NH₃ originated from the catalytic reduction of N₂produced by the electrode.Density functional theory（DFT）calculations confirmed the outstanding contribution of oxygen vacancies（V_o）in Ti O₂ to stabilize Au clusters and adsorb and activate N₂.In addition,the synergy between Au and V_o greatly reduces the reaction barrier,and promotes the NRR reaction process from both electrochemical and thermodynamic aspects.