Study On Design Of Catalyst And Efficiency Improvement Strategy For Electrochemical Ammonia Synthesis Under Ambient Conditions

Ammonia is one of the most basic chemical raw materials in industrial and agricultural production,which supports and solves the problem of food supply for billions of people in the world.At the same time,ammonia is also an ideal energy（hydrogen）carrier,which has a great application prospect in the future hydrogen recycling economy.For a long time,the production of ammonia has mainly depended on the Haber-Bosch process.As we all know,although the synthesis technology has been developed for hundreds of years,it still needs to be carried out under high temperature and high pressure conditions（300～500℃,200～300atm）,which requires great energy consumption.Its annual energy consumption accounts for nearly 2%of the world’s total energy consumption.Moreover,as the hydrogen feedstock of the process mainly comes from non-renewable fossil fuels,it will also cause a large amount of CO₂ emissions.Therefore,under the background of national energy conservation and emission reduction and"dual carbon"goals,it is very necessary to seek a"green"ammonia synthesis technology with mild and sustainable reaction conditions.In contrast,electrochemical ammonia synthesis is considered to be an attractive alternative technology for ammonia synthesis because it can directly use nitrogen and water as raw materials to achieve ammonia synthesis under mild conditions.At present,the main challenge facing the electrochemical ammonia synthesis technology is that it is difficult to activate the nitrogen-nitrogen triple bond under mild conditions,and there is a strong competitive reaction of hydrogen evolution,which leads to a relatively lower ammonia yield rate and current efficiency.In response to the above challenges,this thesis mainly focused on the high-efficiency catalytic activation of the nitrogen-nitrogen triple bond under mild conditions and the balance of the hydrogen evolution competition reaction,carried out relevant basic research,and tried to use nitrate as a replacement nitrogen source and plasma-assisted technical scheme to improve the efficiency of electrochemical ammonia synthesis.The research content and results of this paper are as follows:（1）Firstly,this paper proposes a catalyst design strategy that takes advantage of the preferential adsorption of nitrogen to balance the nitrogen reduction and the competitive hydrogen evolution.Based on the guidance of theoretical prediction,a two-dimensional Ti₃C₂T_x（MXene）nanosheet catalyst that can preferentially adsorb nitrogen and activate nitrogen molecules was designed and prepared.By adjusting the size of MXene nanosheets and loading them on the support with vertical array structure,the maximum exposure of edge active sites and the adjustment of the hydrogen evolution intensity of the two-dimensional Ti₃C₂T_x were achieved.The results show that the ammonia yield rate of Ti₃C₂T_x nanosheets increases from 2.29μg h^-1 cm^-2 to 4.72μg h^-1 cm^-2 when the size of Ti₃C₂T_x nanosheets is reduced.When Fe OOH with poor hydrogen evolution performance was further selected as a carrier,the hydrogen evolution intensity was significantly reduced,and the current efficiency was increased to 5.78%.（2）On the basis of the above research,this paper draws on the metal components of nitrogenase,and further proposes the design strategy of bimetallic MXene catalyst.Based on this academic idea,this paper designed and prepared bimetallic Mo₂Ti C₂T_x（MXene）nanosheet catalyst by introducing Mo site into Ti₃C₂T_x（MXene）catalyst.The experimental results show that Mo₂Ti C₂T_x nanosheets can adsorb and activate N₂ under mild conditions,and their catalytic performance is higher than Mo₂CT_x and Ti₃C₂T_x single metal components,indicating that the synergistic effect of Mo and Ti sites makes the catalyst exhibit good activity of nitrogen reduction activity for ammonia synthesis,providing a new idea for the design and development of efficient nitrogen reduction catalyst.（3）Aiming at the problem of low yield of ammonia by directly reacting nitrogen and water by electrochemical methods under mild conditions,this paper proposes a strategy of recycling nitrate or nitrite in the environment as a nitrogen source to achieve high-efficiency electrochemical synthesis of ammonia.And by selecting cuprous oxide nanowires as the electrocatalyst for the reduction of nitrate or nitrite,the yield and current efficiency of ammonia with potential application are realized at room temperature and pressure.The results show that the ammonia yield rate can reach 3058.23μg h^-1 cm^-2 with nitrite as nitrogen source and 1749.68μg h^-1 cm^-2 with nitrate as nitrogen source using Cu₂O/Cu as catalyst electrode at-0.6 V vs.RHE potential,which is two orders of magnitude higher than the best yield of ammonia synthesis with nitrogen as nitrogen source reported at present.（4）In view of the superiority of nitrate or nitrite as a nitrogen source in electrochemical ammonia synthesis,this paper further proposed a two-step ammonia synthesis strategy of oxidizing nitrogen to nitrogen oxides and then reducing nitrogen oxides（i.e.,"N2→NOx→NH3" two-step ammonia synthesis）,and through the use of non-thermal plasma to assist the synthesis of NOx intermediates,high-efficiency electrochemical ammonia synthesis using nitrogen as the nitrogen source is realized.This thesis mainly explores the composition of absorption liquid,output voltage and feed gas composition on the non-thermal plasma synthesis efficiency of NOx intermediate,and uses cuprous oxide as a catalyst to achieve efficient electrocatalytic reduction of nitrate and nitrite.The results of this paper show that the highest NOx conversion efficiency and energy efficiency can be obtained at 2 k V voltage with neutral PBS buffer solution as absorption solution and air as feedstock gas（nitrite yield rate is 12.08 mg/h,nitrite yield rate is 16.95 mg/h）;The minimum energy consumption of NOx is 3.42 kwh/mol N.The highest ammonia yield rate(2213.4 μg h^-1 cm^-2was achieved at-0.5 V vs.RHE potential with Cu₂O/Cu as electrocatalyst.