Regulation Of Composition And Structure Of Transition Metal Catalysts For Ambient Electrosynthesis Of Nitrogen-Containing Molecules

The excessive consumption of fossil fuels and the sharp increase in energy demand have caused increasingly prominent problems of environment and energy.To get rid of dependence on fossil energy and make full use of clean and renewable energy is a research hotspot for a considerable period of time.As an important chemical,ammonia(NH3)is widely used in various industrial production,such as dyes,synthetic fibers,nitric acid,etc.It plays a vital role in the global economy,and NH3 is also considered to be an ideal fuel for hydrogen storage in the future.In addition,urea is one of the most important nitrogen fertilizers,feeding about 19%of the world’s population over the past century.Urea is also a key feedstock for the manufacture of fine chemicals and a promising energy carrier for sustainable hydrogen production.To date,the Haber-Bosch process,which requires harsh reaction conditions and accounts for 1-2%of world energy consumption and about 1.4%of carbon dioxide emissions.In industry,about 80%of NH3 produced by the Haber-Bosch process is used for urea synthesis.Haber-Bosch process needs to be carried out under extremely harsh reaction conditions such as high temperature and high pressure,which results in high energy consumption and serious pollution.Therefore,it is urgent to explore sustainable,efficient and environmentally friendly technological routes to achieve nitrogen conversion.Electrosynthesis driven by clean electricity has become a hot spot and an attractive alternative method in industry.Thermodynamic,the energy efficiency of electrochemical ammonia synthesis is about 20%higher than that of Haber-Bosch process.In addition,the C-N can be synthesized electrochemically by using CO2(or CO)and N2,nitrite,nitrate or NO in ambient conditions and water systems.This route can not only reduce CO2 emissions,but also reduce the pollution of CO,nitrite,nitrate or NO,which has important scientific value and socioeconomic benefits.However,the electrocatalytic synthesis route still has a big gap from the actual application,in which the activity and selectivity of catalysts to the target products need to be greatly improved.Therefore,the development of high efficiency electrocatalyst is very important for the practical application of electrocatalytic synthesis.In this thesis,to meet the demand of electrocatalytic synthesis of high-value nitrogencontaining compounds,a series of transition metal catalysts with specific composition and structure have been designed and prepared.The obtained catalysts showed excellent performance of electrocatalytic synthesis of ammonia and urea.Combined with theoretical and experimental results,the reaction process and catalytic mechanism were revealed.The main research contents and achievements are as follows:1.An oxygen-coordinated molybdenum single atom catalyst for efficient electrosynthesis of ammonia.The surface of activated carbon(AC)was richly modified with oxygen-containing functional groups,and then the molybdenum(Mo)single atoms were anchored on the surface of AC by O,forming a Mo-Ox site.Due to the attraction of O to electrons,the Mo single atom exhibits a positive charge and increases its activity.The obtained materials were used for electrochemical N2 reduction reaction(NRR)to ammonia.Mo single atom can effectively adsorb and activate N2 molecules.By adding a proton and an electron transfer,the adsorbed N2 undergoes a hydrogenation reaction,first forming a*NNH species that is adsorbed on the Mo site.Next,the distal N atom of the*NNH molecule combines with(H++e)to form a new*NNH2 molecule.Subsequently,the distal N atom of the*NNH2 continued to be attacked by the third(H++e),and the first NH3 was released.The one N atom left on the surface of the catalyst will be further hydrogenated by three other electron coupled protons to form a second NH3 molecule.Performance studies have shown that the prepared Mo-SAs/AC exhibits high NRR electrocatalytic activity.The NH3 yield reached 2.55±0.31 mg h-1 mgMo-1 at 0.40 V(vs.RHE)in 0.1 M Na2SO4 electrolyte,and the Faraday efficiency(FE)was 57.54±6.98%.It also shows good stability.2.Electrochemical reduction of nitrate to ammonia with oxygen vacancy-rich CuOx nanoparticles.Oxygen-rich vacancy CuOx nanoparticles were prepared by laser irradiation and used for nitrate reduction(NO3RR)to ammonia in fluidized electrocatalytic system.The oxygen vacancy on the surface of cuprous oxide can be used as an active site for the reduction reaction,improving the efficiency and activity of the reaction.First,nitrate(NO3-)combines with oxygen vacancies in the material and is activated(*NO3-).The activated nitrate combines with hydrogen ions in the solution and is protonated(H*NO3-).H*NO3-undergoes a continuous dehydration and protonation cycle until it evolves into H2NO*,and the intermediate is further hydrated and deoxidized to form NH3.Performance studies show that the NH3 yield reached 449.41±12.18 μg h-1 mgcat-1 in-0.25 V(vs.RHE)with FE of 74.18±2.27%.In this fluid electrocatalytic system,CuOx nanoparticles with abundant oxygen vacancy are highly dispersed in the electrolyte,which effectively improves the electrochemical mass transfer efficiency and the adsorption and activation efficiency of reactants,and thus improves the selectivity and yield of NO3RR for ammonia.3.Cu2O/Cu(OH)2 heterogeneous catalyst for electrocatalytic ammonia synthesis.Cu2O/Cu(OH)2 heterogeneous nanorods have been successfully fabricated by liquid phase laser irradiation technology.The effects of laser irradiation conditions on the composition and structure of the materials have been systematically investigated.The product prepared in 0.05 M alkali solution not only forms a Cu2O/Cu(OH)2 heterogeneous interface,but also(131)crystal surface with higher activity appears in the Cu(OH)2.The abundant heterogeneous interfaces and highly active crystal surfaces become active sites for catalytic reactions.The catalytic performance of Cu2O/Cu(OH)2 heterogeneous materials was studied using NO3RR to ammonia as a model reaction,in which nitrate(NO3-)combines with heterogeneous interfaces and active crystal surfaces in the material and is activated(*NO3-).NH3 is formed through subsequent protonation and continuous dehydration,protonation,hydration,and deoxidation processes.The results show that the yield of NH3 reached 1630.66±29.72 μg h-1 mgcat-1 with FE of 61.04±8.22%at-0.6 V(vs.RHE).4.Electrosynthesis of Urea over Iron-Based Dual-Sites.A liquid-phase laser irradiation method was used to fabricate symbiotic graphitic carbon encapsulated amorphous iron(Fe(a)@C)and iron oxide(Fe3O4)nanoparticles on carbon nanotubes(CNTs)(Fe(a)@C-Fe3O4/CNTs).Theoretical and experimental results reveal that Fe(a)@C and Fe3O4 formed on carbon nanotubes provide dual active sites for the adsorption and activation of NO3-and CO2,with Fe(a)@C exhibiting lower adsorption free energies of CO2 and NO3-.Although the reduction of NO3-to*NH2 in the three models is a spontaneous process,the Fe(a)@C site has the lowest energy,which is conducive to the formation of*NH2,while Fe3O4 is more conducive to the CO2RR process.The synergistic effect of the two sites greatly promotes the C-N coupling reaction,resulting in a high yield of urea synthesis.Performance studies reveal that the yield of urea from the reaction of NO3-and CO2 catalyzed by Fe(a)@C-Fe3O4/CNTs reaches 1341.3±112.6 μg h-1 mgcat-1 and a faradic efficiency of 16.5±6.1%at ambient conditions.