Development And Application Of Green And Efficient Electrochemical Ammonia Synthesis Catalyst

Synthetic ammonia industry is of great importance to national economy and social development.Ammonia is not only an important chemical product,but also an industrial raw material for the production of nitrogen-containing compounds,which are widely used in fertilizer,transportation,refrigeration,plastics,pharmaceuticals and explosives.In recent years,ammonia has been regarded as a good substance for storing hydrogen energy,which can be used directly in ammonia fuel cells and has an important role in developing hydrogen economy.At present,the production process used in ammonia industry is still the Haber-Bosch（H-B）process,which was put into operation in 1913.The H-B process suffers from harsh conditions,high equipment requirements,high energy consumption,serious pollution and low conversion rate,etc.It is urgent to develop an alternative,sustainable and green process that can reduce N₂ or nitrate to synthesize NH₃ under mild conditions using renewable resources.It is urgent to develop an alternative,sustainable and green process for the synthesis of NH₃ using renewable resources under mild conditions.In recent years,research in ammonia synthesis by electrochemistry has become an industry hotspot due to the success of renewable energy generation technologies.However,the competition between catalytic activity（ammonia yield）and selectivity（Faraday efficiency）of electrochemical catalysts in the aqueous electrocatalytic nitrogen reduction synthesis of NH₃ is a great challenge for electrochemical processes,where higher ammonia yields are often accompanied by lower Faraday efficiencies.In view of the high stability of the N≡N triple bond in the nitrogen molecule,nitrate reduction ammonia synthesis strategies are gradually entering the vision of researchers.In this thesis,we work on both electrocatalytic nitrogen reduction ammonia synthesis and electrocatalytic nitrate reduction ammonia synthesis.（1）For the electrocatalytic reduction of ammonia by nitrogen,an iron-cobalt bimetallic oxide catalyst doped with graphene was designed and synthesized by an electrochemical exfoliation method.The catalysts were characterized by XRD,XPS,SEM,EDS-mapping,and HRTEM to analyze the appearance morphology,phase composition,and chemistry states.The possible theoretical geometrical configurations of cobalt ions and N₂,the site energies corresponding to different configurations of various complexes and the activation energy of the reaction process were explained by DFT calculations.In the electrochemical process,Fe₃O₄ and Co₃O₄ act as adsorption and active sites for N₂,while the doping of graphene increases its conductivity.In the three-electrode system,the ammonia yield reached 2.322×10^-6mol·cm^-2·h^-1 with a Faraday efficiency of 5.2%at-1.5 Vvs SCE voltage and 0.1 M of K₃PO₄as the electrolyte.The advantage of this part of the work is the convenient and efficient preparation process of the catalyst,which provides a new idea for electrocatalytic nitrogen reduction for ammonia synthesis in aqueous systems.（2）For the problem of large bond energy of N≡N triple bond and difficult activation,NO₃^-with relatively low activation energy was used as the nitrogen source.A layer of dense Co₃O₄nanoparticles was deposited on the surface of the electrode material using copper foam as the substrate for electrocatalytic nitrate synthesis of ammonia,and the catalyst was characterized using XRD,XPS,SEM,and EDS-mapping.The ammonia yield was 4.60×10^-5mol·h^-1·cm^-2 at a nitrate concentration of 0.03 M with a potential of-1.1 V vs SCE,and the Faraday efficiency could reach98.17%.In the electrochemical process,Co₃O₄ acts as the active center of nitrate,while the surrounding Cu plays the role of HER inhibition,which results in excellent ammonia production efficiency and selectivity.Moreover,this catalyst can still maintain more than 95%Faraday efficiency after ten cycles of experiments,and the ammonia yield remains basically stable.This part of the work provides both a new strategy for electrochemical catalytic synthesis of ammonia and a new idea for the treatment of nitrate wastewater.