Synthesis Of Carbon Coated Metal Oxides And Their Electrocatalytic Synthesis Of Ammonia

Ammonia,as an important chemical product,plays an important role in the national economy.The industrial synthesis of ammonia mainly relies on the traditional Haber-Bosch ammonia synthesis process,requiring a large amount of energy consumption with a large amount of carbon dioxide released.It is critical to create new synthetic ammonia alternative technologies in order to overcome the high energy consumption and environmental problems in the Haber Bosch ammonia synthesis process.Electrocatalytic nitrogen reduction is a new technology for synthesizing ammonia with low energy consumption,simple operati on,and no pollution.However,nitrogen,as an inert gas,is difficult to activate under mild conditions,resulting in an unsatisfactory ammonia yield rate and Faraday efficiency in electrocatalytic ammonia synthesis.In addition,hydrogen evolution reactions as competition can also inhibit the progress of ammonia synthesis.Targeting the above issues,three types of carbon coated metal oxide catalysts were synthesized in this thesis,exhibiting excellent electrocatalytic performance for ammonia synthesis at mild conditions.The octahedral ZrO₂@C was synthesized via the high-temperature pyrolysis of Ui O-66 under the N₂ atmosphere.ZrO₂ was generated via the reaction between Zr ions and oxygen in terephthalic acid during the high temperature pyrolysis process,and the carbon structure from the carbonization of terephthalic acid limited the agglomeration increases of ZrO₂.The pyrolysis temperature,as the main influencing factor,affected the carbonization of organic ligands and the formation of ZrO₂.With the increase of pyrolysis temperature,the carbonization degree of terephthalic acid increased,and the content and grain size of ZrO₂ also increased.ZrO2@C obtained at different pyrolysis temperatures displayed the morphology,electrochemical impedance,electrochemical active area,and electrochemical performance of the catalyst was investigated.ZrO₂@C-650 obtained at 650℃exhibited the highest electrocatalytic performance,with an ammonia yield rate of 6.80μg h^-1 mg^-1 and a Faraday efficiency of 2.68%（-0.6 V vs RHE）.La₂O₃@C was synthesized via the pyrolysis of the La-citrate nanosheets prepared by the cation exchange of La³⁺and sodium citrate under the N₂ atmosphere.During the high-temperature pyrolysis process,La₂O₃ was generated by the reaction between Zr ions and oxygen in citrate.The pyrolysis temperature was the main influencing factor,and the higher the pyrolysis temperature,the more favorable it is for the carbonization of citric acid and the formation of La₂O₃.La₂O₃@C-800obtained at 800℃exhibited the strongest N₂ adsorption capacity,the smallest electrochemical impedance,and the largest electrochemical active area,also possessed the best electrocatalytic performance for ammonia synthesis with ammonia yield rates of 19.8μg^-1 mg^-1,and Faraday efficiency of 16.8%at-0.3V（vs RHE）.Eu₂O₃@C,Tb₂O₃@C and Er₂O₃@C nanosheets prepared by the same method with La₂O₃@C exhibited the electrocatalytic ammonia synthesis,proving the universality of the method for synthesizing lanthanide metal oxide nanosheets.The DFT calculation results indicate that the surface NRR process of La₂O₃@C followed the alternating association mechanism.Cu@NC nanocubes were prepared by the pyrolysis of Cu₂O with a polydopamine shell,which was formed by the oxidative self-polymerization characteristics of dopamine under alkaline conditions.During the pyrolysis process,polydopamine carbonized into a carbon structure,limiting the aggregation of catalysts and effectively maintaining the morphology of nanocubes.During the pyrolysis process of Cu₂O@PDA,Cu₂O was reduced to Cu⁰,and the higher the pyrolysis temperature,the more favorable the formation of Cu⁰.Compared to Cu₂O,the Cu@NC exhibited enhanced N₂ adsorption capacity,larger electrochemical active area,and lower electrochemical impedance,proving that the introduction of Cu⁰ was conducive to their electrocatalytic activity.Cu@NC at 500℃showed the highest electrocatalytic performance for ammonia synthesis,with an ammonia yield rate of35.54μg h^-1 mg^-1 and a Faraday efficiency of 21.12%（-0.2 V vs RHE）.