Electrocatalytic Performance Of Carbon Dot-Supported Cobalt-Based Materials For The Reduction Of Nitrate To Ammonia Synthesis

Ammonia plays an integral role in every aspect of human life.The traditional Haber-Bosch process still dominates the ammonia synthesis industry due to the abundance of raw materials,low prices and lower costs due to large-scale industrialization.However,the reaction conditions of high temperature and high pressure make the energy consumption of the ammonia synthesis process relatively high.In addition,the ammonia synthesis process not only requires a large amount of energy supply,but also emits a large amount of carbon dioxide,causing environmental problems,which are not friendly to developing countries.Therefore,it is reasonable and urgent to develop a low-energy-consumption,green ammonia synthesis technology.Among them,electrocatalytic reduction of nitrate to synthesize ammonia is a very promising route to synthesize ammonia under ambient conditions.However,most of the nitrate-ammonia electrocatalysts developed to date have limited Faraday efficiency due to the competing hydrogen evolution reaction.Simple preparation and facile modification of carbon dots（CDs）can be used to modify catalysts and have been widely used in electrocatalytic reactions.Modification of catalysts with rational doping of CDs can change their surface acidity and basicity to improve the reaction efficiency and efficiency.Suppresses adverse side effects.Based on this,this thesis focuses on the advantages of carbonized dot-based electrocatalysts,and studies its electrocatalytic performance and catalytic mechanism for the reduction of nitrate to ammonia,and the following works are carried out:（1）Preparation of XCDs using small molecules containing different heteroatoms as precursors and then carbon cloth was added to form a composite material through secondary hydrothermal treatment.Finally,XCDs/CC were obtained after calcination.To explore the performance of CDs with different doping types in electrocatalytic reduction of nitrate to synthesize ammonia in 0.5 M K₂SO₄ solution.Since boron-doped carbon dots（BCDs）provide more active sites and strong nitrate adsorption energy,it possesses higher current density and nitrate reduction activity under the tested conditions.（2）Based on the above studies,we prepared BCDs with boron salt as raw material,mixed it with CoCl₂·6H₂O and urea,and prepared Co₃O₄@BCDs/CC composite catalyst by a combination of hydrothermal and calcination.Co₃O₄ is the main body to catalyze the reduction of nitrate to synthesize ammonia,and the modification of BCDs not only further increases the catalytic sites,but also enhances the adsorption energy of nitrate.Therefore,in 0.5 M K₂SO₄（+200 ppm NO3-）solution,the catalyst exhibits a high Faradaic efficiency（90%）with good stability.（3）Since nitrate reduction involves a multiproton engagement process,the amount of dissociated hydrogen from the water is critical to the ammonia conversion process.In order to further improve the Faradaic efficiency and explore the catalyst reaction path,we introduced metallic nickel as the hydrogen adsorption site to prepare BCDs/NiCo₂O₄/CC catalysts based on the previous work.In the electrocatalytic process,metallic nickel provides a large amount of adsorbed hydrogen required for the reaction,which promotes the hydrogenation process;BCDs and NiCo2O4 jointly provide sufficient catalytic sites.Therefore,the catalyst exhibits high Faradaic efficiency（～100%）;meanwhile,at-0.55 V（vs.RHE）,the yield of ammonia synthesis by electrochemical reduction of nitrate as cathode is 173.9μmol^-1cm^-2.