Electronic Structure Tuning Of Copper Nanomaterials For Ammonia Synthesis From Nitrate Electroreduction In Flow Cell

Ammonia（NH₃）is widely used for production of fertilizers,pesticides,weapons and ammunition,and thus plays an important role in economy and national defense.In industry,ammonia is obtained mainly through the Haber-Bosch Reaction（N₂+3H₂→2NH₃）.The Haber-Bosch reaction,which produces ammonia directly from nitrogen in the air,is considered the greatest invention in the 20th century,and it is the only chemical reaction that has resulted in three Nobel Prize winners.Ammonia synthesis by the Haber-Bosch process requires high temperatures（350-450℃）and high pressures（150-200 Pa）,In addition,the hydrogen required for this reaction is mainly produced by reforming fossil fuels;this process not only consumes 3%of the world’s natural gas each year but also emits a significant amount of carbon dioxide（about 1.3%of global carbon emissions）.Electrocatalytic reduction of nitrogen paves a new path for green and sustainable ammonia synthesis,as it uses renewable energies as the driving source,utilizes nitrogen and water as reactants,and produces ammonia at ambient temperatures and pressures.Unfortunately,due to the ultra-low solubility of nitrogen（0.66 mmol/L）in water and ultra-high N≡N bond splitting energy（941 k J/mol）,the selectivity of electrocatalytic reduction of nitrogen to ammonia is low（less than 10%in most cases）,and the corresponding partial current density is small（generally less than 1 m A/cm²）,which cannot meet the requirements of large-scale practical applications.Compared with nitrogen,nitrate（NO₃^-）has a lower N=O bond dissociation energy of 204 k J/mol,is highly soluble in water,and is widely distributed in agricultural and industrial wastewater.In addition,when nitrate concentrations in drinking water exceed 10 mg/L,the human body is prone to diseases such as methemoglobin disease and non-Hodgkin’s lymphoma.Therefore,electrocatalytic reduction of nitrate is not only expected to achieve high selectivity and high reaction rate ammonia synthesis,but also a potential solution to environmental problems.In the electrocatalytic reduction of nitrate synthesis of ammonia reaction,copper-based nanomaterials show excellent catalytic selectivity at low current density due to the performance enhancement strategies including morphology regulation,monatom modification,alloying,valence design,heterogeneous structure construction,and defect engineering.However,the performance of ammonia synthesis at industrial current density is still limited by weak intermediate adsorption and slow mass transport.To solve these problems,in this thesis,the approach based on electronic structure tuning is used to improve the ability of copper nanomaterials to efficiently catalyze nitrate–to–ammonia conversion,while a flow cell is utilized to accelerate the transportion of nitrates,electrons and protons to realize highly selective electrocatalytic reduction of nitrate to ammonia under industrial-level current densities.The main contents of this thesis are as follows:（1）The first work focus on tuning the electronic structure of copper nanoparticles by constructing a higly alkaline reaction environment.Theoretical calculations and experimental characterizations joinly demonstrate that high concentration of OH^-can improve the d-band center of Cu,promote the adsorption of*NO,and reduce the energy barrier of the rate-decisive step（*NO→*HNO）.In a flow cell,using the mixture of 7 M KOH and 0.75 M KNO₃ as electrolyte,copper nanoparticles can achieve almost 100%Faraday efficiency toward ammonia synthesis at an applied current density of 1500 m A/cm².（2）The second work utilizes doping strategy to tailor the electronic structure of copper nanoparticles.First,copper nanoparticles with tunable boron doping concentration are synthesized by a facile wet chemical method.Then,through structural characterizations,it is found that boron dopants mainly exist within the shallow lattice of copper,causing an enlarged Cu-Cu bond length and thus a tensile strain field.Subsequently,theoretical calculations and in situ spectroscopy show that the tensile strain field can promote the generation of hydrogen radicals,which enables an accelerated hydrogenation kinetics of key intermediates.Besides,boron dopants can also raise the d-band center of Cu and weaken the thermodynamic energy barrier of the intermediate reaction.Benefiting from these,boron-doped copper nanoparticles exhibit a nearly 100%selectivity for ammonia production in a flow cell with an applied 1800 m A/cm² current density.