Main Product Analysis Of Nitrate Electrochemical Reduction And The Improvement Of Ammonia Conversion Efficiency

Ammonia（NH₃）,as one of the basic raw materials for modern industrial and agricultural development,has been considered a potential green energy carrier.Currently,industrial ammonia is synthesized by the Haber-Bosch method,which consumes huge amounts of energy and produces large quantities of CO₂.With the increasing energy shortage and environmental pollution,it is urgent to develop a mild and sustainable technique for ammonia synthesis.Electrocatalytic nitrate reduction to ammonia（NRA）is a promising method for ammonia synthesis,which allows obtaining ammonia from nitrate-rich wastewater at ambient temperature and pressure by utilizing sustainable energy such as solar and wind.NRA not only contributes to alleviating energy shortage but also promotes the development of a circular economy.Accompanied by various side reactions and competing reactions,nitrate reduction is a relatively complex process that involves multiple reaction steps and various intermediates.Therefore,the ammonia synthesis yield and Faraday efficiency are inhibited,which hinders its large-scale applications.The design and synthesis of high activity NRA electrocatalyst is the key to improve the efficiency of ammonia synthesis,which has become the focus of scientific research.In this thesis,we have designed several non-noble metal-based electrocatalytic materials and studied the relationship between the structure and property of the metal sites.Through metal doping,construction of dual active centers,and atom substitution,the effects of active site design and electronic structure modulation on catalytic activity are explored,the NRA catalytic mechanism is preliminarily studied,the mechanism of NRA activity enhancement is revealed.The main contents of this thesis are as follows:1.The Mo-based materials containing the same active center with nitrate reductase show great potential in NRA catalysis.Exploring the relationship between Mo electronic structure and activity is instructive for designing highly active Mo-based catalysts.Through in-situ oxidation,the MoO₂-C nanoball-flower（MoO₂-C NBF）is oxidized to MoO₃-C nanoball-flower（MoO₃-C NBF）.The effect of the electronic structure of the Mo site on the catalytic activity is explored.The experimental results prove that MoO₂-C NBF exhibits a superior NRA activity:NH₃ yield(R_NH3)of 109.28μmol cm^-2 h^-1 and NH₃ Faraday efficiency(FE_NH3)of 99.05%at-0.3 V.Compared with MoO₃-C NBF,the unique electronic structure of Mo（IV）site in MoO₂-C NBF enhances the adsorption capacity of reactive intermediates and reduces the energy barrier,significantly improving the NRA activity.2.To enhance the ammonia yield,Fe/Mo-based nitride nano-octahedra（Fe Mo N@C NO）with bimetallic sites is designed,and the mechanism of NRA activity enhancement by the Fe and Mo dual active centers is investigated.The results demonstrate that the Mo site effectively accelerates the reduction of nitrate to nitrite,while the Fe site exhibits a high selectivity for the reduction of nitrate to ammonia.The dual active centers effectively accelerate the NRA processes and dramatically increase the ammonia yield.At-0.3 V,the FE_NH3 of Fe Mo N@C NO is 98.2%,and the R_NH3is 434.16μmol cm^-2 h^-1.The Fe Mo N@C NO-based Zn-nitrate battery exhibits high activity with a maximum power density of 3.23 m W cm^-2 and the open circuit voltage of 1.47 V.3.Cu-based materials exhibit excellent nitrate reduction activity due to the most compatible orbital with nitrate.However,the large amount of nitrite byproduct produced during the electroreduction processes results in a low selectivity of ammonia.The construction of dual active centers is expected to achieve cascade catalysis that can reduce the byproduct generation and increase the NRA activity.A series of Ni_xCu_（3-x）（HITP）₂ MOFs（HITP:2,3,6,7,10,11-hexaaminotriphenylene）with dual active centers are prepared by replacing Cu sites in Cu（HITP）₂ with Ni,and the synergistic effect of Ni and Cu sites on NRA is investigated.The results show that the Ni_1.5Cu_1.5（HITP）₂MOF exhibits the best R_NH3 of 130.93μmol cm^-2 h^-1 and FE_NH3 of 72.45%.Theoretical study reveals that the bimetallic site strategy realizes the cascade catalysis of NRA:Firstly,NO₃^-is adsorbed at the Cu site and catalyzed to^*NO₂,then the^*NO₂intermediate dissociates from the Cu site and forms NO₂^-.Subsequently,the Ni site captures the free NO₂^-and reduces it to NH₃.The cascade catalysis inhibits the formation of byproducts,significantly improving the selectivity of ammonia.4.A series of Co-doping Cu（OH）₂ nanoflower(Co_xCu_（1-x）（OH）₂)materials are successfully synthesized by controlling the metal ratio in the hydrothermal reaction.The effect of Co-doping on NRA activity is investigated.The electronic interaction between Co and Cu reduces the electron density around the Cu site,which adjusts the Cu electronic structure and inhibits the side reactions and competition reactions,resulting in an improvement of ammonia selectivity and NRA activity.The Co_0.6Cu_0.4（OH）₂ nanoflower shows the highest R_NH3（99.88%）and FE_NH3(22.34 mg cm^-2 h^-1)at-0.5 V.5.Co₃O₄ displays a high NRA activity.To identify the active center of Co₃O₄,the metal site replacement strategy is used to prepare Zn Co₂O₄ and Co Al₂O₄,via substituting the Co O₄ and Co O₆ sites in Co₃O₄ by inert Zn and Al,respectively.The results reveal the trend of NRA activity as follows:Co₃O₄>Zn Co₂O₄>Co Al₂O₄,indicating the dominant active site of Co O₆.The unique electron configuration of Co O₆enhances the interaction between Co 3d orbital and O 2p orbital,which improves the adsorption energy of oxygen-containing intermediates,leading to a higher NRA activity.In addition,the Co O₆ and Co O₄ sites are bridged by O atoms,while the Co O₄ site is not the active center of NRA.Therefore,the electronic structure of Co O₆ can be further regulated by replacing the metal element at the Co O₄ site.The NRA activity is thus can be improved.Through metal site substitution,the Ni Co₂O₄ exhibits a much higher catalytic performance than Co₃O₄ with R_NH3 of 15.49 mg cm^-2 h^-1 and FE_NH3 of99.89%.The results of theoretical calculation reveal that the d-band center moves towards the Fermi level after Ni replacement,and the adsorption capacity is enhanced.The reaction rate of NRA is thus greatly improved.