Application Of Cation-doped Transition Metal Nanocatalysts In Electrocatalytic Nitrogen Reduction

Ammonia（NH₃）is an important chemical with significant applications in the chemical,agricultural and medical fields.Compared with the current Haber-Bosch process for NH₃ synthesis,which requires high pressure and high temperature,electrocatalytic nitrogen reduction reaction（NRR）technology is an extremely effective alternative method for synthesizing NH₃ because it is green and does not require high temperature and pressure.Due to the presence of the extremely difficult to break N≡N triple bond(945 k J mol^-1)and competitive hydrogen evolution reaction（HER）,the electrochemical conversion efficiency from N₂ to NH₃ is still at a low level,and there is an urgent need to develop cheap and efficient catalysts for enhancing the activity and selectivity of electrocatalytic NRR.In this thesis,non-precious metal catalysts were studied to regulate the structure and chemical composition of the catalysts through doping,and then modulate the catalyst performance,and finally design and synthesize efficient and stable new transition metal-based NRR catalyst materials.In addition,this paper systematically investigates the NH₃ production rate,selectivity and stability of the catalysts under ambient temperature and pressure conditions through a series of electrochemical test experiments,and analyzes the effects of doping-induced changes in catalyst structure and composition on the catalytic activity of NRR.The studies described in this paper can be divided into three parts as follows:1.Molybdenum carbide（Mo₂C）has been widely used in electrocatalytic reactions due to its desirable electronic structure and good adsorption ability to electron-rich substances.In this chapter work,Fe-doped Mo₂C nanoparticles loaded on nitrogen-doped carbon（Fe-Mo₂C NPs/NC）electrocatalysts were synthesized by template method and acid etching method.The Fe-Mo₂C NPs/NC achieved the NH₃ production rate of 36.7μg _NH3 h^-1 mg_cat.^-1and the Faraday efficiency（FE）of 5.2%at the optimum potential of-0.4 V vs.RHE in 0.1 M Na₂SO₄ electrolyte at ambient temperature and pressure.The good NRR catalytic activity of the catalyst was analyzed and attributed mainly to the Fe doping-induced phase transition from MoC to Mo₂C and lattice strain,which optimized the electronic structure of the surface of the catalyst material and increased the number of active sites on the catalyst surface.This work provides a novel and feasible idea for the design and development of high-efficiency NRR catalysts.2.Rare earth elements have special electronic structures and dynamically tunable coordination modes.The doping of rare earth elements can enhance the NRR electrocatalytic performance of catalyst materials to a certain extent.In this work,Yb-doped MoO₂（Yb-MoO₂）catalysts were prepared by hydrothermal and calcination methods,and the comparative analysis shows that the doping amount of Yb directly affects the electrocatalytic NRR performance of MoO₂,and the appropriate amount of Yb doping into MoO₂ can effectively regulate the crystal structure of the catalyst,which in turn adjusts its electronic structure and coordination environment and finally improves its electrocatalytic NRR reaction performance.The optimum catalyst was Yb-MoO₂-4%,which could produce NH₃at the rate of 32.7μg _NH3 h^-1 mg_cat.^-1at-0.3 V vs.RHE potential.This work demonstrates that the appropriate doping of rare earth elements can effectively improve the NRR electrocatalytic performance of transition metal oxides,which provides valuable reference information for related research in this field.3.The synergistic effect between the catalytic active sites due to Fe doping plays an important role in the enhancement of NRR activity.In addition,the size effect of catalyst materials is also one of the effective ways to enhance their NRR catalytic activity.In this chapter,a series of Fe-doped CuS_0.8 quantum dots(Fe_x-CuS_0.8 QDs)catalyst materials with different Fe contents were controllably prepared by electrodeposition method.After a series of studies,Fe-CuS_0.8 QDs were shown to be the optimal NRR catalysts with the NH₃ production rate of 29.3μg _NH3 h^-1 mg_cat.^-1at-0.4 V vs.RHE.The good activity of this catalyst can be attributed to the fact that the appropriate amount of Fe doping can increase the number of low-coordinated metal atoms on the surface and edges of Fe-CuS_0.8 QDs,where the low-coordinated Fe and Cuatoms can act as high-quality catalytic active sites in the NRR process.In addition,Fe-CuS_0.8 QDs with quantum dot structures have a significant size effect,which allows full exposure of the active sites.The controlled and facile synthesis method of Fe_x-CuS_0.8 QDs developed in this work can provide a new strategy for the design and development of NRR catalysts prepared by electrodeposition.