Synthesis And Electrocatalytic Performance Of Transition Metal-based Catalysts

Ammonia（NH₃）,one of the most important industrial raw materials in the word,is widely utilized in agriculture,pharmaceuticals,plastics and so on.The energy-intensive Haber-Bosch（H-B）process,which operates at high temperature and high pressure for industrial NH₃synthesis,results in low hydrogen equilibrium conversion,high energy consumption and serious pollution.With the development of artificial ammonia synthesis technologies,the electrochemical ammonia synthesis at ambient conditions has recently gained some interest due to its advantages of low energy consumption and no emmision of CO₂and so on.However,at present,electrochemical ammonia synthesis faces the problems of high cost and low ammonia production efficiency.Therefore,the development of low cost,high activity and high selectivity electrocatalysts is the key to improve the efficiency of electrochemical ammonia synthesis and accelerate the replacement of Haber-Bosch method.So far,the reported NRR electrocatalysts can be mainly divided into three categories:noble metal catalysts,transition-metal-based catalysts and nonmetallic catalysts.But,previous works have demonstrated that nonmetallic catalysts have poor activity and low selectivity,and the noble metal catalysts are expensive and have poor stability,which limit their large-scale application.Transition-metal-based catalysts have been demonstrated as a promising electrocatalyst for NRR because theÏ€-back donation feature of the d-band electrons of transition-metal weakens the N≡N bond,which is necessary for successful N₂molecule activation.However,its d orbital electrons also facilitate the formation of metal-H bond,which intensifies the hydrogen evolution reaction,thus reducing the selectivity of electrochemical ammonia synthesis.To solve this problem,this thesis work mainly focuses on the cheap and efficient transition-metal-based catalysts as the research object.Firstly,a series of micro/nano structures with different morphology were designed and constructed by simple methods,and then these materials were used as electrocatalysts for NRR.Combined with the experimental phenomena and characterization results,the effect of interfacial,doping and bimetal on NRR activity is respectively discussed.The structure-activity relationship between NRR activity and catalyst structure was deeply understood,and the reaction mechanism of NRR was deepened.The specific researches are as follows:（1）We constructed the amorphous ReS₂nanosheets anchored on TiO₂nanowires by hydrothermal method,which exhibited considerable average ammonia production rate and Faradaic efficiency as high as 5.3μg h^-1cm^-2and 49.8%at an applied potential of-0.2 V（vs.RHE）in 0.1 M Na₂SO₄.The high NRR performance could be attributed to the amorphous feature of ReS₂nanosheets,the rich oxygen vacancies in TiO₂nanowires,their semiconducting feature and Ti-O-Re bond formation,which not only facilitate the sufficient exposure of active sites but also efficiently boost the Faradaic efficiency due to the well-regulation of the surface proton or electron accessibility.（2）The Re-doped TiO₂nanofibers were prepared by electrospinning technology through rhenium doping,cleverly combining transition metal doping and defect engineering.The results suggest that the rhenium doping modulates the electronic structure of TiO₂toward the electrochemical N₂reduction reaction,which significantly improves the electrocatalytic activity.The prepared Re-doped TiO₂nanofibers exhibited a considerable average ammonia production rate(22.7μg h^-1mg^-1_cat.)and Faradaic efficiency（18.1%）at an applied potential of-0.3 V（vs.RHE）in 0.1 M Na₂SO₄.（3）Cu₃Ag bimetallic nanosheets were prepared via one-pot co-reduction method.The as-obtained Cu₃Ag bimetallic nanosheets exhibit excellent NRR performance with an average NH₃production rate of 31.3μg h^-1mg^-1_cat.,a Faradaic efficiency of31.3%at-0.2 V（vs.RHE）and outstanding stability due to the synergistic cooperation between Cu and Ag.Theoretical calculations demonstrate that the introduction of Ag regulates the electronic state of Cu₃Ag and the strong orbital hybridization between Ag 3d and Cu 3d enables more active electrons to participate in the NRR,resulting in reducing the rate determining step of energy barrier and efficiently improving the NRR performance.