Synchrotron Radiation Study On Structure And Properties Of Transition Metal Baed Nano-Electrocatalysts

As the basis of the modern chemical industry,catalyst plays an important role in promoting the development of industrial society and environmental governance.During the catalytic reaction process,the catalyst involves complex physical and chemical reactions such as the adsorption of reactants,formation of reactive intermediates,desorption of products,as well as the surface electronic structure reconstruction and the change of chemical components.These physical and chemical processes not only affect the activity and stability of catalysts,but are also directly related to the synthesis of high-performance catalysts.Herefore,tracking the relevant information involving chemical valences,spatial configurations and active intermediates of the catalytic active sites during their reaction processes via synchrotron radiation techniques is very important for establishing connection between micro-structure and macroscopic catalytic performance and understanding their catalytic reaction mechanism on the atomic scale.In this thesis,transition metal based nanomaterials were chosen as the research system.Some novel structures were engineered by introducing lattice strain,building heterojunction and constructing hetero-dicationic pair,and the reaction performance of kinetic,energy conversion efficiency and stability of these nano-electrocatalysts is improved in oxygen evolution reaction(OER)and nitrogen reduction reaction(NRR).Combined with synchrotron radiation X-ray fine structure absorption spectroscopy(XAFS)and synchrotron radiation Fourier transform infrared spectroscopy(SR-FTIR),we have conducted in-depth study of the evolution of coordination configuration,electronic structure and reaction intermediates of catalysts under working conditions and further disclosed their microscopic reaction mechanism.Our findings would provide a theoretical guidance and an experimental basis for designing and synthesizing transition metal-based nanocatalysts with excellent performance.The specific research content of this paper is as follows:1.Operando synchrotron radiation study of lattice strain NiFe-MOFs Micro-reaction mechanism during OER processNiFe-MOFs have acted as promising OER electrocatalysts,owing to their structural tunanilty and intrinsic large surface area.searching an effective method to regulating NiFe-MOFs OER performance and promoting practical application.Here we propose a rational linker scission approach to induce lattice strain in metal-organic framework(MOF)catalysts by partially replacing multicoordinating linkers with nonbridging ligands.Strained NiFe-MOFs with 6%lattice expansion exhibit a superior catalytic performance for OER under alkaline conditions(230 mV@10 mA cm-2).Operando studies by using synchrotron radiation X-ray absorption and infrared spectroscopy identified the emergence of a key*OOH intermediate on Ni3+/4+ sites during OER,providing a strong evidence that the Ni3+/4+sites are the active sites and the formation of*OOH is the rate-limiting step.The first-principles calculations were performed to reveal the strain-induced electronic structure changes of the NiFe-MOFs and the Gibbs free energy during OER.It is found that the optimized Ni 3d eg-orbital facilitates the formation of*OOH,thus enhancing the OER performance of the strained MOFs.2.Operando synchrotron radiation study of Co9S8@Fe3O4 heterojunction structure evolution during OER processTransition metal sulfides have been studied widely as promising electrocatalysts for OER in alkaline media.However,their real active species in the OER process remain elusive.And it is a great challenge to probe the evolution of oxyhydroxides reconstructed from sulfides delivering stable large current density at low applied potentials.Here we report a novel synergistic hybrid catalyst,Fe3O4 acts as "fence",composed of nanoscale heterostructures of Co9S8@Fe3O4.According to OER tests,the Co9S8@Fe3O4 could run 120 h at the 500 mA cm-2.Voltage-dependent soft X-ray absorption spectroscopy(sXAS)and Operando Raman spectroscopy demonstrate the initial Co9S8@Fe3O4 is reconstructed into CoOOH/CoOx@Fe3O4 and further transformed into CoOOH@Fe3O4 totally.Operando sXAS and electron microscopy imaging analysis reveal that the completely reconstructed CoOOH acts as the active species and Fe3O4 components prevent the aggregation of CoOOH.Operando infrared spectroscopy indicates cobalt superoxide species(*OOH)as the active intermediates during the OER process.Density functional theory calculations demonstrate the formation of*OOH can be as the rate-determining step of OER and CoOOH@Fe3O4 exhibits lower energy barrier for OER.3.Operando synchrotron radiation study of hetero-dication in MoO2@MoO3 during NRR processAmong many nitrogen fixation reaction,NRR has received extensive attention due to its environmental friendliness.However,the breaking of nonpolar N≡N bond of dinitrogen is the biggest dilemma for electrocatalytic nitrogen reduction reaction(NRR)application.In theory,we revealed that constructing a unique hetero-dicationic Mo4+-Mo6+ pair could effectively activate N≡N bond with a lying-down chemisorption configuration by an asymmetrical "π back-donation"process,by using density functional theory calculations.In experiment,we adopted a facile localized phase transformation strategy synthesized MoO2@MoO3 heterostructure with double Mo sites(Mo4+-Mo6+).NRR tests show that this hetero-dicationic Mo4+-Mo6+ pair catalysts display more excellent catalytic performance with a high NH3 yield(60.9 μg·h-1·mg-1)and Faradic efficiency(23.8%)under ambient conditions,compared with pristine MoO2 and MoO3.Using synchrotron-based spectroscopic techniques identified the electron transfer between the electrocatalyst could boost the actiavetion of inert N≡N.This finding could provide a theoretical guidance for the synthesis of high performance NRR catalysts.