Synchrotron Radiation High-energy-resolution XES And XAFS Methods And Their Application In The Research Of Electrocatalysis

Electrocatalysis,including electrochemical reduction of CO₂（ECO₂RR）,oxygen reduction（ORR）and other reactions,provides a promising route for utilization and conversion of renewable energy,pollution prevention,and curbing climate change,which is in line with the goals of"carbon emissions peak by 2030"and"carbon neutrality by 2060"proposed in the“Fourteenth Five-Year Plan and the Outline of 2035Vision Goals”.The core issue of electrocatalysis lies in the preparation of catalysts with high activity,high stability and low cost.It is necessary to clarify the influence of catalyst structure on performance at atomic and electronic dimensions.X-ray emission spectroscopy（XES）and X-ray absorption fine structure（XAFS）spectroscopy are powerful characterization methods to explore the local atomic and electronic structures of catalysts.XES originates from the second-order process and it’s energy resolution is determined by the lifetime broadening of the outer orbital.Compared with conventional XAFS,the energy resolution of XES is much higher.XES can directly detect the information of occupied state,while XAFS obtains the information of unoccupied state.These two methods complement each other and play an indispensable role in the interpretation of catalytic mechanism and the establishment of structure-activity relationship.Synchrotron radiation high energy resolution XES and XAFS methods combined with operando analysis can identify the real active sites and monitor the adsorption and desorption of adsorbate in real time.Accordingly,the electrocatalysis reaction mechanism can be explored and the theorical guide of rational design and preparation of efficient catalysts can be offered.This dissertation reports a high energy resolution XES spectrometer built at the BL14W1-XAFS beamline at Shanghai Synchrotron Radiation Facility（SSRF）.Besides,the research of copper and nickel catalysts under ECO₂RR and single atom catalysts under various electrocatalysis reaction was conducted based on synchrotron radiation high energy resolution XES and XAFS methods.The main research results of this dissertation are as follows:（1）Based on BL14W1-XAFS beamline at SSRF,first wavelength dispersion high energy resolution X-ray emission spectrometer based on Von Hamos geometry in China was designed and set up,and a complete set of process including spectrometer construction,spectrometer debugging,data collection and data analysis was established.Based on the theoretical analysis and existing parameters of the station,the parameters of each component,including the pixel size of the detector,the radius of curvature of the cylindrical curved crystal,and the adjustment precision of the motion control device,are determined.A series of XES measurement methods was developed and improved,and the communication and joint control between the spectrometer and the beamline control system are realized.And the data processing program was written,can carry out batch data sorting,pixel extraction,pixel integration,back and bottom deduction and other operations,to realize the rapid integration of data,improve the work efficiency.（2）Based on the spectrometer,the operando high resolution XES was measured to determine the dynamic changes of the atomic and electronic structures of Cu-based catalysts in the process of ECO₂RR.Utilizing resonant inelastic X-ray scattering（RIXS）analysis,the gradual transition of Cu（I）to Cu（0）in the surface of Cu₂O/Cu catalyst under different reaction time was determined.Compared with conventional XAFS,high resolution XES broking the self-absorption effect and averaging effect was confirmed.In addition,based on the operando XES analysis of Cu Pc model catalysts under ECO₂RR,we reveal its structure evolution under different potential and its influence on catalytic properties,i.e.the Cu species of atomic dispersion gathered with potential migration,offering the active site of C-C coupling for the generation of C₂H₄.（3）The efficient catalytic activity of ECO₂RR by Ni single atom catalyst（Ni-N-C）was further explored in this study.The catalytic performance of potential-induced structural change regulation of Ni-N-C was analyzed based on operando XAFS.The prepared Ni-N-C has excellent ECO₂RR performance,and can convert CO₂ to CO with the Faradic efficiency up to 99%in a wide potential range.The unique Ni N₄O₂structure was determined by comprehensive spectroscopy analysis,and the Ni was offset from the N₄ plane.Furthermore,operando XAFS analysis confirmed that Ni gradually moved to the N₄ plane with the negative shift of potential,and Ni diatomic clusters were formed under the extremely negative potential,which was corresponding to its catalytic performance,revealing the structural origin of the inhibition and promotion of ECO₂RR.（4）The structure of single atom catalysts in various electrocatalytic systems was analyzed accurately by XAFS method,and the reaction mechanism was revealed.The real active site of Cu single atom catalyst in ECO₂RR was analyzed and confirmed,and it was confirmed that the active site of C₂₊product was not its initial atomically Cu species,but the particles formed in the reaction.As for the carbon-supported divacancy-anchored Pt single atom catalyst for ORR,the complex catalyst structure of single atom and particle coexisted was accurately analyzed based on the combination of XAFS and electron microscope.The coordination configuration and single atom ratio of monodisperse Pt of the were determined,and the effect of defect site loading on the performance of ORR was explained.As for the Rh-N-C single atom catalyst for catalyzing CO oxidation system,a comprehensive spectral analysis method,including XANES simulation,FT-EXAFS fitting and wavelet transform,the configuration of Rh N4 was determined combined with the electronegativity of N and C,providing a structural explanation for its thermodynamic benefit to CO oxidation.