In Situ Study On The Structural Evolution Of Atomically Dispersed Metal Catalysts By Synchrotron Radiation

With the continuous development of nanomaterial science,a wide variety of nanocatalysts have been produced,among which atomically dispersed metal catalysts have become star materials in recent years and have attracted extensive attention in the field of catalysis.Due to the high dispersion and unique local structure of metal atoms,atomically dispersed metal catalysts exhibit special physical and chemical properties and high activities in various catalytic reactions.Faced with the problems of energy and environment,the design of high-efficient atomically dispersed metal catalysts and the realization of clean energy conversion and storage are an important directions in the field of energy catalysis.However,due to the higher surface energy,the atomic dispersed metal catalysts are more sensitive to chemical environment.Therefore,a series of structural changes occur easily in atomic dispersed metal catalysts during catalytic reactions,including the evolution of morphology,elemental distribution,coordination environment and structure evolution to a near free state,which can bring changes to the activity and selectivity of the catalysts.The characterization of catalysts under reaction conditions and the research of structural evolution and deactivation mechanism will bring us a deeper understanding of the structure-activity relationship of catalysts.In recent years,characterization methods based on high brightness synchrotron radiation(SR)sources have been widely used in the study of catalysis.Such as the Operando XAFS technique is sensitive to the chemical environment and local structure of absorption atoms,and the SR-infrared spectroscopy is sensitive to the surface structure and easy to obtain the data with high signal-to-noise ratio.Thus,the synchrotron radiation based characterizations are effective methods to characterize atomically dispersed metal catalysts.In this thesis,the single atom alloy(SAA)and dual atom catalysts as novel concepts in atomically dispersed metal catalysts,are selected to be investigated.To study the dynamic structural evolution,the metal-metal interaction and structure-activity relationship of the catalysts,we performed the research on the basis of synchrotron radiation light source and combined EXAFS fittings,XANES simulations and DFT calculations together.The research content of this paper is divided into the following parts:1.Operando XAFS technique reveals the dynamic reconstruction process of the Cu-Au single-atom alloy during electrochemical reactionsBimetallic nanoparticles are sensitive to their chemical environment.It is of great significance to the design of high-performance catalysts and monitor the surface reconstruction process during the catalytic reactions.In the third chapter of this paper,we selected Cu single atom/Au nanoparticles with highly homogeneous structure to conduct the operando research of surface structure evolution under electrochemical conditions.Operando XAFS has observed that at a certain reduction potential,Cu single atoms are reducted and migrate from the corner sites of the nanoparticles into the more stable(100)plane.At the end of reduction reaction,Cu atoms return to the original corner sites,and the XANES simulations further confirm the dynamic migration process of Cu single atoms.First-principles calculations show that the electronic structural interaction between Cu atoms and Au nanoparticles is the key factor during the migration process.In this work,the dynamic migration process of Cu single atoms under electrocatalytic conditions was well revealed.This work is helpful to understand the mechanism of surface structure recombination of nanocatalysts at atomic scale.2.Combining several synchrotron radiation operando spectroscopy techniques to research the structure-activity relationship of CoAu-SAA in CO2 reduction reactionSingle atom alloy has a good application prospect in CO2 reduction reaction(CO2RR),and it’s very important to study the structure-activity relationship of catalysts in practical catalytic reactions.In the fourth chapter of this paper,we designed an CoAu-SAA electrocatalyst.Using operando XAFS technique,it is revealed that Co-O4 on the surface of Au nanoparticles(NPs)is partially reduced in CO2RR,and then Co atoms is able to bond with Au atoms directly on the surface.With the increase of the alloying degree between Co and Au,the charge transfer between them appeared,and the interaction between Co and Au is enhanced.Combined operando synchrotron-IR and XAFS,we found that the introduction of Co atoms successfully modulated the atomic and electronic structures of Au NPs,which is beneficial to the adsorption of CO2 molecules and the conversion of*COOH active intermediates.As a consequence,the CO selectivity was increased from 42.5%to 92.2%.In this work,a highly selective CO2 electroreduction catalyst was designed,and the synergistic effect of CoAu-SAA was studied by operando characterization techniques.The structure-activity relationship of CoAu-SAA was well revealed,which provides a new idea for the development of high efficiency catalyst.3.Utilization operando XAFS techniques for the accurately design and identify of atomically accurate binuclear active sites in CO2RRDual-atom catalysts exhibit higher catalytic activity than single atom catalysts in specific reactions,which provides a platform for futher understanding of the structure-activity relationship of materials at atomic scale.In the fifth chapter,we adopt an active-sites pre-synthesis strategy using Ni2(DPPM)2Cl3(DPPM=CH3(PPH2)2)clusters as the precursor.The atomically accurate Ni2 diatomic species were successfully loaded on nitrogen-doped carbon supporter.The operando XAFS measurements clearly revealed the dynamic evolution of atomic and electronic structures of the diatomic sites.The EXAFS fittings and XANES simulations demonstrated the dynamic adsorption of bridging oxygen on the Ni2-N6 site.At the same time,the length of Ni-Ni bond is shortened and the interaction between Ni atoms is enhanced.The theoretical calculations further showed that the structure of O-Ni2-N6 significantly reduced the energy barrier of the activation of CO2 molecules,and thus showed higher activity and selectivity in CO2RR.This work reveals the nature of the enhanced catalytic activity of precise diatomic sites and provides theoretical guidance for the design of multi-site catalysts.