Synchrotron Radiation Study On The Structure-Activity Relationship Of Noble Metal-Based Alloy Catalysts

The electrochemical reduction of small molecules such as carbon dioxide,oxygen and nitrogen into high-value products by renewable power is expected to become an effective solution to realize the reuse of renewable resources.So far,many electrocatalytic reactions still rely on precious metals as catalysts.For example,the catalyst with the highest activity of oxygen reduction reaction（ORR）is Pt group precious metal nano materials.The fundamental reason is that precious metals have special d-band electronic structure,such as Pt,Ru,Rh and Pd unsaturated with coordination of d orbitals,which can produce good adsorption with gas reactant molecules and maintain excellent stability in acid-base corrosive media.However,the scarcity of precious metal resources,high cost and complex synthesis process hinder its large-scale application in the field of energy conversion.Many studies have shown that transition metals also have certain catalytic properties,but the adsorption of most transition metals is often too weak or too strong.For example,the activity and stability of Fe in the actual reaction are far lower than those of noble metals.In order to give better play to the value of precious metals in the direction of electrocatalysis,different transition metals with electron capturing or electron donating properties（such as Fe,Co,Ni,Cu,Mn,etc.）can be introduced to form alloys with precious metals.By changing the components of the alloy,the structure of precious metals can be further regulated,so as to optimize the bonding strength of reaction intermediates on the catalyst surface,which can give precious metals more ideal catalytic performance and higher economic practicability.Therefore,in this paper,a series of binary,ternary and quaternary alloy catalysts based on precious metals were prepared by solvothermal method.The transition metals were used to regulate the atomic electronic states of precious metals to improve the catalytic activity and stability of CO₂RR and ORR.The in-situ synchrotron radiation absorption fine structure spectrum is very sensitive to the local structure.It can combined with in-situ synchrotron radiation infrared spectroscopy（SR-FTIR）,which is particularly sensitive to reactant intermediates.By establishing a synchrotron radiation characterization platform under real reaction conditions,monitoring the dynamic evolution of alloy structure and the adsorption and desorption process of surface key reactant intermediates in real time under working conditions,mastering the working mechanism of the catalyst under harsh conditions,and clarifying the relationship between catalyst activity,stability and active center structure,it provides a solid theoretical and experimental basis for further developing efficient electrochemical reduction catalysts and promoting the large-scale application of new energy conversion reactions.This paper mainly includes the following research contents:1.Exploring the dynamic working mechanism of gold based bimetallic catalyst in CO₂RR by in-situ X-ray absorption spectroscopy.Using the strategy of bimetallic collaborative optimization,Fei/Au nanoparticles were accurately designed and controllably synthesized.They were used in CO₂ electrocatalytic reduction reaction to achieve excellent selectivity of carbon monoxide（CO）products.In 0.5M KHCO3,the Faraday efficiency（FE）of CO₂ conversion to CO of Fe1/Au catalyst is 96.3%at-0.65 V,the mass activity is 399 mA/mg and the conversion frequency（TOF）is 11521 h-1 at-0.9 v.It is proved that Fe is anchored on Au NP in the form of single atom by synchrotron radiation X-ray absorption fine structure spectrum and high angle annular dark field scanning transmission electron microscope.In order to exploring the dynamic structure evolution of the catalyst in the reaction,the changes of the interface electron and geometry of Fe1/Au catalyst under CO₂RR working conditions were monitored in real time by in-situ synchrotron radiation X-ray absorption spectroscopy.The coordination configuration evolution of Fe single atom was revealed by XAFS.With the decrease of reduction potential,the electron transfer process from Au to Fe atoms increases and the average valence state of Fe decreases.Through XANES and EXAFS fitting analysis and in-situ FT-IR experimental analysis,the structural evolution process is obtained:when the catalyst is immersed in the electrolyte,the interface structure O3-Fe1Au2 evolves into O2-Fe1Au3.The synergistic effect of Fei/Au promotes CO₂ activation and enhances the adsorption of key intermediates of*COOH,so as to carry out efficient carbon dioxide electroreduction.These results illustrate that the charge directional transfer between bimetallic elements and the enhancement of monatomic interface interaction in the reaction are conducive to the activation of reactants and the adsorption of key intermediates,emphasize the significance of collaborative modulation in promoting the catalysis of monatomic modified nanoparticles,and provide a new idea for the further design and develop the new alloy catalysts.2.The structure-activity relationship of platinum based ternary alloy catalyst under acidic oxygen reduction reaction was analyzed by synchrotron radiation Xray absorption spectroscopy.PtFeMn alloy with small size,multi orientation and coral dendritic morphology was synthesized by solvothermal method.Then we constructed the general synthesis strategy for PtM（M=Fe\Co\Cu\Mn）coral dendritic nanostructures.Through the characterization of transmission electron microscope and photoelectron spectroscopy,it is found that the doped Mn element effectively regulates the size and morphology of the ternary alloy,the particle aging and size growth are inhibited,and the single branch diameter of multi oriented coral branch is only 2nm.Mn doping induces abundant ladder atoms and crystal surface defects on the surface,which is conducive to the improvement of alloy activity.By comparing the properties,it is found that the PtFeMn dendritic alloy doped with tertiary metal has better oxygen reduction performance than PtFe,the active area of the sample increased by 51%,the half wave potential shifts forward by 40mV,and the ORR mass activity reached 2.2 times that of commercial Pt/C catalyst.After 10000 cycles,the mass activity of PtFeMn remained only 82%of the initial value and had excellent cycle acceleration stability.Further synchrotron radiation characterization showed that the introduction of Mn leaded to the increase of 5d empty orbit of Pt,the increase of oxidation state of Fe,and the change of electronic environment of Pt and Fe atoms.Pt and Fe tend to transfer electrons to Mn,and the interaction between Pt,Fe,and Mn are enhanced.Mn plays an important role as an"glue",which leads to the significant improvement of the stability of the alloy catalyst.The above structure-activity relationship shows that the alloying effect brought by Mn doping further optimizes the electronic structure of the alloy surface,and the strong interaction between elements greatly improves the activity and stability of the alloy.This work emphasizes the regulation of multi-element metal interaction on the electronic structure of alloy atoms,provides a new idea for understanding the structureactivity relationship of complex system and designing efficient multi-element alloy catalysts,and provides a reference for the design of fuel cell alloy catalysts.3.Synchrotron radiation X-ray absorption spectroscopy guides the precise design of PtFeCoCu high entropy alloy catalysts for PEMFCs.Small size and multi-oriented coral dendritic alloy nanocrystals are synthesized by the solvothermal method as the framework.The coral nanocrystals are in-situ reconstructed by "high-temperature reduction" strategy to prepare PtFeCoCu high entropy alloy nanoparticles with short-range order and multi-domain polycrystalline structure.The performance test shows that the high entropy alloy nanoparticles have excellent acid oxygen reduction performance.Through cyclic voltammetry,it is found that the electrochemical active area ECS A of the sample is as high as 134 m2/g,which is far higher than 43.2 m2/g of Pt/C sample.Through further performance test,it is found that in oxygen saturated 0.1 M HClO4,PtFeCoCu sample’s half-wave potential is up to 0.955mV,which has a positive displacement of 96mv and 136mV compared with the half-wave potential of PtCo and commercial Pt/C,respectively.The mass activity of PtFeCoCu at 0.9 V is 3.52 A/mg,which is much higher than that of commercial Pt/C（0.14 A/mg）.In order to further explore the relationship between the activity,morphology,composition and structure,the alloy were analyzed by HAADF electron microscope and X-ray diffraction and X-ray photoelectron spectroscopy.Through the characterization of High Angle Annular Dark Field scanning transmission electron microscope（HAADF-STEM）,we found that the sample maintains a high exposure rate of Pt（111）active surface,and has rich crystal surface defects and short-range ordered structure.Therefore,PtFeCoCu has a much higher electrochemical active area than other samples,which greatly improves the catalytic activity of the sample.The X-ray diffraction peak is widened and the intensity is reduced.It is speculated that the particle size of the sample is small and the surface is full of defects,so it has high activity,which is consistent with the analysis results of morphology characterization.Then,the atomic and electronic structure of PtFeCoCu high entropy alloy is deeply explored by using multiple synchrotron radiation spectro.Through XANES and EXAFS analysis,we found that the elements in PtFeCoCu high entropy alloy are not completely randomly distributed,Pt tends to coordinate with Fe/Cu to form a short-range ordered structure,Co tends to coordinate with Fe/Cu.Due to the strong interaction between Pt and adjacent metals,Therefore,the coexistence of local order and disorder can be maintained,and the special structure can exist stably.Due to the difference of interaction between different elements,short-range order leads to more surface lattice distortion and defects.Through the in-depth exploration of the characterization results in the early stage,we found that the high entropy alloy activity after heat treatment reduction comes from stronger multi-element metal synergy,rich crystal surface defects and short-range ordered structure.The catalyst has also been used to construct PEMFCs with excellent power density and durability.The current density of PtFeCoCu sample reaches 1.5 A cm-2 under the condition of 0.66 V,and the peak power density reaches 1.54 W cm-2.It is found that the power density of PtFeCoCu sample is significantly higher than Pt/C and most reported electrocatalysts.The durability of the sample is further evaluated.Before the accelerated cycle durability test（ADT）,the mass activity of ptfecocu sample reaches 0.94A mg-1 in the initial stage.After 30K cycles of ADT,Its mass activity remained at 83%of the initial stage,exceeding the target requirements of DOE.In this work,we obtain structural information feedback through synchrotron radiation technology to guide the optimization of catalysts,which provides a new perspective for the development of fuel cell cathode catalysts with low cost,high activity and high durability.