Synchrotron Radiation Study On The Structure And Performance Regulation Of Metal Single-atom Photoelectrocatalysts

The rapid development of industrial and increasing population of the world will be accompanied by the rapid consumption of non renewable resources,resulting in an increasingly serious energy crisis and environmental pollution,which has become the two major challenges that human beings have to face in the 21st century.Therefore,the development of new energy resources and the way of efficient energy conversion have become imminent.In the field of new energy resources,photoelectrocatalytic decomposition of water is regarded as the key,efficient and clean way of energy conversion,which has aroused extensive research interest of researchers.The C-N-based catalysts anchored with metal single atoms have the characteristics of reforming the electronic structure of the active metal,exposing the active center,maximizing the utilization rate of metal atom and stabilizing the chemical structure,which is conducive to obtain high-efficiency photoelectrocatalytic performance.In addition,the strong electron interaction between single atom and carbon nitrogen substrate promotes the formation of uniform and stable active structure,which also provides an ideal model and practical basis for the study of energy conversion process.Therefore,the single atom catalyst anchored on carbon and nitrogen substrate has become a hot scientific research.In this thesis,we anchored metal atoms on the carbon nitrogen based materials by the method of amino-induced reduction.A series strategies had been employed to significantly improve the water splitting performance,such as the active site regulation,the local p-n junction establishment and the metal structure design.It also effectively solved the key scientific issues of instability and insufficient kinetics of electrocatalysts in acid electrolyte as well as weak light absorption capacity and easy recombination of electron holes of photocatalysts.Moreover,it provided valuable theoretical guidance and reference for the design of new catalysts with high performance,low price and high utilization.Based on synchrotron radiation UPS and XAFS technologies and other characterization methods,we explored the influence of electronic structure of single-atom active sites,local coordination environment and the interaction between metal active sites and carbon-nitride substrate on the catalytic performance to establish a clearer structure-activity relationship between the electrocatalyst and catalytic performance.Furthermore,with using the established in-situ synchrotron radiation FTIR spectroscopy technology,the dynamic structure evolution of the active sites can be tracked in real time,and the mechanism of catalytic reaction can be understood more deeply.The specific research content of this thesis is composed of the following parts:1?Operando FTIR study on the kinetic process of electrocatalytic oxygen production by single atom CoThe development of noble-metal-free acid-compatible oxygen electrocatalysts and then monitoring their active sites evolution under working conditions are crucial for global renewable energy storage and conversion.Here,we present a new type of hetero-N coordinated Co(HNC-Co)single sites,with Co active centers bonding to hetero pyridinic-and amino-N ligand,as an efficient oxygen evolution reaction(OER)electrocatalyst in acid medium.By using the operando synchrotron infrared spectroscopy,a potential-driven active site evolution of H2N-(*O-Co)-N4 is observed for the first time during the OER process,which greatly promotes thesurface oxo-species transformation for efficient acidic OER catalysis.In 0.5M H2SO4 solution,the atomically-dispersed HNC-Co electrocatalyst could effectively oxidize water at a quite low overpotential of 265 mV at 10 mA cm-2 with an ultra-high turnover frequency of 2.8 s-1,and a huge mass activity of 7.6 A mg-1,which is 80-240 times that of commercial IrO2.2?XAFS study on the structure and photocatalytic hydrogen production of atom level Pt-Au/C3N4 heterojunctionUnderstanding the principle of photogenerated electron hole pair separation at the atomic level for the design and synthesis of homogeneous catalysts with single atom active sites,development of photocatalysts with synergetic bifunctional catalytic centers is of great importance for efficient photocatalytic water splitting towards sustainable conversion and storage of renewable solar energy.Here,we present a conceptual design of atomic-level bifunctional redox single-sites to realize efficient catalytic reaction of photoexcited electron-hole pairs on g-C3N4-based photocatalysts for spontaneous overall water splitting.Through XAFS characterization,we found that the atom dispersed Pt1Nx-Au1Nx redox unit anchored on the g-C3N4 carrier has strong sp2 coupling with the substrate,and successfully introduced the p-n coupling micro region as an effective dual function electron donor receptor center.In the visible light range of 400-450 nm,the efficiency of electron hole separation has increased significantly to 60%,which is 6-10 times that of individual Pt/C3N4 and Au/C3N4,and lowers surface redox reaction barrier by?300 mV.This p-n type Pt-Au cluster/C3N4 photocatalyst could thus effectively split pure water with excellent H2 evolution rate up to 285 ?mol g-1 h-1 and prominent quantum efficiency of 3%at 420 nm.3.XAFS study on the structure and photocatalytic hydrogen production of Au cluster-NP/C3N4Adjusting the electronic structure and band structure of photocatalyst to design photocatalyst with visible light response is very important to improve the hydrogen production performance of photocatalyst.Here,we successfully integrate Au clusters and nanoparticles(NP)into semiconductors(g-C3N4)through the strategy of amino induced reduction and temperature regulation.Synchrotron radiation XAFS and UPS characterizations and theoretical calculations reveal that the synergetic electron coupling between Au NP and cluster significantly decreases the metal/semiconductor interfacial Schottky barrier by 0.6 eV,and thus promotes the plasmoinc hot-electron injection kinetics for high-efficiency hydrogen evolution.This unique Au cluster-NP/C3N4 could effectively shorten the injection time and prolong the average lifetime of energetic hot-electrons by an order of magnitude in the light absorption range of 400-1100 nm.Hence,the as-obtained Au cluster-NP/C3N4 photocatalyst achieves a prominent photocatalytic H2 production rate of 230 ?mol g-1 h-1,which is 6-20 times that of Au NP/C3N4 and Au cluster/C3N4.