Synchrotron Radiation Study on the Structure and Performance of Single-atom Catalysts

Author:Cao Yuan Jie

Supervisor:yao tao wei shi qiang


Degree Year:2018





With the increasing consumption of traditional energy resource,developing clean energy and studying non-fossil fuel conversion become very crucial for achieving the sustainable development of human society.Designing and synthesizing new catalysts have become the key to achieve the efficient energy conversion.As the smallest nanomaterial in size,single-atom catalysts(SACs)exhibit excellent activity and stability in many catalytic reactions due to their unique atomic and electronic structures and maximized atom utilization efficiency.In addition,single-atom catalysts have the relatively uniform active sites,which provide a good material model for experimentally and theoretically researching the catalytic reaction.Single-atom catalysts,with great research value and application prospects,have become one of the research frontiers and hotspots in the field of heterogeneous catalysis.In this thesis,the background of single-atom catalysts and their application in energy conversion were first introduced and taken as the starting point.Then the synthetic method for SACs,local atomic coordination configuration and electronic structure of SACs active sites,metal-support interaction as well as how these factors affect the catalytic performance were studied and discussed by combining synchrotron radiation XAFS and other characterization methods.The well-defined relationship between the structure and performance of SACs was aiming to provide valuable guidance and reference for designing new energy materials with excellent performance and low cost.At the end,the research content of this thesis consists of the following parts:1.Study of Co single-atom catalysts for photocatalytic H2 evolutionThe knowledge of photocatalytic H2 evolution mechanism is of great importance for designing active catalysts toward sustainable energy supply.Here,we report an atomic-level insight,design and fabrication of single-site Co1-N4 composite as a prototypical photocatalyst for efficient H2 production.Correlated atomic characterizations verify that atomically dispersed Co atoms are successfully grafted by covalently forming Co1-N4 structure on g-C3N4 nanosheets,via atomic-layer-deposition technique.Different from the conventional homolytic or heterolytic pathway,we reveal by theoretical investigations that the coordinated donor nitrogen increases the electron density and lowers the formation barrier of key Co hydride intermediate,thereby accelerating H-H coupling to facilitate H2 generation.As a result,the composite photocatalyst exhibits a robust H2 production activity up to 10.8 μmol h-1,11 times higher than that of pristine counterpart.2.Study of Pt single-atom catalysts for photocatalytic H2 evolutionModulating the electronic structure of cocatalysts is critical for designing active sites towards efficient photocatalytic H2 evolution.Here,we report an electronic modulation in atomically dispersed Pt as highly-efficient H2-evolution site on graphitic carbon nitride(g-C3N4)nanosheets.Synchrotron radiation X-ray spectroscopic results confirm the singly dispersed Pt atoms anchored on g-C3N4 via forming "Pt1-N4" moiety,where the strong interaction of Pt with supports leads to the redistribution of Pt valence electrons with the highly vacant 5d orbital.Mechanistic investigations reveal that the immobilization of Pt single atoms with electron-deficient 5d orbitals on g-C3N4 nanosheets not only facilitate the separation of electron-holes pair,but also optimize the water reduction kinetics on the surface.As a result,the Pt single atoms photocatalyst achieves a high intrinsic activity with turnover frequency of 250 h-1,about 13 times higher than that of nanocrystal counterpart.3.Study of Pt single atoms supported on carbon framework derived from MOFs for electrocatalytic H2 evolutionPlatinum-based catalysts are generally considered to be the most effective electrocatalysts for the hydrogen evolution reaction.Improving the intrinsic activity and atoms utilization efficiency of Pt atoms become important for designing effective and low-cost electrocatalysts.Here,we reported a Pt single-atom catalyst anchored on the carbon framework derived from MOFs via ALD technique.XAFS and HAADF-STEM results showed that Pt are atomically dispersed on the carbon frameworks.The HER test suggested that the single-atom catalysts exhibited only 35 mV overpotential at the current density of 10 mA cm-2.Remarkably,the mass activity of the catalyst was 12 times greater than the Pt/C catalyst.