Development And Application Of In-situ XAFS In Different Catalytic Systems

Due to its high brightness,wide wavenumber range,high stability and high intensity,synchrotron radiation has been one of indispensable technique for the characterization of materials.X-ray absorption fine structure（XAFS）spectroscopy is a spectral characterization method that is generated by the transition of the core electron.XAFS exhibits certain elemental selectivity and short ordered range sensitivity due to spectral characterization of interferometric electron and scattered electron interference modulation.The electronic structure and spatial structure of the target element can be obtained simultaneously,so that it has become an important method for studying the local structure of materials in recent years.Catalysis,as the key to accelerate chemical reactions and energy conversions,has attracted lots of researchers’attention to cope with energy shortage and environmental pollution in the modern society.As reported in the future 50-year report of DOE–Manipulation of matter and energy-Grand challenge in science and imagination:The greatest challenge for energy revolution is how to control the atomic-level and electronic-level synthesis of catalyst,and clarify the relationships between the atomic and electronic structure of catalysts and its catalytic performance.In order to predict and understand the dynamic and real atomic transformation of catalysis process,it is crucial to relate the structure of catalysts to their catalytic performances,in-situ monitor their electronic and atomic structures under reactions,track their electronic and atomic structure transformations,and finally clarify their catalytic active sites for the reactions.Thus,deeper understanding and more knowledge to the catalytic mechanism could be achieved by developing new in-situ XAFS methodology towards different catalytic systems,and thus monitoring the structural transformations of catalyst under the mimicked real catalytic reaction conditions.This work was conducted in XAFS beamline at Shanghai Synchrotron Radiation Facility,mainly focusing on fundamental scientific questions in catalysis,designing in-situ XAFS devices,developing in-situ XAFS methods,collecting in-situ XAFS data.The combination of in-situ XAFS and other techniques paves a new avenue for the prediction and understanding of catalytic reaction mechanism.Here are the three main works:1.Development and application of in-situ XAFS in thermal catalytic reactions.1)Targeting at the catalytic reaction mechanism of Co-based catalysts under traditional Fischer–Tropsch（FT）process,two sets of in-situ XAFS devices have been developed for high-temperature,atmosphere-pressure FT process and high-temperature,high-pressure FT process,respectively.For the high-temperature and atmosphere-pressure XAFS measurement set-up,it creates the capability for regular gas-to-solid reactions.During the reaction,gases can be flowed in and out the reaction cell,and the heating temperature can be varied from room temperature to 500?C.Overall,this set-up provides inserted gases,varied and homogeneous temperatures,and convenient operation for users.By optimizing and advancing the high-temperature and atmosphere-pressure XAFS set-up,a high-temperature and high-pressure XAFS set-up was obtained with varied heating temperatures from RT to 900 C and varied pressures from room atmosphere to 10 bar.2)Applying the high-temperature and atmosphere-pressure XAFS set-up to the study of the in-situ growth of Cobalt carbide under FT process.The structure transformation of a model catalyst Na-doped Co₃O₄ was conducted under H₂ and Syngas（H₂ and CO）reaction conditions.With the combination of wavelet transform,two possible reaction pathways were revealed,i.e.1)Co₃O₄ to metallic Co to Co₂C₃ and Co₃O₄ to Co₂O₃;2)Co₃O₄ to Co₂O₃ to Co₂C₃.3)Three model catalysts,Co@MnO,Co/Co₂C@MnO and Co₂C@MnO prepared from precipitation method,were analysed by XAFS simulation,activity evaluation and DFT calculation.We found that Co₂C has a stronger CO absorption capability to that of Co,whilst a weaker H absorption capability.This favours C-C coupling but suppressing hydrogenation.With a combined C-C coupling capability from Co,Co/Co₂C catalyst is able to facilitate the generation of C₅₊products and the prohibition of CH₄,which provides new instructions for the preparation new model catalyst.4)According to the mysterious relationship between FT catalytic active center and their supports,a standard CoMn model catalyst prepared from wet immersion method was investigated by high-temperature and high-pressure XAFS set-up.A combination of in-situ XAFS and other techniques clarified the interaction between the support and catalytic active center,and revealed the catalytic structural transformation leading to the increased activity.2.In-situ XAFS in the development and application of electrocatalysis.Two in-situ XAFS set-ups were developed for multiple electro-catalytic reactions,e.g.OER,ORR,HER,et al.,under Lytle collection mode and solid detector collection mode,respectively.The relationship between the OER performance and structure of V-doped NiFe-LDH was studied,revealing its catalytic active center and its OER mechanism.3.In-situ XAFS in the development and application of photocatalysis.Two in-situ XAFS set-ups were developed for regular in-situ photo catalytic reactions and photo-electro catalytic reactions,respectively.By using regular in-situ photocatalyst XAFS set-up,a model catalyst,Pt nanoparticles on C₃N₄,was investigated.It was found that the decreased oxidation state and increased metallic state of Pt nanoparticles helped its improved photocatalytic performances under photo catalytic reaction.