Study On The Construction Of Active Sites And Catalytic Performance Of Organic-inorganic Composites

Energy shortage and environmental pollution are two major problems today's world is facing.Photocatalysis and electrocatalysis are two green technologies developed in recent years for environmental purification and energy conversion,which display great research significance and industrial application potential,and have attracted wide attention all over the world.The improvement of catalytic performance become the joint research target.In recent years,it has been proved that the size of the metal particles is one of the key factors determining the activity and selectivity of the catalyst.As the decreasing of the size of metal particles,the number of surface atoms exposed is dramatically increased,together with their surface atomic structure,electronic structure and surface defects changed.Consequently,the utilization of metal atoms is greatly improved,together with the catalytic activity.As a result,monodisperse metal-supported catalysts become the research focus and are developed vigorously.However,the interaction between metal atoms and the supports is the key factor affecting the activity and stability of monodisperse metal-supported catalysts.The number of anchoring sites for metal atoms in the support directly affects the number of active sites in the monodispersed metal-supported catalyst,consequently affects the activity of the catalysts.In the mean time,the electronic interaction between the supports and the metal atoms will affect the electronic structure of the catalyst,together with the catalytic activity.In addition,the strength of the interaction between the supports and the metal atoms determines whether the metal atoms are easy to aggregate,thus affecting the stability of the cataly st.Therefore,the structure of the support is very important for the monodispersed metal-supported catalyst due to it will directly affect the interaction between the metal atoms and the support.In consequence,the key to improve the catalytic activity and stability of the catalyst is to select an appropriate support to anchor the metal atoms and avoid the aggregation of metal atoms so as to increase the number of monodisperse active sites.Aiming at above issues,this thesis is devoted to enhance the catalytic activity and stability of monodisperse metal-supported catalysts by developing new supports,construction of novel active sites and selecting appropriate supports to improve the loading capacity of active sites.The specific research content in the thesis are presented as below:1.From the perspective of developing new supports,heterocyclic conductive polymer polythiophene(PTh)was chose as a new substrate material which owns excellent conductivity and flexibility.The sulfur(S)atoms in its polymer main chain can be used to anchor metal atoms.Therefore,PTh is a good candidate as support material for monodisperse metal-supported catalysts.S atoms in PTh were used to anchor Co atoms and construct atomically dispersed CoxOyS4 catalytic sites.The synthesized atomically dispersed catalyst is presented as Co@PTh.high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM)and electron energy loss spectroscopy(EELS)demonstrated that the cobalt-based species in Co@PTh were atomically dispersed.X-ray absorption near edge structure(XANES)and extended X-ray absorption fine structure(EXAFS)characterized the oxidation state and coordination environment of cobalt-based species in Co@PTh.We propose a possible structure of Co@PTh according to the XANES and EXAFS results.Compared with inert PTh,Co@PTh displayed prominent catalytic activity and stability,which surpassed that of the commercial RuO2 and comparable with the state-of-the-art catalyst IrO2.2.The catalytic activity was tried to enhanced by selecting appropriate supports to construct monodisperse active sites and improve the loading capacity of active sites.HITP(HITP=2,3,6,7,10,11-hexaiminotriphenylene)ligands were selected to coordinate with Co and form Co-N4 sites.A ?-d conjugated two-dimensional(2D)MOF Co3(HITP)2 was synthesized.XANES,EXAFS,X-ray photoelectron spectroscopy(XPS)and Fourier transform infrared(FTIR)spectra confirmed the successful coordination of Co with HITP and the formation of Co-N4 sites.The metallic properties and conduction mechanism of Co3(HITP)2 were studied experimentally and theoretically.Due to the outstanding conductivity of Co3(HITP)2 and the extremely high loading capacity of atomically dispersed Co-N4 sites(23.44 wt.%of Co element)in it,the loading capacity of which is far more than do the reported single-atom-based catalysts,Co3(HITP)2 displayed excellent and stable OER activity,which is superior to most traditional single-atom-based catalysts as well as commercial RuO2 and IrO2.3.The strategy of selecting appropriate supports to construct monodisperse active sites and improve the loading capacity of active sites was extended to other HITP related materials.A ?-d conjugated two-dimensional(2D)MOF Fe3(HITP)2 was successfully synthesized by the coordination of Fe with HITP ligands.XANES,EXAFS,XPS,FTIR spectra,Raman spectra and thermogravimetric analysis(TGA)measurements proved the successful coordination of Fe with HITP and the formation of Fe-N4 sites.Fe3(HITP)2 possessed stable chemical structure and large amount of homogeneously dispersed Fe-N4 centers(18.11 wt.%of Fe element).Fe3(HITP)2 was applied in the field of photo-Fenton like degradation of antibiotics.Fe3(HITP)2 displayed superior catalytic performance and stability.In addition,Fe3(HITP)2 could work efficiently in a wide pH range.The concentration of iron ions leached from Fe3(HITP)2 was much lower than those reported before.Therefore,Fe3(HITP)2 was an excellent Fenton like reagent.Fe3(HITP)2 also held great potential in the degradation of industrial wastewater.The mechanism of the photo-Fenton like reaction was proposed through the results of electron paramagnetic resonance(EPR)spectra and the first principles density functional theory(DFT)calculations.This work further demonstrated the importance of improving the loading capacity of catalytic sites and intrinsic material stability for catalytic activity and stability.4.The active sites construction strategy was used to improve the photocatalytic activity of the wide band gap semiconductor CdWO4.CdWO4 was modified on the surface with polar 4-mercaptobenzoic acid(4-MBA)to construct Cd-S and W-S active sites.FTIR spectra,XPS and TGA measurements indicated that 4-MBA was successfully connected on the CdWO4 surface through chemical bonds.The UV-vis diffuse reflectance spectra(DRS)indicated that the absorption band edge of CdWO4 was shifted to the visible region.Photocatalytic hydrogen evolution and rhodamine-B(RhB)degradation by CdWO4 were significantly enhanced.The photoluminescence(PL)emission spectra and time-resolved PL decay spectra,open circuit voltage(Voc)and second harmonic generation(SHG)results confirmed the generation of induced polar surface and built-in electric field at 4-MBA@CdWO4,which was favorable for the separation of photogenerated charge carriers and the improvement of photocatalytic activity.