Study On The Microstructure Of Ionic Liquid And The Reduction Process Of Metal Ion By Synchrotron Radiation X-Ray Absorption Fine Structure Spectroscopy

Synchrotron radiation X-ray absorption fine structure spectroscopy(XAFS)with atomic selectivity is an effective technical method for investigating the local microstructure of elemental species.Moreover,from the spectral analysis,the structure information such as the electronic structure of central atom,the type and number of coordination atoms,the length of coordination bond,the disorder degree of the system,and so on can be obtained.In addition,being independent of long-range ordered crystal structure,XAFS is very suitable for investigating the structure of amorphous and complex ionic liquid and the reduction process of metal ion.Ionic liquid(IL),completely composed of cation and anion,is a kind of special molten salt and is liquid at room temperature.ILs have a series of unique physical and chemical properties,such as low vapor pressure,high thermal stability,wide liquid range,high conductivity and wide electrochemical window etc.,due to their special chemical composition.These excellent properties make ionic liquid widely used in some fields,such as organic synthesis,catalysis,extraction separation and electrochemistry,etc.These applications of ionic liquids often involve the solvent effect,the temperature effect and the interface induced effect of solid material and other factors.By varying the above factors,understanding the microstructure change of ionic liquid is of great significance to expand the application of ionic liquid in some fields.In this paper,synchrotron radiation XAFS was used to study the microstructural changes of ionic liquids with the varied external factors(solvent and temperature).Then,when ionic liquid was confined in or adsorbed on the nano-material surface(laponite or carbon nanotubes),an situ XAFS was used to study the temperature-dependent microstructural changes of ionic liquids confined in nano-materials surface.Additionally,metal nanoparticles confined in nano-materials surface has a good dispersion and can significantly improve its catalytic performance in the catalytic field.Therefore,investigation on the reduction process of confined metal ion provides a theoretical guidance for its industrial applications.In this paper,synchrotron radiation XAFS was used to study the spontaneous reduction process of platinum ions confined in carbon nanotubes.This article mainly includes the following five research contents:(1)The structure of [Bmim]2Cu Cl4 ionic liquids with different ratios of H2 O and C2H5 OH was investigated using X-ray absorption fine structure(XAFS)spectroscopy.Results confirmed that the structure of ionic liquids was gradually transformed from tetrahedral to octahedral configuration with an increase in the volume ratio of H2 O.The structure change can be interpreted that water molecules coordinated with the copper ions of [Bmim]2Cu Cl4 to form the compound.However,results indicated that the structure of ionic liquid had no change and still maintained the tetrahedral configuration in the presence of high concentration of anhydrous ethanol.This result can be interpreted that anhydrous ethanol cannot coordinate with the copper ions of [Bmim]2Cu Cl4 to form the compound.Therefore,anhydrous ethanol is a good solvent for the dilution of ionic liquid.(2)The temperature-dependent microstructure changes of [Bmim]Fe Cl4 ionic liquid were investigated by an in-situ synchrotron radiation XAFS combined with Raman and DFT simulation methods.Raman results show that the anion structure composition of ionic liquid changed with the varied temperature.An in situ XAFS result show that the number of coordination atom Cl around Fe atom gradually decreased and the length of Fe-Cl bond gradually shortened with the increasing of temperature.Further,we performed DFT to calculate the length of Fe-Cl bond with the varied number of coordination atom Cl around Fe atom(4,3,2).Results show that the length of Fe-Cl bond is gradually shortened with the decrease in number of coordination atom Cl around Fe atom,which is consistent with the XAFS results.(3)In this study we investigated the cation–anion relative position and the molecular orientation of C6 mim Br ionic liquid intercalated in LAPONITE? by in-situ XAFS and some traditional methods(TEM,DSC,XRD,and IR).DSC and TEM analyses revealed the formation of C6 mim Br crystals intercalated in LAPONITE?.The XAFS and XRD analyses at ambient temperature revealed that an ordered monolayer structure,with C6mim+ cations intercalated in the interlayer nanospace and Br located on the edge of the LAPONITE?,was formed when ILs were intercalated in LAPONITE?.Moreover,results also demonstrated that the enhanced interactions among ionic liquid,the formed hydrogen bonds between Br anion of the C6 mim Br and O–H group of LAPONITE?,as well as the ordered monolayer arrangement of intercalated C6mim+ cations induced the formation of C6 mim Br crystals intercalated in LAPONITE?.Further,in-situ XAFS analysis with a combination of XRD patterns revealed that the structure orientation of the intercalated C6mim+ cations tends to deflect and maintain the ordered monolayer arrangement with the elevation of temperature.Moreover,the ordered crystal structure shows a significant change after the temperature has risen to the melting point of the confined ionic liquid.(4)In-situ XAFS with combination of some traditional methods(TEM,DSC,Raman and XRD)were performed to investigate structural changes in the C6 mim Br ionic liquid confined within the channels of multiwalled carbon nanotubes(MWCNTs).DSC and TEM analyses revealed the formation of C6 mim Br crystals confined within multiwalled carbon nanotubes.The XAFS spectra at room temperature confirmed that the existence of p-? conjugation and ?-? stacking between ionic liquid and carbon nanotubes,which induced the structure of ionic liquid to be an ordered alternating layer structure.Moreover,results indicated that carbon nanotubes have a compression effect on ionic liquids.Therefore,the ordered structure of ionic liquid and the compression from carbon nanotubes induced the formation of confined C6 mim Br ionic liquid crystals.Further,in-situ XAFS analysis and XRD patterns revealed that the temperature variation induces the structural rearrangement of the confined ionic liquid from the ordered alternating layer structure to a near planar structure.Raman and XRD analysis confirmed that the structure of confined ionic liquid is rearranged with the elevation of temperature,which is consistent with XAFS results.(5)The structure and valence changes of H2 Pt Cl6 confined in multi-walled carbon(MWCNTs)was mainly investigated using XAFS spectroscopy with the aid of TEM,Raman and XRD studies.The XAFS spectra revealed that Pt4+ ion spontaneously reduced into the element of Pt,and transfromed tetravalent into zerovalent,when H2 Pt Cl6 was confined in MWCNTs.The XAFS results indicated that the reduction of Pt4+ was attributed to the electron transfer from MWCNTs to Pt4+ ion.Moreover,the Pt XAFS data displayed that the compression from carbon nanotubes promoted the formation of metal platinum nanoparticles and the Pt–Pt bond length in confined Pt nanoparticles became shorter than that of Pt in the bulk state.Further,by varying the inner diameter of MWCNTs from 15 nm to 10 nm and 5 nm,metal platinum nanoparticles with different sizes can be prepared and the Pt–Pt bond length of nanoconfined Pt nanoparticles decreased gradually.The above results clearly show that the multi-walled carbon nanotubes,which are rich-electron donors,can continuously reduce the confined platinum ions into the metal platinum elements.