Design Of Graphene Oxide-based Composites And Study On Their Catalytic Performances

Since graphene was first discovered with the tape-tearing method in 2004 by Andre Geim and Konstantin Novoselov,interests in graphene and its derivatives have been escalated dramatically.Such amazing carbon materials show extraordinary talents on electrochemistry,biotechnology,energy storage and catalysis because of its unique electronical,mechanical and chemical features.Among the various derivatives of graphene,graphene oxide?GO?stands out from the numerous carbon materials due to its chemical properties and extensive applications.Graphene oxide exhibits many similar characteristics with graphene.For example,it also belongs to the carbonic family and possesses the isolated forms and relatively huge high surface area from graphene,which endows GO extraordinary advantages in the synthesis of heterogeneous catalyst.Beyond these basic similarities,it is very different from graphene.While graphene is only composed of sp² hybridized carbon atoms,graphene oxide is inserted by a variety of oxygen-containing functional groups,partial of the C atoms in the plane sheets present sp³hybridizd.Such functionalization has a profound impact on the properties of GO,particularly compared to its graphene parent.Oxygen functional groups above the surface can be utilized for further modification and grafting various functional groups,which finally improve the catalytic performances of GO-based nanomaterials and solve the challenge of low catalytic efficiency from traditional heterogeneous catalysts.Considering so many attractive features of GO,it becomes rather meaningful to further investigate the effect GO takes on various kinds of catalytic reactions,as well as the mechanisms behind them.In this thesis,a serious of GO composite nanomaterials have been synthesized utilizing abundant oxygen-containing functional groups above GO surface and its excellent nature features for carriers since its 2-D structure.Moreover,the nano-composites were applied to several catalytic reactions,such as esterification,epoxidation and photocatalysis.The introduction of GO not only extends recycling times of catalysts,but also improves their catalytic performances.The internal mechanism of enhanced catalytic activity arose from GO was further discussed through the investigation of catalytic performance and the establishment of catalytic model,which provides theoretical foundation for similar research on GO-based nanocomposite.The main contents and results of this thesis are listed as follows:1)GO powder with super dispersity in organic solvents has been prepared through a modified Hummers method under the optimized preparation conditions,which dramatically extended the acidating time of graphite.FT-IR,XRD,Raman,SEM and TEM were used to confirm the successful exfoliation of as-prepared GO,as well as the investigation of the oxygen functional groups,degree of oxidation,crystallinity and regularity of GO sheets.GO nanaosheets prepared with the modified Hummers method contained various oxygen functional groups with an exhaustive oxidation.The sheets exhibited typical plicated structureand well-shaped layers as seen from the SEM images,which would be an ideal carrier for the heterogeneous catalysts.Moreover,the dispersity experiment was carried out in organic solvent?methanol?.The suspension of GO dispersed in methanol kept uniform for 3 days without any precipitate,indicating its outstanding dispersity and pave a way for the following organic reactions.2)Considering the?-?interactions can facilitate the mass transfer and then increase catalytic performance,rational design concepts are used to develop a novel poly?p-styenesulfonate acid,namely PSSF?grafted multi-walled carbon nanotubes supported on graphene oxide nanomaterial?PSSF-mCNTs-GO?with a simple two-step method.FT-IR,XRD,Raman,SEM,TEM,NH₃-TPD were used to analyze and characterize this inorganic-organic hybrid material.Particularly,doping of GO resulted in remarkably enhanced catalytic properties on the conversion of FAME?fatty acid methyl ester,92%?and benzoate esters?90%?,which mainly due to its strong?-?interaction with substrate.Additionally,PSSF-mCNTs-GO can be separated from the substrate conveniently and has a strong recyclable stability with relatively high catalytic activity even after 6 times recycling.This novel catalyst is promising in synthesis of biodiesel and benzoate esters.3)In this chapter,a 3D ordered macro-mesoporous titania composites with graphene oxide?TiO₂-GO?were synthesized through a confinement self-assembly method and applied to the epoxidation reactions of styrene and its derivatives.XRD,BET,SEM,TEM and XPS were used to characterize the hierarchically ordered composites.The incorporation of GO in the TiO₂-GO composite made the catalyst structure rather stable and dramatically improved the catalytic activity compared with pure mesoporous TiO₂,the conversion of styrene in epoxidation raised from 54%to93%as the TiO₂ combined with GO.Combining the preliminary discussion of promising potential graphene oxide showed in the previous study on facilitating electron transfer which can enhance catalytic properties,the mechanism was proposed to study the?-?interaction between GO and styrene based on the experimental data,which was further confirmed by photoluminescence spectroscopy?PL?analysis,polarization and cyclic voltammetry curves.Owing to the?-?stacking between GO and benzene molecule from olefin substrate,the catalytic performance was improved since the electron transfer of metal active centers was accelerated.In short,combining GO with metallic oxide materials will enhance the catalytic performance in epoxidation reactions and such designed strategy can extend to other catalytic reactions.4)A novel MoO₂ composite supported on mesoporous graphene oxide?MoO₂/m-GO?was designed with one-pot method and applied as an efficient epoxidation catalyst.On the basis of previous experimental results,the Mo element chosen in this chapter with higher selectivity in epoxidation exhibited much better catalytic performance than Ti active centre.Compared with pure mesoporous MoO₂?m-MoO₂?and amorphous MoO₂/graphene oxide?a-MoO₂/GO?,MoO₂/m-GO showed superior catalytic activity.The conversion and selectivity for cyclooctene were both over99%in 6 h.Remarkably,the mesoporous structure in m-MoO₂/GO which derived from SiO₂nanospheres endowed the catalyst better catalytic performance for long chain aliphatic and large annular olefins,the conversion of methyl oleate can be as high as 82%.Such robust catalyst can be easily recycled and reused five times without significant loss of catalytic activity.This novel catalyst is promising in the synthesis of epoxides with long carbon chain or large ring size.5)To overcome the energy consumption problem arised from heating process in epoxidation reactions,GO-based nanocomposite was further investigated in the photocatalytic epoxidation based on background of last two chapters.A novel Schiff?Mo?based complex supported on GO surface was successfully prepared,which further combined with the graphitic carbon nitride?g-C₃N₄?polymer?Mo-GO/g-C₃N₄?to make it photo-activated.The photocatalytic activity of the resulting nano-composite was tested using various olefin substrates under simulated sunlight?AM1.5 G?,which was demonstrated to be dramatically enhanced after the addition of GO.Compared with other similar composites with photocataltic activity,Mo-GO/g-C₃N₄ also presented higher catalytic yields.Remarkably,surface photovoltage?SPV?measurement was applied to explore electron acceleration effect caused by GO and molybdenum.It was demonstrated that both GO and metal active centers can efficiently accelerate the electron transfer,which finally contribute to the superior catalytic performances.