Design Of Graphene And Carbon Nitride-based Composites For Catalytic Application

As the increasingly worse energy and environment issues,how to use the solar energy efficiently has attracted much attention around the world.Photocatalysis can convert solar energy to chemical energy availably without secondary pollution on dyes' degradation.However,the photocatalytic electrons and holes are difficult to separate in the throditional photocatalysts,retarding the catalytic acitivity.In this dissertation,a series of graphene-based materials have been designed and fabricated via soft chemical methods.Graphene's exceptional conduvtivity helps to separate the electrons and holes,further enhancing the photocatalytic activities.As a new carbon nano-material and a monolayer of carbon atoms packed into a dense honeycomb crystal structure,graphene can also been used as a support to load some nanoparticals.Meanwhile,in recent years,carbon nitride has also contributed a lot in photocatalysis with the graphitic struture.Since their intrinsic advantages,Pd-graphene and Pd-carbon nitride have been synthesized and applied in some organic reactions,like nitrophenols hydrogenation and Suzuki-Miyaura coupling reaction.Besides,the modified carbon nitride and carbon nitride hydrogel have also been investigated.The main contributions of this dissertation are described as follows:?1?A magnetically separable P25-CoFe₂O₄-graphene catalyst with enhanced adsorption capacity and visible-light-driven photocatalytic activityA straightforward strategy is designed for the fabrication of a magnetically separable P25/CoFe₂O₄/graphene photocatalyst with differing P25 contents via a facile one step hydrothermal approach.The adsorption capacity and visible-light-driven photocatalytic activity were evaluated in terms of the efficiencies of adsorption and photodegradation of various dyes,including methylene blue?MB?,methyl orange?MO?and neutral dark yellow?DY?.The evaluation results demonstrated that reaction corresponded to the pseudo-second-order kinetic equation and the Langmuir isotherm.P25/CoFe₂O₄/graphene photocatalyst exhibited the best performance on methylene blue?MB?degradation.Furthermore,CoFe₂O₄ nanoparticles themselves have excellent magnetic properties,which are largely maintained in the composite,and therefore,it is much helpful in the recovery of catalysts.?2?Catalytic hydrogenation of nitrophenols and nitrotoluenes over a palladium/graphene NanocompositeHomogeneous palladium nanoparticales are loaded on graphene sheets via a soft chemical method,Pd/G composites are used in the catalytic hydrogenation of nitrophenols and nitrotoluenes.TEM,XRD,XPS all indicated the reduction of graphene oxide and the palladium nitrate,confirming the Pd/G structure.The Pd/G hybrids exhibited much higher activity and higher stability than the commercial Pd/C.Considering inductive and conjugation effects that may affect the reactions,the reactivity of nitrophenols in this study was expected to follow the order m-NP>o-NP>p-NP>2,4-DNP>2,4,6-TNP,which is in good agreement with the experimental results.?3?Green Suzuki-Miyaura coupling reaction catalyzed by palladium nanoparticles supported on graphitic carbon nitrideThe synthesis of biphenyls under mild conditions is really a challenge to green chemistry.Here we report a facile photodeposition strategy to fabricate a Pd/g-C₃N₄ nanocomposite and demonstrate its application as a green catalyst for Suzuki-Miyaura coupling reactions.The Pd nanoparticles with an average size of 2.7 nm are uniformly dispersed on the g-C₃N₄ surface.Troditional Suzuki-Miyaura coupling reactions were conducted in the organic solvent with unstable catalyst in high temperature.While with Pd/g-C₃N₄ nanocomposite,the reaction can be carried out at room temperature without any phase transfer agents,toxic solvents and inert atmosphere,especially,allowing a complete conversion?100%?of bromobenzene and a high yield of 97%for biphenyl.Besides,Pd/g-C₃N₄ is much easy to prepare,more stable,even cheaper and good recovery,compare to the widely used organic catalysts.?4?Supramolecular solvent interactions toward hollow carbon nitride spheres as efficient photocatalystHere we report a new,facile and "template-free" synthetic method to form highly ordered carbon nitride hollow spheres by solvent interaction with cyanuric acid-melamine?CM?supramolecular aggregates as the starting precursor.A detail analysis showed that solvent interactions with the initial sphere-like CM supramolecular aggregates can lead to a new rearrangement of the CM structure from spheres to rods or only to the modification of the aggregate surface while fully preserving the bulk sphere morphology.In the case of only surface rearrangement,upon calcination in high temperatures the entire sphere shape can be preserved to form hollow sphere carbon nitride due to different reactivity of the outer and inner layers of the CM sphere upon heating.Furthermore,the alteration of the surface properties significantly enhances the C₃N₄ photocatalytic activity for hydrogen production due to the creation of energy bands gradient that leads to superior charge transfer properties.The use of the solvent interactions with supramolecular aggregates opens many opportunities for the significant improvement of carbon nitride synthesis,structure and photophysical properties toward an efficient photocatalyst.?5?Design of carbon nitride hydrogel and its application in photocatalysisCarbon nitride hydrogel was generated in an inerratic syringe by inducing white LED light,capable of undergoing large deformation.In comparison with the reference hydrogel without carbon nitride inside,carbon nitride hydrogel shows improved mechanical strength,enhanced deformation restorability and better stability.CryoSEM indicated hydrogels with differing carbon nitride had mutative mechanical properties.Hydrogels with well dispersed carbon nitrdie were homogeneously deformed accompany with peachy elasticity,onced fabricated with higher carbon nitride contents,it turned to be tough but easily fractured.Carbon nitrde hydrogel had superior adsorption capacity and degradation activity,especailly for cationic dyes?MB?,which could be contributed to the negative-charged carbon niteide in the hydrogel.Besides,it was also efficient in the application of hydrogen production with favourable cycle performance.