The Fabrication Of Graphene-based Materials And Their Applications In Heterogeneous Catalysis

Graphene is a kind of new carbon material, which is not only easily controlled in composition and function, but also possesses high utilization efficiency toward large surface area, good resistance to acid/alkali and high temperature. At the same time, the adjustment of composition allows graphene well dispersing in different kinds of solvents. Therefore, compared with the traditional activated carbon and carbon nanotubes, graphene is expected to become a new and efficient catalyst support. Herein, graphene and graphene oxide, nitrogen doped graphene are used as supports for fabricating metal (Pt, Pd), metal oxide (MnO2, Co3O4) and hybrid metal-metal oxide (Pd-Ni2Os) catalysts, and their applications in heterogeneous catalysis are also discussed.Firstly, reduced graphene oxide (RGO)-supported platinum (Pt) catalyst was prepared by simple ethylene glycol (EG) reduction and used for the hydrogenation of aromatic nitro compounds. Characterizations showed that EG as a reductant exhibited many more advantages than the widely used hydrazine hydrate to fabricate monodispersed, small sized Pt nanoparticles on the surface of RGO. The yield of aniline over the Pt/RGO-EG catalyst reached70.2mol-AN/(mol-p1·min) at0℃, which is12.5and19.5times higher than that of multi-walled carbon nanotube-and active carbon-supported Pt catalysts, respectively. When the reaction temperature was increased to20℃, the catalytic activity of Pt/RGO-EG jumped to1138.3mol-AN/(mol-pt·min), and it was also extremely active for the hydrogenation of a series of aromatic nitro compounds. The unique catalytic activity of Pt/RGO-EG is not only related to the well dispersed Pt clusters on the RGO sheets but also the well dispersion of Pt/RGO-EG in the reaction mixture.The active component can be modulated for further exploring the application scope of graphene. Here, Ni2O3-around-Pd hybrid can be fabricated on graphene oxide (Pd-Ni/RGO) by a simple one-pot wet chemical route. These isolated Pd clusters showed significantly improved performance for Suzuki coupling reaction compared to that of pure Pd/RGO, Ni2O3/RGO, popularly reported Pd/AC, as well as homogeneous PdCl2and PdCl2(PPh3)2. Characterizations disclosed that Ni2O3plays a multiple roles in exfoliating graphene sheets, mediating the size as well as stability of Pd clusters. Pd-Ni/RGO dispersed homogeneously in the aqueous reaction mixture, exhibited high enrichment towards reactants as well as extremely high activity and stability for the Suzuki coupling reaction. The best turnover frequencies of all Pd atoms reached38750h-1at80℃for bromobenzene coupling. It was concluded that the intimate interaction between Ni2O3nanoparticles and Pd clusters appears to be beneficial for activating the Pd surface for the catalytic cycle.To meet the sustainable development need, exploring the controllable synthesis of cheap active species on graphene is also carried out. Here, well-organized MnCO2nanorods on graphene oxide (MnO2/GO) were fabricated though a novel and easily controlled chemical route. The rod-like MnO2with5-20nm in diameter and100-600nm in length, uniformly and densely attached on both side of GO sheets. Compared with the bare MnO2, MnO2/GO nanocomposite is an efficient heterogeneous catalyst for a widely applicable synthesis of primary amides from primary alcohols and ammonia as well as for transformation of aldehydes or nitriles. More importantly, water is a superior medium for this reaction than other commonly used organic solvents, which is beneficial for catalyst/product separation. MnO2/GO catalyst has good recyclability in these reactions. High dispersion in water, high MnO dispersion on GO and suitable functional groups on carbon materials are important for a synergistic ammoxidation catalytic activity of MnO2and GO in the hybrid.Attempts to control the dispersion and durability of catalytic metal NPs are essential to catalytic performance and economical feasibility, which implys us to strengthen the interaction between support and active species by introducing dopant and defect into the graphene. Here, a sandwich-like N-doped graphene/Co3O4hybrid was prepared via a simple one-pot hydrothermal reaction in the solution of NH3. Characterizations disclosed that highly dispersed Co3O4nanoparticles with dominant exposed {112} and {110} planes were fabricated on both sides of well-exfoliated N-doped graphene, N-dopants in graphene matrix can prevent re-graphitization of graphene, strengthen the interaction between Co3O4and graphene matrix, and improve the dispersion of Co3O4. This hybrid (Co3O4/RGO-N) exhibited predominant activity and stability for the epoxidation of styrene than bulk Co3O4and N-free graphene supported Co3O4. At the same time, the resulting catalyst also showed high compatibility to various olefins and alcohols with good conversion and high selectivity.In order to further explore the effect of support modification on the active component and its catalytic performance, the catalyst is fabricated by two-step method. Here, N-doped graphene, which is synthesized via a facial one-step hydrothermal reaction of graphene oxide with urea, is especially adapted for anchoring extremely ultrafine and highly stabilized Pd clusters under a very "clean" strategy using H2as reductant. This in situ hydrogen-water strategy allows catalyst self-assembly at atom level, and the adjustment of concentration of doped N in graphene results in a significant variation of Pd morphology. This N-doped graphene supported Pd (Pd/NRGO) is exclusively selective for the activation of C=C or benzene ring in aqueous medium. Our experimental work shows N-doped graphene is not only a good support for strong anchorage of Pd, and its well dispersion in both water and substrate phase, but also a powerful mediator in tuning reaction path. Density functional theory calculations disclosed that N dopants can obviously enhance the binding of Pd atom with graphene, decrease the barrier of H spillover from Pd to support, and promote the electrons transfer from support to substrate that protected C=O group and improved the activation of C=C.In conclusion, this thesis has carried out the preliminary exploration on both the preparation and application of graphene based catalyst supports. We except our research can provide simple, efficient and versatile blue-prints for low-cost fabrication of graphene-based nanocomposites for extending applications where graphene has rarely been exploited and beyond.