Research On Controllable Synthesis And Catalytic Performance Of Graphene Supported Copper Nanocatalysts

In the oxidative carbonylation of methanol to dimethyl carbonate(DMC),carbon supported copper catalysts suffer from deactivation because of agglomeration,leaching and oxidation of copper nanoparticles(NPs).The surface properties of carbon support materials have significant effects on the particle size,morphology and distribution of copper nanoparticles,and then exhibit different catalytic properties.As a novel two-dimensional carbon material,graphene has great electrical and thermal conductivities,large theoretical surface area and good mechanical property,which can act as an ideal catalyst support material.In this dissertation,a variety of synthesis strategies for graphene supported copper nanocatalyst are proposed,and the particle size and dispersion of copper NPs are effectively regulated by nitrogen doping and spatial confinement effect,while inhibiting the agglomeration,leaching and oxidation of copper active species in the reaction.Therefore,the catalysts exhibited excellent catalytic performances in the oxidative carbonylation of methanol.Meanwhile,density functional theory(DFT)calculations are carried out to discusse the effects of nitrogen doping and copper valence on catalytic performance.The main research contents and results of the dissertation are as follows:(1)Nitrogen-doped graphene(NG)was synthesized via hydrothermal method,and NG supported copper catalysts(Cu/NG)were synthesized by wet-impregnation method.Cu/NG catalysts exhibited an excellent activity and remarkable stability in the oxidative carbonylation of methanol,and the catalytic activity was positively correlated with nitrogen content.The results confirmed that nitrogen doping could increase the anchoring sites of graphene support,reduce the particle size of copper NPs,and also enhance the charge transfer of the catalyst and the adsorption of CO gases,therefore,the catalytic activity was significantly improved.In addition,nitrogen doping increases the interaction between copper NPs and graphene support,inhibites the agglomeration and leaching of copper NPs during reaction,and improves the catalytic stability.The DFT calculation results show that nitrogen doping can reduce the energy barrier of CO insertion reaction and enhance the binding energy between copper and support;(2)Carbon black was used as the support,phenanthroline and cupric acetate as precursors to synthesize the carbon supported nitrogen-doped graphene wrapped copper“chainmail catalyst”(Cu@NG/C)via a direct co-pyrolysis approach.The wrapped layer number of NG is 5 and the thickness is 2 nm.The results show that Cu@NG/C-700 exhibits significantly improved catalytic activity and stability than carbon black directly supported copper catalyst(Cu/C).This is due to the spatial confinement effect of chainmail catalyst as well as the shielding and protective effects of the NG layers can effectively inhibit the agglomeration,leaching and oxidation of copper species during the reaction process.In addition,nitrogen doping can also improve the dispersion of copper NPs and the charge transfer ability of catalyst;(3)Few-layer graphene(FLG)and graphene-supported high-dispersion copper catalysts(Cu/Li-PGO)were fabricated via a lithium-promoted thermal expansion exfoliation method.It is found that the addition of lithium can reduce the binding energy of oxygen-containing functional groups such as hydroxyl groups,which are beneficial to the remove of functional groups during thermal expansion,thereby obtaining FLG with a higher degree of exfoliation.The Cu particle size of Cu/Li-PGO is only 4.2 nm,and can expose more active specific surface.In addition,the addition of lithium can increase the charge transfer of the catalysts,promote the adsorption of reaction gases,enhance the binding capacity of active components,and thus enhance the catalytic activity.Furthermore,the valence distribution of copper species changed due to the charge transfer between copper and the support,and resulted in a slightly positive charge(Cu^?+,0<?<1).The DFT calculation results show that Cu^?+(0<?<1)has higher CO adsorption energy than Cu⁰ and Cu₂O,and the rate-determining step(CO insertion reaction)energy barrier is lowest on its surface,which is beneficial to DMC synthesis reaction.