| The nanohollow spheres possess numerous superior physical and chemical properties to solid particles which have potential applications in medicine, photo-electro catalysis, electric battery and other fields. Therefore, the controllable synthesis of nanohollow spheres has attracted enormous attention of domestic and foreign in recent years. In this research, novel activated carbon(AC)-supported highly dispersed hollow Cu nanoparticles(NPs)(h-Cu/AC) have been prepared by microwave irradiation for the synthesis of dimethyl carbonate(DMC) by vapor-phase oxidative carbonylation of methanol. In addition, experimental factors which have effects on hollow copper particle formation and morphology were studied systematically, and its structural properties and growth mechanism were explored to reveal the crystal-face-dependence of catalytic properties, which will lay scientific foundation to prepare highly efficient catalysts for the synthesis of DMC.In addition, we prepared rGO-CB composite as support material using reduced graphene oxide with large surface area doped carbon black, and fabricated Cu/rGO-CB catalyst using as-prepared rGO-CB composite to explore its catalytic stability and performance by the liquid-phase oxidative carbonylation of methanol. The major contents of the paper were summarized as follows:(1) To the synthesis of h-Cu/AC catalyst irradiated by microwave, the inner core began to appear when the temperature is 300 °C. The optimum irradiation temperature for this process is 360 °C, which can prevent high-temperature-induced morphology changes. A high STYDMC value of 596 mg/g·h with CCH3 OH of 5.8% and 100% SDMC are obtained. On further raising the irradiation temperature, the cavities become irregular in shape, some are even unclear compared with others, and catalytic activity reduced correspondingly.(2) When the retention time is 0 min irradiated by microwave, the hollow copper NPs have the largest cavities. As the retention time was prolonged, the volume of the cavity decreased gradually and the catalytic activity reduced accordingly. Moreover, the catalysts roasted by the tubular furnace at 360 °C for 60 min barely have any hollow particles, so express lower reaction rates and selectivity for DMC.(3) Utilizing the advantage of microwave not having thermal inertia, which is not possible in the conductive heating, is the key factor leading to the formation of hollow structure. The formation of hollow copper followed Ostwald ripening mechanism, and the “open-mouth†hollow structure can increases the specific surface area for reactants to some extent, which is more favorable for the reaction.(4) Through the comprehensive research of experimental characterization, density functional theory(DFT) and microcosmic kinetics model, it can infer that the reaction of oxidative carbonylation of methanol has crystal-facet-dependence to Cu particles. Cu(111) planes are more favorable for the weak adsorption of CO, and weakly adsorbed CO can contribute to the synthesis of DMC, namely, the weak adsorption of CO can improve the selectivity of target of DMC.(5) Cu/rGO and Cu/rGO-CB catalysts all showed excellent catalytic performance to the liquid-phase oxidative carbonylation of methanol, the STYDMC were 2334 mg/g·h and 2757 mg/g·h, respectively. Carbon black particles inserted in layers between graphene can not only effectively prevent coalescence of graphene layers, and further increase the active specific surface area accessible for reactants so as to ensure the active sites in graphene surface can be fully utilized, but also make up the defects that graphene surface is smooth, because carbon black containing abundant oxygen element which can promote the stability of the metal nanoparticles through the electronic interaction with metal nanoparticles and further improve the catalyst stability. Even after five consecutive runs, Cu/rGO-CB catalyst exhibited preferable catalytic activity and stability compared with Cu/rGO. |
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