Synthesis and catalytic oxidation performance of gold@mesoporous carbon core-shell and yolk-shell hollow nanomaterials

Core-shell and yolk-shell nanospheres are types of nanostructured materials that have gained significant scientific interests due to their unique structures and wide range of applications in catalysis, energy storage, and sensors. Traditionally, the shells surrounding the cores are dense or solid in nature. Transforming a dense shell into a mesoporous one can result in improved performance because mesopores within the shells can accommodate molecules and facilitate diffusion into and out of the cores. In this study, we present a surfactant self-assembly method for synthesizing gold@mesoporous carbon core-shell and yolk-shell nanospheres. This approach involves using a triblock copolymer called Pluronic F127 as a template and phenolic resol as a carbon source. Initially, spherical phenolic resol-F127 monomicelles are formed. These monomicelles are then coated onto Au cores modified with 3-mercaptopropyl trimethoxysilane via hydrogen bonding interactions. During a hydrothermal process, the F127/resol spherical monomicelles further assemble into a low-energy cubic closed packing mesostructure. Simultaneously, the hydrothermal condition promotes the polymerization of resols to occur within the spheres. After carbonization, the Au cores with a uniform diameter of 15 nm are confined within a mesoporous carbon shell. If a silica shell and a mesoporous carbon shell are step-by-step coated, a hollow cavity between an Au core and a mesoporous shell can be obtained after a successive step of removing silica shells. The average thickness of the shell or cavity carbon layer is approximately 38 and 10 nm, respectively. Both the core-shell nanospheres and yolkshell hollow nanomaterials possess carbon shells with open mesoporous structures (with pore diameters of 2.0-2.8 nm), high surface areas (>640 m(2) g(-1)), and large pore volumes (>0.55 cm(3) g(-1)), which effectively prevent nanoparticle aggregation under high-temperature conditions. These catalysts possess at least three advantages: (1) During hightemperature carbonization, the diffusion of carbon atoms into interstitial sites of the Au lattice leads to an increase in its d charge. (2) The confinement of nanoparticles by the shells prevents the detachment and aggregation of the nanoparticles during the reactions. (3) The open mesopores, along with the short diffusion paths, facilitate easy access of substrates to the metal active centers. In the selective oxidation of benzyl alcohol, the Au@mesoporous carbon core-shell nanospheres, with their increased d charge, exhibit excellent activity, demonstrating a turn-over frequency as high as 5468 h(-1). The catalyst also demonstrates good stability and can be recycled six times with negligible loss of both reaction rate and overall conversion. In comparison, the commercial Au/C catalyst exhibits a nearly 60% reduction in conversion during the second run, suggesting that soluble Au species leach from Au/C and contribute to catalytic activity. This research strategy opens up possibilities for designing core-shell and yolk-shell hollow nanomaterials with mesoporous shell layers, which hold great potential for practical applications.