金@介孔碳核壳和空腔结构纳米材料制备及其催化氧化性能

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

采用表面活性剂自组装技术合成了金@介孔碳核壳纳米球及蛋黄-蛋壳空腔纳米催化材料.高温碳化后,尺寸为15 nm的Au核被限制在碳壳层或空腔内;其壳层或空腔碳层的平均厚度分别约为38和10 nm.核壳和空腔材料都拥有开放的介孔结构(孔径为2.0~2.8 nm)、高的比表面积(>640 m^(2)g^(–1))和大的孔体积(>0.55 cm^(3)g^(–1)).高温碳化过程中,C原子沉积导致Au的d电荷密度增加.在苯甲醇氧化为苯甲酸反应中,d电荷密度增加的金@介孔碳核壳纳米球具有优异的活性,转化频率高达5468 h^(–1).同时,催化剂具有良好的稳定性,循环使用6次后,未见明显Au的流失和活性降低.

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.