Preparation Of Nobel-metal Core-shell Nanostructures And Their Catalytic Properties

Noble metal nanomaterials have attracted wide attention in the field of catalysis due to their good catalytic activity and high selectivity.However,the high specific surface area and large specific surface energy due to its nanometer size make the noble metal nanoparticles very unstable and extremely easy to aggregate and be oxidized,limiting its application in the promotion.In the preparation of precious metal nanocatalysts,the noble metal nanocatalyst with core-shell structure prepared by coating noble metal nanoparticles with inert mesoporous materials can effectively solve this problem.On the one hand,the shell structure of the mesopores can ensure the active sites of the noble metal,simultaneously can prevent the precious metal from being oxidized,agglomerating and scattering,and has the advantages of stabilizing the noble metal nanoparticles and improving the recycling rate of the catalyst.Therefore,the preparation technology based on core-shell structure has great significance for the performance improvement of noble metal catalytic materials.In addition,due to the high cost of precious metals,how to maximize its effective use,such as multi-component noble metal load,is also a very challenging subject.Based on the catalytic activity and stability of the noble metal nanocatalysts,the noble metal nanocatalysts such as Ag@C,Pt@C,Ag-Pt@C and Ag-Pt@SiO₂have been successfully prepared.The main factors influencing the structure and properties of the composites were studied by scanning electron microscopy（SEM）,transmission electron microscopy（TEM）and X-ray powder diffraction（XRD）.Concrete results are as follows:1.Two noble metal nucleocapsid nanostructures were prepared by the hydrothermal method using a noble metal salt as the precursor and glucose as the reducing agent,which can be expressed as M@C.By controlling the reaction temperature,it was confirmed that the role of glucose in the reaction is first as a reducing agent to reduce the Ma+ into a noble metal nucleus,and then carbonized as a carbon source into a shell.And the regulation of the shell thickness of M@C nanospheres was achieved by adjusting the amount of glucose added.It was found that glucose has a certain reducibility to Ma+,and this preparation method is simple and green.Subsequently,we use this method as the guide.Using silver nitrate and acetylacetone platinum as the precious metal precursor,glucose as the reducing agent,we successfully prepared a core-shell Ag-Pt@C nanostructure with simultaneous coating of Ag,Pt bimetal by hydrothermal reduction method.The synthesis process was studied by controlling the reaction time.Silver was first formed by reduction of Ag+,followed by reduction of Pt²⁺ in-situ around the silver nucleus to form Pt nanoparticles around the silver core,and then glucose was carbonized into glucose shells to obtain core-shell multilevel structure of Ag-Pt@C nanospheres.In this structure,Pt nanoparticles which are easy to agglomerate are dispersed around the silver core,which increases the specific surface area of Pt and improves the utilization rate of noble metal.Meanwhile,the carbonaceous shell structure of the mesopore plays a protective role in the noble metal nucleus,and the hydrophilic groups on the surface of the mesoporous carbon are favorable to the dispersion of the catalyst in the solution.The catalytic performance of Ag-Pt@C nanospheres and Pt@C nanospheres was better than that of Ag@C nanospheres in catalytic 4-NP hydride reduction experiments.2.Ag-Pt@meso-SiO₂ nanospheres were prepared using Ag-Pt@C nanospheres as template,then SiO₂ layer was first coated on the surface,and then annealed to obtain yolk-shell structure.Wherein the larger silver core and smaller Pt nanoparticles are in a monodisperse state in the cavity,the outer SiO₂ shell is mesoporous structure,which BET surface area was 552.9 m2/g and the average pore diameter was 3.55 nm.The samples with different thickness SiO₂ shells were obtained by adjusting the amount of TEOS.Ag-Pt@C nanospheres with different sizes of core structures and the corresponding Ag-Pt@meso-SiO₂ nanospheres were obtained by adjusting the amount of AgNO₃ added.The AgNO₃ addition influenced the size of the Ag nucleus and further influence the amount of the coated Pt nanoparticles.The more the amount of AgNO₃ was added,the larger was the Ag nuclei in the template samples,and the more Pt was loaded,the thinner was the carbon shell,correspondingly the smaller was the cavity in Ag-Pt@meso-SiO₂ nanospheres and the worse was the dispersibility of Pt nanoparticles.The nanostructures could maintain the stability of the nanostructures at 800℃ in the high temperature environment,and the Ag and Pt nanoparticles were not agglomerated by adjusting the annealing temperature.The results show that the prepared Ag-Pt@meso-SiO₂ nanospheres have good catalytic performance by the hydrogenation reduction experiments of p-nitrophenol（4-NP）.It is proved that the highloading capacity,large specific surface area and high stability of the structure have great application value in the treatment of water pollution.