Syntheses And Properties Of Hollow Ni(Co)@SiO₂ Nanoparticles

Core-shell nanomaterials,which consist of different structure,composition and arrangement,have attracted considerable attention due to their instinct chemical and physical characteristics.Among which the hollow core-shell materials are more focused.These materials can provide a stable framework for metal nanoparticles,make them stable,and greatly improve the performance of metal nanoparticles.Although precious metals have good performance in many aspects,the price limits their large-scale applications,and people's research is slowly turning to transition metals,such as cobalt and nickel,which play important roles in catalysis and magnets.The chemical synthesis method can accurately control the size of the hollow structure carrier materials.The reverse microemulsion(W/O)method uses micelles as micro-reactor to achieve the purpose of controlling morphology and size of core-shell nanomaterials.In this thesis,by optimizing the traditional sol-gel method,hollow Ni@SiO₂ and Co@SiO₂ composite nanomaterials were prepared by the"one-pot method"in the reverse microemulsion system.By using different nonionic surfactants,composite materials with different structures were prepared,and their catalytic activity in catalytic hydrogenation of nitro-compound were investigated.Firstly,the ternary phase diagrams for Brij-58,L-44 and Triton X-100,water and cyclohexane were made to determine the microemulsion area.Then cobalt salt solution was added to the system composited by Brij-58 and cyclohexane,and dispersed evenly,then hydrazine hydrate was added to complex and reduce the cobalt ion,followed by the addition of ammonia,and TEOS and finally the hollow Co@SiO₂ was formed.By changing the amount of Brij-58 added,the best conditions for making the hollow nanoparticles were determined.The product was centrifuged and washed 3 times with a mixed solvent of isopropanol and water,air dried at 100°C for 10 h,and calcined at 500°C for 2 h in air and subsequent reduction in 5%H₂/95%N₂ to obtain Co@SiO₂ nanospheres with hollow structure.TEM analysis was used to determine the hollow structure.Secondly,hollow Ni@SiO₂ nanospheres were synthesized in the same way as Co@SiO₂.Surfactants with different structures such as Tween-40,L-61,L-62,L-44,L-64,NP-7,AEO-7,and Triton X-100 were selected,based on the same amount of polyoxyethylene chain as Brij-58,to prepare a series of hollow Ni@SiO₂.Among them,L-61 and L-62 could not form a microemulsion,and the microsolution formed by Tween-40 is unstable,and corresponding materials cannot be prepared.Interestingly,tubular Ni@SiO₂was obtained when L-44,L-64,NP-7,AEO-7 and Triton X-100 were used as templates.UV-Vis and infrared spectrum were used to discuss the formation mechanism of hollow structure.Finally,Ni@SiO₂ prepared by Brij-58 as template was selected as a catalyst to study its activity in the gas-phase hydrogenation of nitrobenzene and the reduction of p-nitrophenol with Na BH₄ as a reducing agent.The substrate conversion rate was analyzed by UV-Vis and gas chromatography.The conversion rate of p-nitrophenol was 97.62%,and the selectivity of nitrobenzene was 75.69%.