Morphology Design And Controlled Synthesis Of Noble Metal/Inorganic Support Nanocomposite Nanoparticles In Inverse Minemulsions

Noble metal nanoparticles?NPs?have attracted intensive attention due to their outstanding performance in the fields of optics,thermotics,magnetism,catalysis,and drug delivery.However,the wide application of the noble metal NPs was somehow limited by their high tendency to agglomerate and difficulty in the recycling.These drawbacks may be effectively overcome through immobilizing noble metal NPs on to porous inorganic nanosupports to form noble metal/inorganic support nanocomposite particles?NCPs?.The performance of noble metal/inorganic support NCPs is mainly determined by the type and structure of noble metal,type and structure of inorganic nanosupports,and the composite pattern of noble metal and inorganic supports.Therefore,how to accurately control the structure of noble metal/inorganic support NCPs through the synthesitic parameters has been one of the most important issues in the related fields.In our previous studies,we reported the prepration of noble metal/inorganic support NCPs through combining a sol—gel process of inorganic precursors and in situ reduction of noble metal salts in inverse miniemulsions.In this thesis,first,rice-roll like?RR-like?Au/SiO₂ NCPs were prepared through controlling over the reduction process of the Au salt by introducing acoordination interaction between Au salt and inorganic support NCPs,and through controlling the formation process of the SiO₂ supports by tuning the surface charge of SiO₂.Second,on the basis of the nucleation and growth mechanisms of noble metal NPs during reduction of noble metal salts,we designed a controlled nucleation and fast growth process of noble metal NPs to prepare Janus Pd/SiO₂ NCPs.In the HAuCl₄-containing inverse miniemulsion systems,aminopropyl modified SiO₂ nanosupports were prepared through the sol—gel process of tetramethoxysilane?TMOS?and 3-aminopropyltriethoxysilane?APTES?.Meanwhile,AuCl₄^-was immobilized on to the SiO₂ nanosupports through the coordination interaction between AuCl₄^-and amine group.Subsequently,AuCl₄^-ions were reduced to narrowly-size distributed small Au NPs by the amine groups.These Au NPs were immobilized onto the SiO₂ nanosupports to form RR-like Au/SiO₂ NCPs.Influences of the synthetic parameters including the amounts of APTES and urea,and the reaction temperature,on the structure of the SiO₂ nano-supports and AuNPs were systematically investigated.At the initial stage of reaction,the aminopropyl groups were positively charged due to the strong acidic environment in the dispersed phase.Thus,the aminopropyl modified SiO₂ NPs had a good colloidal stability and sperately distributed in the dispersed phase.However,with the decomposition of urea,the p H value of the disperse phase gradually increased.In turn,the surface charge of SiO₂ NPs decreased because of the deprotonization of the aminopropyl groups.As a result,the colloidal stability of SiO₂ NPs decreased,leading to the aggregation of SiO₂ NPs to form a RR-like SiO₂ nanosupport in the dispersed phase.RR-like SiO₂ nanosupports with a high specific surface area could be synthesized when the APTES content was in the range of 20—80 mol%,the urea amount was higher than 0.72 mg,and the raction temperature was higher than 60 oC.The immobilization of AuCl₄^-ions onto the SiO₂ nanosupports limited the aggregation of Au element,increased the amount of Au nuclei,and finally formed Au NPs with a samll size and a narrow size distribution.In the case of 50 mol% of APTES,RR-like Au/SiO₂ NCPs had a specific surface area of 451 m²·g^-1 and the number average particle sizeof AuNPs was 9.4 ± 1.6 nm.RR-like Au/SiO₂ NCPs displayed an excellent catalytic activity and cycling catalytic performance.Moreover,these NCPs showed a p H-dependent colloidal stability,which faciliates the recycling and redispersing of the NCPs.According to the formation mechanism of noble metal NPs druing reduction of noble metal salts,we design a new strategy to prepare Janus noble metal/SiO₂ NCPs.The strategy includes formation of one noble metal nucleus in one droplet through controlling the reduction rate,and fast growth of ncuelated noble nucleus through an autocatalytic reduction of noble metal to suppress the formation of new noble metal nuclei.In order to realize our design,K₂ PdCl₄,a relativiely noble metal salt with a weak oxidizing capability,was chosen as the model salt to obtain a large rate difference between the non-catalytic reduction and the autocatalytic reduction.K₂PdCl₄/SiO₂ NCPs were firstly prepared through the sol—gel process of TMOS in the inverse miniemulsion system.In order to more accurately controling the reduction rate of K₂ Pd Cl₄,the reductant,NH₂NH₂,was delivered into the dispersion of the K₂ Pd Cl₄/SiO₂ NCPs through a gaseous diffusion method.Under a slow reduction rate,only one Pd NP formed in one K₂PdCl₄/SiO₂ NCP.The Janus structure of the Pd/SiO₂ NCPs was confirmed by the microscopic observation.The nitrogen isothermal absorption—desorption curve was a type IV curve,indicative of the mesoporous structure of the SiO₂ nanosupports.The number average particle size of Pd NPs was 19.9±3.7 nm,which higher than that of the crystallites?10.8 nm?.It indicated the the Pd NPs had a polycrystallines tructure.The Janus Pd/SiO₂ NCPs could be synthesized in a wide range of Pd salt.The Pd salt amount affected the pore structure of SiO₂ nanosupports,and the particle and crystallite size of Pd NPs.In the range of 0.041—0.163 g of Pd salt,the pore size of the SiO₂ nansupports,particle size andcrystallite size of Pd NPs increased with the increase of the Pd salt amount.Janus Pd/SiO₂ NCPs showed a good catalytic activity and cyclic catalytic performance towards the p-NPh reduction.Moreover,the catalytic activity increased with the increase of the PdNPs.In conclusion,the structure of noble metal NPs and inorganic nanosupports,and their composite pattern can be conveniently,accurately tuned through introduction of an additional interaction between the noble metal sals and inorganic nanosupports or through controlling the reduction rate of the noble metal salts in inverse miniemulsions.The supported noble metal nanocatalysts with excellent catalytic performance can be controllably synthesized through our proposed technique.Furthermore,the morphological design and controllable synthesis of novel,versatile supported noble metal nanocatalysts may benefit from the conclusions obtained in this thesis.