Preparation And Application Of Encapsulated Nano Precious Metal Materials

The noble metal nanoparticles show surprisingly catalytic activity when the particle size is less than 5 nm.However,they are trend to sinter and deactivate because of the huge surface atoms occupation and very high surface energy,which makes the adjacent nanoparticles easy to agglomerate.In order to maintain the catalytic activity,one strategy is to separate the adjacent nanoparticles to avoid the migration and agglomeration.The common method of synthesizing the material is to encapsulate the noble metal nanoparticles into suitable materials.We synthesize three kinds of core-shell structures,compare their characteristics,and evaluated their catalytic activities in oxidation of carbon monoxide.In the first part of this paper,gold nanoparticles encapsulated in silica Au@SiO₂ was synthesized by one pot method in a reverse microemulsion.The catalyst has uniform structure,with the gold particle core of 2.3 + 0.5 nm,silica shell of 10 nm.When the material is used for the oxidation of carbon monoxide,the initial reaction temperature is 230?.The conversion only slightly decreased from about 83.4% to 80.5% after 65 hours at 400?.In the second part,Au@SiO₂ core–shell particles are reacted solvothermally with Ce?NO₃?₃ in an ethylene glycol–water–acetic acid mixture to produce multi-core–shell Au@SiO₂@m-CeO₂ nanospheres.After selective removal of silica by NaOH,Pd@SiO₂@m-CeO₂ nanospheres are transformed into the multi-yolk–shell Au@HM-CeO₂ nanocatalyst.The catalyst has excellent initial catalytic activity,but decrease dramatically because the pore diameter of ceria shell is close to that of gold nanoparticles.In the third part,3-Mercaptopropyltrimethoxysilane was used as bifunctional ligand to anchored gold precursor in the zeolite.With the help of hydrogel,nano zeolite encapsulated noble metal catalyst Au@nano-LTA and Au-Pt@nano-LTA were synthesized.Because the pore diameter of the zeolite is between the silica and ceria coated material,the nanometer noble metal particles can be effectively encapsulated,and simultaneously,the effective contact between the catalyst and the reactant molecule is ensured.The catalyst Au-Pt@nano-LTA has the best catalytic activity and stability of the paper.