Catalytic Reduction Of Cationic Dyes With Mesoporous Silica-coated Noble Metal Nanoparticles

The use of metallic nanomaterials as catalysts for the degradation of organic dyes is the main method for the modern printing and dyeing wastewater treatment process.Therefore,it is of great significance to develop highly efficient and recyclable metal nanocatalysts for the treatment of organic dyes.In this paper,three different noble metal nanomaterials were prepared by liquid-phase chemical reduction method.In order to improve the stability of noble metal nanoparticles in aqueous solution,a layer of mesoporous silica was coated on the surface of noble metal nanoparticles by Stober method.The performances of these composite nanoparticles in the catalytic reduction of cationic dyes with NaBH4 were investigated.The sizes and morphologies of the prepared silica-coated noble metal nanocomposite particles were characterized by transmission electron microscopy?TEM?.The catalytic performances of the catalyst were evaluated by using visible absorption spectroscopy.Silver nanoparticles were prepared by sodium citrate reduction method.After coating a layer of mesoporous silica on the outside of the silver nanoparticles,Ag@mSiO₂ particles were synthesized.The TEM image shows that the prepared composite particles have a core-shell structure.The noble metal nanoparticles are located at the center of the particles and the mesoporous silica layera are evenly coated on the surfaces of the noble metal nanoparticles.The shape of the central silver core is spherical,with an average particle size of about 23 nm.The thickness of the mesoporous silica layer is about 38 nm,and the monodispersity of the particles is good.The prepared Ag@mSiO₂ catalyst showed excellent catalytic performance in the experiment of catalyzing sodium borohydride to reduce cationic dyes.With the increase of the amount of catalyst used,the catalytic efficiency increases.Using HAuCl₄ as the precursor,stable gold nanoparticles with particle size of about 20nm were prepared by reduction of HAuCl₄ with sodium citrate.Au nanoparticles with a size of 45 nm were prepared by a seed growth method.After coating a layer of mesoporous silica on the surface of two gold nanoparticles,Au?20 nm?@mSiO₂ and Au?45 nm?@mSiO₂composite nanoparticles were obtain.These two catalysts exhibited excellent catalytic performance in the experiments of catalyzing the reduction of rhodamine B?RhB?and methylene blue?MB?by sodium borohydride.Comparing the two catalysts,the catalyst Au?45 nm?@mSiO₂ possesses better catalytic performance and stability than Au?20nm?@mSiO₂.Using sodium citrate as reducing agent and stabilizer,stable gold-silver core-shell nanoparticles were prepared by liquid-phase chemical reduction method with AgNO₃ as silver source and HAuCl₄ as gold source.A layer of mesoporous silica was coated to prepare Au@Ag@mSiO₂ particles.TEM results show that the size of core of Au@Ag@mSiO₂prepared is about 35 nm with mesoporous silica layer of about 110 nm.The experiment of catalytic reduction of cationic dyes show that the prepared core-shell Au@Ag@mSiO₂ has excellent catalytic performance.The catalytic reduction rate increases with the increase of the amount of catalyst used.Due to the protective effect of the outer silica,Au@Ag@mSiO₂exhibits a relatively high catalytic activity in the recycling experiment.