Fabrication Of Hierarchically Porous Silica Via Freeze-drying And Immobilization Of Ultrafine Ag Nanoparticles For High-performance Catalysis

As a common carrier material,porous silica has a wide range of applications in the field of catalysis.According to different pore sizes,porous silica can be classified into micropores?pore diameter?2 nm?,mesopores?2 nm<pore diameter<50 nm?and macropores?pore diameter?50 nm?silica.Microporous or mesoporous silica has a high specific surface area and is ideal carrier for catalysts.However,due to its small pore size,the transportability of reactants is seriously affected.In contrast,macroporous silica has excellent mass transfer ability and can provide effective transport channels for reactants.However,due to its low specific surface area,the catalytic effect of the catalyst is also seriously affected.Hence,the preparation of hierarchically porous silica?HPS?with micropores,mesopores and macropores is the key to improve the catalytic ability of the catalyst,and it is also one of the important strategies for obtaining excellent carrier materials.In this dissertation,a variety of HPS was prepared by vacuum freeze-drying combined with high-temperature calcination.Then,small size of silver nanoparticles?AgNPs?were loaded on the surface of HPS by in-situ reduction method,and porous nanocomposites AgNPs/HPS were obtained.Further,the catalytic properties of the porous nanocomposites were studied.The specific findings are as follows:?1?Using a mixed liquid of tetraethyl orthosilicate?TEOS?hydrolysis solution and polyvinyl alcohol?PVA?aqueous solution as a precursor,the preparation of HPS was performed by a combination of vacuum freeze drying and high temperature calcination.The morphology and structure of HPS were characterized by SEM,EDAX,XRD,FTIR,TG-DTA and N₂ adsorption and desorption.The results show that the synthesized HPS in the experiment not only has a high specific surface area,but also has rich interconnecting pores and a unique self-supporting structure.Compared to silica products without PVA,the introduction of polymer PVA can increase the specific surface area of silica by more than 229times.With the increase of PVA addition,the specific surface area of HPS increases first and then decreases.?2?Using the HPS as the carrier,AgNPs/HPS composites were further prepared by the in-situ reduction method.The morphology and structure of porous nanocomposites were characterized by SEM,EDAX,TEM,and N₂ adsorption and desorption.The results showed that the concentration of ammonia?NH₃·H₂O?added during the preparation process can effectively regulate the size of AgNPs.With the increase of NH₃·H₂O content,the size of AgNPs gradually decreases.Finally,in order to study the catalytic ability of the above AgNPs/HPS composites,we further studied the catalytic reduction effect of the porous composites on p-nitrophenol.The study found that AgNPs/HPS nanocomposites could be used as a catalyst to reduce p-nitrophenol to p-aminophenol in 30 s,and its excellent catalytic activity may be due to the high specific surface area and the interconnected macroporous structure of HPS which had a positive effect on the catalytic performance of AgNPs/HPS.In addition,due to the unique self-supporting structure,AgNPs/HPS nanocomposites also exhibited excellent recyclability.