Au and Ag nanoparticles encapsulated within silica nanospheres using dendrimers as dual templating agent and their catalytic activity

The synthesis, encapsulation and characterization of Au and Ag nanoparticles within silica nanospheres is described. Generation 4 amine-terminated PAMAM dendrimers were used as a template for both the nanoparticles and the silica nanosphere support synthesis. The synthesis of dendrimer-encapsulated (DENs) or stabilized (DSNs) Au or Ag nanoparticles was carried out first. The prepared DENs were characterized by UV–vis spectroscopy and TEM. The support precursor (silicic acid) is added to the preformed DENs/DSNs to form silica nanospheres for the fabrication of heterogeneous catalysts. In this case, the dendrimer is acting as a template for both the metallic nanoparticles and the support nanosphere. The dendrimer template was subsequently removed via calcination at 500 °C. These prepared catalysts were further characterized using various techniques such as TGA, BET, FTIR, PXRD, SEM, ICP-OES and EDX. The catalytic activity of these prepared catalysts was evaluated in the oxidation of benzyl alcohol. Preliminary catalytic evaluation was performed using two model reactions, namely, 4-nitrophenol reduction and oxidation of methylene blue. These types of catalysts have few advantages over other differently supported catalysts in that: (1) no agglomeration of nanoparticles that occurs during catalytic reaction even at elevated temperatures, (2) very little leaching or no leaching occurs during reaction, and (3) because the nanoparticles are not necessarily sitting on the support surface or within pores, relatively more active sites are available for reaction to take place.