Sep 27, 2012
Gold nanoparticles trapped in mesoporous silica
Mesoporous silica has been used as support material to anchor gold nanoparticles by many research groups. A variety of ligands can stabilize gold nanoparticles in the mesopore of silica. However, in the past, gold/mesoporous silica did not have very high catalytic activity because a large amount of gold nanoparticles block the mesopores and prevent reagent access to catalytically active sites. What’s more, gold nanoparticles are mobile on the surfaces of metal oxides and are prone to aggregating together to form bulk gold and losing their catalytic activity completely. In other words, traditional gold/mesoporous silica structures suffer from poor durability under reaction conditions involving elevated temperatures/pressures, and scientists have been looking at ways to make these catalysts more robust.
Reporting their work in the journal Nanotechnology, scientists from Colorado School of Mines, East China University of Science and Technology, and South-Central University of Nationalities, have synthesized gold/mesoporous silica nanocomposites with gold nanoparticles intercalated in the walls of mesoporous silica, which display useful characteristics.
Thanks to the use of the gold co-ordinating agent bis[3-(triethoxysilyl)propyl]-tetrasulfide (TESPTS), the tetrasulfide structure (-S-S-S-S-) in the centre forms strong bonds with the precursor gold species while the –Si-O-C2H5 structure at two ends, which is the same as that of the silica precursor (tetraethyl orthosilicate, TEOS), can easily condense with TEOS and form mesoporous silica (SBA-15) with the gold precursor trapped in the forming matrix. On calcination, the gold atoms form nanoparticles while the thioether decomposes.
This synthesis method yields gold nanoparticles intercalated in the wall of SBA-15 instead of on the pore wall, as has been verified by transmission electron microscopy (TEM), N2M adsoption-desorption, X-ray diffraction (XRD) and UV-visible spectroscopy (UV-vis).
In addition to the novel structure, this catalyst also exhibits excellent catalytic activity compared with the traditional gold catalysts. To the researchers surprise, this novel catalyst almost has no loss of catalytic activity after recycling three times and shows excellent stability after exposure to elevated temperatures (up to 750 °C without aggregation), thus making it possible for this system to be used as a robust and reusable catalyst in green chemistry (the design of chemical products and processes that reduce or eliminate the use or generation of hazardous substances).
The team examined the effect of various amounts of gold co-ordinating agent (TESPTS) and found that more TESPTS would co-ordinate more gold, but disrupts the regular mesoporous structure resulting in materials ranging from partially mesoporous to foam-like.
About the author
The study was conducted by researchers under the guidance of Prof. Ryan M Richards at the Colorado School of Mines. Xue Wang is a PhD student in the Department of Chemistry and Geochemistry at Colorado School of Mines. She performed the synthesis of the novel-type materials, XRD and N2 sorption characterizations. Dr Meng Shang is a postdoctoral researcher in the Department of Metallurgical and Materials Engineering. He performed TEM and the UV-vis analysis. Feng Lin, a PhD student in the Materials Science programme at CSM, performed low-angle XRD analysis. Dr Lifang Chen and Dr Juncheng Hu are two pioneers of this novel technique and performed some additional theoretical analysis.