A highly reactive and enhanced thermal stability nanocomposite catalyst based on Au nanoparticles assembled in the inner surface of SiO2 hollow nanotubes.

A novel hollow tubular SiO2-Au catalyst with a mesoporous structure (HTMS) was successfully fabricated by a combination of the sol-gel and calcination processes. This method involves the preparation of modified MWCNTs, the sequential deposition of Au and then silica layers through the sol-gel processes, and finally the calcination at the desired temperature to remove the MWCNTs. The obtained samples were characterized by several techniques, such as N2 adsorption-desorption isotherms, transmission electron microscopy, energy-dispersive X-ray spectroscopy analysis, UV-Vis spectra, X-ray diffraction and Thermogravimetric Analysis (TGA). The results established that a different calcination temperature has an obvious influence on the morphology and structure of the final hollow tubular. When the temperature is 550 °C, the obtained materials exhibit the distinctly tubular structure because of the decomposition of MWCNTs and the preservation of hollow tubes. Furthermore, in the catalyst system, the mesoporous silica layer can act as the physical barrier to resist the agglomeration and sintering of Au nanoparticles even after being subjected to harsh treatments up to 650 °C. In our experiments, the catalytic activities of HTMS SiO2-Au were investigated by photometrically monitoring the reduction of p-nitrophenol (p-NPh) by an excess of NaBH4. It was found that the prepared HTMS SiO2-Au catalysts exhibited a high catalytic activity and this sample could be easily recycled without a decrease of the catalytic activities in the reaction.

Synthesis and characterization of a novel Au nanocatalyst with increased thermal stability.

We report the synthesis of a new Au nanocatalyst with increased thermal stability. This catalyst system consisted of gold nanoparticles attached to functionalized TiO2/SiO2 core-shell nanocomposites, together with the encapsulation of mesoporous silica. The synthesis process mainly involved four steps, which included the synthesis of the TiO2/SiO2 core-shell composites, synthesis of the Au/TiO2/SiO2 particles, coating of Au/TiO2/SiO2 with silica, and etching the outer silica layer. TEM images were used to confirm the success of each of the synthesis steps, and both UV-vis adsorption spectra and the catalytic activity evaluation were employed to investigate the degree of re-exposure of Au nanoparticles after the etching treatment. In our experiments, the obtained mesoSiO2/Au/TiO2/SiO2 catalyst showed a superior thermal stability and higher activity for CO conversion compared to the mesoSiO2/Au/SiO2 one. It resisted sintering during the calcination at 500 °C, whereas the unprotected one was found to sinter. Moreover, it was found that on the mesoSiO2/Au/TiO2/SiO2 sample, the outside silica material could hinder the phase transformation of titania to some extent. Thus, small crystalline particles of TiO2 anchored on the silica beads of the core-shell composites, leading to a better dispersion of small Au nanoparticles and improved catalytic capacity to resist sintering.