Sanming Xiang

A highly reactive and magnetic recyclable catalytic system based on AuPt nanoalloys supported on ellipsoidal Fe@SiO2

A facile method has been developed for the synthesis of highly active and well-defined AuPt nanoalloys supported on the surface of ellipsoidal Fe@SiO2 nanoparticles. This method involves the loading of Pt NPs on the Fe2O3@SiO2 nanocapsules via Sn2+ linkage and reduction, then in situ fabrication of Au nanoparticles by the galvanic replacement reaction between Au and Pt, and finally calcination and reduction to convert the nonmagnetic Fe2O3 to Fe core with high saturation magnetization. XRD and XPS analysis demonstrates the alloy structure of AuPt nanoparticles in the final samples. The obtained Fe@SiO2/AuPt samples exhibit a remarkably higher catalytic activity in comparison with the supported monometallic counterparts toward the reduction of 4-nitrophenol to 4-aminophenol by NaBH4. The catalyst can be reused for several cycles with convenient magnetic separation.

Encapsulation of Au nanoparticles with well-crystallized anatase TiO2 mesoporous hollow spheres for increased thermal stability

Uniform Auencap/TiO2 hollow microspheres, in which sub-10 nm Au nanoparticles are coated with a mesoporous anatase TiO2 shell, are prepared by a protected-calcinating process. The method involves the preparation of SiO2@Au@TiO2 colloidal composites, sequential deposition of carbon and then SiO2 layers through solvothermal and sol–gel processes, crystallization of TiO2 by calcination and finally etching of the inner and outer silica layers to produce the hollow structure. The protected crystallization process suppresses the excessive growth of TiO2 and eventually produces mesoporous anatase shells with high surface area. Additionally, by simply controlling the crystallization temperature, it can be convenient to tune the porosity and crystallinity of the TiO2 shell, meanwhile maintaining the small size of Au nanoparticles. When used as the catalyst for the reduction of 4-nitrophenol, the synthesized Auencap/TiO2 catalysts exhibit significantly enhanced catalytic performance. Moreover, the obtained Auencap/TiO2 hollow spheres show a superior thermal stability, as it resists sintering during the additional calcination at 500 °C, whereas the sample prepared by deposited Au nanoparticles on commercial P25(Au/P25) was found to sinter severely.

CeO2 hollow nanospheres synthesized by a one pot template-free hydrothermal method and their application as catalyst support

Uniform ceria hollow nanospheres composed of ceria nanocrystals have been synthesized via a simple one-step hydrothermal method without using any template. Afterwards, these hollow materials were used as support to prepare the Au/CeO2 catalyst for the reduction of 4-nitrophenol (4-NP). It was found that the obtained porous CeO2 hollow nanospheres were morphologically uniform, with an average diameter of 210 nm and high specific surface area of 167 m2 g−1. According to the basis of a time-dependent experiment, a self-assembly process coupled with an Ostwald ripening mechanism was proposed to explain the evolution of CeO2 hollow nanospheres. In comparison with the commercial CeO2 powder supported sample, the synthesized hollow Au/CeO2 nanospheres catalyst exhibited significantly enhanced catalytic activity. In addition, the results of cyclic stability of the catalyst indicated that similar catalytic performance without visible reduction could be found after 7 repeated cycles. As for this catalyst system, the unique porosity structures of the support, uniform distribution of metallic particles together with the high thermal stability of Au NPs were all responsible for the improved reaction properties.

Enhanced catalytic activity with high thermal stability based on multiple Au cores in the interior of mesoporous Si–Al shells

A novel mesoporous Si–Al/Au catalyst with core–shell structure was successfully fabricated by the combination of a sol–gel strategy and calcination process. This method involves the preparation of a gold sol and the capsulation of Si–Al layers. Afterwards, the mesoporous Si–Al/Au catalyst was obtained by calcination at 550 °C to remove the surfactant and other organics. The synthesized samples were characterized by several techniques, including transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy analysis, X-ray diffraction, field emission scanning electron microscopy (FESEM), N2 adsorption–desorption isotherms and UV-Vis spectra. It was found that this Si–Al/Au core–shell catalyst exhibited high thermal stability and the existence of a mesoporous structure could ensure high permeation and mass transfer rates for species involved in a catalytic reaction. After the calcination, the Au nanoparticles still maintained their small size because of the protective effect of the outside Si–Al layers. Moreover, when the samples were treated by a hydrothermal method, the one core was changed to multiple cores, resulting in the high catalytic activities for the reduction of p-nitrophenol (p-NPh). In our experiment, this prepared catalyst could be easily recycled without a decrease of the catalytic activities in the reaction.

Synthesis and characterization of carbon nanotubes supported Au nanoparticles encapsulated in various oxide shells

A novel supported catalyst with various oxide shells (SiO2, TiO2, ZnO) assembled on Au nanoparticles with carbon nanotubes as support has been successfully fabricated. This process involves preparation of modified MWCNTs, sequential deposition of Au and then oxide shells, and finally calcination at high temperature to remove the organics. The obtained samples were characterized by several techniques, including N2 adsorption–desorption isotherms, transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), UV-Vis spectra, X-ray diffraction and thermogravimetric analysis (TGA). The results established that all the oxide shells could serve as effective barriers to prevent the migration and aggregation of Au NPs during calcination. Moreover, different oxide layers have an obvious influence on the distribution of Au nanoparticles. Additionally, the prepared catalyst exhibited a mesoporous structure because of the preservation of carbon nanotubes. In our experiments, the catalytic activities of MOx/Au/CNTs were investigated by photo-metrically monitoring the reduction of p-nitrophenol (p-NPh) by an excess of NaBH4. It was found that the prepared TiO2/Au/CNTs catalyst revealed excellent catalytic activity and the sample could be easily recycled without a decrease of the catalytic activity in the reaction.

Synthesis of a hierarchical SiO2/Au/CeO2 rod-like nanostructure for high catalytic activity and recyclability

Uniform hierarchical SiO2/Au/CeO2 rod-like nanostructures were successfully fabricated by combining three individual synthesis steps, in which sub-5 nm gold nanoparticles (Au NPs) were coated with a mesoporous CeO2 shell. This method involves preparation of rod-like silica particles, deposition of Au NPs through a self-assembly procedure and then sequential deposition of CeO2 layers. To investigate the catalytic structure, the obtained sample was characterized by several techniques, including transmission electron microscopy (TEM), X-ray powder diffraction (XRD), N2 adsorption–desorption isotherms (BET), and UV-vis diffuse reflection spectroscopy. It was found that SiO2/Au/CeO2 possessed an integral core shell structure including encapsulated Au NPs as core and mesoporous CeO2 as shell. Meantime, the inner silica plays an important role in the morphology control and improvement of the catalyst mechanical strength. The sample shows unique features such as uniform rod-like morphology, well dispersed Au NPs, and large specific surface area. The results of reaction performance indicate that the synthesized SiO2/Au/CeO2 catalysts exhibit significantly enhanced catalytic activity. Moreover, the catalytic activity of our as-prepared nanocomposite catalysts was well maintained even after 8 repeated cycles. It is clear that the core–shell composites can be used as effective nanoreactors with superior catalytic activity and recyclability due to their unique structural features.

Hierarchical structures based on gold nanoparticles embedded into hollow ceria spheres and mesoporous silica layers with high catalytic activity and stability

A uniform hollow CeO2/Au@mSiO2 core–shell catalyst with a hierarchical structure was fabricated successfully. The preparation method involves the synthesis of high surface area and porous hollow CeO2 nanospheres, a sequential deposition of sub-10 nm Au nanoparticles to obtain CeO2/Au, coating the particles with mesoporous SiO2 shells through a sol–gel process and a calcination process at a desired temperature to obtain a mesoporous silica shell. The final obtained product was characterized by several techniques, including transmission electron microscopy (TEM), UV-Vis spectroscopy, X-ray diffraction (XRD) and energy dispersion X-ray spectroscopy (EDS). It is found that CeO2/Au@mSiO2 composite multifunctional materials have a multilayer structure and gold nanoparticles can be embedded into the hollow ceria sphere and the mesoporous silica layer. Compared with the solid CeO2/Au@mSiO2 catalyst, the hierarchical structures of the hollow CeO2/Au@mSiO2 possess open unique hierarchical pores to expose the catalytically active component, including the hollow central structure and mesopores from the CeO2 layer and the silica shell. The results of the reduction of 4-nitrophenol (4-NP) indicate that the synthesized hollow hierarchical catalysts exhibit superior catalytic performance to the traditional core–shell structure and can be easily recycled without a decrease of the catalytic activities in the reaction.

Anisotropic growth of SiO2 and TiO2 mixed oxides onto Au nanostructures: highly thermal stability and enhanced reaction activity

Eccentric Au@(SiO2,TiO2) core–shell nanostructure that consists of a 20 nm Au core coated with mixed SiO2–TiO2 oxides shell was fabricated by a facile sol–gel method. The prepared samples were characterized by means of TEM, EDS, FTIR, and N2 adsorption–desorption isotherms. The results showed that incorporation of TiO2 in SiO2 shells led to the displacement of the Au particle from the center to eccentric positions. By tuning the nanostructure of Au core, the mixed oxides also exhibited anisotropic growth on smaller Au nanoparticles (10 nm) and Au nanorods. When used as the catalyst for the reduction of 4-nitrophenol, the mixed oxides coated catalysts exhibited significantly enhanced catalytic performance compared with the pure SiO2 coated one. In particular, the calcined Au@(SiO2,TiO2) particles showed the highest catalytic activity due to the easy mass transfer, improved thermal stability and increased synergy effect of Au with TiO2. Finally, a possible reaction mechanism for the reduction reaction on Au@(SiO2,TiO2) was also proposed.