Xiaoli Sheng

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.

Microstructure, nickel suppression and mechanical characteristics of electropolished and photoelectrocatalytically oxidized biomedical nickel titanium shape memory alloy

A new surface modification protocol encompassing an electropolishing pretreatment (EP) and subsequent photoelectrocatalytic oxidation (PEO) has been developed to improve the surface properties of biomedical nickel titanium (NiTi) shape memory alloy (SMA). Electropolishing is a good way to improve the resistance to localized breakdown of NiTi SMA whereas PEO offers the synergistic effects of advanced oxidation and electrochemical oxidation. Our results indicate that PEO leads to the formation of a sturdy titania film on the EP NiTi substrate. There is an Ni-free zone near the top surface and a graded interface between the titania layer and NiTi substrate, which bodes well for both biocompatibility and mechanical stability. In addition, Ni ion release from the NiTi substrate is suppressed, as confirmed by the 10-week immersion test. The modulus and hardness of the modified NiTi surface increase with larger indentation depths, finally reaching plateau values of about 69 and 3.1GPa, respectively, which are slightly higher than those of the NiTi substrate but much lower than those of a dense amorphous titania film. In comparison, after undergoing only EP, the mechanical properties of NiTi exhibit an inverse change with depth. The deformation mechanism is proposed and discussed. Our results indicate that surface modification by dual EP and PEO can notably suppress Ni ion release and improve the biocompatibility of NiTi SMA while the surface mechanical properties are not compromised, making the treated materials suitable for hard tissue replacements.

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 of immobilized heteropolyanion-based ionic liquids on mesoporous silica SBA-15 as a heterogeneous catalyst for alkylation

Phosphotungstic acid (H3PW12O40) has been successfully loaded onto sulfonate-functionalized ionic liquid-modified mesoporous silica SBA-15 by total anion-exchange. The immobilized catalysts were characterized by XRD, N2 adsorption, TEM, and FTIR spectroscopy. Characterization results show that the mesopore structure of SBA-15 was maintained well even after surface modification and the subsequent anion-exchange step of [PW12O40]3− (PW). In comparison with the task-specific basic ionic liquid (1-(propyl-3-sulfonate) 3-methyl-imidazolium phosphotungstate), the obtained catalyst showed much higher efficiency in alkylation of o-xylene with styrene. More importantly, such an immobilized task-specific basic ionic liquid could be reused without significant loss of catalytic activity even after recycling six times.

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.

Effect of aluminum modification on catalytic properties of PtSn-based catalysts supported on SBA-15 for propane dehydrogenation

Abstract The catalytic properties of PtSn-based catalysts supported on siliceous SBA-15 and Al-modified SBA-15, such as Al-incorporated SBA-15 (A1SBA-15) and alumina-modified SBA-15 (AI2O3/SBA-15), for propane dehydrogenation were investigated. AI2O3/SBA-15 was prepared either by an impregnation method using aluminum nitrate aqueous solution, or by the treatment of SBA-15 with a Al(OC3H7)3 solution in anhydrous toluene. N2-physisorption, FT-IR spectroscopy, solid-state 27Al MAS NMR spectroscopy, hydrogen chemisorption, XRF, NH3 temperature-programmed desorption, X-ray photoelectron spectroscopy and TPO were used to characterize these samples. Among these catalysts, the PtSn-based catalyst supported on AI2O3/SBA-15, which was grafted with Al(OC3H7)3, exhibited the best catalytic performance in terms of activity and stability The possible reason was due to the high Pt metal dispersion and/or the strong interactions among Pt, Sn, and the support.

In-situ formation of supported Au nanoparticles in hierarchical yolk-shell CeO2/mSiO2 structures as highly reactive and sinter-resistant catalysts

A novel strategy was described to construct Au-based yolk-shell SCVmS-Au nanocomposites (NCs), which combined the sol-gel template-assisted process for the assembly of hierarchical SCVmS NCs with modified CeO2/mSiO2 as yolks/shells, and the unique deposition-precipitation (DP) process mediated with Au(en)2Cl3 compounds for the synthesis of extremely stable supported Au nanoparticles (NPs). Characterization results indicated that the obtained SCVmS-Au NCs featured mesoporous silica shells, tunable interlayer voids, movable CeO2-modified cores and numerous sub-5nm Au NPs. Notably, the Au(en)2Cl3 was employed as gold precursors to chemically modify into the modulated yolk-shell structure through the DP process and the subsequent low-temperature hydrogen reduction induced the in-situ formation of abundant supported Au NPs, bestowing these metal NPs with ultrafine grain size and outstanding sinter-resistant properties that endured harsh thermal conditions up to 750°C. Benefiting from the structural advantages and enhanced synergy of CeO2-Au/mSiO2-Au yolks/shells, the SCVmS-Au was demonstrated as markedly efficient catalysts with superior activity and reusability in catalyzing the reduction of 4-nitrophenol to 4-aminophenol, and its pristine morphology still maintained after eight recycling tests.

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.