Qianli Wang

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 Pt magnetic nanocatalysts with a TiO2 or CeO2 layer

The Pt magnetic nanocatalysts with a TiO2 or CeO2 layer have been fabricated successfully. The mSiO2/Pt/MOx/Fe hybrids have a CeO2 or TiO2 layer synthesized through hydrothermal methods and a moveable magnetic Fe core constructed by hydrogen reduction. The obtained nanocapsules were characterized by several techniques, including transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersion X-ray analysis (EDX). The catalytic evaluation was tested on the reduction of 4-NP to 4-AP monitored by UV-Vis spectroscpoy. The mesoporous SiO2 shell served as an effective barrier to prevent the migration and aggregation of Pt NPs during calcination. Besides, the oxide layers have an apparent co-catalysis effect to improve the catalytic activity. The mSiO2/Pt/MOx/α-Fe2O3 samples were calcined at different temperatures and the final samples exhibited entirely different catalytic activity. Additionally, a possible mechanism was proposed to explain the results. Furthermore, the synthesized mSiO2/Pt/MOx/Fe hybrids could be easily recycled using a magnet and the catalytic activity did not decrease obviously after five runs.

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.

Nanocasting synthesis of an ordered mesoporous CeO2-supported Pt nanocatalyst with enhanced catalytic performance for the reduction of 4-nitrophenol

Ordered mesoporous ceria (meso-CeO2) was fabricated by nanocasting employing Ia3d mesoporous silica KIT-6 as the template. For comparison, ceria nanoparticles (nano-CeO2) with non-ordered mesoporous were also synthesized via a sol–gel method. Polyamidoamine (PAMAM) dendrimers were used as stabilizing agents to prepare a dispersed Pt nanoparticle colloidal solution. Afterward, the obtained well-dispersed Pt nanoparticles were immobilized on meso-CeO2 and nano-CeO2, respectively. The prepared samples were characterized through several techniques, such as X-ray diffraction (XRD), nitrogen adsorption–desorption isotherms, transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and energy dispersion X-ray analysis (EDX) with mapping. The results revealed that the as-prepared meso-CeO2 has high crystallinity, a relatively small crystalline size, a well-ordered mesoporous structure and high surface area of 115.3 m2 g−1. In addition, the Pt/meso-CeO2 catalyst showed relatively uniform distribution of Pt nanoparticles with small sizes (∼4 nm). The catalytic performances of the as-synthesized catalysts were evaluated relying on the reduction of 4-nitrophenol monitored by UV-vis spectra. It was found that Pt/meso-CeO2 exhibited better catalytic activity compared with Pt/nano-CeO2. Besides, Pt/meso-CeO2 possessed good reusability and maintained a conversion of no less than 90% even after five cycles.

Synthesis of dendrimer-templated Pt nanoparticles immobilized on mesoporous alumina for p-nitrophenol reduction

Hydroxyl-terminated fourth-generation poly(amidoamine) (G4-OH PAMAM) dendrimers were employed as templates to encapsulate Pt nanoparticles with small size and uniform dispersion, and then Pt nanoparticles were deposited on mesoporous alumina via self-assembly between the dendrimers and the support. The decomposition of dendrimer templates was investigated in nitrogen at different temperatures. The obtained catalysts were characterized through several techniques, such as transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), nitrogen adsorption–desorption isotherms, energy dispersion X-ray analysis (EDX), X-ray diffraction (XRD) and UV-vis spectra. Compared with the catalyst calcined in air, Pt nanoparticles exhibited uniform dispersion and small size when roasted in flowing nitrogen at 550 °C. The catalytic performances of fabricated Pt/MA catalysts were evaluated through the reduction of p-nitrophenol with NaBH4. It was found that the catalyst calcined at 550 °C in nitrogen exhibited the best catalytic activity. Besides, the catalyst was easily recycled without a marked decrease of the catalytic performance.

Catalytic structure and reaction performance of PtSnK/ZSM-5 catalyst for propane dehydrogenation: influence of impregnation strategy

In this study, a series of PtSnK/ZSM-5 catalysts for propane dehydrogenation were prepared by changing the impregnation sequence of platinum and tin precursors (co-impregnation and successive impregnation). To investigate the influence of impregnation strategy on the catalyst structure and the reaction performance, the prepared samples were studied by several techniques, including XRD, nitrogen adsorption, ICP, TEM, NH3-TPD, FT-IR, hydrogen chemisorption, H2-TPR, and TPO. It was found that the prepared sample with co-impregnation method showed the highest reaction stability and selectivity when comparing with the ones prepared by the sequential impregnation. As for the co-impregnated catalyst, high metal dispersion, strong interaction of Pt with Sn oxide together with the decreased catalyst acidity were all responsible for the improved reaction properties and better catalytic capacity to resist the coke. Nevertheless, over the sequentially impregnated catalysts, the increased catalyst acidity and the relatively easy reduction of tin species were found when the Pt components were deposited first. Furthermore, the successive impregnation with inverse sequence led to the wide metallic distribution and the restricted transformation of the active sites. All of these factors were disadvantageous to the reaction to be carried out. Finally, a model for the influences of impregnation sequence was proposed based on the obtained results.

Synthesis and characterization of a multifunctional nanocatalyst based on a novel type of binary-metal-oxide-coated Fe3O4–Au nanoparticle

A novel type of binary-metal-oxide-coated Au nanocatalyst, including a mixed oxide layer, a moveable magnetic Fe3O4 core and some Au NPs of 2–5 nm, has been synthesized successfully by a facile hydrothermal synthesis method. SEM, TEM, EDX, FTIR, XRD, and TGA were employed to characterize the prepared samples. The results showed the mSiO2–TiO2 layer could increase the thermal stability and reactivity of metal nanocatalysts compared to a pure TiO2 or SiO2 layer. The reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) was employed as a model reaction to test catalytic performance in this work. The results showed that the binary-metal-oxide-coated nanocatalyst (550 °C) exhibited significantly enhanced catalytic performance compared with the pure SiO2 (550 °C) or TiO2 (550 °C). In particular, the mSiO2–TiO2/Au/C/Fe3O4 particles calcined at 550 °C showed the highest catalytic activity, compared to the samples calcined at 700 °C, 300 °C and RT. Meanwhile, because of C layer burning, the sample presented a few white spots between the Fe3O4 microsphere and the oxide layer, suggesting that the specific surface area was increased by calcination. The sample (550 °C) still has a certain degree of magnetism, suggesting the desired samples could be separated by magnet. Finally, to explain the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP), a possible reaction mechanism was also proposed.

A 3D hierarchical magnetic Fe@Pt/Ti(OH)4 nanoarchitecture for sinter-resistant catalyst

Recently, novel morphological nanocomposites have attracted increasing attention due to their unique features. In this work, a 3D hierarchical magnetic Fe@Pt/Ti(OH)4 nanoarchitecture has been synthesized successfully. TEM images were used to confirm the success of each of the synthesis steps, and the reduction of 4-NP to 4-AP was employed to evaluate their catalytic performance. Their large surface area and nanorod structure guarantee their good catalytic performance. Furthermore, the as-prepared nanocapsule shows excellent anti-sintering properties for the physical barrier effects of Ti(OH)4 nanorods. The sample calcined at 700 °C showed the highest catalytic activity in our work due to the decomposition of Ti(OH)4. Finally, the synthesized Fe@Pt/Ti(OH)4 nanocomposite could be easily recycled.