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Ferromagnetic nanoparticles (NPs) prepared by conventional routes usually lead to large particle sizes because of their easy aggregation nature, and the low loading amount greatly limits their application in reduction of 4-nitrophenol (4-NP). In this contribution, well dispersed Ni nanoparticles with small diameter of 7 nm are supported on mesoporous silica SBA-15 through a facile in situ decomposition and reduction strategy. It is found that the as-prepared Ni/SBA-15 can be used as a catalyst and exhibits a high specific surface area (461 m2 g−1) and ultra-high loading amount of Ni particles (574 μg mg−1). The well-dispersed and high-loading Ni NPs make the Ni/SBA-15 exhibit excellent catalytic performance in 4-NP reduction, superior to any Ni NPs supported catalysts reported so far and many noble metal supported catalysts. In particular, the magnetic behaviour of Ni/SBA-15 makes it easy to recycle for reuse. This novel synthetic strategy may pave the way for the preparation of other metal nanoparticles supported on different kinds of amorphous silica with high loading capacity and high dispersion.

The high cost of noble metal nanoparticles used for catalytic reduction of 4-nitrophenol (4-NP) leads to an extensive study of Ni nanoparticles (NPs) for their low cost and magnetic properties. However, the conventional routes for preparing the ferromagnetic Ni NPs usually lead to large particle size and aggregation. In this study, we propose a simple two-step method for the synthesis of hierarchical Ni NP supported silica magnetic hollow microspheres (Ni/SiO2 MHMs). Tiny Ni NPs are well dispersed on the supports with high loading amounts (15 wt%). The size of Ni NPs can be tuned from 10 nm to 21 nm with the size of Ni/SiO2 MHMs increasing from 230 nm to 800 nm. The as-prepared samples exhibit excellent catalytic activity in the reduction of 4-NP. Furthermore, the experimental results prove that the size of Ni NPs plays an important role in the catalytic activity. The catalytic activity of small sized Ni NPs is higher than that of large sized and many other supported Ni NP catalysts as reported. In particular, the magnetic properties of Ni/SiO2 MHMs make them easy to recycle for reuse.

The easy aggregation nature of ferromagnetic nanoparticles (NPs) prepared by conventional routes usually leads to a large particle size and low loading, which greatly limits their applications to the reduction of 4-nitrophenol (4-NP). Herein, we developed a novel in situ thermal decomposition and reduction strategy to prepare Ni nanoparticles/silica nanotubes (Ni/SNTs), which can markedly prevent the aggregation and growth of Ni NPs, resulting in an ultra-small particle size (about 6 nm), good dispersion and especially high loading of Ni NPs. It was found that Ni/SNTs, which have a high specific surface area (416 m(2) g(-1)), exhibit ultra-high catalytic activity in the 4-NP reduction (complete reduction of 4-NP within only 60 s at room temperature), which is superior to most noble metal (Au, Pt, and Pd) supported catalysts. Ni/SNTs still showed high activity even after re-use for several cycles, suggesting good stability. In particular, the magnetic property of Ni/SNTs makes it easy to recycle for reuse.

A unique and rational design was presented to fabricate Ni/SiO2@Au magnetic hollow microspheres (MHMs) with interesting structures and well-dispersed metal nanoparticles. Hierarchical nickel silicate hollow microspheres were synthesized using silica colloidal spheres as a chemical template. Then, Ni/SiO2 MHMs with well-dispersed Ni nanoparticles were prepared via an in situ reduction approach. Ni/SiO2@Au MHMs were finally obtained by immobilizing uniform Au nanoparticles onto Ni/SiO2 support through a low-temperature chemical reduction process. It was found that Ni/SiO2@Au MHMs inherit the shape and uniformity of the original silica scaffold, and Ni NPs and Au NPs, which were less than 5 nm in size, were well dispersed on the mesoporous silica shell with narrow size distribution. Both Ni/SiO2 and Ni/SiO2@Au MHMs showed excellent catalytic activity in the 4-nitrophenol reduction reaction. Importantly, introduction of a small amount of Au NPs onto Ni/SiO2 MHMs markedly improved the catalytic activity. In particular, Ni/SiO2@Au MHMs showed high conversion even after re-use for several cycles with magnetic separation. The unique structure, high catalytic performance, and ease of separation make Ni/SiO2@Au MHMs highly promising candidates for diverse applications.