Magnetic iron oxide nanoparticles coated by hierarchically structured silica: a highly stable nanocomposite system and ideal catalyst support

Abstract

A well-defined core–shell nanocomposite with magnetic iron oxide nanoparticles coated by a hierarchically structured silica shell has been synthesized through a sol–gel process and pseudomorphic transformation. The prepared materials were characterized by means of transmission electron microscopy, small and wide angle X-ray diffraction, Mössbauer spectroscopy and N₂ physical adsorption–desorption. It has been shown that the core–shell nanocomposite possesses a magnetic core in the form of Fe₃O₄ or γ-Fe₂O₃ and a unique hierarchically structured silica shell consisting of an inner nonporous shell and outer shell with hierarchical pores. As a result, this nanocomposite exhibits high stability (acid resistance) and a large surface area (447 m² g⁻¹), which will be especially suitable for use as catalyst supports. To demonstrate this point, a functional catalyst consisting of small Pt nanoparticles well-dispersed on the porous surface of the magnetic core–shell nanocomposite was fabricated. In the hydrogenation of nitrobenzene and 2-nitrochlorobenzene to the corresponding aniline compounds, the hierarchically porous catalyst showed superior performances to its counterpart with a monomodal porous structure. In addition, the used catalyst could be separated conveniently from the reaction system with an external magnetic field. Subsequent recycling tests further confirmed the outstanding reusability and regeneration ability of the composite catalyst.