NiFe 2 O 4 @SiO 2 Nanoparticles Stabilized by Porous Silica Shells
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NiFe2O4@SiO2 Nanoparticles Stabilized by Porous Silica Shells Nisha Shukla • Abigail Ondeck • Johanna C. Lee James B. Miller
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Received: 2 November 2011 / Accepted: 1 March 2012 / Published online: 21 March 2012 Ó Springer Science+Business Media, LLC 2012
Abstract NiFe2O4 nanoparticles stabilized by porous silica shells (NiFe2O4@SiO2) were prepared using a onepot synthesis and characterized for their physical and chemical stability in severe environments, representative of those encountered in industrial catalytic reactors. The SiO2 shell is porous, allowing transport of gases to and from the metal core. The shell also stabilizes NiFe2O4 at the nanoparticle surface: NiFe2O4@SiO2 annealed at temperatures through 973 K displays evidence of surface Ni, as verified by H2 TPD analyses. At 1,173 K, hematite forms at the surface of the metallic cores of the NiFe2O4@SiO2 nanoparticles and surface Ni is no longer observed. Without the silica shell, however, even mild reduction (at 773 K) can draw Fe to the surface and eliminate surface Ni sites. Keywords Nanotechnology Nanoparticles Nanostructure Electron microscopy Spectroscopy and general characterisation
1 Introduction Ni and Fe have important applications as catalysts for Fischer–Tropsch (FT) conversion of synthesis gases to N. Shukla J. B. Miller National Energy Technology Laboratory, 620 Cochrans Mill Road, Pittsburgh, PA 15236, USA N. Shukla Institute for Complex Engineered Systems, Carnegie Mellon University, Pittsburgh, PA 15213, USA A. Ondeck J. C. Lee J. B. Miller (&) Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA e-mail: [email protected]
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liquids and as carriers for chemical looping combustion systems. In the FT application, Ni and Fe catalysts have been used in both supported and powdered forms. Two drawbacks of powdered catalysts are low surface area and the lack of uniformity of individual particles. To overcome these challenges, the past decade has seen increased interest in synthesis of monodispersed nanoparticles for applications in catalysis. Initial efforts in synthesis of nanoparticles were aimed at control of nanoparticle size and shape [1–3]. Recently, efforts have turned towards synthesis of alloy nanoparticles [4–6]. Alloy nanoparticles can be more effective catalysts or have better magnetic properties than pure component nanoparticles [1, 7, 8]. Performance of alloy nanoparticles in applications such as FT depends on control of both composition and particle size [9–13]. However, one disadvantage of subjecting small metallic nanoparticles to the high temperatures required in catalytic reactors is that they tend to sinter or aggregate into larger particles. To prevent aggregation, there has been significant interest and success in the preparation of metal nanoparticles coated with thin shells of silica and other ceramic materials [8, 14–17]. A core–shell multi-component structure can be catalytically active if the ceramic shell is sufficiently porous to allow transport of gases to
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