Preparation of Highly Dispersed Iron Oxide Nanoparticles in Amine-Modified SBA-15
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Preparation of Highly Dispersed Iron Oxide Nanoparticles in Amine-Modified SBA-15 Bing Tan, Wentao Xu†, Alan Dozier, and Stephen E. Rankin †
Department of Physics and Astronomy, University of Kentucky and Department of Chemical and Materials Engineering University of Kentucky, Lexington, KY 40506-0046 (U.S.A.) Email: [email protected]; Phone: +1-859-257-9799 Abstract Silica-supported iron oxide nanoparticles are prepared by precipitation within the pores of amine-functionalized SBA-15 silica. The loading of the iron oxide possible by this method is at least 11 wt%. STEM and TEM images show that the supported particles have a uniform diameter (average ~ 4.0 nm) and are well dispersed. The supported iron oxide nanoparticles are amorphous after calcination at 300 °C and, consistent with their nanoscale dimensions, are superparamagnetic at room temperature. Introduction Highly dispersed nanosized metal oxide particles are of particular interests in catalysis, optics, magnetics, and medicine.[1-4] For these applications, the product activity is determined not only by the size and crystallinity of the nano-scale particles, but also by the dispersion of these particles on a porous support. Mesoporous silica has been widely used as a support since it has high surface area, adjustable pore size, controllable pore structure, and is chemically inert. However, loading of metal oxides into these supports can be difficult to control. Iron (III) nitrate is a classic precursor used to prepare iron oxide particles. In solution, the first step in oxide synthesis is usually to precipitate iron hydroxide particles by raising the pH of the solution. The iron hydroxide can subsequently be transformed to the oxide by calcination. This process generates particles with a wide distribution of sizes and irregular shapes. The reason may be the rapid, uncontrolled precipitation and agglomeration that occur after adding a base such as concentrated aqueous ammonia. For supported oxides, the nitrate can also be loaded directly onto the support and converted to the oxide by calcination at high temperature, but the level of loading that can be achieved is limited.[5] Here, we demonstrate an improved process to prepare dispersed nano-sized iron oxide by using amine-modified SBA-15 as the support. When placed in aqueous solution, the amine groups should be rapidly protonated, to generate hydroxide species locally. The hydroxide reacts quickly with ferric ions to produce iron (III) hydroxide. At the same time, some of the ferric ions may form complexes with silanols at the surface of the material. By either mechanism, widespread rapid nucleation at the surface of the pores of the support is expected to lead to the formation of small, well dispersed nanoparticles within the uniform cylindrical pores of SBA-15. Previously, amine-functionalized silica supports have been used to generate supported CdS particles by complexation and reaction,[6] but in the present case, the mechanism for particle formation does not rely on
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direct amin
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