High-Resolution TEM Characterization of Carbon Aerogels as Catalyst Supports
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High-Resolution TEM Characterization of Carbon Aerogels as Catalyst Supports Dafei Kang1, Ying Zhang2, Carl Saquing2, Can Erkey2 and Mark Aindow1 Department of Metallurgy and Materials Engineering, Institute of Materials Science, University of Connecticut, U-3136, 97 N. Eagleville Rd., Storrs, CT 06269-3136. 2 Department of Chemical Engineering, University of Connecticut, U-3222, 191 Auditorium Rd., Storrs, CT 06269-3222. 1
ABSTRACT Carbon aerogels with tailored pore sizes were prepared by using a combination of sol-gel processing, supercritical CO2 extraction and pyrolysis. Ruthenium particles were incorporated via impregnation of organometallic precursors using supercritical CO2, followed by in situ thermal reduction. TEM studies reveal that the carbon aerogels are characterized by a continuous pore structure and that the ruthenium particles are highly dispersed with a narrow size distribution. It is found that under certain circumstances the carbon aerogels undergo a structural transition under the electron beam, in which the original tortuous aerogel network transforms into onion-like structures. INTRODUCTION Carbon-supported ruthenium has been studied extensively as a catalyst for a series of reactions such as ammonia synthesis [1,2], hydrogenation of carbon monoxide and carbon dioxide [3] and hydrodesulfurization of petroleum fractions [4]. Due to its high energy density [5,6], carbon-supported ruthenium is also being considered as an electrode component for electrochemical capacitors. Carbon aerogel is a novel type of porous carbon material, which is receiving increasing attention because of an attractive combination of properties including: low density, large surface area, continuous pore structure and high electrical conductivity. In this work, we report a preliminary TEM study on the structure of ruthenium-carbon aerogel nanocomposites prepared at the University of Connecticut using novel extraction and deposition techniques with supercritical CO2. EXPERIMENTAL A detailed description of the preparation of pure and metal-loaded carbon aerogels can be found elsewhere [7,8]. In a typical trial, a mixture of calculated amounts of resorcinol and formaldehyde undergoes condensation polymerization in water such that a sol is formed. Upon gelation and curing, the sol transforms into a hydrogel. After replacing the water in the matrix by solvent exchange, the gels are supercritically dried. The resulting resorcinol-formaldehyde aerogels are eventually converted to carbon aerogels by controlled pyrolysis in an inert atmosphere at high temperatures. Such pure carbon aerogels are black in color and usually are made in the form of a cylindrical monolith. The incorporation of ruthenium particles starts with the impregnation of carbon aerogels with appropriate organometallic precursors from supercritical carbon dioxide solutions. The precursors are subsequently reduced at elevated
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temperature, yielding ruthenium particles dispersed throughout the volume of the carbon aerogel monolith. Specimens for T
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