Effect of Catalyst Supports in the Synthesis of Graphite Nanostructures
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A current goal in nanotechnology research is the ability to control not only the growth, morphologies but also the surface characteristics of carbon nanostructures. The controlled catalytic decomposition of hydrocarbons and/or carbon monoxide over heated metal particles is a very efficient method, since all the catalyst is utilized in the process and the structure of the solid carbon product can be tailored by careful control of temperature and reaction conditions over a given metal surface [5]. The details of synthesis procedures can be found in the literature, where the surface composition and state of the catalyst is of the utmost importance and diffusion of the carbon through the metal particle is the rate determining step [6]. The most profound changes occur when different support materials are used or certain metals are combined with other catalytically active or inactive metals. Under such conditions the metal particle size is greatly reduced, as is the size of the resulting carbon nanostructure. Further changes in nanofiber characteristics occur as a result of metal/support effects, as interactions between the metal and support can induce changes in the particle crystallography. Support materials, especially various forms of carbon, can exert a profound influence on the metal surface through a number of unique interactions [7]. Metal-support interactions can be classified into two broadly defined categories: Geometric effects - including template and surface interactions, which physically force the metal to adopt stacking arrangements yielding crystallographic faces that are not normally observed on other support materials or in bulk. Electronic effects - which are manifested through changes in the rates of reactions and are related to the ability of the metal/support system to transfer electrons to or from the reacting molecule. In an attempt to probe these effects, two different supports were used in this study; platelet graphite nanofibers (P-GNF) and amorphous fumed silica (Cab-o-sil). The P-GNF consists entirely of graphite edge sites (the {1120) and {1010} faces), which can exert both geometric (from the 45 Mat. Res. Soc. Symp. Proc. Vol. 593 © 2000 Materials Research Society
atomic on the surface) electronic conductive In contrast,electronic silica hasand no and effects. as a consequence, inherently an insulator and is and regular arrangement crystalline orientation geometric effects will not be operative to any significant degree. EXPERIMENTAL Two bimetallic systems, Fe-Ni and Fe-Cu were prepared from the respective metal nitrates, mixed in the desired ratios and introduced onto silica and P-GNF supports via aqueous and nonaqueous incipient wetness techniques, respectively to give a 5 wt.% metal loading. The impregnated samples were calcined, reduced and passivated. A similar procedure was followed for the preparation of supported 5 wt.% iron catalysts. Carbon nanofibers were grown onto the supported catalyst systems using a CO/rH2 (4:1) feed gas at 550°C in a flow reactor system. Details of the e
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