The Impact of the Graphite Nanofiber Structure on the Behavior of Supported Nickel
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supported on a variety of carbonaceous materials was extremely sensitive to the degree of crystalline perfection of the substrate; the highest activity for the hydrogenation of 1-butene and 1,3-butadiene at 100°C being found for samples in which the metal particles were supported on carbon nanofibers possessing a high graphitic content. These results were in accord with those reported by other workers [4] who claimed that for the hydrogenation of selected hydrocarbons, graphite supported palladium particles exhibited the highest activity and selectivity compared to that found when the metal was dispersed on less ordered carbonaceous solids. Gallezot and coworkers [5] showed that significant differences in both activity and selectivity were achieved for the hydrogenation of cinnamaldehyde when platinum particles were supported on graphite compared to that found when the metal was dispersed on high surface area amorphous carbon. In the current study we have used the hydrogenation of ethylene and crotonaldehyde as probe reactions in an attempt to assess any modifications in catalytic behavior induced by supporting nickel on three types of nanofibers, where there were significant differences in the arrangements of the graphite platelets [6]. It was speculated that the atomic arrangement of the surfaces of nickel particles that nucleated on these different graphite edges would be dictated to a large degree by the interaction with the atoms in nanofiber supports. Under such circumstances one might reasonably expect that different crystallographic faces of nickel would be exposed to the reactant gas depending on which type of nanofiber structure was used as the supporting medium. The success of this approach could open up the possibility of a tailoring the morphological characteristics of metal particles in such a fashion so to achieve a desired catalytic performance. The selective hydrogenation of xf3-unsaturated aldehydes to give the corresponding unsaturated alcohol presents a challenge both from the point of view of thermodynamic and 145 Mat. Res. Soc. Symp. Proc. Vol. 497 ©1998 Materials Research Society
kinetic considerations [7]. Existing catalysts are based on alumina supported noble metals and do not offer a high selectivity towards the desired product [8]. A theoretical study concludes that both the nature of the metal and the type of exposed crystal face are critical factors in determining the performance of the catalyst system for this reaction [9]. English and coworkers [10] demonstrated that the hydrogenation of crotonaldehyde over Pt/SiO 2 and Pt/TiO 2 catalysts was strongly influenced by the size of the metal particles and the promotional effects of surface oxides. They reported that the selectivity to the unsaturated alcohol in the gas phase hydrogenation reaction increased with increasing metal particle size. EXPERIMENTAL Materials Figure 1 is a schematic presentation of the three types of graphite nanofibers used in this work and were grown according to the procedures outlined previously [6]. The nano
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