Catalytic Behavior of Graphite Nanofiber Supported Nickel Particles. 3. The Effect of Chemical Blocking on the Performance of the System

Graphite nanofibers are a newly developed type of material produced by the catalytic decomposition of carbon containing gases at high temperatures. The individual components of these conformations, small-sized graphite crystallites, are arranged in such a manner that only the edge regions are exposed. The carbon atoms at these sites that are arranged in two conformations, “armchair” or “zigzag”, act as templates for the nucleation of metal crystallites. Treatment of graphite with certain phosphorus compounds is a process that is known to result in preferential blocking of the “armchair” faces, whereas boron oxide selectively substitutes into the “zigzag” faces. In the current investigation pretreatment in phosphorus oxide was found to exert little or no effect on the subsequent catalytic performance of graphite nanofiber supported nickel with respect to hydrogenation of ethylene and 1-butene. In contrast, incorporation of boron into the carbonaceous support, which resulted in blockage of the “zigzag” sites of the graphite nanofibers rendered the supported metal system virtually inactive toward hydrogenation of either of the olefins. These results suggest that the active state of nickel is one where the particles are preferentially located on the “zigzag” faces of the nanofiber structures. Under these conditions the metal particles adopt a crystallographic arrangement that is favorable toward reaction with both reactant molecules. It is evident that one can control the catalytic behavior of a given metal by careful tailoring the support structure at the atomic level.