Carbon Nanofibers as Macro-Structured Catalytic Support

Abstract

Since their discovery as a by-product of the arc-discharge process (Iijima, 1991), carbon nanotubes and their related materials, i.e. nanofibers and onion-like particles, have received an increasing academic and industrial interest due to their exceptional mechanical and electronic properties (Dresselhaus et al., 2001). Most commonly, carbon nanofilaments are produced by evaporating solid carbon in an arc discharge, by laser beams or by catalytic chemical vapour deposition (CCVD) of carbon-containing gases (Ebbesen, 1997). Depending on the nature of the metal catalyst, the composition of the carburizing mixture, and the synthesis temperature, carbon nanostructures with different shapes, i.e. nanotubes or nanofibers, can be prepared (de Jong & Geus, 2000). A carbon nanotube structure consists of cylindrical graphene layers with a hollow internal cavity, whilst a carbon nanofiber structure consists of a stacking of different graphite sheets oriented at an angle with respect to the fiber axis. The exposed surface of the carbon nanofibers mainly consists of prismatic planes with high surface reactivity when compared to the graphite basal planes of the carbon nanotubes. Among the different potential applications of these materials, catalysis either within the gas or the liquid phase seems to be the most promising according to the results recently reported in literature (Salman et al., 1999). Metals supported on carbon nanofibers or nanotubes exhibit unusual catalytic activity and selectivity patterns when compared to those encountered with traditional catalyst supports such as alumina, silica or activated carbon. The extremely high external surface area displayed by these nanomaterials significantly reduce the mass transfer limitations, especially in liquid phase reactions (Pham-Huu et al., 2001), and the low interaction between the impregnated metallic phase and the exposed planes of the support which leads to the formation of active metallic faces (Rodriguez et al., 1994) were advanced to explain these catalytic behaviours. However, these materials have only been synthesized in a powder form, making difficult their handling and large scale use, especially in a conventional fixed-bed catalytic reactor. The handling of the carbon nanostructures is hampered by the formation of dust and a severe pressure drop along the catalyst bed. It is of interest to find a method allowing (i) the synthesis of carbon nanostructures on a large scale and (ii) with a macroscopic shape in order for it to be used as a catalyst support. It is expected that the macroscopic shaping of such nano-structured materials will open-up a real opportunity for their use as a catalyst support in relation to the traditional catalysts carriers (Vieira et al., 2004). The macroscopic