Читать книгу Carbon Nanofibers - Группа авторов - Страница 80

It is well known that the mechanical, physical, and chemical properties of nanoparticles may differ considerably from those of bulk substances. The deposition over support often endows particles with novel or improved properties. These changes are provided by the change of electronic and structural properties, strong interaction with the support, and increase of the particle/support interface length taking place with metal dispersion. These factors are closely interrelated and determine, for example, the number of catalytically active sites and adsorption energies during catalytic reactions. Thus, the dispersion of a supported metal can be extremely high, making almost every metal atom accessible to reactants. Advantages of metal dispersing over support widely used for catalytic applications are also applicable for different fields of nanotechnology.

Because of their novel properties, which can be realized in a variety of ways in numerous applications, CNFs have received a great deal of attention from both the research and industrial communities. Several academic and industrial research groups have directed their efforts toward synthesizing and optimizing the growth of these CNFs. In its simplest form, the catalytic synthesis of CNFs consists of formation of these fibers on metallic catalysts in the form of powders, foils, gauzes, or supported particles. The process consists of reducing the catalyst sample in a hydrogen-inert gas stream at a somewhat lower temperature, followed by heating the catalyst up to the reaction temperature, subsequent to which the reaction mixture, consisting of hydrocarbon, hydrogen, and inert gas, is introduced into the system [25–30]. The reaction proceeds for periods ranging from a few minutes to several hours. Several different metallic and bimetallic catalysts can be used. The most commonly used catalysts for CNF synthesis are iron, cobalt, nickel, and copper, both in bulk and in supported form. Lower hydrocarbons, such as methane, ethylene, acetylene, or benzene, and carbon monoxide are the common sources of carbon material. The most common mechanism proposed in the literature for the synthesis of CNFs consists of decomposition of the hydrocarbons on the metal surface, releasing carbon atoms. These carbon atoms then form metal carbides that dissolve and diffuse through the bulk of the metal, resulting in the deposition of CNFs at the other end of the metal particles [31–33]. Despite great advances in synthesis methods and efforts to understand the mechanism of nucleation and growth of CNFs, continuous production of these fibers has proven to be challenging.