Large scale synthesis of carbon nanofibers by catalytic decomposition of ethane on nickel nanoclusters decorating carbon nanotubes Cuong Pham-Huu,* a Nicolas Keller, ab Vladimir V. Roddatis, b Gerhard Mestl, b RobertSchlo¨gl b and Marc J. Ledoux a a Laboratoire des Mate ´riaux, Surfaces et Proce´de´s pour la Catalyse, Universite ´ Louis Pasteur, UMR 7515 CNRS, 25, rue Becquerel, 67087 Strasbourg Cedex 02, France. E-mail: cuong.lcmc@ecpm.u-strasbg.fr b Fritz-Haber-Institut der Max-Planck-Gesellschaft, Abteilung der Anorganische Chemie, Faradayweg 4-6, 14195 Berlin, Germany Recei v v ed 20th July 2001, Accepted 9th Nov v ember 2001 First published as an Adv v ance Article on the web 12th December 2001 A large scale synthesis of carbon nanofibers with a controlled diameter of about 50 nm has been achieved at relatively low temperatures (550–650  C) by the decomposition of ethane on a carbon nanotube supported nickel catalyst. The carbon nanofibers can be used as a catalyst or a catalyst support without subsequent purification, due to the use of carbon nanotubes as support, the high nanofiber yields, and the purity obtained. Introduction Solid carbon is the most widely used material for many industrial applications, i.e. as a catalyst support, or for gas adsorption and gas separation processes. 1 The preparation and use of supported carbon catalysts have been well-reported in the literature for some decades. 2 Carbon materials can be tailored in a variety of macroscopic shapes such as powder, spheres, extrudates or honeycombs, etc. with surface areas ranging from a few square meters for graphite, to several hundred square meters for activated charcoal, depending on the application. For use as a catalyst support, carbon based materials show strong resistance to alkaline and acidic envir- onments compared to classical oxidic supports such as alumina or silica. However, they contain a highly detrimental micro- porosity which could hinder the accessibility of the reactants to the active phase and induce harmful mass transfer limitations. Recently, an increasing number of studies have been devoted to the synthesis and characterisation of carbon nanotubes (CNTs), 3–7 discovered by Iijima in 1991 during arc-discharge experiments, 8 which seem to hold much promise for future applications. It has been reported by different authors that carbon nanofibers can be efficiently used as catalyst supports for several catalytic reactions, either in the gas phase or in liquid phase media. 9–11 In both cases, the carbon nanofiber based catalysts exhibited better catalytic performance than observed on equivalent conventional catalysts. This high catalytic activity and the unusual selectivity were attributed to: (i) the high external surface area displayed by the nanometric-scale support which allowed a significant decrease in mass transfer limitations, generally encountered in liquid phase reactions, 12– 16 and (ii) the existence of a strong interaction between the supported active phase and the graphite edges of the support which could be responsible for the peculiar selectivity. However, such nanostructured carbon materials have up to now been produced in too small quantities to be suitable for large scale use. It is consequently of interest to find simple processes which would permit large scale production of such materials in order to render them attractive as catalyst support materials or for other applications. The aim of the present article is to report the large scale (several hundred grams per gram of active phase) synthesis of uniform carbon nanofibers (average diameter ranging between 40 and 60 nm) by the catalytic decomposition of a mixture of ethane and hydrogen over a nickel catalyst supported on carbon nanotubes at relatively low synthesis temperatures, 4650  C. Experimental Materials and catalyst characteristics Purified carbon nanotubes supplied by Applied Science Ltd (Ohio, USA) were used as received. Table 1 summarizes their main characteristics obtained by transmission electron micro- scopy (TEM), BET surface area measurements and inductively coupled plasma-mass spectrometry (ICP-MS) analysis. High resolution TEM observations have shown that the residual metallic iron (used for the carbon nanotube synthesis) was encapsulated in the graphite layers and thus could not be accessible to the gas phase during the carbon nanofiber synthesis (Fig. 1). Indeed, the ability of iron to produce carbon nanotubes instead of nanofibers is well known in the litera- ture. 3,6 The blank test performed on the pristine carbon nanotubes without nickel showed no activity for carbon nanofibers formation. Table1 Main characteristics of the carbon nanotubes used as catalyst support Mean outer diameter 100–150 nm Mean inner diameter 50–80 nm Length Several dozen micrometers Residual iron concentration 0.2 wt.% BET surface area 10 m 2 g  1 514 Phys. Chem. Chem. Phys., 2002, 4, 514–521 DOI: 10.1039/b106512m This journal is # The Owner Societies 2002