New efficient Fe2O3 and FeMo supported OMCVD catalysts for single wall carbon nanotubes growth
Introduction
Nowadays, carbon nanotubes have become a strategic material in the area of nanotechnologies. Among the different kind of filamentous carbon, single wall nanotubes (SWNT), multi wall nanotubes (MWNT) and graphite nanofibers (GNF) can be distinguished [1]. SWNT are often considered as perfect 1D nanostructures so that their peculiar properties have been intensively studied by physicists and chemists, and numerous potential applications have been proposed [1]. A key point for carbon nanotubes industrial manufacture still remains their low cost and large scale production. As far as the synthesis processes are concerned, catalytic chemical vapor deposition (C-CVD) seems to be the most promising technique in view of an industrial scale production. In that process the nanotubes growth involve the use of aggregates of metals or semi-metals that act as germs for carbon precipitation. Even if it does not fit stricto sensu with the exact definition of a catalyst, these germs are usually called “the catalyst”.
Currently, GNF [2], [3] and MWNT [4], [5], [6] can be produced selectively by C-CVD on a large scale. Consequently, even if they do not present the same performances than SWNT, mainly due to the presence of structural defects, these products should be the first ones to appear on the market. Indeed, in the case of SWNT, catalytic systems performances in terms of selectivity and activity are still relatively poor. One of the main challenge for the catalytic growth of SWNT is the control of the catalyst nanoparticles size distribution during the high temperatures (800–1200 °C) process. Indeed, several studies have shown that SWNT grow on catalysts with small size particles (typically <10 nm) and narrow particles size distribution [7]. In order to obtain well dispersed and active catalysts for SWNT synthesis, different strategies have been followed, including the use of colloidal solutions of monodispersed nanoparticles [7], catalyst preparation by coprecipitation techniques when nanoparticles are produced during the high temperature treatment [8], and the use of bimetallic catalysts [9]. In this latter case, addition of an excess of molybdenum to cobalt catalysts allows the stabilization of active nanoparticles.
Chemical vapor deposition is today a well established technique for the preparation of highly dispersed supported metal catalysts [10]. However, only scarce reports described the use of CVD catalysts for carbon nanotubes growth [11], [12]. Thus, MWNT were produced from acetylene on FeMo/Al2O3 catalysts [11]; in that case [Fe(CO)5] and [Mo(CO)6] were used as catalyst precursors. We have also reported a very active and selective Fe/Al2O3 catalyst prepared from [Fe(CO)5] for MWNT growth from ethylene [12]. In order to grow SWNT from methane on CVD catalysts we have decided to follow two strategies : the control of bimetallic systems or the definition of optimal CVD conditions in order to obtain well dispersed monometallic systems. In this work, we report for the first time results dealing with SWNT growth by C-CVD on catalysts produced by fluidized bed OMCVD.
Section snippets
Catalyst preparation
CAUTION: [Fe(CO)5] is a highly toxic compounds, all manipulation should be carried out under a well ventilated fume-hood.
Fluidized bed CVD of iron and molybdenum on γ-Al2O3 (158 m2 g−1, Aldrich) starting from iron pentacarbonyl [Fe(CO)5] and molybdenum hexacarbonyl [Mo(CO)6] was performed in a specially design glass vessel (Fig. 1). For FeMo/Al2O3 systems, [Mo(CO)6] was introduced as a powder in the sublimator and the liquid precursor [Fe(CO)5] into a bubbler. A mass of grains of Al2O3 was poured
Results and discussion
Three types of supported CVD catalysts have been prepared (Table 1): (i) monometallic Fe/Al2O3 and Mo/Al2O3 catalysts (type I), (ii) bimetallic FeMo/Al2O3 catalysts with different Fe:Mo weight ratio (type II) and (iii) monometallic iron oxide catalysts (type III) prepared by introduction of oxygen during the iron pentacarbonyl decomposition step. It is important to note that in order to reach an homogeneous deposition on the whole surface of the support grains a careful sieving of the alumina
Conclusion
In the present paper we report for the first time the use of OMCVD catalysts for the selective growth of single wall carbon nanotubes. We demonstrate that monometallic Fe/Al2O3 and Mo/Al2O3 systems are not active for this reaction and that the SWNT synthesis requires the production of small iron particles from the reduction of an highly dispersed oxide phase. Such an highly dispersed oxide phase has been obtained either from oxidation of bimetallic FeMo catalysts or, more easily from the one
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