This review analyses the literature from the early 1990s until the beginning of 2003 and covers the use of carbon nanotubes (CNT) and nanofibers as catalysts and catalysts supports. The article is composed of three sections, the first one explains why these materials can be suitable for these applications, the second describes the different preparation methods for supporting metallic catalysts on these supports, and the last one details the catalytic results obtained with nanotubes or nanofibers based catalysts. When possible, the results were compared to those obtained on classical carbonaceous supports and explanations are proposed to clarify the different behaviors observed.

Carbon nanotubes and nanofibers in catalysis

The influence of cobalt particle size in the range of 2.6-27 nm on the performance in Fischer-Tropsch synthesis has been investigated for the first time using well-defined catalysts based on an inert carbon nanofibers support material. X-ray absorption spectroscopy revealed that cobalt was metallic, even for small particle sizes, after the in situ reduction treatment, which is a prerequisite for catalytic operation and is difficult to achieve using traditional oxidic supports. The turnover frequency (TOF) for CO hydrogenation was independent of cobalt particle size for catalysts with sizes larger than 6 nm (1 bar) or 8 nm (35 bar), while both the selectivity and the activity changed for catalysts with smaller particles. At 35 bar, the TOF decreased from 23 x 10(-3) to 1.4 x 10(-3) s(-1), while the C5+ selectivity decreased from 85 to 51 wt % when the cobalt particle size was reduced from 16 to 2.6 nm. This demonstrates that the minimal required cobalt particle size for Fischer-Tropsch catalysis is larger (6-8 nm) than can be explained by classical structure sensitivity. Other explanations raised in the literature, such as formation of CoO or Co carbide species on small particles during catalytic testing, were not substantiated by experimental evidence from X-ray absorption spectroscopy. Interestingly, we found with EXAFS a decrease of the cobalt coordination number under reaction conditions, which points to reconstruction of the cobalt particles. It is argued that the cobalt particle size effects can be attributed to nonclassical structure sensitivity in combination with CO-induced surface reconstruction. The profound influences of particle size may be important for the design of new Fischer-Tropsch catalysts.

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Cobalt particle size effects in the fischer- : Tropsch reaction studied with carbon nanofiber supported catalysts

Carbon nanotubes as adsorbents in environmental pollution management: A review

Characterization of surface oxygen complexes on carbon nanofibers by TPD, XPS and FT-IR

The thermal stability and the reducibility of oxygen-containing functional groups on the surface of nitric acid-treated multiwalled carbon nanotubes (CNTs) have been studied using temperature-programmed desorption and reduction (TPD and TPR) and high-resolution X-ray photoelectron spectroscopy (XPS). The thermal treatments up to 720 °C were carried out in the XPS setup, either under ultrahigh vacuum (UHV) or in diluted hydrogen. Deconvoluted XP spectra were used for the quantitative determination of the amount of the different functional groups on the CNT surfaces as a function of the pretreatment. The number of the oxygen atoms per unit surface area was obtained from the oxygen to carbon (O/C) ratio derived from the corresponding peak areas in the XP spectra. The results obtained by XPS agree quantitatively with the observations by TPD and TPR. The acid treatment not only introduced carboxyl, carbonyl, and phenol groups on the surface but also generated ether-type oxygen groups between the graphitic laye...

Thermal Stability and Reducibility of Oxygen-Containing Functional Groups on Multiwalled Carbon Nanotube Surfaces: A Quantitative High-Resolution XPS and TPD/TPR Study

Jürgen M.P van Heeswijk

Papers

The influence of oxidation on the texture and the number of oxygen-containing surface groups of carbon nanofibers