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

Carbon nanotubes are promising materials for various applications. In recent years, progress in manufacturing and functionalizing carbon nanotubes has been made to achieve the control of bulk and surface properties including the wettability, acid–base properties, adsorption, electric conductivity and capacitance. In order to gain the optimal benefit of carbon nanotubes, comprehensive understanding on manufacturing and functionalizing carbon nanotubes ought to be systematically developed. This review summarizes methodologies of manufacturing carbon nanotubes via arc discharge, laser ablation and chemical vapor deposition and functionalizing carbon nanotubes through surface oxidation and activation, doping of heteroatoms, halogenation, sulfonation, grafting, polymer coating, noncovalent functionalization and nanoparticle attachment. The characterization techniques detecting the bulk nature and surface properties as well as the effects of various functionalization approaches on modifying the surface properties for specific applications in catalysis including heterogeneous catalysis, photocatalysis, photoelectrocatalysis and electrocatalysis are highlighted.

Carbon nanotube catalysts: recent advances in synthesis, characterization and applications.

Intermetallic catalyst for carbon nanotubes (CNTs) growth by thermal chemical vapor deposition method

Metal-supported carbon-based materials: opportunities and challenges in the synthesis of valuable products

The feasibility of multi-walled carbon nanotubes (MWCNTs) as support to NiMo catalysts for hydrotreating of gas oil derived from Athabasca bitumen was tested in a trickle bed reactor at industrial conditions. High quality MWCNTs were prepared by CVD method using ferrocene as catalyst and toluene as carbon source. The produced MWCNTs were characterized by XRD, TEM, TGA and Raman spectroscopy in order to reveal the morphological and structural characteristics. Using functionalized MWCNTs as support, NiMo catalysts were prepared with varying Ni and Mo content by pore filling impregnation method. The calcined NiMo/MWCNTs catalysts were characterized by ICP-MS, N 2 adsorption, XRD and TPR and the sulfide form of the catalysts was examined by DRIFT spectroscopy of adsorbed CO. The XRD patterns confirm the enhanced dispersion of MoO 3 particles when increasing the Ni content from 0 to 4.5 wt.% over 12 wt.% Mo/MWCNTs. The TPR profiles indicate the two step reduction characteristics of Mo 6+ to Mo in lower oxidation state such as Mo 4+ and Mo 0 . The promoted and unpromoted MoS 2 sites were clearly differentiated with the help of DRIFT of adsorbed CO over sulfided catalysts. The number of Ni promoted MoS 2 (NiMoS phase) sites is increased significantly with increasing Ni addition up to 3 wt.% over 12 wt.% Mo/MWCNTs. The HDN and HDS activities of sulfided NiMo/MWCNTs using bitumen derived light gas oil were carried out at different temperatures under industrial condition. The HDN and HDS activities of the catalysts increased with increasing Ni content up to 3 wt.% and Mo content up to 12 wt.%. Based on weight of the catalyst, the HDN and HDS activities of 3 wt% Ni–12 wt.% Mo/MWCNTs are significantly higher than those over conventional Al 2 O 3 -based catalyst under the experimental conditions studied. The introduction of 2.5 wt.% P to MWCNTs-based catalyst found to show a fall in hydrotreating activity.

Application of multi-walled carbon nanotubes as efficient support to NiMo hydrotreating catalyst

Carbon nanotubes (CNT) were used as support medium to prepare Co catalysts for the selective dehydrogenation of cyclohexanol. Due to the unique structure and electronic property of CNT, Co/CNT possessed less acid sites than Co/activated carbon (AC). It seems to be easier to form multi-point complexes on Co/CNT than on Co/AC, resulting in a little higher selectivity to cyclohexanone and different distribution of by-products. The effect of K addition, attributed to electronic promotion, was also stronger on Co-K/CNT than on Co-K/AC.

Weixiang Chen

Papers

Cyclohexanol dehydrogenation over Co/carbon nanotube catalysts and the effect of promoter K on performance