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

Dry (CO2) reforming of methane (DRM) is a well-studied reaction that is of both scientific and industrial importance. This reaction produces syngas that can be used to produce a wide range of products, such as higher alkanes and oxygenates by means of Fischer–Tropsch synthesis. DRM is inevitably accompanied by deactivation due to carbon deposition. DRM is also a highly endothermic reaction and requires operating temperatures of 800–1000 °C to attain high equilibrium conversion of CH4 and CO2 to H2 and CO and to minimize the thermodynamic driving force for carbon deposition. The most widely used catalysts for DRM are based on Ni. However, many of these catalysts undergo severe deactivation due to carbon deposition. Noble metals have also been studied and are typically found to be much more resistant to carbon deposition than Ni catalysts, but are generally uneconomical. Noble metals can also be used to promote the Ni catalysts in order to increase their resistance to deactivation. In order to design catalysts that minimize deactivation, it is necessary to understand the elementary steps involved in the activation and conversion of CH4 and CO2. This review will cover DRM literature for catalysts based on Rh, Ru, Pt, and Pd metals. This includes the effect of these noble metals on the kinetics, mechanism and deactivation of these catalysts.

A review of dry (CO2) reforming of methane over noble metal catalysts

Lithium- or potassium-doped carbon nanotubes can absorb approximately 20 or approximately 14 weight percent of hydrogen at moderate (200 degrees to 400 degrees C) or room temperatures, respectively, under ambient pressure. These values are greater than those of metal hydride and cryoadsorption systems. The hydrogen stored in the lithium- or potassium-doped carbon nanotubes can be released at higher temperatures, and the sorption-desorption cycle can be repeated with little decrease in the sorption capacity. The high hydrogen-uptake capacity of these systems may be derived from the special open-edged, layered structure of the carbon nanotubes made from methane, as well as the catalytic effect of alkali metals.

https://science.sciencemag.org/content/sci/285/5424/91.full.pdf

High H2 uptake by alkali-doped carbon nanotubes under ambient pressure and moderate temperatures

Solid Oxide Fuel Cells (SOFCs) are one of the most promising technologies for future efficient conversion of the chemical energy stored in fuels to electrical energy. One of the primary advantages of SOFCs is the potential to operate with a wide variety of fuels. While H2 is the fuel of choice for most fuel cells, operation with fossil-derived and bio-derived hydrocarbon fuels would bypass the costly requirement of a new H2 infrastructure, and accelerate adoption of fuel cell technology. This is feasible as SOFCs transport oxygen anions from the air electrode (cathode) to the fuel electrode (anode). The primary barrier to the realization of fuel flexible SOFCs is the anode material set. Traditional SOFC anodes are based on Ni composites. While these are very efficient for H2 and CO fuels, Ni catalyzes graphite formation from dry hydrocarbons, leading to rapid degradation of cell performance and possible mechanical failure. This motivates the development of new anode materials and composites. The principle requirements of an anode are oxygen anion conductivity, electronic conductivity, and electrocatalytic activity toward the desired reaction. This entry reviews the primary issues in direct hydrocarbon anode development and discusses the new materials and composites that have been developed to meet this challenge.

Direct hydrocarbon solid oxide fuel cells.

Catalyst deactivation is usually inevitable, although the rate at which it occurs varies greatly. This article discusses the causes of deactivation and the influence on reaction rate. Methods for minimising catalyst deactivation, by tailoring catalyst properties and/or process operations, are presented, as well as reactor configurations suitable for the regeneration of deactivated catalysts. Alkane dehydrogenation is used as an example to demonstrate the variety of engineering solutions possible.

Catalyst deactivation: is it predictable?: What to do?

Growth of carbon nanotubes by catalytic decomposition of CH4 or CO on a NiMgO catalyst

Two kinds of carbon nanotubes grown catalytically, as a novel material for catalyst carrier, were prepared and characterized. Propene hydroformylation catalyzed by the Rh–phosphine complex catalysts supported by carbon nanotubes was investigated, and compared to that catalyzed by the Rh–phosphine complex catalysts supported by SiO 2 (a silica gel), TDX-601 (a carbon molecular sieve), AC (an active carbon), and GDX-102 (a polymer carrier). Activity assay of the catalysts showed that the carbon nanotubes-supported Rh–phosphine complex catalysts displayed not only high activity of propene conversion but also excellent regioselectivity to the product butylaldehyde. Under the reaction conditions of 393 K, 1.0 MPa, C 3 H 6 /CO/H 2  = 1/1/1 (v/v), GHSV = 9000 ml (STP) h −1  (g catal.) −1 , P/Rh = 9–12 (molar ratio), and Rh-loading at 0.1 mmol Rh (g carrier) −1 , the molar ratio of normal/branched ( n / i ) aldehydes reached 12–13 at a turnover frequency (TOF) of 0.12 s −1 , corresponding to propene conversion of ∼32%. The characterization by using TEM, HRTEM, XRD, Raman, XPS, BET and temperature programmed desorption (TPD) methods indicated that the carbon nanofibers prepared were quite even nanotubes with the outer diameters at 15–20 nm and the inner diameters (i.e., pore diameters) at ∼3 nm. Each tube wall was constructed of many layers of carbon with graphite-like platelets in a cross-section orientation of ‘parallel type’ or ‘fishbone type’; their C (1 s) electron binding energy was about 0.5 eV lower than that of graphite. These results, together with the results of comparative studies of the Rh–phosphine complex catalysts supported by several other carriers, implied strongly that the tubular channels with the inner diameter of ∼3 nm in the carbon nanostructures and its hydrophobic surface consisting of six-membered C-rings played important roles in enhancing the activity of propene hydroformylation, especially the regioselectivity of butylaldehyde on the Rh–phosphine complex catalysts supported by them.

Preparation, characterization and catalytic hydroformylation properties of carbon nanotubes-supported Rh–phosphine catalyst

Addition of small amount of trivalent-metal oxides, Cr 2 O 3 and La 2 O 3 , to a Ni Mg O (Ni/Mg=1/1, mol/mol) catalyst for partial oxidation of methane (POM) and CO 2 -reforming of methane (MCR) reactions has been found to improve the performance of the catalyst for coking-resistance. The POM operation at 1053 K for 50 h, or the MCR operation at 1100 K for 6 h, did not leave any detectable amount of carbon deposit on the surface of the catalyst. Studies of XRD, XPS, and H 2 -TPR spectroscopies showed that the doping of small amounts of Cr 3+ and La 3+ to the Ni Mg O system led to the formation of a host-dopant-type Ni Mg Cr La O solid solution, with a considerable number of Schottky defects in the form of cationic vacancies. An increase in the degree of disorder in the solid solution due to Cr 2 O 3 and La 2 O 3 dissolved in Ni x Mg 1− x O lattice would be expected to enhance the mobility of the lattice oxygen anions. This would be in favor of speeding up the reaction between the carbon-containing species and reactive oxygen species via migration of the lattice O 2− so as to inhibit the deposition of carbon on the surface of the catalyst. On the other hand, part of the Schottky defects in the form of cationic vacancies may diffuse to the surface, where Ni + -species can be well accommodated and stabilized, thus, forming a rich-in-Ni (with mixed valence states) surface layer. As a result, the proportion of the reducible Ni-species was pronouncedly increased, but the temperature for their reduction was considerably raised, so that the surface Ni-species were maintained with higher possibility in positive valence states under POM and MCR reaction conditions. This would, to some extent, lead to the reduction of the rate of deep dehydrogenation of methane to carbon, therefore tending to reduce, if not avoid, coking caused by an excess of carbon on the surface.

Development of coking-resistant Ni-based catalyst for partial oxidation and CO2-reforming of methane to syngas

The catalyst consists of at least one transition metal oxide and one alkali earth or RE metal oxide in the contents of 1.0-7.0% and 30.0-99.0% separately. During the preparation of nanometer carbon pipe, methane of air speed 600-6,000/hr or carbon monoxide of air speed 400-4,000/hr is led into pre-reduced catalyst at 573-1073K, and the product afte reaction for 10-40 min is collected, acid soakedand dried to obtain pure carbon nanometer pipes with outer diameter of 10-50nm. The present invention has simple operation and low cost.

Ping Chen

Papers

Growth of carbon nanotubes by catalytic decomposition of CH4 or CO on a NiMgO catalyst

Preparation, characterization and catalytic hydroformylation properties of carbon nanotubes-supported Rh–phosphine catalyst

Development of coking-resistant Ni-based catalyst for partial oxidation and CO2-reforming of methane to syngas

Transition metal catalyst and its use method in preparing uniform-caliber nanometre carbon pipe