Carbon nanotube supported catalyst
The exceptional physical properties of carbon nanotubes (CNTs) such as large specific surface areas, excellent electron conductivity incorporated with the good chemical inertness, and relatively high oxidation stability makes it a promising support material for heterogeneous catalysis.
[5] To unveil more molecular details of the extensive interactions between precursors and supports in an aqueous environment, studies of adsorption and precipitation chemistry must be taken into account.
An early purpose of the support was to obtain a solid granular material coated with catalytic component, providing a hard and stable structure to withstand disintegration under gas or liquid flows.
[6][7] During the same time frame, it was noticed that the catalyst and the support were cooperating in some cases to produce two simultaneous and mutually beneficial reactions.
Regarding to its elemental form, although there is no catalytic properties ascribed to diamond, graphite is known to be an active catalyst in some oxidation reactions.
Graphitic carbon is also used as a support material where other catalytic components may be dispersed, resulting in an increase of the surface area they expose to the chemical reactants.
CNTs possess a very large Young's modulus, as well as a great tensile strength, and their flexibility property makes them an ideal component for applications in composite materials.
CNTs also have good thermal conductivity, which helps to prevent the agglomeration and growth of small nanoparticles during post-annealing treatments, and stabilize newly formed phases.
[16] CNTs are generally produced by four main techniques: arc discharge, laser ablation, molten salt intercalation, and chemical vapor deposition.
[17] Since the impurities interfere with most of our desired properties and influence biocompatibility of CNTs, impairing the catalytical performance and limiting the application, they need to be purified and separated.
As a result, the major challenge is to develop cheap and facile methods to improve the uniformity in lengths, diameters and chirality of CNTs.
[23] Hydrophilic metal oxides such as MnO2,[24] MgO,[25] TiO2[26] and Zr(SO4)2[27] can be directly attached to the carboxyl groups, averting the use and separation of linking agent.
The surfactant sodium dodecylsulfate (SDS) is widely used to attach diverse nanoparticles including Pt,[28] EuF3, TbF3[29] and SiO2[30][31] to multi-walled carbon nanotubes (MWCNTs).
In another approach which utilizes hydrophobic capping agents, for instance, octanethiols[32] and dodecanethiols,[33][34] both coverage and morphology of the hybrid materials can be well controlled by modifying the length and functional groups of the chains.
A similar route is to make use of the delocalized π electrons of CNTs as well as those in aromatic organic compounds containing polar group terminated alkyl chains.
In another simple and facile approach where electrostatic interactions are utilized, ionic polyelectrolytes are deposited on CNTs so as to attract charged nanoparticles.
This is a cheap technique avoiding the requirement of high temperature, enabling fine controls in chemical composition as well as lowest concentration of dopants.
But it also shows the weakness that the product will typically contain an amorphous phase, thus crystallization and post-annealing steps are required and increase the complexity of preparation.
Many of these preparations suffer from deposition scarcity, unwanted large size or aggregates of catalyst particles even at a relatively low loading content.
On the other hand, in order to decorate CNTs with catalytic particles, a functionalization process is generally required beforehand: this makes the preparation more complex and increases the cost.
Although at an early stage of research, CNTs supported metal-nanoparticle catalysts as transition metals Ru, Co, Ag, Pt, Pd, and Au shed new light to catalysis reactions in many fields such as batteries, flat panel displays, and chemical sensors.
For example, catalytic hydrogenation of CO2 to produce methanol has been considered as one of the most economical and effective ways to chemically fix huge amount of emitted CO2 and also to improve climate conditions.
Among the many electrocatalysts, Pt enjoys high electrocatalytic efficiency and has been proved to be the most effective catalysts for alcohol oxidation reactions.
Bimetallic catalysts including Pt and a second precious or non-precious metal (like Ru, Rh, Sn, Pb, Sb, Ni, etc.)
are often applied to enhance the electrochemical activity of Pr and at the same time avoiding its deactivation when exposed to poisoning intermediates by the bifunctional or ligand mechanisms.
Besides, CNTs not only enjoy a highly electrochemically accessible surface area but can also offer a remarkable electronic conductivity due to its multi-wall structure, which properties render it a competitive electrocatalyst support for Pt-catalyst.
[74] Among them, Co catalysts are preferred because of their high activity and selectivity to linear hydrocarbons for FTS, more stable, and low cost compared to Ru.
Activated carbon has many advantages, such as resistance to acidic or basic media, stable at high temperatures, etc., serving as FTS catalyst support.
