Carbon nanotube supported catalyst is a novel supported catalyst, using carbon nanotubes as the support instead of the conventional alumina or silicon support. 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.
The catalyst is a substance, usually used in small amounts relative to the reactants, that increases the rate of a chemical reaction without itself undergoing any permanent chemical change.IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "catalyst" One or more kinds of catalysts can be loaded on another material with a high surface area, which serves as the support, to form a supported catalyst as a whole system. In a supported catalyst system, the significance of using the support are to increase the dispersion of the active phases, to have a better control of the porous structure, to improve mechanical strength, to prevent sintering and to assist catalysis. There is a wide spectrum of supports ranging from conventional and most commonly alumina to novel various kinds of activated carbon. Synthesis methods and functions vary greatly due to different kinds of support and catalytic materials.
The challenge in making a supported nanoparticulate catalyst is to avoid agglomeration. This can be achieved by using a poly-functional anchoring agent, and drying under a relatively low temperature. Relative research are deposition of palladium and platinum particles on activated carbon, using a poly-acrylate anchor.Hwang, Gan-Lin (2003) "Carbon nanocapsule supported catalysts" 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. Progress is being made in the use of chemical vapor deposition for the synthesis of supported catalysts. Combinatorial techniques have seen their contributions to solid catalyst synthesis.
Background
Catalyst supports and supported catalysts
Catalysts are widely used in various chemical reactions. The activity, stability, selectivity, and regeneration ability are the most important properties to be considered in catalyst design.
The catalyst supports can improve specific properties such as mechanical strength, distribution, stability, catalytical reactivity and selectivity of catalysts. The definition of the support is broad: the shape of support varies, including granular, powdered, colloidal, coprecipitated, extruded, pelleted, spherical, wires, honeycombs, and skeletal supports. Catalyst supports can be either inert or active in reactions. The ensemble of the catalyst and its support can be regarded as an entirety: supported catalyst.
In pre-1940 publications, the supports were only considered as physical carriers on which the catalytic metal or oxide was disposed as broadly and uniformly as possible. But over the years, a better understanding of the cofunctioning of catalysts and their supports has been achieved. It was recognized that the support was actually a promoter in many cases. In Catalysis (Berkman et al. 1940), the difference between a promoter and a support is described as the difference in quantity: when the support exceeds the quantity of the catalyst, it is a support; otherwise it is a promoter. This view was more or less simplistic, but implied the recognition that even at this early year the support was a catalytic component in the broadly construed catalytic composition.
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. Another purpose to load catalytical noble metal on supports is to dilute noble metals in a larger volume. Some supports act as a stabilizer to prevent lower-melting-point materials from agglomeration. Another use of the support was to serve as a reservoir for semi-molten salts.
Many experiments about alumina were conducted at the early period, which helped people to realize that catalysts supported on different species of alumina have different catalytic properties. (4-5-66) "Method of Preparing a Catalyst Composition Consisting of Eta-Alumina and the Product Thereof" (6-1-65) "Method of Preparing a Nickel Oxide Alumina Composition and the Product Thereof" 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. This was called the dual-functioning catalyst and was observed in those hydrodenitrogenation, hydrodesulfurization, and reforming catalysts reactions.
Traditional carbon materials as supports
Carbon is a ubiquitous element that forms millions of compounds, ranging from simple carbon monoxide to highly complex enzymes. 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.
The applications of graphite, carbon black and activated charcoal manufactured annually as catalyst support are relatively few. The major catalytical use of charcoals and carbon blacks is to support metals. In addition, charcoals are sometimes used to support compounds such as sulfides and halides. Some graphite is used to support metals, but the most important feature of graphite is its ability to form intercalates, which are the catalysts for some hydrogenation, dehydrogenation, isomerization, alkylation, hydrodealkylation, polymerization and ammonia formation reactions. For charcoal and carbon black-supported metals with various industrial uses, the methods of manufacture can be divided into three broad groups based on the catalytic metal loaded: wet impregnation, hydrolysis impregnation and chemical vapor deposition (CVD).
Carbon nanotubes as supports
Properties
Many textbooks describe carbon nanotubes (CNTs) in easily understood terms: carbon nanotubes are tubular structures made entirely of rolled-up layers of graphene. The diameters of CNTs ranges from about one nanometer to tens of nanometers, while their lengths can be up to centimeters, much greater than the diameters. In general, the hollow geometry of CNTs leads to large specific surface areas, which makes CNTs extremely attractive supports for heterogeneous catalysts. Another advantage of CNTs is their relatively high oxidation stability which is induced by their structural integrity and chemical inertness. Additionally, CNTs have exceptional physical properties including electrical conductivity, mechanical strength and thermal conductivity, which are important factors for catalyst supports. CNTs can be either metallic or semiconductive, depending on their helicity and diameter, and this property can greatly affect charge transfer processes. 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.
In summary, CNTs are tough and tensile, owning good electron conductivity and chemical inertness, as well as thermal conductivity. Therefore, they become ideal and unique templates for nanoparticle immobilization, which allows the construction of designed nanoarchitectures. This makes CNTs extremely attractive supports for heterogeneous catalysts and related technologies.
Preparation of carbon nanotubes CNTs are generally produced by four main techniques: arc discharge, laser ablation, molten salt intercalation, and chemical vapor deposition. Since as-produced CNTs usually contain a variety of impurities such as graphene fragments, amorphous carbon, fullerenes and metal catalyst particles. 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. CNTs need to be functionalized with different groups before being used in hybrid materials based on their purpose as well. Furthermore, in many applications we require uniform and stable dispersions of CNTs, but pristine single-walled CNTs (SWCNTs) are insoluble in most solvents, leading to aggregation between individual tubes. We also want to separate CNTs based on whether they are semiconducting or metallic.
The production of CNTs has become easier and cheaper by the years, and the quality of as-prepared CNTs has improved since the contaminated impurities are considered. As a result, the major challenge is to develop cheap and facile methods to improve the uniformity in lengths, diameters and chirality of CNTs. CNTs has great potential as an important bridge connecting the molecular realm and the macroscopic world.
Characterization The characterization techniques of CNTs supported catalysts are varied. The most common methods include X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), X-Ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), high-resolution TEM (HRTEM), near-infrared spectroscopy, UV absorption spectroscopy, photoluminescence spectroscopy and fluorescence microscopy. These techniques and methodologies have been employed to not only comprehensively characterize CNT–nanocrystal heterostructures, but also thoroughly investigate the properties of these hybrid materials.
Preparation
To obtain a CNTs supported catalyst, the first step is to load the catalytic materials onto CNTs. A variety of synthesis strategies for the CNT/metal nanoparticle hybrids can be classified as ex situ and in situ techniques.
Ex situ approaches Ex situ approaches utilize different interactions including covalent interactions, noncovalent interactions, π-π stacking and electrostatic interactions.
Covalent interactions are used to attach inorganic nanoparticles with various function group terminals to CNTs treated by acids. One typical example is amide bond. Because of the excellent biological compatibility of Au, they are so frequently used in biosensing, medical and other related fields. Au nanoparticles have been linked to acid treated CNTs by aminothiols, bifunctional thiols or thioether bonds. Hydrophilic metal oxides such as MnO2, MgO, TiO2 and Zr(SO4)2 can be directly attached to the carboxyl groups, averting the use and separation of linking agent. On the other hand, however, the interactions are relatively weak, and the distribution of the nanoparticles are not as uniform.