According to theoretical predictions, the geometry of the ring structures imparts either a metallic or semi-metallic nature to CNTs 48 , This naturally has an effect on the properties of metals loaded onto the support. Duca et al. Their findings showed that Pd 9 clusters may have a stronger interaction with a CNT than with traditional flat graphite sheet bilayers due to the curvature of the nanotube which has an effect on metallic properties. The high mechanical strength of CNTs makes them not only favourable in composites 47 , 48 , but also in mechanically taxing catalytic environments such as vigorously stirred liquid phase reactions.
On more conventional supports, high levels of friction may lead to attrition, effectively altering the surface area of the exposed metal catalyst Moreover, the relatively lower mechanical stability of common commercial carbon-based supports initiates the formation of fine particulates during operation. The existence of a large amount of micropores can reduce the full accessibility of the reactant particles to the active site.
The high strength of CNTs results from the covalent sp 2 bonds, which form a honeycomb lattice between the individual carbon atoms. Carbon supports are often unreactive and stable in many acidic and basic media, and consequently find use in a wide range of industrial applications 3 , 53 while many other supports are rendered useless after reaction.
However, traditional carbon-based supports have limitations not only when it comes to mechanical stability but also in terms of resistance to oxidising atmospheres at high temperatures Chiang et al. However, impurities in carbonaceous supports, such as metal residues from CVD processes, as well as defects in the structure, may not only limit the temperature at which hydrogenation takes place, but may also poison the active metal catalyst, such as Pd, and lower the temperature at which carbon oxidises 4.
Temperature can affect the performance of the catalyst on the carbon support and more research is needed to explore this aspect. Microwave irradiation was used to drive the hydrogenation of cinnamate esters with ammonium formate as a reductant. Liang et al. The high conversion and selective formation of methanol from CO 2 by the MWCNT-supported catalysts was due to their favourable ability to reversibly adsorb a greater amount of hydrogen, thus increasing the rate of surface hydrogenation reactions.
SWCNTs, which are inherently microporous, are considered by some to be very suitable supports for metal catalysts. The SWCNT structure is such that every atom is exposed to not one but two surfaces — both the inner and outer surfaces of the nanotube.
Table I shows the approximate surface areas of various carbon supports. In the case of AC, the micropores are large in quantity and their size may actually slow the progress of substrate molecules into the pores The ACs can have macro-, meso- and micropores, which can decrease the reproducibility of metal loading. The authors of that study suggest that this is due to the larger pore volumes and surface area of the AC, and thus the correspondingly larger number of sites on which nucleation can occur.
A much researched model compound of this class of aldehydes is cinnamaldehyde. Pd's selectivity for cinnamyl alcohol is low when compared to Pt or ruthenium, however, using CNTs as supports seems to provide advances in overcoming this challenge. Corma et al. This was thought to be due to the larger variety of functional groups on AC. MWCNTs are inherently mesoporous structures. The pore sizes of MWCNTs allow for diffusion, reaction and desorption of chemical species and thus are good supports for catalysts These supports sometimes give better activity than microporous supports such as AC.
Janowska et al. The explanation given was the lack of micropores as well as the high surface area of the CNTs. Lack of micropores affects the time spent by reactants and products on the support surface as well as affecting the manner in which desorption occurs. Also, Pd crystallites and CNTs may interact to alter the adsorption and selectivity properties.
In this work it was shown that at a higher hydrogen flow rate, the rate of hydrogenation increased, but more so for the AC support than for the MWCNTs. The AC-supported catalyst showed an equal selectivity for both products. Neither catalyst showed any selectivity for cinnamyl alcohol. It was concluded that the MWCNT-supported catalyst was superior in terms of selectivity for hydrocinnamaldehyde and that the MWCNTs had a higher surface area some micropores on the traditional support being inaccessible to some substrates , resulting in a higher catalytic activity.
Zhang et al. An intermediate loading 8. This was despite the fact that the specific surface area of the MWCNTs was lower than those of the zeolite and AC as determined after acid treatment. It was also shown that diluted Pd salt solutions yielded smaller Pd particles inside tubes and more concentrated solutions yielded larger particles, which would influence catalytic rates.
Both the surface area:volume ratio and thus catalytic activity were higher for the CNF-supported catalyst. This was assumed to be due to the large number of micropores on AC leading to diffusion problems which affect the catalytic rate. This cannot occur in CNFs because they have no micropores. The AC catalyst produced a mixture of all products.
However, the nature of the carbon support was shown to have negligible effect on the hydrogenation of a variety of polar aromatic compounds in a recent article by Anderson et al. They showed that the nature of the solvent is a bigger factor in selectivity. The high price of precious metals makes the recovery and reuse of such catalysts economically important. Carbon supports make recovery more economical compared to metal oxide supports as they can be burnt off the metal catalyst and the catalyst recovered from a small volume of ash.
The solid waste is minimal. The metal can then be dissolved and recovered from acidic solutions. Oxidation of AC over long periods of time can decrease its usefulness as a support 3.
The inertness of CNTs, on the other hand, enables them to resist oxidation over long periods of time and allows them to be recycled many times. CNT-metal hybrids can be dispersed to a fairly uniform degree by stirring in organic solvents. Hence, as a support, CNTs are economical and stable when it comes to the processes of recovery and reuse of catalysts.
Common loading techniques for Pd include impregnation 42 , 43 , 64 , deposition precipitation 69 , electroless deposition 70 , 71 and electrochemical deposition 72 , Less commonly used methods include CVD 74 and microemulsion techniques Supercritical CO 2 has been used instead of conventional solvents to effect a greener approach to Pd loading techniques One of the main disadvantages of carbon nanotubes is the lack of solubility in aqueous media, and to overcome this problem, scientists have been modifying the surface of CNTs, i.
Its molecules are made up of 60 carbon atoms joined together by strong covalent bonds. Molecules of C 60 are spherical. There are weak intermolecular forces between molecules of buckminsterfullerene. These need little energy to overcome, so buckminsterfullerene is slippery and has a low melting point. Over the past several decades, carbon materials have been regarded as promising lubricating materials due to their versatile structures, and superior mechanical, thermal, electrical, and chemical properties.
Carbon nanotubes have emerged as unique carbon allotropes that bear very interesting prospects in catalysis. Their use is mostly related to that of supports for inorganic metal catalysts, including molecular catalysts, metal nanoparticles, metal oxides or even more complex hierarchical hybrids.
Understanding graphene Graphene is a single layer monolayer of carbon atoms, tightly bound in a hexagonal honeycomb lattice. It is an allotrope of carbon in the form of a plane of sp2-bonded atoms with a molecular bond length of 0. Carbon nanotubes were going to change the world. The ball structure means that this type of fullerene structure can be used to carry small molecules. This can be used to carry medical drugs into the body. Graphene and fullerenes Nanoparticles have very small sizes and large surface area to volume ratios.
Graphene Graphene is an allotrope of carbon. Fullerenes A fullerene is a molecular allotrope of carbon. Nanotubes A nanotube resembles a layer of graphene, rolled into a tube shape. Nanotubes can be several millimetres long but only a few nanometres wide Nanotubes can be added to other materials, for example in sports equipment, to make them stronger.
Jump to site search. You do not have JavaScript enabled. Please enable JavaScript to access the full features of the site or access our non-JavaScript page. Issue 8, Carbon nanotubes and catalysis: the many facets of a successful marriage. Marchesan , a M. Prato a and P. Fornasiero a. You have access to this article. Please wait while we load your content
0コメント