Carbon nanotubes reactive sites

Section I reviews the new concepts and applications of nanotechnology for catalysis. Chapter 1 provides an overview on how nanotechnology impacts catalyst preparation with more control of active sites, phases, and environment of actives sites. The values of catalysis in advancing development of nanotechnology where catalysts are used to facilitate the production of carbon nanotubes, and catalytic reactions to provide the driving force for motions in nano-machines are also reviewed. Chapter 2 investigates the role of oxide support materials in modifying the electronic stmcture at the surface of a metal, and discusses how metal surface structure and properties influence the reactivity at molecular level. Chapter 3 describes a nanomotor driven by catalysis of chemical reactions. 

To summarize, one can say that the electrochemical performance of CNT electrodes is correlated to the DOS of the CNT electrode with energies close to the redox formal potential of the solution species. The electron transfer and adsorption reactivity of CNT electrodes is remarkably dependent on the density of edge sites/defects that are the more reactive sites for that process, increasing considerably the electron-transfer rate. Additionally, surface oxygen functionalities can exert a big influence on the electrode kinetics. However, not all redox systems respond in the same way to the surface characteristics or can have electrocatalytical activity. This is very dependent on their own redox mechanism. Moreover, the high surface area and the nanometer size are the key factors in the electrochemical performance of the carbon nanotubes. 

Palladium metal particles with an average diameter of ca. 5 nm were homogeneously dispersed inside carbon nanotubes. Such nanostructured material was an extremely active and selective catalyst for the hydrogenation of the C=C bond of cinnamaldehyde. The high external surfece area of the carbon nanotubes could explain the high reactivity of the catalyst despite its relatively low specific surfece area, i.e. 20 m. g". On the other hand, the high selectivity towards the C=C bond hydrogenation was attributed to the absence of a microporous network and of residual acidic sites in the carbon nanotube catalyst as compared to a commercial activated charcoal. 

Banks CE, Davies TJ, WUdgoose GG, Compton RG (2005) Electrocatalysis at graphite and carbon nanotube modified electrodes edge-plane sites and tube ends are the reactive sites. Chem Commim 7 829-841... 

Stability of, for example, (10,10)-nanotxibes, but kinetic reasons contribute as well to the effect. In achiral nanotubes, and especially in those of the armchair type, the replacement of metal atoms by carbon is much easier due to the orientation of the lattice structure (see below). What is more, several SWNTs at a time wiU emerge from very reactive sites. With the prevaiUng temperature being constant for all tubes nucleating in these zones, their respective diameters wiU also be more or less the same. Consequently, they may form a symmetric packing, which is why the most stable bundles are observed in these cases. 

For carbon nanotubes just as well as for the fuUerenes discussed in Chapter 2, there is a direct correlation between their structure and their respective reactivity. Further important conclusions can be drawn from the relation to graphite. Considering a nanotube quickly reveals that there are three clearly distinct sites available for a chemical reaction (Figure 3.62) the most reactive tips, the outer side... 

Furthermore, there are interactions between neighboring walls within a multi-walled tube that cause an additional stabilization. Still it is no serious problem to derivatize multiwalled carbon nanotubes as well. In doing so, one takes advantage of the increased reactivity at the tubes ends and of the vulnerability of defect sites in the outer wall of the MWNT. 

Compared to the bromination, the chlorination of carbon nanotubes should be easier due to the higher reactivity of chlorine. This effect is observed indeed. However, the chlorination chiefly occurs at the ends of the tubes or at defects of the side wall. The direct reaction of the side wall itself has not yet been described as a preparative method. Still the chlorination at defect sites provides materials with modified characteristics as well. 

Within the bundles of single- or multiwalled carbon nanotubes, cavities exist that may be filled with atoms or molecules. It has already been mentioned in the introductory chapter on the reactivity of carbon nanotubes that different sites are available for this intercalation. In detail, these are the channels in between the tubes, the interstice between individual layers in a multiwalled nanotube, and the central cavity of the tube. Examples of all three types have been proven by experiment (Figure 3.93). 

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