Carbon nanofiber growth

Atomic-scale Monitoring of Carbon Nanofiber Growth in Steam Reforming Catalysts... 

Examples of HRTEM movies of the carbon nanofiber growth from Reference (52) can be found at http //www.haldortopsoe.com/site.nsf/pages/nanotechnology. 

Abild-Pedersen F, Norskov JK, Rostrup-Nielsen JR, Sehested J, Helveg S (2006) Mechanisms for catalytic carbon nanofiber growth studied by ab initio density functional theory calculations. Phys Rev B 73... 

Celebi, S., Nijhnis, T., van der Schaaf, J., et al. (2011). Carbon Nanofiber Growth on Carbon Paper for Proton Exchange Membrane Enel, Carbon, 49, pp. 501—507. 

Fig. 7. Growth mechanism of graphitic carbon nanofibers. The illustration highlights the observation of spontaneous nickel step edge formation at the carbon-nickel interface. The observations in Reference (52) are consistent with a growth mechanism involving surface transport of carbon and nickel atoms along the graphene-nickel interface.

As reported elsewhere [22], similar to those found on other catalysts, the forms of carbon materials deposited on Fe-loading zeolite molecular sieves are carbon nanotube, carbon nanofiber and amorphous carbon. One obvious phenomenon of the carbon nanotubes formed on Fe/NaY or Fe/SiHMS catalysts is that almost all tips of these tubes are open, indicating the interaction between catalyst particles and supports is strong [23]. On the other hand, the optimal formation time of carbon nanotubes on Fe/SiHMS is longer than that on Fe/NaY. However, the size of carbon nanotubes is easily adjusted and the growth direction of carbon nanotubes on the former is more oriented than on the latter. 

Four different aryldiazonium salts have been used to functionalize SWCNTs through electrochemical reduction. By XPS and Raman diffusion measurements, the growth of aryl chains on the sidewalls of the nanotubes was observed [178]. Electrically addressable biomolecular functionalization of SWCNT electrodes and vertically aligned carbon nanofiber electrodes with DNA was achieved by elec-trochemically addressing (reduction) of nitrophenyl substituted nanotubes and nanofibers. Subsequently, the resulting amino functions were covalently linked to DNA forming an array of DNA-CNT hybrid nanostructures (Scheme 1.28) [179],... 

In the elementary reactions of the pyrolysis, the atomic carbon is formed first. Then it transforms into the final product, whether it be soot, graphite, carbon nanofibers, or so forth. Why does the presence of catalysts make it possible to grow carbon nanofibers or nanotubes instead of soot In many cases, this is the so called carbide cycle that is characteristic of the catalytic process of hydrocarbon pyrolysis that is responsible for the growth of the elongated structures but not soot particles. The primary car bon atoms produced by pyrolytic decomposition of the hydrocarbon molecules are dissolved in the metal particle of the active catalyst compo nent to form a nonstoichiometric carbide (the carbon solution in the... 

Figure 5.2 A schematic of the mechanism of the nanofilaments growth at catalytic pyrolysis of hydrocarbons. Hydrocarbon decomposition on the metal nanoparticle (the dark area) surface produces chemisorbed atomic carbon Cg species with a high chemical potential. In the (pseudo)fluidized catalyst particle, the atomic carbon is capable of diffusing through the metal nanoparticles toward the interphase boundary between the active component and the growing face of the carbon nanofiber (the light areas).

The reaction often is accompanied by the growth of carbon nanofibers of different textures (see Figure 5.3). Notice that the supported metal cat alysts are normally used for the purpose and that processes Hke (5.20) can lead to the increase of the catalyst granules weight by a factor of 300 and more compared to their initial weight to allow extremely pure carbonaceous filamentous materials to be produced. 

The growth mechanism of carbonaceous nanostructural materials is generally thought to be via VLS. Cite some recent examples of carbon nanombe/nanofiber growth at temperatures far below the melting point of the nanoparticulate catalyst species. 

Figure 1. A Ni catalyst particle inside a carbon nanofiber. The direction [100] is oriented along the fiber growth axis.

Fig. 3 shows the TEM micrographs of the carbon nanofibers as a function of the synthesis temperature at 600 and 700 °C. At low synthesis temperature the carbon nanofiber structure was more disordered, with a non-regular diameter (Fig. 3a). The structure became more ordered when the synthesis temperature was increased to 700 °C (Fig. 3b). High-resolution TEM micrographs (not shown) also revealed the presence of a nickel particle at some fiber tips which indicated that during the synthesis both tip and base growth mechanisms occurred.

The oxidation behavior of the composite after 2 h of growth allowed the calculation of the carbon nanofiber stoechiometry inside the composite used as catalyst support for the low temperature H2S oxidation, -i.e. about 20 wt.%. 

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