Filamentous carbon formation studies

A series of kinetic studies on the carbon filament formation by methane decomposition over Ni catalysts was reported by Snoeck et al. [116]. The authors derived a rigorous kinetic model for the formation of the filamentous carbon and hydrogen by methane cracking. The model includes the following steps ... 

Kinetics studies on carbon formation, using the TEOM, have established that both CH4 in the reactants and CO in the products, may serve as the source of carbon formation in the C02 reforming reaction. In an equimolar C02-CH4 reforming condition, CO in the product stream is most likely the major source of carbon formation. However, carbon formed in C02 reforming from a feed without containing CO shows a more filamentous morphology, which is more difficult to be oxidized this contrasts with carbon formed from a feed containing CO, which mostly encapsulates metal particles in the catalyst and is relatively easily oxidized. Unfortunately, it is currently unclear as to whether these differences result from the presence of CO in the feed, or from the different times on-stream to accumulate similar amounts of carbon due to different carbon formation rates when the feeds contain CO or no CO. 

Inhibition of filamentous carbon growth on metal surfaces by addition of titanium oxide has been reported previously (10). In these experiments the titanium oxide was in the fully oxidized state and was an effective physical barrier toward filament formation, provided that the temperature was kept below 920 K. Above this temperature, the oxide tended to spall, and as bare metal was exposed to the gas phase, prolific filament growth occurred. In the present study we have combined electron microscopy studies with... 

Carbon formation from different hydrocarbons over metals has been studied by several workers. The presence of carbon filaments on the surface of the metal, containing metal crystals, has been reported under conditions similar to those of these experiments. 

Figure 8.15 The most common mechanism of carbon filament formation from the pyrolysis of acetylene (CaHa) on a metal particle (M) where (C) denotes carbon. Source Reprinted with permission from Baker RTK, Electron microscopy studies of the catalytic growth of carbon filaments. Carbon Fibers Filaments and Composites. Copyright 1990, Springer. Figueiredo JL, Bernardo CA, Baker RTK, Hiittinger KJ eds., Kluwer, Dordrecht, 419, 1990, Baker RTK, Barber MA, Harris PS, Feates FS, Waite RJ, J Catal, 80, 86, 1972.

Deviations from the above classification are also reported in the literature [93,94]. Synthesis of filamentous carbon from Cr3C2 was studied under hydrothermal condition at 100-200 MPa and 350-800°C. The formation of spherical and filamentous carbon was observed in the presence of organic compounds (ascorbic acid, malonic acid, glycolic acid, oxalic acid, citric acid, and stearic acid) at... 

It was established that the electrical conductivity of DWNTs/CA increases with nanotube concentration and reaches 8.1 S/cm at the highest content studied (8 mass%). A similar trend was observed by the same authors for the thermal conductivity of DWNTs/CA (Worsley, Satcher, and Baumann, 2009). Further, the study was extended to the fabrication of SWNTs/CA containing up to 55 mass% nanotubes (Worsley, Pauzauskie et al., 2009). Interestingly, above 20 mass%, the microstructure of aerogel changed from a network of carbon nanoparticles to a network of randomly oriented filament-like formations. Hence, at high SWNT concentrations, the majority of nanoparticles are located on the surface of the SWNTs and interconnect the nanotube bundles. In other words, above concentrations of 20 mass%, SWNTs dominate the microstructure of aerogels and determine their... 

Amorphous carbon thin films have also been studied/ as have a number of other amorphous and graphite-derived systems. " There is Raman evidence for the formation of filamentous carbon from paraformaldehyde at high temperatures and pressures. 

In most publications, Iijima is given credit for the discovery in 1991 of the nanotube structure of carbon (Iijima, 1991 Bethune et al., 1993 Iijima and Ichihashi, 1993). However, it has been said that Oberlin et al. (1976) also imaged carbon nanotubes, perhaps even SWNTs. Incredibly, nearly a century earlier, there was a study on the thermal decomposition of methane that resulted in the formation of long carbon strands, which were proposed at the time as a candidate for filaments in light bulbs (see Bacon and Bowman, 1957). 

Snoeck, J., Froment, G., and Fowles, M., Kinetic study of the carbon filament formation by methane cracking on a nickel catalyst, /. Catal., 169, 250,1997. 

A second problem of catalyst regeneration is often the modification of the dispersion of the active component. Several studies [24, 230] clarify that carbon deposition originating from hydrocarbons not only covers an active particle but may remove it from its support. This mode of carbonization occurs effectively with metals catalyzing the formation of carbon filaments (see above). Figure 35 summarizes this effect. A metal... 

IR spectroscopy appears to be a very promising tool for oxidation studies. The rate of formation of carbon monoxide, carbon dioxide and formic acid and the disappearance of ozone was measured this way with the O3/O2 + CH4 system . The system used by Burt and Minkoff for the combustion studies is shown in Fig. 61. Light from a Nernst filament is split in two and passed alternately through two heated cells and F2 containing either fuel + N2 or fuel + Oj. The beams are rejoined and fed into a Wadsworth monochromator containing a CaF2 prism and finally focussed onto a thermopile, from which a particular signal may be amplified and recorded. 

The use of mass spectrometers suggests itself as a convenient method for the investigation of unstable reaction intermediates. The mass spectrometer can not only measure the concentration of an intermediate but also establish its chemical identity. Eltenton first applied the technique to the study of free radicals, and from the earliest experiments useful results have been obtained. Thus Eltenton in his first paper showed that the pyrolysis of methane on a carbon filament leads to the formation of methyl radicals, and not methylene 2is believed formerly, and he... 

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