Carbon filament formation

The activity and stability of catalysts for methane-carbon dioxide reforming depend subtly upon the support and the active metal. Methane decomposes to carbon and hydrogen, forming carbon on the oxide support and the metal. Carbon on the metal is reactive and can be oxidized to CO by oxygen from dissociatively adsorbed COj. For noble metals this reaction is fast, leading to low coke accumulation on the metal particles The rate of carbon formation on the support is proportional to the concentration of Lewis acid sites. This carbon is non reactive and may cover the Pt particles causing catalyst deactivation. Hence, the combination of Pt with a support low in acid sites, such as ZrO, is well suited for long term stable operation. For non-noble metals such as Ni, the rate of CH4 dissociation exceeds the rate of oxidation drastically and carbon forms rapidly on the metal in the form of filaments. The rate of carbon filament formation is proportional to the particle size of Ni Below a critical Ni particle size (d<2 nm), formation of carbon slowed down dramatically Well dispersed Ni supported on ZrO is thus a viable alternative to the noble metal based materials. 

As the metal particle size decreases the filament diameter should also decrease. It has been shown that the surface energy of thirmer filaments is larger and hence the filaments are less stable (11,17-18). Also the proportion of the Ni(l 11) planes, which readily cause carbon formation, is lower in smaller Ni particles (19). Therefore, even though the reasons are diverse, in practice the carbon filament formation ceases with catalysts containing smaller Ni particles. Consequently, well dispersed Ni catalysts prepared by deposition precipitation of Ni (average metal particle size below 2-3 nm) were stable for 50 hours on stream and exhibited no filamentous coke [16]. 

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 ... 

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. 

Figure 2. Schematic of the proposed mechanism of carbon filament formation.

Finally, the CAEM studies show that Fe3C is not an active catalyst for carbon filament formation. This observation is also supported by the Mossbauer spectroscopy data which show that the... 

Snoeck et al., [16] have proposed that the driving force for carbon filament formation is the difference in chemical potential between the gas phase and the carbon filament, and this causes different solubilities in Ni at the gas side and the support side of the particle and a concentration gradient leading to diffusion of carbon. 

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.

Figure 8.19 Mechanism of carbon filament formation in the PVFe system by extruded filament growth. Source Reprinted with permission from Baker RTK, Waite RJ, Formation of carbonaceous deposits from the platinum-iron catalyzed decomposition of acetylene, J Catal, 37, 101-105, 1975. Copyright 1975, Elsevier.

The rigorous kinetic modeling with the incorporation of the diffusion step allows explaining the deactivation of the carbon filament growth and the influence of the affinity for carbon formation on the nucleation of the filamentous carbon. 

The decomposition of methane is an important process since it can produce two valuable products hydrogen and carbon filaments. Wayne Goodman (Texas A M University) and Tushar Choudhary (ConocoPhillips) show that methane decomposition may be a viable alternative to conventional steam reforming as a source of hydrogen, without the formation of COx as a byproduct. The authors examine the effects of catalyst support and promoters, as well as the inevitable regeneration of the catalyst. The formation of carbon fibers, under certain conditions, makes this process an attractive one. 

Figure 35. Mode of operation for the removal of a melt particle from an oxidic support by growing a carbon filament. Stage (I) initial saturation of the particle with carbon atoms from dissociation of a hydrocarbon and subsurface dissolution of the resulting free atoms. Stage (2) is after about 1 h on stream the particle exsolutes at the most active faces carbon which grow in a concentric set of graphene bands and remove the particle from the support. Stage is (3) after some time on stream the particle has reshaped such that surfaces within the carbon tube also become active for graphene formation and are deposited as little flakes inside the tube.

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... 

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