Filaments activated carbon

Carbon fibers, porous and activated Carbon filaments and whiskers Carbon films... 

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. 

In contrast to the Pt catalysts discussed above, Ni based catalysts (i.e., also when supported on ZrO usually form coke at such a rapid rate that most fixed bed reactors are completely blocked after a few minutes time on stream (see Fig. 8) [16], The coke formed with the Ni catalysts is filamentous. The Ni particle remaining at the tip of the filament hardly deactivates as the coke formed on its surface seems to be transported through the metal particle into the carbon fibre, but the drastic increase in volume causes reactor plugging and prevents use of the still active catalyst (see Fig. 8). The TEM photographs indicate that the carbon filaments have similar diameters to those of the Ni particles. 

At higher temperatures, out of the typical FT regime, carbon could encapsulate the active metal, thereby blocking access to reactants. In extreme cases carbon filaments can also be formed that can result in the breakup of catalyst particles.42... 

A critical factor for biotechnology application is the stability of the enzyme electrode. Hydrogenase immobilized into carbon filament material has high level of both operational and storage stability. Even after the half year of storage with periodical testing, the enzyme electrode preserved more than 50 % of its initial activity [9,10], Thus, it is possible to achieve appropriate stability of the enzyme electrode, suitable for hydrogen fuel cells development. 

Busofit is a universal adsorbent, which is efficient to adsorb different gases (H2, N2, 02, CH4, and NH3). Figure 2 shows the texture of the active carbon fiber filament. The carbon fiber refers to microporous sorbents with a developed surface and a complicated bimodal structure. The material can be performed as a loose fibers bed or felt or as monolithic blocks with binder to have a good thermal conductivity along the filament. 

The micropore distribution is performed mostly on the carbon filament surface. Nowadays a program was undertaken to examine the parameters of an active carbon fiber to optimize both the mass uptake of ammonia, methane and hydrogen and the carbon density. 

A large number of intermediate pathways arc possible when catalytic reactions interfere with the polymerization-dehydrogenation steps. A common scenario is the catalytic dehydrogenation of hydrocarbons on nickel surfaces followed by dissolution of the activated carbon atoms and exsolution of graphene layers after exceeding the solubility limit of carbon in nickel. Such processes have been observed experimentally [40] and used to explain the shapes of carbon filaments. In the most recent synthetic routes to nanotubes [41] the catalytic action of in situ-prepared iron metal particles was applied to create a catalyst for the dehydrogenation of cither ethylene or benzene. 

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... 

In three-phase reactors, one of the main problems is often the mass transport limitations, which may reflect internal as well as external mass transfer resistances. The use of filamentous catalytic materials for multiphase reactions may help reduce or even avoid mass transfer limitations [63,132,133]. Filamentous woven cloths made of glass, composite mixed oxides, metallic alloys, or activated carbon (Figure 18) can be used as supports for active components such as platinum, palladium, or transition metal oxides. The diameters of the filaments are of the order of several micrometers and correspond to the typical diameters of catalysts that are suspended in the reaction medium. By using such small diameters, internal mass transfer limitations can be avoided. 

E. Joannet, C. Homy, L. Kiwi-Minsker, A. Renken, Palladium supported on filamentous active carbon as effective catalyst for liquid-phase hydrogenation of 2-butyne-1,4-diol to 2-butene-l,4-diol, Chem. Eng. Sci. 57 (2002) 2453. 

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