Introduction to SkeletonC
Carbon materials produced from metal or metalloid carbide, also called as Carbide-Derived Carbon (CDC) or SkeletonC, can be assorted to the group of the templated carbons. These carbons are formed during extraction of non-carbon atoms from the carbide structural network and therefore, the nanostructure and properties of carbon significantly depend on the template, i.e. precursor carbide. Carbides used to produce carbon may be as powders or pellets or films. From the point of view of structural order the precursor may be for instance monolithic crystal or polycrystalline or porous biomorphic carbide.
There are several methods to extract the non-carbon atoms from carbide; the most wide-spread though is a chemical extraction with chlorine at high temperature.
MCx + y/2 Cl2 → MCly + xC
Chlorination of carbide can be realised whether in steady state horizontal furnaces, or in dynamic reaction medium, which is a case for rotating reactor tubes or vertically aligned fluidised bed reactors. The atmosphere in the reaction zone, where the carbide is placed, must be dry that is achieved by flushing the reactor tube with inert-gas. After that the temperature is raised to the desired value and chlorine gas is introduced to the reaction zone. When the reaction is completed, the reaction zone is again flushed with inert-gas to remove the by-produced chlorides and residual chlorine. As-produced carbon can be further purified from the chemically bound chlorine by using reducing reagents such as hydrogen or ammonia, or the surface of carbon can be chemically functionalised by oxidation or decomposition reactions in both, the gas phase or liquid phase.
According to the mass-balance of chlorination reactions theoretical yield of carbon from different carbides may range from ~6% (wt.) in the case of molybdenum carbide to almost 30% (wt.) for silicon carbide.
Chlorination reaction of carbide and its products can be easily controlled by the process parameters such as reaction temperature and rate of the gas flow. General feature is that the higher is chlorination temperature, the higher is structural order and the lower is porosity of resulting carbon material as exemplified by the boron carbide derived carbons [A.E. Kravchik et al. Carbon 44 (2006) 3263].
However, the precise influence of temperature on carbon structure is very “personal” for different precursor carbide. The exact effect is determined by the chemical reactivity, crystal structure, stoichiometry of elemental composition of carbide, etc.
The table below gives few comparative examples about some physical properties of carbons made from several different carbides at variable discrete temperatures.
|Carbide||Crystal structure||BET Surface, m2 g-1||Pore Volume, cm3 g-1|
Special interest to the carbon materials produced from carbides is caused by the fact that even small changes in carbide chlorination temperature can affect the average pore size and pore-size distribution of resulting carbon. And even more so – the results are well reproducible. This makes the carbide-derived carbons very attractive to the majority of adsorption based applications such as adsorbents, electrode materials in energy storage devices, molecular sieves for separation/purification processes etc.
Examples of fine-tuned SkeletonC materials
Silicon carbide derived SkeletonC is highly nanoporous carbon, which has high apparent density and noticeably narrow pore size distribution.
Molybdenum carbide derived SkeletonC may be fine-tuned from microporous to mesoporous carbon and is therefore usuful as a substrate for catalysts or as the adsorbent for selective adsorption.
Aluminum carbide derived SkeletonC may have a nanostructure from amorphous to graphitic nanobarrels. Formation of nanobarrels can be promoted with catalytic d-metal compounds dispersed into the carbide.