Carbon microcrystalline, surface groups

The concentration of active sites depends to a large extent on the microcrystalline structure of the carbon material. Small crystallites can expose more edges therefore, more surface groups can be formed, while the role of the basal planes and the K electrons will become more important with large crystallites. Thus, the orientation of the graphene layers and the ratio between prismatic and basal plane areas may affect the catalytic performance, and such effects have indeed been reported in the case of carbon nanohbers (see Section 6.3.1). 

The most common adsorbant used is granular or powdered activated carbon. This material, which is available from almost all forms of organic carbon-containing matter, is a microcrystalline nongraphite form of carbon. The production of activated carbon can be achieved by use of rotary kilns, hearth furnaces, or furnaces of the vertical shaft or fluidised bed type, and each is suitable for the generation of different pore size and the source of carbon. The pore volume and size are influenced by both the carbon source and method of production. The adsorption properties are directly related to the pore volume, pore size distribution and the nature of the functional groups on the surface of the carbon. Activation is achieved chemically, by treatment by dehydration with zinc chloride or phosphoric acid, or by treatment with steam, hot carbon dioxide or a mixture of both. The activated carbon is available in three basic forms, powder, granules or as cylindrical or spherical pellets. For solvent recovery systems the carbon is usually obtained from either wood charcoal, petroleum residues or coconut shells and is often used in the form of pellets. 

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