Introduction

Carbon material is a versatile substance because of its variety of bonding character. The typical allotropie forms are graphite and diamond, in which carbon element consists of sp2 and sp3 hybridization, respectively. In general, carbonaceous products categorized in the former carbon are obtained when many organic compounds and those of biological origin such as wood, peat, coal, and biomass are pyrolyzed in inert atmosphere at high temperature. The products contain a lot of elemental carbon and a small amount of oxygen, nitrogen, and so on.

Franklin classified the carbonaceous materials into two distinct groups (e.g., nongraphitizable and graphitizable carbon) on the basis of X-ray diffraction study [1]. Also hard and soft carbons are often referred to under another nomenclature, respectively [2]. The structural models proposed are represented schematically in Figure 1. Thereafter the great advance of the high resolution transmission electron microscope (TEM) has made it possible to visualize the more precise structure of the carbons. Jenkins and Kawamura observed TEM images of a glasslike carbon and illustrated a schematic model for the network of ribbon stacks, as shown in Figure 2a [3]. In addition Shiraishi proposed another model for nongraphitizable carbon (Fig. 2b) [4]. Figure 3

shows typical lattice images of nongraphitizable carbon from phenol formaldehyde resin and of graphitizable carbon from petroleum pitch coke. They reflect the skeleton of the hexagonal aromatic carbon layers [4]. The resin carbon does not form a structure like Figure 3d even at high temperatures like 3273 K, whereas the number of parallel layers increases considerably in the pitch carbon with heat treatment at much lower temperature, indicating a better graphitic structure.

It is said that such a structural difference originates both from the structure of the raw material and from the carbonization process: the cross-linking develops between the randomly oriented crystallites during carbonization of the resin leading to a rigid structure and to a charcoal which is hard carbon and has a well developed porous structure. On the other hand, the pitch proceeds in a liquid state during the carbonization around 627-773 K and aromatic hydrocarbons produced at this temperature range can arrange in parallel with each other to lead a formation of small spherical droplets with optical anisotropy called carbonaceous mesophase [5]. It has been shown that oxygen-rich precursors inhibit graphitization whereas high C-H ratios enhance the formation of a three-dimensional graphitic structure with less developed porous structure. The nongraphitizing carbon contains a large number of pores between small crystallites as can be seen from Figure 1a. Most of these pores originally included are located inside far from the surface. Thus, in order to generate the open pores on the surface of the particles, carbon atoms are partially burned off at high temperature using oxidative agents such as steam, carbon dioxide, and so on. The process and the resultant product are referred to as activation and activated carbon. Since most of the pores are mainly the spaces between a few stacked lamellae or crystallites, the pore shape in the carbon materials is basically of slitlike structure.

Activated carbon is one of the well-known porous substances and a collective name for a group of porous carbons. In 1911, Norit Company began the commercial production

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Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 7: Pages (237-262)

Figure 1. Schematic representation of (a) nongraphitizable and (b) graphitizable carbons. Reprinted with permission from [1], R. E. Franklin, Proc. Roy. Soc. London Ser. A 209, 196 (1951). © 1951, The Royal Society of London.

5 nm

Figure 2. (a) Structural model for the network of ribbon stacks in glassy carbon by Jenkins and Kawamura, La: lattice constant of a-axis, Lc: lattice constant of c-axis. Reprinted with permission from [3], G. M. Jenkins and K. Kawamura, Nature 232, 175 (1971). © 1971, Nature. (b) Structural model for nongraphitizable carbon by Shiraishi. Reprinted with permission from [4], M. Shiraishi, in "Introduction to Carbon Materials" (Carbon Society of Japan, Ed.), revised version, p. 29. Science and Technology Publishing, Tokyo, 1984 [in Japanese]. © 1984, The Carbon Society of Japan.

5 nm

Figure 2. (a) Structural model for the network of ribbon stacks in glassy carbon by Jenkins and Kawamura, La: lattice constant of a-axis, Lc: lattice constant of c-axis. Reprinted with permission from [3], G. M. Jenkins and K. Kawamura, Nature 232, 175 (1971). © 1971, Nature. (b) Structural model for nongraphitizable carbon by Shiraishi. Reprinted with permission from [4], M. Shiraishi, in "Introduction to Carbon Materials" (Carbon Society of Japan, Ed.), revised version, p. 29. Science and Technology Publishing, Tokyo, 1984 [in Japanese]. © 1984, The Carbon Society of Japan.

Figure 3. Electron microdiffraction (a) and their TEM lattice image (b) of nongraphitizable carbon from phenol formaldehyde resin, and those (c and d) of graphitizable carbon from petroleum coke. Reprinted with permission from [4], M. Shiraishi, in "Introduction to Carbon Materials" (Carbon Society of Japan, Ed.), revised version, p. 29. Science and Technology Publishing, Tokyo, 1984 [in Japanese]. © 1984, The Carbon Society of Japan.

Figure 3. Electron microdiffraction (a) and their TEM lattice image (b) of nongraphitizable carbon from phenol formaldehyde resin, and those (c and d) of graphitizable carbon from petroleum coke. Reprinted with permission from [4], M. Shiraishi, in "Introduction to Carbon Materials" (Carbon Society of Japan, Ed.), revised version, p. 29. Science and Technology Publishing, Tokyo, 1984 [in Japanese]. © 1984, The Carbon Society of Japan.

of activated carbons from peat by steam activation. During the first decades of the last century, activated carbon was used mainly for the purification of products of the chemical, pharmaceutical, and food industries. Purification of drinking water was also an important application from the outset. In recent years, the production of carbon has been increasing for the prevention of environmental pollution and for meeting the constantly increasing demands for purity of natural and synthetic products [6].

Porous carbon materials have so far played an important role in many industries, including air and water purification, gas separation, catalysis, chromatography, and so on. However, there are numerous other potential applications in which materials with carbonaceous surfaces would be attractive, for instance adsorption of large hydropho-bic molecules such as vita dyes, humic acids, dextrines, chromatographic separation, electrochemical double layer capacitors (EDLC), and lithium batteries. In these cases, the presence of wider pores, preferably mesopores, would be more advantageous to the adsorption of these large compounds.

To meet such requirements many novel approaches have been proposed to control pore structure and to produce different types of porous carbons. And it has been demonstrated that such porous carbons are very useful and exhibit outstanding performance in many new applications such as catalytic supports [7] and EDLC with very high power [8].

In the first section of this chapter the general definition of a pore in solid materials and the characteristics of the pores included in carbon materials are briefly represented to help readers understand the contents described in the following sections. Second, the importance of the nanopores in the carbons is pointed out and then special emphasis has been put on the novel preparations of nanoporous carbons and development of new utilization areas of these carbons. Throughout the chapter readers can survey the state of the art preparation and application for nanoporous carbons.

From this point of view carbon nanotubes and nano-horns are also promising materials because the pores have a unique shape and are more uniform than those in the conventional carbons.

Here the authors point out the importance of meso-pores (or nanopores) in the porous carbons. All the micropore entrances in fibrous activated carbon [or activated carbon fiber (ACF)] can access the outer surface of the fiber, whereas most of the micropores existing in granular activated carbons are placed deep inside far from the surface of the carbon particles through the mesopores. Such a difference in the circumstance of the pores in fibrous and granular carbons is depicted in Figure 4. Of course, micropores are important to the adsorption ability of small molecules because most of the adsorption takes place in the micropores. In the case of granular carbons the species to be adsorbed have to approach to the micropores through meso- or macropores. In other words, these relatively large pores can serve as a passage for the transport of the adsor-bates. In particular the presence and the quantity of the mesopore are very important in designing nanoporous carbons suitable for heterogeneous catalysis and supercapacitor applications.

fibrous activated carbon granular activated carbon

Figure 4. Structural models of pores in fibrous activated carbon and granular activated carbon.

fibrous activated carbon granular activated carbon

Figure 4. Structural models of pores in fibrous activated carbon and granular activated carbon.

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