BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 51

1.1.9 PORE FORMATION IN CARBON MATERIALS, PORE
CHARACTERIZATION AND ANALYSIS OF PORES
About pore formation in carbon materials, this is accepted that all carbon materi-
als, except highly oriented graphite, contain pores, because they are polycrystal-
line and result from thermal decomposition of organic precursors. During their
pyrolysis and carbonization, a large amount of decomposition gases is formed
over a wide range of temperatures, the profile of which depends strongly on the
precursors. Since the gas evolution behavior from organic precursors is strongly
dependent on the heating conditions, such as heating rate, pressure, etc., the pores
in carbon materials are scattered over a wide range of sizes and shapes. These
pores may be classified as shown in Table 1.3 [1-3].

TABLE 1.3 Classification of Pore Formation in Carbon Materials

1) Based on their origin 2) Based on their size 3) Based on

their state

Intra-
particle
pores

Intrinsic intra-
particle pores
Extrinsic
intraparticle
pores

Micropores < 2 nm Open pores

Inter-
particle
pores

Rigid interpar-
ticle pores
Flexible inter-
particle pores

Mesopores 2~50 nm Ultramicro-

pores <0.7 nm

Closed pores
(Latent pores)

Macro-
pores

> 50 nm Supermicro-

pores 0.7-2 nm

Table 1.3 shows that based on their origin, the pores can be categorized into
two classes, intra particle and inter particle pores. The intra particle pores are
further classified into two, intrinsic and extrinsic intra particle pores. The former
class owes its origin to the crystal structure, that in most activated carbons, large
amounts of pores of various sizes in the nanometer range are formed because of
the random orientation of crystallites; these are rigid inter particle pores. A clas-
sification of pores based on pore sizes was proposed by the International Union
for Pure and Applied Chemistry (IUPAC). As illustrated in Table 1.3, pores are
usually classified into three classes: macropores (>50 nm), mesopores (2-50 nm)
and micropores (<2 nm). Micropores can be further divided into supermicropores
(with a size of 0.7-2 nm) and ultra micropores (<0.7 nm in size). Since