Porous Materials Controlled in Shape
Porous materials have been widely used for industrial applications as well as in product of daily use. The classification of porous materials is based on the morphology of the internal pore structure. The factors of classification are as follows: 1. size
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5.1 Classification of Porous Materials Porous materials have been widely used for industrial applications as well as in product of daily use. The classification of porous materials is based on the morphology of the internal pore structure. The factors of classification are as follows: 1. size and distribution, 2. shape, 3. dimension and connectivity. The size is the most significant factor to characterize the pore structure. The International Union of Pure and Applied Chemistry (IUPAC) recommended a classification [1] as shown in Table 5.1. Similarly to the case of particle size, the definition of the pore size is not so simple unless the pores are spherical or cylindrical. Actual pore shapes are not so regular, and the size is distributed. Among several characterization methods, visualization is the most direct one although the observation in sub-nanometer-to-sub-micrometer scales must be performed by high-resolution microscopy such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM). On the nanometer scale, the visualization itself is challenging. The characterization has been, therefore, performed by an alternative, indirect method via adsorption of probe molecules. Nitrogen has been most often used as a probe because the interaction with pore surface is less specific or significant. The standard characterization by nitrogen adsorption has been carried out at the temperature of liquid nitrogen (77 K). When pores are smaller, condensation of vapor in the pore is observed below the saturation pressure. This unique phenomenon is designated as “capillary condensation”. The relationship between the pore size and the Table 5.1. Classification of pores by size Macropore Mesopore Micropore
> 50 nm 2–50 nm < 2 nm
Y. Waseda et al. (eds.), Morphology Control of Materials and Nanoparticles © Springer-Verlag Berlin Heidelberg 2004
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T. Okubo, M. Matsukata
Fig. 5.1. Potential profiles across mesopore and micropore
condensation pressure is given by the Kelvin equation (5.1) when the pore is cylindrical, (5.1) ln(p/p0 ) = −2γVL /(rp RT ), where p denotes the vapor pressure, p0 the saturation pressure, γ the surface tension, VL the molar volume of liquid, and rp the pore radius. In much smaller pores, the interaction between the nitrogen molecules in the pores is less significant than that between a nitrogen molecule and the pore surface, and the capillary condensation is not observed. Instead, each molecule is trapped in the deep potential field in the pore as shown in Fig. 5.1. This region is designated as the micropore. On the contrary for larger pores, the pressure where capillary condensation is observed is closer to the saturation pressure (1 atm at 77 K), and it is difficult to determine the point of condensation precisely. Accordingly, the upper limit of mesopores is fixed at 50 nm. The definition in Table 5.1 is, thus, based on the adsorption behavior of nitrogen at the temperature of liquid nitrogen. The characterization by adsorption should be influenced not only by the size of the pores but also
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