Page 403 - Book Hosokawa Nanoparticle Technology Handbook

6.8 CATALYTIC PROPERTY FUNDAMENTALS
The optical fiber (e) with photonic crystal structure can about 400 Mpa, which corresponds to about 0.003 m
guide light efficiently along the central core. Silica of the diameter of the pore. The relation between the
fibers and glass capillaries were bundled by wire draw- pressure and the volume of intruded mercury can be
ing at high temperature. Diamond type photonic crys- converted into that of pore size and pore volume.
tals (f) composed of TiO , SiO or Al O can be Over 90 of the contact angle between a sample and
2
3
2
2
fabricated by using stereolithograpy and successive sin- mercury is necessary as a precondition. Pore size dis-
tering process. The wider perfect band gap is obtained tribution of a material cannot be measured if the pre-
in microwave and terahertz wave frequency ranges. condition would not be satisfied. Porosimetry without
mercury is possible if the combination of a porous
sample and liquid pressure media satisfy the precon-
References dition in which the contact angle of the pressure
media on the sample is larger than 90 . However, no
[1] K. Otaka: Phys. Rev. B, 19, 5057–5067 (1979).
porosimetry technique is achieved through other
[2] E. Yablonovitch: Phys. Rev. Lett., 58, 2059–2062
liquid pressure media than mercury for practical use.
(1987).
On the other hand, for the case of some porous mate-
[3] K.H. Ho, C.T. Chan and C.M. Soukoulis: Phys. Rev. rials which have a mechanically weak framework,
Lett., 65, 3152–3155 (1990). porous material itself is consolidated by the crum-
pling up of its framework under the pressure at which
mercury is intruded into its pores. In that case, meas-
6.8 Catalytic property
ured data shows only the information of the consoli-
dation of the sample. Pore distribution of the sample
This chapter explains catalytic property of nanoparti- cannot be measured. When data show no distinctive
cle. The evaluation of catalytic properties is classified peak and show intrusion in increments in a wide pres-
mainly into following three kinds. They are physical sure range, there is a possibility of consolidation of
property, chemical property and testing method of the sample. In such a case, pore distribution measure-
catalytic activity. The physical properties are, for ment must be done not only in the direction of pres-
example, particle morphology or microstructure; sure increase but also in the direction of pressure
chemical properties are for example density or qual- decrease and must be done for two or more cycles. If
ity of active site. Each evaluation technique is consolidation happens, the tendency of incremental
explained as follows. intrusion does not happen after second cycle meas-
Crystal structure, crystal morphology and pore size urements. If the data shows real intrusion, the ten-
distribution are enumerated as physical properties. dency of incremental intrusion is observed even after
The crystal structure is evaluated by powder X-ray second cycle measurements reproducibly. Tubular
diffraction method, and crystal morphology is ana- type pore-morphology model in which one end is
lyzed by HR-TEM (High Resolution Transmission open and another end is closed is used as standard in
Electron Microscopy). However, they are not touched mercury porosimetry. When hysteresis behavior exists
in this chapter because there are many established in the pressure increase side and the pressure decrease
textbooks. In the physical properties, pore size distri- side, even though, the measurement is normally done,
bution especially affects diffusivity of reactant, heat the sample would have ink-bottle-type pore structure
resistance and reaction selectivity. For the effective in which inner-size of the pores are larger than their
catalytic reaction, reactant has to come at active site entrance. Specific surface area of the sample also can
promptly and reaction product has to eliminate from be estimated in mercury porosimetry by using the
there promptly, too. For the evaluation of the reaction tubular type pore-morphology model.
efficiency, diffusivity of gas molecule must be meas- In the case to measure pore size distribution in the
ured, however, no effective method has been estab- size range smaller than 0.003 m, or the case to meas-
lished to evaluate diffusivity of them. Therefore, the ure that of the material which has too weak framework
evaluation of the pore size distribution of a catalyst is to be measured by mercury porosimetry, gas adsorp-
necessary, because the pores would act as the route of tion method is available. The pore size is estimated by
the diffusion of reactant and product. hysteresis behavior in the adsorption isotherm of
Pore size distribution can be measured by mercury adsorption and desorption of gas molecule. Pore size
porosimetry in the range of several hundred microm- and pore volume are calculated with the Kelvin equa-
eters to 0.003 m [1]. The procedure of mercury tion based on volume of the gas molecules which is
porosimetry is as follows. Air in the pores of the sam- condensed by the capillary condensation effect [2].
ple is evacuated by vacuum pump. Then, mercury is Physical adsorption method of nitrogen molecule at
injected into a sample cell. Then pressure is added lit- the boiling point of liquid nitrogen is being used gen-
tle by little on the injected mercury, and simultane- erally. BJH (Brrett, Joyner, Halenda) method is one of
ously the volume of mercury which is intruded in the the analysis methods using the capillary condensation
pores is measured. The higher pressure corresponds to theory. There are some analysis methods which have a
the smaller pore size. The highest available pressure is difference in estimating the thickness of adsorbed gas

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