BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 59

(DME) solution of alkali metals naphthalene complexes at room temperature. S BET
of the resultant carbon was 1000-1800 m 2 /g and no effect of alkali metals on pore
structure was observed.
1.1.10.4 CARBON AEROGELS
Carbon aerogels, which have been well known as one of mesoporous carbons,
were prepared from the pyrolysis of organic aerogels of resorcinol and formal-
dehyde. Extensive studies focused on their pore structure and also on doping of
some metals were carried out. Primary carbon particles have the size of approxi-
mately 4-9 nm and interconnected with each other to forma network. Adsorp-
tion isotherms belong to type IV and have a clear hysteresis. Pore structure pa-
rameters calculated through a analysis are listed up on carbon aerogels prepared
from resorcinol and formaldehyde at different temperatures can reported. These
carbon aerogels contain predominantly inter particle mesopores formed in a three-
dimensional network of interconnected minute carbon particles, and only small
amount of intra particle micropores were formed in primary carbon particles. Car-
bon aerogels could be activated in order to increase micropores by CO 2 at 900°C.
The activation for 5 h increased both of micropores and mesopores: pore volume
of 0.68 and 2. 04 mL g and surface area of 1750 and 510 rn 2 /g, respectively. The
detailed studies on adsorption of N 2 at 77 K and of water vapor at 303 K on acti-
vated carbon aerogels, whose surface functional groups were showed clearly that
the amount of adsorbed water corresponded only to the micropore volume and not
to the mesopore volume. Doping of Ce and Zr into carbon aerogels was found to
result in micropore rich carbon materials. Pore structure of carbon aerogels was
known to be governed by that of precursor organic aerogels, which was controlled
by the mole ratios of resorcinol to formaldehyde (R/F), to water (R/W) and also
to basic catalyst Na 2 CO 3 (R/C). Aqueous gels synthesized were dried under super-
critical CO 2. Pore size distributions in mesopore region for both original organic
aerogels and resultant carbon aerogels as a function of R/W, the other factors R/F
and R/C being the constant as 0.5 and 75, are reported. Pore size distributions of
organic aerogels are rather sharp and their maxima decrease with increasing R/W
ratio. By carbonization of these organic aerogels, pore size distributions shift to a
little smaller size, mainly because of the shrinkage of gels during thermal decom-
position. Instead of supercritical drying of aqueous gels, freeze-drying method
was also applied. On the gels prepared through freeze drying (cryogels), much
smaller shrinkage in pore size during carbonization was observed. Conditions for
the preparation of resorcinol-formaldehyde gels through solgel condensation and
those for freeze-drying were studied in detail in order to control mesoporosity
in resultant carbons. Effect of drying process of gels, such as freeze drying, mi-
crowave drying, and hot air drying, on pore structure were also studied. The first
two drying methods are effective in order to get mesoporous carbons. Resorcinol-
acetaldehyde cryogels could be also a precursor for mesoporous carbons. Pore