BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 56

mesopores increases slightly, which result in the decrease in surface areas. This
change in pore volumes might suggest the gradual enlargement of pore size from
ultramicropore through supermicropore to mesopore and possibly to macropores
by prolonged oxidation above 100 h. At very beginning of activation, 1 h oxida-
tion at 400°C, BET surface area reaches 400 m 2 /g of which the predominant part
is microporous surface area. This was experimentally proved to be caused mainly
by the change of closed pores, which were formed during carbonization of the
precursor phenol resin, to open pores. To understand the activation process from
the view point of gasification, it was proposed to normalize the fractional weight
loss in different atmospheres (different oxidizing agents, such as steam and CO 2 ,
and their different pressures) as a function of t/t o 5, where to is the time giving
fractional weight loss of 0.5. The experimental data of mass loss obtained at a
constant temperature for each oxidizing agent were successfully unified to one
curve. The activation process of glass-like carbon spheres in air at different tem-
peratures and residence times was shown to be understood by master curves for
the yield and pore structure parameters. On the same carbon spheres, adsorption
behaviors of various organics in their aqueous solutions were also understood by
the master curves for each adsorbates as functions of oxidation temperature and
time. Unification curves are shown as a function of dimensionless time t/t o 5 and
master curves are expressed by real time at a reference temperature. The former
seems to be useful to compare the activation (gasification) of various carbona-
ceous materials and to discuss its mechanism, but the latter might be useful to
discuss the activation conditions to prepare activated carbons.
The derivation procedure of master curves suggests that the conversion be-
tween oxidation temperature and time was possible for pore structure parameters
as well as activation yield through air oxidation. Activation processes have been
pointed out to have some demerits. The mesopores can usually be created by the
enlargement of micropores. In other words, by some expense of micropores, and
certain part of carbon atoms has to be gasified to CO and/or CO 2 during activation
process, in other words, the final yield of activated carbons becomes low. These
demerits were pointed out to be one of the barriers for cost down of the industrial
production of activated carbons. Also these demerits of activation process were
one of the motive forces for the development of new preparation processes of
porous carbons, as described in the following sections. Activation in two steps
in air was reported to be efficient, the first step at a high temperature for a short
time, followed by the second step at a low temperature for a long residence time.
On glass like carbon spheres, two step activation at 500°C for 3 h for the first step
and at a temperature below 415°C for different periods for the second step. In the
beginning of activation, activation yields larger than 60 wt.%,higher S BET and S mw
are obtained with the same yield, in other words, the same surface areas can be
obtained with about 10 wt.% higher yield by two step activation than by one step
activation.