Environmental Engineering Reference
In-Depth Information
activation agent. Recently, a great success to get a high surface area reaching to
approximately 3600 m 2 g −1 was obtained by using KOH for activation and applied
for the preparation of some of activated carbons. During activation process, the
creation of micropores is the most important. In most of carbon materials, how-
ever, macropores and mesopores coexisted with micropores. In other words, mac-
ropores and mesopores had to be formed during the activation process in order to
develop a large number of micropores. On adsorption and desorption procedure,
however, these pores play an important role as diffusion pathways for adsorbates
to micropores. For the carbons prepared from biomasses, such as coconut shells
and wood chips, many macropores are already formed during carbonization as
a memory of cell structure of the original biomasses, which seems to make mi-
cropore development by activation easier. In activated carbon fibers, however,
micropores are formed on the surface of thin carbon fibers. Such a direct exposure
of micropores to adsorbates gives an advantage of fast adsorption/desorption. The
pore development in carbon materials in nanometric scale by air oxidation was
studied in detail, which is an activation process with the simplest in the equip-
ments, the mildest in thermal conditions, and also energy and resources saving
among different activation processes. The commercially available carbon spheres
having the diameter of approximately 15 µm were activated at different tempera-
tures of 355-430°C for various residence times of 1-100 h in a flow of dry air.
The original carbon spheres had negligibly small BET surface area, no pores
on their surface were observed with high magnification scanning electron micros-
copy, and their structure was amorphous with non-graphitizing random nanotex-
ture, so called glasslike carbon. Therefore, oxidation is supposed to start only on
the physical surface of each sphere and proceed to the inside of the spheres by
forming macropores, mesopores, and micropores. Each experimental point on ox-
idation yield against logarithm of residence time, log t, at different temperatures
are superimposed on the curve for a reference temperature of 400°C by the trans-
by the trans-
lation along the log t axis to give a smooth curve, being called the master curve.
Plot of shift factors against inverse of oxidation temperature can be approximated
to be linear and gives apparent activation energy AE of approximately 150 kJ
mo1 -1 from its slope. In wet air, however, AE of approximately 200 kJ mo1 -1 was
obtained at the same temperature range. For different pore parameters measured
by BET, as BJH, and DFT methods, the master curves could be obtained by apply-
ing the same shift factors. Master curves for micropore volume determined by as
plot, ultramicropore and supermicropore volumes by DFT method and mesopore
volume by BJH method are shown in order to make the comparison easier, togeth-
er with some SEM images to show the appearance of the spheres. In the beginning
of oxidation, up to 10 h oxidation at 400°C, the main process is the formation of
ultramicropores. Above 10 h up to approximately 60 h, relative amount of ul-
tramicropores decreases but supermicropores increase with increasing oxidation
time. Above 65 h, both ultramicropores and supermicropores decrease rapidly but
°C by the trans-
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