Page 97 - Adsorbents fundamentals and applications

82 ACTIVATED CARBON

With steam or CO 2 at atmospheric pressure, the kinetically controlled regime is
◦
◦
below approximately 900 C; hence this step is usually performed at 750–900 C.
The mechanisms of these gasification reactions are different and have been stud-
ied extensively (Walker et al., 1959; Yang, 1984). A main function of gasification
is to widen the pores, creating large mesoporosity. These reactions are catalyzed
further by minerals that catalyze the reactions by undergoing a variety of actions
such as tunneling, channeling, and pitting (Yang, 1984; Baker, 1986), hence
helping create and propagate fine pores. Another major function by chemical
activation is the removal of oxygen and hydrogen. With the added catalysts,
oxygen and hydrogen are selectively and nearly completely removed at below
◦
700 C (Jankowska et al., 1991). The main catalysts are zinc chloride, phosphoric
acid, and potassium sulfide. The mechanism of activation by phosphoric acid has
been studied (Jagtoyen et al., 1992; 1993; Solum et al., 1995). Studies by use of
13
C NMR have shown that phosphoric acid promotes cross-linking reactions and
dehydration at low temperatures, thereby bonding otherwise volatile compounds
and hence increasing the carbon yield. Different chemical agents also yield dif-
ferent pore structures; for example, carbons activated by zinc chloride were more
mesoporous, whereas those activated by KOH were more microporous, although
2
both yielded high surface areas around 1000 m /g, with that from ZnCl 2 slightly
higher (Ahmadpour et al., 1998).
A breakthrough was made by Wennerberg and O’Grady (1978) by using a
novel activation method, after a series of patents by Wennerberg beginning in
1971 (Wennerberg, 1971). The activation process is accomplished in molten
KOH. The method was initially aimed at petroleum coke (a rather pure form
of coke generated as the tail-end product in petroleum refining, used as anodes
in aluminum smelting). It works also with other precursors for activated carbon,
such as coals and nutshells. In this method, KOH and coke are mixed at a ratio
◦
of about 3/1 KOH/coke, heated to ∼700–800 C (the melting point of KOH is
◦
360 C) in an inert atmosphere (or in a closed system) for about 2 h. A small
amount of water is used for pasting. A large microporosity is formed during the
activation, with “cage-like” pores, mainly <2 nm. The BET surface area is typ-
2
2
ically 2800 m /g and reaches as high as 4057 m /g, depending on the activation
condition. The mechanism of activation is not understood. The 3/1 KOH/coke
ratio is equivalent to ∼0.6 KOH/C. This amount of KOH is adequate to pro-
vide oxygen for gasification of carbon to create the micropores. Activation with
a 1/1 mole mixture of KOH and NaOH is also effective (Audley and Holder,
1988). These carbons have been proven to be the best for methane storage, as
will be discussed in Chapter 10 (10.4). Further developments and the commercial
status of these carbons are also discussed in Chapter 10.4.

5.2. PORE STRUCTURE AND STANDARD TESTS
FOR ACTIVATED CARBON
Activated carbons are characterized by a large surface area between 300 and
2
∼4000 m /g, as measured by the BET method, and are the largest among all