Page 103 - Adsorbents fundamentals and applications

88 ACTIVATED CARBON

layers, other neighboring groups, and their charges. However, such influence
is minor, so the different types of groups can be differentiated by the simple
titration technique. By careful continuous titration, the distribution of different
acid groups on the same carbon can be delineated by different peaks at different
acidity constants (pK) (e.g., Contescu et al., 1997).
Continuous titration can also show the existence of basic sites of different pK
or basicity strengths on the surface of activated carbon (Contescu et al., 1998).
Two types of basic groups that have been proposed are shown in Figure 5.5.
The pyrone-type group (first suggested by Boehm and Voll, 1970; Voll and
Boehm, 1971) contains two non-neighboring oxygen atoms to constitute one basic
site. Upon contact with a proton in the aqueous medium, the semiquinone oxy-
gen is protonated to a hydroxyl, leaving a positive charge on the other oxygen.
The two oxygen atoms are located at two different graphite rings so as to favor
resonance stabilization of the positive charge. The net positive charge gives rise
to its anion exchange capacity and basicity. Evidence for the pyrone-type site as
the basic site has been cumulating, including acid titration and TPD (Tempera-
ture programmed desorption) (Leon y Leon et al., 1992) as well as theoretical
calculations (Suarez et al., 1999; 2000; Boehm, 2002). The chromene model as
a basic site was suggested by Garten and Weiss (1957). Upon reaction with a
proton and O 2 , a positive charge is introduced to the aromatic ring, hence the
basicity. Beside the pyrone and chromene types, it has long been thought that
the basic behavior of carbon surfaces may derive from the π basicity (or Lewis
basicity) of the aromatic rings (Boehm and Voll, 1970; Leon y Leon et al., 1992;
Montes-Moran et al., 1998). However, the basicity from the aromatic rings is
weak; and the main basicity is still attributed to oxygen-containing groups.
Much has been learned from thermal decomposition or TPD of surface oxides.
When the TPD results are coupled with titration and chemical analysis, one can
determine the specific forms of surface oxides that are being desorbed (e.g.,
Cookson, 1978). During TPD, water, carbon dioxide, carbon monoxide, and small
amounts of hydrogen are the evolved gases. Generally, the evolution of CO 2 and
◦
◦
H 2 O commences in the 200–300 C range, ending in the 400–500 C range for
◦
H 2 O and the 700–800 C range for CO 2 . The evolution of CO commences in

O

O O H
R
or
C

O H R

Pyrone Chromene
Figure 5.5. Schematic of possible basic groups on activated carbon.