Page 104 - Adsorbents fundamentals and applications

SURFACE CHEMISTRY AND ITS EFFECTS ON ADSORPTION 89
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the 500–700 C range and ends around 1000 C. Evolution of small quantities of
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hydrogen occurs at the 600–1000 C range, and some hydrogen is retained even
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after heating at 1200 C. This is generally because the C−H bond is stronger than
the C−C bond. It is generally believed that carboxylic groups and their deriva-
tives, such as lactones, decompose to give CO 2 , whereas quinone or semiquinone
groups give CO and hydroquinones and phenols give CO and H 2 O.
Acidic surface oxides can be generated by oxidation with oxygen at elevated
temperatures or with liquid oxidants. Aging can also generate such oxides. How-
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ever, aging in air or water (at temperatures <200 C; Puri, 1970) after the carbon
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is degassed at 700–950 C would create surface oxide that is basic in nature (Puri,
1970; Boehm, 2002). Obviously, little is understood about the aging process. The
surface oxides can also be removed, as seen in TPD, at high temperatures in
vacuo or in an inert atmosphere. Adsorption on these cleaned carbons has also
been studied.
Basicity of carbon surfaces can also be introduced by “nitriding,” that is, react-
ing the carbon with anhydrous ammonia at elevated temperatures (Radovic et al.,
1996). Unlike oxygen, only very small amounts of nitrogen can be incorporated
as functionalities, which, however, can yield basicity on the surface.

5.4.1. Effects of Surface Functionalities on Gas Adsorption

Adsorption of gas molecules on activated carbon is dominated by the van der
Waals forces (i.e., dispersion and repulsion forces). The electrical charges on
activated carbon (or modified carbons) are too weak (compared with other sor-
bents) or the positive and negative charges are too close to each other to exert
any significant electric field or field gradient on the surface. In addition, the
small pore sizes and the large surface area of activated carbon play the major
role in gas adsorption. Consequently, the surface groups have significant effects
only for adsorption of polar gases. The most extensively studied gas adsorbate
molecule is water. Other polar molecules that have been studied are alcohols,
amines, ammonia, SO 2 (Jankowska et al., 1991), NO, and other “supercritical”
gases (Kaneko, 1998).
With polar molecules that have strong permanent dipole moments, the field-
dipole interaction term (see Chapter 2) becomes significant. Hence the adsorption
of these molecules can be significantly increased by introducing oxide groups on
the surface. An example for water adsorption is shown in Figure 5.6. For the
adsorption of water, hydrogen bonding between water and the surface oxide has
been proposed (Puri, 1966). The amount of increase in adsorption seems to be
correlated to the amount of the oxygen-containing groups that release CO 2 upon
thermal desorption, and there appears to be one water molecule per such group
(Puri, 1966).
Figure 5.6 shows the significant increase in adsorption of water molecules at
low vapor pressures by oxidation (Barton et al, 1984). This increase is caused by
the introduction of surface oxygen functionalities. Changes in pore volume and
surface area also accompany oxidation. The ultimate capacity (at P/P 0 near 1)