Page 285 - Adsorption by Powders and Porous Solids

CHAPTER 9. ADSORPTION BY ACTlVE CARBONS

Table 93. EEective pore sizes of activated carbons
Carbon Effective pore volume, Pore range, w (nm)
vn(mic) (an3 g-')

'For propam.
For nitm~en.
U, ul~cropomus; S, supermicropomus.

pore size or volume (Rouquerol et al., 1994) and also specify the adsorptive and
operational temperature.

9.53. Adsorption of helium
Helium is often used in adsorption manometry for the determination of the 'dead
space' volume (see Chapter 31, but this procedure is based on the presupposition that
the gas is not adsorbed at ambient temperature and that it does not penetrate into
regions of the adsorbent structure which are inaccessible to the adsorptive molecules.
In fact, with some microporous adsorbents, significant amounts of helium adsorption
can be detected at temperatures well above the normal boiling point (4.2 K). For this
reason, the apparent density (or so-called 'true density') determined by helium
pycnometry (Rouquerol et al., 1994) is sometimes dependent on the operational
temperature and pressure (Fulconis, 1996).
Because of its small size (collision diameter = 0.20 rim), helium would appear to
be a useful probe molecule for the study of ultramicroporous carbons. The experi-
mental difficulty of working at liquid helium temperature (4.2 K) is the main
reason why helium has not been widely used for the characterization of porous
adsorbents. In addition, since helium has some unusual physical properties, it is to
be expected that its adsorptive behaviour will be abnormal and dependent on
quantum effects.
Ln their recent investigations of helium adsorption by microporous carbons,
Kaneko and his co-workers (Kuwabara et al., 1991; Setoyama et al., 1993; Setoyama
and Kaneko, 1995; Setoyama er al., 1996) have obtained strong evidence that the
density of physisorbed helium is not the same as that of bulk liquid helium at 4.2 K
(i.e. 0.102 g cm"). By adopting the value 0.202 g ~rn-~, which had been proposed on
theoretical grounds by Steele (1956), Kaneko and his co-workers were able to obtain
fairly good agreement between the corresponding uptakes of He and N, by certain
microporous carbons - as indicated in Figure 9.23. With some other porous carbons,
the presence of narrow ultramicropores was demonstrated by the much larger appar-
ent pore volumes available for helium adsorption.
The shapes of a series of helium and nitrogen isotherms are compared in Figures
9.24 and 9.25. To facilitate comparison, the amount adsorbed is expressed in the form