Page 276 - Adsorption by Powders and Porous Solids

264 ADSORPT~ON BY POWDERS AND POROUS SOWDS

very different in size: the kinetic diameter of N, is 0.36 nrn and that of CO, is
0.33 nm; and the corresponding minimum dimensions are 0.30 and 0.28
However, by far the most important factor in causing the greater uptake of CO, is the
higher operational temperature (Gregg and Sing, 1982, p. 230).
This effect is exploited to an even greater extent when the carbon dioxide mea-
surements are undertaken at 273 or 298 K. At these temperatures the carbon dioxide
saturation pressures are extremely high (e.g. at 298 KpO = 63.4 bar) so that the range
of p/pO is limited to 20.02 at sub-atmospheric pressures. This has the advantage that
the initial part of the isotherm can be determined with a much greater accuracy than
is nonnally possible with nitrogen at 77 K, and in addition the DR plots are generay
more linear (Rodriguez-Reinoso, 1989).
Rodriguez-Reinoso and his co-workers have identified three groups of porous
carbons: (a) carbonized materials and activated carbons of low burn-off, giving much
larger CO, uptake, because of restricted diffusion of nitrogen into very narrow pores;
(b) activated carbons of low-to-medium bum-off, having fairly narrow micropores
and giving approximately equivalent uptakes of CO, and N,; and (c) activated
carbons of medium-to-high burn-off, having a range of wider micropores and giving
larger uptakes of N, than CO,.
Recent experiments by Cazorla-Amoros et al. (1996) have involved the measure-
ment of CO, isotherms up to pressures of 4 MPa at 273 and 298 K. These investiga-
tors have confirmed that CO, adsorption at subatmospheric pressures is a useful
complementary technique for the characterization of very narrow micropores and
that at higher pressures the adsorption of CO, is similar to nitrogen.

9.5.2. Adsorption of organic vapours
Benzene was the most popular adsorptive in many early studies of the pore structure
of activated carbons (Dubinin, 1958, 1966; Cadenhead and Everett, 1958; Smisek and
Cemy. 1970). Indeed, in order to construct the characteristic curve for a given micro-
porous carbon, Dubinin and his co-workers (Dubinin, 1966) originally adopted
benzene as the standard adsorptive: thus, in the context of the Dubinin theory of the
volume filling of micropores (TVFM), the scaling factor 0 (C,H,) = 1 (see Chapter 8).
Polanyi's concept of the temperature invariance of the characteristic curve became
an important feature of the TVFM proposed by Dubinin (1966). The approach pro-
vided a way of bringing together a family of isotherms determined at different tem-
peratures. The resulting common curve may be regarded as the relation between the
fractional filling of the pores of a microporous carbon by a particular adsorptive and
the 'adsorption potential', defined as RTln (pO/p). An example of a typical
characteristic curve is shown in Figure 9.17, where it is evident that for the active
carbon CK the common characteristic curve is given by all the benzene isotherms
determined over the temperature range of 20-140°C.
Not all characteristic curves are temperature invariant (Aranovich, 1991;
Tolmachev, 1993). Invariance over a wide temperature range has thermodynamic
implications, which are unlikely to be consistent with the behaviour of many systems
- especially when strong adsorbent-adsorbate interactions or a combination of