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Chapter 9. Adsorption by Active Carbons
Figure 9.4. Standard nitrogen isotherms for carbons (a), and c o w d i n g FHH plots (b) (circles, Carrott et d.,1988a; diamonds, Voloshchuk et al., 1988,
squares, Isidcyan and Kiselev, 1961; triangles down, Pierce, 1968; triangles up, Rdiguez-Reinoso et d.,1987). Reproduced from C m t t and Sing (1989).
9. ADSORPTION BY ACTIVE CARBONS
couldserve as standard for both graphitized and non-graphitized carbons. There are
three main reasons why there are significant discrepancies between proposed standard data to be found in the literature. First, any significant differences in the
pphitic surface structure will have some effect on the isotherm shape - especially
low surface coverage. Second, any interparticle capillary condensation will
an upward deviation in the multilayer/capillary condensation region. Third,
any microporosity will enhance the adsorption in the sub-monolayer region and will
tend also to reduce the isothenn slope in the multilayer region.
As explained in Chapters 6 and 8, by applying the as-method we have a simple
way of checking the validity of the BET area and detecting the presence of microporosiv. Many carbon blacks have been found to be essentially non-microporous
(Cmott et al., 1987; Bradley et al., 1995), in which case the corresponding values of
BET area and as area are in good agreement. However, in a few cases the backof the as-plot has given a positive intercept on the adsorption axis
which is an indication of some microporosity. The microporous nature of some
carbon blacks has been confmed in several recent investigations (Stoeckli et al.,
1994a; Kruk et al., 1996). As one might expect, oxidation leads to a considerable
increase in the level of the microporosity (Bradley et al., 1995).
The strong energetic heterogeneity exhibited by Spheron 6 was first shown calorimetrically by Beebe and his co-workers (Beebe el al., 1947; Kington et al., 1950).
Tbis work also revealed that the surface of Graphon was much less heterogeneous
than that of the original carbon black. The results of a more detailed investigation of
the effect of thermal treatment of carbon black on the energetics of nitrogen adsorption (i.e. variation of ~ , h with coverage 19)are shown in Figure 9.5. Microcalorimetric measurements were undertaken on a sample of heat-treated Sterling
FT-FF (i.e. a thermal black).
. 1 0~5~
. 1 ~ 'o 6.70-'
5.10~o S . ~ O - ~
AgI em' g"
Nz I Sterling - 77 K
A , h / kl.moil
Figure 9.5. Differential enthalpies of N, adsorption at 77 K on heat-treated blacks (at temperatures
from 1500 to 2700°C). as a function of coverage (Grillet er al.. 1979).
ADSORPTION BY POWDERS AND POROUS S O m s
The energetic heterogeneity of the original ungraphitized surface was revealed by
the careful adsorption calorimetric measurements undertaken by Beebe et al., (1953).
By comparing the changes in the differential energies of adsorption for argon on
Spheron and Graphon, these investigators found that the initial steep decline in 'differential heat of adsorption' was largely removed as a result of graphitization.
Instead, an increase in the differential energy was observed at a higher surface coverage: it is now generally agreed that this was due to the adsorbate-adsorbate interaction becoming apparent as the degree of energetic heterogeneity was reduced.
A systematic study of laypton adsorption on exfoliated graphite was subsequently
undertaken by Thomy and co-workers (Thorny and Duval, 1969; Thorny et a!.,
1972). Their stepwise isotherm, determined at 77.3 K, is shown in Figure 4.1. The
layer-by-layer nature of the physisorption process is clearly evident - at least up to
four molecular layers. This isotherm shape is remarkably similar to that of the
krypton isotherm on graphitized carbon black reported by Arnberg er al., (1955).
The work of Thorny and co-workers (Thorny and Duval, 1969; Thorny et al., 1972)
provided the first well-documented evidence for the presence of a sub-step in the
krypton isotherm. The effect of temperature on the shape and the location of the substep is shown in Figure 9.7. The fact that the riser of the sub-step remained vertical
over the temperature range of 77.3-96.3 K served to confirm that the sub-step was
Figure 9.7. Adsorption isotherms of krypton on exfoliated graphite. Curves labelled from 1 to 10,
obtained at 77.3, 82.4, 84.1,85.7,86.5,87.1, 18.104.22.168.90.1 and 90.9 K, respectively (courtesy Thorny