Page 302 - Adsorption by Powders and Porous Solids

CHAPTER 10. ADSORPTlON BY METAL OXIDES

Table 10.2. Standard data for the adsorption of argon at 77 K on non-porous
hydroxylated silica
Relative pressure p/pO as ( = n/na,) Relative pressure p/po a, ( = n/n,,)

FHH plots have also been constructed from the isotherms of isobutane and neopen-
tane on the pyrogenic silicas (Carrott et al., 1988). Derived values of the FHH
exponent, s, for neopentane s (np) and nitrogen s (N,) are recorded in Table 10.3. '
Also included in this table are the BET-nitrogen surface areas a(BET-N,), and the
BET C values for nitrogen and neopentane C (N,) and, C (np). As expected, the high-
temperature treatment of TK800 and Aerosil 200 resulted in the removal of a high
proportion of the surface OH groups: this in turn led to a signifkant reduction in the
nitrogen-adsorbent interaction energy and consequently a decrease in the values of
c (N,).
The remarkable constancy of the values of s(N3 and s(np) in Table 10.3 follows
from the fact that the linear FHH plots are parallel in the multilayer range. These
results confi that the multilayer character of each adsorptive is rather insensitive to
changes in surface chemistry.
The as-method has been used for the analysis of the isotherms of the following
gases on porous and non-porous silicas: nitrogen (Bhambhani et al., 1972; Carron and
Sing, 1984) argon (Carruthers et al., 1971; Payne et al., 1973) carbon tetrachloride
(Cutting and Sing, 1969) and neopentane (Carrott et al., 1988). A consistent pattern of
behaviour has emerged from the study of different samples of the pyrogenic silicas.
The derived a,-plots are all linear over the monolayer and lower multilayer range. In