Page 305 - Adsorption by Powders and Porous Solids

ADSORPTION BY POWDERSAND POROUS SOLJDS

Table 10.4. Comparison of surface areas of pyrogenic silicas eval-
uated by the BET and as-method.
Silica Outgassing T(K) a(BET) 4s)
(m2 P-'1 (mZ g-'1

area a(N2) determined by nitrogen adsorption. Thus,

The five a(np) values in Table 10.3 are in close agreement (i.e. 0.595 * 0.015 nm2),
but they are much larger than the value, 0.40 nm2, calculated for a close-packed
monolayer of freely rotating neopentane molecules. However, it must be kept in
mind that the C(BET) values are very low (i.e. 6-8) and therefore it is unlikely that
n,(np) represents the true statistical monolayer capacity.
Prolonged storage of a pyrogenic silica can result in slow ageing, as indicated by a
loss of BET area. For example, the original specific surface area of a master batch of
TK800, as evaluated from the measurements of Payne et al. (1973) and Baker and
Sing (1976), was 163-166 m2 g -'. A loss of area of c. 6 m1 g-l, which occurred over
a period of eight years, was evidently due to the development of some interparticle
mesoporosity since this led to the development of a narrow hysteresis loop in the
nitrogen isotherm at high nitrogenp/po (Carrott and Sing, 1984). A partial restoration
in area was achieved by raising the outgassing temperature from 25°C to 140°C (see
Table 10.4). This restoration was probably due to the removal of water from the par-
ticle interstices: it is likely that the presence of residual water and compaction were
together responsible for the loss of area.
A number of detailed studies have been made of the physisorption of gases on
compacted silica powders (Avery and Ramsay, 1973; Gregg and Langford, 1977).
The changes in the character of nitrogen adsorption as a result of increasing the com-
pacting pressure are shown in Figure 10.4.
It is striking that the isotherms in Figure 10.4 change fist from Type I1 to IV and
finally to Type I. Thus, the results of Avery and Ramsay (1973) have clearly demon-
strated that mesopores and micropores can be produced by the progressive com-
paction of a non-porous powder. However, a drastic loss of surface area accompanied
this change (from 630 to 219 m2 g-') and Avery and Ramsay point out that this was
associated with a marked increase in the particle packing density.
The importance of surface silanol (hydroxyl) groups in controlling the specific
physisorption interactions was studied by Kiselev (1958. 1971). Kiselev and his co- .
workers found that fully hydroxylated silica has a surface OH concentration of
7-9 pmol m-', which corresponds to a surface population, N(OH), of c. 5 OH nm-I