Page 289 - Adsorption by Powders and Porous Solids

CHAPTER 9. ADSORPTION BY ACTIVE CARBONS 277

until p/pO > 0.5. Another interesting difference between the two water isotherms in
Figure 9.26 is the size of their hysteresis loops: the loop given by Carbosieve is steep
and narrow, while the loop obtained with AX21 is broader and more rounded.
A qualitative explanation for isotherms of the type shown in Figure 9.26 was first
put forward by Pierce and Smith (1950), who postulated that initially a few water
molecules are adsorbed on polar sites (e.g. oxygen complexes) and, with increase in
p/p", clusters of molecules are then hydrogen-bonded around these favourable sites.
The clusters grow with increased pressure until they merge together and the pores are
filled. According to Pierce and Smith, the hysteresis is due to a difference between
the steps involved in pore filling and emptying, the latter representing a more stable
state. The two-stage process was the basis of the model adopted by Dubinin and
Serpinski (1981) and further developed by Barton and Koresh (1983).
The latter authors found that a reversible water isotherm was obtained after the
low-temperature (i.e. 40°C) evacuation of a carbon cloth, which had been activated
by oxidative HNO, treatment. The molecular sieve character of this material was
reduced by evacuation at 400°C and this also led to the appearance of hysteresis in
the water vapour isotherm. Barton and Koresh (1983) conclude that such hysteresis
is mainly due to the concentration of surface oxides 'which dictate the adsorption
value at which the change from cluster adsorption to a continuous adsorbed phase
takes place'. The relationship between the adsorption of water and the surface con-
centration of chemisorbed oxygen was first established by Walker and Janov (1968).
Bansal et al. (1978a,b) also investigated the influence of the surface oxygen on the
adsorption of water: they concluded that at p/pO < 0.5 the level of water uptake is
determined by the concentration of surface oxygen-containing structures.
Truly reversible Type V isotherms are quite rare. It is significant that the example
reported by Dubinin (1980) was obtained on a low bum-off (5.7%) carbon, which
was certainly ultramicroporous. It was pointed out that with an activated carbon
obtained by 20% bum-off, the hysteresis extended over virtually the whole range of
pore filling - the water isotherm then having a very similar appearance to that of the
Carbosieve isotherm in Figure 9.26.
A study of the hydrophilic sites on the surface of activated carbon fibres has been
made recently by Kaneko et al. (1995) with the aid of X-ray photoelectron spec-
troscopy (XPS). In this work cellulose (CEL)- and polyacrylonitrile (PAN)-based acti-
vated carbon fibres were used and samples were either chemically treated with H,02 or
heated in H, at 10OO0C. As expected, surface oxidation by the H,02 treatment
increased the initial uptake of water, while the H, reduction caused a marked decrease
in the amount of water adsorbed at lowp/pO. Measurement of the peak areas of the XPS
spectra provided a means of determining the fractional surface coverage by the
hydmphilic sites. In this way a linear relationship was found between the low-pressure
adsorption of water vapour and the number of hydrophilic sites (mainly -COOH).
The low affinity of the surface of pure carbon for water is associated with the
unusually weak non-specific interactions between the non-polar surface and the
adsorbate. When certain functional groups are present on the carbon surface, specific
interactions come into play and the adsorption affinity is thereby increased (see
Chapter 1).