Page 19 - Adsorption by Powders and Porous Solids

4 ADSORPTION BY POWDERS AND POROUS SOLIDS

Another important stage in the history of gas adsorption was the work of Brunauer
and Emmett, which preceded the publication of the Brunauer-Emmett-Teller (BET)
theory in 1938. In 1934 Emmett and Brunauer made their first attempt to use low-
temperature adsorption of nitrogen to determine the surface area of an iron synthetic
anunonia catalyst. They noted that the adsorption isotherms of a number of gases,
measured at temperatures at, or near, their respective boiling points, were all
S-shaped with certain distinctive features. Others, including Langrnuir, had recog-
nized that this type of adsorption was not always restricted to monolayer coverage
and an empirical approach was adopted by Emmett and Brunauer (1937) to ascertain
the stage at which the mdtilayer adsorption began. They eventually decided that
completion of the monolayer was characterized by the beginning of the middle nearly
linear section of the adsorption isotherm (designated Point B - see Figure 1.7). The
surface area was then evaluated from the amount adsorbed at Point B by making the
further assumption that the completed monolayer was in a close-packed state. In
1938 the publication of the BET theory appeared to provide a sound basis for the
identification of Point B as the stage of monolayer completion and the onset of
multilayer adsorption.
It would be difficult to overestimate the historical importance of the BET theory
since for over 50 years it has attracted an enormous amount of attention (Davis,
1991). Indeed, the BET method is now accepted as a standard procedure for the
determination of the surface area of a wide range of fine powders and porous
materials. On the other hand, it is generally recognized that the theory is bas* on an
oversimplified model of multilayer adsorption and that the reliability of the BET
method is questionable unless certain conditions are fulfilled.
There was a growing awareness in the early 1930s that a distinction could be made
between physical adsorption (i.e. physisorption) in which the van der Waals interac-
tions are involved and chemical adsorption (i.e. chemisorption) in which the
adsorbed molecules are attached by chemical bonding. Taylor (1932) introduced the
concept of 'activated adsorption' which, by analogy with the familiar idea of an
energy of activation in chemical kinetics, attempted to explain the marked increase in
rate of adsorption with rise in temperature in terms of surface bond formation. The
activated adsorption theory aroused a good deal of early criticism and with the sub-
sequent improvement of high vacuum techniques it was established that chemisorp-
tion of certain gases can take place very rapidly on clean metal surfaces. However,
there are other chemisorption systems which do appear to exhibit some features of
activated adsorption.
In his 191 6 paper, Langmuir had stated that with highly porous adsorbents such as
charcoal 'it is impossible to know definitely the area on which the adsorption takes
place' and that 'there are some spaces in which a molecule would be closely sur-
rounded by carbon atoms on nearly all sides'. He concluded that equations derived
for plane surfaces were not applicable to adsorption by charcoal. Unfor!xnately, these
observations have been overlooked by many investigators, who have applied the
simple Langrnuir monolayer equation to adsorption data obtained with zeolites and
activated carbons.
The significance of Langmuir's comments was appreciated, however, by Dubinin