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2.3 Adsorption, Ion Exchange, and Catalysis: Three Related Phenomena 43

Table 2.5 ( Continued )

Scientist(s) name(s) Breakthrough Year
G. Natta Continues the study initiated by Karl Ziegler on 1953
metal-organic catalysts for polymerization of alk enes
P. Kisliuk Publishes a theory of precursor kinetics for 1957
chemisorption.
Catalytic converters are introduced in ne w cars 1975
in the United States.
R. Kelley, Measure the rate of a reaction catalyzed by a single 1982
D. Goodman crystal (methanation, Ni single crystals)

2.3 ADSORPTION, ION EXCHANGE, AND CATALYSIS:THREE RELATED
PHENOMENA

At a first glance, adsorption, ion e and catalysis are three different phenomena
xchange,
er
ferences,
, with dierse characteristics. Ho despite these dif there are many common
v
we
v
features among these processes. In the following sections, a relationship between them
will be attempted.
2.3.1 Adsorption and ion exchange
Ion exchange is similar to adsorption, since mass transfer from a fluid to a solid phase is
common in both processes, i.e. the fusion processes. Ion e xchange is also y are basically dif
a sorption process, but ions are the sorbed species in contrast to adsorption, where electri-
cally neutral species are sorbed (Noble and Terry, 2004; Perry and Green, 1999). It is gen-
erally accepted that adsorption and ion exchange can be grouped together as sorption for
a unified treatment in practical applications.
Most of the mathematical theories and approaches hae been deeloped originally for v v
sorption rather than ion ethey are suficiently general to be applicable er v, we xchange. Ho f
with minor, if an modifications to a number of similar phenomena such as ion e , y xclusion
and ligand exchange. According to Helfferich (1995), the applicability of a simplified theory
depends more on the mode of operation than on the particular mechanism of solute uptake.
A significant feature of physical adsorption is that the rate of the phenomenon is generally
too high and consequently, the overall rate is controlled by mass (or heat transfer) resistance,
Thus, rather than by the intrinsic sorption kinetics (Ruthven, 1984). sorption is vie wed and
termed in this book as a “diffusion-controlled” process. The same holds for ion exchange.

2.3.2 Catalysis and adsorption

As discussed earlier, the f irst step in heterogeneous catalysis is the adsorption of the mole-
cules of the reactants on the surface of the adsorbent or of the catalyst (inner and outer sur-
faces). Then, molecular dissociation of at least one or two reacting components tak es
fusion.
place, usually preceded by surface difThe next step is a surface reaction, which is