Page 124 - Principles of Catalyst Development
P. 124
112 CHAPTER 6
o
UJ
III
a:
o
C/)
o
c(
I-
Z
::J
o
::!'
c(
CONCENTRA TlON
Figure 6.14. Isotherm for adsorption of catalytic ions.
Adsorption from solutions may be either cationic or anionic depending
on the properties of the surface. In general, zeolites are strong cation
exchangers, silica is a weak cationic adsorber, alumina adsorbs both cations
and anions weakly, magnesia is a strong anion adsorber and carbon prefers
to form charge-transfer complexes with electron donation but also weakly
adsorbs cations. (157) With the exception of zeolites, that follow a different
mechanism, the governing processes for ionic adsorption involve equili-
brium reactions of the type
(6.6)
and
(6.7)
Equilibrium is determined by the concentrations ofC C and A- in the solution
and by the pH. For cations, relative adsorption strength is largely a function
of the polarizing power of the ions. A cation adsorber C, has affinities in
the order C + > C3+ > C + > C+ - H+. Adsorption of anions depends on
2
4
anionic polarizability and ionic charge, so that SO~- > r > Br- > Cl- >
F-, etc. Ionic adsorption characteristics of catalytically important groups
are given in Table 6.3.