Page 126 - Principles of Catalyst Development
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114 CHAPTER 6
Figure 6.15. Shell adsorption of chloroplatinic acid.
This technique may be extended with strongly adsorbing acids such as
oxalic, tartaric, and citric. These adsorb preferentially on the outside, forcing
platinum ions deeper into the particles, as shown in Fig. 6.16.
Profile (a), for example, is useful in cases such as automobile exhaust
oxidation. Platinum-poisoning lead contamination adsorbs in the outer
platinum-free shell of alumina, which acts as a guard.(169) Applications for
(b) and (c) include reactions with negative order and diffusionally limited,
consecutive reactions.
6.4.3. Ion Exchange
Ion exchange in catalyst preparation is very similar to ionic adsorption
but involves exchange of ions other than protons. Lower valency ions, such
as Na t, exchange with ions having higher charge, for example Ni ' ,accord-
2
ing to the equilibrium
(6.8)
Ion exchange is useful in removing harmful agents and adding promoters.
During washing with ammonium solutions, NH~ ions are exchanged with
impurities such as Na' to remove potential poisons. For controlled loadings
of active components or promoters, the catalyst is soaked in excess solutions
containing the exchange ions. Loading or extent of exchange is controlled
with soaking time. Even multiple exchange is possible. Figure 6.17 shows
---INCREASING ACID STRENGTH •
(a) (b) (c) (d)
Figure 6.16. Pellet concentration rrotiles with adsorbing acids.