Page 216 - Principles of Catalyst Development

CATALYST DEACTIVATION 205
economics. Figure 8.15 shows activity decline when the poison encounters
diffusion resistance. Once again, if the reaction has a low value of 1>R
then decline is uniform. However, with strong diffusion limitations, the
reaction concentrates in the outer layers of the particle. Only a small amount
of poison is needed to give substantial decreases in activity.
Thus, in the diagnosis and analysis of poisoning deactivation, site
heterogeneity, stoichiometry, and diffusion play important roles. Each situ-
ation must be considered on its own merits, with all these factors taken
into consideration. Techniques for mathematical modeling of poisoning
phenomena have advanced to the point where they are useful tools in this
endeavor. (259)
We now examine in more detail the poisoning of catalytic materials,
maintaining the convenient classification of metals, semiconductors, and
insulating acids.

8.3.7.1. Poisoning of Metals
Catalytic metals have d orbitals available for adsorption. This is the
key both to activity and susceptib ility to poisons. Metal poisons fall into
three classes: (l) nonmetallic ions, (2) metallic ions, and (3) unsaturated
molecules. (273)
8.3.7.1 a. Nonmetallic Ions (s and p Orbitals). Most commonly encoun-
tered are those in group Vb (N, P, As, and Sb) and group Vlb (0, S, Se, Te).
The degree of toxicity depends on the availability of empty valence orbitals
or unshared electron pairs. If these are not present, then the ion is nontoxic.
Examples are given in Table 8.6. In certain environments, ions are toxic;
in others they are nontoxic.

8.3.7.1 b. Metallic Ions (d Orbitals). Ions of metals with occupied d
orbitals possess electrons that bond with empty orbitals of catalytic metals.
In applying this concept to platinum, metal ions were found to be toxic or
nontoxic as shown in Table 8.7.

8.3.7.1c. Unsaturated Compounds. Metals have a strong preference to
adsorb unsaturated compounds such as CO or C 2 H 4 • Ifretained irreversibly
in the molecular form, this is the same as poisoning. However, if decomposi-
tion or dissociation occurs, it is more properly treated as part of a coking
mechanism.
Reversibility of these poisons depends on process conditions. Sulfur-
poisoning of nickel catalysts, for example, is irreversible at lower tem-
peratures. Methanation catalysts beds cannot be regenerated even with