Page 40 - Principles of Catalyst Development

26 CHAPTER 2
For both processes, carbon formation is serious. (41) In reforming it
originates mostly from CH 4 decomposition, in methanation from CO dispro-
portionation. Preventing nickel from catalyzing these conversions is difficult
without affecting the main reactions. The solution is to remove the carbon
through reaction with steam before polymerization to graphite occurs. This
is done by adding another catalyst (in this case a promoter) to the
formulation. (42) Alkalis catalyze carbon removal, the most effective being
potassium. The fact that they also lower nickel activity for the main reaction
is another compromise that must be accepted.
In this example, the delicate balance between three catalyst factors and
process conditions is clearly demonstrated. Other examples will be seen
over and over again as we examine successful, proven catalyst developments.

2.3. CATALYST COMPONENTS

Although some catalytic materials are composed of single substances,
most catalysts have three types of easily distinguishable components: (1)
active components, (2) a support or carrier, and (3) promoters. (43) In Fig.
2.2 we again invoke a triangle to demonstrate mutual dependencies.

2.3.1. Active Components
Active components are responsible for the principal chemical reaction.
Selection of the active component is the first step in catalyst design. As
knowledge of catalytic mechanisms on various materials advances, methods
for selection are becoming more scientific, if perhaps still empirical. These
are examined in subsequent chapters.
Historically, it has been convenient to catalog active components
according to the type of electrical conductivity (Table 2.1). Table 2.1 is not
intended to be exhaustive but to give perspective to the classification. More
examples are given in Chapter 4.
The main reason for classifying active components by conductivity
type is one of convenience. No relationship between conductivity and
catalysis should be assumed. However, both depend on atomic electronic
configurations. Each of the three, metals, semiconductors, and insulators,
has a theoretical and experimental background useful in deriving catalytic
models. With metals, overlapping electronic energy bands promote electron
transfer with adsorbing molecules. (44) Redox or charge transfer reactions
such as hydrogenation, hydrogenolysis, and oxidation are found. Systematic
variation with atomic electron configurations explains trends in adsorption
and catalysis. Similarities in groups within the periodic table are rationalized.
Orbital considerations, such as type, occupancy, and symmetry, may be