Page 76 - Principles of Catalyst Development
P. 76
CATALYTIC MATERIALS 63
TABLE 4.4. (continued)
Group Metal Reaction
Ru NH, synthesis
Hydrogenation of CO, CO 2 (Fischer-Tropsch), olefins, oximes, nitriles,
phenols, benzyl alcohols, aryl amines, aromatic heterocycles, cyclopen-
tane, aliphatic ketones, nitrobenzenes, oxidation of hydrocarbons
Os Ammonia synthesis
VIIB Mn
Tc
Re Hydrogenation of olefins, aromatic, carboxylic acids, amides, nitrogen-
zenes, ketones, dehydrogenation of cyclohexanes, alcohols
" Note: Reverse reactions are also catalyzed. Similarities within groups are expected,
from a fundamental point of view, the role of these phenomena in practical
catalysis remains to be demonstrated.
Another emerging area of investigation is the behavior of supported
metal cluster compounds. (I t3.114) For research purposes, these materials
provide well-defined models. However, they have not yet led to any major
developments in industrial applications.
We leave the topic the metal catalysis with a listing of observed activities
(Table 4.4). Patterns of behavior are also important. Examples of these are
given in Chapter 5, and the reader should also refer to many excellent
sources in the literature.(115)
4.3. SEMICONDUCTORS
Catalytically interesting semiconductors are oxides and sulfides of
transition elements. (45.116.117) Unlike metals, electronic bands do not overlap
but form separated regions, shown in Fig. 4.14.
The lower band, the valence band, contains levels normally filled by
electrons participating in valence bonds of the crystal. (99) Separated by an
energy E g , the second band is empty unless electrons are promoted by heat
or radiation from their valence positions. The upper band is called the
conduction band, since electrons with these energies are sufficiently free to
migrate with the application of an electric field, i.e., conduction. Simul-
taneously, the "hole" left by the promoted electron conducts electrical
energy in the opposite direction. Two types of current carriers are involved.
Semiconductors of this type are called intrinsic but are not important in
catalysis, since the temperature range, 300-700°C, is usually too low to
generate a sufficient number of promoted electrons.