Page 78 - Principles of Catalyst Development
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CATALYTIC MATERIALS 65
Much more significant are the impurity "-type and p-type shown in
Fig. 4.15. In an "-type semiconductor, for example ZnO, the structure is
nonstoichiometric, and an excess of Zn + ions exists. The small cation is
easily accommodated in interstitial positions, generating energy levels that
are filled ( donor) with energy values close to the conduction band. Only a
small amount of thermal or radiation energy is necessary for promotion
and conduction. Conductivity occurs via electrons in the conduction band.
With p-type semiconductors, there is an excess of anions and cationic
vacancies. These produce empty levels (acceptor) close to the valence band,
from which an electron is easily promoted. Conductivity occurs through
the resulting hole in the valence band.
Donor and acceptor levels are also created by introducing (doping)
altervalent cations into interstital or cationic lattice positions. Both may
coexist within the same crystal.
The Fermi level is the electrochemical potential, midway between the
highest filled and the lowest empty levels. Since the Fermi level also responds
to doping (increases with more donor levels, decreases with acceptor levels)
and is easily measured, it is a convenient index with which to follow changes
in electronic structure.
Table 4.5 gives the type of semiconductivity found in common transition
oxides and sulfides. Many of these will be recognized as catalytically
important materials.
The role of these electronic structures in catalytic reactions is best
demonstrated with a much-studied probe reaction, decomposition of nitrous
oxide(117):
(4.2)
The mechanism is believed to proceed as follows:
(4.3 )
(4.4)
where donation of electrons to the catalyst is the rate-determining step.
Transfer of an electron from adsorbed 0- only takes place if the Fermi
level of the surface is below the ionization potential of 0-. This is most
likely to occur in p-type semiconductors, where the Fermi level is sufficiently
low as shown in Fig. 4.16.
Relative activities for reaction (4.2) over a wide range of semiconductors
are given in Table 4.6. Several important points emerge from the data in
Table 4.6. First, p-type oxides are more active than "-type oxides. This is
consistent with the model of electron transfer to the catalyst. Second, the