Product Description:
Among the numerous applications of the rare-earth
elements, the field of catalysis accounts for a large number. Catalysis
represents approximately 20% of the total market sales of rare earths worldwide.
As a matter of fact two main applications
have been prominent in the last decades: zeolite stabilization for fluid
cracking catalysts, and automotive post-combustion catalytic treatment.
The oldest use of rare earths in catalysis deals
with the structural and chemical stabilization of the zeolites for petroleum
cracking applications. For a long time this has been an area of application for
non-separated rare earths. The addition of several percent of rare earths in the
pores of the zeolite results in a strong surface acidity,
which is essential for an efficient conversion of high-weight molecules into
lighter species, like low-octane fuel, even in the very aggressive conditions of
the petroleum
industry.
The popular demand for high-quality air in spite
of the traffic congestion in large cities resulted in larger and larger
constraints in the emission exhaust from cars. Thus highly efficient catalysts
have had to be designed, and due to the combination of its redox properties and
very good thermal stability, cerium oxide has been since the beginning, early in
the 1980s, a major component of the three-way catalysts (TWC) now used in all
modern gasoline cars.
The future
of rare earths in catalysis is probably bright. The fact that approximately 400
patents are applied for yearly in the area since 1992 is an illustration of a
very active area. Usage of rare earths in catalysis is expected to grow due to
their highly specific properties. Instead of the physical properties used in
electronic applications, one deals now with redox properties, water and thermal
stability, coordination numbers and so forth. The rare earths are so specific in
these properties that their use can hardly be avoided, not only for the beauty
of academic
studies
but also for the development of industrial applications with immediate influence
on everyday life.
Careful control of the synthesis conditions and
the definition of optimum composition in each case are the keys to the
preparation of highly performing compounds for catalytic applications. They must
actually be considered as high performance products with functional properties,
and not just chemical species.
Chapters devoted primarily to catalysis have been
published in earlier volumes of the Handbook. In this volume several more are
added. The first is an extension of the earlier chapter 43, on interactions at
surfaces of metals and alloys, to reactions such as hydrogenation, methanation,
ammonia synthesis, saturated hydrocarbon reactions, dehydrogenation of
hydrogenated materials, hydrodesulfurization, and carbon
monoxide
oxidation. The second chapter reports on the wide variety of catalyzed reactions
involving metals and alloys in the innovated form of metal overlayers or
bimetallic compounds with some transition
metals produced from ammonia solutions. This is followed by a chapter on
catalysis with mixed oxides usually having perovskite or perovskite-related
structures.
Then follows a comprehensive discussion on the
background and current role of cerium oxide and associated materials for
post-treatment of exhaust gases for pollution
control.
These three-way catalysts (TWC) are designed to render harmless the CO, NOx, and
unburned hydrocarbons
from internal combustion engines. The next chapter considers the wide field of
zeolite catalysts containing rare earths from their historic use in petroleum
refining in the 1960s to other petrochemical and fine chemical applications
today. The final chapter documents the use of the triflates (the
trifluoro-methane-sulfonyl group which is a hard Lewis acid in both aqueous and
organic solutions) as versatile catalysts in carbon-carbon bond-forming
reactions. Their stability in the presence of water, in spite of their being
hard Lewis acids, enhances their growing usefulness.