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Encyclopedia of Physical Science and Technology EN014A-653 July 28, 2001 20:55

20 Rare Earth Elements and Materials

The catalyst formulation comprises three key constituents:
precious metals (platinum, palladium, rhodium), alumina,
andrare-earth-basedmaterialswhichenhancecatalyticac-
tivity of the metals. Ceria is typically used in this capac-
ity. It is white when pure, but is usually pale yellow on
account of the nonstoichiometric phases between Ce 2 O 3
FIGURE 15 Molecular structures of ligands of rare-earth-based and CeO 2 . The nonstoichiometric ceria is responsible for
contrast agents for magnetic resonance imaging (MRI).
the following functions to remove the pollutants from the
motor vehicle engine exhaust:
the contrast agents will certainly be playing accordingly
critical roles for high-resolution diagnosis.
a. Promotion of the water-gas shift reaction: CO +
H 2 O ⇒ CO 2 + H 2 .
D. Other Miscellaneous Applications b. Enhancement of the NO x reduction capability of
rhodium.
1. Hydrogen Storage
c. Oxygenstorage.Inthisrole,ceriaprovideselemental
Considering the decreasing resource of petroleum and oxygen in fuel-rich but air-poor conditions, to ensure oxi-
growing environmental concern, the use of hydrogen as dation of unburnt hydrocarbons and the removal of CO. It
fuel is most attractive because it is abundant, nearly inex- accomplishes this by going nonstoichiometric to CeO 2−x .
haustible, and environmentally benign. However, liquid In leaner (fuel-deficient, air-rich) conditions, it re-oxides
H 2 is difficult to store and transport because of its low to CeO 2 , i.e., it stores oxygen during the air-rich periods.
density and low boiling point. In addition, H 2 forms ex-
plosive mixtures with air, adding further difficulty to its Catalytic converters are an integral part of all automobile
storage. The rare earth intermetallic compounds provide exhaust systems manufactured since the mid-1970s (in the
a means to store H 2 as a solid compound (hydride) from United States as a result of the 1970 Clean Air Act).
which it can be re-extracted. These properties of the al-
loys can be utilized for the hydrogenation of unsaturated 3. Superconductors
organic compounds. The more important applications of
rare earth intermetallic hydrides are in the technologically Few events of recent times have received as much atten-
important areas. For example, the nickel rare earth hy- tion, both from the scientific community and the general
dride battery provides a source of energy storage to the public, as the discovery and characterization during the
new generation of electric vehicles. They are also exten- period late 1986 to 1988 of the so-called “high tempera-
sively used in cordless tools, camcorders, cellular phones, ture superconductors.” These are a series of structurally
and laptop computers. These rechargeable batteries are related, complex copper oxides, most of which contain
improving the performance of portable electronic devices rare earth elements as an integral component. Although
by reducing their size and weight, while increasing the en- the first material was discovered in early 1986, the report
ergy storage capacity. Moreover, because they contain no was not published until late in that year. The authors of this
heavy metals such as cadmium, lead, and mercury, these report, G. Bednorz and A. Muller, had found signs of “pos-
batteries contribute to the protection of the environment. sible” superconductivity at temperatures as high as 29 K
(the previous record was 23 K for Nb 3 Ge) in a multiphase
mixture of lanthanum, barium, and copper oxides. Other
2. Catalytic Converter
workers in Tokyo quickly identified the single compound
A well-tuned engine still produces many pollutants caused responsible for the superconductivity as La 1.85 Ba .15 CuO 4 .
by the incomplete combustion of the fuel. The three reg- Ironically, this exact compound had been synthesized
ulated harmful compounds from exhaust gas mixtures are earlier by several other groups, in one case in the
hydrocarbons (in the form of unburned gasoline), car- mid-1970s, but no one had measured electrical conduc-
bon monoxide (formed by the combustion of gasoline), tivity down to sufficiently low temperatures until Bednorz
and nitrogen oxides (created when the heat in the engine and Muller.
forces nitrogen in the air to combine with oxygen). Carbon This discovery generated much frenetic activity world-
monoxide is a poison for any air-breathing animal. Nitro- wide, and in early 1987 a large group at the Universities of
gen oxides lead to smog and acid rain, and hydrocarbons Houston and Alabama-Huntsville reported another multi-
produce smog. phase mixture with a superconducting transition temper-
A catalytic converter is a device that uses a catalyst to ature above 90 K. This was recognized as another break-
convert the three harmful compounds into harmless ones. through, as the transition temperature now exceeded 77 K,