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Encyclopedia of Physical Science and Technology EN010b-481 July 14, 2001 18:45

Noble Metals (Chemistry) 485

electroplating, and synthetic chemistry (e.g., the prepa- The precious metals have been used successfully in
ration of organic and organometallic compounds). They most types of catalytic reactions, and as their chemistry
are also used in the commercial production of zinc, man- becomes better understood new applications should open
ganese, and cobalt. up as well as improvements to the state of the art.

G. Fuel Cells 1. Gold

The fuel cell, a special case of a primary battery, involves Gold has been studied frequently as a catalyst or cocata-
the continuous feeding of high-energy reactants such as lyst, and it can show good or even improved selectivities
hydrogen, hydrocarbons, ammonia, or hydrazine and an butislessactivethanothernoblemetalsystems.Supported
oxidant such as pure oxygen, air, or peroxide. Fuel cells gold catalysts are usually prepared from HAuCl 4 by re-
have been used in the space program and show promise in duction. Gold is used primarily as an activity modifier with
areas such as peaking and intermediate power for electric other PGMs (e.g., in Pd–Au used in vinyl acetate produc-
utility generation and onsite cogeneration for residential, tion). The catalysts can hydrogenate olefins, dienes, and
commercial and industrial buildings. The direct use of acetylenes, but severe conditions are required and coking
hydrocarbon fuels is not currently efficient, but the fuels problems result. Oxydehydrogenations can also be carried
can be converted to hydrogen-rich streams in situ, e.g., by out with gold. For example, ketones can be converted to α,
steam reforming. β-unsaturated ketones in 70–94% selectivity and 22–48%
◦
The system closest to commercialization is the conversion at 400–600 C and ethyl benzene can be con-
hydrocarbon–air–phosphoric acid electrolyte cell. It is tol- verted to styrene in 94% selectivity and 53% conversion
◦
erant of carbon dioxide impurities. The molten carbonate at 700 C, but more effective catalysts are available.
electrolyte fuel cell is in an earlier stage in its develop-
ment cycle, but it has the potential for higher efficiency.
2. Silver
The solid oxide electrolyte fuel cell is also relatively early
in its development cycle, and, because of its high operat- Silver catalysts are uniquely suited for direct oxidation
ing temperature, it is expected that hydrocarbon fuels can of ethylene to the important industrial chemical ethylene
be converted directly to hydrogen bearing gas at the fuel oxide. Silver will also catalyze the dehydrogenation of
electrode.Alkalineelectrolytefuelcellshavebeenusedef- methanol to formaldehyde or other appropriate alcohols
fectively and at very high efficiency in the space program; to yield acetone or methylisobutylketone. Ethylene glycol
but commercially viable versions will require additional and air over Ag/Ag 2 O yield glyoxal. Commonly used ox-
development, and their use with carbonaceous fuels will
idation catalysts for sugars, olefins, and diols are Ag 2 CO 3
be limited because of their carbon dioxide intolerance. and Ag 2 CO 3 /Celite. Ag 2 SO 4 is used as a catalyst to deter-
To date, the catalysts for low-temperature fuel cell elec- mine the chemical oxygen demand (COD) in waste water,
trodes (phosphoric acid and alkaline cells) have been the and it has also been used for the reduction of aromatics to
precious metal blacks and, more recently, precious metals cyclohexanes. Silver has also shown activity in the isomer-
on carbon supports. Development of fuel cell catalysts us- ization of carbon–carbon bonds in strained ring systems
ing precious metals remains very active. Also, some work (e.g., cubane can be converted to cuneane). Acrylates and
is being done on systems that may be substituted for the acrylamides can be polymerized with AgClO 4 .
noble metals. For example, tungsten carbide based anode
catalysts have been shown to have good durability over
long periods, but they are not as active as platinum. 3. Platinum
In commercial applications probably the platinum cata-
lyst most widely known to the general public is that used
H. Catalysis
in the automobile catalytic converter, which currently ac-
Noble metals are widely used in homogeneous and het- counts for approximately 60% of U.S. platinum usage.
erogeneous catalysis. Their advantages are high activity The two-way converter, used between 1975 and 1980 to
under mild reaction conditions and improved selectivities remove carbon monoxide and hydrocarbons, is a Pt–Pd
compared to base metal catalysts. Precious metal catalyst catalyst with roughly a 7:3 ratio of metal. In 1980, the
systems are expensive initially because of the metal costs, three-way catalyst (Pt–Pd–Rh or Pt–Rh), which removes
but because PGMs can be recovered, the overall cost of carbon monoxide, hydrocarbons and nitrogen oxides, was
using these catalysts may actually be lower than that of a introduced. It is well documented that lead has significant
less active or selective base metal system. effects on the catalyst, from changing product selectivity