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

486 Noble Metals (Chemistry)

to complete deactivation. This is seen when leaded gaso- difficult. Like platinum, the palladium systems are widely
line is used in a car equipped with a catalytic converter. used for reductions. For example, nitro, olefin, acetylene,
Platinum on a monolithic honeycomb support has been diene, aldehyde, and ketone groups can readily undergo
used in a large number of odor and pollution abatement hydrogenation and substituted systems having benzyl or
applications. The Pt–Re and Pt–Ir catalysts on acid sup- halide functions can undergo hydrogenolysis. Palladium
ports are used in the petroleum industry for re-forming is used in petrochemical purifications to remove acetylene
low-octane naphthas into high-octane branched hydrocar- or diolefins from olefins and unsaturates from a saturate
bons and aromatics. The platinum acts as a hydrogena- stream. Palladium is also used in the synthesis of hydrogen
tion/dehydrogenation catalyst, and the acid support is re- peroxide. It has been used to control or eliminate oxygen
sponsible for the isomerization, cracking, and cyclization by catalyzing the hydrogen–oxygen combination reaction.
reactions. This process also serves as a source of aromat- On the exploratory front, the palladium glasses mentioned
ics for other purposes. Product stream octane ratings as earlier are being tested in hydrogenation reactions when
high as 104 are produced. A 90Pt–10Rh gauze is used to it is thought that their disordered structure might lead to
produce nitric acid from ammonia and oxygen. The ni- some novel chemistry.
tric acid is used in fertilizers and explosives. The alloy
90Pt–5Rh–5Pd is also used. During HNO 3 production,
5. Iridium
platinum loss can be reduced by the use of a getter, which
is a palladium-based gauze, downstream. The 90Pt–10Rh Iridium has little commercial use in catalysis. Its main
is used to convert ammonia, methane, and oxygen into function is as a synergist with other PGMs (e.g., Pt–Ir,
HCN. Platinum is used on glass wool or asbestos in cat- which was mentioned earlier as a re-forming catalyst).
alytic heaters to burn hydrocarbons or alcohols. It was Iridium compounds, mainly homogeneous, have been
first used for H 2 SO 4 production, in which it catalyzed the shown to catalyze olefin and acetylene hydrogenation,
oxidation of SO 2 to SO 3 . This is one of the few areas in olefin isomerization, hydroformation, and oxidation reac-
which noble metals have lost ground, with the platinum tions, as well as H–D exchange in saturated hydrocarbons.
being replaced by a base metal system. In general, iridium is not as good a catalyst as rhodium.
Platinum is used in hydrogenation reactions. The most
commonly used catalytic systems for chemical synthe-
6. Rhodium
sis are PtO 2 , Pt/C, and Pt/Al 2 O 3 . Platinum will catalyze
reductive alkylations, NO x to NH 2 OH, cyclohexanone Rhodium is an excellent catalyst for many reactions. It
oxime to caproiactam for use in Nylon 6, and selectively is used commercially as a homogeneous catalyst (e.g.,
reduce nitro groups in halonitroaromatics. Homogeneous [Rh(CO) 2 I 2 ] and CH 3 I) to produce acetic acid or acetic
−
systems [e.g., Pt(PPh 3 ) 4 ] will catalyze oxidation reactions anhydride from methanol and carbon monoxide. An-
such as the conversion of cyclohexanone to adipic acid and other process is the hydroformylation of propylene using
carbonmonoxidetocarbondioxide.Pt/Al 2 O 3 isaversatile RhH(CO)(PPh 3 ) 3 to yield mainly the desired n-butanal
dehydrogenation catalyst useful for producting alkenes and 2-methylpropanal. It has been shown that the product
and aromatics. Platinum(II) and platinum(IV) will activate isomer ratio can be varied by changing the alkyl or aryl
C—H bonds in saturated molecules for H–D exchange in group on the phosphine. As discussed earlier, rhodium is
D 2 O. This type of system is used to produce heavy water. now used in catalytic converters to control NO x pollution.
Platinum catalyzes many other reactions such as dimeriza- The complex RhCl(PPh 3 ) 3 , Wilkinsons catalyst, was the
tions, hydrosilations, oxygen removal, and water gas shift. first homogeneous catalyst that would hydrogenate olefins
and other unsaturated compounds at ambient temperatures
and 1 atm of pressure. The system was found to be ver-
4. Palladium
satile since it will also catalyze olefin isomerizations, hy-
Commercial uses for palladium catalysts include hydro- droformylations, decarboxylations, hydrosilylations, and
cracking, vinly acetate production, and the Wacker pro- oxidations. Work is being done to anchor catalysts such
cess, which is used to convert olefins into aldehydes or as Wilkinson’s onto a polymer backbone to avoid the sep-
ketones. The best known example is the conversion of aration problems inherent in a homogeneous system.
ethylene to acetaldehyde. The process is catalyzed by An impressive application of rhodium is in asymmet-
[PdCl 4 ] , which is reduced to palladium(0) as the olefin ric synthesis. A bis-phosphine rhodium complex using an
2−
is converted to the aldehyde. The metal is reoxidized with asymmetric phosphine is used to prepare L-DOPA (di-
CuCl 2 . The homogeneous system is attractive here be- hydroxyphenylalanine), a chiral compound used to treat
cause the product is easy to separate from the catalyst. This Parkinson’s disease, in high optical yield. Asymmetric
is an advantage that heavier products may not enjoy in ho- synthesis is a very active and promising area of research.
mogeneous systems in which separation would be more Rhodium has been used as a dimerization catalyst in