Page 333 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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

Noble Metals (Chemistry) 487

the preparation of 1-butene from ethylene and trans-1,4- dizable material, a potential fire hazard exists, as is often
hexadiene from butadiene and ethylene. In a reaction sim- noted in the literature. For safety, the catalysts should be
kept under an inert atmosphere and wet if possible. The re-
ilar to that in the Wacker process, RhCl 3 and Cu(ClO 4 ) 2
or Cu(NO 3 ) 2 will convert alkenes and oxygen into methyl duced catalysts are available commercially in a water-wet
ketones. The cluster complex Rh 6 (CO) 6 will catalyze the form to reduce the fire hazard.
oxidation of cyclohexanone to adipic acid and CO to
CO 2 . Cubane can be converted to syn-tricyclooctadiene
I. Dental Applications
as an example of strained ring isomerization. Rhodium
will also catalyze H–D exchange with saturated hydro- Noble metal alloys, wires, and solders are used in dentistry
carbons. Cluster compounds (e.g., [Rh 12 (CO) 34 ] 2− and because they are nontoxic, biocompatible, stress and wear
[Rh(CO)I] x ) will catalyze the water gas shift reaction. resistant, noble in the oral environment, and have good
Cluster compounds have also been shown to form ethylene casting properties. The alloys selected for use must have
glycol from synthesis gas. good fluidity and low gas absorption when molten for
good reproduction in the preparation of partials, bridges,
crowns, and orthodontic appliances and in filling cavities.
7. Osmium
The standards for dental alloys are set by the American
Osmium has only minor uses as a catalyst. The most Dental Association Specifications.
useful reaction is the conversion of olefins to cis diols
using OsO 4 . In the process, the OsO 4 is converted
1. Gold
to OsO 2 (OH) 2 , but the tetroxide can be regenerated
using H 2 O 2 or ClO − to make the reaction catalytic. The use of gold is decreasing because more economical
HOsCl(CO)(PPh 3 ) 3 can convert alkynes to cis alkenes and metalsarebeingused,butitisstillthepreferredcolor.Gold
Os 3 (CO) 12 shows some activity for the hydroformylation foil and matte gold have been used in the direct filling of
of olefins. certain types of cavities but these fillings are not as strong
as those made with other alloys. Gold used in dentistry
involves various alloyed combinations with platinum, pal-
8. Ruthenium
ladium, silver, iridium, rhodium, copper, zinc, and nickel.
Ruthenium can be used alone or as a synergist. It is a good The exact composition depends on the end use. Copper
catalyst for reducing carbonyls to alcohols, aromatics to and silver are added to obtain a solid solution and hard-
cyclohexanes,andforthemethanationofCO.Otherreduc- ening; palladium and platinum are used to increase the
tions for which ruthenium catalysts are well suited include potential for precipitate hardening; palladium preserves
the selective reduction of 1-alkenes and nitriles to amides. the nontarnish property as gold content decreases and sil-
Alcohols can be dehydrogenated to aldehydes or ketones ver content is increased; zinc is a deoxidizer; and irid-
using RuCl 3 . Primary amines can be oxidatively dehydro- ium and ruthenium are used to reduce the grain size in
genated to nitriles with RuCl 3 . Ruthenium is also a good castings. Gold alloys also use palladium and platinum to
catalyst for a variety of coupling reactions, hydroformy- raise the solidus temperature and lower the coefficient of
lation, and the Fischer–Tropsch reaction. The examples expansion to match ceramics when a porcelain covered
listed can be accomplished with homogeneous catalysts crown is prepared. Dental solders commonly have com-
or heterogeneous catalysts such as Ru/C or Ru/Al 2 O 3 . positions involving Au–Ag–Cu–Sn–Zn, with the copper
Ruthenium tetroxide is a stronger oxidizing agent than and silver varied to control the color and working char-
OsO 4 and can perform many oxidation reactions. It will acteristics. For crown and bridge work 22-karat wrought
cleave olefins to aldehydes or ketones, oxidize aldehydes gold and high-strength wires are assembled and soldered.
to acids, alcohols to aldehydes or ketones, ethers to es- Low-temperature solders are preferred to limit the effect
ters, and amides to imides. Its use can be made catalytic of temperature on the part. Solders having a high silver
by regenerating the RuO 4 from lower valent ruthenium content flow freely and high copper solders attach to the
species with strong oxidizing anions (e.g., hypochlorite, substrate, which is useful in building up a deficient site.
chlorine, bromate, prime or permanganate). Ruthenium
in [tris(2,2 -bipyridine)ruthenium(II)] 2+ type complexes

2. Silver
and in Pt/TiO 2 /RuO 2 is photolytically active. The systems
have potential for the production of hydrogen and oxygen The greatest use of silver is in dental amalgams for restor-
from water because of their photo-activity. ing lost tooth structure. The amalgams, involving 65 min %
A word of caution should be mentioned concerning the Ag–29 max %Sn–6 max %Cu–3 max %Hg–2 max %Zn,areused
use of dispersed reduced noble metal catalysts. When they because of their good mechanical properties but they
are allowed to come into contact with oxygen and an oxi- are attacked by inorganic acids and can undergo slight