Page 268 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN009M-428 July 18, 2001 1:6

Metal Particles and Cluster Compounds 525

in surface regions of high curvature. The resulting defects support.Generally200–600 Cistherangewherecommon
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may play an important role with regard to cluster stabil- catalysts may age by sintering.
ity in dictating a change from tetrahedral to face-centered Of course, the bonding of the support to the metal parti-
cubic indium. cle will affect the ease of sintering, and may also affect the
Further studies of supported clusters by X-ray diffrac- electronic properties of the metallic particle. Thus, strong
tion and electron microscopy have shown that Ni clus- support effects are the subject of much current research.
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ters of about 21 A diameter supported on Al 2 O 3 or SiO 2 The support TiO 2 has the most pronounced support ef-
have a great deal of strain attributed to internal pressure fect because it is capable of shuttling electrons to and
buildup in small particles to counter the surface tension from the metallic particle. This is apparently due to the
or deformation of fcc structures. Using the most sophis- fact that TiO or TiO 2 (Ti 2+ or Ti ) is available on the
4+
ticated technology available, rhodium clusters of as few surface.
as five atoms have been observed by high-resolution elec- One of the most pronounced differences between small
tron microscopy. The outline of each atom was observable, particles and bulk ferromagnetic metals (e.g., Fe, Co, Ni)
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and the atomic “spots” measured 2.7 ± 0.2 A in diameter is their magnetic behavior. Such differences were first re-
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compared with the atomic diameter of Rh as 2.74 A. The ported in 1896. By 1930 small metal particles were re-
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spacings between the spots measured 3.5–4.0 A, which is ferred to as “magnetic drops,” which were predicted to be
in good agreement with the lattice spacings given for the the smallest units which display ferromagnetism and were
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fcc structure of Rh of 3.8 A. a minimum of 10 −4 cm. in diameter. By 1938 there was
So modern technology is beginning to allow the physi- evidence that small metal particles behaved as tiny mag-
cal study and observation of the smallest metal particles. nets. By 1946 the smallest domain structure of ferromag-
The future holds a great deal of progress, information, and netic bodies was calculated to be 2 × 10 −6 cm. Smaller
new uses for these interesting species. particles should therefore exhibit quite different magnetic
Most metal particles are supported or attached to a sur- properties from bulk specimens. Theories then developed
face. This surface is often a high surface area powder, such and experiments were performed showing that particles
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as SiO 2 ,Al 2 O 3 , MgO, or C. How mobile are metal parti- below 300 A in diameter act like paramagnetic “atoms”
cles on these surfaces? What are the electronic effects of with very large magnetic movement. This phenomenon
the surfaces (supports) on the small metal particles? These has been termed superparamagnetism or collective para-
are vital questions in the field of heterogeneous catalysis. magnetism.
The growth of supported metal particles by movement This knowledge has allowed magnetic measurements to
and agglomeration is called sintering. Sintering can occur be used to measure metal particle sizes, even down to less
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by two different mechanisms: (1) migration and coales- than 50 A (modern electron microscopy now allows mea-
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cence and (2) dissociation of single atoms and movement surements down to 1 A). During the years that magnetic
of the atoms to other particles. It can be seen that these measurements were used for particle size measurements
are the same phenomena discussed prevously for parti- it was noted that chemisorbed hydrogen had a significant
cles on surfaces. However, in catalysis other reactants are effect. Oxygen and nitrogen also had effects. These ob-
present and it is possible for mechanism (2) to operate servations led to the use of magnetic measurements to
by chemical reaction of surface metal atoms to yield a elucidate chemisorption mechanisms on particles.
volatile compound which can be transported to another The Curie temperature, or that temperature where fer-
cluster, decompose, and deposit the atom. For example, romagnetic substances become merely paramagnetic, has
Ni particles are easily sintered in a CO atmosphere: also been used to study small metal particles. Although
magnetic effects are of great use in the study of small
(Ni) n + 4CO → (Ni) n−1 + Ni(CO) 4 particles of ferromagnetic metals, great care must be ex-
(Ni) m + Ni(CO) 4 → Ni (m+1) + 4CO ercised since chemisorbed species and the oxidation state
of surface atoms can have dramatic effects on the exper-
Generally these mechanisms and theories of sintering pre- imental results. A few additional comments about elec-
dict a movement of atoms from smaller particles to larger tronic properties, as they depend on particle size, are in
ones, eventually reaching unisized particles. Indeed, ex- order.
perimentally it has been found that the broader the starting Theoretical treatments have shown that the average
particle size distribution, the faster the sintering. Likely spacing δ between energy levels in a statistical collection
driving forces are the reduction of internal strain and re- of small metal particles is inversely proportional to the size
duction of surface free energy. of the particles. In bulk metals δ is smaller than any of the
Sintering of supported metal particles can occur over a relevant energy parameters such as kT . In small particles,
wide range of temperatures depending on the metal and the however, δ may no longer be considered small. Thermal