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

Metal Particles and Cluster Compounds 527

were found with Mg atoms versus Mg 2 and Mg 3 reacting to form Nb-carbide species with loss of hydrogen. It was
with CH 3 Br at 10 K. Only Mg 2 and Mg 3 reacted whereas concluded that the closed structure of Nb 8 and Nb 10 render
Mg atoms and bulk Mg did not. In addition, the recent de- them less reactive, while the reactive clusters were capable
velopment of techniques for producing and studying gas of a lower activation energy entry to benzene, and the
phase metal clusters has led to a great deal of new knowl- reaction went to completion driven by the exothermicity
edgeaboutclusterreactivities.CarefulstudiesofFe x ,Nb x , of Nb–C bond formation and H 2 bond formation.
Pt x , C x , Al x , and others (where x = 2−100) produced in In an investigation of CO 2 reactions, reaction channels
gaseous streams of helium followed by reaction with CH 4 ,
H 2 , H 2 O, NH 3 , or O 2 have led to many intriguing results. OCNB x O (larger clusters)

Reactivity appears to be sensitive to cluster size and proba-
Nb x + CO 2 → (OCNb x O)
bly structure as well. In some cases, the ionization energy

of the metal cluster is important in determining relative
Nb x O + CO (smaller
reactivity. Also, it has been noted that heating of the gas clusters)
phase clusters sometimes leads to a lowering in relative
reactivity (for example, Fe x with H 2 ). This finding is com- were dependent on cluster size.
patible with the idea that the gas phase cluster growth Interestingly, small Nb clusters favored Nb x O forma-
process may lead, by kinetic control, to structures with tion while larger ones favored OCNb x O. Possibly this re-
many reactive defect sites. Upon raising their tempera- sults because larger clusters can internally stabilize the
ture,theclustersmayannealtomorecompact,lessreactive “hot” OCNb x O intermediate while the smaller adducts fly
structures. apart since energy cannot be dissipated well enough.
Thus, recent experimental evidence seems to support These studies again clearly showed that geometrical
theideathatgrowingsmallparticleshavemaximumchem- structure coupled with electronic structure (open shell
ical reactivities, and certain sized/shaped small particles with unpaired electrons, or closed with no unpaired elec-
may have the highest reactivities. What size and/or shape trons) are important in determining relative reactivates.
varies with the metal in question and the reaction in ques- However, it still remains in large part a mystery why cer-
tion? This information strongly supports three ideas: (1) tain clusters are reactive or unreactive, or why ionization
structure sensitivity in chemical reactions on metal sur- energies vary so much with cluster size.
faces is very important, (2) more than one atom is neces- Reactivity of much larger gas phase metal clusters have
sary to carry out at least some bond breaking processes, also been examined in recent years. An example is the
and (3) defect sites on growing small particles are ex- determination of binding sites for NH 3 gas on Ni x and
tremely reactive (see Fig. 9). It has also been possible Fe x where x = 50–147. The adsorption of ammonia on
by pulsed laser vaporization to produce many types of gas phase metal clusters can lend information, since the
gas phase metal clusters. Particularly interesting have number of binding sites would vary with cluster structure.
been reactivity studies of niobium clusters Nb x where For Ni clusters, the number of ammonia molecules ad-
x = 5–20. A definite cluster size dependence on reactivity sorbed showed pronounced minima in the 50–116 atom
was observed. Exposure range for those specific clusters that are particularly sta-
ble (those that are a “magic number” are in larger amounts
Nb x + C 6 H 6 → Nb x C 6 + 3H 2
than statistically predicted (see Fig. 10)).
of Nb 8 or Nb 10 to benzene caused no reaction. On the These clusters probably arise from closing of shells and
other hand Nb 5 , Nb 6 , Nb 9 , and Nb 11 reacted vigorously subshells of the MacKay icosohedra; indeed the ammo-
nia adsorption data support this idea. However, for clus-
ters with a more open structure, more than the predicted
amountofammoniawasadsorbed.Evidenceforsuchopen
shell structures was found more prevalently for Fe n than
Ni n . It seems that d-electron-rich metal atom clusters more
readily form close-packed structures (Ni) but d-electron-
poor clusters growth is more controlled by kinetic factors,
and many unusual, metastable structures appear to be pos-
sible even at as high a room temperature.
In another example of the interesting behavior of small
metal clusters, the ability to produce small gas phase metal
FIGURE 9 Illustration of possible reactive sites on a growing clusters, and to slow them down so they can be “soft-
metal particle (low-temperature growth, not in equilibrium). landed” on a surface, has been utilized in an ingenious