Introduction to Nanotechnology

10.2. CATALYSIS 277
based on the proposed zirconium ion Zr,804(0H)36(S04),4, the trivalent titanium
ion [Ti(CH3COO)6,(OH)o~4~~l,~]~~ . 1 lH,O, hexavalent chromium octahedra forming
the ions [Cr4(OH),(H20), ,I6+ and [Cr40(OH)6(H20),o]5+, an alumina-silica
A1203-SiO, combination, and silica Si02 supplemented by some titania Ti02. The
availability of these nonaluminum pillars, which differ in their dimensions, can
provide catalysts with a wide range of pore sizes. These catalysts have been studied
for their capability in carrying out various chemical reactions, such as cracking, in
which hydrocarbons or other molecules are broken up and their fragments are
recombined into desirable product molecules. An example is crude oil and gas
cracking to produce gasoline. One of the liabilities of these pillared catalysts is their
tendency toward coke formation whereby the surface becomes coated with carbon,
and acid sites become deactivated or unable to function.
10.2.5. Colloids
Nanosized particles of metals are ordinarily insoluble in inorganic or organic
solvents, but if they can be prepared in colloidal form, they can function more
readily as catalysts. A colloid is a suspension of particles in the range from 1 nm to
1 pm @e., 1000 nm) in size, larger than most ordinary molecules, but still too small
to be seen by the naked eye. Many colloidal particles can, however, be detected by
the way they scatter light, such as dust particles in air. These particles are in a state of
constant random movement called Brownian motion arising from collisions with
solvent molecules, which themselves are in motion. Particles are kept in suspension
by repulsive electrostatic forces between them. The addition of salt to a colloid can
weaken these forces and cause the suspended particles to gather into aggregates, and
eventually they collect as a sediment at the bottom of the solvent. This process of the
settling out of a colloid is calledflocculution. Some of the colloidal systems to be
discussed are colloidal dispersions of insoluble materials (e.g., nanoparticles) in
organic liquids, and these are called orgunosols. Analogous colloidal dispersions
in water are called hydrosols.
In Section 2.1.3 we discussed the formation of face-centered cubic nanoparticles
such as Au,, with structural magic numbers of atoms, in this case 55. This
nanoparticle has been ligand-stabilized in the form Au,,(PPh3),,C1, to make it
less reactive, and hence more stable. This sturdiness is brought about by adding
atomic or organic groups between the atoms of the cluster, or on their surfaces.
These FCC metallic nanoparticles can be stabilized as colloids by the use of
surfactants, which can operate, for example, by lowering the surface tension. The
ring compounds tetrahydrofuran (THF) and tetrahydrothiophene, with structures
sketched in Fig. 10.20, have been used to stabilize metallic nanoparticles as colloids.
Figure 10.21 shows a Til3 nanocluster coordinated with the oxygen atoms of six
THF molecules in an octahedral configuration. In this cluster the Ti-Ti distance
(0.2804 nm) is slightly less than that (0.289 nm) in the bulk metal.
A way to obtain colloidal dispersions in organic liquids is to stabilize a metallic
core using a lipophilic surfactant tetraalkylammonium halide NhX, where X is a
halogen such as chlorine (Cl) or bromine (Br), and R represents the alkyl group