Introduction to Nanotechnology

264 SELF-ASSEMBLY AND CATALYSIS
passivation by protecting the underlying surface from corrosion. Alkanediols can
assist in colloid preparation by controlling the size and properties of the colloids, and
this application can be very helpful in improving the efficacy of catalysts.
10.2. CATALYSIS
10.2.1. Nature of Catalysis
Catalysis involves the modification of the rate of a chemical reaction, usually a
speeding up or acceleration of the reaction rate, by the addition of a substance, called
a catalyst, that is not consumed during the reaction. Ordinarily the catalyst
participates in the reaction by combining with one or more of the reactants, and
at the end of the process it is regenerated without change. In other words, the catalyst
is being constantly recycled as the reaction progresses. When two or more chemical
reactions are proceeding in sequence or in parallel, a catalyst can play the role of
selectively accelerating one reaction relative to the others.
There are two main types of catalysts. Homogeneous catalysts are dispersed in the
same phase as the reactants, the dispersal ordinarily being in a gas or a liquid
solution. Heterogeneous catalysts are in a different phase than the reactants,
separated from them by a phase boundary. Heterogeneous catalytic reactions usually
take place on the surface of a solid catalyst, such as silica or alumina, which has a
very high surface area that typically arises from their porous or spongelike structure.
The surfaces of these catalysts are impregnated with acid sites or coated with a
catalytically active material such as platinum, and the rate of the reaction tends to be
proportional to the accessible area of a platinum-coated surface. Many reactions in
biology are catalyzed by biological catalysts called enzymes. For example, particular
enzymes can decompose large molecules into a groups of smaller ones, add
functional groups to molecules, or bring about oxidation-reduction reactions.
Enzymes are ordinarily specific for particular reactions.
Catalysis can play two principal roles in nanoscience: (1) catalysts can be
involved in some methods for the preparation of quantum dots, nanotubes, and a
variety of other nanostructures; (2) some nanostructures themselves can serve as
catalysts for additional chemical reactions. See Moser (1996) for a discussion of the
role of nanostructured materials in catalysis.
10.2.2. Surface Area of Nanoparticles
Nanoparticles have an appreciable fraction of their atoms at the surface, as the data
in Tables 2.1, 9.1 demonstrate. A number of properties of materials composed of
micrometer-sized grains, as well as those composed of nanometer-sized particles,
depend strongly on the surface area. For example, the electrical resistivity of a
granular material is expected to scale with the total area of the grain boundaries. The
chemical activity of a conventional heterogeneous catalyst is proportional to the
overall specific surface area per unit volume, so the high areas of nanoparticles
provide them with the possibility of functioning as efficient catalysts. It does not