Nanomaterials: Properties, Preparation and Applications - Environmental and Human Health Impacts of Nanotechnology - page 45

1

Silver

0.9

0.8

0.7

0.6

0.5

Gold

0.4

0.3

0.2

0.1

0

300 350 400 450 500 550 600 650 700

Wavelength (nm)

Figure 2.11 Surface plasmon bands for gold and silver.

2.4.2 Catalysis

As already discussed, nanoparticles have large specifi c surface areas, which makes
them excellent candidates as catalysts. There are two main forms of catalyst, het-
erogeneous and homogeneous. Many processes are catalysed by heterogeneous
catalysts which are insoluble in the reaction medium. Many of these are based on
precious metals such as rhodium, platinum or palladium. The determining factor
relating to the rate of a reaction in such systems is the diffusion of material to and
from the catalyst surface. This issue is not simply related to the surface area of the
catalyst but also to the structure of the material. For example, certain processes,
such as butane hydrogenolysis on rhodium, show signifi cantly increased activity
when the reaction occurs on one face of a crystal compared to the others. Therefore,
if the material can be prepared such that it contains a greater proportion of that
phase the reaction rate can be increased. Furthermore, if it is a porous structure
there will be limitations to the rate of the reaction related to the rates at which
material can diffuse into and out of the structure. It is therefore of interest to be
able to prepare free fl oating particles which may have high levels of specifi c crystal
faces and present a very large surface area to the reaction medium. In practice
generally the particles are present as a highly open agglomerated mass, which also
has the benefi t of being easier to retain in the reaction vessel as well as catalysing
the reaction. Interestingly, some metals have been shown to have unusual catalytic
effects compared to the bulk material. A good example of this is the ability of gold
to catalyse reduction of organic materials. It is well documented that the reduction
of the dye eosin is catalysed by gold nanoparticles in the presence of sodium boro-
hydride. It has been shown that the ability of gold to participate in the reactions is
closely related to particle size and that the reaction rate changes signifi cantly when
the particles are less than 10 nm (Sau et al. , 2001). Clearly these properties could
have a signifi cant impact on the production of fi ne chemicals and commodity
materials.