Page 112 - Science at the nanoscale

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June 9, 2009
Surfaces at the Nanoscale
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Masatake Haruta first reported the catalytic activity of gold
nanoparticles towards oxidation and reduction of hydrocarbons.
Gold is traditionally considered an inert metal and catalytically 7 ch05
inactive, however the increase in its catalytic activity has been
attributed to various factors, e.g. the low-coordination sites on
its surfaces, the higher mobility of surface atoms, the higher elec-
tronegativity and higher oxidation potential. The relatively higher
surface mobility of corner and edge atoms at room temperature is
supported by the lowering of its melting point at the nanoscale —
gold nanoparticles of 2.5 nm in diameter have been found to melt
at ∼600 C compared to 1320 C for the bulk gold. In addition, the
◦
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catalytic activity of gold nanoparticles towards CO reduction has
been reported to be optimum at the diameter of ∼3.5 nm, when
the metal to non-metal transition is observed. In a recent report,
nano-sized gold particles of 2–15 nm diameters have been shown
to demonstrate higher catalytic activity for many selective hydro-
carbon oxidation reactions that are used to make compounds con-
8
tained in agrochemicals and pharmaceuticals. For an overview of
this exciting and growing field of nanocatalysis, the readers may
refer to a recent reference on the subject. 9

5.3 SURFACE STABILISATION

Due to the large energy associated with their high surface areas,
nano-sized objects can be considered to be thermodynamically
unstable (or “metastable”) as there is a natural drive towards
reduction of free energy via processes such as agglomeration etc.
How was it possible then, as mentioned in Chapter 1, for gold
nanoparticles to be prepared in ancient times and used in beauti-
ful stained glass windows in medieval churches?
Since the early days of alchemists in the 17th century, it was al-
ready known that adding certain salts or chemicals allows stable
colloids to be prepared. A colloid refers to a suspension of fine par-
ticles in liquid phase such as water or organic solvents. Thus it is
a two-phase system and a resultant interfacial potential develops.

7 M. Haruta, Catalysis Today 36, 153 (1997).
8
M. D. Hughes, Y. Xu, P. Jenkins, P. McMorn, P. Landon, D. I. Enache, A. F. Carley,
G. A. Attard, G. J. Hutchings1, F. King, E. H. Stitt, P. Johnston, K. Griffin and C. J.
Kiely, Nature 437, 1132 (2005).
9 U. Heiz and U. Landman, Nanocatalysis, Springer, 2007.