Since the discovery that nanosized-gold particles, highly dispersed on
certain oxide supports, are active catalysts for a variety of reactions1, numerous studies have addressed the structure and
mechanisms associated with this activity2,3 and references therein. The unique activities of these
nano-sized Au particles1-4 are found to strongly
correlate with the gold particle size, the highest reactivity being associated
with a particle size of ~3 nm.
In an early study, Professor Goodman and his team used a model system, gold
supported on single crystalline titania, to investigate this unusual catalytic
phenomenon under realistic reaction conditions2.
Using carbon monoxide oxidation as a probe reaction, similar results were
obtained for the model catalyst as was found for the corresponding high surface
area gold supported on titania in that the reactivity depended critically on the
gold particle size2. The maximum reaction rate was
observed for a surface with gold particles of a mean particle size of ~3 nm and
a particle thickness of two atomic layers as determined by scanning tunneling
Oxygen defects on the support surface have been shown to play a key role in
the nucleation and growth of gold nanoparticles as well as in defining their
electronic properties4. These observations are
consistent with previous work showing that nanoparticles of gold on defect-rich
titania are much more chemically active than on defect-deficient titania5 .
Because of facile gold cluster sintering as a deactivation mechanism of
supported gold catalysts, considerable effort has focused on the design and
synthesis of sinter-resist oxide supports. A recent example of this is
titania-doped silica where the titanium atom incorporates into the surface
forming titanium-oxygen-silicon linkages6. Gold was
found to nucleate primarily at the titanium defects when deposited on the
titanium-doped silica surface. Thermal sintering of gold particles on either of
the titanium-doped surfaces was significantly inhibited compared to the
corresponding sintering characteristics of gold on silica6.
Figure 1. Structural
model and atomic resolved STM image of (a) one of gold on a highly reduced
titania surface; and (b) one and one-third monolayers of gold on a highly
reduced titania surface
The recent synthesis of highly ordered gold mono- and bilayer structures
suppported on a highly reduced titania surface (Figure 1) and the exceptionally
high catalytic activity for carbon monoxide oxidation observed for the bilayer
structure are important milestones in defining the nature of the active site for
supported Au catalysts7.
These studies show that ultrathin gold films on a reduced titania surface
exhibit catalytic activity comparable to the most active gold nanoparticles,
i.e. the thickness of the particle rather than the particle diameter is the
critical structural feature with respect to catalytic activity. The wetting of
gold on a reduced titania surface to form an active bilayer film (two atomic
layers in thickness) increases the active site density by 1~3 orders of
magnitude compared to typical high-surface-area supported gold catalysts.
A bilayer gold structure has been shown to be a critical feature for
catalytically active gold nanoparticles, with low coordinated gold sites,
support-to-particle charge effects, and quantum size effects being contributing
factors to the activity. The relative contributions of each of these factors to
the special catalytic properties of nanosized Au particles are being assessed in
ongoing research around the world.
1. Haruta M, Yamada N, Kobayashi T,
Iijima S, Au catalysts prepared by coprecipitation for low-temperature oxidation
of hydrogen and of carbon-monoxide. J. Catal., 115, 301-309 (1989).
2. Valden M, Lai X, Goodman DW, Onset of
catalytic activity of gold clusters on titania with the appearance of
nonmetallic properties, Science, 281, 1647-1650 (1998).
3. Meyer R, Lemire C, Shaikhutdinov Sh.K, Freund HJ, Surface
Chemistry of catalysis by Au, Gold Bulletin, 37, 72-124 (2004).
4. Chen MS and Goodman DW, Catalytically
Active Gold: From Nanoparticles to Ultrathin Films, Acc. Chem. Res., 39, 739-746
5. Lopez N, Janssens TVW, Clausen BS, Xu Y, Mavrikakis
M, Bligaard T, Norskov JK, On the origin of the catalytic activity of Au
nanoparticles for low-temperature CO oxidation. J. Catal., 223, 232-235
6. Min BK, Wallace WT, Goodman DW, Synthesis of a
sinter-resistant, mixed-oxide support for Au nanoclusters, J. Phys. Chem. B,
108, 14609-14615 (2004).
7. Chen MS, Goodman DW, The structure
of catalytically active Au on titania, Science, 306, 252-255 (2004).
Copyright AZoNano.com, Professor D. Wayne Goodman (Texas A&M