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 microscopy (STM).
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 (2006).
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 (2004).
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).
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