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Advanced Composite Materials Facility

Bifunctional effects in oxide supported metal catalysts for oxidation reactions

For many years, gold has been considered to be catalytically inert, unlike the platinum group metals. In 1987, however, Haruta et al [1] discovered that utilising gold nanoparticles supported on various reducible metal oxides greatly increases the reactivity of gold for a variety of reactions, most notably the oxidation of carbon monoxide. CO oxidation is of great interest in a number of situations, most notably the issues caused in fuel cell applications due to reduced efficiency caused by the CO poisoning of catalysts.

Conventional and inverse catalyst systems [5].
Conventional and inverse catalyst systems [5].

Increased reactivity of the metal oxide supported gold.

Much has been done to try to explain this increased reactivity, with a number of theories being put forward over the years. The most common being: an increased concentration of more reactive edge/corner atoms stabilised by the support [2], electronic changes in the gold caused by the support [3] and spill-over of reactants between the support and the gold/interface effects [4] (otherwise known as bifunctional effects) allow for improved reactivity.

The current work is focussed on the investigation into the increased reactivity of the metal oxide supported gold. The main area of focus is on using inverse catalyst systems, where the gold and metal oxide positions are switched, giving metal oxide nanoparticles on a gold support.

STM image of a TiOx/Au (111) surface, 287x297nm.
STM image of a TiOx/Au (111) surface, 287x297nm.

By using the inverse system, it can be seen that the methods for increased reactivity are removed or severely reduced: there are no edge/corner gold atoms and the electronics of the gold are as/close to bulk.

As such, an investigation into the electrochemical behaviour of this system can give great insights into the underlying cause of the observed effect and further our understanding of the interaction between the support and the metal.

References

1. Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N., Novel Gold Catalysts for the Oxidation of Carbon-Monoxide at a Temperature Far Below 0-Degrees-C. Chem Lett 1987, (2), 405-408.

2. Lemire, C.; Meyer, R.; Shaikhutdinov, S.; Freund, H. J., Do quantum size effects control CO adsorption on gold nanoparticles? Angew Chem Int Edit 2004, 43 (1), 118-121.

3. Okazawa, T.; Kohyama, M.; Kido, Y., Electronic properties of Au nano-particles supported on stoichiometric and reduced TiO2 (110) substrates. Surf Sci 2006, 600 (19), 4430-4437.

4. Boccuzzi, F.; Chiorino, A.; Manzoli, M.; Lu, P.; Akita, T.; Ichikawa, S.; Haruta, M., Au/TiO2 nanosized samples: A catalytic, TEM, and FTIR study of the effect of calcination temperature on the CO oxidation. J Catal 2001, 202 (2), 256-267.

5. Rodriguez, J. A.; Hrbek, J., Inverse oxide/metal catalysts: A versatile approach for activity tests and mechanistic studies. Surf Sci 2010, 604 (3-4), 241-244.

 

 

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