Small metal particles on the size range of 10-500 nanometer can be fabricated and precisely positioned on flat wafers using electron-beam lithography. The particles can be made with any arbitrary shape and positioned on the supporting wafer in any fashion imaginable.

Such man made nanometer metal samples are excellent model catalysts. By fabricating samples with platinum nano particles on silica-ceria and alumina-ceria supports, well defined model catalysts are made. Catalytic studies performed on these samples enables the systematic study of the reaction as a function of the geometry of the model catalysts, such as shape and distance between particles.

An example of how these model catalysts can be used is the study of morphological (size and shape) changes. A commonly used catalysts, for example the automobile exhaust catalyst, consists of fine powder partilces of platinum and rhodium dispersed on a silica based support. The purpose of making fine powder of the metals is to increase the surface to volume ratio. Since, only the surface of the metals contribute to the catalytic reaction, it is desirable to make as large a surface area as possible for a given amount of metal. The role of the silica support is partly to hold the metal particles in place and partly to act as storage containers for the reactants, e.g. oxygen.

Freshly made catalysts from the factory are highly efficient in removing harmful gases from our automobile exhausts. However, as the catalyst ages the efficiency drops. This is mainly due to the sintering phenomenom. Small particles coalesce with each other to form larger particles, hence reducing the surface area available for the catalytic reaction. Detailed studies of the morphology of small metal particles and how they changes in catalytic reactions can therefore lead to better understanding of the catalytic aging process. Eventually this could help to make better catalyst designs with longer usability.

Platinum discs made by the lithography method transform into 3D crystallites when exposed to the catalytic reaction of water formation from hydrogen and oxygen. This conversion does not occur or occur at a much slower rate when the particles are subject to either hydrogen gas alone or oxygen gas alone even at high temperature. Notice how the underlying layer of ceria cracks apart after the reaction.

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