Japan's National Institute for Materials Science (NIMS) developed a technology to prevent the thermal aggregation of metal catalyst by forming spherical shell-shaped catalyst.
Its exhaust gas treatment capability is 10 times as high as that of traditional metal catalysts. And the capability lasts longer because it is less likely to cause thermal aggregation.
The newly-developed material is the "Metallic Cell," a spherical shell whose diameter is about 0.01mm. There is a fine pore (channel) with a diameter of about 0.001mm on its surface, and gas comes in and out of the pore. Exhaust gas is oxidized and resolved by catalytic elements inside the shell.
Many normal catalysts lose their activity at high temperatures due to thermal aggregation. On the other hand, the catalytic elements of the new material are protected by the shells even when thermal aggregation is occurring. Therefore, its exhaust gas treatment capability lasts longer than those of existing catalysts.
The gas treatment capabilities of existing catalysts (per amount) are low, and they further lower due to thermal aggregation. To make up for the lowered gas treatment capabilities, it is necessary to increase the amounts of catalysts such as platinum (Pt), palladium (Pd) and rhodium (Rh). But those rare metals are expensive and limited in availability.
The Metallic Cell is formed through the following processes. First, in an alcohol solvent at ordinary temperature and pressure, platinum coating is deposited on the surface of commercially available polystyrene microparticles by chemical reduction. Then, it is heated in the atmosphere until its temperature reaches 500°C so that the polystyrene evaporates. As the polystyrene evaporates, the platinum becomes hollow, leaving a pore on the surface.
This time, NIMS used platinum for both the shell and the catalyst. But it is possible to use stronger and less expensive materials such as iron (Fe) for the cell and other metals such as rhodium (Rh) for the catalyst.