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Posted: Nov 08, 2010
Nanotechnology exhaust gas catalyst has 10x higher thermal agglomeration resistance than conventional materials
(Nanowerk News) A research team headed by Dr. Hideki Abe, Senior Researcher of the Advanced Electronic Materials Center and Dr. Katsuhiko Ariga, Principal Investigator of the International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science developed an exhaust gas catalyst material with approximately 10 times greater thermal agglomeration resistance than conventional materials. This dramatic improvement in thermal agglomeration resistance opens the road to a large reduction in the amount of rare metals used in exhaust gas purification technologies.
Fig. 1: Newly-developed artificial "Metallic Cell." Automotive exhaust gas is sucked into the cell through the channel 1/1000mm in diameter opened in the cell wall, and discharged after purification.
Environmental and energy technologies, represented by automotive exhaust gas purification,1 are necessary and indispensible for human society in the 21st century for satisfying both abundant energy supplies and safe and healthy life. Metal catalysts,2 which are the most critical element materials in environmental and energy technologies, are confronted with the problem of thermal agglomeration, in which the catalyst loses its activity as a result of bonding/fusion of the catalyst due to heat and the accompanying reduction in the number of catalytic active sites.3
As catalytic active sites of metal catalysts, mainly platinum, palladium, rhodium4, and other rare metals5 are used. To compensate for the reduction in catalytic activity caused by thermal agglomeration, the current technologies unavoidable require consumption of large amounts of rare metals, as there is no other method of introducing a large excess of active sites in the catalyst.
Therefore, in this research, the NIMS team developed a metal catalyst with high resistance to thermal agglomeration by controlling the topology6 of the material at the nano-scale, which is completely different from the conventional approach.
The developed material, called "Metallic Cell" (Fig. 1), consists of metal spheres with a cavity approximately 1/100mm in diameter, which is surrounded by a thin cell wall containing pores (channels) 1/1000mm in diameter that enable transmission of substances and energy to and from the outer world. Because "Metallic Cell" has a special topology by which the catalytic active site in the cell is protected by the cell wall, it demonstrates excellent long-term catalytic properties, even under high temperature conditions in which ordinary catalyst materials would lose their activity due to thermal agglomeration (Fig. 2).
Fig. 2 :Principle of Metallic Cell.
Metallic Cell is synthesized by precipitating a platinum film on the surface of commercially-available polystyrene powder by chemical reduction in an alcohol solution at normal temperature and pressure, followed by heating to 500°C to vaporize the polystyrene. Accompanying vaporization of the polystyrene, the hollow topology is formed and pores through which the polystyrene gas escapes are opened in the platinum film, resulting in natural formation of the unique morphology of Metallic Cell shown in Fig. 1. The method used to synthesize Metallic Cell is extremely simple and can be applied not only to platinum, as described here, but also to a number of other metals which display catalytic activity, beginning with rhodium, which shows high activity in NOx purification. The applications of Metallic Cell are not limited to exhaust gas purification technology. Taking advantage of its excellent heat resistance and high scalability, a large reduction in the amount of rare metals used in many environmental and energy technologies is also possible, beginning with fuel cell technology7.
Automotive exhaust gas purification performance of Metallic Cell (red line in figure). Due to its special topology, Metallic Cell demonstrates heat resistance characteristics greatly exceeding those of the conventional catalyst materials (blue and black lines in figure).
1. Automotive exhaust gas purification: Automobile engines emit highly concentrated toxic gases, beginning with carbon monoxide and nitrogen oxides (NOx) which are harmful to the human body. When the exhaust gases from engines are released into the atmosphere, these toxic gases must be removed and purified by some method. Exhaust gas purification using metal catalysts is a representative method.
2. Metal catalyst: Generally indicates solid catalysts in which mainly transition metals are used as the material. Metal catalysts are used in important industrial applications, such as purification of exhaust gas from automobiles and power plants, desulfurization of petroleum, etc.
3. Catalytic active site: Indicates the atoms which play the main role in the catalytic reaction, among the atoms comprising the surface of a metal catalyst; these atoms are called catalytic active sites. In general, the activity of a catalytic material is high in proportion to the number of catalytic active sites per unit of weight.
4. Rhodium (Rh): A transition metal element located directly below cobalt in the Periodic Table; atomic number 45. Rh demonstrates high catalytic activity for NOx in exhaust gas purification. Production is small (approximately 10 tons/year), and Rh is also known as the most expensive of the noble metals.
5. Rare metal: General term for metal elements which are produced in small amounts and have a small distribution scale in the market, in contrast to base metals such as iron, copper, aluminum, zinc, and lead. The rare metals occupy an extremely important position as functional materials, including, for example, noble metal elements (platinum, palladium, rhodium, etc.), which are indispensible in exhaust gas purification technology, and rare earths, which are indispensible in high coercivity permanent magnets. Because the distribution of these mineral resources is uneven, it is also known that supplies and prices are easily affected by the political conditions in countries which possess these resources.
6. Topology: Word which expresses the 3-dimensional shape of a body or image in terms of phase geometry.
7. Fuel cell: Device which electrochemically burns small molecules such as hydrogen, methanol, etc. and supplies the charge transfer generated accompanying this reaction for use outside the reaction in the form of electricity. Fuel cells are a new technology which has attracted intense interest as a next-generation energy source.