[Click to enlarge image]
A TEM image of the catalyst after the polyol reduction. The black particles in the image are platinum nanoparticles.
A TEM image of the catalyst after the polyol reduction. The black particles in the image are platinum nanoparticles.
[Click to enlarge image]

Japan's National Institute of Advanced Industrial Science and Technology (AIST) developed a catalyst that enables to halve the amount of platinum metals such as platinum (Pt) and palladium (Pd) used for diesel exhaust oxidation catalysts.

With the "surface polyol reduction method," a catalyst is supported by precipitation, not by impregnation. It improves heat resistance and productivity. AIST plans to test volume production of the catalyst and use it for an actual vehicle on a trial basis in the aim of commercializing the catalyst.

The surface polyol reduction method uses a phenomenon in which polyol (polyalcohol) reduces ions such as of precious metals to metals. It is known as a method to make precious metal nanoparticles.

This time, AIST developed a method to precipitate and support nanoparticles compounded of multiple precious metals such as Pt and Pd directly on the surface of a catalyst carrier. The catalyst made with this method has a high heat resistance and, therefore, enables to drastically reduce the amount of platinum metals, compared with conventional catalysts made by impregnation.

In the first half of this development project, AIST found that a nanoparticle catalyst compounded of Pt and Pd is effective in improving heat resistance. Therefore, it started to develop a production process suited for volume production, aiming to commercialize the catalyst.

As a process to mass-produce the Pt-Pd nanoparticle catalyst, AIST used the surface polyol reduction method. Specifically, it is produced by the following process.

(1) A small amount of polyol reducing agent (e.g. ethylene glycol) is added to a precious metal salt solution to make a mixed aqueous solution.
(2) A catalyst carrier (Al2O3 or alumina powder) is impregnated with the mixed aqueous solution.
(3) The resultant suspension is heated to make dry powder.
(4) The powder is heated in a nitrogen gas stream.
(5) As a result, the polyol reducing agent remaining on the surface of the powder facilitates the reduction reaction, precipitating precious metal salt as precious metal nanoparticles on the surface of the carrier.
(6) Finally, when the powder is heated at a high temperature, burning the remaining polyol reducing agent, etc, a catalyst supporting precious metal nanoparticles is made.

When the catalyst was examined with a transmission electron microscope (TEM), Pt nanoparticles with almost the same diameter (about 3nm) were precipitated directly on the surface of the alumina powder. Even though the amounts of Pt and Pd were reduced by half, the cleaning performance of the Pt-Pd nanoparticle catalyst for hydrocarbon is equivalent to or higher than that of catalysts made with the conventional method.