Experiments on ceria (cerium oxide—CeO₂) nanoparticles carried out at Brookhaven National Laboratory may lead to catalytic converters that are better at cleaning up auto exhaust, and/or to more-efficient ways of generating hydrogen.

Current generations of three-way converter (TWC) catalysts use noble metals (rhodium, platinum and/or palladium) finely dispersed on a honeycomb surface. (Earlier post on non-noble metal catalysts.) This honeycomb is made of alumina (Al₂O₃) surface-treated with cerium and zirconium oxides.

In a catalytic converter, ceria acts as a buffer, absorbing or releasing oxygen depending on the conditions of the engine to maintain the catalyst in its optimum operating condition for converting harmful emissions such as carbon monoxide and nitrogen oxide to carbon dioxide and nitrogen gas.

—Brookhaven chemist Jose Rodriguez

After using a novel technique to synthesize the ceria nanoparticles, BNL researchers studied how their composition, structure, and reactivity changed in response to doping with zirconium.

Adding zirconium improves ceria’s ability to store and release oxygen. The Brookhaven work discovered why: zirconium changes the ceria’s structure to increase the number of oxygen “vacancies”—places for oxygen uptake and release.

Furthermore, they found that the ceria nanoparticles have much better performance—higher chemical reactivity—than the bulk form of ceria currently used in catalytic converters. This could lead to more-efficient catalytic converters and cleaner air.

In a separate study, the research team deposited gold on the surface of ceria nanoparticles and studied the catalyst’s “active phase” in the conversion of water and carbon monoxide to hydrogen gas and carbon dioxide. This water-gas shift reaction is a critical component in some processes for generating hydrogen.

Rodriguez is presenting the results of both studies today at the 229^th National Meeting of the American Chemical Society.

Resources:

• Driving Down Emissions, Chemistry World, Vol. 1, No. 3, March 2004

• Three-way Catalyst Technology for Automobile Exhausts