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Energy

Cheaper Auto Exhaust Catalysts

High-throughput screening reveals promising low-cost Co-Ba catalyst

by Mitch Jacoby
January 31, 2005 | A version of this story appeared in Volume 83, Issue 5

CATALYSIS

Catalytic converters for highly fuel-efficient automobiles may one day be free of expensive noble metals. Researchers in Delaware have discovered a catalyst formulation that includes no precious metals yet rivals the performance of catalysts containing platinum and other costly materials.

Today's automobile engines are designed to combust fuels in proportions that are quite close to the stoichiometric air-to-fuel ratio (roughly 14.7). Exhaust emissions from these types of engines are cleaned up in catalytic converters that feature noble-metal-based "three-way" catalysts. The name describes the catalysts' ability to facilitate three types of reactions simultaneously: oxidation of carbon monoxide and of unburned hydrocarbons and reduction of nitrogen oxides (NOx).

Fuel efficiency could be bumped up from today's standards by burning fuel in excess oxygen using newly designed "lean-burn" engines. To capitalize on the potential for increased gas mileage, researchers have been developing new types of catalysts because conventional three-way catalysts are unable to reduce NOx in an oxygen-rich, fuel-lean environment.

NOx storage and reduction (NSR) catalysts that alternately trap the oxides on a barium component under one set of engine conditions and reduce them under other conditions are a promising solution to the lean-burn NOx problem. But until now, these alternative catalysts, which are being road-tested in Japan and other countries, have relied on expensive noble metals such as platinum and rhodium.

Now, a group of chemical engineers at the University of Delaware, including Jochen Lauterbach, Rohit Vijay, Christopher M. Snively, and their coworkers, has demonstrated that an NSR catalyst that uses cobalt as an oxidizing metal and contains no noble metals is just as effective at treating NOx as platinum-based NSR catalysts [Catal. Commun., 6, 167 (2005)].

Using a high-throughput reactor to test 16 catalysts in parallel, the Delaware team examined the effect of small concentrations of manganese, iron, and cobalt on the performance of NSR catalysts. They found that an alumina-supported catalyst containing 5% cobalt and 15% barium was just as effective as conventional NSR catalyst formulations that contain 1% platinum. In addition, the researchers observed that adding 1% platinum to the cobalt-barium catalyst produced a material with twice the NOx-storage capacity of traditional platinum-based NSR catalysts.

On the basis of X-ray diffraction studies, the team proposes that the enhancement in NOx storage derived from the presence of cobalt is due in part to Co3O4, which plays a role in oxidizing NO to NO2 (a key step in the NOx storage mechanism). In addition, the team suggests that the close proximity of Co3O4 to barium storage sites increases the interfacial contact between the oxidizing and storage components, which in turn boosts NOx-storage efficiency.

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