Cheaper Auto Exhaust Catalysts

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 (NO_x).

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 NO_x in an oxygen-rich, fuel-lean environment.

NO_x 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 NO_x 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 NO_x 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 NO_x-storage capacity of traditional platinum-based NSR catalysts.

On the basis of X-ray diffraction studies, the team proposes that the enhancement in NO_x 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 NO_x 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 NO_x-storage efficiency.