New NO_x Reduction Catalyst for Lean-Burn Engines

8 March 2005

Most gasoline engines today run at a chemically correct (stoichiometric) air-to-fuel ratio (about 14.7:1) that allows the three-way catalysts used for emissions control to operate at high efficiency.

Lean-burn engines mix more air with the fuel under lighter load conditions. The enhanced oxidation improves the thermal efficiency of the engine, reduces fuel consumption, and lowers HC and CO emissions. But the higher combustion temperatures increase the amount of NO_x, making it very difficult for current three-way catalysts to handle the burden.

The Honda Insight is one example of a car with a lean-burn gasoline engine. It can operate at a fuel-air ratio of up to 22:1, but uses a dual catalyst system that includes an extra NO_x storage and reduction catalyst to handle the extra emissions load.

NSR catalysts that alternately trap the oxides on a barium component under one set of engine conditions and reduce them under other conditions are one approach to the lean-burn NO_x problem. These alternative catalysts, which are being road-tested in Japan and other countries, still rely on expensive noble metals such as platinum, palladium or rhodium.

Researchers at the University of Delaware may have discovered another approach.

The group demonstrated that a NO_x Storage and Reduction (NSR) catalyst using Cobalt (Co) as an oxidizing metal and without noble metals is just as effective at treating NO_x as platinum-based NSR catalysts. [Catal. Commun., 6, 167 (2005)]

After running through a series of tests, the chemists found that an alumina-supported catalyst containing 5% cobalt and 15% barium was just as effective as conventional NSR catalysts containing 1% platinum.

Adding 1% platinum to the cobalt-barium catalyst created a material with twice the NO_x-storage capacity of current platinum-based NSR catalysts.