Building a cheaper catalyst

Chemical engineers have found a cheaper and possibly better material than platinum for cleaning up the exhaust streams of diesel vehicles.

Many automobiles use platinum-containing catalytic converters to help clean their exhaust streams of various pollutants, notably the nitrogen oxide compounds that can contribute to smog. But the high and volatile price of the precious metal makes it difficult to build an economical catalyst.

An alternative material, known as perovskite,  is far less expensive than platinum and may do the job more effectively, engineers from the research arm of General Motors report in the March 26 Science.

“It’s excellent work, really groundbreaking to be able to have an alternative to platinum-based catalysts,” says Louise Olsson, a catalysis researcher at Chalmers University of Technology in Göteborg, Sweden, who was not involved in the research. “It’s going to save a lot of money.”

Diesel cars can cost $1,000 to $5,000 more than comparable gasoline models because of the need to modify the engine and add more expensive catalytic converters to meet emissions standards. Platinum, used in many of those converters, sells for about $1,590 per troy ounce.

In Europe, about half of passenger vehicles run on diesel. The percentage is far lower in the United States, although many large vehicles and freight trucks run on diesel because of the high fuel efficiency of those engines. The problem is that diesel engines need to burn “lean,” or in the presence of extra air, compared with regular gasoline engines. The additional oxygen makes it harder to remove the resulting nitrogen oxide compounds.

A team led by Wei Li at GM’s Global Research and Development branch in Warren, Mich., decided to focus on a particular chemical reaction in diesel exhaust streams, in which platinum is used to convert NO to NO2, which can be further processed and released to the atmosphere as nitrogen gas.

“This is a critical reaction required in the diesel system,” explains Li. “For most gas applications people have already moved away from platinum, but for diesel we cannot.”

In their laboratory the researchers replaced a commercial platinum-based catalyst with one based on perovskite oxides made of cobalt or manganese combined with oxygen. By adding a bit of strontium and lanthanum into the mix, Li’s team showed that the manganese-based perovskite catalysts converted NO to NO2 about as well as  platinum-based ones did.

The cobalt-based perovskite catalyzed the reaction at rates significantly higher than platinum. “We were looking for a good catalyst, but we were not expecting it to be that good,” says Li.

The new catalysts are not, however, entirely free of precious metals. The team had to add a bit of palladium – which  goes for about one-quarter the cost of platinum – to eliminate some sulfur buildup.

There’s a long way to go before perovskite-based catalysts appear in automobiles, notes Jim Parks, a catalyst researcher at the Oak Ridge National Laboratory in Knoxville, Tenn. Li’s team performed its tests in simulated exhaust streams; now other GM researchers are doing experiments to see how the perovskite catalysts handle real pollutants, Li says.

The new work is “a step in the right direction, but there will be more to do with developing this technology,” says Parks.