Clean Diesel: Not So Precious After All

Diesel engines use less fuel and emit less carbon dioxide than their gasoline counterparts, but they are rare in the United States, in part because they often fail U.S. standards for the emissions of smog-producing pollutants. Now scientists have produced a new type of catalytic converter that could make it cheaper for diesel engines to overcome that regulatory hurdle.

Early diesels belched clouds of smoke. To solve that problem, engineers designed engines that draw in far more air than needed for the combustion of fuel. But there was a side effect: The leftover oxygen in the exhaust makes it hard to remove smog-forming nitrogen oxides. Scientists have been working for some time on ways to remove these nitrogen oxides from diesel exhaust.

One solution is to add a metal such as barium to the catalytic converter. This reacts with any nitrogen oxide to form barium nitrate, which can be easily removed from the engine without affecting performance. But the barium-based reaction only works for one form of nitrogen oxide. Removing all the others requires platinum to catalyze the oxidation of nitric oxide to nitrogen dioxide so that the barium can remove it. Platinum, unfortunately, is one of the most precious metals on the planet. This is one reason clean diesel engines are more expensive than their gasoline equivalents.

There's a dark horse. A cheaper metal oxide called perovskite can replace platinum, but it's generally much less efficient than platinum and tends to be deactivated by the sulfur in diesel. The sulfur can be removed by heating the catalytic converter to above 700°C, but this often breaks down the perovskite as well.

In the new research, published in the 26 March issue of Science, chemical engineer Wei Li and colleagues at General Motors Global Research and Development in Warren, Michigan, report success with a mixture of palladium (which costs 70% less than platinum) and perovskite containing lanthanum, strontium, and manganese. At a diesel engine's cruising temperature, this blend removes pollutants at least as well as a traditional platinum catalyst. (It is slightly less effective when the engine is cold.) Better still, the mixture can survive sulfur-purging temperatures in the exhaust system.

The team has continued to develop and refine its catalytic converter design over the past year and is now preparing to test it in prototype vehicles. "The main challenge is to improve the low-temperature performance," says Li.

Chemical engineer Jan Stepanek of the Institute of Chemical Technology in Prague foresees another potential problem. "It is well known that, due to automotive catalyst decay, there are appreciable concentrations of precious metals near roads," he says. This hasn't caused environmental or health problems so far because platinum, for example, is very stable. But the team's new design contains strontium, which is thought to stunt the growth of children. If this were released from an aging catalytic converter, says Stepanek, it might be more dangerous.