NEW DIESEL CATALYST DEVELOPED
Diesel engines use less fuel and emit less carbon dioxide than their gasoline counterparts, but a problem is the emission of smog-producing pollutants. With catalysts and DPF’s these problems could be solved. But a new problem was created due to the use of platinum, one of the most precious metals in the world. Now scientists have produced a new type of catalytic converter that could make catalysts cheaper for diesel engines.
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. However, 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.
