Since its introduction more than 20 years ago, the catalytic converter has sharply reduced automotive emissions. Using catalysts containing palladium, platinum and rhodium, a converter breaks down harmful carbon monoxide, nitrogen oxides and hydrocarbons before the exhaust leaves the car's tailpipe. But these precious metals are expensive and are produced through the intensive and polluting chemical processing of sulphide ores extracted from often treacherous underground mines.
On page 164 of this issue, Yasuo Nishihata et al. (Nature 418, 164–167; 2002) propose a new catalyst for automotive-emissions control that lasts longer and accomplishes its task more effectively than conventional catalysts. The material, a perovskite containing small amounts of palladium (Pd), could reduce by 70–90% the amount of precious metals needed to meet today's car emission standards.
Catalytic converters consist of a highly porous ceramic structure coated with finely divided catalytic material. To ensure immediate action when the car is started, converters are placed close to the hottest part of the car, the engine. Over time, heat exposure causes the tiny particles of precious metal to agglomerate, thus reducing the catalyst's overall surface area and hence its activity. To counter this effect, conventional catalytic converters are loaded with an excess of precious metal, ensuring that performance targets are met for vehicle use over the expected range, usually 80,000 km.
Nishihata et al. demonstrate that excess-metal loading is not needed if the perovskite LaFe0.57Co0.38Pd0.05O3 is used as the catalyst. This material, first investigated for catalytic-converter applications in the 1970s, maintained its high metal dispersion and high catalytic activity during a 100-hour test in engine exhaust. In the same test, the activity of a conventional catalyst (alumina impregnated with palladium) decreased by 10%.
The resilience against metal-particle agglomeration results from the perovskite's ability to respond structurally to the fluctuations in exhaust-gas composition that occur in modern petrol engines. These fluctuations switch the exhaust environment continuously from an oxidizing to a reducing atmosphere. In separate tests, the authors established that Pd is firmly incorporated into the perovskite lattice of the oxidized LaFe0.57Co0.38Pd0.05O3 catalyst (see figure, left). But the Pd atom (red) moves out of the structure, being replaced by an iron atom (blue), in the reduced material (see figure, right). It is this fully reversible hopping of Pd into and out of the perovskite structure that seems to suppress the agglomeration of the metal and thus the slow deactivation of the catalyst.
Although catalytic converters have already made a significant contribution to emissions control, Nishihata et al. show that more can still be done to address the environmental impact of car use.
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Helmer, M. Cleaning up catalysts. Nature 418, 138 (2002). https://doi.org/10.1038/418138b
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