Credit: © 2010 AAAS

Diesel engines are more fuel efficient than petrol ones but, because they run at higher air-to-fuel ratios, they give off more harmful NOx (NO and NO2). Removing NOx in this oxygen-rich environment is challenging, but made easier by increasing the proportion of NO2 compared with NO. Current exhaust systems require the use of expensive platinum-based catalysts to do this, and these also have poor stability at the high operating temperatures.

Now, Wei Li and colleagues from General Motors Global Research and Development in Michigan have developed1 perovskite oxide catalysts that do the job as well or better than the commercially available ones. The oxides were LaCoO3 and LaMnO3 with some of the lanthanum replaced with a small amount of strontium. This doping not only almost doubled the surface area of the solids, but also promoted the catalysis itself in LaCoO3, by increasing the number of weakly bonded oxygen atoms.

A useful exhaust catalyst also must be able to oxidize carbon monoxide and unburnt hydrocarbons. Although the perovskite catalysts on their own were less effective at catalysing these reactions, combining them with palladium particles made them as effective as commercial platinum catalysts. The palladium additive also helped prevent the new catalysts from becoming poisoned by sulfur, a common problem in exhaust catalysts. The manganese-based catalyst was structurally stable enough to withstand the high temperatures as well as the reducing environments required to regenerate them.