Modern diesel engines are more fuel efficient than gasoline engines. Cleaning up their exhaust is a bit more challenging, though, due to the large amount of oxygen involved in the combustion. In particular, removing the nitrogen oxides (NO x ) formed as oxygen and nitrogen in the air reacting at high temperatures requires specialized systems and expensive catalysts like platinum. While everyone would like to get rid of the platinum, no materials have been found that match its catalytic performance in diesel engine exhaust.

Until now, apparently. Research published recently in Science describes a new catalyst, a complex mixture of metal oxides including manganese, mullite, and the rare earth metals samarium and gadolinium (Mn-mullite (Sm, Gd)Mn 2 O 5 , to be precise), that actually performs better than platinum. And it’s cheaper.

The work was performed by scientists at the nanotechnology startup Nanostellar, with collaborators at the Huazhong University of Science and Technology, University of Kentucky, and the University of Texas at Dallas.

Let’s take a step back: why do we need these catalysts in the first place? At the high temperatures inside engines (around 1900K), oxygen and nitrogen in the air begin to dissociate and react to form nitric oxide (NO). Nitrogen dioxide (NO 2 ) can then form by the oxidation of NO, but this is much slower—meaning most of the NO x in the exhaust is just NO.

There are two common ways to remove NO x from diesel exhaust. The first is selective catalytic reduction (SCR), which reacts NO and NO 2 with ammonia or urea in the presence of a (non-platinum) catalyst, forming nitrogen and water. The second is the NO x trap, where zeolites absorb the molecules like a sponge. Later, the stored molecules can be reacted with excess fuel to release nitrogen.

In both cases, you want to convert some of the NO into NO 2 (the SCR reaction is fastest when you have equal amounts of the two, and the NO x trap stores NO 2 more efficiently). This is where the platinum comes into the picture: a catalyst is needed to jumpstart the reaction between oxygen and NO to form NO 2 .

Back to the new catalyst: how did it perform? The researchers exposed the material to a gas mixture of 450 parts per million NO and 10 percent oxygen (the rest was inert helium). Over a range of temperatures, the new catalyst performed better than platinum (around 64 percent better at 300 degrees C, and 45 percent better at 120 degrees C).

Diesel engines also contain catalytic converters to remove carbon monoxide and unburned hydrocarbons. The authors added their new material to a commercial catalyst (based on platinum and palladium), and found that it oxidized NO without impeding the catalyst's original function. The next step, then, would be to create a catalyst (or modify this new one) that could replace the entire system—removing platinum altogether.

Science, 2012. DOI: 10.1126/science.1225091 (About DOIs)