Breakthrough could lead to reduced nitrogen oxides in diesel exhaust

A newly discovered reaction mechanism could be used to improve catalyst designs for pollution control systems in vehicles, reducing the emissions of nitrogen oxides in diesel exhaust.

The research from Purdue University focused on a type of catalyst known as zeolites, already used in petroleum and chemical refineries and in emissions control systems for diesel engines. Current emission reduction technologies only work well at relatively high temperatures, meaning new catalyst designs are needed.

"The key challenge in reducing emissions is that they can occur over a very broad range of operating conditions, and especially exhaust temperatures," said Rajamani Gounder, the Larry and Virginia Faith Assistant Professor of Chemical Engineering in Purdue University's Davidson School of Chemical Engineering. "Perhaps the biggest challenge is related to reducing NOx at low exhaust temperatures, for example during cold start or in congested urban driving."

Increased efficiency means that in the future most vehicles will work at lower temperatures. "So we're going to need catalysts that perform better not only during transient conditions, but also during sustained lower exhaust temperatures," Gounder said.

Gounder and colleagues have uncovered an essential property of the catalyst for it to be able to convert nitrogen oxides. The researchers’ findings have been published in the journal Science.

"The results here point to a previously unrecognised catalytic mechanism and also point to new directions for discovering better catalysts," said William Schneider, the H. Clifford and Evelyn A. Brosey Professor of Engineering at the University of Notre Dame. "This is a reaction of major environmental importance used to clean up exhaust."

Zeolites have a crystalline structure containing tiny pores about 1 nanometer in diameter that are filled with copper-atom "active sites" where the chemistry takes place. The researchers discovered that ammonia introduced into the exhaust "solvates" these copper ions so that they can migrate within the pores, find one another, and perform a catalytic step not otherwise possible.

These copper-ammonia complexes speed up a critical bond-breaking reaction of oxygen molecules, which currently requires an exhaust temperature of about 200 degrees Celsius to occur effectively. Researchers are trying to reduce this temperature to about 150 degrees Celsius.

"The reason this whole chemistry works is because isolated single copper sites come together, and work in tandem to carry out a difficult step in the reaction mechanism," Gounder said. "It's a dynamic process involving single copper sites that meet to form pairs during the reaction to activate oxygen molecules, and then go back to being isolated sites after the reaction is complete."