New catalyst designs are needed to reduce the emission of nitrogen oxides - NOx - because current technologies only work well at relatively high temperatures.
“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.”
In addition to these transient conditions, future vehicles will operate at lower temperatures due to improved efficiency. “So we’re going to need catalysts that perform better, not only during transient conditions, but also during sustained lower exhaust temperatures,” Gounder said.
He co-led a team of researchers from Purdue University, Notre Dame University and diesel engine manufacturer Cummins who uncovered an essential property of the catalyst for it to be able to convert nitrogen oxides. Their findings have been published in 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, Indiana. “This is a reaction of major environmental importance used to clean up exhaust.”
The team worked with zeolites, which have a crystalline structure containing pores about 1nm in diameter that are filled with copper-atom active sites. In the new findings, the researchers discovered that ammonia introduced into the exhaust solvates these copper ions so that they can migrate within the pores 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.”
This rate-limiting step might be accelerated by fine-tuning the spatial distribution of the copper ions, leading to lower nitrogen oxide emissions at cooler temperatures than now possible.