A new reaction mechanism that could be used to improve catalyst designs for pollution-control systems has been discovered. It could further reduce emissions of smog-causing nitrogen oxides in diesel exhaust.
“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, Assistant Professor of Chemical Engineering at Purdue University in Indiana.
“Perhaps the biggest challenge is related to reducing NOx at low exhaust temperatures, for example during cold start or in congested urban driving.”
Current catalysts only work well at relatively high temperatures but future vehicles will operate at lower temperatures thanks to improved efficiency. So the current research was directed at zeolites, which have a crystalline structure containing tiny pores about 1 nm 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.”
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.
[Purdue University Assistant Professor Rajamani Gounder co-led research that could bring improved catalyst designs. Photo: Erin Easterling/Purdue University]