Purdue, Notre Dame, Cummins discovery could lead to new SCR catalyst design for improved NOx control
18 August 2017
Researchers at Purdue University, the University of Notre Dame and Cummins have discovered a new reaction mechanism that could be used to improve SCR catalyst designs for pollution-control systems to further reduce emissions of smog-causing nitrogen oxides in diesel exhaust. The research focuses on zeolites—workhorses in petroleum and chemical refineries and in emission-control systems for diesel engines.
“The key challenge in reducing emissions is that they can occur over a very broad range of operating conditions, and especially exhaust temperatures,” explained 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.” Current NOx reduction technologies only work well at relatively high temperatures.
However, in addition to these transient conditions, future vehicles will naturally operate at lower temperatures all the time because they will be more efficient. New catalyst designs that perform better, not only during transient conditions, but also during sustained lower exhaust temperatures are thus needed to reduce NOx emissions.
Gounder co-led a team of researchers who have uncovered an essential property of the catalyst for it to be able to convert nitrogen oxides. Findings are published in the journal Science.
The results here point to a previously unrecognized catalytic mechanism and also point to new directions for discovering better catalysts. This is a reaction of major environmental importance used to clean up exhaust.
—William Schneider, the H. Clifford and Evelyn A. Brosey Professor of Engineering at the University of Notre Dame
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. 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, 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.
Copper ions exchanged into zeolites are active for the selective catalytic reduction (SCR) of NOx with NH3, but the low temperature rate dependence on Cu volumetric density is inconsistent with reaction at single sites. We combine steady-state and transient kinetic measurements, x-ray absorption spectroscopy, and first-principles calculations to demonstrate that under reaction conditions, mobilized Cu ions can travel through zeolite windows and form transient ion pairs that participate in an O2-mediated CuI → CuII redox step integral to SCR. Electrostatic tethering to framework Al centers limits the volume that each ion can explore and thus its capacity to form an ion pair. The dynamic, reversible formation of multinuclear sites from mobilized single atoms represents a distinct phenomenon that falls outside the conventional boundaries of a heterogeneous or homogeneous catalyst.
—Paolucci et al.
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. 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.
—Rajamani Gounder
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.
To make these discoveries, the researchers needed techniques that could see the copper atoms while the catalytic reaction was happening. No one technique is able to accomplish this, so they combined information from studies using high-energy X-rays at a synchrotron at Argonne National Laboratory, with molecular-level computational models performed on supercomputers at the Notre Dame Center for Research Computing and the Environmental Molecular Sciences Laboratory at Pacific Northwest National Laboratory.
Although the project focuses on “on-road” pollution abatement applications, the largest market share for zeolite catalysts is in petroleum refineries. The discovery has implications for “heterogeneous catalysis,” which is widely used in industry.
The research has been funded by the National Science Foundation and by Cummins Inc.
Resources
Christopher Paolucci, Ishant Khurana, Atish A. Parekh, Sichi Li, Arthur J. Shih, Hui Li, John R. Di Iorio, Jonatan D. Albarracin-Caballero, Aleksey Yezerets, Jeffrey T. Miller, W. Nicholas Delgass, Fabio H. Ribeiro, William F. Schneider, Rajamani Gounder (2017) “Dynamic multinuclear sites formed by mobilized copper ions in NOx selective catalytic reduction” Science doi: 10.1126/science.aan5630
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