PNNL team finds correlation between reaction mechanism for zeolite SCR catalyst for NOx aftertreatment and bacterial enzyme catalysis
11 September 2013
|Computer model of Cu-SSZ-13 shows nitric oxide (ball-and-stick) interacting with a positively charged copper ion (copper ball) at an unexpected angle (red dotted lines). Photo courtesy of Kwak et al. Click to enlarge.|
A team of researchers in the Institute for Integrated Catalysis at Pacific Northwest National Laboratory led by chemist János Szanyi has proposed a reaction mechanism for a highly active zeolite catalyst (Cu-SSZ-13) used in selective catalytic reduction (SCR) NOx aftertreatment systems for diesel emissions. A paper on their work is published in the journal Angewandte Chemie International Edition.
Although the catalyst is in use, exactly how it converts NOx to nitrogen and water with the help of ammonia (urea) hasn’t been entirely clear. The new research finds that the catalyst works much the same way that similar bacterial enzymes do: by coming at the target from the side rather than head on. The finding provides insight into how to make better catalytic converters.
Combined FTIR and NMR studies revealed the presence of a side-on nitrosyl species in the zeolite Cu-SSZ-13. This intermediate is very similar to those found in nitrite reductase enzyme systems. The identification of this intermediate led to the proposal of a reaction mechanism that is fully consistent with the results of both kinetic and spectroscopic studies.—Kwak et al.
What I find exciting is the correlation between this artificial catalysis and enzyme catalysis. Nature is telling us what to do. Nature’s been at it for many millions of years, and it does this beautifully.—János Szanyi
Zeolites are crystalline alumino-silicate minerals that can accommodate metal ions—metal atoms with a slight charge—for catalytic applications. In some catalytic zeolites, the metal ions can break down the pollutant nitric oxide in vehicle emissions. However, the zeolites crumble and clog easily, leading to early failure. In addition, they produce as a byproduct the greenhouse gas nitrous oxide (known to dental patients everywhere as laughing gas).
Recently, researchers have produced a new zeolite that is surprisingly stable and makes very little nitrous oxide from nitric oxide, a chemical that depletes ozone. The zeolite produces mainly water and atmospheric nitrogen—the main component of air—but it needs to be fed ammonia, such as from urea.
Some of the diesel vehicles in Europe are now using this catalyst, and drivers must top off their urea tank as well as their diesel. Called Cu-SSZ-13, the zeolite uses copper as its added metal and has smaller spaces in its alumino-silicate scaffolding compared to other zeolites.
Researchers have assumed that this zeolite would break down nitric oxide in the same way that other zeolites do, following the same series of chemical reaction steps. However, something else must be going on because researchers can make the older zeolites work faster by adding nitrogen dioxide—but Cu-SSZ-13 doesn’t respond in the same way. This indicates Cu-SSZ-13 must be taking a different chemical route.
To explore how Cu-SSZ-13 breaks down nitric oxide, the team of researchers investigated the structure of the zeolite in the process of performing the reaction. Using tools designed to find such answers at EMSL, DOE’s Environmental Molecular Sciences Laboratory on PNNL’s campus, team members first looked at what molecules stuck to the surface of the zeolite.
There they unexpectedly found a charged nitric oxide molecule bound to the copper ions. This molecular combination could only happen one of two ways, the more common of which requires the presence of nitrogen dioxide. Because the researchers saw no nitrogen dioxide, they ruled out that common reaction pathway.
That left the copper metal itself directly hooking up with nitric oxide. In the process, copper borrows one of nitric oxide’s electrons, giving nitric oxide a charge. This early theft sets the stage for ammonia to react with the charged nitric oxide in the first of several chemical steps, ultimately pumping out atmospheric nitrogen and water.
Zooming in on the zeolite’s structure and reconstructing it with NWChem, software that models molecular chemistry, the team found something unusual. In most zeolite catalysts, nitric oxide is essentially a barbell combining a nitrogen atom and an oxygen atom. The barbell is bound to the metal atom on its head, most often at the nitrogen end of the molecule.
However, in Cu-SSZ-13, the copper metal bonded with both the nitrogen and the oxygen halves of the nitric oxide barbell, as if the copper and nitric oxide formed a three-membered ring. Chemists refer to this orientation as “side-on”.
The side-on complex is uncommon in this type of synthetic catalysis. But bacteria have an enzyme called nitrite reductase that works this way. This enzyme breaks down nitrites into atmospheric nitrogen.—János Szanyi
The chemists also determined that the side-on angle causes the barbell to bend slightly. With no bend, the angle is 180 degrees, but positioned within the zeolite, the angle between the nitrogen and oxygen is about 146 degrees.
The computer reconstruction of Cu-SSZ-13 in action showed the spaces within the aluminum and silicon lattice can only hold one nitric oxide molecule. Other zeolites have almost twice as much space for the nitric oxide to move around.
The small pore size just fits the reactants and provides precise control. This reaction mechanism explains the prior studies—things like why we don’t get nitrous oxide.—János Szanyi
The researchers are continuing to explore whether this side-on intermediate is common in other catalyst materials and in other reactions.
This work was supported by the Department of Energy Office of Energy Efficiency and Renewable Energy.
Ja Hun Kwak, Jong H. Lee, Sarah D. Burton, Andrew S. Lipton, Charles H. F. Peden, and János Szanyi (2013). A Common Intermediate for N2 Formation in Enzymes and Zeolites: Side-On Cu-Nitrosyl Complexes, Angewandte Chemie International Edition, doi: 10.1002/anie.201303498
János Szanyi, Ja Hun Kwak, Haiyang Zhua and Charles H. F. Pedena (2013) Characterization of Cu-SSZ-13 NH3 SCR catalysts: an in situ FTIR study. Phys. Chem. Chem. Phys. doi: 10.1039/C2CP43467A
Feng Gao et al. (2013) Cu-CHA Materials For The Selective Catalytic Reduction Of NOx With NH3: Catalyst Structure/Function and Mechanistic Studies (13 AICHE meeting)
Kwak, J. H. et al. (2012) Cu-SSZ-13 Catalysts for the Selective Catalytic Reduction of NOx with NH3: Catalyst Characterization and Reaction Mechanisms (12 AIChE meeting)
Lei Ma, Yisun Cheng, Giovanni Cavataio, Robert W. McCabe, Lixin Fu, Junhua Li (2013) Characterization of commercial Cu-SSZ-13 and Cu-SAPO-34 catalysts with hydrothermal treatment for NH3-SCR of NOx in diesel exhaust, Chemical Engineering Journal, Volume 225, Pages 323-330 doi: 10.1016/j.cej.2013.03.078
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