Researchers at Pacific Northwest National Laboratory (PNNL) have discovered a previously unknown key mechanism that could inform the development of new, more effective catalysts for abating NO_x emissions from combustion engines burning diesel or low-carbon fuels. An open-access paper on their work is published in Nature Communications.

SCR of NO_x for diesel vehicles uses a reductant (typically ammonia) and a catalyst to convert NO_x to nitrogen, water, and carbon dioxide. The researchers were comparing the efficacy of a series of best-in-class copper-based catalysts when they noticed that the performance of one of the catalysts—denoted Cu/LTA—was 40% less effective at 180 °C than its counterparts, even when more reaction sites were added. The researchers couldn’t explain the observation based on their prior studies.

The team employed electron paramagnetic resonance spectroscopy to get a closer look at the problematic catalyst. They detected a substantial amount of the copper—meaning that it was accumulating, rather than reacting—and combined that with theoretical calculations to identify the culprit.

Its acidity is lower than the other two. Mainly, it is the lower acidity that makes the intermediate less reactive.

—Feng Gao, a staff scientist in PNNL’s Catalysis Science Group and the lead author

The researchers then used hydrothermal aging to reduce the acid sites in the other catalysts; those catalysts, in turn, showed reduced efficacy, confirming the finding.

A lot of the research has focused on the role of copper: how copper has to form complexes, and actually has to move around in this structure. Then there’s long been a debate as to, okay, what’s the role of the acidity?

—Kenneth Rappe, a chief engineer and Applied Catalysis team leader at PNNL

Before this research, the researcher community had broadly understood the role of the acid sites to be storing ammonia and then providing that ammonia to the copper when needed.

It’s more than that. It actually plays an active, participating role. The active copper complex that forms, in the absence of acidity, actually doesn’t drive the reaction—it gets confined in space.

—Kenneth Rappe

Without acidity, the copper accumulates rather than reacting, rendering the catalyst less effective.

With this new understanding in-hand, manufacturers and researchers will be better equipped to pursue more efficient catalytic reduction of NO_x in industrial combustion engines burning diesel or low-carbon fuels.

The acid sites are an important component to drive this reaction at low temperatures and a key consideration for designing superior catalysts that will be more active at lower temperatures. It is a major development. This field has been so intensely studied. This is a significant advancement because it gives us another tool to actually improve these catalysts.

—Kenneth Rappe

We consider this publication a fundamental study, but this research topic is highly oriented toward applications.

—Feng Gao

The next step for the researchers will be working with catalyst manufacturers, engine manufacturers, or both to improve the current state of the art in SCR for combustion engines burning diesel or low-carbon fuels.

This work was supported by the Department of Energy, Office of Energy Efficiency and Renewable Energy’s Vehicle Technologies Office.

Resources

• Wu, Y., Zhao, W., Ahn, S.H. et al. (2023) “Interplay between copper redox and transfer and support acidity and topology in low temperature NH₃-SCR.” Nat Commun 14, 2633 doi: 10.1038/s41467-023-38309-8