Customized EMSL Spectroscopy Leads to Discovery About Catalysts

Five years ago, Eric Walter custom built a flow cell for the electron paramagnetic resonance (EPR) spectrometer at the Environmental Molecular Sciences Laboratory (EMSL) at the request of a scientist with a user proposal.

Walter, a chemist in the Terrestrial-Atmosphere Processes Integrated Research Platform and EMSL’s EPR user contact, developed the special cell to allow catalytic materials to be tested with flowing gas at high temperatures—a unique device that is not commercially available and has no standard design. The cell is one of a small number of these devices available globally and has been used at EMSL to study lignan decomposition and biofuels.

Over the last three years, a collaborative team of scientists led by Pacific Northwest National Laboratory (PNNL) used the EPR spectrometer and the flow cell to study the atomic-level behavior of advanced copper catalysts. In 2019, scientists found the mechanism behind the diminishing performance of catalysts at high temperatures. The same team recently discovered how catalysts deactivate, which could lead to improvements in the design of more efficient, longer-lasting catalysts that play a role in reducing diesel emissions. Their findings are featured on the cover of the journal ACS Catalysis.

“Without the advanced EMSL instrumentation, most molecular-level knowledge that we gained in the past three years would have been impossible and we could do no better than catalysis researchers 50–100 years ago, that is, learning by trial and error,” said Feng Gao, a staff scientist in PNNL’s Physical Sciences Division who co-authored the research. “In this sense, I feel very lucky to be a scientist at this time. There are so many new tools for us to apply in scientific research, allowing us to ‘watch’ how catalysts work, which, decades ago, could only be imagined in researchers’ minds.”

In 2017, the researchers used the EPR spectrometer and a technique using the cell to study a copper-zeolite catalyst known as Cu/SSZ-13. This unique instrumentation and technique allowed researchers to see the various components of the gases interacting—a phenomenon that can’t be seen in static experiments. By replicating the conditions that a catalytic converter in an engine would experience, the team was able to see how the catalyst performed at high temperatures in a mixture of gases. The study showed that Cu/SSZ-13 is stable under high temperatures, which makes it a good option for chemically converting nitrogen oxide (NOx) emissions from engine exhaust.

“I think this has been important,” Walter said. “There have been a series of articles that have looked at the stability of these catalysts. They work fine when they’re new, but then the catalysts’ performance goes downhill as they age.”

More recently, the team, which also included researchers from Washington State University and Tsinghua University, performed a variety of experiments using imaging and theoretical modeling to further understand why the catalyst’s copper ions were changing as it aged. The study found that some copper ions were moving within the catalyst’s support cages as well as moving closer together.

The imaging was repeated and copper was observed in redox cycling, which means it was moving to and from the oxidation state and losing and gaining electrons in the process. Cycling of fresh catalyst samples happened quickly, whereas older samples cycled slowly because copper got stuck in one oxidation state.

This research allows scientists to continue working on the design of these advanced catalysts, like the copper-zeolite known as Cu/SSZ-13, so their performance doesn’t diminish as they age. The findings also demonstrate EMSL’s flexibility to meet the needs of users who come to conduct their research at the facility.

“We have multiple instruments that collect data under reaction conditions,” Walter said. “These instruments are not commercially available, and the collection of capabilities makes EMSL unique.”

Read more about EMSL’s capabilities here.