Chemists uncover reaction mechanism for clean energy catalysts

UPTON, NY—Hydrogen, the simplest element on Earth, is a clean fuel that could revolutionize the energy industry. Accessing hydrogen, however, is not a simple or clean process at all. Pure hydrogen is extremely rare in nature, and practical methods for producing it currently rely on fossil fuels. But if scientists find the right chemical catalyst, which can separate the hydrogen and oxygen in water molecules, pure hydrogen can be produced from renewable energy sources such as solar power.

Now, scientists are one step closer to finding that catalyst. Chemists at the University of Kansas and the US Department of Energy’s (DOE) Brookhaven National Laboratory have uncovered the entire reaction mechanism for a major class of water-breaking catalysts. Their work was published today in Proceedings of the National Academy of Sciences (PNAS).

“It’s very rare that you can get a complete understanding of a full catalytic cycle,” said Brookhaven chemist Dmitry Polyansky, one of the paper’s authors. “This reaction takes many steps, some of which are very fast and not easily observable.”

The rapid intermediate steps make it difficult for scientists to decipher exactly where, when, and how the most important parts of a catalytic reaction occur — and therefore, if a catalyst is suitable for large-scale applications.

At the University of Kansas, associate professor James Blakemore was researching possible candidates when he noticed something unusual about one particular catalyst. This catalyst, called the pentamethylcyclopentadienyl rhodium complex, or Cp*Rh complex, shows reactivity in areas where the molecule is normally stable.

“Metal complexes—molecules containing metal centers surrounded by organic scaffolds—are important for their ability to catalyze difficult reactions,” said Blakemore, who is also one of the paper’s authors. “Normally, the reactivity occurs directly at the metal center, but in the system we are interested in, the ligand scaffold appears to directly take part in the chemistry.”

So, what exactly do ligands react with? Did the team actually observe an active step in the reaction mechanism or was it just an unwanted side reaction? How stable is the resulting intermediate product? To answer questions like these, Blakemore teamed up with chemists at the Brookhaven Lab to use a special research technique called pulse radiolysis.

Pulse radiolysis harnesses the power of particle accelerators to isolate the fast and difficult to observe steps in the catalytic cycle. Brookhaven’s Accelerator Center for Energy Research (ACER) is one of only two locations in the United States where this technique can be performed, thanks to the Lab’s advanced particle accelerator complex.

“We accelerate electrons, which carry significant energy, to very high speeds,” said Brookhaven chemist David Grills, another co-author of the paper. “When these electrons pass through the chemical solution we are studying, they ionize the solvent molecules, producing charged species that are intercepted by the catalyst molecules, which rapidly change their structure. We then used time-resolved spectroscopy tools to monitor chemical reactivity after these rapid changes occurred.”

Spectroscopic studies provide spectral data, which can be considered as fingerprints of the molecular structure. By comparing these signatures with known structures, scientists can decipher physical and electronic changes in the short-lived intermediate products of catalytic reactions.

“Radiolysis of pulses allows us to pick a step and view it on a very short time scale,” says Polyansky. “The instrumentation we use can resolve events in billionths to millionths of a second.”

By combining pulse radiolysis and time-resolved spectroscopy with more general electrochemistry and stopped-flow techniques, the team was able to decipher every step of the complex catalytic cycle, including details of the unusual reactivity that occurs in the ligand scaffold.

“One of the most remarkable features of this catalytic cycle is the direct involvement of the ligand,” says Grills. “Often, these areas of the molecule are mere spectators, but we observed reactivity in the ligands that has not been proven for this class of compounds. We were able to show that the hydride group, the intermediate product of the reaction, jumps to the Cp* ligand. This proves that Cp* ligands are an active part of the reaction mechanism.”

Capturing these precise chemical details will greatly facilitate scientists to design more efficient, stable, and cost-effective catalysts for producing pure hydrogen.

The researchers also hope their findings will provide clues to decipher the reaction mechanism for other classes of catalysts.

“In chemistry, findings like ours can often be generalized and applied to optimize other systems, but obtaining critical details about fast reactivity, as we have done here, is a key step,” said Blakemore. “We hope other research groups will take our insights and build on them, perhaps by using ligand-promoted reactivity to build better catalysts.”

This study is just one set of experiments among the vast amount of clean energy work that scientists at the University of Kansas and Brookhaven Lab are doing.

“We are building on basic chemical knowledge that will one day help scientists design optimal catalysts for producing pure hydrogen,” said Polyansky.

This work was supported by the National Science Foundation and the DOE Office of Science.

Brookhaven National Laboratory is supported by the US Department of Energy’s Office of Science. The Office of Science is the largest supporter of basic research in the physical sciences in the United States and works to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.

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