Chemists reveal the reaction mechanism of a clean energy catalyst

Hydrogen, the simplest element on earth, is a clean fuel that could revolutionize the energy industry. However, accessing hydrogen is by no means a simple or clean process. 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 from each other, then pure hydrogen can be produced from renewable energy sources such as solar energy.

Now, scientists are one step closer to finding that catalyst. Chemists at the University of Kansas and the US Department of Energy’s Brookhaven National Laboratory have revealed the complete reaction mechanism for a key class of water splitting catalysts. Their work is published today in Proceedings of the National Academy of Sciences (PNAS).

“It’s very rare that you get a complete understanding of the entire catalytic cycle,” said Brookhaven chemist Dmitry Polyansky, a co-author on the paper. “These reactions go through many steps, some of which are very fast and not easily noticed.”

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

At the University of Kansas, assistant professor James Blackmore was researching potential candidates when he noticed something unusual in one catalyst in particular. This catalyst, called a pentamethylcyclopentadienyl rhodium compound, or Cp*Rh complex, was showing the reaction in a region where the molecules are normally stable.

“Metallic compounds — molecules that have a metal center surrounded by an organic scaffold — are important for their ability to catalyze challenging reactions,” said Blakemore, who is also a co-author of the paper. “Normally the reactivity occurs directly at the center of the metal, but in our system of interest the bonding scaffold appears to be directly involved in the chemistry.”

So, what exactly was interacting with the ligand? Was the team really noticing an active step in the reaction mechanism, or just an unwanted side reaction? How stable are the intermediate products produced? To answer questions like these, Blakemore teamed up with chemists at the Brookhaven Lab to use a specialized research technique called pulsed radiolysis.

Pulsed radiolysis harnesses the power of particle accelerators to isolate fast, hard-to-observe steps within a catalytic cycle. Brookhaven’s Accelerator Center for Energy Research (ACER) is one of only two locations in the United States where this technology can be performed, thanks to the lab’s advanced particle accelerator complex.

“We’re accelerating electrons, which carry a lot of energy, to very high speeds,” said Brookhaven chemist David Grylls, another co-author on the paper. “When these electrons pass through the chemical solution we’re studying, they ionize the solvent molecules, generating charged species that are intercepted by the catalyst molecules, which quickly change in structure. We then use time-resolved spectroscopy tools to monitor the chemical reaction after this rapid change has occurred.”

Spectral studies provide spectral data, which can be considered fingerprints of a molecule’s structure. By comparing these signatures to known structures, scientists can decipher the physical and electronic changes within the short-lived intermediate products of catalytic reactions.

“Pulse radiometric analysis allows us to pinpoint a single step and look at it on a very short timescale,” Polyansky said. “The hardware we used can resolve events in a millionth to a billionth of a second.”

By combining pulsed radiolysis and time-lapse spectroscopy with the most common electrochemistry and stopped-flow techniques, the team was able to decipher each step in the complex catalytic cycle, including details of the unusual interaction that occurs in the bonding scaffold.

“One of the most notable features of this motivational cycle is the direct involvement of the links,” Grylls said. “This region of the molecule is often just a bystander, but we have observed an interaction within ligands that has not yet been demonstrated for this class of compounds. We were able to show that the hydride group, an intermediate product of the reaction, hopped onto the Cp* ligand. This proves that the Cp* ligand It was an active part of the reaction mechanism.”

Capturing these fine chemical details will make it easier for scientists to design more efficient, stable, and cost-effective catalysts for producing pure hydrogen.

The researchers also hope that their findings will provide clues for deciphering reaction mechanisms for other classes of catalysts.

“In chemistry, results like ours can often be generalized and applied to improve other systems, but getting important details about the fast reaction, as we did here, is an essential step,” Blackmore said. “We hope that other research groups will take our insight and build on it, perhaps by using linker-enhanced interaction to build better catalysts.”

This study is just one set of experiments among a large body of clean energy work being conducted by scientists at the University of Kansas and Brookhaven Lab.

“We are building the fundamental chemical knowledge that will one day help scientists design the optimal catalyst for pure hydrogen production,” Polyansky said.