Capturing a Snapshot of a Complex Catalyst

Photo courtesy of Argonne National Laboratory

Ryan Hadt, along with other Argonne chemists Dugan Hayes and Lin Chen, worked with Daniel Nocera from Harvard University to understand one step of the water-splitting reaction in a promising catalyst.

A hummingbird flapping its wings may seem impossibly fast to capture on camera. But that's nothing compared to how fast chemical reactions occur. While high speed cameras can capture a flap of tiny wings, scientists supported by the Department of Energy's Office of Science need even more specialized equipment to record snapshots of complex chemical catalysts in action.

The Science of Splitting Water

Catalysts are materials that speed up chemical processes while not being consumed themselves in a reaction. They range from enzymes in organisms to platinum used in catalytic converters to reduce cars' emissions.

Developing catalysts with the right types of properties could enable people to create sustainable liquid fuels using sunlight. Producing fuel from sunlight by mimicking plants requires scientists to split water into oxygen and hydrogen gases. While this reaction isn't a problem for trees, splitting water industrially requires incredible amounts of energy. Catalysts could lower these energy requirements by providing new reaction pathways.

One promising catalyst candidate for splitting water is a cobalt phosphate compound, which is based on compounds that are both readily accessible and inexpensive. One of the original developers of this catalyst, Daniel Nocera at Harvard University, focuses on understanding how the bond between the oxygen atoms forms during the splitting of water into its elemental gases. He looks at individual, elementary steps in the catalytic process so scientists can better understand the whole.

"We need to know how it works so we can get insights to making it better and better in the future," he said.

A Simplification That's Not-So-Simple

But the complexity and rate at which the catalyst acts obscures the single step in the process they wanted to study. Nocera and his colleagues then needed to simplify the process to attempt to observe the reaction despite its speed.

Finding a "molecular model" of the original catalyst helped solve the complexity problem. A molecular model is a simpler version of the catalyst that retains the key components that make the catalyst function. The model allows scientists to gain a lot of information about the real catalyst's behavior. In this case, the molecular model contained a single cobalt unit that made up the extended structure of the target catalyst.

"We noticed that there was a basic building block of this very popular catalyst," said Nocera. "Maybe we can get a lot of insight … by studying the most basic building block."

Producing that building block was still difficult. This particular molecular model is so unstable scientists must apply strong electrical currents to stabilize it. The molecular model for the water-splitting reaction also conducts a charge within itself in less than a billionth of a second.

"The challenges were really in ‘how do you study this thing experimentally?'" said Ryan Hadt, who was a postdoctoral fellow working with Nocera at the time. (He's currently an assistant professor at CalTech.) "You couldn't really isolate it and study it."

Tracking the Electrons

Though the equipment to perform these intricate experiments wasn't available at Harvard, DOE's Office of Science's user facilities could provide exactly what they required.

"We knew we needed special instrumentation to probe at that time scale. That led us to the facility at Argonne," said Nocera, referring to the Advanced Photon Source (APS), an Office of Science user facility at DOE's Argonne National Laboratory.

Using the x-ray capabilities at the APS, the scientists tracked where and how the electrons moved in the molecular model during the catalytic reaction. That analysis revealed the details behind the key step in the water splitting process they were looking for.

But the researchers also learned more than they expected.

Examining the results, Nocera and his colleagues found the catalyst was absorbing the energy from the x-ray in surprising ways.

"You're like reading a book and then you get a twist in terms of the plot. We saw some plot changes," he said. "You're looking for something and then you see something over there that's just as interesting."

The APS's measurements gave them more information than they expected about where and how the electrons moved within the molecular model. That discovery expanded researchers' understanding of what the APS could do. Combined with the simulation they ran, the measurements also suggested a role electron spin may play in bonding during the water splitting process.

"It did open our eyes to some new phenomena that we hadn't considered in great detail," said Hadt. "That was exciting to think about—not only what we accomplished but what we could do in the future."

Building on the results of this study, Nocera is delving further into how these catalysts operate.

"That's what scientists live for, that you can see something that nobody else has," he said. "It had all of the elements that we live for in science—the wonderment of discovery."

The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information please visit https://science.energy.gov.

Shannon Brescher Shea is a senior writer/editor in the Office of Science, [email protected].