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Chemists at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory have designed a new way to convert abundant carbon dioxide (CO₂) into formate (HCO₂^-), an industrial chemical used as a fuel, as an antibacterial/antifungal agent, and for making pharmaceuticals. Their reaction uses a light-activated metal-centered catalyst to facilitate the transfer of electrons and protons needed for the chemical conversion.

"We are taking something cheap and abundant like CO₂ and adding electrons and protons to convert it into something useful," said Sai Puneet Desai, the lead author of a paper describing the research just in the Journal of the American Chemical Society.

In some ways, the process mimics photosynthesis, the series of reactions plants use to convert CO₂ and water into sugar, their primary source of fuel. "In both our reaction and photosynthesis, the transfer of protons and electrons is promoted directly or indirectly by light," Desai said.

"You can think of it as storing light energy in the chemical bonds," co-author Andressa Müller added.

Adding ligands adds control

Compared to other attempts to convert CO₂ to useful chemicals, the method developed by Desai, Müller, and other members of Brookhaven Lab's Artificial Photosynthesis group has a twist—or rather, an extension.

"Typically, in these types of CO₂ conversions, you need to bind CO₂ to a metal center on the catalyst," said group leader Javier Concepcion. "That means there are empty spaces for other competing molecules to come in and react with the metal. That can lead to decomposition of the catalyst, and it limits the selectivity over the kind of products you can make."

To control the selectivity and avoid unwanted side reactions, the team surrounded their metal center with ligands.

"The catalyst is like a flower: The metal is the center of the flower and the petals are the ligands," Müller said. "We can tune the properties of the catalyst with these ligands, and all the chemistry takes place at one of the ligands instead of at the metal."

In this new mechanism, all the binding sites on the metal are occupied, so the metal is fully protected from engaging in unwanted side reactions. And by precisely designing the ligands, the scientists can carefully control the product.

"This mechanism is highly selective; only formate is produced," Concepcion said. "Oftentimes, there is competition towards making hydrogen, and/or making carbon monoxide, and sometimes it's difficult to control which of these products you are making. But to make these products, you need open sites at the metal center. In this case, because the mechanism is ligand based, there is no chance for these other products to be generated."

In addition, Müller noted, "Since the chemistry happens on the ligands and not on the metal, this opens the possibility of using other metals at the core of the catalyst."

The current paper reports findings with a ruthenium-centered catalyst. But the scientists have tried a similar approach using inexpensive metals like iron and found that it also works well.

"This paper demonstrates that this ligand-based strategy is generalizable to other metals," Concepcion said. "Our goal is to move toward Earth-abundant metals. It doesn't get more abundant than iron!"

Computation, experiments contribute key insights

The scientists relied heavily on theory and computational chemistry, both in the catalyst design stage and to help them understand their entire reaction mechanism.

"We basically studied the whole mechanism using density functional theory, a computational technique that uses a series of calculations based on electron density to help determine the most likely arrangements and interactions of atoms," added Mehmed Ertem, a principal investigator in the group who specializes in computational chemistry. "The modeling revealed all the steps by which electrons and protons are captured to transform the catalyst into its active form and how the catalyst ultimately delivers these electrons and protons to transform CO₂ into formate."

"The mechanism is very straightforward," Desai said. "It starts with a photosensitizer, which absorbs light and acts as a relay for electrons within our catalytic system."

The system includes another organic molecule called an organohydride, which contributes both the electrons and the protons in two separate steps. This study detected the existence of a "radical cation" of the organohydride, which forms as an intermediate after the electron-donation step. This intermediate is essential for the subsequent proton donation that transforms the catalyst into its active form.

Importantly, once the electrons and protons are delivered to the CO₂, all components of the system can revert to their original forms to be used again.

"This recyclability is really important because we want to make this system as efficient as possible and we don't want to introduce waste," Desai said.

The scientists also conducted laboratory experiments to check the theory-based predictions and track reaction components in real time. One key result was proof that the radical cation intermediate "lived" long enough to engage in the reaction. These results came from the Laser Electron Accelerator Facility (LEAF) within Brookhaven Lab's Chemistry Division, which combines very short pulses of electrons and laser light to produce, excite, and examine transient molecular and atomic species with high time resolution.

"Previously, people thought that radical cations of these organohydrides were very short-lived species that could not stay around for too long," Concepcion said. "We used LEAF to demonstrate that they can survive for hundreds of microseconds, which sounds short on ordinary timescales but is rather long for chemistry."

The team also used a single-crystal X-ray instrument in the Chemistry Division to study the structure of the catalysts. Laser techniques available at the Chemistry Division were also employed to understand all the steps in the conversion of CO₂ to formate.

"These facilities and tools allowed us to study processes that take place over nanoseconds, microseconds, and even longer timescales," Concepcion said. "We can see what happens from the moment the light is introduced and follow all the chemical processes over the entire catalytic cycle."