The exciting thing about rare-earth metals, from a fundamental perspective, is that they have a set of additional electrons that their transition metal counterparts do not have. This gives them interesting opto-magnetic properties – but chemists don’t fully understand if and how the electrons might be used in reactions. Examining reactions involving rare earth metals is an attractive tool for understanding their electronic structures and how their structures can apply to new reactivity.

Rare earths have been known to bind molecular nitrogen since the 1990s. However, until now, researchers have not been able to use them to create nitrogen-functionalized chemicals like ammonia or amines catalytically from N₂. Wong, Arnold, and their colleagues designed compounds that joined two rare-earth metals with simple linkages made from phenolates based on a simple antioxidant used widely in food. The resulting structure formed a rectangular cavity. Molecular nitrogen that diffused into the cavity formed bonds with the metals at either end, which activates the gas. Then, electrons introduced into the cavity from a potassium source attacked the activated nitrogen, cleaving its bonds. In all its standard forms, converted nitrogen forms three covalent bonds to hydrogen atoms, or other reactants, resulting in symmetrical ammonia or amines.

A cavity made from linked rare-earth metals, such as zirconium and titanium, can convert abundant molecular nitrogen (N2) into useful nitrogen compounds including ammonia or tris(silyl)amines at room temperature. (Credit: Amy Kynman/Berkeley Lab)

“Our catalysts activate and hold the dinitrogen, while different reagents come in and react to form different products,” says Arnold. She intends next to use electrodes instead of the potassium reagent as a source of electrons, since these can be renewable if they derive from solar cells, for example.

The scientists will next explore how to use rare earths to synthesize additional nitrogen-containing products by tuning the shape and size of the letterbox-shaped cavity. “Our next step is to explore and understand which rare-earth metal properties impact the chemistry,” said Wong.

The new process is not going to replace the widespread industrial Haber-Bosch process. Global ammonia production has hovered around 200 million metric tons annually since 2020, and the existing tools are optimized and extremely efficient at large scale. But the process consumes around 2% of the world’s energy use and creates geographic inequities in the availability of ammonia. “That’s not food justice,” said Arnold. Wong added, “we need better ways of producing ammonia that are less energy intensive and can be conducted at ambient temperatures and pressures to help with food and energy security.” Their patented technology could bring fertilizers and chemically specific nitrogen products to regions without a pipeline, and at a much lower cost.

“Nobody expected rare earth metals to do this reaction. They’ve expanded our arsenal of potential ambient condition catalysts.”

– Polly Arnold

Some of this research was conducted at the Advanced Light Source, a Department of Energy Office of Science User Facility located at Berkeley Lab.

The technology is now available for licensing; contact [email protected].

This research is funded by the Department of Energy’s Office of Science.

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