Uranium-based catalyst turns air nitrogen into ammonia

Ammonia (NH₃) is vital for agriculture, as it is the basis for fertilizers that are needed to feed the world's population. Currently, ammonia is mostly produced by the Haber-Bosch process, which turns nitrogen gas (N₂) from the air into ammonia. The problem is that this process requires enormous amounts of energy while generating significant gas emissions.

Scientists have long searched for more efficient and environmentally friendly ways to produce ammonia. Nature does this efficiently through enzymes called nitrogenases, but replicating these biological processes on an industrial scale has proven challenging.

“All molecular catalysts developed so far typically attach nitrogen molecules – which are composed of two nitrogen atoms bonded together – to a single metal center in a linear, ‘end-on’ arrangement. It means that one nitrogen molecule binds only one metal, via one of its two atoms,” says Professor Marinella Mazzanti at EPFL. “In contrast, nature uses a multimetallic approach, where nitrogen molecules bind to more than one metal. It has been proposed that the nitrogen binds in a ‘side on’ way, meaning both nitrogens atoms bind two metals, making it easier to break their strong bonds.”

Inspired by nature

Now, a team led by Mazzanti has developed the first molecular uranium catalyst that can bind nitrogen gas in a similar "side-on" way and convert it into ammonia. The work reveals a new catalytic pathway, bridging biological efficiency and industrial feasibility, and opening doors for more sustainable ammonia production methods.

The scientists built a special molecule using uranium combined with a triamidoamine ligand, producing a molecular complex that can hold nitrogen gas (N₂) sideways. They then progressively reduced the nitrogen gas by adding electrons step-by-step, breaking the powerful bond between the gas's two nitrogen atoms. The researchers carefully studied and isolated various stages of this reduction process, creating intermediate molecules (nitrogen forms like N₂²⁻, N₂³⁻, and N₂⁴⁻) until finally splitting nitrogen completely into two separate nitride ions (N³⁻).

A different way to make ammonia differently

Their experiments showed the uranium complex could run repeatedly in a cycle, effectively turning nitrogen gas into ammonia multiple times; specifically, up to 8.8 equivalents of ammonia per uranium catalyst. This demonstrated for the first time that side-on nitrogen binding—a likely binding mode in nature's enzymes—can provide a viable route for producing ammonia.

The catalyst clarifies previously unknown steps in nitrogen conversion chemistry and shows that uranium, historically among the first metals used industrially to make ammonia, still holds untapped potential.

This discovery provides crucial insights into nitrogen chemistry and shows how uranium-based systems can offer new avenues for future ammonia production technologies.

Other contributors

• University of Manchester (UK)
• Université de Bretagne Occidentale (France)