New way to make hydrogen from ammonia

RESEARCHERS at Princeton University, US, have developed a new way to break down ammonia to make hydrogen, which could lead to the use of ammonia as a hydrogen fuel source.

Ammonia is a low-cost, common material. Research leader and Princeton chemistry professor Paul Chirik explained to The Chemical Engineer that ammonia has a high volumetric energy density and is relatively easy to store, unlike hydrogen. The N-H bond within ammonia is extremely strong, so ammonia has not been widely explored as a potential carrier of hydrogen. Chirik’s team’s new catalyst, however, can break that bond under relatively mild conditions.

Ammonia-metal complexes have been known for more than 100 years, but Chirik says that this is the first time that the physical properties of the bond-weakening that occurs when this happens has been understood. The researchers use a terpyridine bis(phosphine) molybdenum(I) cation, which binds to ammonia. This weakens the N-H bond to the extent that it becomes thermodynamically favourable for one of the H atoms to break away and bind to another H atom, forming H₂ gas. The reaction takes place at around 60?C.

The reaction leaves behind a molybdenum-amido complex – [Mo]-NH₂. Chirik says that the complex can be converted back into the original molybdenum-amido complex – [Mo]-NH₃ – using strong acids and reductants, for reuse.

“It is a demonstration of a fundamental concept in chemistry and allows chemists to realise after a century of progress, there is much to be learned about ammonia coordination and our understanding of the chemical bond,” said Chirik, adding that any applications are likely to be more in the long-term as the method is a new approach for activating the typically inert N-H bond.

He and the team are currently working on a way to alter the thermodynamics of the reaction to liberate the second and third hydrogen atoms from ammonia. At present, the N-H bond strength of NH₂ is too high for that to happen.

Science DOI: 10/btcm