For the first time ever, scientists have used light energy to create a rare molecular uranium nitride (U-N) complex containing a discrete terminal U-N unit, where the nitrogen atom is bonded only to the one uranium atom, versus prior work where the nitrogen atom has always been bonded to two or more uranium atoms.
Scientists at Los Alamos National Laboratory used photolysis on a uranium azide–a molecule containing one uranium atom and three nitrogen atoms–exposing it to ultraviolet light and using the energy from a photon to break off nitrogen gas, resulting in a molecule with a single uranium nitride group.
This breakthrough is important because uranium nitride materials show promise as advanced nuclear fuels due to their high density, high stability, and high thermal conductivity–enabling them to run cooler in advanced reactors.
Discovery News – uranium nitride can also break carbon-hydrogen bonds, which are very strong. Unfortunately the new molecule is destroyed when it rips hydrogen atoms off a carbon atom. For uranium nitride to become commercially viable for fossil fuel cars, it would have to knock one hydrogen atom after another and not destroy itself in the process. The scientists would, in other words, have to turn uranium nitride into a catalyst. That should be possible, said Kiplinger, but right now it is not.
Uranium nitride is a ceramic compound that contains many repeating units of U-N. In contrast, the new uranium nitride molecule contains only one U-N, which is the smallest unit observed in the ceramic solid. The uranium nitride molecule derived from the photolysis process is well defined, unlike solid-state compounds from alternative processes, making it ideal for the controlled study of its physical and chemical properties, a longstanding challenge in uranium chemistry and materials science.
“Actinide nitrides are candidate nuclear fuels of the future, particularly in next-generation reactors developed to meet the energy needs of the 21st century, such as a small modular nuclear power reactors, and for future space missions,” said Jaqueline Kiplinger of Los Alamos National Laboratory’s Materials Physics & Applications Division. “Now we’ve created a molecular model that can help us better understand the functional properties, electronic structure, and chemical reactivity of a single isolated uranium nitride unit, opening a new chapter in uranium chemistry.”
The rare molecule is reactive, able to attack strong carbon-hydrogen bonds to form new nitrogen-hydrogen and nitrogen-carbon bonds. This important discovery demonstrates that the molecular uranium nitride structure is not inert and can undergo reactions with strongly bonded molecules.
“Synthesis of a discrete terminal uranium-nitride functionality has been a holy grail for actinide chemists for the past several decades,” explained David Clark, director of the G.T. Seaborg Institute for Transactinium Science. “Its ultimate discovery is a testimony to the tenacity and skill of the research team, and its chemical and physical properties will teach us a great deal about the nature of chemical bonding in this unusual and fascinating molecular U-N bond.”