Hiroshima researchers improve kinetics of ammonia synthesis by chemical looping of lithium hydride; targeting small-scale NH3 production
Researchers at Hiroshima University have greatly improved the performance of ammonia (NH3) synthesis via the chemical looping process of lithium hydride by using a lithium oxide (Li2O) scaffold to improve the reaction kinetics. In a paper published in The Journal of Physical Chemistry C, the Hiroshima team reports that with the scaffold, the reaction of LiH and N2 is completed within 20 minutes—a significantly shorter time than the conventional reaction time of more than 1,000 minutes.
Ammonia has recently been recognized as an outstanding energy carrier molecule. In 1918, German chemist Fritz Haber won the Nobel Prize for synthesis of ammonia from its elements, paving the way for ammonia’s significant role in industrial fertilizers. However, use of ammonia in renewable energy applications has been limited by the processes available to synthesize it. The Haber-Bosch process, used in industrial production of ammonia, requires high temperature and pressure, conditions not typically available in renewable energy storage and transport infrastructure.
NH3 synthesis via chemical looping using lithium hydride (LiH) starts by combining LiH with N2 (molecular nitrogen) at ambient pressure and temperatures up to 500 °C to yield a lithium imide product (LiNH2). The lithium imide then reacts with hydrogen gas (H2) to yield ammonia.
The reaction time for ammonia synthesis from its constituent molecules in this process is more than 1,000 minutes. Its speed is limited by the agglomeration of the products of the reaction into large particles (more than 200 μm) that don’t have much surface area exposed to the hydrogen gas. For its practical application in distributed renewable energy, this prolonged reaction, requiring extreme conditions, is an impediment to ammonia production.
In the new study, researchers experimented with using lithium oxide (Li2O) as a molecular scaffold to synthesize ammonia under ambient pressure and temperatures below 400 °C—conditions easy to mimic in nonindustrial settings.
(Left) Reaction yield as a function of time for N2 dissociation process of LiH and LiH+Li2O, (center) microscopic images of LiH and LiH+Li2O after the N2 dissociation reaction, (right) Reaction yield as a function of time for NH3 synthesis/regeneration process of both samples. (Image courtesy of Hiroki Miyaoka, Hiroshima University)
They combined the reactant lithium hydride with lithium oxide and found that the lithium hydride prevented clumping, leaving smaller particles (less than 50 μm) with more surface area exposed for chemical reactions. Using these non-agglomerated reactants and adding the gaseous hydrogen used in the final step of ammonia synthesis, they were able to produce ammonia more quickly; the reaction substantially sped up.
If ammonia can be produced quickly with relatively simple equipment under modest temperature and pressure conditions, it paves the way for smaller-scale ammonia production.
The chemical looping process is useful to establish the small-scale NH3 synthesis process, which can be operated under lower pressure and temperature with higher conversion yield than the conventional catalytic process.—Hiroki Miyaoka, corresponding author
The new process also obviates the need for expensive metal catalysts—such as elemental ruthenium (Ru)—used in industrial synthesis of ammonia.
Schematic image of small-scale and distributed NH3 synthesis processes required to effectively utilize the fluctuated and localized renewable energy. (Image courtesy of Hiroki Miyaoka, Hiroshima University)
Kentaro Tagawa, Hiroyuki Gi, Keita Shinzato, Hiroki Miyaoka, and Takayuki Ichikawa (2022) “Improvement of Kinetics of Ammonia Synthesis at Ambient Pressure by the Chemical Looping Process of Lithium Hydride” The Journal of Physical Chemistry C doi: 10.1021/acs.jpcc.1c09902