Current processes for synthesizing ammonia require costly processes at high temperatures. Now, researchers are turning to ruthenium as a more cost-effective alternative.
Nitrogen is an important nutrient for plants, and while it comprises 80 percent of the planet, it naturally occurs in a gaseous form that plants can't use. In agricultural applications, chemically-produced nitrogen fertilizers are used to encourage plant growth. In the production of these nitrogen fertilizers, the synthesis of ammonia requires a catalyst to speed up a chemical reaction between nitrogen and hydrogen.
Researchers published their findings in the journal Advanced Energy Materials.
Finding Alternatives for Use in Ammonia Synthesis Processes
However, conventional methods for synthesizing ammonia uses the "Haber-Bosch" process, allowing the mass production of plant fertilizer. It is also among the first industrial processes that used high pressure to catalyze the chemical reaction. Despite its scale and efficiency, the process requires high temperatures, from 400 to 500 degrees Celsius, with achieving and maintaining the temperature being a costly process.
A study led by the Tokyo Institute of Technology proposes ruthenium - a rare transition metal - as the catalyst for ammonia synthesis since it can operate under less extreme conditions compared to traditional iron-based catalysts. Its catch is that nitrogen molecules have to stick first to the ruthenium surface before it reacts with hydrogen molecules and for ammonia.
In the case of the new catalyst, the absence of high temperature means that hydrogen tends to stick to the ruthenium surface instead - a chemical reaction called hydrogen poisoning - and hampers ammonia production. To make ruthenium a feasible catalyst for ammonia synthesis, researchers need to work around the hydrogen poisoning problem.
Suppressing Hydrogen Poisoning
The Tokyo Tech researchers discovered materials that can boost ruthenium's catalytic activities, including a group of lanthanide hydride materials.
"The enhanced catalytic performance is realized by two unique properties of the support material," notes Masaaki Kitano, an associate professor at Tokyo Tech. First is that the support materials donate electrons, guiding the dissociation of nitrogen on the catalyst surface. The second is that the support electrons also combine with hydrogen molecules to create hydride ions, readily reacting with nitrogen to form ammonia without the hydrogen attaching itself to ruthenium, preventing hydrogen poisoning.
Theorizing that hydride ion mobility affects the ammonia production process, researchers then investigated lanthanide oxyhydrides as a potential support material for the new catalyst, looking for a connection between hydride ion mobility and ammonia synthesis.
The experiment revealed that while "bulk" hydride ion conductivity had little effect on the synthesis, the "local" mobility of hydride ions is important since it creates resistance against hydrogen poisoning on the ruthenium. Additionally, lanthanum oxyhydrides require a lower onset temperature compared to other support materials tested. It also displayed higher catalytic activity.
Researchers also discovered that oxygen further stabilized the oxyhydride framework as well as the hydride ions from nitridation - a chemical reaction that turns lanthanum oxyhydride to lanthanum nitride, deactivating the support material - that also impedes catalysis and subsequently, synthesis of ammonia.