Transforming ammonia for food security and economic growth

With a new electrochemical synthesis via an electrochemical nitrogen reduction reaction (NRR), achieving carbon-free ammonia production is closer to reality through work from Drs. Abdoulaye Djire and Perla Balbuena, chemical engineering professors at Texas A&M University, and graduate students David Kumar and Hao En Lai. 

A topic outlined in their recent paper published in the Journal of the American Chemical Society (JACS) introduces NRR, which produces ammonia in a cleaner and simpler way by using renewable electricity. 

The research branches off of the team’s previous work, where they looked further into enabling two-dimensional materials in renewable energy. 

“The current process of making ammonia is energy intensive and emits a lot of carbon dioxide, so if you can make ammonia electrochemically, then you can avoid these two negative effects,” Djire said. “During the electrochemical NRR process, water provides the hydrogen atoms, which combine with nitrogen from the air to form ammonia, all powered by electricity.” 

Because nitrogen gas contains a strong triple bond, it is very difficult to convert it directly into ammonia. According to the team, it is necessary to introduce a catalyst that attracts nitrogen from the liquid environment and provides enough energy to weaken and break that triple bond.

“This process doesn’t rely on coal or natural gas to produce hydrogen,” Kumar said. “Instead, we use earth-abundant resources like water and atmospheric nitrogen. This is essential for building a sustainable future, especially for food production.”  

To get around this, the team used a two-dimensional titanium nitride called MNene that already contains nitrogen atoms within its crystal structure. During the reaction, some of these built-in nitrogen atoms are converted into ammonia, leaving tiny vacancies in the material. 

Nitrogen gas from the air can then refill those vacancies, allowing the process to repeat, a mechanism the team has labeled “Lattice Nitrogen Mediated Ammonia Synthesis.” 

“Most catalysts struggle to activate nitrogen because the triple bond is extremely hard to break,” Djire said. “Once a vacancy forms, it becomes an energetic hot spot, and when nitrogen gas from the air arrives, it naturally binds to that site. This binding weakens the nitrogen triple bond and eventually breaks it and allows the reaction to move forward.” 

The key was designing the MNene so that the bond between titanium and nitrogen is just right,  strong enough to keep the material stable, but not so strong that lattice nitrogen can never be released as ammonia. 

With these strategic designs, the team was able to synthesize and create the material that satisfied this requirement. Using spectroelectrochemistry, a technique that couples voltage-controlled reactions with real-time spectroscopic tools, they identified exactly where nitrogen reduction occurs.

The experimental work was complemented by first-principles theoretical calculations, performed by Balbuena and Lai, that provide mechanistic insights and details about the atomistic motion, bond breaking and forming, leading to surface and species evolution. 

Atomistic simulations based on laws of nature demonstrated vacancy formation, molecular nitrogen binding to the surface, breaking of the nitrogen triple bond, and other reaction events that helped interpret the experimental outcomes and develop a clear understanding of the catalytic reaction.    

“Our initial molecular dynamics simulations revealed a critical requirement at the MNene material's edge sites,” Lai said. “We found that for the cycle to work, the nitrogen gas molecule must partially embed into the lattice vacancy. This specific configuration creates the necessary instability to weaken the strong nitrogen-nitrogen bond. If the nitrogen molecule embeds fully, it becomes trapped and slows down the reaction cycle.” 

An important factor is the electrolyte, typically a liquid solution characterized by specific acidity levels.

“When such a solution is in contact with the surface, depending on its chemistry, it can promote or impede the catalytic reaction,” Balbuena said. “This effect is also complemented with the presence and strategic location of surface chemical groups.”   

These complexities were also captured with great fidelity using a combined experimental-theoretical approach. 

One of the primary challenges in this field is that protons in the solution typically prefer to combine with each other to form hydrogen gas rather than reacting with nitrogen.

However, the team discovered that the lattice nitrogen on the MNene edges acts as a proton trap. The thermodynamics of the MNene material naturally favor the protonation of the lattice nitrogen over the hydrogen evolution reaction, which explains the high efficiency they observed experimentally.

“Our work challenges the traditional view that electrochemical reactions depend solely on the metal,” Djire said. “We show that nonmetals, such as the nitrogen intrinsically built into our material, can also play an active role in electrochemical transformations.”

By Raven Wuebker, Texas A&M University College of Engineering

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