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There's a good chance you owe your existence to the Haber-Bosch process.
This industrial chemical reaction between hydrogen and nitrogen produces ammonia, the main ingredient in synthetic fertilizers that supply much of the world's food supply and enabled the population explosion of the last century.
It can also endanger the existence of future generations. The process uses about 2% of the world's total energy supply, and most of the hydrogen needed for the reaction comes from fossil fuels.
Inspired by the way nature – including lightning – produces ammonia, a team led by the University of Buffalo has developed a reactor that produces the chemical raw material from nitrogen in the air and water, without a carbon footprint to leave behind.
This plasma electrochemical reactor is described in a study published in the Journal of the American Chemical Societycan maintain a high ammonia production rate of about 1 gram per day for over 1,000 hours at room temperature, directly from the air.
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The researchers say this is a significant advance toward green ammonia synthesis with an industrially competitive production rate and reaction stability.
“Ammonia is often viewed as the chemical that feeds the world, but we must also face the realization that the Haber-Bosch process has not been modernized since its invention 100 years ago. It still uses high-temperature, high-pressure processing and creates a large carbon footprint, making it unsustainable in the long term,” says study corresponding author Chris Li, PhD, assistant professor of chemistry in the UB College of Arts and Sciences. “Our process only requires air and water and can be powered by renewable electricity.”
Mimicking nature's nitrogen cycle
Nature has its own way of producing fertilizers.
During nitrogen fixation, the electrical energy from a lightning strike breaks down nitrogen molecules in the atmosphere to form various nitrogen oxide species. After falling as rainwater, nitrogen oxides are converted into ammonia by bacteria in the soil, providing nutrients to plants.
In the UB-led team's two-stage reactor, the role of lightning is replaced by plasma and the role of bacteria by a catalyst made of copper-palladium.
“Our plasma reactor converts humidified air into nitrogen oxide fragments, which are then placed into an electrochemical reactor that converts them into ammonia using the copper-palladium catalyst,” says Li.
What is crucial is that the catalyst is able to adsorb and stabilize the numerous nitrogen dioxide intermediates produced by the plasma reactor. The team's graph theory algorithm revealed that most nitric oxide compounds must undergo an intermediate step through nitric oxide or amine before becoming ammonia. This allowed the team to intelligently design a catalyst that combines well with these two compounds.
“When plasma energy or a lightning strike activates nitrogen, a soup of nitric oxide compounds is created. In our case, converting up to eight different chemical compounds into ammonia at the same time is incredibly difficult,” says Xiaoli Ge, lead author of the study and a postdoctoral researcher in Li’s lab. “Graph theory essentially allows us to map all the different reaction pathways and then identify a bottleneck chemical. We then optimize our electrochemical reactor to stabilize the bottleneck chemical so that all the different intermediates are selectively converted into ammonia.”
Scaling
Li's team is currently in the process of scaling up its reactor and is exploring both a startup and industry partnerships to help with commercialization. The UB Technology Transfer Office has submitted a patent application for the reactor and the methods for using it.
Over half of the world's ammonia is produced by four countries – China, the United States, Russia and India – while many developing countries are unable to produce their own ammonia. While the Haber-Bosch process must be carried out on a large scale in a central power plant, Li says their system can be carried out on a much smaller scale.
“You can imagine our reactors as something like a medium-sized shipping container with solar panels on the roof. This can then be placed anywhere in the world and generate ammonia for that region if necessary,” he says. “This is a very exciting advantage of our system and will allow us to produce ammonia for underdeveloped regions that have limited access to the Haber-Bosch process.”
Reference: Ge X, Zhang C, Janpandit M, et al. Control of reaction pathways of mixed NOxHy reactants in plasma electrochemical ammonia synthesis. J Am Chem Soc. 2024. doi: 10.1021/jacs.4c12858
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