Taking a cue from lightning, eco-friendly reactor converts air and water into ammonia

This industrial chemical reaction between hydrogen and nitrogen produces ammonia, a key ingredient in the synthetic fertilizers that supply much of the world’s food and contributed to the population explosion of the last century.

It could also threaten the survival of future generations. This process consumes about 2% of the world’s total energy supply, and the hydrogen required for the reaction mainly comes from fossil fuels.

Inspired by how nature, including lightning, produces ammonia, a team led by the University at Buffalo has developed a reactor that can use nitrogen from air and water to produce chemical commodities without creating any carbon footprint.

The plasma electrochemical reactor is described in a study published Journal of the American Chemical Society, High ammonia production rates of approximately 1 gram per day can be maintained for more than 1,000 hours at room temperature and directly from air.

The researchers say this is a major advance in green ammonia synthesis, with industrial competitiveness in productivity and reaction stability.

“Ammonia is often thought of as the chemical that feeds the world, but we also have to face the reality: the Haber-Bosch process has not been modernized since its invention 100 years ago. It still uses high temperatures and pressures and creates a large carbon footprint , is unsustainable in the long term. “Our process only requires air and water and can be powered by renewable electricity. “

Mimicking nature’s nitrogen cycle

Nature has her own way of producing fertilizer.

During the nitrogen fixation process, the electrical energy of lightning strikes decomposes nitrogen molecules in the atmosphere to form different nitrogen oxide species. After falling as rain, nitrogen oxides are converted by bacteria in the soil into ammonia, which provides nutrients to plants.

In the University at Buffalo-led team’s two-step reactor, the role of lightning is replaced by plasma and that of bacteria by a copper-palladium catalyst.

“Our plasma reactor converts humid air into nitrogen oxide fragments, which are then put into an electrochemical reactor to convert them into ammonia using a copper-palladium catalyst,” Li said.

Crucially, the catalyst is able to adsorb and stabilize the large amounts of nitrogen dioxide intermediates produced in the plasma reactor. The team’s graph theory algorithm found that most nitrogen oxide compounds must pass through the nitric oxide or amine cycle as an intermediate step before becoming ammonia. This allowed the team to cleverly design a catalyst that combines well with both compounds.

“When nitrogen gas is activated by plasma energy or a lightning strike, a cloud of nitrogen oxide compounds is produced. In our case, it was very difficult to convert up to eight different compounds into ammonia at the same time,” said the first author of the study. The author is a postdoctoral researcher in Li’s laboratory. “Graph theory essentially allows us to map out all the different reaction pathways and then identify the bottleneck chemical. We then optimize the electrochemical reactor to stabilize the bottleneck chemical so that all the different intermediates will be selectively converted to ammonia.”

Expand scale

Li’s team is currently scaling up their reactor and is exploring startups and partnerships with industry to help commercialize it. The University at Buffalo’s Technology Transfer Office has filed a patent application for the reactor and methods of using it.

More than half of the world’s ammonia is produced by just four countries: China, the United States, Russia and India, while many developing countries are unable to produce it themselves. While the Haber-Bosch process must be done at scale in centralized power plants, Li says their system can be done on a much smaller scale.

“You can think of our reactor as a medium-sized shipping container with solar panels on the roof. It can then be placed anywhere in the world and produce ammonia based on the demand in that area,” he said. “This is a very exciting advantage of our system and will allow us to produce ammonia for less developed regions where the Haber-Bosch process is limited.”