Ammonia is the most extensively manufactured compound globally today, utilized chiefly as a source of nitrogen fertilizer. Its creation also constitutes a significant contributor to greenhouse gas emissions — the highest in the entire chemical sector.

Recently, a group of researchers at MIT has devised a groundbreaking method for synthesizing ammonia without the conventional fossil-fuel-based chemical plants that necessitate elevated temperatures and pressures. Instead, they have discovered a technique to leverage the Earth itself as a geochemical reactor, generating ammonia underground. This procedure utilizes the Earth’s naturally occurring heat and pressure, provided at no cost and without emissions, alongside the reactivity of minerals inherently existing underground.

The innovative strategy the team developed involves injecting water underground into an area rich in iron-containing subsurface rock. The water transports a nitrogen source and metal catalyst particles, enabling the water to interact with the iron to produce clean hydrogen, which subsequently reacts with the nitrogen to form ammonia. A second well is then employed to draw that ammonia back to the surface.

This method, which has been validated in the laboratory but not yet tested in a natural environment, is elaborated upon today in the journal Joule. The co-authors of the paper include MIT professors of materials science and engineering Iwnetim Abate and Ju Li, postdoctoral researcher Yifan Gao, along with five other MIT contributors.

“When I initially produced ammonia from rock in the laboratory, I was incredibly thrilled,” recalls Gao. “I recognized that this indicated a completely novel and previously unreported method for ammonia synthesis.”

The traditional approach to ammonia production is known as the Haber-Bosch process, which was created in Germany during the early 20th century to substitute natural nitrogen fertilizer sources like mined deposits of bat guano, which were becoming exhausted. However, the Haber-Bosch process is notably energy-intensive: It demands temperatures of 400 degrees Celsius and pressures of 200 atmospheres, necessitating vast installations to operate efficiently. Some regions, like sub-Saharan Africa and Southeast Asia, have limited or no such facilities in function. Consequently, the scarcity or exorbitant cost of fertilizer in these areas has restricted agricultural output.

“The Haber-Bosch process ‘is effective. It functions,’” states Abate. “Without it, we wouldn’t have been able to sustain 2 out of the total 8 billion individuals in the world right now,” he adds, alluding to the fraction of the global population whose food is cultivated with ammonia-based fertilizers. “However, due to the emissions and energy requirements, an improved method is essential,” he emphasizes.

The combustion of fuel to produce heat accounts for roughly 20 percent of the greenhouse gases released from facilities employing the Haber-Bosch process. The production of hydrogen is responsible for the remaining 80 percent. Yet, ammonia, the molecule NH₃, consists solely of nitrogen and hydrogen. There’s no carbon in its composition; hence, where do the carbon emissions originate? The conventional method of obtaining the necessary hydrogen involves processing methane gas with steam, breaking the gas down into pure hydrogen for use, and carbon dioxide gas that is emitted into the atmosphere.

Alternative methods exist for generating low- or zero-emission hydrogen, such as utilizing electricity derived from solar or wind sources to split water into oxygen and hydrogen, but this method can be costly. This is why Abate and his team focused on developing a mechanism for producing what they refer to as geological hydrogen. Certain regions globally, including some in Africa, have been found to spontaneously generate hydrogen underground through chemical interactions between water and iron-rich rocks. These pockets of naturally formed hydrogen can be extracted like natural methane deposits, yet the extent and locations of such reserves remain relatively uncharted.

Abate recognized that this process could be initiated or augmented by injecting water, infused with copper and nickel catalyst particles to expedite the reaction, into regions where iron-rich rocks were already present. “We can use the Earth as a factory to produce clean streams of hydrogen,” he asserts.

He remembers contemplating the issue of emissions from hydrogen production for ammonia: “The ‘aha!’ moment for me was considering how we might connect this geological hydrogen production process with the method of creating Haber-Bosch ammonia.”

This connection would address the primary challenge of the underground hydrogen production process, which is capturing and storing the gas once it is generated. Hydrogen is an exceptionally small molecule — the smallest of them all — making it difficult to contain. However, by executing the entire Haber-Bosch process underground, the only substance needing to be transported to the surface would be the ammonia itself, which is straightforward to capture, store, and move.

The sole additional component required to finalize the method is the introduction of a nitrogen source, such as nitrate or nitrogen gas, into the water-catalyst mixture being injected into the ground. Then, as hydrogen is released from water molecules after interacting with the iron-rich rocks, it can promptly bond with the nitrogen atoms also carried in the water, with the deep underground environment supplying the high temperatures and pressures necessary for the Haber-Bosch process. A second well located near the injection point then extracts the ammonia and directs it into surface tanks.

“We refer to this as geological ammonia,” Abate states, “because we are utilizing subsurface temperature, pressure, chemistry, and naturally occurring rocks to produce ammonia directly.”

While transporting hydrogen necessitates costly equipment to cool and liquefy it, and virtually no pipelines are available for its transport (aside from those near oil refinery sites), transporting ammonia is simpler and more economical. It costs approximately one-sixth of what it takes to transport hydrogen, and there are already over 5,000 miles of ammonia pipelines and 10,000 terminals established in the U.S. alone. Furthermore, Abate elaborates, ammonia, unlike hydrogen, already has a significant commercial market established, with production expected to increase two to threefold by 2050, as it is used not merely for fertilizers but also as feedstock for an array of chemical processes.

For instance, ammonia can be combusted directly in gas turbines, engines, and industrial furnaces, offering a carbon-free alternative to fossil fuels. It is being explored for maritime shipping and aviation as a potential fuel, as well as a prospective propellant for space travel.

Another advantage of geological ammonia is that untreated wastewater, inclusive of agricultural runoff, which is generally rich in nitrogen already, could be utilized as the water source and treated during the process. “We can address the issue of managing wastewater while also producing something valuable out of this waste,” Abate states.

Gao concurs that this method “incurs no direct carbon emissions, offering a pathway to reduce global CO2 emissions by as much as 1 percent.” To reach this outcome, he notes, the team “overcame numerous obstacles and gained insights from many unsuccessful attempts. For example, we experimented with a wide variety of conditions and catalysts before determining the most effective one.”

The project received initial funding under a flagship initiative of MIT’s Climate Grand Challenges program, the Center for the Electrification and Decarbonization of Industry. Professor Yet-Ming Chiang, co-director of the center, remarks, “I don’t believe there has been any prior instance of intentionally utilizing the Earth as a chemical reactor. That’s one of the key novel aspects of this approach.” Chiang highlights that even though it is a geological process, it occurs rapidly, not over geological timescales. “The reaction is fundamentally completed in a matter of hours,” he states. “The reaction is so swift that this resolves one of the fundamental questions: Do you have to wait for geological durations? And the answer is absolutely no.”

Professor Elsa Olivetti, a mission director of the recently established Climate Project at MIT, comments, “The innovative thinking by this team is invaluable to MIT’s capacity to create a significant impact. Merging these promising outcomes with, for instance, an advanced understanding of the geology surrounding hydrogen accumulations illustrates the comprehensive efforts the Climate Project strives to promote.”

“This represents a major breakthrough for the future of sustainable progress,” says Geoffrey Ellis, a geologist at the U.S. Geological Survey, who was not involved in this research. He adds, “While there is undoubtedly more work needed to verify this at the pilot stage and to scale it for commercial use, the concept that has been demonstrated is genuinely transformative. The method of engineering a system to enhance the natural process of nitrate reduction by Fe²⁺ is ingenious and will probably lead to further advancements along these lines.”

The initial experiments on the process have been conducted in the laboratory, so the subsequent step will be to validate the process utilizing an actual underground site. “We believe that kind of experiment can be accomplished within the next one to two years,” Abate states. This could pave the way for employing a similar technique for other chemical production processes, he adds.

The team has submitted a patent application and aims to work toward commercializing the process.

“Moving ahead,” Gao states, “our emphasis will be on refining the process conditions and scaling up tests, with the objective of enabling practical applications for geological ammonia in the near future.”

The research group also included Ming Lei, Bachu Sravan Kumar, Hugh Smith, Seok Hee Han, and Lokesh Sangabattula, all affiliated with MIT. Additional funding was provided by the National Science Foundation and was facilitated, in part, through the use of MIT.nano resources.