The technique could allow cheaper fertilizer production

Most of the world’s fertilizer is produced in large manufacturing plants, which require large amounts of energy to generate the high temperatures and pressures necessary to combine nitrogen and hydrogen in ammonia.

MIT chemical engineers are working to develop a smaller-scale alternative, which they imagine could be used to produce fertilizers locally for farmers in remote and rural areas, such as sub-Saharan Africa. Fertilizer is often difficult to obtain in such areas due to the cost of transporting it from large manufacturing facilities.

In a step toward that type of small-scale production, the research team has devised a way to combine hydrogen and nitrogen using electrical current to generate a lithium catalyst, where the reaction takes place.

“In the future, if we imagine how we want this to be used someday, we want a device that can breathe air, drink water, have a solar panel connected and be able to produce ammonia. This could be used by a farmer or a small community of farmers “says Karthish Manthiram, assistant professor of chemical engineering at MIT and lead author of the study.

Graduate student Nikifar Lazouski is the lead author of the article, which appears today in Catalysis of nature. Other authors include graduate students Minju Chung and Kindle Williams, and undergraduate student Michal Gala.

Smaller scale

For more than 100 years, fertilizer has been manufactured using the Haber-Bosch process, which combines atmospheric nitrogen with hydrogen gas to form ammonia. The hydrogen gas used for this process is generally obtained from methane derived from natural gas or other fossil fuels. Nitrogen is very little reactive, so high temperatures (500 degrees Celsius) and pressures (200 atmospheres) are required for it to react with hydrogen and form ammonia.

Through this process, manufacturing plants can produce thousands of tons of ammonia per day, but they are expensive to run and emit a large amount of carbon dioxide. Among all the chemicals produced in high volume, ammonia is the largest contributor to greenhouse gas emissions.

The MIT team set out to develop an alternative manufacturing method that could reduce those emissions, with the added benefit of decentralized production. In many parts of the world, there is little infrastructure to distribute fertilizers, making it costly to obtain fertilizers in those regions.

“The ideal feature of a next generation method of making ammonia would be that it is distributed. In other words, it could make ammonia close to where you need it,” says Manthiram. “And ideally, it would also remove CO₂ footprint that otherwise exists. “

While the Haber-Bosch process uses extreme heat and pressure to force the reaction of nitrogen and hydrogen, the MIT team decided to try using electricity to achieve the same effect. Previous research has shown that the application of electrical voltage can change the balance of the reaction to favor the formation of ammonia. However, it has been difficult to do this economically and sustainably, the researchers say.

Most of the previous efforts to perform this reaction at normal temperatures and pressures have used a lithium catalyst to break the strong triple bond found in nitrogen gas molecules. The resulting product, lithium nitride, can react with hydrogen atoms in an organic solvent to produce ammonia. However, the solvent that is typically used, tetrahydrofuran or THF, is expensive and is consumed by the reaction, so it must be continually replaced.

The MIT team devised a way to use hydrogen gas instead of THF as the source of hydrogen atoms. They designed a mesh-shaped electrode that allows nitrogen gas to diffuse through it and interact with hydrogen, which dissolves in ethanol, on the surface of the electrode.

This stainless steel mesh structure is coated with the lithium catalyst, produced by plating lithium ions from the solution. Nitrogen gas diffuses throughout the mesh and is converted to ammonia through a series of lithium-mediated reaction steps. This configuration allows hydrogen and nitrogen to react at relatively high speeds, despite the fact that they are generally not very soluble in any liquid, making it more difficult to react at high speeds.

“This stainless steel cloth is a way to very effectively contact nitrogen gas with our catalyst, while still having the necessary electrical and ionic connections,” says Lazouski.

Water division

In most of their ammonia production experiments, the researchers used nitrogen and hydrogen gases that flow from a gas cylinder. However, they also demonstrated that they could use water as a source of hydrogen, first electrolyzing the water, and then flowing that hydrogen into their electrochemical reactor.

The overall system is small enough to sit on a lab table, but could be expanded to produce large amounts of ammonia by connecting many modules, Lazouski says. Another key challenge will be improving the energy efficiency of the reaction, which is now only 2 percent, compared to 50 to 80 percent for the Haber-Bosch reaction.

“We have a general reaction that finally seems favorable, which is a big step forward,” he says. “But we know that there is still an energy loss problem that needs to be resolved. That will be one of the main things we want to address in the future work that we will undertake.”

In addition to serving as a production method for small batches of fertilizers, this approach could also lend itself to energy storage, says Manthiram. This idea, which some scientists are now pursuing, requires the use of electricity produced by wind or solar energy to power the generation of ammonia. Ammonia could serve as a liquid fuel that would be relatively easy to store and transport.

“Ammonia is such a critical molecule that it can wear many different hats, and this same method of producing ammonia could be used in many different applications,” says Manthiram.

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site covering news on MIT research, innovation and teaching.