Nabbing nitrogen from the air to make fertilizer on the farm | May 1, 2017 Issue - Vol. 95 Issue 18 | Chemical & Engineering News
Volume 95 Issue 18 | pp. 22-23
Issue Date: May 1, 2017

Nabbing nitrogen from the air to make fertilizer on the farm

More sustainable approaches to synthesizing ammonia could re-revolutionize agricultural fertilizer production
Department: Science & Technology
Keywords: ACS meeting news, fixation, sustainability, Haber-Bosch process, ammonia, enzymatic fuel cell, fertilizer
[+]Enlarge
Two bunches of radishes allow comparison of a control crop grown naturally (left) and an experimental crop with enhanced growth through ammonia supplied by Nocera’s bionic leaf (right).
Credit: Nocera lab/Harvard University
A pair of images compares radishes grown with and without an ammonia-producing bacterial treatment.
 
Two bunches of radishes allow comparison of a control crop grown naturally (left) and an experimental crop with enhanced growth through ammonia supplied by Nocera’s bionic leaf (right).
Credit: Nocera lab/Harvard University

British chemist Humphry Davy is best known for being the first person to isolate several elements, including sodium and calcium. Davy carried out his work electrolytically more than 200 years ago as an early experimenter with batteries. Less known about Davy is that during experiments on water electrolysis, in which water is split into hydrogen and oxygen, he found that ammonia formed on the cathode in his electrolysis cell. Davy had unexpectedly coupled N2 dissolved in the water from the air with the H2 being formed to make NH3 (Phil. Trans. R. Soc. Lond. 1807, DOI: 10.1098/rstl.1807.0001).

Scientists and engineers have been fixated on this so-called nitrogen-fixing process ever since, primarily as a means to make NH3 to prepare fertilizer. We now know that bacteria in the soil produce nitrogenase enzymes to pull N2 from the air to make NH3. The ammonia is subsequently converted to nitrate by other bacteria in the soil so that it can be used by plants.

A new set of NH3 production strategies now combines the best of both these worlds: Davy’s electrochemical observation and nature’s enzymatic approach. If these experimental technologies prove successful on a larger scale, they could one day usher in a new revolution in agricultural fertilizer production, much in the way that the Haber-Bosch industrial process currently used to make ammonia ushered in a fertilizer revolution when it was developed 100 years ago.

In one example, postdoctoral researcher Ross D. Milton and chemistry professor Shelley D. Minteer of the University of Utah and coworkers have developed a bioelectrochemical process in which an enzyme-based fuel cell produces NH3 from N2 and H2 at room temperature and pressure. In another example, chemistry professor Daniel G. Nocera at Harvard University, biochemistry professor Pamela A. Silver at Harvard Medical School, and coworkers are developing engineered bacteria that incorporate H2 from water electrolysis with N2 from the air to produce NH3. Minteer and Nocera presented details of their research at the recent American Chemical Society national meeting in San Francisco.

[+]Enlarge
In this enzymatic fuel cell designed by Utah’s Milton and Minteer, a hydrogenase enzyme oxidizes H2 and a nitrogenase reduces N2 in an overall process that produces NH3 and a small surplus of electricity (MV = methyl viologen, ATP = adenosine triphosphate, ADP = adenosine diphosphate).
Credit: Ross Milton
A schematic diagram shows the components of an enzymatic fuel cell that produces ammonia.
 
In this enzymatic fuel cell designed by Utah’s Milton and Minteer, a hydrogenase enzyme oxidizes H2 and a nitrogenase reduces N2 in an overall process that produces NH3 and a small surplus of electricity (MV = methyl viologen, ATP = adenosine triphosphate, ADP = adenosine diphosphate).
Credit: Ross Milton

Following Davy’s discovery, chemists began trying to develop electrosynthesis procedures to produce ammonia on a large scale. And with an understanding of the biological nitrogen-fixing process, chemists have been trying to develop metal catalysts that mimic enzymes. But researchers haven’t quite figured out how to make these approaches work efficiently on a large enough scale to be practical.

For that, we have the Haber-Bosch chemical synthesis process. This brute-force industrial method employs a metal catalyst to couple H2 with N2 at high temperature and pressure to prepare NH3. Much of the NH3 is then converted to nitric acid by the Ostwald process to make the fertilizer ammonium nitrate.

The Haber-Bosch-Ostwald pathway requires a substantial industrial infrastructure that consumes massive amounts of energy and creates great volumes of carbon dioxide and other pollutants. Researchers have therefore sought out environmentally friendlier and more sustainable approaches to producing ammonia, ranging from thermal solar reactors to engineered plants that make their own ammonia. The new strategies from Minteer and Nocera could be part of the solution everyone is looking for.

“If electrolysis should become a viable strategy for nitrogen fixation, it could be a means of circumventing the Haber-Bosch and Ostwald processes,” says chemistry professor Robert H. Crabtree of Yale University, who specializes in catalytic strategies for alternative energy generation.

Last year, Crabtree and chemistry intellectual property specialist Michael Jewess published a perspectives paper recounting Davy’s discovery and proposing that scientists ramp up efforts to develop sunlight-driven bioelectrochemical systems for making NH3 (ACS Sustainable Chem. Eng. 2016, DOI: 10.1021/acssuschemeng​.6b01473). In a big picture way, electrocatalytic nitrogen fixation for distributed fertilizer production would be a more sustainable method for individual farms to harvest N2 from the air and make their own fertilizer, bypassing the current industrial production and distribution systems, Crabtree suggests.

“This is leading the chemical side of the nitrogen-fixation problem to progress beyond prior mechanistic and biomimetic concerns and take on real practical significance, as the recent work of the Minteer and Nocera groups shows,” Crabtree says.

Although many researchers have explored enzymatic approaches to NH3 production, Minteer’s group at Utah has provided the first evidence for bioelectrochemical NH3 production by a complete nitrogenase, rather than just one subunit of the enzyme (Angew. Chem. Int. Ed. 2017, DOI: 10.1002/anie.201612500). The Utah team’s enzymatic fuel cell consists of two compartments. On one side, a hydrogenase enzyme oxidizes H2 supplied to the cell to form hydrogen ions and electrons. Because the enzyme is not efficient at directly interacting with the electrode, the researchers need to provide a redox-active compound as a go-between to pick up and drop off the electrons. They chose methyl viologen, a versatile compound often used for this electrochemical role.

In the other compartment of the fuel cell, they use a nitrogenase enzyme to reduce N2 from the air to make NH3, using electrons supplied from the hydrogenase side of the cell through an external circuit. Methyl viologen again acts as a redox mediator to shuttle the electrons between the electrode and the enzyme. The hydrogen ions, needed to form NH3, migrate through a membrane separating the two compartments. As a bonus, the overall reaction generates a small excess of electricity.

There’s a few catches to the bioelectrochemical system, Minteer explains. Nitrogenases are not commercially available, and when isolated from cultured bacteria they must be handled with care because the enzymes can be irreparably damaged by oxygen. In addition, nitrogenases require the coenzyme adenosine triphosphate (ATP) to operate; ATP undergoes hydrolysis to mediate energy transfer for nitrogen reduction. Minteer’s group had to devise a way to continuously supply ATP to the enzyme, which the researchers did by adding creatine phosphate to recycle adenosine diphosphate (ADP) into more ATP.

The team has been able to produce small amounts of NH3 so far, Minteer says, and several challenges remain before scaling up. But she thinks those will be mostly related to materials design and enzyme engineering. One challenge is to address the oxygen sensitivity of nitrogenase and the lifetime of the enzymes. Another is to develop a workaround to avoid the need for ATP.

Looking to the future, Minteer is thinking about small-scale systems in which every farmer could use a solar cell to run an enzymatic bioelectrosynthesis cell or set of cells to make ammonia, rather than buying it delivered in trailer-mounted tanks as many currently do. Farmers could use the excess electricity to help power their operations, or they could sell it to the power grid.

“There has been a growing interest in alternative, greener methods that move away from centralized ammonia production.”

John J. Watkins, chief executive officer, Fulcrum Bioscience

“We generally think of Haber-Bosch as an intensive process that consumes energy. But with the right catalytic system design, we can actually generate energy,” she says. “Our technology would definitely enable us to decentralize fertilizer production and avoid building and running large industrial plants.”

Nocera’s group is already known for developing an “artificial leaf,” a wireless solar-cell device that mimics a natural leaf by splitting water into H2 and O2. The H2 can be stored and used as needed to run fuel cells to generate electricity. The team has recently been taking the concept a step further to develop systems for making liquid fuels, and now a hybrid artificial leaf-microbial system to produce NH3.

The new approach, a construct called a “bionic leaf,” is actually an engineered bacterium that effectively carries out Haber-Bosch in a single microbial cell. The researchers designed a Xanthobacter species to take H2 from the artificial leaf and use a carbohydrogenase enzyme to couple it with CO2 from the air to make the bioplastic polyhydroxybutyrate. A number of bacteria are known to produce such bioplastics that they store as a fuel source, like people store fat. But the team also integrated the ability for the microbe to absorb N2 from the air and use its nitrogenase to couple it with H2 from the polyhydroxybutyrate to make NH3. This trick replaces the need for ATP to power the enzyme.

The researchers spray a solution containing the polyhydroxybutyrate-storing bacteria onto the soil like a nutrient, where NH3 is produced and expelled into the ground, reminiscent of the way farmers apply liquid ammonia to fields. Natural bacteria in the soil do the rest, converting the NH3 into nitrate that plants can absorb through their roots. Nocera’s group tested the strategy on radishes, showing that plants treated with the bacteria weigh 150% more than untreated plants. “We can grow big radishes, really big radishes,” Nocera exclaims.

The technology is still at an early stage and nowhere near being put into practical use, Nocera stresses. “I just wanted to find out if we could actually do it,” he says. “The answer is yes.”

Nocera’s team is now exploring ways to speed up NH3 production. The proof of concept also points to the possibility of modifying the microbes to synthesize other compounds. “One day we might be able to make everything we need by tailoring these bugs,” Nocera says. “You would have a solar-based manufacturing lab.”

Nocera notes the primary beneficiaries of the Haber-Bosch process have been people living in developed countries with established infrastructure. He views the bionic leaf instead as a means of boosting agriculture and food production in developing regions. In fact, the strategy assumes developing an infrastructure won’t be needed at all.

“The Haber-Bosch process is one of the greatest scientific achievements in the 20th century,” says John J. Watkins, chief executive officer of Fulcrum Bioscience. “Industrial-scale nitrogen fixation allowed agriculture production to increase enormously and feed an ever-increasing world population. However, there has been a growing interest in alternative, greener methods that move away from centralized ammonia production.”

Watkins’ company is working toward increasing the nitrogen-fixation rate of algae biofilms using genetic modification and electrochemical methods. Part of his evaluation includes looking at the dynamics of the fertilizer market.

“The current system of purchasing fertilizer is straightforward for farmers: Fertilizer is purchased, delivered, and then applied,” Watkins says. Because agriculture operates on narrow margins, farmers must balance increased yields against increased cost to remain profitable, he adds. That means any new production process, like the technologies being developed by the Minteer and Nocera groups, must be cost competitive, Watkins says. And they must be simple to operate with low maintenance demands. “Growing up in Iowa, I never knew any farmers with an abundance of free time or money.”

 
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Comments
Mr Michael T Deans (Wed Apr 26 18:33:03 EDT 2017)
My research introduces the H-bond-lined trans-membrane channels in tRNA analogue 'transport DNA'. An ATP-driven ratchet mechanism propels protons to bond with nitrogen and nicotinamide, fixing nitrogen. It releases hydrogen and is more efficient than the Haber process. Vats of bugs would work better with a smaller carbon footprint. Read SCIENCE UNCOILED, Melrose Press.
Stephen Lord (Wed Apr 26 19:22:59 EDT 2017)
The Haber process itself does not generate large amounts of CO2. It is the production of hydrogen from natural gas via SMR that produces the CO2. Thus the CO2 reduction comes from use of electrolysis which requires considerable amounts of electricity which in turn generates large amounts of CO2. These kind of unacknowledged substitutions are a problem in science today as it reduces credibility of all researchers. The laws of chemical engineering scaleup are well known and a single world scale plant is always much cheaper. Were the electricity to come from solar panels on the farm then land would be taken out of crop use. This is a real problem with all biofuel programs which is essentially what this is. PS I am a Chem Eng if you has not guessed.
KM (Thu Apr 27 08:45:32 EDT 2017)
1. The Haber process is also a high temperature, high pressure, equilibrium-limited process, and thus energy intensive (requires heat + compression). Hence the process itself does generate large amounts of CO2 on top of that generated by the requisite H2 production.
2. There is no reason to place the solar panels (or wind- or hydro-electricity generation capacity) on valuable cropland.
Steve Ritter (Thu Apr 27 10:02:35 EDT 2017)
As generally cited, the Haber-Bosch process consumes about 1% of all global energy production, most of which comes from burning fossil fuels. The Haber-Bosch and Ostwald processes combined are estimated to account for about 3% of global CO2 emissions http://dx.doi.org/10.1021/acssuschemeng.6b01473.
Madhav Acharya (Thu Apr 27 09:33:17 EDT 2017)
Very interesting read. ARPA-E has launched a new program "REFUEL" focused on conversion of renewable energy into liquid fuels including ammonia - many projects are doing electrochemical synthesis with different metal catalysts. more information may be found at

https://arpa-e.energy.gov/?q=arpa-e-programs/refuel
Michael King (Tue May 02 11:01:19 EDT 2017)
www.sciencemag.org/lookup/doi/10.1126/science.1254234
Richard Kohn, Ph.D. (Thu May 11 10:28:10 EDT 2017)
Though interesting, I don't understand why this process is greener or more sustainable. The problem with fertilizer is the leaching and runoff into water and volatilization into air. These losses contribute to smog, eutrophication, and climate change. Simply decentralizing fertilizer production, if anything might increase fertilizer use including overapplication. That would only make the problem worse.
Steve Ritter (Mon May 15 12:42:59 EDT 2017)
This is a good point. As with any technology, a complete life cycle assessment is needed to look at the pluses and minuses. In this case, decentralizing fertilizer production would greatly reduce energy needs and use of fossil fuels, and the associated CO2 and other pollution costs. More fertilizer is needed, especially in developing regions where fertilizer access is now limited, to meet future (sustainable) needs for global food production. The trade off will be more problems with agricultural runoff and perhaps atmospheric pollution, but that can be moderated with education about more efficient use of fertilizers. Overall, the ability to create a sustainable world is far from perfect.

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