Next-generation ammonia tech: biohybrid nanoparticles

Sustainable ammonia can be produced today: doing so would use electrolyzers to make hydrogen to feed the traditional Haber-Bosch process. In a very few years, new technologies will skip this hydrogen production phase altogether and make ammonia directly from renewable power in an electrochemical cell. Further down the pipeline, next generation technologies will mimic nature, specifically the nitrogenase enzyme, which produces ammonia naturally.

One of these next generation technologies is currently producing impressive results at the US Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL).

The NREL research began as a project to produce hydrogen, mimicking the enzymes in algae, but soon developed into a project to produce ammonia – and this presented a greater challenge.

Green algae absorb sunlight and use that energy to generate carbon and hydrogen. NREL scientists had already found that, by replacing the algae’s normal hydrogen-producing enzyme — called hydrogenase — with a ferredoxin and hydrogenase fusion protein, they could trigger the algae to increase hydrogen production.

But what works for one biochemical reaction may not work for another. Compared to the tight bond between nitrogen molecules, hydrogen molecules are relatively easy to separate. Researchers needed greater understanding of nitrogenases — enzymes that direct the conversion of nitrogen to ammonia — before they could begin to work on a renewable method for ammonia production.
NREL article: NREL Scientists Upend Century-Old Ammonia Production Method, 07/26/2017

The method that NREL has developed uses sunlight to power the reaction that breaks the triple bonds of the nitrogen molecule. This sunlight-driven reaction is different from most of the renewable ammonia production technologies I’ve written about because those are generally designed to use solar power, sunlight transformed into electricity, or another renewable power source, to fuel an electrochemical reaction.

NREL’s technology doesn’t need the electricity, just the sunlight itself. In other words, these are not electrocatalysts, but photocatalysts.

Nitrogenases … require a large amount of chemical energy that comes from adenosine triphosphate (ATP) molecules. NREL solved this problem by replacing ATP with sunlight, captured by nanocrystals of cadmium sulfide (CdS). The solar rays power a catalytic reaction within the enzyme, allowing it to break apart the nitrogen bonds and create ammonia.
NREL article: NREL Scientists Upend Century-Old Ammonia Production Method, 07/26/2017

Click to enlarge. Illustration by Al Hicks via NREL Scientists Upend Century-Old Ammonia Production Method. NREL’s biohybrid approach harvests light to produce electrons (e) to drive the dinitrogen (N2) reduction reaction for generating ammonia (NH3).
The initial results are very close to those seen in nature: “the nanocrystals produced 63% as much ammonia as the system powered by ATP.”

(I would argue that 63% of a baseline is, for a new technology, excellent: many other projects are orders of magnitude away from their target production rates. My caveat follows below.)

This latest work, to produce ammonia using nitrogenase, builds upon their previous results, which also relied on CdS nanocrystals, to produce hydrogen using hydrogenase – all of which is possible because of interdisciplinary collaboration between NREL, Utah State University, University of Colorado, and Montana State University.

The earlier project demonstrated the need for “a sacrificial electron donor” so that the nanocrystal could be regenerated and “repeatedly serve as a catalyst for the chemical reaction.”

“We’ve done a lot of work to understand how the hydrogenase interacts with the nanoparticle and we were able to translate a lot of that knowledge to the nitrogenases … we’ve been able to understand how nanoparticles and enzymes generally react …

“In addition to reducing nitrogen, [nitrogenase] can also make hydrogen … It can also take acetylene and make ethylene, and we looked at that. We worked our way up to nitrogen reduction, optimizing at each step, and took this systematic approach instead of trying to hit the bullseye for nitrogen right away. We built up slowly to a system that looked like it would work for nitrogen.”
Katherine Brown, quoted in NREL Scientists Upend Century-Old Ammonia Production Method, 07/26/2017

So the system produces ammonia at a rate not dissimilar to ATP-powered nitrogenase – but Thomas Hager, who wrote The Alchemy of Air about the invention of the Haber-Bosch process, addressed the shortcomings of nitrogenase in his keynote speech at the DOE’s ARPA-E Energy Innovation Summit, on March 24, 2017.

In his keynote, Hager praised nitrogenase: “It’s a beautiful system, but very slow.”

Slow is, however, a relative term: until we introduced intensive farming, nitrogenase was fast enough to supply all the nitrogen our planet required, more or less. Time will tell whether the team at NREL will be able to engineer a way of speeding up their catalyst, or if they will find a scale and market application for which its natural speed is optimal.

NREL is by no means the only research group taking this approach to sustainable ammonia synthesis. Indeed, understanding nitrogenase’s speed limit was the first challenge identified in the DOE’s 2016 funding announcement for sustainable ammonia technology development (using the phrase “turnover frequency” to describe the speed of the reaction):

Nitrogenase functions at room temperature and ambient pressure albeit at low turnover frequencies (about 2 NH3 s-1). Understanding how the nitrogenase and its ancillary components gate and control the flow of electrons to the catalytic site could open up intriguing possibilities for the delivery of electrons from alternate sources.
US Department of Energy, Sustainable Ammonia Synthesis (DE-FOA-0001569), 04/12/2016

The projects that the DOE funded in that round included one, from UC Irvine, that aimed to construct and study “a functional MoFe protein equivalent,” using “a synthetic biology approach.”

A different but notable project that also powers nitrogenase with sunlight is Daniel Nocera’s “bionic leaf” at Harvard University, “coupling solar-based water splitting to a nitrogen fixing bioorganism in a single reactor.”

The team at NREL is not aiming to build a reactor filled with bioorganisms, however. They say their research will be most useful if it leads to the development of nitrogenase-inspired catalysts, instead of scaling up a system that uses the enzymes themselves:

A device that uses light-driven enzymes to make ammonia would not be the best idea. The enzymes, she said, “are labor-intensive to make and to isolate. Ideally, you would want some kind of artificial catalyst that is based on what we understand about the enzyme. That’s the value of this work.”
Katherine Brown, quoted in NREL Scientists Upend Century-Old Ammonia Production Method, 07/26/2017

In other words, as their paper in Science concludes, “the CdS:MoFe protein biohybrids provide a photochemical model for achieving light-driven N2 reduction to NH3.”

The 2016 Science paper is here, and NREL has published promotional articles here (2016) and here (2017), as well as hosting an information page about the technology here.

And finally, for the full history of ammonia production technology in 42 minutes, here’s the video of Thomas Hager’s keynote speech at the 2017 ARPA-E Energy Innovation Summit.

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