Coauthor and CU-Boulder assistant professor Gordana Dukovic [Credit: CU-Boulder]
A group of U.S. scientists report a new light-driven nitrogen-fixing process for converting dinitrogen (N2) into ammonia (NH3)—the main ingredient in fertilizer (Science, doi: 10.1126/science.aaf2091). The authors say their process, which makes use of nanomaterials and naturally occurring enzymes, could be the next revolution in fertilizer production, reducing agriculture’s dependence on fossil fuels.
There are two common methods for converting the nitrogen in the atmosphere into a form that’s usable to plants and animals. In the first, bacteria in plant roots catalyze atmospheric nitrogen into ammonia with an enzyme called nitrogenase and a large amount of chemical energy from adenosine triphosphate (ATP) molecules. The second is an energy-intensive industrial process called the Haber-Bösch method, which revolutionized fertilizer production a century ago—but which consumes two percent of the world’s fossil fuel and releases “appreciable amounts” of carbon dioxide into the atmosphere.
Light-harvesting nanocrystals
The new light-driven process integrates nanoscience and biochemistry, eliminating the need for massive amounts of chemical energy from ATP or environmentally unfriendly fossil fuels. It uses cadmium sulfide nanocrystals to harvest energy from sunlight or artificial light. In the system, these nanorods—tuned to absorb photons at a 405-nm wavelength—are combined with molybdenum-iron (MoFe) hydrogenase, the same enzyme used in natural bacterial nitrogen fixation.
As sunlight strikes the nanocrystals, the absorbed photons kick out photoexcited electrons, which the nitrogenase uses for catalytic reduction of dinitrogen to ammonia. After the reaction, an organic chemical buffer, HEPES, serves as a “sacrificial electron donor” that resets the electrons in the nanorods, allowing them to catalyze the next round of the reaction.
Approaching bacterial reaction rates
Thus, under the new process, photon-absorbing nanorods, rather than bacterial ATP, provide the chemical energy and electron source driving ammonia production. During demonstrations, the team found that the system could produce some 315 ± 55 nanomoles of NH3 per milligram of MoFe protein. That’s 63 percent of the natural ATP-coupled reaction rate used by nitrogen-fixing bacteria under optimal conditions. And, the authors note, “the light-harvesting properties of nanomaterials are highly tunable,” a property that sets up the possibility of using such nanomaterials as a customizable electron source to drive other difficult catalytic transformations.
The authors believe their light-driven process could also contribute to the development of cleaner fuel technologies, including fuel cells to store solar energy.
The study was funded by the U.S. Department of Energy’s National Renewable Energy Laboratory and involved researchers from the University of Colorado Boulder (CU-Boulder), Utah State University and Montana State University.