Light sparks conversion of dinitrogen to ammonia

In an effort to improve on the way both nature and industry run chemical reactions, researchers have developed a technique that uses light to spur a bacterial enzyme to convert dinitrogen (N₂) into ammonia (NH₃). The method might one day lead to a more environmentally friendly industrial process for making NH₃.

Currently, the iron-catalyzed Haber-Bosch process, used to make NH₃ from N₂, is one of the world’s largest industrial processes in terms of tonnage produced. Ammonia is a major ingredient of fertilizer, and in the past century, the Haber-Bosch process has boosted crop production so much that the world population would be unsustainable without it. Ammonia is also a potential energy source for NH₃-based fuel cells that power motor vehicles.

But the Haber-Bosch process requires high temperatures and pressures, is powered by fossil fuels, and emits large amounts of the greenhouse gas carbon dioxide. Nitrogen-fixing bacteria carry out the same process at room temperature with nitrogenase enzymes, but the eight-step biological process is slow and requires large amounts of the cellular fuel adenosine triphosphate (ATP).

Researchers have been trying for years to harness the best of both worlds by developing ways to combine the high efficiency of the Haber-Bosch process with the mild conditions, low greenhouse gas emissions, and reduced fossil-fuel dependence of biological nitrogen fixation. Now, Paul W. King of the National Renewable Energy Laboratory and coworkers have come up with a potentially fruitful approach (Science 2016, DOI: 10.1126/science.aaf2091).

They tested various redox-active nanocrystals before coming up with a cadmium sulfide (CdS) nanorod that efficiently transfers electrons to one of the nitrogenase complex’s two proteins—molybdenum-iron (MoFe) protein, which uses electrons to power N₂-to-NH₃ conversion. The nanorod replaces nitrogenase’s other protein, Fe protein, by substituting light-driven energy for Fe protein’s ATP-driven energy generation process. The hybrid technique could use solar power to reduce N₂ to NH₃ and would produce no CO₂. It works 63% as fast as native nitrogenase right now. But it might never reach the speed of the Haber-Bosch process because even native nitrogenases are slow in comparison.

Even so, biohybrid catalysis could be an effective route “to solar-driven reduction of small molecules such as N₂ and CO₂, reactions that are traditionally difficult to activate,” says nanocrystal catalysis specialist Peidong Yang of the University of California, Berkeley. “It takes advantage of the best of two worlds: the great light-harvesting capability of synthetic semiconductor nanostructures and the catalytic power of biology.”