Iron-sulfur gel provides possible green route to ammonia

To produce the large quantities of fertilizer needed to sustain global food production, industry relies on the Haber-Bosch process to split dinitrogen from the air to synthesize ammonia. But to generate the high temperatures and pressures needed for the reaction, ammonia plants consume fossil fuels and, as a result, contribute significantly to greenhouse gas emissions.

Researchers want to develop a green alternative to the Haber-Bosch process. In a new effort, Mercouri G. Kanatzidis and George C. Schatz of Northwestern University and coworkers used iron-sulfur (Fe₄S₄) clusters, found naturally in the nitrogenase enzymes bacteria use to split dinitrogen, to make a light-driven catalyst that converts N₂ to NH₃ in water at ambient temperature and pressure (Proc. Natl. Acad. Sci. USA 2016, DOI: 10.1073/pnas.1605512113).

After synthesizing the Fe₄S₄ clusters, the researchers link them with [Sn₂S₆]^4– anions and form the complex into a chalcogel, a foamy gel containing the chalcogen element sulfur. The black material absorbs visible light and uses that energy to break the N≡N triple bond, one of the strongest in chemistry. The nitrogen atoms then combine with water-derived hydrogen to make NH₃.

Other researchers have developed transition metal-N₂ complexes that convert N₂ to NH₃, but those systems typically require organic solvents, low temperatures, and/or high pressures. Some previously developed light-based systems also can reduce N₂, but the chalcogel is 10 times as efficient

Hybrid synthetic-biological approaches, such as a nanorod-nitrogenase system (Science 2016, DOI: 10.1126/science.aaf2091, C&EN, April 25, page 9), are much more efficient than the chalcogel system. But the hybrid approach has limitations: Obtaining nitrogenase from organisms is difficult, and the enzymes are not stable for more than a few hours after isolation, Kanatzidis says.

The chalcogel is orders of magnitude less efficient than the Haber-Bosch process. But unlike the Haber-Bosch process or metal-N₂ complexes, the chalcogel catalyst can fix N₂ under quite mild conditions using light as the only driving force, comments photocatalysis specialist Lizhi Zhang of Central China Normal University. Understanding of the chalcogel mechanism remains fragmentary at this point, he says, so further study is still needed to find ways to optimize the catalyst.

Catalysis expert Patrick Holland of Yale University says it isn’t fair to compare the efficiency of the chalcogel to the Haber-Bosch method because the industrial process has been optimized for more than a century. “The chalcogel already makes reasonable amounts of ammonia, so it’s a valid first step toward using light to drive N₂ reduction,” he says. “The catalyst is very modifiable, so it could in principle be improved a lot in the long run.”