New Ways To Nab Nitrogen

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. 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 co-workers 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 co-workers 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 in April at the American Chemical Society national meeting in San Francisco. 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 have not 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