Conventionally used Haber process generates ammonia from hydrogen and nitrogen gases. The process is still in use today and ushered in a revolution in agriculture. However, the method consumes around one percent of the world’s energy to achieve the high pressures and temperatures that drive the chemical reactions to produce ammonia.
Recently chemists from University of Utah published an alternative method using enzymes derived from nature. This method helps to generate ammonia at room temperature. As a bonus, the reaction generates a small electrical current.
Chemistry and materials science and engineering professor Shelley Minteer and postdoctoral scholar Ross Milton have only produced small quantities of ammonia so far. However, their method could lead to a less energy-intensive source of the ammonia, which is used worldwide as a vital fertiliser.
In biological system conversion of gaseous nitrogen to ammonia is called nitrogen fixation and is accomplished through several pathways, including through an enzyme called nitrogenases. Nitrogenases are the only known enzymes to reduce nitrogen to ammonia. However, only nitrogenase cannot bring about ammonia production. Even in a biological process, it requires another enzyme hydrogenase which strips electrons from hydrogen gas and provides them to the nitrogen-reducing reaction.
Minteer and Milton envisioned a fuel cell system that replicated the nitrogen fixation, using nitrogenase and hydrogenase. They received hydrogenase from collaborators at the Instituto de Catalisis y Petroleoquimica in Spain. The cell consists of two compartments, connected via carbon paper electrodes. In one vial, hydrogen gas is oxidised by hydrogenase and electrons are carried to the anode. In the other, electrons come off the cathode and are combined with nitrogen, via nitrogenase, to create ammonia.
Protons travel through a membrane between the anodic and cathodic chambers, supplying the hydrogen atoms needed to synthesise ammonia. The movement of the electrons creates current and is the source of the small amount of electrical power generated by the reaction. However, several challenges remain to be overcome before the researchers’ small-scale process can find the application at an industrial scale.