Electrochemical synthesis of ammonia directly from N2 and water over iron-based catalysts supported on activated carbon

Abstract

A new green methodology for the CO₂-free synthesis of ammonia from air and water is presented. The conventional production of H₂ utilizes fossil fuels and causes a massive greenhouse gas release, making ammonia production one of the most energy intensive and highest CO₂ emitting manufacturing processes. In 2014 we introduced an alternative method for efficient ammonia synthesis that utilizes water (along with N₂) instead of H₂ based on electrolysis of nano-structured catalyst suspensions of Fe₂O₃ in low temperature aqueous or higher temperature molten hydroxide electrolytes. Here, this is replaced with a solid Fe₂O₃ catalyst confined to activated charcoal opening pathways to improve the rate and efficiency of ammonia production. Cyclovoltammetric studies show that Fe₂O₃/AC catalysts can inhibit competing hydrogen reduction and enhance reduction of iron. This iron-based catalyst supported on activated carbon (Fe₂O₃/AC) was prepared for use as an electrocatalyst for the electrochemical synthesis of ammonia in molten hydroxide (NaOH–KOH) directly from wet N₂ at atmospheric pressure. XRD analysis shows that the catalyst exhibits a Fe₂O₃ structure. At 250 °C, a voltage of 1.55 V with a current density of 49 mA cm⁻² yielded the highest rate of ammonia formation, 8.27 × 10⁻⁹ mol (s cm²)⁻¹. The highest coulombic efficiency for the 3e⁻ per ammonia formation, 13.7%, was achieved at 1.15 V with a lower average current density of 11 mA cm⁻². This is a promising simple technology for the sustainable synthesis of ammonia in the future.