Synthesis of ammonia directly from air and water at ambient temperature and pressure

Abstract

The N≡N bond (225 kcal mol−1) in dinitrogen is one of the strongest bonds in chemistry therefore artificial synthesis of ammonia under mild conditions is a significant challenge. Based on current knowledge, only bacteria and some plants can synthesise ammonia from air and water at ambient temperature and pressure. Here, for the first time, we report artificial ammonia synthesis bypassing N2 separation and H2 production stages. A maximum ammonia production rate of 1.14 × 10−5 mol m−2 s−1 has been achieved when a voltage of 1.6 V was applied. Potentially this can provide an alternative route for the mass production of the basic chemical ammonia under mild conditions. Considering climate change and the depletion of fossil fuels used for synthesis of ammonia by conventional methods, this is a renewable and sustainable chemical synthesis process for future.

Recent progress towards the electrosynthesis of ammonia from sustainable resources

“…53 At 25 ⁰C and a cell bias of 1.8 V, the maximal rate of ammonia production using air as the nitrogen source was measured as being 0.04 mol h -1 m -2 (up to 0.126 mol h -1 m -2 was possible when pure N 2 was used in place of air). The Faradaic yield for ammonia production from air was around 0.5%, largely on account of competing hydrogen evolution from the aqueous reaction medium (Pt/C is an excellent hydrogen evolution electrocatalyst).…”

Electrochemical Synthesis of Ammonia: A Low Pressure, Low Temperature Approach

“…In addition, the acidic environments require costly materials of construction compared to a basic environment and severely limit the options for catalyst materials, potentially eliminating many highly active and selective catalysts from the design. While PEMs have shown extremely long lifetimes and fast ion transport in other electrochemical applications, ammonia is a weak base, and it is expected that it will react with acidic membranes to reduce proton conductivity 18 and, speculatively, membrane lifetime. In contrast, using alkaline chemistry reduces the membrane reactivity with ammonia, enables low-cost materials of construction, and allows the utilization of a wider array of low-cost and active catalysts.…”

“…Proton exchange membrane (PEM) materials are well established and have been recently incorporated into a number of ammonia synthesis devices. [18][19][20][21][22][23] In addition, the Energy and Environmental Research Center (EERC) in Grand Forks North Dakota has also some highly relevant work in this area, with demonstration of large reductions in energy usage by using an integrated acid-based electrochemical-thermal ammonia production process that operates at a reaction temperature of 200-400 °C. 24 Work at elevated temperatures (200 °C) has also been conducted using nanoscale Fe 2 O 3 in molten hydroxide and basic electrolyte.…”

“…The field of catalyst development for electrochemical ammonia synthesis is small but growing, with ammonia production demonstrated on a variety of electrode materials from precious metals such as platinum 18 and ruthenium 9,29 to non-precious metal-based copper, iron, and nickel materials. 20,30,31 Most recently, Licht and co-authors demonstrated significantly higher ammonia production rates and faradaic efficiencies in a molten hydroxide electrolyte cell with a nano-Fe 2 O 3 catalyst and at slightly elevated temperatures (105 °C-200 °C).…”

Ambient nitrogen reduction cycle using a hybrid inorganic–biological system

“…A distributed approach toward NH 3 synthesis from renewable energy sources at ambient conditions would enable on-site deployment and reduce CO 2 emissions. To this end, significant effort has been devoted to promoting the reduction of nitrogen to NH 3 with the use of transition metal catalysts (3)(4)(5), electrocatalysts (6), photocatalysts (7-11), purified nitrogenases (N 2 ases) (11,12), and heterotrophic diazotrophs (13,14), potentially powered by renewable energy and operating at ambient conditions. Such approaches, however, typically use sacrificial reductants to drive conversion at low turnover or suffer poor selectivity.…”