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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.22257
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Given the need to feed a growing world population whilst simultaneously reducing global carbon emissions, it is desired to break the link between industrial production of agricultural fertilisers based on ammonia and the use of fossil fuels. On the other hand, energy storage is a big challenge for renewable electricity. To synthesis basic chemicals such as ammonia from renewable electricity through electrochemical processes is a good option to save on carbon emissions and to reduce the pressure on renewable energy storage1.
Globally 131 million tons of ammonia were produced in 20102. The dominant ammonia production process is the Haber-Bosch process invented in 1904 which requires high temperature (~500°C) and high pressure (150–300 bar), in addition to efficient catalysts3,4. Natural gas or coal is used as the energy source of the ammonia industry. 1.87 tons of CO2 is released per ton of ammonia produced5. Globally 245 million tons of CO2 were released by the ammonia industry in 2010 equivalent to about 50% of the UK CO2 emissions (495.8 million tons) in that year6. In the Haber-Bosch process, the presence of ppm level oxygen may poison the commonly used Fe-based catalysts. In industry, extensive purification of N2 and H2 is needed and this remarkably increases the overall cost of the process7,8. Therefore researchers have been seeking a simpler way for synthesis of ammonia from nitrogen separated from air. To the best of our knowledge, the first report on synthesis of ammonia from nitrogen at room temperature is through the reduction of ligating molecular nitrogen9. Following this pioneering work, there are several key reports on synthesis of ammonia under mild conditions through complex intermediates3,10,11,12,13,14,15. On the other hand, ammonia can be synthesised at room temperature through electrochemical synthesis. In 1985, for the first time, Pickett et al. reported the electrochemical synthesis of ammonia at room temperature through protolysis of cis-[W(N2)2(PMe2Ph)4]16. There were reports on electrochemical synthesis of ammonia from N2 and H2 using Na2SO4 aqueous solution as the electrolyte but the current was quite small17,18. This could be related to the low proton conductivity of Na2SO4 solution. It is expected that the current density and ammonia production rate would be much higher if a conductive electrolyte is applied.
Proton conductors are important electrolytes for electrochemical devices19,20. Some perovskite oxides exhibit high proton conductivity and have been used in solid oxide fuel cells19,21,22. Stoukides reported the electrochemical synthesis of ammonia from N2 and H2 at 570°C based on a solid proton-conducting oxide SrCe0.95Yb0.05O3−δ23. The authors also further reported the synthesis of ammonia directly from N2 and H2O bypassing the process of H2 production24. There are other reports on electrochemical synthesis of ammonia from N2 and H2O in molten salts at a temperature ~300°C25,26. Recently we reported electrochemical synthesis of ammonia at ~500°C based on oxide-carbonate composite electrolytes4,27. Ammonia tends to decompose at ~500°C28 therefore low temperature synthesis is necessary to avoid ammonia decomposition; however, most good low temperature proton conducting materials are based on acidic materials29. Ammonia is a weak base and readily reacts with an acidic membrane reducing the proton conductivity. Sulfonated Nafion has been demonstrated as the best proton-conducting polymer which has been widely used in proton exchange membrane fuel cells (PEMFCs)29. In 2000, Kordali et al. reported the synthesis of ammonia from N2 and H2O based on a Ru/C cathode, Pt anode, 2 M KOH aqueous solution as the electrolyte using Nafion as a separation membrane (not electrolyte) and an ammonia formation rate of ~170 ng h−1 cm−2 (2.78 × 10−8 mol m−2 s−1) was achieved at 20°C30. Although ammonia was synthesised from N2 and H2O at room temperature using 2 M KOH solution as electrolyte, N2 cannot be replaced by air as CO2 in air may react with KOH to form K2CO3. It was reported that ammonia has been synthesised from N2 and H2 based on acidic H+-form Nafion membrane, with Ni-SDC (Sm-doped CeO2) as anode, SmBaCuMO5+δ as cathode with maximum formation rate of 4.1 × 10−9 mol cm−2 s−1 at 25°C31; however, the chemical compatibility of active metal Ni with the strongly acidic Nafion membrane is concerning. The reaction between active Ni and H+-form Nafion may form Ni2+-form Nafion and thus lose proton conduction therefore the reaction would not be sustainable32. In addition, to synthesise ammonia directly from air without the N2 separation stage would be a better choice.