The Feasibility of Electrochemical Ammonia Synthesis in Molten LiCl-KCl Eutectics.
The Feasibility of Electrochemical Ammonia Synthesis in Molten LiCl-KCl Eutectics.
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DOI：
10.1002/anie.201909831
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发表时间：
2019-11
影响因子：
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通讯作者：
I. McPherson;Tim Sudmeier;Joshua Fellowes;Ian Wilkinson;Tim Hughes;Edman Shik Chi Tsang
中科院分区：
文献类型：
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作者：
I. McPherson;Tim Sudmeier;Joshua Fellowes;Ian Wilkinson;Tim Hughes;Edman Shik Chi Tsang
已知熔融的LiCl和相关的低共熔电解质允许以高效率将N2直接电化学还原成N3-。有人提出，这可以与电解池中的H2氧化相结合，以在环境压力下产生NH3。在此，在LiCl-KCl-Li-3 N电池中测试了该提议，发现情况并非如此，因为之前关于N3-直接电化学氧化为NH3的假设过于简化。我们发现，添加到熔融电解质中的Li 3 N促进了在不存在外加电流的情况下H2（H氧化态0）自发且同时地化学还原成H-（H氧化态-1）和NH 2-/NH 2-/NH 3（H氧化态+1）形式的H+，导致NH 3的非法拉第释放。进一步观察到NH 2-和NH 2-具有它们自己的氧化还原化学。然而，这些自发反应使我们能够提出一种替代的，真正的催化循环。通过添加LiH而不是Li 3 N，N2可以被还原成N3-，而化学计量量的H-被氧化成H-2。然后，H2可以自发地与N3-反应形成NH3，再生H-并关闭催化循环。初步测试表明，在最大电流效率为4.2%时，NH3合成速率峰值为2.4×10-8 mol cm-2 s-1。用15 N2进行同位素标记证实所产生的NH3来自催化N2还原。
Molten LiCl and related eutectic electrolytes are known to permit direct electrochemical reduction of N2 to N3- with high efficiency. It had been proposed that this could be coupled with H2 oxidation in an electrolytic cell to produce NH3 at ambient pressure. Here, this proposal is tested in a LiCl-KCl-Li-3N cell and is found not to be the case, as the previous assumption of the direct electrochemical oxidation of N3- to NH3 is grossly over-simplified. We find that Li3N added to the molten electrolyte promotes the spontaneous and simultaneous chemical disproportionation of H2 (H oxidation state 0) into H- (H oxidation state -1) and H+ in the form of NH2-/NH2-/NH3 (H oxidation state +1) in the absence of applied current, resulting in non-Faradaic release of NH3. It is further observed that NH2- and NH2- possess their own redox chemistry. However, these spontaneous reactions allow us to propose an alternative, truly catalytic cycle. By adding LiH, rather than Li3N, N2 can be reduced to N3- while stoichiometric amounts of H- are oxidised to H-2. The H2 can then react spontaneously with N3- to form NH3, regenerating H- and closing the catalytic cycle. Initial tests show a peak NH3 synthesis rate of 2.4×10-8 mol cm-2 s-1 at a maximum current efficiency of 4.2%. Isotopic labelling with 15N2 confirms the resulting NH3 is from catalytic N2 reduction.