Materials for electrochemical ammonia synthesis McPherson, Ian James; Sudmeier, Tim; Fellowes, Joshua ...
Dalton transactions : an international journal of inorganic chemistry,
02/2019, Letnik:
48, Številka:
5
Journal Article
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Direct electrochemical synthesis of ammonia is proposed as a means of reducing the carbon footprint of the fertiliser industry, as well as providing new opportunities for carbon-free liquid energy ...storage. We review the current status of research into materials for electrochemical ammonia synthesis and evaluate the reported rates and efficiencies in terms of recent US Department of Energy targets. Surprisingly, development of electrocatalysts has only recently received much attention, and despite a number of promising rates, the target values remain distant. A number of theoretical studies suggest a range of candidate materials yet to be explored.
Direct electrochemical synthesis of ammonia is proposed as a means of reducing the carbon footprint of the fertiliser industry, as well as providing new opportunities for carbon-free liquid energy storage.
The wheel‐shaped, potassium‐templated 27‐tungsto‐3‐arsenate(III) K⊂{(β‐AsIIIW8O30)(WO(H2O))}314– (1) was synthesized by simple one‐pot condensation of the trilacunary B‐α‐AsIIIW9O339– precursor in ...aqueous, acidic KCl solution. Polyanion 1 comprises three β‐{AsW8O30} units linked via three trans‐{WO(H2O)} groups, forming a cyclic assembly with a potassium ion located in the central cavity. The rubidium‐analogue Rb⊂{(β‐AsIIIW8O30)(WO(H2O))}314– (2) could also be prepared, by addition of a small amount of Rb+ ions to a solution containing a large excess of K+ ions. This indicates that the rigid {(β‐AsIIIW8O30)(WO(H2O))}315– (As3W27) host exhibits high selectivity for Rb+ ion guests compared to K+ ions. Polyanions 1 and 2 were characterized in the solid state by single‐crystal XRD, FT‐IR, TGA, and elemental analysis.
We have prepared a trimeric, wheel‐shaped 27‐tungsto‐3‐arsenate(III) host, M⊂As3W27 (M = Rb, K), which exhibits high selectivity for Rb+ ions, in analogy to organic crown ethers.
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–Li3N 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 H2. 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.
Die zentralen Herausforderungen und Möglichkeiten der Verwendung geschmolzener LiCl‐Eutektika als Medium für die direkte elektrochemische Reduktion von N2 zu Ammoniak werden identifiziert. Durch Zugabe von LiH wird N2 zu N3− reduziert, während stöchiometrische Mengen von H− zu H2 oxidiert werden. Das H2 reagiert dann spontan mit N3− zu NH3, H− wird regeneriert und der katalytische Zyklus geschlossen.
There is an exciting possibility to decentralize ammonia synthesis for fertilizer production or energy storage without carbon emission from H2 obtained from renewables at small units operated at ...lower pressure. However, no suitable catalyst has yet been developed. Ru catalysts are known to be promoted by heavier alkali dopants. Instead of using heavy alkali metals, Li is herein shown to give the highest rate through surface polarisation despite its poorest electron donating ability. This exceptional promotion rate makes Ru–Li catalysts suitable for ammonia synthesis, which outclasses industrial Fe counterparts by at least 195 fold. Akin to enzyme catalysis, it is for the first time shown that Ru–Li catalysts hydrogenate end‐on adsorbed N2 stabilized by Li+ on Ru terrace sites to ammonia in a stepwise manner, in contrast to typical N2 dissociation on stepped sites adopted by Ru–Cs counterparts, giving new insights in activating N2 by metallic catalysts.
Lithium treatment: Introduction of Li+ on Ru‐based catalysts can polarize and stabilize adsorbed dinitrogen on the metal surface, which facilitates the non‐dissociative pathway to produce ammonia under mild conditions. The Li–Ru catalysts are suitable for new green ammonia synthesis at lower pressure, and many times better than the commercial Fe counterparts.
Here we demonstrate the preparation of enzyme-metal biohybrids of NAD
reductase with biocatalytically-synthesised small gold nanoparticles (NPs, <10 nm) and core-shell gold-platinum NPs for tandem ...catalysis. Despite the variety of methods available for NP synthesis, there remains a need for more sustainable strategies which also give precise control over the shape and size of the metal NPs for applications in catalysis, biomedical devices, and electronics. We demonstrate facile biosynthesis of spherical, highly uniform, gold NPs under mild conditions using an isolated enzyme moiety, an NAD
reductase, to reduce metal salts while oxidising a nicotinamide-containing cofactor. By subsequently introducing platinum salts, we show that core-shell Au@Pt NPs can then be formed. Catalytic function of these enzyme-Au@Pt NP hybrids was demonstrated for H
-driven NADH recycling to support enantioselective ketone reduction by an NADH-dependent alcohol dehydrogenase.
Here we demonstrate the preparation of enzyme‐metal biohybrids of NAD+ reductase with biocatalytically‐synthesised small gold nanoparticles (NPs, <10 nm) and core‐shell gold‐platinum NPs for tandem ...catalysis. Despite the variety of methods available for NP synthesis, there remains a need for more sustainable strategies which also give precise control over the shape and size of the metal NPs for applications in catalysis, biomedical devices, and electronics. We demonstrate facile biosynthesis of spherical, highly uniform, gold NPs under mild conditions using an isolated enzyme moiety, an NAD+ reductase, to reduce metal salts while oxidising a nicotinamide‐containing cofactor. By subsequently introducing platinum salts, we show that core‐shell Au@Pt NPs can then be formed. Catalytic function of these enzyme‐Au@Pt NP hybrids was demonstrated for H2‐driven NADH recycling to support enantioselective ketone reduction by an NADH‐dependent alcohol dehydrogenase.
Under mild conditions, small (<10 nm), spherical, monodisperse metal nanoparticles (NPs) can be synthesised using an NAD+ reductase enzyme as reductant and stabilizer, forming Au NPs and Au@Pt (core‐shell) NPs. The enzyme‐Au@Pt NP hybrids demonstrated promising activity as a H2‐driven NADH cofactor recycling system, coupled with an alcohol dehydrogenase (ADH).
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–Li3N 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 H2. 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.
The key challenges and opportunities of using molten LiCl eutectics as media for the direct electrochemical reduction of N2 to ammonia are identified. By adding LiH, rather than Li3N, N2 can be reduced to N3− while stoichiometric amounts of H− are oxidised to H2. The H2 can then react spontaneously with N3− to form NH3, regenerating H− and closing the catalytic cycle.
This thesis explores the activity of single atom metal catalysts on graphene-like support (SACs) and metal nitrides for the nitrogen reduction reaction (NRR). Furthermore, extensive analysis of HER ...activity and in-situ characterisation are carried out to derive a structure-activity relationship of SACs in the hydrogen evolution reaction (HER). To that end, a library of single atom transition metals on nitrogen-doped graphene (M-NGs) with comparable compositions is synthesised by a carbon backbone method. Their HER activity is assessed showing Ru-NG and Co-NG to be highly active. The data is correlated with the φ factor, a computationally suggested descriptor for electron density on the metal site constituting the first experimental substantiation of this design principle. Additionally it is found that HER activity on cobalt-containing SACs can be further tuned by changing the coordination environment of the metal via the inclusion of oxygen into the first coordination shell. In in-situ XANES and IR spectroelectrochemical thiocyanate poisoning experiments three distinct active sites are observed and it is suggested that distortion from the square planar Co-N₄ environment caused by axial oxygen ligands leads to decreased HER activity. In the second part of this thesis Co₃Mo₃N was identified as a potential electrocatalyst for the direct reduction of N₂ to N³- in molten chloride eutectics. Based on experimental evidence and previous literature reports, a Mars van Krevelen-type mechanism is suggested for the reaction in which Li⁺ ions facilitate the activation of dinitrogen, as well as enable participation of the lattice nitrogen of Co₃Mo₃N.
Abstract Here we demonstrate the preparation of enzyme‐metal biohybrids of NAD + reductase with biocatalytically‐synthesised small gold nanoparticles (NPs, <10 nm) and core‐shell gold‐platinum NPs ...for tandem catalysis. Despite the variety of methods available for NP synthesis, there remains a need for more sustainable strategies which also give precise control over the shape and size of the metal NPs for applications in catalysis, biomedical devices, and electronics. We demonstrate facile biosynthesis of spherical, highly uniform, gold NPs under mild conditions using an isolated enzyme moiety, an NAD + reductase, to reduce metal salts while oxidising a nicotinamide‐containing cofactor. By subsequently introducing platinum salts, we show that core‐shell Au@Pt NPs can then be formed. Catalytic function of these enzyme‐Au@Pt NP hybrids was demonstrated for H 2 ‐driven NADH recycling to support enantioselective ketone reduction by an NADH‐dependent alcohol dehydrogenase.
The catalytic synthesis of NH 3 from the thermodynamically challenging N 2 reduction reaction under mild conditions is currently a significant problem for scientists. Accordingly, herein, we report ...the development of a nitrogenase-inspired inorganic-based chalcogenide system for the efficient electrochemical conversion of N 2 to NH 3 , which is comprised of the basic structure of Fe–S 2 –Mo. This material showed high activity of 8.7 mg NH 3 mg Fe −1 h −1 (24 μg NH 3 cm −2 h −1 ) with an excellent faradaic efficiency of 27% for the conversion of N 2 to NH 3 in aqueous medium. It was demonstrated that the Fe 1 single atom on Fe–S 2 –Mo under the optimal negative potential favors the reduction of N 2 to NH 3 over the competitive proton reduction to H 2 . Operando X-ray absorption and simulations combined with theoretical DFT calculations provided the first and important insights on the particular electron-mediating and catalytic roles of the Fe–S 2 –Mo motifs and Fe 1 , respectively, on this two-dimensional (2D) molecular layer slab.
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