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potential of interconversion of ammonia and dinitrogen is known to References
be E=+0.092V versus SHE (E=–0.53V versus Fc)36,37, and the over-
potential of this ammonia oxidation system is estimated to be 0.73V
from the onset voltage. The overpotential is relatively high compared
to Pt-based alloy catalysts such as Pt–Ir, which have achieved the low-
est overpotentials of ammonia oxidation reactions thus far4,6, while
the overpotential is low to comparable to Ni-based catalysts, which
are among the most densely studied ammonia oxidation electrode
catalysts4,6. Under the catalytic conditions shown in Fig. 4b(iv) the
that the present Ru catalyst maintains its reactivity for ~4h under the
current electrochemical conditions.
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We investigated the catalytic current in more depth accord-
ing to equation (1) (icat, catalytic current; n, number of electrons;
F, Faraday constant; A, surface area of working electrode; C, bulk
concentration of catalyst; D, diffusion coefficient of catalyst) to
determine the rate constant of catalysis (kobs)38. Under the catalytic
conditions (Fig. 4b(iv)), we observed a plateau-shaped voltammo-
gram with a constant current of icat =200μA, irrespective of the
scan rate below 1.0mVs−1. With the values of icat and other vari-
ables (n=6, F=96,485Cmol−1, A=0.071cm2, C=1.0μmolcm−3
and D=8.38×10−6 cm2 s−1) in hand, we estimated a rate constant
of kobs =2.8s–1 for the production of a dinitrogen molecule. We also
confirmed that the present electrocatalysis works with direct use of
ammonia solution with a rate constant of kobs =0.5s–1 (Fig. 4b(vi,vii)).
These electrochemical data suggest that ruthenium complex 1a
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works as a new anode catalyst for ammonia oxidation4,6,39
:
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(1)
icat = nFAC Dkobs
In summary, we have succeeded in developing a novel reaction
system to convert ammonia into dinitrogen, catalysed by ruthenium
complexes supported by 2,2′-bipyridyl-6,6′-dicarboxylate ligands.
This process has long attracted attention in various scientific con-
texts, such as molecular models of reversibility in the Haber–Bosch
and utilization of ammonia as an energy carrier1–3. It is noteworthy
that the present ammonia oxidation is achieved through a combina-
tion of single-electron transfer-type oxidants and bases. We believe
that the present findings have opened up a new field of catalysis to
achieve an energy extraction process from ammonia that leads to
the utilization of ammonia as an energy carrier.
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methods
A representative procedure for the ammonia oxidation reaction is as follows. To
a Schlenk flask (internal volume of 95ml) are added 2a (735mg, 0.900mmol),
NH4OTf (501mg, 3.00mmol) and 1a (6.0mg, 0.010mmol). MeCN (5ml,
degassed under Ar) is added to the mixture at −40°C. A microtube containing
2,4,6-collidine (0.40ml, 3.0mmol) is placed in the Schlenk flask to separate the
2,4,6-collidine from other reagents. After the Schlenk flask is sealed, the Schlenk
flask is evacuated and backfilled with Ar 10 times. The 2,4,6-collidine is mixed with
other reagents. The reaction mixture is then stirred at –40°C for 2h, and further
stirred at room temperature for 4h. After the reaction, a part of the headspace
gas (100μl) is taken with a gas-tight syringe through a silicone rubber septa,
and the gas is analysed with GC. The amount of generated N2 is then quantified
after correction for aerobic contamination estimated from the O2 signal (aerobic
contamination is estimated by assuming air to be an 8:2 mixture of N2/O2).
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Data availability
Crystallographic data for the structure reported in this Article have been
are available within the Article and its Supplementary Information, or from the
corresponding author upon reasonable request.
Received: 19 November 2018; Accepted: 13 June 2019;
Published online: 24 July 2019
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