Electrochemical Synthesis of Ammonia By a Transition Metal Complex in Ionic Liquid
Authors: Katayama Akira , Inomata Tomohiko , Ozawa Tomohiro , Masuda Hideki 
Published: February 2020

A number of researchers in various chemical fields have investigated the conversion of N2 to NH3 under mild conditions. A transition metal complex has previously been found to promote electrochemical generation of NH3 as the catalysts. For example, Pickett and co-worker reported the electrochemical synthesis of NH3 through protonation of cis–[W(N2)2(PMe2Ph)4] under ambient conditions [1], in which the reaction was carried out in THF–0.2 M [NBu4][BF4] using a toxic Hg–pool cathode as the working electrode at –2.6 V (vs. Fc/Fc+). Furuya and co−workers demonstrated electrochemical reduction of N2 to NH3 using a gas diffusion electrode modified by Fe−phthalocyanine, but the current efficiency of NH3 production was less than 0.1% after controlled−potential electrolysis for 10 min [2]. Becker and co−workers reported that titanocene dichloride, Cp2TiCl2, could reduce N2 to NH3 when the controlled−potential electrolysis (CPE) was carried out at −2.2 V (vs. Ag wire) in MeOH solution containing 0.3 M LiClO4 and 0.25 M catechol [3]. This reaction was proceeded at room temperature under 1 atm using hydrogen atoms from catechol and/or MeOH, but the yield of NH3 per Cp2TiCl2 and the current efficiencies were both found to be quite low (1.45% and 0.28 %, respectively). In order to improve this reaction, we have decided to carry out CPE using a solid polymer electrolyte cell (SPE cell), which is composed of a working electrode (W.E.) and a counter electrode (C.E.) separated by a proton exchange membrane. Electrochemical synthesis of NH3 in an SPE cell using a Ru cathode as W.E. was previously reported [4]. In this case, a proton was generated by oxidation of H2O at C.E., which was transferred to W.E. to react with N2. It is advantageous that the proton originated from H2O oxidation has been employed and the generated O2 has been separated from W.E. by proton exchange membrane. Thus, it is possible to use H2O as the hydrogen source. Furthermore, in order to use Cp2TiCl2 as the metal complex in the SPE cell, we investigated an ionic liquid as the supporting material.

An ionic liquid, which is a salt in a liquid state under ambient conditions, has recently been employed in a number of different research fields, because it has several unique properties such as low volatility, large electrochemical window, high thermal and chemical stabilities, and high electric conductivity [5]. In particular, 1−butyl−1−methylpyrrolidinium tris(pentafluoroethyl)trifluoro−phosphate, [C9H20N]+[(C2F5)3PF3]–, is appropriate for use as a supporting material because of its high stability [6]. The W.E. is conveniently fabricated by coating the ionic liquid [C9H20N]+[(C2F5)3PF3]– supported with a transition−metal complex.

 We have reported the first example of the electrochemical reduction of N2 to NH3 using the W.E. coated with Cp2TiCl2–supported ionic liquid, [C9H20N]+[(C2F5)3PF3]–
, under ambient conditions [7]. When the controlled potential electrolysis was carried out at -1.5 V (vs. Ag/AgCl), the yield of NH3 per Cp2TiCl2 and current efficiency were 27% and 0.2%, respectively, which are significantly higher in comparison with those reported previously [3, 7].

 In this paper, we will report the controlled potential electrolysis by Cp2TiCl2-supported [C9H20N]+[(C2F5)3PF3]– in other experimental conditions.

[1] C. J. Pickett et al., Nature, 1985, 317, 652–653.

[2] N. Furuya et al., J. Electroanal. Chem.,
1989, 263, 171–174.

[3] J. Y. Becker et al., J. Electroanal. Chem.,
1987, 230, 143–153.

[4] C. Lambrou et al., Chem. Commun.,
2000, 1673–1674.

[5] P. Hapiotet et al., Chem. Rev.,
2008, 108, 2238–2264.

[6] N.V. Ignat’ev et al., J. Fluor. Chem.,
2005, 126, 1150–1159.

[7] A. Katayama et al., Electrochem. Commun., 2016, 67, 6–10.