Electrolytic Ammonia Synthesis from Hydrogen Chloride and Nitrogen Gases with Simultaneous Recovery of Chlorine under Atmospheric Pressure

Abstract

It was confirmed that ammonia was synthesized by bubbling hydrogen chloride gas into a LiCl-KCl-CsCl melt containing nitride ions at 573 K. Then, anodic galvanostatic electrolysis was conducted at 20 mA cm −2 using a glassy carbon working electrode and a nitrogen gas counter electrode, which showed that chlorine evolved at the anode by the oxidation of chloride ions, and that nitrogen was reduced to nitride ions at the cathode. The obtained results confirmed that ammonia and chlorine were produced from hydrogen chloride and nitrogen as the overall reaction.Hydrogen chloride is produced as a by-product in various industrial chlorination processes. 1 Approximately half of the chlorine produced in the world, which amounts to tens of millions of metric tons per year, is wasted as hydrogen chloride or chloride salts in the current world chlorination processes. Therefore, effective utilization of the hydrogen chloride by-product is required. Since the recovery of chlorine is often the best way from the viewpoints of economy and the environment, several chlorine recovery processes have been reported. 2-5 On the other hand, the fixation of nitrogen as ammonia is also an important process because ammonia is an essential ingredient of fertilizer, chemicals, and so on. 6 The authors have proposed an electrolytic ammonia synthesis method from nitrogen and various hydrogen sources, such as hydrogen, 7 methane, 8 hydrogen sulfide, 9 and water, 10 in molten salts under atmospheric pressure. Here, the authors propose a novel electrochemical method that utilizes hydrogen chloride as a hydrogen source for ammonia synthesis and simultaneously recovers chlorine from hydrogen chloride. The principle is shown in Fig. 1. The electrolyte is molten alkali metal halides. Nitrogen gas is reduced to nitride ion ͑N 3− ͒ at the cathode 11Hydrogen chloride gas is introduced into the melt between the anode and the cathode, preferably in the vicinity of the cathode. Then, the nitride ions formed at the cathode react with hydrogen chloride in the melt to form ammoniaAt the anode, chlorine gas is produced by the oxidation of chloride ionsIn total, ammonia and chlorine are produced from hydrogen chloride and nitrogen gases under atmospheric pressureThe theoretical standard electrolysis voltage is calculated to be 1.06 V at 573 K from the standard Gibbs energy change of Reaction 4 306 kJ mol −1 . 12 Since the standard electrode potential of N 2 /N 3− couple was reported to be 0.281 V ͑vs. Li + /Li͒ 13 in LiClKCl-CsCl melt at 573 K, the standard Gibbs energy of formation of Li 3 N in the melt can be evaluated as −81.3 kJ mol −1 . The standard Gibbs energies of formation of NH 3 ͑g͒, HCl͑g͒, and LiCl͑l͒ at 573 K are reported as 13.1, −97.8, and −345.0 kJ mol −1 , respectively. 12 Then, the equilibrium constant of Reaction 5 is calculated to be 1.0 ϫ 10 59 3HCl + Li 3 N → NH 3 + 3LiCl ͓5͔

Recent progress towards the electrosynthesis of ammonia from sustainable resources

“…The Ito group have also demonstrated that ammonia can be produced by an analogous process, but using HCl as the proton donor. 37 In this case, the nitride reacts with bubbled HCl gas to produce ammonia and chloride ions. These chloride ions can then be removed from the melt electrolytically as Cl 2 .…”

Progress in the Electrochemical Synthesis of Ammonia

“…The same cell also operated for short durations with different hydrogen sources such as methane [43], hydrogen sulfide [59] and hydrogen chloride [60]. In a similar study, the effect of replacing hydrogen with steam on the ammonia formation rate was investigated [57,58].…”

Alternative Strategies Toward Sustainable Ammonia Synthesis

“…In the presence of N 3− , however, a portion of NH 3 could dissolve to form imide (NH 2− ) and amide (NH 2 − ) anions, which might lower the NH 3 production rate [61]. Apart from H 2 , other hydrogen sources, such as H 2 O [62,63], HCl [64], H 2 S [65] and CH 4 [66], were explored for electrochemical NH 3 synthesis in LiCl-KCl-CsCl eutectic melts.…”