Lithium-Ammonia Nitrogen Fixation
6 Li0 + N2 (g) -----------------> 2 Li3N
2 Li3N + 6 H2O ----------------> 6 LiOH + 2 NH3 (g)
6 LiOH + electricity ----------------> 6 Li0 + 1.5 O2 (g) + 3 H2O
Net : N2 (g) + 3 H2O + electricity --------------> 2 NH3 (g) + 1.5 O2 (g) ;
This is the reverse of combustion : 4 NH3 (g) + 3 O2 (g) -------------> 2 N2 (g) + 6 H2O .
Step 1 : Lithium metal reacts with nitrogen gas at an elevated temperature (200 450 ºC) to give a quantitative yield of lithium nitride :
6 Li0 + N2 (g) -----------------> 2 Li3N (100% yield)
m.p. 180.5 ºC reddish-brown hexagonal crystals, m.p. 813 ºC
E. Masdupuy and F. Gallais , Inorg. Synth. 4 , pp. 1-5 (1953) ; F. Schönherr , A. Köhler , and G. Pfrommer , Inorg. Synth. 30 , Nonmolecular Solids , pp. 38-45 (1995) .
Step 2 : The lithium nitride is dissolved in water to give ammonia and lithium hydroxide . The ammonia is distilled from solution , dried , and liquefied for storage . The water is evaporated from the solution to give dry lithium hydroxide . Schönherr et al. mention (p. 41) , The compound [lithium nitride] forms NH3 in humid air ; that is , it is rapidly and quantitatively hydrolysed :
Li3N + 3 H2O -----------------> 3 LiOH + NH3 (g)
Step 3 : The dry lithium hydroxide is melted (m.p. 450 ºC) and electrolyzed to give molten lithium metal and hydrogen gas at the cathode , and oxygen gas at the anode . Analogous electrolyses of molten sodium and potassium hydroxides were used in 1807 by the British chemist Sir Humphry Davy (1778-1829) to isolate the alkali metal elements sodium and potassium , respectively , for the first time . The redox reactions involved are as follows :
Cathode : 4 Li1+ + 4 e- -----------------> 4 Li0 E0red = 3.040 V
Anode : 4 OH- 4 e- -----------------> O2 (g) + 2 H2O (g)
E0ox = 0.401 V
The water molecules diffuse to the cathode , where they react with the lithium to give hydrogen gas and lithium hydroxide (which is recycled internally) :
2 Li0 + 2 H2O (g) ---------------> 2 LiOH + H2 (g)
Net : 2 Li+ + 2 OH- ---------------> 2 Li0 + H2 (g) + O2 (g) ; E0T = 3.441 V .
The molten lithium metal (m.p. 181 ºC) is tapped off from the electrolysis cell and recycled back to the nitrogen reactor . The oxygen and hydrogen by-products can be either vented , or collected , dried , and liquefied for storage and sale .
The lithium electrolysis cell could be designed and operated in a manner similar to the Castner cell , in which sodium hydroxide is melted and electrolysed to produce sodium metal . The Castner cell is described in the following references :
J.R. Partington , A Text-Book of Inorganic Chemistry , sixth ed. , Macmillan , London (UK) , 1957 ; pp. 683-685 ; and ,
C.H. Lemke , Sodium and Sodium Alloys, pp. 181-204 in the Kirk-Othmer Encyclopedia of Chemical Technology , vol. 21 , third ed. , M. Grayson and D. Eckroth (eds.) , John Wiley , New York (1983) ; see especially pp. 187-189 for a description of the Castner cell .
Note that lithium metal usually isn't produced in a Castner cell , but rather only in a Downs cell , which uses a molten chloride salt electrolyte consisting of 55% LiCl and 45% KCl :
R. Bach and J.R. Wasson , Lithium and Lithium Compounds, pp. 448-476 in the Kirk-Othmer Encyclopedia of Chemical Technology , vol. 14 (1981) ; esp. p. 456 .
I think it prudent in this approach to ammonia to avoid any chemistry involving halide salts or halogens .
The high theoretical cell potential , 3.441 V (the thermodynamic ideal , ignoring the inevitable overvoltages) , indicates a considerable amount of electricity will be required for the electrolysis of molten lithium hydroxide . Additionally , heat energy will be required for all three steps (the nitriding reaction , drying the ammonia and LiOH , and melting the LiOH in the electrolysis cell) , and in general plant operations . The overall lithium-ammonia nitrogen fixation process will thus be very electrical and thermal energy-intensive .
The ammonia product would probably be quite costly , and could not compete economically with the ammonia produced from the hydrogenation of atmospheric nitrogen , usually by the Haber-Bosch or a related process . Unfortunately , the hydrogen used for ammonia production is almost entirely obtained from the steam reforming of either natural gas (mostly in North America) , or coal (elsewhere in the world) . The by-product of such steam reforming of carbon or hydrocarbons is carbon dioxide , which is usually vented to the atmosphere . The above lithium-mediated ammonia synthesis is entirely carbon-free , and produces no atmospheric pollutants . However , it would need large quantities of green energy (electrical and thermal) to become a practical reality . The production of such green energy from hydroelectric , solar , wind , geothermal , tidal , and ocean thermal (OTEC) sources , replacing the present-day fossil fuel economy , lies well in the future .
Recently I became aware of research carried out in this area by Japanese electrochemists (several of their papers are cited below) , who discovered that nitrogen gas can be cathodically reduced to nitride anions , in a high yield , in molten salt mixtures similar to those used in the production of lithium metal in the Downs cell . They extended this finding to design a practical synthesis of ammonia by forcing steam into the cathode chamber , to hydrolyse the nitride anions and distil ammonia gas out of the cell . A possible modification of their process is sketched below , in which the three steps discussed above for the lithium-mediated synthesis of ammonia are combined in a single electrolysis cell :
Lithium and its compounds , including lithium hydroxide , are quite expensive . It might be possible to use a mixture of LiOH diluted down with the much cheaper NaOH (and maybe also KOH) as the molten salt electrolyte , which remains unchanged in the process . The added NaOH and KOH might also form a eutectic mixture with the LiOH , which would melt at a much lower temperature than pure LiOH (450 ºC) . Note that under normal conditions , only lithium metal is reactive enough to reduce nitrogen molecules to nitride anions . The other alkali metals don't usually react at all with nitrogen gas , even when heated . Thus , the presence of lithium in the molten salt electrolyte may be critical to the success of the overall process , with the formation of the transient , but crucial , lithium metal at the cathode .
D.R. Safrany , Nitrogen Fixation, Scientific American 231 (4) , pp. 64-80 (October , 1974) ; discussion of lithium nitride on p. 74 ; economic analysis of the Li3N process on p. 80 .
T. Goto and Y. Ito , Electrochemical Reduction of Nitrogen Gas in a Molten Chloride System, Electrochim. Acta 43 (21-22) , pp. 3379-3384 (1998) .
T. Murakami , T. Nishikiori , T. Nohira , and Y. Ito , Electrolytic Synthesis of Ammonia in Molten Salts Under Atmospheric Pressure, J. Amer. Chem. Soc. 125 (2) , pp. 334-335 (2003) .
T. Murakami et al. , Electrolytic Ammonia Synthesis from Water and Nitrogen Gas in Molten Salt Under Atmospheric Pressure, Electrochim. Acta 50 (27) , pp. 5423-5426 (2005) .
R.B. Steele , A Proposal for an Ammonia Economy, Chemtech 29 (8) , pp. 28-34 (August , 1999) . In this article (p. 34) I proposed another ammonia synthesis scheme involving the reduction of nitrogen gas in aqueous solution by V(OH)2 and Mg(OH)2 [catalyst] at ~ 70 ºC .
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