Tikalon Blog by Dev Gualtieri

Ammonia Synthesis

March 6, 2017

Ammonia is an important industrial chemical, with nearly 200 million tons being produced annually. About 14 million tons of ammonia are used to make nitrate fertilizers,[1] and the rest is used for such industrial applications as nitriding of metals, neutralization of acids and stack emissions, manufacture of nitric acid for the synthesis of a variety of chemicals. As anyone who's ever painted with latex paint knows, ammonia is used also to stabilize latex to prevent premature coagulation.

As a consequence of its low boiling point (-33.34 °C, -28.0 °F) and high enthaly of vaporization (23.35 kJ/mol, 5.56 kcal/mol at its boiling point), ammonia is used as an industrial refrigerant for the preservation of food, beverages, and as a way to freeze skating rinks. For comparison, water has an enthaly of vaporization of 40.68 kJ/mol, or 9.72 kcal/mol at its boiling point. Because of its toxicity, you won't find ammonia in your home refrigerator, but home refrigerators using ammonia or the likewise toxic gases, methyl chloride and sulfur dioxide, were used about a century ago.

The hazards of such toxic refrigerator gases inspired Einstein and his friend, Leo Szilard, to invent a safe alternative. As Szilard related to MIT physicist, Bernard Feld, Einstein read how an entire family, the parents and several children, had been killed by leaking refrigerator fumes as they slept.[3] Spurred by this tragedy, the "Einstein refrigerator" was invented and patented.[4] While this refrigerator still used the standard gases, it was safer since it had no moving parts and did not require rotary seals (see figure).

Figure from US Patent No. 1,781,541, 'Refrigeration,' by Albert Einstein and Leo Szilard, November 11, 1930

Figure from US Patent No. 1,781,541, "Refrigeration," by Albert Einstein and Leo Szilard, dated November 11, 1930.

The "Einstein refrigerator" is an absorption refrigerator in which the cooling evaporate is absorbed by another liquid, which is run through a heat exchanger to recover the refrigerant liquid for another refrigeration cycle.

(Via Google Patents.[4])

Compounds of ammonia were known to the ancients, and ammonia is named after the Greek god, Ammon. Deposits of the ammonia salt, ammonium chloride (sal ammoniac), were found near the Temple of Ammon in Libya. Pliny the Elder gives a brief mention to ammonium chloride crystals of that region in his Naturalis Historia (Natural History) (see figure).

Pliny's Natural History, Book XXXI, Chapter 39, line 79, mentioning ammonium chloride

Ammonium chloride (sal ammoniac), as mentioned in Pliny's Natural History, Book XXXI, Chapter 39, line 79.[5] My translation of this reads, "For example, the region of Cyrenaica is notable, also, for hammoniacum, which is found beneath the sands. It is similar in color to the alum which is called schiston, it consists of long masses, not transparent, has a foul taste, but is useful as medicine." (Simulated manuscript created using GIMP.)

Sal ammoniac, as an easily procured chemical, was often used in alchemy, and it is was mentioned by Albertus Magnus. It wasn't until 1774 that Joseph Priestley first isolated gaseous ammonia. Claude Berthollet determined the composition of ammonia NH₃ in 1785. Ammonia was produced by simple reaction of nitrates until the 20th century.

They say, "necessity is the mother of invention," and such was true for the industrial scale development of the 1909 Bosch-Haber process for ammonia synthesis from nitrogen gas. Germany's supply of the potassium nitrate (saltpeter) from Chile was thwarted by its enemies during World War I, but ammonia was an essential compound for synthesis of explosives. That's when Carl Bosch of BASF decided to industrialize the ammonia synthesis discovered by German chemist, Fritz Haber.

Any schoolgirl can write the equation for synthesis of ammonia from its elements; viz., N₂ + 3H₂ -> 2NH₃. While the reaction is exothermic (-45.9 kJ/mol, -10.97 kcal/mol) and has a favorable Gibbs free energy change (-16.4 kJ/mol, -3.92 kcal/mol), it does not occur spontaneously, as the existence of free hydrogen and free nitrogen in the atmosphere affirm.

In 1905, Haber noted that use of a catalyst will produced small quantities of ammonia from the elements at 1000 °C. The catalyst for the Bosch-Haber process was potassium-doped iron, formed by addition of potassium hydroxide to iron, but many other catalysts will work, including osmium and uranium. The synthesis is conducted at about 450 °C under 15-25 MPa (2,200 3,600 psi) pressure. Ruthenium, when used as a catalyst, allows for a lower pressure reaction. Presently, about 180 million metric tons of ammonia are synthesized annually by the Bosch-Haber, and similar, processes.[6]

Why we need a catalyst.

Breaking nitrogen and hydrogen molecules into their constituent atoms takes a lot of energy, but all this energy is recovered in the end.

(Wikimedia Commons image by Marsupilami.)

In our energy-conscious world, there are always scientists researching ways to make more energy-efficient chemical syntheses. Since the Bosch-Haber process is an energy-intensive process that's in widespread use, it's a prime candidate for improvement. This does not appear to be an easy task, since no major improvements in ammonia synthesis have appeared in a century. Recently, chemists from the University of Utah (Salt Lake City, Utah), the National University of Ireland Galway (Galway, Ireland), and the Instituto de Catalisis y Petroleoquimica (CSIC, Madrid, Spain) have developed a new method of ammonia synthesis using a bioelectrochemical hydrogen-nitrogen fuel cell.[7-8]

Schematic diagram of a bioelectrochemical hydrogen-nitrogen fuel cell.

(Modified University of Utah image by Ross Milton.)

The key to this novel fuel cell are the natural enzymes, called nitrogenase and hydrogenase, that split nitrogen and hydrogen molecules into their constituent atoms at room temperature. Nitrogen-splitting is common in nature, where nitrogen fixation is the natural method of converting nitrogen to ammonia. One small problem with nitrogenase is that it's sensitive to oxygen, so it must be handled in an oxygen-free environment.[8] The hydrogenase was provided by research team members at the Instituto de Catalisis y Petroleoquimica in Spain.[8]

The nitrogenase and hydrogenase are coupled to the fuel cell electrodes by an intermediary chemical, methyl viologen (N,N'-dimethyl-4,4'-bipyridinium), a common herbicide, that acts an electron transport medium.[7] The anode chamber contains hydrogen, the methylviologen, and a purified metalloenzyme as the hydrogenase. The cathode chamber contains nitrogen, methylviologen, and a purified metalloenzyme as the nitrogenase.[8] The chambers are coupled by a proton exchange membrane, and they contain carbon paper electrodes through which a low voltage is applied, causing the cell to generate a small current by the chemical reaction.[8]

Working hydrogen-nitrogen fuel cell.

Carbon paper electrodes are used for the anode (left) and cathode (right).

Methyl viologen is used as an electron carrier in each half-cell.

(University of Utah image by Ross Milton.)

In experiments, passage of a 60 millicoulomb charge through the cell produced 286 nmol NH₃ per milligram of MoFe protein, corresponding to an efficiency of 26.4 %.[7] Although the ammonia yield at this time is very small, it's a process that produces, rather than consumes, energy. Says Shelley Minteer, a chemistry and materials science and engineering professor at Utah and an author of this study,

"It's a spontaneous process, so rather than having to put energy in, it's actually generating its own electricity."[8]

Ammonia Synthesis

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